Methodologies for determination of antimony in terrestrial environmental samples

Contents

• 1 Introduction

• 2 The determination of antimony in aqueous samples
  • 2.1 Total antimony determinations
  • 2.2 Speciation determinations

• 3 Determination of antimony in soils, sediments and biological materials
  • 3.1 Total antimony determinations
  • 3.2 Solid state partitioning

• 4 Conclusions

• References

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Abstract

Methodologies for the environmental analysis of total antimony and aqueous chemical speciation are critically reviewed, including preparation techniques for aqueous and solid matrices and the determination of solid state partitioning and recommendations are given for future research directions. Concentrations of total antimony commonly present in aqueous and solid environmental samples are readily determined using present day analytical techniques. This has resulted primarily from technological advances in microwave digestion for solid matrices and the development of plasma based analyte detection systems. ICP-AES and ICP-MS techniques are both utilised for the environmental analysis of total antimony concentrations. However, ICP-MS is increasingly favoured as a result of reduced spectral interferences and the potential for analyte detection in the pg mL⁻¹ range. Determination of aqueous antimony speciation presents a number of complex analytical challenges and highly selective separation and identification techniques are required prior to detection. The majority of published techniques including common applications of hydride generation are insufficiently selective for the determination of intrinsic chemical speciation and often only oxidation state data are obtained. The recent in-line applications of HPLC-ICP-MS offer the potential for highly selective separations of aqueous antimony species and determination of detailed chemical speciation data. However, considerable development work is required to optimise chromatographic separations and identify uncharacterised species resident in environmental systems. Analytical techniques to aid the determination of antimony's associations with solid environmental matrices include the application of chemical extraction procedures and leaching experiments. To date, this area of analytical research has received little attention and further studies are required to elucidate this aspect of antimony's environmental chemistry.

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1 Introduction

As evidence for antimony's toxicity continues to accumulate,^1–7 there is increasing concern relating to the concentrations and chemical forms of this element in environmental systems. Particular concerns relate to the issue of chronic antimony exposure and the associated long-term effects that may prevail. As a result of this, the determination of antimony in environmental samples has received growing interest in recent years.

In this review the analysis of total antimony is described in both aqueous and solid environmental samples with emphasis on freshwaters, soils, sediments and biological materials. The relative merits of a range of sample preparation and analysis techniques are discussed including aqueous sample preconcentration and matrix digestion. In addition to this, an assessment is undertaken of analytical techniques for the determination of aqueous and solid state speciation of antimony. The extent of progress in this field of analytical research is discussed including recommendations for future research directions.

Natural sources of antimony in the environment result from rock weathering and soil runoff processes whilst anthropogenic sources include fossil fuel combustion, mining and smelting activity, and the application of `superphosphate' fertilisers to agricultural soils. The release of antimony from these sources has led to almost ubiquitous concentrations throughout environmental systems, and global soil concentrations⁸ are estimated at 1 mg kg⁻¹ whilst fresh and saline waters^9–13 exhibit concentrations between 0.01–5.6 ng mL⁻¹.

The determination of antimony in environmental samples can be problematic due to the very low concentrations often present. Particular difficulties may be encountered with the analysis of solid samples as a result of both analytical interference effects and analyte loss by volatilisation. Therefore, the chosen sample preparation and analysis technique is critical and the intrinsic nature of the sample must be carefully considered prior to analysis. The elucidation of aqueous antimony speciation presents further complex analytical challenges and in recent years attention has begun to focus on this area of analytical research. Environmental samples may contain antimony in the (III) and (V) oxidation states and both organic and inorganic species can be formed. The toxicity of antimony is largely dependent upon chemical speciation and the most toxic characteristics are displayed by the inorganic antimony (III) oxyanion.^2,3,5 Improved analytical data relating to both total antimony and chemical speciation are essential if we are to understand the significance of antimony concentrations in environmental systems.

2 The determination of antimony in aqueous samples

Most analytical techniques have an intrinsic requirement for liquid sample introduction and this often allows the analysis of aqueous environmental samples with minimal sample preparation. Total antimony concentrations are determined using a range of analytical instrumentation. However, highly selective separation and identification techniques are required prior to the detection of particular aqueous antimony species. Sections 2.1 and 2.2 focus on sample preparation and analytical techniques for total antimony and species specific determinations in aqueous samples, respectively.

2.1 Total antimony determinations

The determination of total antimony in naturally derived water samples is often problematic due to the low analyte concentrations, often in the ng mL⁻¹ range. However additional difficulties may result from high matrix ion concentrations and subsequent analytical interference. Preconcentration or separation of the analyte prior to analysis can alleviate these problems, however, improvements in analyte detection depend upon the original sample volume used and the sample concentration factor.¹⁴ Analytical techniques for the preconcentration and extraction of total antimony in aqueous samples are shown in Table 1.

Table 1 Analytical techniques for the preconcentration and extraction of total antimony in aqueous samples^a

Application	Extraction/preconcentration technique	Detection system	LOD/ng mL^−1	Ref.
a N.R., not reported; CPC, cetylpyridinium chloride; TX-100, Triton X-100; HG, hydride generation; GF-MIP-AES, graphite furnace microwave induced plasma atomic emission spectrometry; AAS, atomic absorption spectrometry; GC-PID, gas chromatography photoionisation detection; PGM, palladium group metals; GFAAS, graphite furnace atomic absorption spectrometry.
Rain water	Preconcentration with CPC and TX-100 and complexation with Brilliant Green	Spectrophotometry	3	17
Synthetic solutions	HG with cold trapping	MIP-AES	0.4	15
Synthetic solutions	HG with hot trapping	GF-MIP-AES	0.35	15
River water	Acidification and elution through Chelex 100 resin	AAS	N.R.	20
Synthetic solution	Continuous HG with cold trapping	GC-PID	0.1–0.2	14
Synthetic solutions	In situ chemisorption with PGM substrates	GFAAS	0.0029	16
Sea water	Preconcentration of SbH3 with hot aqua regia	HG-X-ray fluorescence	N.R.	19

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The incorporation of temperature reliant collection traps in conjunction with hydride generation (HG) can be effective in-line techniques.^14,15 These trapping systems promote the preconcentration and separation of stibine (SbH₃) required for analysis and can improve detection limits of conventional hydride generation from 3 ng mL⁻¹ to around 0.35 ng mL⁻¹ with atomic emission spectrometric (AES) detection.¹⁵ In addition to these techniques, novel in situ chemisorption trapping has also been developed.¹⁶ This in-line preconcentration technique is coupled to an atomic absorption spectrometric (AAS) detection system and enables detection limits of around 0.0029 ng mL⁻¹ although applications to environmental analysis are not reported.

Aside from in-line preconcentration, separate pre-analysis techniques are also described and include solvent extrac-tion^17–19 and selective adsorption.²⁰ These techniques promote trace level determination by reducing analytical interference, and particular advantages may result when used in conjunction with hydride generation. Common interferences of hydride generation result from the presence of additional hydride-forming elements including As, Bi and Sn and precipitation of elements such as Cu, Pb, Cr, Fe, Co and Ni in the sample solution. Solvent extraction and selective adsorption can successfully remove many problematic elements prior to analysis allowing improved quality of analytical data.^18,20 In one particular study, a selective adsorption procedure was utilised for the analysis of river water samples and analyte concentrations were determined at around 0.1 ng mL⁻¹, however, the detection limit was not quoted.²⁰ Whilst the advantages of aqueous sample preparation are evident, their application to environmental samples requires careful consideration. Aqueous sample introduction is preferable for most analytical techniques and lengthy sample preparation may be avoided by the application of an appropriate analyte detection system. Analytical techniques for the determination of total antimony in aqueous samples are shown in Table 2.

Table 2 Analytical techniques for the determination of total antimony in aqueous samples^a

Application	Method of determination	LOD/ng mL^−1	Ref.
a N.R., not reported; HG, hydride generation; FAAS, flame atomic absorption spectrometry; ICP-AES, inductively coupled plasma atomic emission spectrometry; ICP-MS, inductively coupled plasma mass spectrometry; AAS, atomic absorption spectrometry; ETV-ICP-MS, electrothermal vaporization inductively coupled plasma mass spectrometry.
Synthetic solutions	Selective continuous HG-FAAS	N.R.	21
Water samples	Selective continuous flow HG-ICP-AES	0.41	30
Water samples	Flow injection HG-ICP-MS	0.017	23
Water samples	In-line HG-ICP-AES	3	29
River water	Continuous HG-AAS	N.R.	20
Sea water	Flow injection HG-ICP-MS	N.R.	25
Reference waters	ETV-ICP-MS	0.01	26
Water samples	HG-ICP-MS	0.06	27
Cloud water	ICP-MS	0.005–0.020	24
Synthetic solutions	HG-ICP-MS	0.005	22
Sea water	HG-ICP-MS	0.0022	28

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Graphite furnace and flame AAS techniques (GFAAS and FAAS) are used for total antimony determinations from aqueous samples,^20,21 however, technological advances have led to the use of inductively coupled plasma atomic emission spectrometry (ICP-AES) and more recently inductively coupled plasma mass spectrometry (ICP-MS) for analyte detection.^22–30 The application of ICP-AES and ICP-MS detection systems enables rapid, multi-element capability with large linear ranges and significantly improved detection limits. To date the most favoured technique for determination of antimony in aqueous environmental samples is ICP-MS. This technique allows analyte detection in the pg mL⁻¹ range and total antimony determinations using these detection systems are widely reported.^22–28

Atomic absorption, atomic emission and mass spectrometry are often used in conjunction with hydride generation for total antimony analysis. The use of this approach can improve analytical sensitivity by increasing the efficiency of analyte transport, help separate the analyte from the matrix and reduce associated non-spectroscopic interference effects. The technique requires acidic media or pre-reducing agents to convert all Sb(V) to Sb(III) within the sample solution prior to analysis. This is essential if representative results are to be obtained since only the Sb(III) species is efficiently reduced to the required volatile hydride, SbH₃, for subsequent detection. The coupling of hydride generation to ICP-MS can reduce detection limits for conventional ICP-MS by 20-fold,²² allowing detection of antimony at around 0.005 ng mL⁻¹. However, efficient hydride generation is often problematic, and interference effects from a high background signal, severe memory effects and plasma instability are commonly encountered.^22,24,31 These effects result from matrix ions in the sample solution, chemical reagents for hydride generation and the adhesion of antimony to the inner surfaces of glass components inside the instrument. Problems with matrix ions and memory effects can be reduced by the use of sample preparation techniques as described above and allowing longer washout times between consecutive samples. The alleviation of additional interference effects can be achieved by optimising the chemical conditions for hydride generation and this is currently an active area of analytical research.^32–39

A range of acidic media have been evaluated for total antimony determinations with hydride generation and these include hydrochloric, perchloric, nitric and sulfuric acid reagents.^34,35 These studies indicate that both the acidic medium used and its concentration are critical to the performance of hydride generation. Schramel and Xu³⁴ reported 40% inhibition of the antimony signal in 50% HCl (v/v) in comparison to 5% HCl (v/v). However, in the same study, the use of concentrated acidic medium (containing 30% HCl) reduced significant interference effects from transition metals. Furthermore, a mixed solution of 30% HCl and 20% HNO₃ is reported as the most effective medium for removal of large scale interference from Ni and Cu ions.³⁴ This demonstrates the need for compromised acidic conditions to minimise overall interference effects and enable optimum sensitivity.

Whilst optimised acidic conditions contribute largely to the quality of the analytical data, the addition of a pre-reductant and interference-masking agents can also be significant. Potassium iodide, thiourea and L-cysteine are used as pre-reducing agents for Sb(V) and these reagents may also contribute to the reduction in analytical interference along with EDTA, 1,10-phenanthroline and thiosemicarbazide.^32–39 Potassium iodide is perhaps the most commonly used pre-reductant. The addition of this reagent is shown to increase the signal intensity from Sb(V) by 40% when using the HG-ICP-AES technique and reduces interference from copper ions.³⁴ However Welz and Sucmanova³⁵ note that potassium iodide requires preparation in highly acidic media, which can subsequently result in damage to instrument components and creates difficult handling procedures. The use of thiourea and L-cysteine instead of potassium iodide is suggested to improve analytical determinations as a result of reduced interference effects.^32,33,35 However, L-cysteine is considered the most beneficial reagent as a result of enhanced Sb(III) stability in solution, plasma stability and reduced interference effects.^32,35 Improved sensitivity with this reagent is suggested to result from faster SbH₃ production in the hydride generation phase although the extent of this improvement was not quantified.³²

Less common analytical techniques for the determination of total antimony in aqueous samples are presented in Table 3. A range of detection systems are utilised including electroanalytical techniques,^40–43 neutron activation analysis (NAA),⁴⁴ spectrophotometry,^17,45–47 photoionisation,¹⁴ X-ray fluorescence (XRF) spectrometry,¹⁹ and microwave induced plasma atomic emission spectrometry (MIP-AES).¹⁵ Many of these techniques alone are insufficiently sensitive for total antimony determinations in environmental samples. As a result of this, sample preconcentration or hydride generation is often incorporated for low level determinations. NAA techniques have an intrinsic requirement for a reactor source and this often prevents wide-scale applications. Furthermore, recommended sample preparation involves the evaporation of liquid samples onto a solid matrix prior to analysis.⁴⁸ This may promote analytical errors through the loss of antimony by volatilisation (see section 3.1.1). Electroanalytical techniques are widely reported for total antimony determinations in aqueous environmental samples. Techniques such as anodic stripping voltammetry (ASV) allow analyte detection at around 10 µg kg⁻¹ although Cámara and de la Calle-Guntiñas⁵ indicate HG-ASV can further improve sensitivity, enabling detection limits of 0.05 µg kg⁻¹. However these techniques have not proved as popular as AAS and ICP technology.

Table 3 Less common analytical techniques for the determination of total antimony in aqueous samples^a

Application	Method of determination	LOD/ng mL^−1	Ref.
a N.R, not reported; ASV, anodic stripping voltammetry; NAA, neutron activation analysis; DPP, differential pulse polarography; CSV, cathodic stripping voltammetry; HG-GC-PID, hydride generation gas chromatography photoionisation detection; MIP-AES, mcrowave induced plasma atomic emission spectrometry; GF-MIP-AES, graphite furnace microwave induced plasma atomic emission spectrometry.
Snow samples	ASV	0.02	41
Water samples
Sorption studies with synthetic solutions and commercial grade humic acid	ASV	N.R.	40
Brackish water	NAA	N.R.	44
Industrial waste water	DPP	20	42
Estuarine waters	CSV	N.R.	43
Synthetic solutions	HG-GC-PID	0.1–0.2	14
Rain water	Complexation with Brilliant Green and spectrophotometric detection	3	17
Sea water	Total-reflection X-ray fluorescence spectrometry	N.R.	19
Synthetic solutions	HG with cold trapping-MIP-AES	0.4	15
Synthetic solutions	HG with hot trapping-GF-MIP-AES	0.35	15
Natural waters (inc. sea waters and effluents)	Coprecipitation and complexation with crystal violet-spectrophotometric detection	0.005	46
Synthetic solutions	Extraction with bis(2-ethylhexyl) phosphoric acid-spectrophotometric detection	N.R.	45
Waste waters	Complexation with 2-(3,5-dibromo-2-pyridylazo)-5-diethylaminophenol(3,5-dibromo-PADAP)-spectrophotometric detection	N.R.	47

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2.2 Speciation determinations

Aqueous antimony species can be characterised in terms of oxidation state or intrinsic chemical form and analytical techniques for their determination require high species-specificity and low detection limits. Determinations of aqueous speciation involve selective species separation prior to analytical detection. Whilst pre-analysis separations are widely reported, the development of in-line separation is a growing area of analytical research and the application of coupled techniques is increasingly utilised. A review of analytical techniques (chemical, chromatographic and electrochemical) for antimony speciation in waters is presented by Smichowski et al.⁴⁹ However, considerable analytical method development has occurred since this publication, particularly with in-line chromatographic separations and the application of hydride generation systems. Sections 2.2.1–2.2.3 discuss the current range of analytical techniques for the determination of aqueous antimony species, emphasising recent method developments with state-of-the-art in-line techniques and their scope for future speciation studies. The merits of less common analytical methods are also discussed including recent developments with in-line and pre-analysis species separations.

2.2.1 Hydride generation techniques.. The use of hydride generation coupled with AAS, AES and MS detection systems is widely reported for the determination of aqueous antimony concentrations. Whilst a number of applications for total antimony determinations are described, the selective production and analysis of volatile SbH₃ can also permit oxidation state speciation data. Table 4 shows hydride generation techniques for the determination of antimony speciation in aqueous samples.

Table 4 Hydride generation techniques for the determination of antimony speciation in aqueous samples^a

Application	Hydride generation technique	Detection system	Sb species identified	LOD	Ref.
a N.R., not reported; HG, hydride generation; FAAS, flame atomic absorption spectrometry; AAS, atomic absorption spectrometry; GC-AAS, gas chromatography atomic absorption spectrometry; ICP-AES, inductively coupled plasma atomic emission spectrometry; GC-MS, gas chromatography mass spectrometry.
Sea water	Selective batch HG	FAAS	Sb(III)	1 ng	54
Sea water	Selective flow injection analysis HG	Electrothermal heating (quartz cell) and AAS	Sb(III)	0.007 ng	58
Sea water	Selective continuous flow HG	Electrothermal heating (quartz cell) and AAS	Sb(III)	0.21 ng mL^−1	58
Synthetic solutions	Batch HG	GC-AAS	SbH3	N.R.	61
			CH3SbH2	N.R.
			(CH3)2SbH	N.R.
			(CH3)3Sb	N.R.
Sea water	Selective batch HG	ICP-AES/AAS	Sb(III)	N.R.	75
Fly-ash leachates				N.R.
Synthetic solutions	Selective continuous flow HG	FAAS	Sb(III)	1.85 ng mL^−1	52
Sea water	Tandem in-line separation HG	ICP-AES	Sb(III)	3 ng mL^−1	50
			Sb(V)	8 ng mL^−1
Sb(III) and(V) in spiked water samples	Pre-reduction kinetics HG	ICP-AES	Sb(III)Sb(V)	1.2 ng mL^−14.5 ng mL^−1	51
Freshwater plant extracts	Semi-continuous HG	GC-MS	SbH3	15 ng	62
			CH3SbH2	N.R.
			(CH3)2SbH	N.R.
			(CH3)3Sb	N.R.
Water samples	FIA-HG	AAS	Sb(III)	0.007 ng	59
Water samples	Continuous HG	AAS	Sb(III)	0.21 ng mL^−1	59
Water samples	Batch HG	AAS	Sb(III)	2.97 ng	59

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Aqueous Sb(III) is readily reduced to SbH₃ in the presence of NaBH₄ over a pH range of 2–9, however, Sb(V) requires pre-reduction to Sb(III) before efficient SbH₃ formation can be achieved. This requires more acidic conditions, often ≤pH 1, or the use of a pre-reducing agent. By adjusting the chemical conditions in the hydride generation cell it is possible to generate SbH₃ specifically from Sb(III) in the presence of Sb(V). It is also possible to generate SbH₃ from species in both oxidation states simultaneously. As a result of this both total Sb and Sb(III) species can be determined, enabling the determination of Sb(V) by subtraction. However, caution is required with the application of this technique since incomplete reduction of Sb(III) and Sb(V) species or interference from Sb(V) during selective reduction of Sb(III) may produce inaccurate speciation data. The application of hydride generation to speciation studies has led to the development of novel techniques enabling simultaneous Sb(III) and Sb(V) determinations. These techniques utilise continuous in-line extraction of Sb(III) prior to SbH₃ generation and the application of pre-reduction kinetics.^50,51 Other areas of research focus on the optimisation of chemical conditions to improve efficiency and selectivity of SbH₃ generation for speciation studies. However, the majority of research in this field relates to the choice of acidic medium or the influence of pH. An evaluative study⁵² examined hydrochloric, perchloric, nitric, sulfuric and phosphoric acids for selective SbH₃ generation and identified phosphoric acid at pH 1.8 as the most suitable. However, other studies utilise citric acid and citrate buffers at pH 2, borate buffers at pH 8, tris-HCl at pH 6–7, 1.0 mol L⁻¹ HCl [with 0.1 mol L⁻¹ HF to mask Sb(V)], and 0.35 mol L⁻¹ HCl with 4 mg mL⁻¹ of Zr(IV).^53–57 The choice of media and pH is critical for the application of hydride generation to aqueous speciation studies. Whilst these conditions are fundamental to species differentiation they also influence the extent of analytical interference, as with total Sb determinations.^34,35,54

Aside from the optimisation of analytical components and chemical conditions the underlying procedure for sample and reagent introduction can be a critical factor. Batch, flow injection analysis (FIA) and continuous flow systems are commonly used for sample and reagent introduction to the hydride generation cell. Evaluative work on these techniques^58,59 demonstrated lower detection limits for continuous flow and FIA; detection limits with AAS are reported as 2.97 ng, 0.21 ng mL⁻¹ and 0.007 ng for batch, continuous flow and FIA, respectively. Furthermore the FIA technique is recommended by de la Calle-Guntiñas et al.⁵⁹ for speciation studies because of its continuous non-equilibrium state resulting in less favourable production of SbH₃ from Sb(V).

Despite extensive method development, the application of hydride generation alone coupled to AAS, AES and MS detection systems is somewhat limited for speciation studies, as only the Sb(III) and Sb(V) species can be identified. Whilst oxidation state data are essential there is an increasing requirement for data relating to the intrinsic chemical form of antimony and its organic and inorganic functionality. Hydride generation applications that address this requirement include coupling with chromatographic separation techniques including high performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS),⁶⁰ gas chromatography atomic absorption spectrometry (GC-AAS),⁶¹ gas chromatography mass spectrometry (GC-MS)⁶² and gas chromatography inductively coupled plasma mass spectrometry (GC-ICP-MS).^63–65 When using the HPLC-ICP-MS technique, the sensitivity of HPLC separations for inorganic Sb(III) and Sb(V) is enhanced by post column hydride generation, leading to near 100% analyte transport efficiency, although only oxidation state speciation data are determined.⁶⁰ In the above GC applications, hydride generation of aqueous methylated and inorganic antimony species is undertaken prior to separation and subsequent detection.^61–65 These GC techniques are presently advantageous over HPLC in that they enable separations of a range of methylated antimonial species (mono-methylated, di-methylated and tri-methylated antimony) from inorganic species with detection limits in the lower pg g⁻¹ range.^64,65 However, problematic molecular rearrangements of methylated antimony hydrides have been reported^61,62 giving rise to misleading and inaccurate analytical data. Dodd et al.⁶² alleviated such problems by flushing the reaction coil (in the hydride generation phase) with acetic acid, NaBH₄ and distilled water prior to analysis. However, further qualitative HPLC separations are required to determine accurate speciation data.

Hydride generation alone, as a species separation technique, is clearly inadequate for studies requiring the determination of the intrinsic chemical form. However the coupling of hydride generation to chromatographic techniques may enable significant improvements in future antimony speciation studies. Further method development with these coupled techniques will determine their limitations for species determinations in environmental matrices.

2.2.2 Chromatographic techniques..
2.2.2.1 Chromato-graphic separations.. Chromatographic techniques for separation and identification of antimony species prior to detection are a developing area of analytical research. The intrinsic nature of HPLC separations makes them particularly suitable for aqueous speciation and a number of studies identify aqueous antimony species using anion exchange columns, suggesting the predominance of anionic antimony species in aqueous environments.^60,66–71 Cationic and reversed-phase HPLC techniques have been evaluated for the determination of tri-methylated Sb and Sb(V) species.⁶⁷ However, in these cases, species separation is unsuccessful and at the very best no improvements are observed by comparison with anion exchange.

The success of HPLC separation is dependent upon a number of critical parameters. This is further complicated when coupled techniques are utilised, since the limitations of the detection system must also be considered. For successful chromatographic separations and subsequent application to speciation studies, the aqueous species of interest require an affinity for the stationary phase and the chemical structure of both components must be considered. In addition to this, factors such as sample and mobile phase composition, flow rate, column width, column length, injection volume and external temperature can be influential. HPLC techniques for the determination of antimony speciation in aqueous samples are shown in Table 5.

Table 5 HPLC techniques for the determination of antimony speciation in aqueous samples^a

Application	HPLC column	Mobile phase	Detection system	Identified Sb species	LOD/ng mL^−1	Ref.
a N.R., not reported; EDTA, ethylenediaminetetraacetic acid; KHP, potassium hydrogen phthalate; ICP-MS, inductively coupled plasma mass spectrometry; HG-AAS, hydride generation atomic absorption spectrometry; HG-ICP-MS, hydride generation inductively coupled plasma mass spectrometry; ICP-AES, inductively coupled plasma atomic emission spectrometry.
Sewage and landfill extracts	Hamilton PRP-X100150 × 4.6 mm	Tetramethylammonium hydroxide 12 mmol L^−1	ICP-MS	(CH3)3SbCl2Sb(V)	0.005  0.005	68
Sewage and landfill extracts	Dionex Ion PacAS4A-SC4 mm	Tetramethylammonium hydroxide 3 mmol L^−1	ICP-MS	(CH3)3SbCl2Sb(V)	0.005  0.005	68
River waterSea water	Hamilton PRP-X100250 × 4.1 mm	Phthalic acid 0.002 mol L^−1 (pH 5)	ICP-MS	Sb(III)Sb(V)	7.5  0.9	60
River waterSea water	Hamilton PRP-X100250 × 4.1 mm	Phthalic acid 0.002 mol L^−1 (pH 5)	HG-AAS	Sb(III)Sb(V)	50  6	60
Natural waters	Supelcosil LC-SAX 120 × 4.6 mm	Ammonium tartrate 50 mmol L^−1 (pH 5.5)	HG-AAS	Sb(III)Sb(V)	2.0  1.0	66
River waterSea water	Hamilton PRP-X100250 × 4.1 mm	Phthalic acid 0.002 mol L^−1 (pH 5)	HG-ICP-MS	Sb(III)Sb(V)	0.4  0.08	60
Synthetic solutionsSoil extracts	Hamilton PRP-X100150 × 4.6 mm	Potasium hydroxide 2 mmol L^−1	ICP-MS	(CH3)3SbSb(V)	0.6  0.5	67
Synthetic solutionsSoil extracts	Hamilton PRP-X100150 × 4.6 mm	EDTA 20 mmol L^−1KHP 2 mmol L^−1 (pH 4.5)	ICP-MS	Sb(III)Sb(V)	0.8  0.5	67
Water extractsSoil extracts	Hamilton PRP-X100150 × 4.1 mm	Phthalic acid 2 mmol L^−1 (pH 5)	ICP-MS	Sb(III)Sb(V)(CH3)3SbO	3.11  0.51  0.59	69, 70
Water samplesSoil extracts	Hamilton PRP-X100150 × 4.1 mm	4-Hydroxybenzoic acid 2 mmol L^−1 (pH 5.5)	ICP-MS	Sb(III)Sb(V)(CH3)3SbO	3.19  0.72  0.81	70
Water extractsSoil extracts	Hamilton PRP-X100150 × 4.1 mm	Phthalic acid 2 mmol L^−1 (pH 5)	ICP-AES	Sb(III)Sb(V)(CH3)3SbO	409102151	70
Water samplesSoil extracts	Hamilton PRP-X100150 × 4.1 mm	4-Hydroxybenzoic acid 2 mmol L^−1 (pH 5.5)	ICP-AES	Sb(III)Sb(V)(CH3)3SbO	457107156	70
Synthetic solutions	Dionex AS4 A	20 mmol L^−1 ammonium chloride	ICP-AES	α-Hydroxy acid-Sb complexes	N.R.	71

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Recent development work using anion exchange for antimony speciation has lead to the successful separation of aqueous Sb(III) and Sb(V) species using Hamilton PRP-X100 and Supelcosil LC-SAX 1 columns.^60,66,67 Elution of Sb(V) is achieved relatively easily because of its existence as a mononegatively charged species, [Sb(OH)₆]⁻, over a wide pH range (2.7–10.4).⁶⁷ However in the case of Sb(III), problems with chromatographic separation have been encountered including irreversible retention and peak tailing.^60,67 These events are thought to occur as a result of on-column precipitation or the influence of dinegative charge in some species which have a greater affinity for the stationary phase and resist elution.⁶⁷ Problems associated with the elution of Sb(III) species are alleviated by the use of mobile phases containing phthalate, although tailing peak shapes are still observed.^60,67 Suggested reasons for these observations include the preferential formation of negatively charged Sb(III)-phthalate complexes with reduced affinity for the stationary phase.⁶⁷ Other separation techniques for Sb(III) and Sb(V) species utilise mobile phases containing tartrate buffers⁶⁶ or ethylenediaminetetraacetic acid (EDTA) in conjunction with potassium hydrogen phthalate.⁶⁷ However, mobile phases containing EDTA are unsuitable for specific Sb(III) speciation due to the preferential formation of EDTA complexes. Other factors shown to influence elution of Sb(III) species relate to column specifications. An early attempt at Sb(III) and Sb(V) speciation failed to resolve Sb(III) on a 5 cm IC-Pack column and determination of these species was reported using a longer 25 cm PRP-X100 column with a phthalate mobile phase.⁶⁰ However, this resulted in lengthy retention times and poor peak shapes. Zhang et al.⁶⁶ proposed the use of a shorter column length to reduce the retention times reported by Smichowski et al.⁶⁰ This is considered a sensible approach particularly when Sb(III) species are strongly retained on the column. Zhang et al.⁶⁶ demonstrated a method for Sb(III) and Sb(V) separation using a 2 cm guard column. Sb(III) was determined by initial conversion to the tartrate species with an optimised tartrate concentration in the mobile phase. This technique enabled elution of both Sb(III) and Sb(V) species with very short retention times although the peak shape obtained for Sb(III) was still significantly broader than that obtained for Sb(V).

Tri-methylated Sb and Sb(V) separations are reported using Hamilton PRP-X100 and Dionex Ion Pac AS4A columns with potassium hydroxide, carbonate buffer, or tetramethylammonium hydroxide in the mobile phase.^67,68 Using synthetic solutions both columns enabled detection limits of 5 ng L⁻¹ for both tri-methylated Sb and Sb(V).⁶⁸ The Dionex column is more susceptible to matrix interference and this is demonstrated by the influence of model sodium chloride solutions.⁶⁸ Matrix concentrations of sodium chloride above 0.05% promote significant interference with the Dionex AS4A column by comparison with a tolerance of 0.1–0.5% sodium chloride for the PRP-X100.⁶⁸

Whilst Sb(V) and tri-methylated Sb are easily chromatographed with these columns, the elution of inorganic Sb(III) is considerably problematic. The elution of Sb(III) is not mentioned with the Dionex AS4A column⁶⁸ however it is irreversibly retained on the PRP-X100 column when phosphate buffer, carbonate buffer and potassium hydroxide mobile phases are used.⁶⁷ Tri-methylated Sb species are eluted in the solvent front when carbonate buffer and potassium hydroxide mobile phases are used and it is suggested that cationic or neutrally charged species are formed.⁶⁸ The use of phosphate buffer as a mobile phase component is shown to lengthen retention times for tri-methylated Sb species, however, a loss in sensitivity is also observed.⁶⁷ This is suggested to result from the formation of anionic tri-methylantimony-phosphate compounds with an enhanced affinity for the stationary phase. Unfortunately these chromatographic conditions are unable to differentiate between different tri-methylated Sb species, although an interesting separation between organic and inorganic Sb(V) is achieved.

Separations of inorganic Sb(III), Sb(V) and methylated Sb using a single chromatographic system are problematic due to the variations in chemical structure, charge and subsequent retention time of these species. However, separations of this kind have been attempted in studies by Ulrich using a Hamilton PRP-X100 column with phthalic and 4-hydroxybenzoic acid eluent systems.^69,70 These studies describe the development of a method for chromatographic separations of inorganic Sb(III), Sb(V) and dissolved tri-methylstiboxide and the subsequent application of this technique to environmental samples. The initial separation of these species is achieved using synthetic solutions although a broad, tailing chromatographic peak is observed for inorganic Sb(III).⁶⁹ As a result of this, the sensitivity for inorganic Sb(III) is poor by comparison with both inorganic Sb(V) and tri-methylated Sb and detection limits are reported as 3.11, 0.51 and 0.59 ng mL⁻¹, respectively.⁶⁹ Separations with synthetic solutions were relatively successful in comparison with the application to soil and water samples from contaminated sites.⁷⁰ In the analysis of these environmental samples only Sb(V) is clearly resolved. The identification of both inorganic Sb(III) and tri-methylated Sb is much less well defined since the peaks are poorly resolved.

Whilst a number of problems with HPLC techniques result primarily from Sb(III), the progression from identification of Sb(III) and Sb(V) oxidation states to organic tri-methylated Sb species has been achieved. However, future aqueous antimony speciation studies using HPLC may be limited due to the lack of commercially available soluble antimony compounds. At present the only antimony compounds widely available and suitable for separation studies are potassium antimony(III)tartrate and potassium hexahydroxyantimonate(V). Tri-methylated Sb species for development work are currently prepared by in-house laboratory synthesis, which can raise questions about the purity of the compound and subsequent reliability of the analytical results obtained.

Naturally occurring humic and organic acids along with chloride and carbonate species in environmental systems are thought to significantly influence the solubility of antimony compounds as a result of complexation.⁶⁹ Future work should address, therefore, the identification and separation of uncharacterised aqueous antimony complexes. Inorganic Sb(III), Sb(V) and dissolved tri-methylstiboxide complexation with chloride, citric and tartaric acids have been examined by changes in chromatographic retention times.⁶⁹ However, this study produced spurious complexation data and no Sb(V) complexes were observed. A likely reason for this observation relates to the use of an inappropriate phthalic acid mobile phase, which has a complexing capacity stronger than the complexing agents under examination. Guy et al.⁷¹ reported an extensive investigation of aqueous Sb(V) interactions with α-hydroxy acids and evidence for the formation of new antimony complexes. In these studies, a Dionex AS4A ion-exchange column and an ammonium chloride mobile phase were successfully applied to the separation of Sb(V) compounds associated with lactic, mandelic, DL-malic and citric acid. Furthermore, a chromatographic application was utilised for a series of controlled mole-ratio complexation experiments. This determined the extent of ligand exchange and the stoichiometric ratio of α-hydroxy acid ligands: Sb(V) in the separated complex molecules. The separation and mole-ratio experiments for the Sb-α-hydroxy acid complexes were performed using a HPLC-ICP-AES technique and stoichiometric data were obtained by monitoring the atomic emission lines of both Sb (204.597 nm) and C in the α-hydroxy acid ligand (193.026 nm). This, in turn, aided the identification of the molecular structure by the further application of electrospray ionisation mass spectrometry (ESI-MS).

The application of chromatographic separation techniques for identification of aqueous antimony species in environmental samples is currently an exciting area of analytical research. However, to date, the aqueous chemical speciation of antimony in environmental matrices is poorly understood and this is likely to hinder method development with chromatographic separations. Guy et al.⁷¹ reported the application of HPLC-ICP-AES in conjunction with nuclear magnetic resonance (NMR) and electrospray ionisation mass spectrometry to enable the identification of uncharacterised antimony complexes. Future studies could further apply this technique and focus upon separation and controlled complexation of Sb species with other naturally occurring organic and inorganic ligands prior to real time sample analysis. The application of these techniques will elucidate antimony's potential for complex formation in environmental matrices and enable an improved understanding of separation chemistry. This in turn, will advance the chromatographic determination of aqueous antimony species from complex environmental matrixes.

2.2.2.2 Detection systems for chromatographic techniques.. The quantification of aqueous antimony species requires a highly sensitive detection system since total antimony concentrations are typically found in the ng mL⁻¹ range. Coupling of a chromatographic separation technique with a sensitive detection system promotes both qualitative and quantitative aqueous speciation data and chromatographic techniques for antimony speciation utilise HG-AAS,^59,63 ICP-AES⁶⁷ and ICP-MS^59,64–67 detection. At the time of the review, Smichowski et al.⁴⁹ noted that a number of analytical techniques for determination of aqueous antimony speciation in environmental matrices required preconcentration steps to achieve detection limits in the low ng mL⁻¹ range and that applications to spiked water samples, rather than environmental samples, were most commonly reported. Since the publication by Smichowski et al.,⁴⁹ a number of developments are noted in this field of analytical research and this is reflected in recent literature. The coupling of HPLC to sensitive detection systems such as ICP-MS^64–67 and HG-AAS⁶³ has facilitated collection of aqueous antimony speciation data with detection limits in the lower ng mL⁻¹ and pg mL⁻¹ range. However, to date, the most popular technique is HPLC-ICP-MS and a number of environmental applications have been observed in recent years.^59,63–67 The optimisation of mobile phase composition and intrinsic ICP-MS parameters is fundamental to high quality analytical data when using HPLC-ICP-MS techniques. However, further improved analytical sensitivity may result from the incorporation of post-column hydride generation (HPLC-HG-ICP-MS).⁶⁰ This application of hydride generation promotes efficient sample transport to the detection system and enables detection limits of 0.40 and 0.08 ng mL⁻¹ for Sb(III) and Sb(V), respectively.⁶⁰ Comparative detection limits for HPLC-ICP-MS alone are reported as 7.5 and 0.9 ng mL⁻¹ for Sb(III) and Sb(V), respectively.⁶⁰

2.2.3 Other less common techniques.. Aside from the described hydride generation and chromatographic techniques, a number of less common analytical techniques are also utilised for the determination of aqueous antimony speciation. These techniques utilise either pre-analysis or in-line species separations. (Tables 6 and 7).

Table 6 Less common analytical techniques utilising pre-analysis species separations for the determination of antimony speciation in aqueous samples^a

Application	Pre-analysis separation technique	Identified Sb species	Detection system	LOD	Ref.
a N.R., not reported; DPBA, diphenylbenzamidine; APCDT, ammonium pyrrolidinecarbodithiolate; DDTC, diethyldithiocarbamate; DBDTC, dibenzyldithiocarbamate; FDDC, fluorinated dithiocarbamate; NAA, neutron activation analysis; GFAAS, graphite furnace atomic absorption spectrometry.
Water samples	Extraction using DPBA and Brilliant Green	Sb(V)	Spectrophotometric	10 ng mL^−1	76
Natural waters	Preconcentration and separation using thionalide-loaded acrylic polymer	Sb(III)	NAA	0.0025 ng mL^−1	77
Natural waters	Preconcentration and separation using thionalide-loaded acrylic polymer	Sb(III)	NAA	0.023 ng mL^−1	78
Tap water	Selective sorption onto Polyorgs 31	Sb(III)	GFAAS	0.03–0.034 ng mL^−1	79
Snow samples
Sea water	Selective adsorption onto activated alumina	Sb(III) + Sb(V)Sb(III)	GFAAS	0.004–0.026 ng	80
Natural waters	Extraction with APCDT	Sb(III)	NAA	0.001 ng mL^−1	72
Sea water	Chelation using Na-DDTC and thionalide cocrystalisation	Sb(III)Sb(V)	NAA	N.R.N.R.	73
Water samples	Selective extraction with lactic acid and Malachite Green	Sb(III) + Sb(V)Sb(III)	GFAAS	0.01 ng	59
Natural waters	Complexation with FDDC and extraction with dichloromethane	Sb(III)	Supercritical fluid chromatography	0.011 ng	74
Fly-ash leachates	Coprecipitation with DBDTC	Sb(III)	NAA	0.5 ng mL^−1	75

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Table 7 Less common analytical techniques utilising in-line species separations for the determination of antimony speciation in aqueous samples^a

Application	Method of determination	Identified Sb species	LOD/ng mL^−1	Ref.
a N.R., not reported; HG, hydride generation; AAS, atomic absorption spectrometry; GC-PID, gas chromatography photoionisation detection; GC-MS, gas chromatography mass spectrometry; AFS, atomic fluorescence spectrometry.
Natural waters	Selective continuous HG with cryogenic trapping-AAS	Sb(III)	0.23–0.24	81
Natural waters	Selective batch HG with cryogenic trapping-AAS	Sb(III)	0.001–0.01	82
Natural waters	Selective HG with cold trapping-GC-PID	Sb(III)	0.0004	83
Volatile Sb compounds	Capillary gas chromatography-fluorine induced chemiluminescence and GC-MS-head space analysis	(CH3)3Sb	N.R.	85
Tap water, sea water	HG-AFS	Sb(III)	0.3	84
Air samples
Waste water samples	HG-AFS	Sb(III)	N.R.	53
Natural waters	Selective batch HG with cold trapping-AAS	Sb(III)(CH3)SbH2	0.0003–0.0012	56
		(CH3)2SbH

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Pre-analysis species separation techniques differentiate between Sb(III) and Sb(V) by exploiting species-specific complexation or adsorption mechanisms prior to analyte detection. The reactions of Sb(III) or Sb(V) with complexing reagents (lactic acid and Malachite Green, diphenylbenzamidine and Brilliant Green, ammonium pyrrolidinecarbodithiolate, sodium diethyldithiocarbamate, fluorinated dithiocarbamate and dibenzyldithiocarbamate) facilitate quantitative species determination by selective extraction prior to AAS, NAA and supercritical fluid chromatographic detection^59,72–75 or in situ spectrophotometric detection.⁷⁶ Detection limits with these techniques are typically in the ng mL⁻¹ range, although a 1.0 pg mL⁻¹ detection limit is reported using NAA instrumentation.⁷² Adsorption techniques utilise species-specific elution from thionalide-loaded acrylic polymer, Polyorgs 31 and activated alumina to determine quantitative data.^77–80 These techniques allow detection limits similar to those obtained from complexation reactions (down to 2.5 pg mL⁻¹) and are used in conjunction with both NAA,^77,78 and GFAAS^79,80 detection systems. Less common in-line species separations utilise selective hydride generation in conjunction with cryogenic trapping together with AAS and gas chromatography photoionisation detection (GC-PID).^56,81–83 These trapping systems enable enhanced preconcentration of the analyte prior to detection and subsequent detection limits are reported around 0.3 pg mL⁻¹.⁵⁶ Additional in-line techniques include hydride generation coupled with atomic fluorescence detection (HG-AFS)^53,84 and GC with fluorine induced chemiluminescence and mass spectrometric detection.⁸⁵

The detection limits for these less common techniques are generally acceptable, although the extent of the qualitative data are often limited, as only a single aqueous species is selectively determined. The majority of techniques differentiate between total antimony and a single oxidation state enabling determination of a second oxidation state by subtraction. These techniques are inadequate for most present day speciation studies since more detailed, chemical speciation data are required. A less common analytical technique with enhanced species selectivity utilises chelation and co-crystalisation for isolated Sb(III) and Sb(V) determination.⁷³ However this technique relies upon extensive pre-analysis species separation and the in-line hydride generation or chromatographic techniques are generally considered more favourable.

3 Determination of antimony in soils, sediments and biological materials

3.1 Total antimony determinations

3.1.1 Sample preparation.. The majority of analytical systems for trace metal determination require sample introduction in the liquid state. For determination of metals in solid materials it is necessary therefore to use matrix digestion techniques that sufficiently dissolve the solid sample matrix and release the analyte of interest into solution. Whilst these digestion techniques allow total antimony determinations for solid samples, data for solid state partitioning are often unobtainable because most digest reagents chemically alter the sample integrity. Matrix digestion techniques for the determination of total antimony in solid samples are shown in Table 8.

Table 8 Matrix digestion techniques for the determination of total antimony from soils, sediments and biological material^a

Application	Matrix digestion technique	Open/sealed vessel	Method of determination	Ref.
a HG-ICP-AES, hydride generation inductively coupled plasma atomic emission spectrometry; ICP-MS, inductively coupled plasma mass spectrometry; HG-AAS, hydride generation atomic absorption spectrometry; GFAAS, graphite furnace atomic absorption spectrometry; ICP-AES, inductively coupled plasma atomic emission spectrometry. HG-GC-PID, hydride generation gas chromatography photoionisation detection; DPASV, differential pulse anodic stripping voltammetry.
Herbage	Dry ashing with Mg(NO3)2-furnace technique	Open	HG-ICP-AES	97
Soils and sediments	Concentrated HCl-hot block technique	Sealed	HG-ICP-AES	87
Plant and biological tissue	Concentrated HCl-hot block technique	Open	HG-ICP-AES	88
Soil	Concentrated HCl-hot block technique	Open	HG-ICP-AES	88
Silicon	HNO3, HF, H3BO3 and H2O2-microwave technique	Sealed	ICP-MS	89
Soil	Dry ashing with Mg(NO3)2-furnace technique	Open	ICP-AES	98
Herbage	H2SO4, HNO3 and sodium sulfite-hot block technique	Open	HG-AAS	90
Algal cells	HNO3 at 80 °C	Open	GFAAS	91
Biological tissue	HNO3 and H2O2-microwave technique	Sealed and open focussed	ICP-MS	92
Biological tissue	Acetic acid or H2SO4 with KI-microwave technique	Open focussed	Slurry sampling- HG-AAS	93
Botanical samplesSoil	HNO3, HF, H2O2 and H3BO3 in a Teflon bomb-microwave technique	Sealed	AAS/ICP-AES	94
Botanical samples	HNO3 in a PFA bomb-microwave technique	Sealed	ICP-MS	95
Geological samples	HNO3, HF and HClO4 in a PFA bomb followed by hot plate evaporation to dryness with HClO4 and dissolution in HNO3	Open and sealed phases	ICP-AES/MS	96
Soils	Boiling in 6 M HCl using a Liebig condenser	Open	HG-AAS	86
Sea weed	Reflux with HNO3, evaporation to reduce volume, addition of H2SO4 with continued heating until white fumes evolve	Sealed	Spectrophotometric	46
Silicate rocks	Reflux with HF, evaporate to dryness, add HNO3 and evaporate to dryness, re-dissolve in HCl	Open and sealed phases	Spectrophotometric	46
Sediment	Reflux in HNO3 followed by evaporation to dryness, reflux in potassium persulfate followed by evaporation to dryness, heating with HCl	Sealed	HG-GC-PID	83
Biological tissue	Pressurized ashing with HNO3	Sealed	HG-ICP-AES	34
Botanical samples
Geological samples	Reflux in HNO3 and HClO4, addition of HF and HNO3 and evaporate to reduced volume, addition of HCl, KI and ascorbic acid	Open and sealed phases	HG-ICP-AES	99
Geological samples	HCl and HNO3 at 90 °C	N.R	HG-ICP-MS	100
Geological samples	Digest with HCl, HF, HClO4, evaporate to dryness, further digestion with HCl and HNO3	Open and sealed phases	HG-ICP-MS	100
Soil	HNO3-microwave digestion	Sealed	DPASV	103
Biological tissueSoil	Heating with HNO3, HClO4 and H2SO4-hot plate technique	Open	HG-ICP-AES	34

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For antimony determination, solid sample matrices are often decomposed at high temperature in acidic media,^{34,46,83,86–96} although dry-ashing with Mg(NO₃)₂ is also used.^97,98 The choice of acid for matrix digestion depends largely upon the matrix composition and to be effective, sample digestion must efficiently destroy the matrix and release antimony in a form compatible with the chosen analytical method. The determination of antimony in biological samples is often undertaken using mixtures of oxidising acids (H₂SO₄, HNO₃, HClO₄) and H₂O₂ to decompose matrices with high organic content.^{30,34,46,83,88,90–93,95} However, when these reagents are used to digest materials including soils and sediments with high silicate content, poor recoveries in the region of 10–56% are reported.^{30,34,59,83,86,87} Suggested reasons for these observations include the formation of insoluble Sb(V)–silicate complexes in the presence of oxidising acids and the specific formation of insoluble SbOCl compounds when perchloric acid medium is used.^30,86 Improved sample dissolution is achieved by using HF in conjunction with the above oxidising reagents. These digest mixtures enable mean recoveries in the region of 94–132% and both microwave and hot-plate heating systems are utilised.^{46,89,94,96,99,100} The mineral acid, HCl, is also used for matrix digestion and the application of this reagent can improve the recovery of antimony from soils and sediments by up to 90% in comparison with some oxidising acid techniques and recoveries are reported in the range of 90–110%.^30,88,98 However, matrices with high organic content, such as biological tissues, may be insufficiently decomposed by HCl alone and this factor may contribute significantly towards low antimony recoveries for some environmental samples. Anderson and Isaacs³⁰ evaluated the use of HCl (sequential procedure utilising 12 and 5 mol L⁻¹ HCl, respectively) for open-vessel, high temperature (up to 150 °C) digestions of reference materials including pine needles and liver tissue, although surprisingly recoveries of 0% antimony were reported from these samples. In the absence of significant evidence, these anomalous results were attributed solely to the inadequate acid strength of HCl and insufficient matrix decomposition. However, other sources of analytical error, including the potential for loss of antimony by volatilisation, appear to have been overlooked and preventative measures to control volatilisation are discussed below.

Heating systems reported for HCl digestion include furnace,^97,98 hot block^30,87,88 and sand bath technologies.⁸⁶ The furnace technique utilising dry-ashing with Mg(NO₃)₂ is shown to be suitable for matrices with variable organic content and the successful digestion of both soil and herbage samples is reported.^97,98 Analytical errors during soil digestion for total antimony are reported as <10%⁹⁸ and recovery of antimony from herbage reference materials ranged between 96–107%.⁹⁷ HCl digestion with sand bath and hot block heating systems are evaluated for total antimony determination from a range of geological samples including rocks, soils and sediments.^30,86–88 However, these digestion techniques are shown to produce variable analytical results and their subsequent application to future environmental analysis may require careful consideration if accurate data are to be obtained. Asami et al.⁸⁶ reported HCl digestion with a sand bath heating technique and obtained recoveries from soil samples in the region of 44–97%. Pahlavanpour et al.⁸⁷ reported HCl digestion with a sealed vessel hot block technique and obtained recoveries from soil and rock samples in the region of 36–100% and 103–177%, respectively. Anderson and Isaacs^30,88 reported HCl digestion with an open vessel hot block technique and obtained recoveries from soil and sediment samples in the region of 80–123%.

Low or variable recoveries of total antimony from solid samples may result from the loss of antimony by volatilisation in addition to insufficient acid strength and incomplete matrix decomposition. A number of reported digestion techniques involve rigorous heating in open vessels and sample evaporation.^{30,34,46,83,88,90,91,96} Loss of volatile antimony species, including SbCl₃ and SbCl₅, may be promoted with these techniques, particularly if high antimony concentrations are present in the sample. The volatilisation of antimony can be prevented by the use of carefully adapted digestion techniques with physical or chemical mechanisms to retain volatile antimony at high digest temperatures. Suitable techniques include the use of closed vessel-microwave,^{34,89,92,94,95} open-vessel dry-ashing with Mg(NO₃)₂^97,98 and open-focussed microwave^92,93 technology. The majority of recently published techniques utilise closed-vessels with microwave heating systems to physically prevent volatilisation of antimony at high temperatures. These techniques are advantageous because high temperatures and pressures are developed inside the digestion vessel and rapid matrix decomposition is enabled. Furthermore, a number of samples may be digested simultaneously. However, the use of specialised digestion vessels is essential to these techniques since they prevent analyte loss through venting or vessel rupture during the heating phase. The use of open-focussed microwave techniques avoids high-pressure build-up during digestion and loss of volatile antimony is prevented by the use of vapour condensing reflux systems. These techniques enable digestion of larger samples by comparison with closed vessel systems (Lamble and Hill¹⁰¹ suggest up to 15 g of sample per vessel) although potential for multiple and simultaneous sample digestions are limited with focussed microwave systems.¹⁰¹ The furnace technique utilising dry-ashing with Mg(NO₃)₂ oxidises organic material in the sample during the heating phase and the formation of involatile Sb-complexes is suggested to occur.^97,98 These chemical reactions are thought to prevent volatilisation of antimony at high temperatures avoiding the necessity for a closed vessel system, although application of this technique may be inappropriate for samples with low organic matter content. Ash generated in the heating phase is re-dissolved in HCl at room temperature prior to analysis. This technique enables multiple and simultaneous digestion of samples (sample number depending upon the size of the furnace) and the technique is relatively inexpensive by comparison with the described microwave techniques. However, precautions are required to prevent cross contamination during the heating phase and loss of residual ash prior to dissolution.

Whilst a number of digestion techniques are available for total antimony determination in solid environmental samples, recent improvements in matrix digestion techniques have resulted largely from advances in microwave technology, and microwave heating systems have become increasingly popular in recent years. These techniques offer the advantages of less reagent and sample usage, reduced sample cross contamination, reduced analyte loss by volatilisation and improved safety features during digestion.¹⁰² Microwave digestion techniques currently make a significant contribution to the analysis of solid sample matrices and particular advantages are observed with volatile elements. Further development with these techniques will improve the accuracy of digestion procedures and enable more efficient total antimony determinations from solid matrices.

3.1.2 Detection systems.. A range of analytical techniques exist for the determination of antimony in solid samples and this results largely from the application of matrix digestion prior to analysis. The use of these sample preparation techniques can allow total antimony determination from solids using detection systems more commonly used for aqueous samples.^{18,30,34,86,99,100,103–106} However, other analytical techniques are also available allowing sample introduction in a dry or slurried form.^59,107–113Table 9 shows analytical techniques for the determination of total antimony in soils, sediments and biological material.

Table 9 Analytical techniques for the determination of total antimony in soils, sediments and biological material^a

Application	Method of determination	LOD (solid or processed sample)	Ref.
a N.R., not reported; HG-AAS, hydride generation atomic absorption spectrometry; HG-ICP-AES, hydride generation inductively coupled plasma atomic emission spectrometry; GFAAS, graphite furnace atomic absorption spectrometry; HG-ICP-MS, hydride generation inductively coupled plasma mass spectrometry; NAA, neutron activation analysis; ASV, anodic stripping voltammetry; PAA, photon activation analysis.
Ores, rocks, soils and sediments	Xanthate complexation-extraction-HG-AAS	0.02 µg g^−1	18
Soils, sediments and biological materials	Selective continuous flow HG-ICP-AES	0.41 ng mL^−1	22
Soils	Automated HG-Electrothermal heating-AAS	0.007–0.02 µg g^−1	86
Soils and sediments	Slurry sampling GFAAS	30 ng mL^−1	107
Geological reference materials	Oblique HG–ICP-AES	N.R.	99
Geological samples	HG-ICP-MS	0.006 µg g^−1	100
House and street dust	NAA	N.R.	108
Soil	ASV	1.08 ng mL^−1	103
Peat bog sample	NAA	N.R.	109
Flood plain soils	PAA	N.R.	110
Dust and grasses	ICP-MS	0.03 µg g^−1	104
Biological samples	HG-X-ray fluorescence spectrometry	0.03 µg g^−1	105
Marine sediments	Selective continuous flow HG-ICP-AES	0.8 ng mL^−1	106
Sea weed, silicate rocks, red clay, biological tissue	Co-precipitation and complexation with crystal violet with UV spectrophotometry	0.005 ng mL^−1	34
Soil	Slurry sampling with batch HG-AAS	2.97 ng	59
Airborne particulates	Slurry sampling GFAAS	15 ng mL^−1	111
Soils	NAA	0.5 µg g^−1	48
Marine sediments	Slurry sampling with HG-AAS	0.00835 µg g^−1	112
Geological materials	Automated ultrasonic slurry sampling with ETAAS	0.008 µg g^−1	113
Biological samples	Continuous HG-ICP-AES	0.8 ng mL^−1	34
Soil

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Matrix digestion, liquid sample introduction and subsequent detection with AAS, ICP-AES and ICP-MS is often used for the determination of total antimony in rocks, soils and sediments.^{18,30,34,86,99,100,104,106} The use of hydride generation in conjunction with ICP-AES is the most commonly used detection system and improved sensitivity resulting from the production of volatile SbH₃^30,34,99,106 allows detection limits for digested solids in the region of 0.4–0.8 ng mL⁻¹. Slurry sampling techniques are also used for total antimony determinations in solid matrices and the analyses of airborne particulate material, soils and sediments are reported using AAS detection systems.^{59,107,111–113} These techniques require minimal sample preparation and detection limits of around 8–30 ng mL⁻¹ are generally reported. However rapid sample analysis, choice of suspension medium and intrinsic particle size are critical for stability of the slurry, reduction of background noise and overall accuracy of the analytical data.^{107,111–113}

Less common analytical techniques for the determination of total antimony in solid samples include the use of NAA,^48,108,109 photon activation analysis (PAA),¹¹⁰ ASV,¹⁰³ HG-XRF¹⁰⁵ and UV spectrophotometry.⁴⁶ Applications of NAA and PAA are advantageous for antimony determination in solid matrices because samples are analysed in their solid state and minimal sample preparation is required. However, these techniques can be insufficiently sensitive for determination of naturally occurring antimony concentrations. Detection limits for PAA are not reported, although for NAA⁴⁸ analyte detection is achieved at 0.5 µg g⁻¹. ASV, HG-XRF and UV spectrophotometry require liquid sample introduction and detection limits are reported around 1, 30 and 0.005 ng mL⁻¹, respectively. The described UV spectrophotometric technique enables excellent limits of detection for total antimony determination and this is attributed to the formation of a highly absorptive antimony complex with the triphenylmethane dye, Crystal Violet, prior to analysis.⁴⁶

The majority of analytical techniques described can be used for total antimony determinations in both solid and aqueous samples with varying degrees of sample preparation. However, enhanced interference effects resulting from high matrix ion concentrations often compromise the quality of analytical data for solid sample analysis. Detection limits for ICP techniques can deteriorate by around 25 ng mL⁻¹ when the digested product of a solid matrix is analysed. However, interference effects with MS detection are less severe by comparison with AES techniques and this results from the high m/z values of Sb isotopes and subsequent low spectral interference. The analysis of solid sample digestion products with HG-ICP-AES is prone to enhanced interference effects during the hydride generation phase and particular problems result from high transition metal concentrations in the sample solution.⁹⁹ In addition to this, high Fe concentrations (often present in soil and sediment samples) prevent the complete reduction of Sb to SbH₃ and this can lead to low Sb recoveries, particularly when KI is used as a pre-reductant.³⁰ Problematic spectral interferences with AES detection result from Cu, Ni, Co and Cr ions^30,34,99,106 and an interference study by Anderson and Isaacs³⁰ demonstrated complete signal suppression at 231.147 nm by Ni and Co when present in concentrations around 2%. Furthermore, a signal enhancement of 50% at 206.833 nm was observed with a 2% concentration of Cr ions.³⁰ Additional mutual interference effects may result from Se ions⁹⁹ and this element reduces the Sb signal by 50% when present at concentrations around 10 µg mL⁻¹. Novel techniques for the alleviation of hydride generation interference with solid sample analysis include the application of oblique hydride generation.⁹⁹ However, the reduction of specific chemical interferences derived from Fe, Ni, Co and Cu ions is achieved by adjusting chemical conditions, as described previously with hydride generation for antimony determinations in aqueous samples. (See sections 2.1 and 2.2.1.)

Interference effects with AAS analysis can also be significant and detection limits with these techniques may deteriorate by around 20 ng mL⁻¹ as a result of solid sample dissolution and high matrix ion concentrations. The presence of high As content in solid samples is reported to interfere with Sb determination using HG-AAS as a result of depressive gas phase reactions during the atomisation.¹⁸ However, other interferences are reported to result from elements such as Al, Ge, Ni, Sn and Si.^86,107 The removal of matrix ions prior to analysis can alleviate interference effects resulting from solid sample analysis and this is achieved by the application of selective extraction or co-precipitation procedures.¹⁸ Spectral interferences from Si during slurry sample analysis can also be removed by use of HF at concentrations of 40–60% in the suspension medium. This facilitates the removal of Si from the sample during the heating phase with no detrimental effects to the analytical instrumentation.¹⁰⁷

Analytical interference effects with less common analytical techniques are infrequently reported in comparison with ICP and AAS techniques. However, interference effects with HG-XRF, ASV and UV spectrophotometry are reported for total antimony determinations in solid samples.^46,103,105 HG-XRF suffers from interferences derived from hydride generation although contributory effects from Cu ions are removed by co-precipitation of antimony with lanthanum hydroxide.¹⁰⁵ ASV techniques tolerate most interfering metal ions up to concentrations around 1 × 10⁻⁵ mol L⁻¹ but concentrations of 1 × 10⁻⁵ mol L⁻¹ Ge(IV) and 5 × 10⁻⁵ mol L⁻¹ Ta(V) depress the Sb signal by 36% and 54%, respectively.¹⁰³ In addition to this, interference from Hg is reported at concentrations around 1 × 10⁻³ mol L⁻¹ and the formation of a Sb–Hg amalgam is suggested to be responsible.¹⁰³ However, specific interferences from Hg may be masked by the addition of sodium citrate. Furthermore, the use of chemically modified carbon paste electrodes are suggested to alleviate interference effects from metal ions with ASV by increasing the affinity of the electrode to the target analyte.¹⁰³ Interference effects with UV spectrophotometric techniques may result from Hg, Tl and Au ions, although the most significant interference is suggested to result from fulvic acid.⁴⁶ This substance competes with the essential complexing agent (Crystal Violet) and reduces the sensitivity of the technique by forming antimony–fulvic acid complexes. Particular problems may result therefore with the analysis of environmental samples when this technique is utilised.

The determination of antimony in solid samples can be undertaken utilising solid, liquid or slurry sample introduction and careful choice of an analytical technique is required, since this pre-determines the extent of sample preparation. To date the application of matrix digestion prior to HG-ICP-AES detection is most commonly used for the analysis of solid sample components and this is reflected in the recent literature. However, the advantages of ICP-MS over ICP-AES detection are becoming increasingly recognised in terms of both reduced analytical interference and potential sensitivity. Future application of ICP-MS detection combined with technological advances in solid sample preparation will subsequently improve the quality of analytical data for total antimony determinations in solid matrices.

3.2 Solid state partitioning

Techniques for the determination of total antimony in soils, sediments and solid wastes are an essential tool for environmental analysis, however, these techniques provide little information with regard to biotic and abiotic environmental interactions. The solid state partitioning of antimony, therefore, requires elucidation, since this identifies the association of antimony with specific particulate environmental phases. Indirect techniques to estimate solid state partitioning in operationally defined fractions include both single and sequential chemical extractions. However, chemical fractionation cannot be accurately identified due to the nonselectivity of the extractants and the possibility of element redistribution during extraction. Brannon and Patrick¹¹⁴ describe an extraction technique to examine the fixation and mobilisation of antimony in sediment samples. In this study, operationally defined sediment fractions were determined by sequential extraction with water, ammonium sulfate, hydroxylamine and oxalate reagents. This enabled an estimation of antimony concentrations in interstitial water, exchangeable phases, easily reducible phases and moderately reducible phases and mean coefficients of variation for fractionation varied in the range of 0–7.5%.

The applications of extraction procedures for solid state partitioning of antimony are reported to a lesser extent by comparison with aqueous leaching experiments. These techniques enable estimation of mobile and bioavailable trace-element fractions, and studies with antimony examine both sediment and fly-ash incineration waste samples.^75,114,115 Long-term leaching of antimony was assessed in sediment samples using distilled-deionized water at natural pH.¹¹⁴ However, only short-term leaching studies (6–24 h) have been conducted on fly-ash incineration waste.^75,115

To date the solid state partitioning of antimony in environmental samples remains incompletely understood. Whilst considerable research is directed at analysis techniques for total antimony and aqueous speciation, few studies focus on the associations of antimony with intrinsic soil and sediment components. Further application of leaching experiments at variable pH together with chemical extraction procedures will assist in the elucidation of long-term stability of antimony in environmental samples and these studies are being undertaken by the authors. However, careful consideration is required with regard to the application of Tessier-based extractions^116,117 since high temperature extractions may promote loss of antimony by volatilisation.

4 Conclusions

The determination of total antimony concentrations commonly found in aqueous environmental samples is readily achievable as a result of technological advances with analyte detection systems and the development of ICP-AES and ICP-MS techniques. Total antimony determinations from solid samples also benefit from these detection systems although liquid sample introduction is often required and pre-analysis matrix digestion can be problematic. Advances in microwave digestion technology provide a great deal of scope for determination of antimony in solid matrices and prevention of analyte loss by volatilisation. However, the choice of digest reagents requires careful consideration to ensure adequate matrix dissolution and prevent the formation of insoluble antimony-bearing precipitates. Enhanced analytical interference effects are also problematic during the determination of total antimony in solid sample digest products. The reduced spectral interferences resulting from MS detection are increasingly recognised and the application of ICP-MS detection is likely, therefore, to gain in popularity.

Available techniques for the determination of aqueous antimony species utilise both pre-analysis and in-line species-specific separations prior to detection. However, the majority of published techniques, including common applications of hydride generation, are insufficiently selective for the elucidation of detailed chemical speciation. As a result of this, the development of in-line techniques with improved species specificity has become an active area of analytical research. At present the application of chromatographic techniques coupled with ICP-MS detection (HPLC-ICP-MS) is in the early stages of development for aqueous antimony speciation. However, these techniques offer the potential for highly selective aqueous species separations and identification of detailed chemical speciation data with superior detection limits. The majority of studies utilising chromatographic techniques for determination of aqueous antimony speciation focus on the separation of Sb(III), Sb(V) and tri-methylated Sb species. However, the problems encountered with simultaneous separation of multiple aqueous antimony species are difficult to overcome and often only Sb(III) and Sb(V), or Sb(V) and tri-methylated Sb species are separated on a single chromatographic system. Future studies should focus, therefore, upon the optimisation of chromatographic separations to facilitate the determination of more than two aqueous species, in-line.

Whilst the separation and identification of Sb(III), Sb(V) and tri-methylated Sb species are currently receiving considerable interest, little research is focused upon the elucidation of uncharacterised aqueous antimony species in environmental matrices. In the absence of many commercially available, soluble antimony compounds, future studies should investigate the potential for complexation and chromatographic separation of Sb(III) and Sb(V) associated with naturally occurring organic and inorganic ligands. Thorough application of these complexation-type studies will improve the understanding of separation chemistry for aqueous antimony species. Furthermore, this will facilitate the development of advanced chromatographic techniques for the separation and identification of complex antimony species in environmental matrices.

To date, the majority of published analytical research relating to environmental analysis for antimony is focused largely upon the determination of total antimony and aqueous antimony speciation. However, few studies are directed at the long-term stability and bioavailability of antimony in solid environmental matrices, particularly soils, sediments and solid wastes. Future studies in this area of analytical research should address, therefore, the development of chemical extraction procedures to aid the elucidation of antimony's solid state partitioning.

An improved knowledge of antimony's aqueous speciation and solid state partitioning is fundamental to understanding the significance of antimony concentrations in environmental systems. The associations of antimony with solid environmental matrices including soils, sediments and solid waste influence the extent of release and mobility in environmental systems. Furthermore, the intrinsic chemical speciation of mobilised antimony can determine bioavailability and the potential for toxicity. The majority of antimony's environmental chemistry is at present, incompletely understood. However, there is considerable scope for analytical research within this field. Extensive research in relation to solid state partitioning and continued developments with chromatographic separations will prove essential to the elucidation of antimony's environmental chemistry.

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