Multielemental analysis of Mn–Fe nodules by ICP-MS: optimisation of analytical method

Abstract

Two acid digestion procedures (microwave-assisted and room temperature) were developed for the quantitative analysis of ferromanganese nodules by inductively coupled plasma double focusing sector field mass spectrometry (ICP-SFMS). Different compositions of the acid mixture, dilution factors and corrections for spectral interferences were tested. A combination of nitric, hydrochloric and hydrofluoric acids is necessary for complete sample digestion, with lowest acid to sample ratios (v/m) of 15 and 1.5, respectively, for the last two acids. Sample dilution factors higher than 2 × 10⁴ should be used in order to decrease matrix effects and provide robust long-term instrumental operation. In spite of high dilution, method detection limits in the sub-μg g⁻¹ range were obtained for 54 out of 71 elements tested, due to the high detection capability of ICP-SFMS, as well as the special care taken to ensure the purity of reagents, to clean the instrument sample introduction system and to minimise sample handling. Owing to the presence of unresolved (at the resolution available) spectral interferences, accurate determination of Au, Hg, Os, Pd, Re and Rh is impossible without matrix separation. The accuracy of the entire analytical method was tested by the analysis of two nodule reference materials. The results generated agreed to within ±2% for about 10, within ±10% for more than 40 and within ±20% for about 50 of 53 elements for which certified, recommended or literature values are available. A precision better than 3%, expressed as the between-digestion relative standard deviation (n = 4), was obtained for the majority of elements, except in cases limited by low analyte concentrations.

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Introduction

Ever since ferromanganese nodules were first found in the deep-sea during the Challenger expedition in 1872–1876, their mode of formation and the origins of the elements have been vehemently discussed.¹ When it was realised that the trace metal concentrations in nodules constituted a substantial economic potential, many countries launched manganese nodule research programmes. Research within this field was therefore intensive during the period 1965–1980, as exemplified elsewhere.^2,3 This activity also resulted in a need for manganese nodule reference samples⁴ to validate data generated by the diverse analytical techniques used. When it became clear that nodules were not the mineral bonanzas they once were thought to be, a scaling down of exploration, commercial activity and academic research followed.

However, interest in ferromanganese deposits has been rekindled during the past few years because trace element geochemistry provides a key to understanding how the Earth works. Many trace metals are present at extremely low levels in seawater, but reach readily measurable concentrations in ferromanganese nodules and crusts. Therefore, manganese crusts have been extensively used to study the historical development of the Earth–ocean system. The analysis of various trace metal isotope systems by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has opened up several new research fields,⁵ as many hypotheses concerning the Earth–ocean system are based on trace element determinations in ferromanganese crusts. In the Baltic Sea region, increasing attention is being directed to the use of concretions for the long-term monitoring of heavy metal pollution.⁶ Consequently, there is a need for more detailed studies of trace metals and their isotopes in ferromanganese samples of various kinds.

A variety of analytical techniques have been used in the past for the determination of elemental concentrations in ferromanganese nodules: neutron activation analysis, gravimetry, coulometry, atomic absorption spectrometry, spectrophotometry and X-ray fluorescence to name but a few.⁴ Inductively coupled plasma double focusing sector field mass spectrometry (ICP-SFMS) is a perfect choice for multielemental analysis due to a combination of low instrumental detection limits, wide dynamic range (9–10 orders of magnitude) and relative freedom from spectral interferences. Several investigations have been performed in which ICP-SFMS has been used to study geological matrices, such as sediments, soils or plant tissues.^7–9 In a recent report,¹⁰ it has been shown that about 70 elements, ranging from a few ng g⁻¹ to per cent levels, can be determined simultaneously in geological materials using less than 5 ml of sample digest solution, with an instrument time of 8 min per sample, employing ICP-SFMS. As the technique, in standard configuration, requires samples in dissolved form, considerable effort should be devoted to developing simple and rapid sample preparation procedures, although direct determination of the platinum group and rare earth elements (REEs) in nodules by laser ablation ICP-MS has been suggested as an alternative.¹¹

In general, in order to obtain multielemental information from nodule samples, various combinations of digestions and analytical methods have been used. If a single method was available, it should save time and effort and decrease the required instrument time. A number of different digestion procedures for nodules have been reported in the literature, the majority relying on traditional hotplate digestion using acid combinations, such as HF and HCl, HNO₃, HCl and H₂SO₄, HNO₃, HF, HClO₄, HCl and H₂O₂ or HNO₃, HF and HCl, in a PTFE beaker followed by ICP-MS and ICP-optical emission spectrometry (OES) analysis.^4,12–14 The use of large volumes of acids, or combinations of acids, carries a risk of contamination, precipitation losses and spectral interferences, especially if the reagent to sample ratio is large. Moreover, the majority of reported digestion methods require sample sizes of more than 100 mg. This limits the usefulness of such procedures for zoning analysis within nodules using micro-sampling. Spatially resolved sampling can provide more detailed information on the growth rates of nodules, as well as on the historical record of environmental conditions in the region of interest. Therefore, the analytical requirements for the optimal digestion procedure can be summarised as follows: the ability to obtain accurate (elemental composition) and precise (elemental distribution) data for as many elements as possible, using a limited sample size (less than a few milligrams), combined with high sample throughput.

In this paper, two different approaches to nodule sample digestion prior to analysis by ICP-SFMS are described: microwave (MW)-assisted and room temperature acid digestion using combinations of HNO₃, HCl and HF. The figures of merit of these analytical procedures were evaluated using two commercially available nodule reference materials. The optimised and validated (for about 60 elements) method has the potential to replace the multitude of sample preparation and instrumental analytical techniques previously used to determine specific groups of elements in ferromanganese nodules.

Experimental

Instrumentation

The ICP-SFMS instrument used was an Element (Thermo Finnigan, Bremen, Germany), which can be operated in low- (LRM, m/Δm about 300), medium- (MRM, m/Δm 4500) and high- (HRM, m/Δm 9200) resolution modes. The typical instrument sensitivity in LRM for a middle mass element such as In is in the range 250–300 MHz. In this work, only LRM and MRM have been used. Details of the instrumental operating conditions and measurement parameters are reported in Table 1. It is possible to acquire analytical signals using both ion counting and analogue detection modes, with automatic switching between modes at an ion current of about 4.5 × 10⁶ cps. This option allows increased dynamic range of the instrument to about nine orders of magnitude. Optimisation and mass calibration of the ICP-SFMS instrument were carried out as reported previously.^15,16 A microwave system MDS-2000 (CEM, Matthews, USA), equipped with 12 low-volume perfluoroalkoxy (PFA)-lined vessels (ACV 50) with safety rupture membranes (maximum operating pressure, 1380 kPa), was used for MW dissolution.

Table 1 ICP-SFMS operating conditions and measurement parameters

a Per cent of peak width. b Internal standard.
Rf power/W	1400
Sample uptake rate/ml min^−1	0.3
Argon gas flow rates/l min^−1
Coolant	15
Auxiliary	0.85
Nebuliser	0.85–0.90
Ion sampling depth/mm	9
Ion lens settings	Adjusted to obtain maximum signal intensity
Torch	Fassel 1.5 mm id injector
Nebuliser	Mikromist GlassExpansion
Spray chamber	Scott type PFE (double-pass)
Sample cone	Nickel, 1.1 mm orifice diameter
Skimmer	Nickel, 0.8 mm orifice diameter
Isotopes
Low-resolution mode (LRM)	^7Li,^9Be,^11B,^75As,^82Se,^85Rb,^88Sr, ^89Y,^90Zr, ^93Nb,^98Mo,^99,101,102Ru, ^103Rh, ^105,106,108Pd, ^107,109Ag, ^111,114Cd, ^115 In^b, ^118,120Sn, ^121Sb, ^125,126Te, ^127I, ^133Cs, ^138Ba, ^139La, ^140Ce, ^141Pr, ^143Nd, ^147,149Sm, ^151,153Eu, ^157,160Gd, ^159Tb, ^163Dy, ^165Ho, ^167Er,^169Tm,^171,173Yb, ^175Lu^b, ^178,180Hf,^181Ta, ^184W, ^185,187Re, ^190,192Os, ^191,193 Ir, ^194,195,196Pt, ^197Au, ^202Hg, ^205Tl, ^206,207,208Pb,^209Bi,^232Th,^238U
Medium-resolution mode (MRM)	^26Mg, ^27Al, ^28Si, ^31P,^32S, ^39K, ^44Ca, ^45Sc, ^47,49Ti,^51V, ^52Cr,^55Mn,^56Fe,^59Co,^62Ni,^63Cu,^64Zn, ^69,71Ga,^72,74Ge, ^79Br, ^115In^b
Acquisition mode	E-scan
No. of scans	15 for each resolution
Acquisition window (%)^a	50 in LRM; 120 in MRM
Search window (%)^a	50 in LRM; 80 in MRM
Integration window (%)^a	50 in LRM; 60 in MRM
Dwell time per sample/ms	10 in LRM; 20 in MRM
No. of samples per nuclide	30 in LRM, 25 in MRM

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Samples and reagents

Distilled ‘Milli-Q’ water (DDW, Millipore Milli-Q, Bedford, USA) was used for the dilution of samples and standards. All calibration and internal standard solutions used were prepared in acid-washed (hot mixture of HNO₃–HCl, followed by soaking in 10% HNO₃ overnight; then just before use the tubes were rinsed in fresh Milli-Q water) 10 ml polystyrene test tubes (Nalge Nunc International, Rochester, NY, USA) by dilution of 1 g l⁻¹ single-element standard solutions (SPEX Plasma Standards, Edison, NJ, USA). Analytical grade nitric acid (65%, Merck, Darmstadt, Germany) was used after additional purification by sub-boiling distillation in a quartz still (Heraeus, Karlsruhe, Germany). The hydrofluoric (40%, Merck, ‘Suprapur’ grade) and hydrochloric (30%, Fluka, Steinheim, Germany, ‘PA+’ grade) acids were used without additional purification. The two different ferromanganese nodule reference materials used were Nod-A1 and Nod-P1 (Branch of Geochemistry, US Geological Survey, 923 National Center, Reston, VA 22092, USA). Material used in the preparation of Nod-A1 was collected from the Atlantic Ocean along the Blake Plateau (31°02′ N, 78°22′ W) at a depth of 788 m, and, for Nod-P1, from the Pacific Ocean (14°50′ N, 124°28′ W) at a depth of 4300 m.⁴

Description of analytical procedures

MW-assisted digestion. During preliminary experiments, it appeared that complete digestion (assessed by visual inspection of the digest solutions) of the reference materials could be obtained by using a combination of concentrated nitric, hydrochloric and hydrofluoric acids. Exclusion of any of the acids resulted in an undigested residue. For the MW-assisted digestion, sample amounts of about 50 mg and a total reagent volume of 2.1 ml (1.5 ml HNO₃, 0.1 ml HF and 0.5 ml HCl) were used. The nodule material was weighed and transferred into the MW vessels and the reagent mixture was added. The vessels were then closed and mounted in special outer vessels and heated at 325 W power for 30 min, cooled to room temperature and carefully vented in a fume hood. The solution was then transferred to a test tube and about 5 ml of DDW was used to rinse the MW vessel. The rinse was combined with the digestion solution followed by the addition of DDW up to a total volume of 10 ml.

Further dilution of these solutions was based on the following considerations. The determination at sub-μg g⁻¹ concentrations (trace and ultra-trace elements) requires low sample dilution. However, this may be accompanied by problems with the simultaneous determination of major elements, e.g. Mn, Fe, Al, due to detector overload even in analogue mode, severe matrix effects, rapid clogging of cone apertures, need for frequent re-tuning and rapid loss of instrumental response which is impractical for long runs. It appears that, in order to maintain stable instrument performance over periods of a few hours, a sample dilution higher than 10⁴ should be used. The three stepwise dilution factors used in this work for MW-assisted digestions were 2 × 10⁴, 2 × 10⁵ and 2 × 10⁶. These dilutions were performed gravimetrically rather than on a volume basis, providing a combined uncertainty for sample preparation below 0.5%, even for the highest dilution. By using variable dilutions, it is possible to determine all elements at comfortable concentrations in the measuring solutions, using only the ion counting detection mode. Major elements were determined in the most dilute solutions and ultra-trace elements in the least dilute.

The dynamic range of ICP-SFMS can be further extended by measuring major elements in MRM, which reduces ion transmission by a factor of 10–20. Nevertheless, the need for three separate analyses of each nodule sample greatly reduces instrumental sample throughput. Additional potential limitations of MW-assisted digestion are: the rather extensive and laborious handling of vessels (including between-batch washing procedure); increased contamination risks; and the total volume of acid mixture should be at least 2 ml even when using low-volume (50 ml) digestion vessels, leading to a high acid to sample ratio while digesting small sample amounts. Assuming that only one set of digestion vessels (12 vessels) is available and at least one of them will be occupied by the preparation blank, the sample throughput with this method is about 30 samples in an 8 h shift.

Room temperature digestion. As an alternative to MW-assisted digestion, room temperature digestion of Nod-A1 and Nod-P1 materials in test tubes was evaluated using different acid combinations, varying only the composition at constant total volume. Special attention was paid to find the lowest possible volumes of HCl and HF required for complete sample digestion. This is necessary in order to: (i) keep the blank contribution from the acids at a minimum; (ii) reduce possible spectral interferences from HCl (such as ArCl⁺ on As⁺, BaCl⁺ on Yb⁺); (iii) reduce corrosive effects of excess HF on glass parts of the sample introduction system; and (iv) prevent analyte losses via the formation of volatile (such as SiF₄) or insoluble (such as CaF₂) species owing to excess HCl and HF.

In this optimisation study, the sample to acid mixture ratio was kept constant (2 ml per 20 mg of nodule reference material), while the volume fractions of HCl and HF were gradually changed. The volume of HNO₃ was varied in the range 1–2 ml, HCl in the range 0–1 ml and HF in the range 0–80 μl. The weighed nodule material was transferred into the test tube followed by the addition of acid mixture. The closed test tubes were placed in a holder and left on a slow speed turntable overnight followed by the addition of DDW to 10 ml. From these solutions, further dilution was made to reach the final dilution factor of 5 × 10⁴. The use of single compromise dilution makes this procedure more sensitive to blank levels for ultra-trace elements, especially from DDW and sample introduction parts. Moreover, the determination of some major elements requires the use of combined detection mode, and certain errors from cross-calibration between counting and analogue modes may arise. There are a number of important advantages of room temperature digestion compared to MW-assisted digestion: it is less labour intensive, requires no supervision and acid mixture volumes as low as 100–200 μl can be easily handled. Digestion overnight in capped vials at room temperature carries low contamination and volatilisation risks. The digestion tubes are also disposable and therefore there is no need for extensive washing procedures between samples and no carry-over effects between samples. Some tubes are used for both the dissolution and storage of solutions, which leads to reduced contamination from containers. Moreover, practically unlimited numbers of samples can be prepared simultaneously, providing high sample throughput.

Prior to each measurement sequence, all glass parts of the ICP-SFMS introduction system were carefully cleaned by overnight soaking in 10% HNO₃. Samples were analysed using In for internal standardisation (added to all solutions at 25 ng ml⁻¹) and external calibration using a set of multielement standards in the expected concentration range. In order to test the stability of instrumental performance, control standards were analysed after every 12 samples. Due to high sample dilution, changes in In response were less than 10–15% during a 10 h measurement sequence.

Results and discussion

The method detection limits (MDLs) were defined as three times the standard deviation for 11 preparation blanks assuming a dilution factor of 5 × 10⁴ (Table 2). The MDLs were found to be independent of acid mixture and vary from the sub-ng g⁻¹ level up to a few μg g⁻¹ (major elements). Hence, assuming that the minimum necessary volume of analytical solution is 5 ml, the sample size can actually (at least theoretically) be reduced to 0.1 mg.

Table 2 Detection limits (μg g⁻¹)

Element		Element		Element
Ag	0.09	Hg	0.3	Ru	0.007
Al	4	Ho	0.0004	S	100
As	1	I	0.5	Sb	0.04
Au	0.009	Ir	0.0005	Sc	0.2
B	10	K	500	Se	2
Ba	0.1	La	0.03	Si	100
Be	0.07	Li	1	Sm	0.002
Bi	0.005	Lu	0.0009	Sn	0.2
Br	6	Mg	4	Sr	0.2
Ca	50	Mn	0.6	Ta	0.004
Cd	0.05	Mo	0.2	Tb	0.0006
Ce	0.06	Na	300	Te	0.025
Co	0.1	Nb	0.008	Th	0.004
Cr	0.1	Nd	0.02	Ti	1
Cs	0.008	Ni	2	Tl	0.007
Cu	4	Os	0.0006	Tm	0.0008
Dy	0.001	P	4	U	0.004
Er	0.0005	Pb	0.3	W	0.05
Eu	0.002	Pd	0.02	V	0.07
Fe	2	Pr	0.003	Y	0.02
Ga	0.02	Pt	0.002	Yb	0.002
Gd	0.002	Rb	0.2	Zn	2
Ge	0.5	Re	0.0007	Zr	0.03
Hf	0.001	Rh	0.002

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Spectral interferences

Along with preparation blanks, the formation of spectral interferences may severely limit the detection capabilities of ICP-MS, especially for trace elements.¹⁷ During multielemental analysis, these interferences originate from plasma gases (such as ArO⁺ on Fe⁺, Kr⁺ on Se⁺, Xe⁺ on Te⁺, etc.), acids (N₂⁺ on Si⁺, NO⁺ on P⁺, ArCl⁺ on As⁺) and from the sample matrix (such as SiO⁺ on Sc⁺, MoO⁺ on Cd⁺, etc.). The majority of interferences in the mass region from 24 to 81 u can be resolved in MRM, providing a straightforward approach for interference-free determination.¹⁸ There are, however, numerous interferences in the higher mass region (such as LuO⁺ on Ir⁺; Cd⁺, MoO⁺, ZrO⁺, ZnAr⁺ on Pd⁺, etc.) whose separation requires resolution unattainable with commercially available instrumentation. In the present work, mathematical correction has been applied for these and other potentially affected isotopes. This requires the measurement of interfering isotopes and a knowledge of the formation rates for interfering species. These ratios were determined on a daily basis from the analysis of standard solutions. Detailed information on the mathematical correction procedures is given elsewhere.^19,20 In total, isotopes of about 20 elements were corrected. As an example, uncorrected and corrected concentrations for selected isotopes are shown in Table 3, where elements are grouped in accordance with the magnitude of the correction. For Cd, Eu and Pt, it is possible to find at least one isotope for which mathematical correction accounts for less than 10% of the uncorrected signal. This group of elements also includes Sb, Te, Ta, Bi and some heavy REEs. Correction by 10–80% is required for Ag and Ir isotopes (as well as for As and Ru). In spite of this relatively high degree of correction, the agreement between corrected results for different isotopes (where available) is still acceptable. When spectral interferences contribute to more than 90% of the initial signal or corrected concentrations are below the MDLs, there is poor agreement between corrected results for different isotopes and the accuracy of the results can no longer be guaranteed. Therefore, concentrations for such elements (Pd, Rh, Os, Re, Au and Hg) will be reported as below 20% of the uncorrected concentration obtained for the least affected isotope. For these elements (which are as a rule present at ultra-trace levels), lower sample dilution will provide no improvements in detection capabilities. Chemical separations of analytes from interfering elements or a significant decrease in the formation of oxide species by aerosol desolvation are mandatory for the accurate determination of these six elements, although neither of these approaches has been tested in this study.

Table 3 Interference corrections

Isotope	Concentration in A1		Concentration in P1		Corrected interferences
	Uncorrected	Corrected	Uncorrected	Corrected
^111Cd	8.66	7.49	24.5	22.9	^95Mo^16O^+
^114Cd	8.13	7.5	23.5	22.6	^114Sn^+, ^98Mo^16O^+
^151Eu	5.31	5.19	7.85	7.58	^135Ba^16O^+
^153Eu	5.52	5.23	8.09	7.6	^137Ba^16O^+
^194Pt	0.578	0.522	0.151	0.124	^178Hf^16O^+
^195Pt	0.545	0.519	0.13	0.119	^179Hf^16O^+
^196Pt	0.609	0.525	0.176	0.12	^180Hf^16O^+, ^196Hg^+
^107Ag	0.838	0.181	0.692	0.196	^91Zr^16O^+
^109Ag	0.608	0.173	0.328	0.207	^93Nb^16O^+
^191Ir	0.048	0.016	0.027	0.006	^175Lu^16O^+
^193Ir	0.038	0.015	0.018	0.007	^175Lu^18O^+, ^176Hf^16OH^+
^105Pd	3.62	<0.020	2.31	<0.020	^89Y^16O^+, ^88Sr^16OH^+, ^65Cu^40Ar^+
^106Pd	5.88	0.197	5.14	<0.020	^106Cd^+, ^90Zr^16O^+, ^89Y^16OH^+, ^66Zn^40Ar^+
^108Pd	2.35	0.146	2.41	<0.020	^108Cd^+, ^92Mo^16O^+, ^92Zr^16O^+, ^68Zn^40Ar^+
^185Re	0.011	<0.0007	0.0086	<0.0007	^169Tm^16O^+, ^184WH^+
^187Re	0.0018	<0.0007	0.0014	<0.0007	^171Yb^16O^+, ^169Tm^18O^+, ^186WH^+

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Accuracy

ICP-SFMS results obtained for the Nod-A1 and Nod-P1 nodule reference materials prepared by MW-assisted digestion are presented in Table 4, together with a compilation of certified, recommended and published ranges. All obtained results in this work are corrected for moisture content, determined in separate subsamples of Nod-A1 and Nod-P1 material. For both of these certified reference materials, 19 out of 29 element concentrations are certified values, whereas other data have to be handled as information or recommended values. Concentrations of about 20 additional elements in these nodule materials have previously been reported,^4,21–24 and these data provide the published ranges compiled in Table 4. In total, certified, recommended and literature data are available for 53 elements.

Table 4 Results for Nod-A1 and Nod-P1 and comparison with certified and published data (all concentrations in μg g⁻¹)

	Nod-A1					Nod-P1
	Found		CRM^4^b	Published ranges^4,17–20		Found		CRM^4^b	Published ranges^4,17–20
	ICP-SFMS	s ^a		Min	Max	ICP-SFMS	s ^a		Min	Max
a s corresponds to one standard deviation based on at least four separate analyses. b CRM, certified reference material values. c Recommended values.
Ag	0.17	0.02		<5		0.21	0.03		<5
Al	20800	300	20480	17000	32600	24600	1000	25400	18000	38500
As	310	8		298		88.5	3.9		39
Au	<0.009					<0.009
B	120	0.2		94		95.0	5.4		120
Ba	1530	30	1670	978	2700	2690	20	3350	1040	6700
Be	5.60	0.04		5.6	8.5	2.30	0.02		1.5	2.8
Bi	10.2	0.1				5.80	0.40
Br	40.9	0.7				30.3	2.2
Ca	114000	3000	110100	109700	113000	22400	100	22160	20700	23100
Cd	7.50	0.16		6.5		22.6	0.3		22.3
Ce	720	2	730^c	656	930	305	2	290^c	280	310
Co	3180	160	3110	1090	3400	2290	50	2240	1100	2500
Cr	20.9	0.7		14	25.9	13.3	0.01		11.8	20
Cs	0.61	0.010				1.80	0.01
Cu	1130	30	1110	800	1800	11200	100	11500	10400	13500
Dy	23.8	0.4	23^c	20	25.8	27.1	0.4	27^c	25	29.2
Er	14.4	0.4	12^c	12	15.6	13.6	0.1	12^c	12	15.2
Eu	5.20	0.11	5^c	1	6.1	7.60	0.10	7.5^c	3.3	9.04
Fe	112000	2000	109100	97000	110800	58900	400	58050	47600	64700
Ga	6.30	0.04				28.1	0.9
Gd	25.4	0.4	26^c	22	34.3	30.4	0.5	28^c	21	33.8
Ge	<0.5					0.54	0.50
Hf	5.80	0.66		6.2		4.20	0.02		4.3
Hg	<0.39					<0.3
Ho	5.00	0.01		4.7	5.8	5.00	0.01		4.73	6.5
I	47.7	3.2				31.4	0.3
Ir	0.016	0.001				0.006	0.004
K	4940	80	4981	1700	5200	10000	400	9960	2800	10500
La	115	2	120^c	104	152	105	2	104^c	82	120
Li	76.1	0.8		72	76.3	140	1		138	142
Lu	2.10	0.05		1.92	2.8	1.80	0.01		1.66	1.85
Mg	28300	600	28700	27400	32000	20300	700	19900	18000	20800
Mn	183000	4000	185000	137000	186400	296000	13000	291300	252000	299300
Mo	390	0.3	448	230	567	675	1	760	630	982
Na	8660	180	7420	7420	8800	17100	240	16300	16100	17300
Nb	43.1	1.1		43.2	43.8	21.3	0.1		21.2	21.4
Nd	98.0	2.0	94^c	81	105	130	2	120^c	83	143
Ni	6450	40	6360	3150	7100	13500	280	13400	12400	15600
Os	<0.004					<0.002
P	5920	120	6110	4200	6110	2060	50	2010	1700	2100
Pb	860	3	846	346	1323	475	7	560	436	715
Pd	<0.47			0.003		<0.48			0.006
Pr	25.0	0.01		21.7	23	31.0	0.2		27	27.5
Pt	0.52	0.02		0.453		0.12	0.01		0.123
Rb	10.6	0.1		10	11	23.7	0.7		21	23
Re	<0.0007					<0.0007
Rh	<0.07					<0.11
Ru	0.022	0.005		0.018		0.012	0.014		0.005
S	3350	40				1000	40
Sb	33.8	0.1		33.5		49.4	1.4		50.1
Sc	12.4	0.3	13	10.8	13.0	9.70	0.10	7.6	7.6	9.47
Se	3.5	1.5				3.9	2.0
Si	17400	400	17800	14200	20000	58500	1900	64980	63600	69700
Sm	21.9	0.1	21^c	18b	24.7	31.0	0.3	30^c	20	35.3
Sn	3.00	0.08				1.90	0.03
Sr	1630	20	1750	1060	1800	670	2	680	330	1080
Ta	0.76	0.03				0.33	0.01
Tb	4.00	0.07		3.8	4.87	4.90	0.02		4.2	5.31
Te	30.9	0.1				4.80	0.20
Th	25.1	0.6		25.8	27.4	16.7	0.2		17	17.7
Ti	3030	70	3177	2800	3800	2720	40	2998	2800	3700
Tl	120	1		61	148	210	2		154	265
Tm	2.00	0.06		1.75	2.19	1.90	0.02		1.72	1.77
U	7.00	0.08		5.95	6.86	4.00	0.04		3.5	4.28
V	660	5	770	260	916	510	3	570	270	673
W	87.0	0.8				57.8	1.1
Y	120	1		97	138	90.0	0.6		55	89
Yb	13.9	0.4	14^c	13.2	16.3	12.9	0.2	13^c	8.6	13.8
Zn	800	20	590	460	870	2020	60	1600	1467	1998
Zr	310	3		317	498	280	1		260	280

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Excellent agreement (deviation between found and certified/recommended concentrations below 2%) was found for 12 elements in Nod-A1 (Al, Ca, Ce, Co, Cu, Fe, K, Mg, Mn, Ni, Pb and Yb); for Nod-P1, the list was one element longer (Ca, Co, Cu, Dy, Eu, Fe, K, La, Mg, Mn, Ni, Sr and Yb) and overlapped for nine elements. For 33 and 28 additional elements in Nod-A1 and Nod-P1, respectively, agreement is either better than 10% (compared to single certified, recommended or published values) or the ICP-SFMS results fall within published ranges where such are available.

Slightly poorer accuracy (results within ±20% of certified, recommended or single published values or marginally outside published ranges) was found for Cd, Pr, Pt, Ru, Th, U and Zr in Nod-A1 and for Pr, Rb, Sc, Si, Th, Ti, Tm, Y and Zn in Nod-P1. Fig. 1 shows graphs of chondrite-normalised REE concentrations²⁵ for each nodule material plotted using found as well as recommended values. For some of the elements (Pr, Tb, Ho, Tm and Lu), no data are specified in the reference material documentation, and a single published value or an average from the published range was used. The ICP-SFMS values in both Nod-A1 and Nod-P1 materials result in smooth curves, thus confirming the internal consistency of found concentrations. Finally, differences of more than 20% between ICP-SFMS results and published values were observed for B in Nod-A1 and for B, As and Ru in Nod-P1. As each of these published values is based on a determination by a single technique performed by only one laboratory each, the significance of these differences is difficult to evaluate.

Fig. 1 Chondrite-normalised REE patterns for Nod-A1 and Nod-P1 reference materials using MW-assisted dissolution method.

To the best of our knowledge, there are no literature data available on the concentrations of 18 elements (Au, Bi, Br, Cs, Ga, Ge, Hg, I, Ir, Os, Re, Rh, S, Se, Sn, Ta, Te and W) in these two specific nodule materials. Hence, the accuracy for these elements cannot be evaluated. The precision, assessed as the relative standard deviation (RSD) for three replicate digestions/analyses of each reference material tested, was generally better than the 3% RSD level with a few notable exceptions (Ru, Se, Ge, 20–120%), attributable to concentrations which are close to the MDL and/or large spectral interference corrections.

Optimisation of room temperature acid digestion

The main goal of this optimisation was to identify an acid mixture providing an accuracy similar to MW-assisted digestion while keeping the proportions of HCl and HF at a minimum. Therefore, the total acid volume was set to 2 ml and the acid amounts were varied according to the x axis in Figs. 2 and 3. The results from these room temperature digestions are evaluated in terms of recovery relative to the successful MW-assisted method described above. The relative recovery for La for both reference materials [Fig. 2(A)] is independent of the acid mixture (being within the 0.90–1.05 range); a similar behaviour was obtained for the rest of the REEs, Y and U. The importance of HCl is evident for Cd and W [Figs. 2(B) and 2(C)], selected as representative elements for the groups Ag, Bi, Ba, Cd, Co, Cu, Li, Mg, Mn, Ni, P, Pb, Pt, S, Sr, Te, Zn and I, Mo, Sn, W, respectively. It is interesting to note that, for Cd, even trace amounts of HCl are sufficient to increase the recovery from 0.6 to 1.0. Tungsten, I, Mo and Sn all require either a large volume of HCl or a combination of HCl and HF to reach recoveries close to 1.0. Fig. 3 shows examples of elements whose complete recoveries require the addition of HF. There are obvious differences in Zr (as well as B, Be, Ca, Fe, Hf, Na, Nb, Sc, Th and V) recovery between the two nodule reference materials using the same acid combination [Fig. 3(A)]. In Nod-A1, a combination of HNO₃ and HCl results in Zr recovery of about 0.9, while it is about 0.5 for Nod-P1. The addition of the lowest amount of HF (5 μl) increases the recoveries to above 0.95 in both materials. For Rb [Fig. 3(B)], the relative recovery without using HF is below 0.6 and at least 10 μl of HF is required in order to obtain more than 0.9 recovery; a similar behaviour was obtained for Cs, Ga, K and Sb. For Si (selected as a representative element for the group Al, Cr, Ge, Si, Ta and Ti), recovery from Nod-P1 is about 0.1 until HF is added [Fig. 3(C)]. At least 30 μl of HF is necessary to achieve an Si recovery better than 0.95 in this material.

Fig. 2 Recovery of La (A), Cd (B) and W (C) after room temperature dissolution relative to MW-assisted digestion method (total volume of acid, including HNO₃, always 2 ml). Volumes on the x axis are in µl. Open circles denote data for Nod-A1 and filled circles for Nod-P1.

Fig. 3 Recovery of Zr (A), Rb (B) and Si (C) after room temperature dissolution relative to MW-assisted digestion method (total volume of acid, including HNO₃, always 2 ml). Volumes on the x axis are in µl. Open circles denote data for Nod-A1 and filled circles for Nod-P1.

The explanation for these differences in dissolution behaviour for different elements may be found in the geochemical composition. Deep-sea ferromanganese nodules consist essentially of three major phases: aluminosilicates, Fe-oxyhydroxides and Mn-oxyhydroxides.^26–28 The specific association of elements with these three phases can be explained by differences in the chemical forms of elements in seawater and by fundamental differences in physicochemical properties (e.g. the pH of the zero point of charge and the dielectric constant) of the three phases.²⁹ Adsorption is dependent on the pH of the zero point of charge (pH_zpc) of the sorbing medium. For ferrihydrite, the pH_zpc is about 8.0,³⁰ and, for Mn-oxyhydroxides, about 2.2.³¹ In seawater, the adsorption of anionic species would be expected to be stronger on Fe(III)-oxyhydroxides than on Mn-oxyhydroxides. Hence, the elements preferentially incorporated into the Fe-oxyhydroxide phase are mainly present as oxyanions or anions (e.g. As, B, Br, I, P, S, Se, Sn, Te and V), hydroxide complexes of tri- and tetravalent cations (e.g. Am, Bi, Ge, Hf, Ti, In, Nb, Pa, Pd, Pu, REE, Th, Y and Zr) and carbonate complexes of U and Pb. The elements associated strongly with the Mn-oxyhydroxide phase are mono- and divalent cations (e.g. Ag, Ba, Ca, Cd, Co, Cu, Mg, Ni, Ra, Tl and Zn).²⁸ Tungsten is often associated with Mo in Mn-rich nodules.³² Using factor analysis, Li^28,29 demonstrated that Mo and Sb are preferentially associated with the Mn-oxyhydroxide phase in nodules, and tentatively suggested that the oxyanions of Mo, Sb and W have a strong affinity for manganese and possibly exist in the phase as manganese compounds rather than as adsorbed species.^28,29

The ionic species predominant in the aluminosilicate phase are the alkali elements and crystal lattice components, such as Be, Cr, Ga, Sc and sometimes Ta, Ti or Th. Enrichment factors of elements in the world’s average ferromanganese nodules relative to the world’s average pelagic clays show that Al, Be, Cr, Cs, Ga, K, Li, Na, Rb, Sc and Si are close to or below unity. This indicates their association with the admixed sediment or aluminosilicates.²⁸

Varying combinations of acids in the digestion mixture will affect the degree of dissolution of these three phases, hence resulting in different behaviour of the associated elements. For example, differences in recoveries between Nod-A1 and Nod-P1 materials for selected elements (Fig. 3) can be explained by the presence of a significant fraction of pelagic clay in the latter material,⁴ manifest as a higher content of aluminosilicates (compare Al and Si concentrations in Table 4).

As follows from Figs. 2 and 3, the use of an HCl to sample ratio (v/m) of 15 and an HF to sample ratio (v/m) of 1.5 provides relative recoveries in the 0.95–1.05 range for all elements present at detectable concentrations in both nodule reference materials tested. Hence, the accuracy for room temperature digestion is comparable with that for MW-assisted digestion for each element. However, if the fraction of aluminosilicate phase in the nodules under investigation is suspected to be higher than that in Nod-P1, the adoption of a higher HF to sample ratio should be considered.

Conclusions

Both MW-assisted and room temperature digestions, when combined with the powerful analytical technique of ICP-SFMS, provide quantitative information on more than 60 elements in ferromanganese nodules with an accuracy better than 100 ± 10% for the majority of elements studied. Microwave-assisted digestion has the advantage of a shorter digestion time, but it is rather laborious and requires relatively expensive equipment. Room temperature digestion is a simpler and more cost-effective approach that allows high sample throughput and the potential for reducing the sample size below 1 mg without increasing the acid to sample ratio. This will be of particular value for studying elemental distributions in nodules with high spatial resolution, thus providing information on the growth rate and changing environmental conditions in the sampling region. Effects such as incomplete analyte recovery, volatilisation losses and formation of insoluble species do not constitute first-order problems, given that the composition of the digestion mixture is carefully optimised. The combination of optimised room temperature acid digestion and ICP-SFMS thus provides a straightforward and unified approach to complete elemental and, if required, spatial mapping of ferromanganese nodules for the first time.

Acknowledgements

Special thanks are due to Dr Douglas Baxter for valuable comments and for linguistic revision of the manuscript. This study was partly financed by Norrbottens forskningsråd.

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