Elemental fractionation in laser ablation-inductively coupled plasma-mass spectrometry: evidence for mass load induced matrix effects in the ICP during ablation of a silicate glass

Abstract

To study mass load dependent matrix effects in LA-ICP-MS, different masses of aerosols from the NIST 610 glass were generated using a 193 nm ArF excimer laser and a 266 nm Nd:YAG laser and introduced into an ICP-MS. The different mass loads in the ICP were achieved using different crater diameters, the application of an aerosol dilutor, a tandem ablation setup and mixing of a single element matrix (desolvated solution) with the laser generated aerosols. The comparison of results acquired using these different experimental setups supports the existence of a significant matrix effect dependent on the mass load of the ICP. The proof of such effects on 266 nm laser generated aerosols was limited by temporal changes of element ratios due to the generation of a broad particle size distribution and the related incomplete vaporization within the ICP. However, the differences in the particle size distributions measured for various crater diameters as well as the temporal changes of the element ratios using the 193 nm ArF excimer laser were insufficient to explain mass load related changes of the element ratios. It is shown that an increase of the mass load of the ICP by a factor of 16 (crater diameter from 30 to 120 μm) leads to a decrease in certain intensity ratios (e.g., Cu/Ca, Zn/Ca, Cd/Ca, Pb/Ca) up to 25%. Even after taking into account the fact that smaller crater diameters might be partially influenced by laser-induced fractionation, an excessive mass load of the ICP using two 193 nm laser ablation systems demonstrates a further decrease in ion signal intensities of volatile elements in comparison with Ca. In contrast, applying dilution up to a factor of 30 to the aerosols generated using different crater diameters leads to a stabilization of the intensity ratios (e.g. Cd/Ca) to a constant value. The mass load enhanced matrix effect is element dependent and most severe for elements with low melting points (e.g., Cu, Zn, Ag, Cd, Pb). Based on the changes of e.g. As/Ca, Sn/Ca and the constant Be/Ca ratios, an explanation of the plasma induced effects dependent on parameters such as 1st or 2nd ionization potential was impossible. Furthermore, the mass load dependence on easily ionizable elements (Cs, Rb) indicates the absence of incomplete vaporization related effects. In contrast, Al/Ca, Ti/Ca, Th/Ca ratios, for example, remained constant (within 2–3%) with and without dilution. The induction of matrix effects independently of the ablation process by adding various Rb concentrations to the laser aerosols indicates that elements (Cu, Zn, Cd, Pb, U) previously described to be dominantly influenced by laser-induced elemental fractionation undergo significant ICP-induced matrix effects. These matrix effects are mass load dependent and for most elements exceed the contribution of laser-induced fractionation.

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Introduction

Since the first report of using laser ablation sample introduction with ICP-MS by Gray¹ it has already been demonstrated that some elements (e.g. Cd, Zn) show an increase in signal intensity relative to a less volatile element (e.g. Ca) as a function of time during a prolonged ablation. In 1995, Fryer et al.² published a “landmark” LA-ICP-MS paper on elemental fractionation. The authors described the time dependent behaviour of intensity ratios (Ca intensity used as a reference) of a signal acquired over 4 min using the NIST 610 glass. The quotient of the intensity ratio of the second and the first time interval has been defined as the fractionation index and represents a relative measure of fractionation. It has been observed that for elements such as Cu, Zn, Cd, Ag, Sb, Tl and Pb, elemental fractionation relative to Ca occurs as the ablation progresses deeper into the sample. The underlying phenomenon causing elemental fraction effects can originate during the ablation, aerosol transport, or during the vaporization, atomization, and ionization within the ICP. Owing to the fact that the ablation process is primarily responsible for the aerosol generation, it has been considered to be the most likely source of elemental fractionation. This was further supported by proposed changes of the ablation protocols which resulted in reduction of elemental fractionation (e.g., active focusing, soft ablation).^3,4 Today it can be stated that these techniques have not been routinely applied. Further evidence for laser ablation induced elemental fractionation has been provided by Mank and Mason.⁵ The authors demonstrated that a critical crater depth to diameter ratio of >6 (this, for a given number of pulses, is reached faster for smaller crater diameters) leads to an enhanced mobilization of volatile elements and therefore to pronounced elemental fractionation. Several studies have been carried out to correlate physical properties of elements to the degree of elemental fractionation (e.g. boiling points, melting points, ionization potential or condensation temperature).^6–8 However, none of them provided sufficient correlation.

In contrast to this, the change from IR towards UV and deep UV laser wavelengths reduced elemental fractionation significantly.^9–15 These improvements have been mainly attributed to the particle size distribution generated at different wavelengths. On aerosols generated at 266 nm it has been verified that the elemental composition of the ablated samples is not maintained within a broad particle size distribution (from nano-particles and their agglomerates to splashed microspheres).^16–18 Analysis around the ablation crater by SIMS revealed further evidence for laser induced changes of particle composition,¹⁹ which further explains their size dependent elemental composition.^16,17 Investigations on particles and within the ablation crater generated using a 266 nm laser demonstrated phase separation during ablation.^19,20

Furthermore, it has been shown that the size of the ablated particles has a significant influence on the vaporization, atomization and ionization within the ICP.^17,21,22 Therefore, size dependent composition and incomplete vaporization of particles larger than 150 nm (glass particles²¹) within the ICP leads to elemental fractionation effects. Based on this, matrix matched calibration^19,23–26 or matching in particle size distribution (by similar absorptivity of sample and standard) has been proposed for precise and accurate quantification. Wavelengths below 200 nm generate particle sizes and agglomerates,²⁷ which have been reported to be representative for the sample ablated¹⁷ and more suitable for complete vaporization and ionization within the ICP. Moreover, a number of non-matrix matched calibration approaches have been reported.^28–32 However, considering the effects of particle sizes within the ICP, it is surprising that IR-laser generated data by Jackson et al.³³ (REEs, Th, Ca internal standard) are similar in accuracy to the data reported using deep UV lasers and that the progress made for volatile or “fractionating” elements remains rather small.

The incomplete conversion of particles into ions in the plasma is one effect which occurs in the ICP. However, sample matrix induced changes in signal intensities (enhancement or depression) could be another source of uncertainty influencing quantification and has been reported for LA-ICP-OES^34,35 and also assumed for LA-ICP-MS.^19,23,26,36

Because the mass introduced into the ICP in laser ablation is far lower than the typical mass introduced by solution nebulization and also due to the magnitude of mass load related matrix effects demonstrated in an earlier work,³⁷ matrix effects in dependence on mass load have not been of major concern in LA-ICP-MS.

However, non-spectroscopic matrix effects in solution nebulization ICP-MS have been extensively reported.^37–40 These matrix effects are generally defined as a change in elemental sensitivity of analytes as a function of a concomitant and its concentration in the ICP. These effects have been commonly corrected using internal standardization.^41,42 Furthermore, standard addition and isotope dilution have also been applied to reduce the influence of the matrix.^43,44 A review containing almost 400 citations of the pioneering studies on matrix effects in ICP-MS can be found in ref. 40. Investigations on the analyte’s response in the presence of various types and concentrations of matrix have been reported.^39,45,46 The fundamental reasons for the matrix effects are not well understood. Up to the present, there has been no unifying theory which allows the prediction of the magnitude of the effect according to Evans and Giglio.⁴⁰ However, the general conclusions from the experimental observations summarized in ref. 40 are: (a) the most severe matrix effects are caused by high mass matrix elements with low ionization potentials; (b) light elements with high ionization potentials are most affected; (c) the magnitude of the matrix effect depends on plasma operating conditions; (d) the matrix effect depends on the concentration of the matrix element and not on the relative concentration of the matrix element to the analyte element; and (e) dilution has been always proposed as a suitable strategy to minimize matrix effects within the ICP. Applying the conclusions to LA-ICP-MS, the study of (e) could provide some insights into the occurrence of matrix effects in direct solid sampling.

The aim of this study was to investigate mass load related non-spectroscopic matrix effects in LA-ICP-MS. To separate particle effects and matrix effects, two laser wavelengths (266 nm and 193 nm) were used for aerosol generation. Craters of various diameters, mass load studies using tandem of two 193 nm ablation systems and an aerosol dilutor were applied. In addition, an easily ionizable element (Rb) was nebulized, desolvated and added to the laser-generated aerosol before entering the plasma to investigate its influence on signal intensities. Matrix effects were studied on a wide range of elements. Since we wanted to compare the results with the previous studies on elemental fractionation,² NIST 610 was used as a matrix and Ca as an internal standard. All experiments were based on commonly used laser parameters, ICP operating conditions and acquisition times in order to discover the impact on routine analysis.

Experimental

Instrumentation

To investigate the ICP response at various mass loads, SRM NIST 610 was used as the sample. This glass was ablated using either a 193 nm ArF excimer laser (GeoLas C or Geolas M, MicroLas, Goettingen, Germany) or a 266 nm Nd:YAG (LSX 500, CETAC Technologies, Omaha, NE, USA). Both excimer laser systems are equipped with apertures, which image the laser beam onto the sample surface. This optical configuration allows selection of a constant fluence which is independent of crater diameters.⁴⁷

The lasers were coupled to an Elan 6100 DRC+ ICP-MS (PerkinElmer SCIEX, Thornhill, Canada) where transient signal intensities were acquired. The response of the instrument was tuned for optimum sensitivity, low background intensities and a ThO⁺/Th⁺ ratio of less than 1%. Furthermore, the U⁺/Th⁺ ratio was included in the optimization procedure as previously described elsewhere.⁴⁸ Data acquisition and analysis were performed according to the protocol of Longerich et al.⁴⁹ Particle size distribution measurements were carried out using a HS-LAS optical particle system (Particle Measuring System, Denver, CO, USA). This instrument detects particles from 65 nm up to 1 μm in 32 channels using laser light scattering. A summary of the operating conditions used for laser ablation, ICP-MS and particle size distribution measurements is given in Table 1.

Table 1 Summary of the operating conditions used for LA-ICP-MS measurements without/with aerosol dilution

Parameter	ArF-Excimer	Nd:YAG
Wavelength	193 nm	266 nm
Energy density	28 J cm^−2 (GeoLas M)	13 J cm^−2
Pulse length	15 ns	4 ns
Ablation duration	60 s	60 s
Repetition rate	10 Hz	10 Hz
Ablation spot sizes	120, 80, 60 and 30 μm	200, 150, 100 and 50 μm
Ablation cell gas	He	He

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ICP-MS conditions
Carrier gas flow	1 L min^−1 He
Make up gas flow	0.8 L min^−1 Ar
Auxiliary gas flow	0.8 L min^−1 Ar
Plasma gas flow	18 L min^−1 Ar
Rf power	1400 W
Autolens	On
Detector	Dual (pulse and analog counting)
Isotopes measured	^27Al, ^42Ca, ^49Ti, ^59Co, ^63Cu, ^65Cu, ^66Zn, ^111Cd, ^208Pb, ^232Th, ^238U
Values of 1st ionization potential/eV	Al = 6.0; Ca = 6.1; Ti = 6.8; Co = 7.9; Cu = 7.7; Zn = 9.4; Cd = 9.0; Pb = 7.4; Th = 7.0; U = 6.9

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Optical particle sizer
Internal flow rate	0.24 L min^−1 He

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Aerosol dilutor
Frequency of rotations/rpm	Dilution	Dilution factor
83	Dilution 1	12
62	Dilution 2	16
48	Dilution 3	21
32	Dilution 4	31
4	Dilution 5	250

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Aerosol dilution

The influence of the plasma load on the measured ion signal intensities was studied using an aerosol dilutor. The dilution unit (Matter Engineering AG, Wohlen, Switzerland) allows the dilution of the aerosol without affecting the original particle size distribution of the aerosol. The dilution unit contains two separate gas channels. Dilution was achieved by the transformation of a specified gas volume from the raw gas channel to the diluted aerosol channel via cavities in a rotating disc. The dilution factor is a function of the cavity volume, the number of the cavities in the disc (in this work the number of cavities was 10) and the frequency of disc rotations. The frequency of the disc rotations is adjusted by a potentiometer and in this work it was from 83 rpm (the lowest dilution) to 4 rpm (the highest dilution). In this study, four dilutions were used. The list of the settings and the corresponding dilution factors are in Table 1. Calculations of the dilution factors were performed according to the operating instructions for the aerosol adjustable unit type MD19-1i for 10 cavity discs.

Two different experimental setups were used. In the first setup, the laser generated aerosol was sampled into the ICP-MS without being diluted (Fig. 1, line A). At the same time a small portion of the aerosol was diluted and used for measuring the particle size distribution (PSD). The highest applicable dilution was applied for PSD measurements (dilution 5, approximately 250 times) to prevent the optical particle counting device from being saturated. In the second setup, the ablated aerosol was diluted prior to its introduction into the ICP-MS (Fig. 1, line B) using four different dilutions (dilutions 1–4; 12–30 times). It is important to note that the carrier gas and dilution gas was always helium and that the gas flow rate into the ICP-MS remained unchanged at all dilution settings and for both experimental configurations (lines A and B). Argon was used as the make-up gas, providing constant excitation conditions within the ICP. The influence of the gas combination on laser aerosols ICP was investigated. Since 1 L min⁻¹ of helium has a similar effect on the ICP as approximately 0.3 L min⁻¹ of argon,⁵⁰ addition of 0.7 L min⁻¹ of argon provides a total gas flow of 1.0–1.1 L min⁻¹. In addition to the ICP operating conditions, ablation conditions within individual experiments (repetition rate, number of shots per crater and fluence) were also kept constant.

Fig. 1 Experimental setup used for mass load related experiments. Aerosol dilution unit was implemented into the set up to introduce undiluted and diluted laser generated aerosols into the ICP-MS. The setup allows on line monitoring of the particle size distribution.

Mass load and aerosol mixing

Two 193 nm ArF excimer lasers were connected in series to perform the ablation at different spot sizes and/or their combinations. A similar approach was described by Chan et al.³⁴ for matrix effect studies in LA-ICP-OES. The use of two lasers allowed extension of the mass load of the ICP-MS. Signal intensities for different plasma mass loads were measured and evaluated. Operating the two lasers in series allowed the use of identical gas flow conditions (1 L min⁻¹, He) to those used for the previously described studies. The ablation cells used for the experiment were identical in shape and volume (approximately 30 cm³). The energy density of both lasers was adjusted to the value of 9 J cm⁻² to cover the range of typical laser ablation conditions. The experimental conditions are summarized in Table 2. The schematic drawing of the experimental set-up is shown in Fig. 2.

Fig. 2 Experimental set-up to extend the mass load capabilities. A tandem ablation system consisting of two ArF 193 nm laser ablation systems was installed to mix variable amounts of ablated aerosols and transport them into the ICP-MS. This configuration transports all aerosols using one carrier gas flow, which is necessary to maintain the excitation conditions within the ICP constant.

Table 2 Operating conditions of the tandem ablation set-up using two ArF 193 nm excimer laser ablation systems for “excessive” mass load studies

ICP-MS conditions
Carrier gas flow	1 L min^−1 He
Make up gas flow	0.7 L min^−1 Ar
Auxiliary gas flow	0.9 L min^−1 Ar
Plasma gas flow	18 L min^−1 Ar
Rf power	1420 W
ThO^+/Th^+	<1%
Autolens	On
Detector	Dual (pulse and analog counting)
Isotopes measured	^7Li, ^9Be, ^23Na, ^25Mg, ^27Al, ^29Si, ^42Ca, ^45Sc, ^49Ti, ^53Cr, ^55Mn, ^57Fe, ^59Co, ^60Ni, ^63,65Cu, ^66Zn, ^71Ga, ^75As, ^85Rb, ^88Sr, ^89Y, ^90Zr, ^107Ag, ^111Cd, ^120Sn, ^121Sb, ^133Cs, ^137Ba, ^139La, ^140Ce, ^146Nd, ^147Sm, ^151Eu, ^157Gd, ^159Tb, ^165Ho, ^169Tm, ^173Yb, ^175Lu, ^178Hf, ^181Ta, ^184W, ^208Pb, ^209Bi, ^232Th, ^238U

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Laser parameters
Wavelength	193 nm ArF (GeoLas M and C)
Energy density	9 J cm^−2
Pulse length	15 ns
Ablation duration	60 s
Ablation spot sizes	32, 40, 60, 63, 80, 95, 120 and 127 μm
Repetition rate	10 Hz

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Matrix addition

To investigate matrix effects within the plasma, an Aridus (CETAC Technologies, Omaha, NE, USA) was used (Fig. 3), as described in ref. 51. This allows the mixing of the laser generated aerosol with different concentrations of Rb (10–1000 μg g⁻¹) without changing the gas flow conditions commonly used for laser ablation. This also allowed the study of the variation of the signal intensities from laser generated aerosols at different crater sizes in the presence of a single “synthetic” matrix element within the plasma. However, to match the excitation conditions for the laser aerosols (dry) and the addition of a desolvated Rb matrix (most likely not completely dry) as closely as possible, the laser generated aerosol was simultaneously introduced into the ICP with a desolvated blank solution (3% nitric acid). The operating conditions are given in Table 3.

Fig. 3 Experimental set-up for mixing laser generated aerosols and desolvated solutions.

Table 3 Operating conditions for the addition of desolvated Rb solutions to laser generated aerosols before their introduction into the ICP-MS

ICP-MS conditions
Carrier gas flow	1 L min^−1 He
Nebulizer gas flow for Aridus	0.7 L min^−1 Ar
Auxiliary gas flow	0.8 L min^−1 Ar
Plasma gas flow	18 L min^−1 Ar
Rf power	1480 W
Aridus sweep gas flow	2.2 L min^−1 Ar
Oxide rate	<1%
Autolens	On
Detector	Dual (pulse and analog counting)
Isotopes measured	^7Li, ^9Br, ^25Mg, ^27Al, ^42Ca, ^45Sc, ^49Ti, ^52Cr, ^55Mn, ^57Fe, ^59Co, ^60Ni, ^63,65Cu, ^66Zn, ^71Ga, ^75As, ^89Y, ^90Zr, ^111Cd, ^120Sn, ^121Sb, ^133Cs, ^137Ba, ^139La, ^140Ce, ^146Nd, ^147Sm, ^151Eu, ^157Gd, ^159Tb, ^163Dy, ^165Ho, ^166Er, ^169Tm, ^173Yb, ^175Lu, ^178Hf, ^181Ta, ^208Pb, ^209Bi, ^232Th, ^238U

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Laser parameters
Wavelength	193 nm ArF
Energy density	28 J cm^−2
Pulse length	15 ns
Ablation duration	60 s
Ablation spot sizes	40 and 80 μm
Repetition rate	10 Hz
Concentration of Rb solutions	10 and 1000 μg g^−1

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Results and discussion

A first experiment aimed to reproduce the elemental fractionation index (FI) published by Fryer et al.² The similarity in the relative degree of fractionation for most of the elements is summarized in Fig. 4a and it needs to be mentioned that the same laser wavelength and similar ablation conditions, but different instrumentation, was applied. In addition, when ablating the NIST 610 glass at different crater diameters for 4 min, the critical depth to diameter⁵ becomes most pronounced for smaller crater diameters, which leads to an increase in the fractionation index for elements such as Cu, Zn, Ga, As, Ag, Cd, Sn, Sb, Pb, Bi. However, a more detailed evaluation of the fractionation index dependence on time (Fig. 4b) indicates that the FI is progressively changing from below 1 to above 1. The FIs calculated for three time intervals were selected to demonstrate the existence of different fractionation origins. According to Kuhn et al.,²¹ larger particles, which occur for this wavelength at the beginning of the ablation and are enriched in Ca,¹⁷ are not completely vaporized within the ICP. Therefore, the FI falls below 1. Similar results have been reported in ref. 13 but, as indicated in the paper, presented as 1/FI. However, the elements similar to Ca remain at FI ∼1. During the second integration period (2 min) the FI stabilizes at around 1 for almost all elements. The FI for the 4 min ablation is dominated by increasing aspect ratios, as described by Mank and Mason,⁵ and becomes most severe for longer ablation times since the particle effect (first 30 s of the ablation) becomes negligible in comparison with the total time of ablation. Owing to the fact that an ablation of 60 s using a 266 nm Nd:YAG laser is dominated by particle effects, as partially shown by Rodushkin et al.,⁵² the following results (although acquired for both wavelengths) will focus on the experiments using the 193 nm ArF excimer only.

Fig. 4 (a) Fractionation indices (FI) of NIST 610 in dependence on crater diameter. The element/Ca ratios were calculated from 4 min ablations using a 266 nm Nd:YAG laser. As a result of the similarity of the acquisition protocol, data were compared with the fractionation index reported by Fryer et al.² (b) Fractionation indices of NIST 610 were derived from a 4 min ablation using a 100 μm crater (266 nm Nd:YAG). In contrast to Fryer et al.,² the FIs were calculated for equal parts of different time periods (1, 2 and 4 min).

Constant ICP mass load

Ablation studies on NIST 610 (20 repeated measurements, 60 s ablation) using the ArF 193nm excimer laser ablation system (80 μm crater diameter used for calibration and analysis) were carried out. The concentrations were calculated using Ca as an internal standard for better comparison with the work by Fryer et al.² The concentrations of more than 40 elements analyzed within the sample differed by less than 1.6% and less than 3.1% for Be, K, and Fe from their reference concentrations.⁵³ These data indicate that the ablation and ionization process for constant mass load into the ICP is reproducible within few %. A selection of these values (Table 4, 1st row) were acquired under standard operating conditions (see Table 1) and were used as a reference for the following mass load studies.

Table 4 Determined concentrations of selected elements of NIST 610 which were acquired at different crater diameters using a 193 nm ArF excimer laser. Calibration was carried out using the intensities measured at 80 μm. Ca was used as internal standard. The intensities of the other crater diameters were treated as samples. The 3rd row (80 μm) represents the theoretical composition. Measurements performed according to the experimental set-up in Fig. 1, line A

Ablation spot		Element/unit
		Al2O3 (wt%)	TiO2 (wt%)	Co/μg g^−1	Cu/μg g^−1	Zn/μg g^−1	Cd/μg g^−1	Pb/μg g^−1	Th/μg g^−1	U/μg g^−1
a All 20 acquisitions were carried out using 80 μm (4 acquisitions for calibration and 16 acquisitions were treated as samples).
80 μm quantif. at constant mass load^a	Average	1.89	0.07	408	434	462	263	418	454	462
	SD	0.01	0.003	3	4	7	4	6	2	3
120 μm	Average	1.84	0.07	391	405	421	244	390	446	436
	SD	0.02	0.003	3	4	2	1	5	3	12
80 μm	Average	1.89	0.07	405	430	457	259	413	451	457
	SD	0.01	0.003	2	3	2	2	2	1	3
60 μm	Average	1.93	0.07	417	450	483	267	434	455	470
	SD	0.01	0.003	2	3	6	3	4	2	3
30 μm	Average	1.93	0.07	427	471	543	294	456	457	480
	SD	0.02	0.003	1	2	7	7	3	3	2

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Variable mass load

To investigate mass load effects in LA-ICP-MS, aerosols generated using various ablation crater diameters (30, 60, 80 and 120 μm) were introduced into the ICP. The analysis sequences were always started with 2 ablations of NIST 610 (80 μm) followed by the measurements at different crater diameters and finished by 2 ablations of NIST 610 (80 μm). The 80 μm crater signals were used for calibration and the others were treated as samples.

The concentrations determined for the various crater diameters are summarized in Table 4. Each data point represents the average of 4 replicate measurements. The average concentrations for Al, Ti and Th deviate in the range of −3 (high mass load) to +2% (low mass load) from their reference values, which was less accurate than for the constant mass load. The precision for each mass load remained unchanged in comparison with the constant mass load (approximately 1%). However, the concentrations determined for volatile elements varied significantly from their reference values, e.g., Cu (−6 to +9%), Zn (−8 to +16%), Cd (−6 to +12%), Pb (−5 to +10%) and U (−5 to +5%). The given ranges represent the deviation from the reference value (80 μm) determined for high mass load (120 μm) and low mass load (30 μm). Based on the relative standard deviation (RSD) of the 4 replicates per crater diameter (<2.7% for Cd, U and <1.3% Cu, Zn, Pb) the deviations between the measured concentrations and the reference values are significant. Considering a penetration rate per pulse (0.25 μm per pulse) and the number of pulses within 60 s (10 Hz), the depth to diameter ratio changed from <5 (30 μm) to 1.25 (120 μm). Therefore, the mass load dependent changes in the Cu, Zn, Cd, Pb and U concentrations cannot be attributed to crater depth to diameter induced elemental fractionation effects, as described by Mank and Mason.⁵ Further evidence of minor crater depth to diameter effects is given in Fig. 5, where fractionation indices were calculated after Fryer et al.,² using two equal time intervals of 30 s. It can be seen that the elements Cu, Zn, Cd and Pb show a temporal change in their intensity/Ca ratios. However, the temporal changes of elemental ratios for each crater are significantly less than the deviations from the reference concentration determined for different crater diameters. To demonstrate the influence of temporal changes on quantification, concentrations were determined for both time intervals separately. Their relative deviations from the reference values were lower than the mass load induced variation in concentration (Cu (−1 to +5%), Zn (−3 to +3%), Cd (−4 to +1%), Pb (−3 to +7%) and U (−1 to +0.5%)). Amongst these elements, the Pb concentration determined for the smallest crater diameter represents the only exception, where mass load and temporal induced effects were not distinguishable. Another explanation for the mass load dependent changes in element ratios could be a crater diameter dependent particle size distribution. It has been reported that the elemental composition of various particles sizes is heterogeneous.¹⁷ In the case of incomplete vaporization, such particle effects could induce the changes observed within this study. Therefore, the particle size distribution measurements for the smallest and the largest craters are shown in Fig. 6. Crater diameter dependent changes in the particle size distribution were not observed for the ablation at 193 nm. The relative change of mass transported using a 120 μm and 30 μm crater was calculated from the particle size measurements and follows almost exactly the relative difference in the ablated mass (factor 15.3 instead of 16). In addition, the background corrected signal intensities for Ca measured in the ICP also follow the mass transported to the ICP (factor 16.1). Therefore, differences in the concentrations of Cu, Zn, Cd, Pb and U measured using different crater diameters are not fully explainable by the time dependent changes in the element ratios, particle size distribution effects or crater diameter related changes in transport efficiency. However, the presence of laser-induced fractionation, where the heat affected zone around the ablation crater increases the mobilization of more volatile elements, cannot be judged from the results given above.

Fig. 5 Fractionation indices for a selection of elements calculated using the definition described in Fryer et al.² Data were generated on NIST 610 using a 1 min ablation (193 nm ArF).

Fig. 6 Particle size distributions measured for NIST 610. Data represent particle sizes for minimum and maximum crater diameters, which were generated using the 193 nm ArF excimer laser.

Aerosol dilution

To further elucidate the previous observations, the same mass load experiments (4 crater diameters) were carried out under identical ablation conditions. In contrast, aerosol dilution, as described in (Fig. 1, line B), was applied to change the mass entering the ICP for each individual crater diameter. It needs to be mentioned that the dilution process is ablation process independent. However, all aerosols generated (30 μm to 120 μm) were diluted using 4 dilution settings between 12 and 30 times before the aerosols entered the ICP. The calculated intensity ratios were averaged and compared with intensity ratios measured without dilution. It is important to note that the undiluted and diluted elemental ratios were acquired within the same run and that the ICP operating conditions remained constant.

Fig. 7 shows a selection of element/Ca intensity ratios of undiluted and diluted aerosols in dependence on their relative mass load change normalized to the undiluted mass ablated at 80 μm (100%). For example, the Cd/Ca ratios determined after aerosol dilutions were more constant as a function of different crater diameters than for the ratios measured for the undiluted aerosols. Interestingly, the Cd/Ca ratios measured in the dilution mode were most similar to the ratio measured for the smallest crater size without dilution (30 μm). Similar effects were observed for Cu, Zn and Pb. The Cu/Ca ratios indicate that only the dilution of the mass generated using the 30 μm and 60 μm crater leads to a mass load in the ICP where the ratios become constant. The U/Ca ratios show the same trend at a lower order of magnitude. Excluding the 30 μm crater (already discussed in the FI index), the Pb/Ca ratio also becomes stable. All other element ratios (Al, Th) which showed no or minimum mass load dependent changes in concentrations (Table 4) remained constant with and without dilution. In some cases, the relative changes in the ratios between undiluted and diluted aerosols represent a combination of laser induced fractionation and ICP-induced matrix effect. However, the Cd/Ca, Zn/Ca, Cu/Ca and U/Ca ratios provide evidence that the ablation process (under the given operating conditions) cannot be fully responsible for the observed changes in the element ratios and that the increase of the ratios towards lower mass load is most likely an ICP-induced matrix effect. It could be argued that the dilution leads to a change in the particle size distribution as indicated in ref. 24. In the case of a change in the particles size distribution due to dilution it is unlikely that the U/Th ratio would remain constant between diluted and undiluted aerosols (see Fig. 7).

Fig. 7 Element/Ca intensity ratios measured on undiluted and diluted aerosols. Ratios were normalized to the measurements using an 80 μm crater diameter without aerosol dilution (●, without dilution; ○, with dilution).

Mass load and aerosol mixing

To validate the existence of an ICP-induced matrix effect, the following experiments were focused on “excessive” mass load. This was achieved by using two 193 nm laser systems (experimental setup, see Fig. 2). This approach allows one to change the total mass load transported into the ICP by mixing aerosols generated with different crater diameters (e.g., in this study, 40, 40 + 32, 60, 80, 60 + 63, 40 + 95, 120, 127, 80 + 95, 60 + 127 μm) without changing the operating conditions of the ICP. The single craters in-between the double crater ablation experiments were added to the sequence to make this experiment directly comparable to the undiluted data given in Fig. 7. To provide more information about the mass load dependent behaviour of the elements in NIST 610, the element/Ca ratios as a function of the total mass introduced into the plasma are given for 46 elements (Fig. 8a). For better illustration of the influence of the mass load on the intensity ratios, maximum and minimum deviations (40 and 60 + 127 μm normalized to 80 μm) are shown. The greater the mass ablated and introduced into the plasma, the lower is the element/Ca ratio for elements such as Cu, Zn, Ag, Cd, Sn, Sb, Pb, and Bi, whereas elements which showed already constant ratios during aerosol dilution (see Fig. 7) were also not influenced by the aerosols mixing studies. Fractionation indices calculated for these experiments are shown in Fig. 8b. The FIs for 45 of 46 elements (except Sn) indicate that the changes in element/Ca ratios summarized in Fig. 8a were not caused by time-dependent fractionation. The slopes of different element ratios (Fig. 9) represent the mass load dependent differences in ionization within the ICP or ion sampling into the mass spectrometer. Evaluating various physical properties of individual elements it becomes clear that no single parameter dependence (e.g., 1st and 2nd ionization potential, condensation temperature) exists. In such a case, Be should be as much influenced as Cd (high 1st IP), whereas Ag should not be influenced. Furthermore, the Rb and Cs should not show a matrix effect. Also, no m/z dependence as indicator for space charge effects exists. However, the correlation of fractionating elements to the Goldschmidt classification has been demonstrated on time-dependent element/Ca ratios (FI).⁸ This correlation is very similar for mass load induced matrix effects within the ICP. Since these elements (chalcophile elements) show high affinity for sulfur (which is less than a few μg g⁻¹ within the NIST 610) it might be possible that they are transported in their elemental form into the plasma. This could have some consequences on their vaporization and ionization behaviour within the ICP. Unfortunately, the species formed during ablation and entering the plasma are currently unknown. However, assuming an elemental form of the mass load affected elements (Zn, Ag, Cd, Tl, Pb) when entering the ICP, their melting points are lower than those of CaO, SrO, REE-oxides and others. Melting of the “volatile” elements takes place immediately at the entrance point of the plasma, whereas oxides travel deeper into the plasma before melting, which affects the ion sampling through the interface. Therefore, this assumption could support a different type of “zone” model, as proposed in ref. 54, which is melting point dependent rather than m/z dependent on gas flows. The existence of differences in a zone model between solution nebulization and laser aerosol introduction into the ICP has already been demonstrated by Rodushkin et al.⁵²

Fig. 8 a, Changes of the element/Ca ratios in dependence on the mass load of the ICP-MS acquired using tandem laser ablation set-up. Ratios were normalized to values measured using an 80 μm crater. b, Fractionation indices (representing the temporal changes of element/Ca ratios during ablation) in dependence on the mass load of the ICP-MS. Data were normalized to values measured using an 80 μm crater.

Fig. 9 Element/Ca intensity ratios measured for various mass loads using the tandem 193 nm laser ablation set up. The figure contains a representative selection of elements for the entire mass range studied.

Matrix addition

In addition to the various laser ablation experiments, a single element solution containing different concentrations of Rb was nebulized, desolvated, mixed to laser aerosols (40 μm and 80 μm crater diameters) and introduced into the ICP. Since the desolvation process using an Aridus is not complete, a blank solution was added to the desolvation unit (3% nitric acid) during the ablation of the 40 and 80 μm craters (without Rb addition). This allows the maintainance of constant excitation conditions within the ICP. Therefore, these data were used as a reference for all further Rb addition experiments. Fig. 10a shows a signal depletion of the laser aerosol as a function of the concentration of Rb. A 10 μg g⁻¹ addition of desolvated Rb to a 40 μm crater leads to a matrix suppression, where all element intensities were depleted by approximately 15% (approximately constant for all m/z). The explanation for the rather constant depletion can be found in ref. 55, where it has been proposed to add water to laser induced aerosols in order to reduce matrix effects.

Fig. 10 a, ICP-MS background corrected intensities of NIST 610 aerosol generated using a 40 μm crater diameter in presence of a desolvated 3% HNO₃, blank, and in the presence of 10 and 1000 μg g⁻¹ desolvated Rb matrices. Data of the Rb addition were normalized to 40 μm crater diameter in presence of a desolvated 3% HNO₃. b, ICP-MS background corrected intensities of NIST 610 aerosol generated using an 80 μm crater diameter in the presence of a desolvated 3% HNO₃, blank, and in the presence of a 10 and 1000 μg g⁻¹ desolvated Rb matrix. Data of the Rb addition were normalized to 80 μm crater diameter in presence of a desolvated 3% HNO₃.

However, the addition of 1000 μg g⁻¹ of Rb leads also to a constant matrix suppression (50% depletion for most of the elements) and, furthermore, to an element-dependent matrix effect (approximately additional 25–30% for Zn and Cd). Considering the results using the 80 μm crater ablation (Fig. 10b), it can be concluded that the matrix effect depends only on relative mass difference within the ICP. Therefore, the mass of 10 μg g⁻¹ of Rb induced a matrix effect on the aerosol mass generated from a 40 μm and not on the mass from an 80 μm crater. In contrast, 1000 μg g⁻¹ Rb induced a significant matrix effect for both crater diameters, at the same relative difference, however, as was induced by 10 μg g⁻¹. Here it is important to note that the generation of this matrix effect is ablation independent (compare FI for 80 μm, Fig. 8b).

Surprising results were achieved when dividing the element intensities normalized to Ca measured for the 40 and 80 μm ablation (low mass load) by the same intensity ratios measured for the Rb mixing (1000 μg g⁻¹) to the laser aerosol (high mass load), which is a similar approach as was used for calculating the fractionation index by Fryer et al.² (see Fig. 11). The similarity between the two independent results clearly indicates that the mass load dependent ICP-induced matrix effects exist. It also allows one to question the laser-induced fractionation studies where high ablation rates and fast crater deepening causes a significant mass load change within the plasma. Since these effects are most pronounced for small crater diameters, FI calculations might represent, in particular for these craters, a mass load effect and laser-induced fractionation.

Fig. 11 Comparison of the “Fractionation Index” data published by Fryer et al.² generated using a 4 min ablation at 266 nm and the element/Ca changes induced on a 1 min ablation using a 40 μm ablation crater (193 nm) in absence (low mass load) and presence (high mass load) of 1000 μg g⁻¹ desolvated Rb.

266 nm Nd:YAG results

Owing to the amount of data presented in this manuscript, the details of the 266 nm Nd:YAG results cannot be given in detail. However, identical experiments and data evaluation as performed in section ‘variable mass load and aerosol mixing’ using a 193 nm laser ablation were carried out using the 266 nm laser (Table 1 and Fig. 1). As indicated, particles size related effects, i.e., incomplete ionization of the aerosol during first 20 s of the 266 nm ablations, superimposes the mass load related matrix effects in the ICP. However, if only the last 30 s of the transient signals are used, where the particle size effect becomes significantly reduced, results are in surprisingly good agreement with those obtained at 193 nm LA-ICP-MS (without and with aerosol dilution).⁵⁶

Conclusions

Previous studies have demonstrated that various processes are contributing to elemental fractionation in LA-ICP-MS. However, Fryer’s fractionation index,² which has been extensively used in the literature, is probably not an ideal measure of elemental fractionation. The index would only be useful if the elemental ratio changes in one direction, which was demonstrated not to be the case. A detailed time dependent calculation of the index, however, provides the possibility of determining the presence of particle size effects within the ICP (FI < 1).

The various experimental set-ups used within this study demonstrate the presence and magnitude of mass load induced matrix effects within the plasma. It was shown that these effects were most severe for elements with low melting points. Enhancing the matrix effect in the ICP due to Rb addition supports the view that the majority of element/Ca ratio depression observed for high mass loads is not dominated by the ablation process. Furthermore, it was shown that the matrix effects become less severe using aerosol dilution.

However, it needs to be emphasized that the results of this study were obtained using one matrix and the order of magnitude of such effects may differ from matrix to matrix. In addition, the results do not explain all details of the processes causing such matrix effects within the plasma. Because of the fact that we kept the most important parameter constant (necessary to study these effects), discussion of temperature changes and diffusion processes are very speculative. However, it is likely that vaporization and ion generation of different elements (elemental form and oxides) takes place in different zones of the plasma (even for a narrow and small particle size distributions). As a consequence, the ion sampling efficiency could then also be different for various mass loads. This is most critical in LA-ICP-MS, due to the fact that this technique relies commonly on one internal standard. Therefore, internal standardization using a single internal standard is not suitable for correcting mass load dependent matrix effects and element dependent ion sampling efficiency into the mass spectrometer, especially when ions are generated within different zones of the plasma. Therefore, accurate quantitative analyses should only be carried out using the same crater diameters and ablation times for standard and sample. Furthermore, a similar absorptivity of sample and standard will not only provide a similar particle size distribution, but also a similar mass load of the plasma.

In further investigations we will focus on the radial distribution of elements within the ICP in dependence on mass load, which could provide evidence for the ion sampling efficiency. In addition, the application of raster/scanning ablation mode, which provides a constant mass load over time, also needs to be re-investigated.

Acknowledgements

The authors would like to thank ETH for financial support of the project (TH-14/01-4). Furthermore, we greatly appreciate critical and supportive comments from H. Longerich, J. Kosler, and S. Houk. We also like to thank two anonymous reviewers for their constructive comments, which certainly helped to improve this manuscript.

References

1. A. L. Gray, Analyst, 1985, 110(5), 551–556.
2. B. J. Fryer, S. E. Jackson and H. P. Longerich, Can. Mineral., 1995, 33, 303–312.
3. T. Hirata and R. W. Nesbitt, Geochim. Cosmochim. Acta, 1995, 59(12), 2491–2500.
4. T. Hirata, J. Anal. At. Spectrom., 1997, 12(11), 1337–1342.
5. A. J. G. Mank and P. R. D. Mason, J. Anal. At. Spectrom., 1999, 14(8), 1143–1153.
6. H. P. Longerich, D. Günther and S. E. Jackson, Fresenius’ J. Anal. Chem., 1996, 355(5–6), 538–542.
7. Z. X. Chen, J. Anal. At. Spectrom., 1999, 14(12), 1823–1828.
8. S. E. Jackson, Laser-Ablation-ICPMS in the Earth Sciences, Short Course Series, St. John’s, Newfoundland, Canada, ed. Paul Sylvester, Mineral Association of Canada, Ontario, Canada, 2001, vol. 29, pp. 29–46.
9. T. E. Jeffries, W. T. Perkins and N. J. G. Pearce, Analyst, 1995, 120(5), 1365–1371.
10. T. E. Jeffries, S. E. Jackson and H. P. Longerich, J. Anal. At. Spectrom., 1998, 13(9), 935–940.
11. D. Günther and C. A. Heinrich, J. Anal. At. Spectrom., 1999, 14(9), 1369–1374.
12. J. Gonzalez, X. L. Mao, J. Roy, S. S. Mao and R. E. Russo, J. Anal. At. Spectrom., 2002, 17(9), 1108–1113.
13. M. Guillong, I. Horn and D. Günther, J. Anal. At. Spectrom., 2003, 18(10), 1224–1230.
14. I. Horn, D. Günther and M. Guillong, Spectrochim. Acta, Part B, 2003, 58(10), 1837–1846.
15. P. Telouk, E. F. Rose-Koga and F. Albarede, Geostand. Newsl., 2003, 27(1), 5–11.
16. M. Guillong, H. R. Kuhn and D. Günther, Spectrochim. Acta, Part B, 2003, 58(2–3), 211–220.
17. H. R. Kuhn and D. Günther, J. Anal. At. Spectrom., 2004, 19(9), 1158–1164.
18. M. Motelica-Heino, P. Le Coustumer and O. F. X. Donard, J. Anal. At. Spectrom., 2001, 16(6), 542–550.
19. J. Kosler, M. Wiedenbeck, R. Wirth, J. Hovorka, P. Sylvester and J. Mikova, J. Anal. At. Spectrom., 2005, 20(5), 402–409.
20. D. Bleiner and P. Gasser, Appl. Phys. A: Mater. Sci. Process., 2004, 79(4-6), 1019–1022.
21. H. R. Kuhn, M. Guillong and D. Günther, Anal. Bioanal. Chem., 2004, 378(4), 1069–1074.
22. D. B. Aeschliman, S. J. Bajic, D. P. Baldwin and R. S. Houk, J. Anal. At. Spectrom., 2003, 18(9), 1008–1014.
23. Z. S. Yu, M. D. Norman and P. Robinson, Geostand. Newsl., 2003, 27(1), 67–89.
24. M. Guillong and D. Günther, J. Anal. At. Spectrom., 2002, 17(8), 831–837.
25. O. Bruguier, P. Telouk, A. Cocherie, A. M. Fouillac and F. Albarede, Geostand. Newsl., 2001, 25(2–3), 361–373.
26. S. E. Jackson, N. J. Pearson, W. L. Griffin and E. A. Belousova, Chem. Geol., 2004, 211(1–2), 47–69.
27. H. R. Kuhn and D. Günther, Anal. Bioanal. Chem., 2005, 383(3), 434–441.
28. S. M. Eggins, R. L. Rudnick and W. F. McDonough, Earth Planet. Sci. Lett., 1998, 154(1–4), 53–71.
29. C. Pickhardt, J. S. Becker and H. J. Dietze, Fresenius’ J. Anal. Chem., 2000, 368(2–3), 173–181.
30. D. Günther, A. von Quadt, R. Wirz, H. Cousin and V. J. Dietrich, Microchim. Acta, 2001, 136(3–4), 101–107.
31. D. B. Aeschliman, S. J. Bajic, D. P. Baldwin and R. S. Houk, J. Anal. At. Spectrom., 2003, 18(8), 872–877.
32. K. P. Jochum, M. Willbold, I. Raczek, B. Stoll and K. Herwig, Geostand. Geoanal. Res., 2005, 29(3), 285–302.
33. S. E. Jackson, H. P. Longerich, G. R. Dunning and B. J. Fryer, Can. Mineral., 1992, 30(4), 1049–1064.
34. G. C. Y. Chan, W. T. Chan, X. L. Mao and R. E. Russo, Spectrochim. Acta, Part B, 2000, 55(3), 221–235.
35. G. C. Y. Chan, W. T. Chan, X. L. Mao and R. E. Russo, Spectrochim. Acta, Part B, 2001, 56(1), 77–92.
36. T. Morishita, Y. Ishida and S. Arai, Geochem. J., 2005, 39(4), 327–340.
37. J. A. Olivares and R. S. Houk, Anal. Chem., 1986, 58(1), 20–25.
38. D. Beauchemin, J. W. McLaren and S. S. Berman, Spectrochim. Acta, Part B, 1987, 42(3), 467–490.
39. S. H. Tan and G. Horlick, J. Anal. At. Spectrom., 1987, 2(8), 745–763.
40. E. H. Evans and J. J. Giglio, J. Anal. At. Spectrom., 1993, 8(1), 1–18.
41. C. Vandecasteele, M. Nagels, H. Vanhoe and R. Dams, Anal. Chim. Acta, 1988, 211(1–2), 91–98.
42. F. Vanhaecke, H. Vanhoe, R. Dams and C. Vandecasteele, Talanta, 1992, 39(7), 737–742.
43. J. Riondato, F. Vanhaecke, L. Moens and R. Dams, J. Anal. At. Spectrom., 2000, 15(4), 341–345.
44. K. G. Heumann, Anal. Bioanal. Chem., 2004, 378(2), 318–329.
45. D. C. Gregoire, Appl. Spectrosc., 1987, 41(5), 897–903.
46. D. C. Gregoire, Spectrochim. Acta, Part B, 1987, 42(7), 895–907.
47. D. Günther, R. Frischknecht, C. A. Heinrich and H. J. Kahlert, J. Anal. At. Spectrom., 1997, 12(9), 939–944.
48. Z. Wang, B. Hattendorf and D. Günther, ICP Inf. Newsl., 2004, 30, 1042–1046.
49. H. P. Longerich, S. E. Jackson and D. Günther, J. Anal. At. Spectrom., 1996, 11(9), 899–904.
50. Z. Wang, B. Hattendorf and D. Günther, J. Am. Soc. Mass Spectrom., 2006, 17(5), 641–651.
51. L. Halicz and D. Günther, J. Anal. At. Spectrom., 2004, 19(12), 1539–1545.
52. I. Rodushkin, M. D. Axelsson, D. Malinovsky and D. C. Baxter, J. Anal. At. Spectrom., 2002, 17(10), 1223–1230.
53. N. J. G. Pearce, W. T. Perkins, J. A. Westgate, M. P. Gorton, S. E. Jackson, C. R. Neal and S. P. Chenery, Geostand. Newsl., 1997, 21(1), 115–144.
54. F. Vanhaecke, R. Dams and C. Vandecasteele, J. Anal. At. Spectrom., 1993, 8(3), 433–438.
55. C. J. O’Connor, B. L. Sharp and P. Evans, J. Anal. At. Spectrom., 2006, 21(6), 556–565.
56. I. Kroslakova, Plasma related matrix effects in LA-ICP-MS, Dissertation ETH No. 16751.

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