Precise Determination of Trace Elements in Geological Samples by ICP-MS Using Compromise Conditions and Fine Matrix- Matching Strategy

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1 ANALYTICAL SCIENCES DECEMBER 2000, VOL The Japan Society for Analytical Chemistry 1291 Precise Determination of Trace Elements in Geological Samples by ICP-MS Using Compromise Conditions and Fine Matrix- Matching Strategy Soulin LIN,* Man HE,* Shenghong HU,** Honglin YUAN,** and Shan GAO** *Faculty of Material Science and Chemical Engineering, China University of Geosciences, Wuhan , P. R. China **Faculty of Earth Science, China University of Geosciences, Wuhan , P. R. China A simple method for multi-element analysis of geological samples has been developed. In this work, compromise conditions involving optimizing the operating parameters of the ICP, adjustment of ion optics voltages and selection of internal standards were carefully investigated. Matrix trace element-matching procedures were employed, in which the external calibration solutions were prepared by dividing the elements of interest into seven groups of different concentration levels according to their average contents of 20 typical standard reference rock samples. The proposed method offered simple procedure of sample preparation, instrument calibration, improved precision and reasonable element coverage. Detection limits ranged from µg g 1 for heavier mass elements to µg g 1 for lighter mass elements. Precision was near % RSD (lighter mass elements) and % RSD (heavier mass elements), respectively, with the exceptions of tungsten, molybdenum and thorium. Good agreement between certified and found values were achieved for standard reference rock samples AGV-1, BHVO-2 and GSR-3. Analytical data for rare earth elements were in conformity with values from the chondrite normalized curve. (Received November 26, 1999; Accepted August 24, 2000) Inductively coupled plasma mass spectrometry (ICP-MS) has rapidly become an established technique for multi-element analysis of geological materials 1 5 since the midst of the 1980 s, offering rapid analysis capability, an ability to measure most elements in the periodic table, low detection limits, a large dynamic range and relative simple sample preparation. However, it is increasingly apparent that ICP-MS did suffer from mass spectral interferences such as isobaric, polyatomic ions and doubly charged ions, 6,7 and from the influence of matrix effects due to different types of samples, as well as the variation in mass-dependent instrument sensitivity during the course of an analytical run. 5,8,9 Therefore, fully quantitative measurement by ICP-MS can only be achieved provided that all these effects are minimized, and are monitored or corrected for. Measures which must be taken include selection of compromise experimental conditions, optimization of the ion optics voltages, spiking of samples with standard internal isotopes and employing proper matrix-matching procedures. Optimizing the performance of an ICP-MS system is the most important prerequisite for accurate and precise determination of trace elements. But the system is rarely optimized for any single element because this will adversely affect the measurement of other analytes in a multi-element procedure. 10 It is more important to establish compromise conditions which allow adequate determination of a wide range of elements, with more stress on improving the sensitivities of trace elements that are close to detection limits than on higher abundant elements which may be readily determined under non-optimum To whom correspondence should be addressed. soulin@cug.edu.cn conditions. Non-spectroscopic matrix effects generally refer to those related to the behavior of solutions during sample introduction. Differences in acid concentration and total dissolved solids (TDS) significantly affect sensitivities and precision. 11 Some matrix effects may occur during excitation. In ICP-MS analysis, a large excess ( 1000 µg ml 1 ) of one single element can cause suppression or enhancement of analyte signals; these effects are most extreme with heavy mass elements such as uranium and may be also with light mass elements such as sodium. 12 Fortunately, such matrix effects are not usually a problem in geological samples since any one single element does not generally occur at a high concentration in these samples. In general, non-spectroscopic matrix effects in ICP- MS analysis are best overcome by using more diluted solutions. In an attempt to improve the accuracy and precision for multielement analysis, we have developed a simple method employing compromise conditions along with a fine matrixmatching procedure. Herein the matrix effects are referred to as the influence of co-existing elements on the analyte element. Analytical results demonstrated that the method we have developed is straightforward, simple and rapid, because it does not require diverse corrections compared to alternative analytical strategies. Experimental Instrumentation The instrument used was a POEMS III ICP-OES/MS which is a combination of a PQ3 mass spectrometer (VG Instrumental,

2 1292 ANALYTICAL SCIENCES DECEMBER 2000, VOL. 16 Table 1 Operating parameters of instrument Argon flow: Torch 14.0 L min 1 Nebulization 0.88 L min 1 Auxiliary 1.0 L min 1 Sampling depth 100 step Nebulizer meinhard, glass, concentric Spray chamber cylindric, with spoiler RF power 1350 W Scan number 10 Scan points, amu 16 Integration time 2 s Scan points 5 Examination points 5 Table 2 solution Composition of fine matrix-matched external calibration Element Concentration/µg g 1 Fig. 1 Selecting the compromise operating parameters of the ICP. a,b: RF power 1350 W, sampling depth 100 steps, auxiliary gas flow 1.0 L min 1. c,d: RF power 1350 W, nebulizer gas flow 0.88 L min 1, auxiliary gas flow 1.0 L min 1. e,f: RF power 1350 W, nebulizer gas flow 0.88 L min 1, sampling depth 100 steps., sensitivity;, precision (%RSD);, doubly charged ion;, oxides. UK) and an Iris optical emission spectrometer (Thermal Jarrell Ash Corporation, USA), with an ICP source common to both of them. (MASS only) mode was used in the present work. Operating conditions for the experiments are shown in Table 1. Instrument optimization It is most important to optimize the instrument prior to determination by carefully adjusting the ion optics voltages of the mass spectrometer to obtain a smooth response curve for the whole mass range. A group of isotopes which cover the whole mass range, for example, Be[9], Co[59], In[115], Tb[159] and Bi[209], were used for performing this adjustment. To lay stress on either the lighter mass or heavier mass isotopes is not preferred. Optimizing the operating parameters of the ICP in ICP-MS analysis was described in the literature elsewhere. 13 In this work, the operating parameters of the ICP source were selected by varying one parameter while keeping the others fixed. A standard solution containing 5 µg g 1 of Be[9], Co[59], In[115], Tb[159] and Bi[209], were used for this purpose. In[115] was used as the internal standard element for compensating any possible variation of analyte signals. The precision (RSD) of determination was calculated based on 10 measurements of the standard solution. Figure 1 shows the scheme of selecting the compromise conditions in respect of nebulizer gas flow, Ba, Sr 1000 Cr, Rb, Zn, Zr 500 Li, Ni, Cu, V 100 B, Ca, Ga, Nb, Pb, Th, Sc 40 Be, Cs, Ge, Hf, Mo, Sn, Ta 20 U, W, Cd, Sm, Y La, Nd 80 Ce 160 Pr 32 Eu, Tb, Er, Yb 4 Ho, Tm 2 Gd, Dy 8 Lu 0.8 sampling depth, auxiliary gas flow, with RF power fixed at 1350 W. It can be seen from Figs. 1a and 1e that the analytical precision is greatly affected by both the nebulizer and auxiliary gas flow, while sensitivities, yields of oxides and doubly charged ions are affected to much less extent. On the other hand, the yields of oxides and doubly charged ions are greatly affected by the sampling depth, which has only minor effects on sensitivities and precision. Therefore, all analytical performance, including sensitivities, precision, and yields of oxides and polyatomic ions must be taken into account in selecting the compromise conditions, depending on which performance is most emphasized. In the present work, compromise conditions of 0.88 L min 1 nebulizer gas, 1 L min 1 auxiliary gas, 100 steps in sampling depth and an rf power of 1350 W were the best choices. Fine matrix-matching strategy As analytical solutions for rock and mineral analyses were usually highly diluted, the inherent non-spectroscopic matrix effects on the behavior of sample introduction were considered not to be a serious problem. It is most probable that the matrix effects can be minimized by using more diluted solutions. We mainly focused on the inter-element interference effects. The main point of the present fine matrix-matching strategy is to take into account the interference of inter-element oxides such as oxides of light rare earth elements in heavy rare earth elements and that of oxides of the seven isotopes of barium in rare earth elements. The calibration standard solutions for matrix matching were prepared by adopting the statistic average contents of elements of interest in 20 typical standard reference

3 ANALYTICAL SCIENCES DECEMBER 2000, VOL Table 3 Comparison of analytical data (effects of internal standards) AGV-1 Element Certified/ µg g 1 Internal standard 115 In 103 Rh 115 In Rh Found/µg g 1 Ca/Cs, % Found/µg g 1 Ca/Cs, % Found/µg g 1 Ca/Cs, % Be[9] Sc[45] V[51] Cr[53] Co[59] Ni[60] Cu[65] Zn[66] Ga[71] Rb[85] Sr[88] Y[89] Zr[91] Nb[93] Sn[118] Cs[133] Ba[135] La[139] Ce[140] Pr[141] Nd[146] Sm[147] Eu[151] Gd[157] Tb[159] Dy[161] Ho[165] Er[166] Tm[169] Yb[172] Lu[175] Hf[178] Ta[181] Pb[208] Th[232] U[238] Ca, found value. Cs, certified value. rock samples, including basalt, granite, granodiorite, sediment, marine mud and andesite. These average concentration values so calculated rather well simulated the relative element contents, particularly for rare earth elements in real rock samples, and provided more flexible calibration with solution standards than with rock standards. The relative concentration levels of elements so estimated in the resulting standard calibration solutions are listed in Table 2. Aliquots of each single-element stock solutions (preferably HNO 3 medium) were mixed and diluted with 2% HNO 3 to prepare a series of working solutions. All reagents were prepared using distilled acids and ultrapure water (>18 MΩ). Sample preparation Weigh out 50 mg of the rock sample into a Savillex screw-top Teflon beaker, moisten with a few drops of ultra-pure water, then add 1 ml of HNO 3 and 3 ml of HF. Tighten the screw-top and place in an ultrasonic bath for 1 h. The beaker is then put on a hot plate and heated at 140 C, followed by evaporating to incipient dryness. During the digestion, the beaker is placed in the ultrasonic bath several times in order to disaggregate granular materials and make it more susceptible to acid attack. Repeat the digestion procedure at 120 C to ensure complete dissolution, evaporate to incipient dryness again. Add 3 5 ml of 5% HNO 3, tighten the screw cap and allow to stand at 105 C to dissolve the salt. Finally, the resulting solution is transferred to a polyethylene bottle and diluted to a known weight with 2% HNO 3 to a sample/solution weight ratio of 1:2000 (50 mg sample in 100 g of final solution). This dilution factor is usually a compromise between required detection limits and tolerable total dissolved solid contents (<0.2%). Internal standard Spiking of samples with identical concentrations of an internal standard element has been unexceptionally applied in ICP- MS 5,14 16 in order to monitor and compensate variation in massdepending instrument sensitivity, excitation of the analyte species and other matrix effects. In this work, we have used indium and rhodium as internal standard elements because these two elements tend to occur at very low levels in geological samples and have masses which fall in the midst of the whole mass range, with the excitation energy 17 of kj mol 1 for In (similar to those of heavy mass elements) and 720 kj mol 1 for Rh (similar to those of light mass elements). This makes them rather suitable for the whole mass range of elements, though not

4 1294 ANALYTICAL SCIENCES DECEMBER 2000, VOL. 16 Table 4 Detection limits and limits of quantitation (acid digestion) Element Detection limit/µg g 1 Limit of quantitation/µg g 1 Be[9] Sc[45] V[51] Cr[53] Co[59] Ni[60] Cu[65] Zn[66] Ga[71] Rb[85] Sr[88] Y[89] Zr[91] Nb[93] Mo[97] Sn[118] Cs[133] Ba[135] La[139] Ce[140] Pr[141] Nd[146] Sm[147] Eu[151] Gd[157] Tb[159] Dy[161] Ho[165] Er[166] Tm[169] Yb[172] Lu[175] Hf[178] Ta[181] W[182] Pb[208] Th[232] U[238] Fig. 2 Analytical precision values of elements based on six replicate analyses. Fig. 3 Relative errors (R. E%) for three SRMs. Fig. 4 Chondrite normalized curve of REEs. Solid symbol and solid line, found data; open symbol and dotted line, certified value. perfectly. Table 3 shows the effects of different internal standards and their combination on the analytical data for rock sample AGV-1. It can be observed that the values of C a/c s (found value/certified value) ratio were obviously less than 100 for heavy rare earth elements, Pb, Th and U when using only In as the internal standard element. This can not be explained by similar values of excitation energy between In and these elements, it can be attributed to the variation of mass-dependent sensitivity which is often a complex function of mass. In contrast, the recoveries of heavy mass elements are better when using only Rh as internal standard element, though the recoveries of light mass elements were a little bit worse. It was shown that best results were obtained by using a mixture of indium and rhodium, in which indium was used for improving the precision and accuracy of analyte elements with masses <Tb[159] and rhodium for elements with masses >Dy[161], although, for elements with masses >Cs[133] and <Tb[159], both single In and single Rh worked well. Notable improvements in overall precision and accuracy were achieved by using mixed internal standards. Results and Discussion Detection limits, accuracy and precision Practically, detection limits of ICP-MS are low, typically <1 ng g 1 solution (1 µg g 1 sample equivalent) for the lightest elements and better than 0.05 ng g 1 solution for heavy elements, REEs, Th and U. The content of TDS is generally limited to 2000 µg ml 1. Table 4 shows the detection limits (3σ) and limits of quantitation (10σ) of ~40 elements for real samples by running the analytical procedure. The measured concentrations of the trace elements of interest in our laboratory along with certified values of three international standard reference materials and inter-laboratory values are listed in Table 5. Also reported are relative standard deviation (RSD) values based on six replicate analyses in different time periods from October, 1998 to July, The RSD values for most elements are between 1 8% with the exceptions of Mo for BHVO-2, Pb for BHVO-2, Th for BHVO- 2, and W for AGV-1, BHVO-2 and GSD-3. The large deviation may be attributed to the volatility during the digestion (Pb), lack of enough wash time from determination to determination (Mo and W) and counting failure near detection limits (Th and U), as shown in Fig.2.

5 ANALYTICAL SCIENCES DECEMBER 2000, VOL Table 5 Certified values, inter-laboratory values, measured element concentration values and analytical precision for three international standard reference materials (µg/g) Element AGV-1 BHVO-2 GSR-3 Certi. a Eggins b Found c RSD, % Certi. d Found RSD, % Certi. a Found RSD, % Be[9] Sc[45] V[51] Cr[53] e Co[59] Ni[60] Cu[65] Zn[66] Ga[71] Rb[85] Sr[88] Y[89] Zr[91] Nb[93] Mo[97] e Sn[118] Cs[133] e 1.8 Ba[135] La[139] Ce[140] Pr[141] Nd[146] Sm[147] Eu[151] Gd[157] Tb[159] Dy[161] Ho[165] e Er[166] e Tm[169] Yb[172] Lu[175] Hf[178] Ta[181] W[182] f 12 Pb[208] Th[232] U[238] a. Ref. 19. b. Ref. 5. c. Average value of six analytical runs. d. Ref. 20. e. n = 4. f. n = 5. Figure 3 illustrates the method s accuracy of the proposed technique by making reference to the certified values of the SRMs. It can be observed that most measured concentration values show reasonable agreement with certified values of the SRMs, with relative errors (R.E.%) of <10%. Chondrite normalized curve of REEs Plotting the chondrite normalized curve of the REEs 18 is a very effective way to check the internal conformity of the results of chemical analysis of geological samples. The analytical data were normalized to the chondrite s by dividing the measured concentration values for the rock samples with the average elemental contents in chondrites. Figure 4 shows the plot of log(sample/chondride) versus the atomic number of the REEs. The solid symbols represent the plot for found data and the open symbols stand for the plot for certified values. Smooth curves and excellent co-incidence between the found curves and the reference curves in the figure suggest that the fine matrix-matching procedure we have developed is essentially of utility for the analysis of rare earth elements. Conclusions This article describes a simple method for routine ICP-MS analysis of geological sample for ~40 trace and ultratrace elements. The proposed method possesses the following attributes: simple analytical procedure, minimal matrix interferences without diverse corrections, accurate and precise, reasonable element coverage and low cost/effect compared to existing analytical strategies. Acknowledgements The authors express their thanks to National Foundation of China for Natural Science for financial support ( ). References 1. A. R. Date and A. L. Gray, Spectrochim. Acta, 1985, 408, 115.

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