Determination of Major and Trace Elements in Plant Samples by Inductively Coupled Plasma Mass Spectrometry

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1 Communications in Soil Science and Plant Analysis ISSN: (Print) (Online) Journal homepage: Determination of Major and Trace Elements in Plant Samples by Inductively Coupled Plasma Mass Spectrometry P. Masson, T. Dalix & S. Bussière To cite this article: P. Masson, T. Dalix & S. Bussière (2010) Determination of Major and Trace Elements in Plant Samples by Inductively Coupled Plasma Mass Spectrometry, Communications in Soil Science and Plant Analysis, 41:3, , DOI: / To link to this article: Published online: 05 Feb Submit your article to this journal Article views: 5040 Citing articles: 23 View citing articles Full Terms & Conditions of access and use can be found at

2 Communications in Soil Science and Plant Analysis, 41: , 2010 Copyright # Taylor & Francis Group, LLC ISSN: print / online DOI: / Determination of Major and Trace Elements in Plant Samples by Inductively Coupled Plasma Mass Spectrometry P. MASSON, 1 T. DALIX, 1 AND S. BUSSIÈRE 2 1 USRAVE, Centre de Recherches INRA de Bordeaux, Villenave d Ornon, France 2 UMR TCEM, Centre de Recherches INRA de Bordeaux, Villenave d Ornon, France The determination of several trace elements [arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), molybdenum (Mo), nickel (Ni), and lead (Pb)] in plant samples using inductively coupled plasma mass spectrometry (ICP- MS) was evaluated. It was established experimentally that moderate amounts (0.2 2%) of dissolved solids decreased the analyte signals significantly. Internal standardization with Rh was efficiently used to compensate for these matrix effects. The accuracy of the method was verified using reference materials digested according to two different procedures: dry ashing and microwave digestion. No significant differences were observed between measured concentrations and certified values. The investigation was next extended for the determination of major elements [aluminum (Al), boron (B), calcium (Ca), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), sodium (Na), phosphorus (P), and zinc (Zn)] on the same reference materials. The ICP-MS values agree well with the values supplied. However, it appeared that Sc was the most useful internal standard for major elements. Keywords Inductively coupled plasma atomic mass spectrometry, internal standardization, major elements, matrix interference, plant sample analysis, trace elements Introduction Low detection limits and the speed of inductively coupled plasma mass spectrometry (ICP-MS) make the technique well suited to analyze trace elements in environmental samples. The ICP-MS method overcomes the sensitivity limitations of inductively coupled plasma atomic emission spectrometry (ICP-AES) and the time consumption of electrothermal atomic absorption spectrophotometry by providing multielement measurement capability in a single run. In the past 15 years, it has become a widely used technique in environmental monitoring. Therefore, the more recent instruments propose a wide dynamic range of 10 9 units, allowing the theoretical determination of elements present at the ng L 21 level and others at the g L 21 level. Received 28 August 2007; accepted 21 September Address correspondence to P. Masson, USRAVE, Centre de Recherches INRA de Bordeaux, B.P. 81, Villenave d Ornon cedex, France. masson@bordeaux.inra.fr 231

3 232 P. Masson, T. Dalix, and S. Bussière The ICP-MS method has become a very interesting technique to analyze major and trace elements in plant samples in the same run. Nevertheless, ICP-MS is subject to both spectral and nonspectral interferences. Isobaric overlaps occur with some regularity, especially for elements between 28 and 80 daltons (Soltanpour et al. 1998; Broekaert 2000). The more serious practical problems are due to the formation of polyatomic ions in the plasma. In many cases, these interferences can be avoided by choosing alternative isotopes. However, ion signal intensity can be also modified significantly by concomitant concentrations. Although both suppression and enhancement have been observed, suppression is more commonly reported (Beauchemin, McLauren, and Berman 1987; Gregoire 1987; Tromp et al. 2000). It is assumed that matrix effects in ICP-MS are caused by a combination of physical and chemical interactions in the plasma and changes in the fraction of ions transported from the ICP to the quadrupole mass filter in the ion optical train (Fraser and Beauchemin 2000). These nonspectroscopic matrix effects have important ramifications in practical analysis. It makes the problems of calibration more difficult, and a significant analytical error can result for samples with relatively high matrix composition such as mineralized plant solutions. The objective of this study is to further investigate matrix effects resulting from concomitant elements present in plant samples for the determination of eight elements [arsenic (As), cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), molybdenum (Mo), nickel (Ni), and lead (Pb)]. Plant samples have been frequently analyzed with ICP-AES, but few reports are available for ICP-MS (Rodushkin, Ruth, and Huhtasaari 1999; Krachler et al. 2002; Sucharova and Suchara 2006). One of the largest remaining problems is the lack of understanding concerning the effect brought by plant matrices. In these conditions, a synthetic stock matrix solution was prepared containing the major elements [potassium (K), calcium (Ca), magnesium (Mg), and phosphorus (P)] encountered in plant digests. The concentrations were chosen to represent a real plant matrix sample. The effects of concomitant elements on the analyte signals were studied at various concentrations. This simulation method has been used for ICP-AES (Masson 1999; Hoenig, Docekalova, and Baeten 1998) but not in ICP- MS. Therefore, it is assumed that the matrix effects are much more severe in ICP-MS that in ICP-AES (Soltanpour et al. 1998). This approach avoids the consumption of reference materials, which remain expensive and involve time-consuming preparation. The aim of this investigation was also to define appropriate means of correction to obtain accurate analytical results. Matrix matching in standard solutions is not possible because the matrix is different from sample to sample in routine analysis. Standard addition is time-consuming. Isotope dilution is not applicable to mono-isotopic elements such as As or Co. The only remaining alternative in such conditions is the use of internal standard (IS) correction, which compensates for variation in analyte signal intensities affected by phenomena occurring in the ICP-MS (Thompson and Houk 1987; Vandecasteele et al. 1988). However, the choice of internal standard for each analyte must be judicious when quantitative multi-element analysis on plant samples is performed. During the experiment, various internal standards were then applied to the analyte signals, and their respective efficiency was discussed. The final accuracy of the method was verified using four standard reference materials. The possible extents of interferences and their corrections were next controlled for the determination of major elements [aluminum (Al), Ca, iron (Fe), K, Mg, manganese (Mn), sodium (Na), P, and zinc (Zn)] in plant samples. The investigation of the whole procedure on major elements was conducted by analyzing the same reference materials.

4 Elemental Determination in Plants 233 Materials and Methods Apparatus The ICP-MS instrument used was a Thermo Electron X Series II (Thermo Elemental, Winsford, England) equipped with a CETAC (Omaha, Neb.) autosampler model ASX 500. The standard torch includes a 1.5-mm-diameter injector. The instrument used a Ni sampler and Ni skimmer cones. Prior to sample evaluation, the instrument was allowed to equilibrate (from the start up) for 30 min, and experimental conditions and mass calibration of the instrument were checked. A short-term stability test was performed using a tuning standard solution containing 10 mgl 21 of barium (Ba), beryllium (Be), bismuth (Bi), cesium (Ce), Co, indium (In), lithium (Li), Ni, Pb, and uranium (U) covering the whole range of masses. This autotune function enables optimization of ion optics voltage and also the sampling zone of the plasma to provide a compromise between high sensitivity, stability of signals, and low levels of cluster ions and doubly charged ions. Details of experimental conditions are summarized in Table 1. They were quite satisfactory for the trace-element analysis in plant samples and stable from day to day. By lowering the temperature of spray chamber at 3 uc, polyatomic interferences under normal ICP-MS conditions decreased. Details of the preferred isotopes used for elemental determination are given in Table 2. The determination of As can be disturbed by the presence of chloride ions. The polyatomic ion 40 Ar 35 Cl interferes with 75 As. In many cases, spectroscopic interferences can be avoided by choosing alternative isotopes, but this method is not applicable to mono-isotopic elements such as As. An elemental equation of correction was then used (Cai, Georgïadis, and Fourqurean 2000). The equation uses the naturally occurring isotopes ratios of relevant elements to estimate and allow the subtraction of isobaric interferences: 75 As~1 75 C {3: C z0: C where 75 C, 77 C, and 82 C are counts on m/z 75, 77, and 82, respectively. Reagents All reagents were analytical grade. All vessels were washed with 10% v/v nitric acid (HNO 3 ) and rinsed three times with purified water before use. Water purified (18 MV) with an ELGA (Elga, Bucks, UK) Purelab Ultra water-purification system was used for all solutions. Commercial stock solution of elements (1000 mg L 21 ) was obtained from Merck (Merck, Darmstadt, Germany). All solutions were acidified with 5% v/v Table 1. Analytical parameters of ICP-MS Parameter Value Power (kw) 1.4 Solution flow rate (L min 21 ) 0.9 Plasma gas flow (L min 21 ) 13 Auxiliary gas flow (L min 21 ) 0.5 Peltier temperatures (uc) 3

5 234 P. Masson, T. Dalix, and S. Bussière Table 2. Spectroscopic data of measured isotopes Isotopes Abundance (%) Ionization potential (ev) Limit of detection a Trace elements Cr Co Ni Cu As Mo Cd Pb Major elements B Na Mg Al P K Ca Mn Fe Zn Internal standards Sc Rh Bi a Measured in ng 21 for trace elements and mg 21 for major elements. of concentrated HNO 3 (69 70% Baker analyzed). For the digestion of plant samples, the reagents were hydrogen peroxide (H 2 O 2, 30% Baker analyzed), concentrated HNO 3, and concentrated hydrofluoric acid (HF, 48% Baker analyzed). The ICP-MS instrument was calibrated with a blank and four multielement standard solutions prepared from stock solutions. The final concentrations of trace elements in the standard solutions were respectively 200, 100, 50, and 25 mgl 21. For major elements, solutions contained respectively Al (0, 5, 10, 20, and 40 mg L 21 ), B (0, 5, 10, 20, and 40 mg L 21 ), Ca (0, 12.5, 25, 50, and 100 mg L 21 ), Fe (0, 2, 4, 8, and 16 mg L 21 ), K (0, 50, 100, 200, and 400 mg l 21 ), Mg (0, 10, 20, 40, and 80 mg L 21 ), Mn (0, 2, 4, 8, and 16 mg L 21 ), Na (0, 5, 10, 20, and 40 mg L 21 ), and Zn (0, 2, 4, 8, and 16 mg L 21 ). The linearity of standards was satisfactory for all species (r 2 > 0.999) over these concentration ranges. An internal-standard stock solution of 100 mgl 21 of Bi, rhodium (Rh), and scandium (Sc) was prepared from single-element stock solutions. Details of internal-standard elements applied to the experiment are given in Table 2. For the determination by ICP-MS, all the samples were diluted two times to obtain solutions containing 2.5% (v/v) of acid. All solutions were spiked with the internalstandard solution for a final concentration of 10 mgl 21. In this way the test solutions were in the same conditions as the calibration solutions, acidified to 2.5%

6 Elemental Determination in Plants 235 v/v with concentrated HNO 3 and spiked with the internal-standard mix at the 10 mgl 21 level. Interference Studies Interference studies were performed on the eight analytes(as,cd,co,cr,cu,mo,ni,and Pb) present at the concentration of 4 mgl 21 in the matrix solutions. The solutions were issued from a stock solution containing the main matrix elements as encountered in mineralized plant samples: K 1000 mg L 21,Ca500mgL 21,Mg200mgL 21,andP 100 mg L 21. Parts of the synthetic matrix solution were added to aliquots of the trace multielement solution in the following proportions: 0% (pure trace multielement solution), 10% v/v, 40% v/v, and 80% v/v before completion with purified water to a constant volume. All solutions were spiked with the internal-standard mix at the 10 mgl 21 level. The solutions were quantified in triplicate with external standardization. Element recoveries were expressed as the ratios of the concentrations obtained in the matrix solutions to those obtained in the pure solution. For each analyte, the results were calculated without and with various internal standards. To overcome the analytical bias due to possible contamination from matrices, the matrix element solutions were also analyzed as blanks for all elements studied, and the values obtained were considered in the final calculations. This procedure determines the effects of progressively increasing the amount of plant matrix on the analytical response of trace elements studied and their correction by internal standardization. Sample Preparation Four types of NIST (National Institute of Standard and Technology, Gaithersburg, Md.) standard reference materials (SRMs) were used: SRM 1573a (tomato leaves), SRM 1575a (pine needles), SRM 1568a (rice flour), and SRM 1547 (peach leaves). Each sample was digested with five replicates for each one and blanks. Digestions were carried out according two ways: dry ashing and wet digestion. The SRMs 1573a and 1575a were incinerated in a silicon capsule at 480 uc for5h in a muffle furnace. After incineration, the sample was transferred to a Teflon capsule, and 5 ml hydrofluoric acid (HF) were added. The HF treatment allows dissolving elements, which may be retained by the insoluble silica residue. After evaporation on a hot plate, the residue was taken up with 5 ml HNO 3, filtered to a 100-mL volumetric flask, and adjusted with purified water. Approximately 1.0 g of dried plant material was weighed. For SRMs 1568a and 1547, approximately 0.5 g of dried plant materials was weighed in a Teflon pressurized bomb. Then 2 ml concentrated HNO 3 were added with CEM microwave system marsx (CEM, Matthews, N.C.). The acid mixture was thereafter filtered through ash-free paper filter into a 50-mL volumetric flask. Results and Discussion Interference Studies In Figure 1, the recoveries of analytes are calculated without internal standardization and presented depending on the addition rate of the matrix solution. Accurate analyses require that any change in the plant matrix composition does not result in a significant variation in the analytical signal (max. 10%). However, accuracy was

7 236 P. Masson, T. Dalix, and S. Bussière Figure 1. Effect of the matrix concentration on signal recoveries of the eight trace elements. degraded when matrix mismatch occurs between the standard and the unknown solutions. Without exception, suppression of the analytical signals was a function of increased concomitant concentration. The results were in accordance with previous observations in ICP-MS (Gregoire 1987; Vandecasteele et al. 1988; Tromp et al. 2000). In the most unfavorable cases (for 60 Ni), the suppression may attain about 40% depression in the presence of the heaviest matrix. Nonspectroscopic interference is the general term adopted in ICP-MS for describing the suppressions observed. It is a complex issue, with several factors contributing to the reduction of analytical signals. Only the ions present in a small area of the plasma were extracted. Small changes in any aerosol or plasma properties may modify the spatial ionic distribution in that plasma region. Also, a shift in the ionization equilibrium, induced by changes in the axial distribution of ions, was the first explanation for suppressions (Fraser and Beauchemin 2000). A substantial mass of particles was introduced into the plasma in the presence of any high salt plant matrix type. Vaporization interference occurred in the ionization source in which the solute did not have sufficient time and/or the source did not have sufficient energy to dissociate and ionize the solute before the analytical species moved through the region of extraction. This would produce a significant change in the plasma properties such as reduction in the ionization temperature (Hobbs and Olesik 1993). In addition, analytes were susceptible to ion suppression from easily ionizable elements (EIEs) such as K, Ca, or Mg, rich in plant samples. The EIEs are the most commonly identified sources of nonspectroscopic interferences in ICP (Beauchemin, McLaren, and Berman 1987; Fraser and Beauchemin 2000; Gillson and Horlick 1986; Olesik and Williamsen 1989). The EIEs were strongly ionized in the plasma and caused a significant increase in electron density along the central axis of the plasma, which shifted the ionization equilibrium back toward the atomic form by ion-electron recombination reactions (Gillson and Horlick 1986; Olesik and Williamsen 1989). In ICP-MS, the interference mechanisms also involve the ion extraction optics. Several authors have suggested space-charge effects as an important source of matrix-induced suppressions in ICP- MS (Gregoire 1987; Tanner 1992; Li, Duan, and Hieftje 1995). Effect of Internal Standardization A consequence of the variable response of analytical elements to matrix amount was the need to apply several internal standards to compensate for these interferences.

8 Elemental Determination in Plants 237 Figure 2a. Effect of various internal standards on the signal recoveries of 75 As, 114 Cd, 59 Co, and 52 Cr. According to Thompson and Houk (1987), internal-standard choice must be based on similarity of ionization potentials and mass numbers between the analyte and internal reference. The recoveries of trace elements were studied as a function of various internal standards (Bi, Rh, and Sc). The typical results obtained with internal standards are presented in Figures 2a and 2b. With any elements (Sc, Rh, or Bi), the error on the recoveries was reduced. However, even with using Sc as an internal standard, the error on the recoveries remained severe, with the exceptions of Cr and Mo. The Sc ion production was certainly less affected by the matrix effect than the analyte ions because of the difference of ionization potential between the metals. Conversely, ionization potentials of Cr, Mo, and Sc are close, so the result was logical. The Rh and Bi gave similarly results. The recoveries for the 80% matrix solution appeared sometimes overestimated (for Mo or Pb). Nevertheless, 80% of matrix concentration is too high to represent real sample; in plant samples, they may attain Figure 2b. Effect of various internal standards on the signal recoveries of 63 Cu, 98 Mo, 60 Ni, and 208 Pb.

9 238 P. Masson, T. Dalix, and S. Bussière only half of the value in the more mineralized samples, that is to say, 40%. In these conditions, the correction was satisfactory with Rh or Bi as internal standard for all elements. However, at this stage, it was difficult to define the best internal standard in practice. The quantitative results obtained on certified plant sample reference materials were necessary. Standard Reference Materials The correction of the matrix effect was verified by the analysis of elements in the SRMs. The quantitative results of trace elements obtained on samples are given in Table 3. The Rh appeared as the most polyvalent internal standard and was used in practice. In these conditions, the correction of analytic signals was satisfactory. The measured concentrations on the digested SRMs solutions were in agreement with the reference values in the 95% confidence interval, which corresponds to two times the standard deviation. When Sc was used as internal standard, the general tendency was to underestimate the concentrations (data not shown). Conversely, using Bi as internal standard, signals of elements were significantly improved by concomitant concentrations (data not shown). This result indicated than signal of element with higher mass was easily degraded with few matrix contents in solution. Nevertheless, the analytical results obtained without internal standard or measured with Bi were typically improved in comparison to the reference values. It can be noted that the two digestion procedures allowed good agreement of measured concentrations for all elements investigated except for Mo with the wet digestion. Yet, the final concentration of HNO 3 in the sample digests achieved with wet digestion may be different than in standard solutions. In fact, some HNO 3 participates in the destruction of organic matter with oxidative and hydrolysis reactions (Gorsuch 1970). Previous studies indicated a reduction in the ICP signal with an increase in the inorganic acid concentration (Farino et al. 1987). It has been suggested that the effect is connected with the changes in density, surface tension, and viscosity of the solutions at different HNO 3 concentrations (Chudinov, Ostroukhova, and Varvanina 1998; Todoli and Mermet 1999). However, the concentrations calculated with Rh observed on the digested SRMs showed satisfactory efficiency of the determination method. Concerning Mo, incomplete dissolution was suspected. The HF was not used in the microwave system. Good results were also obtained for As after calcination of samples. The results were logically acceptable for routine analysis. Arsenic is generally regarded as a volatile element requiring wet oxidation of samples. However, this result was in agreement with previous observations of Vassileva et al. (2001), which indicated that plants of terrestrial origin may be mineralized using dry-ashing method without As (and Se) losses. Major Elements The possible extension of the whole method for the determination of major elements was investigated by analyzing the same reference materials. Results are presented in Table 4 for elements with concentration weaker than 1 mg g 21 (mesoelements) and in Table 5 for elements with concentration stronger than 1 mg g 21 (macroelements). Results indicated clearly that the measured concentrations (calculated with Sc

10 Table 3. Results for trace elements in standard reference materials (IS 5 Rh) Parameter 75 As (ng g 21 ) 114 Cd (mgg 21 ) 59 Co (mgg 21 ) 52 Cr (mgg 21 ) 63 Cu (mgg 21 ) 98 Mo (mgg 21 ) 60 Ni (mgg 21 ) 208 Pb (mgg 21 ) NIST 1573a Mean (n 5 5) St. dev. (n 5 5) Certified value [0.46] NIST 1575a Mean (n 5 5) St. dev. (n 5 5) Certified value [ ] NIST 1568a Mean (n 5 5) St. dev. (n 5 5) Certified value [0.018] [0.01] NIST 1547 Mean (n 5 5) St. dev. (n 5 5) Certified value [0.070] [1] Notes. The certified values and uncertainties are given in the certificates delivered with each sample. The values in the brackets are not certified but only indicative. Elemental Determination in Plants 239

11 Table 4. Results for meso elements in standard reference materials (IS 5 Sc) Parameter 27 Al (mgg 21 ) 11 B(mgg 21 ) 57 Fe (mgg 21 ) 55 Mn (mgg 21 ) 23 Na (mgg 21 ) 64 Zn (mgg 21 ) NIST 1573a Mean (n 5 5) St. dev. (n 5 5) Certified value NIST 1575a Mean (n 5 5) St. dev. (n 5 5) Certified value NIST 1568a Mean (n 5 5) St. dev. (n 5 5) Certified value NIST 1547 Mean (n 5 5) St. dev. (n 5 5) Certified value Note. The certified values and uncertainties are given in the certificates delivered with each sample. 240 P. Masson, T. Dalix, and S. Bussière

12 Elemental Determination in Plants 241 Table 5. Results for macroelements in standard reference materials (IS 5 Sc) Parameter 44 Ca (mgg 21 ) 39 K(mgg 21 ) 24 Mg (mgg 21 ) 31 P(mgg 21 ) NIST 1573a Mean (n 5 5) St. dev. (n 5 5) Certified value [12000] NIST 1575a Mean (n 5 5) St. dev. (n 5 5) Certified value NIST 1568a Mean (n 5 5) St. dev. (n 5 5) Certified value NIST 1547 Mean (n 5 5) St. dev. (n 5 5) Certified value Notes. The certified values and uncertainties are given in the certificates delivered with each sample. The values in the brackets are not certified but only indicative. as internal standard) were in accordance with the reference values. Signal intensities were significantly improved for all elements by using Rh or especially Bi as internal standard. The analytes and Sc are close in terms of mass and ionization potential. Consequently, their behavior in ICP-MS was close, allowing a good correction of matrix effects. Calcium and P concentrations appeared sometimes too high (for SRM 1573a). In this case, the concentrations obtained in volumetric flask were widely greater than concentrations in standard solutions, and this can explain the difference. Of course, it was always possible to dilute the solutions. However, for the simultaneous determination of major and trace elements, the rate of dilution must be selected carefully to analyze trace elements in good conditions. As for trace elements, it should be noted that the two digestion procedures allowed good agreement of measured concentrations for all elements investigated. In particular, the measured concentrations of Al remained comparable to the certified values with the wet-digestion procedure. Generally, the digestion method must be continued with HF attack to obtain the total amount of Al in solution. The HF treatment allows dissolving elements, which may be retained by the insoluble silica residue of sample. Detection Limits Table 2 shows the detection limits obtained for the method. The detection limits were calculated as three times the standard deviations of the blanks, which were determined by measuring 10 times a sample of purified water in 2.5% v/v HNO 3.It should be mentioned that detection limits were calculated in the operating conditions of routine analysis. Matrix effects may decrease the signal to background ratios and

13 242 P. Masson, T. Dalix, and S. Bussière can affect slightly the detection limits. However, it was clear that the performances showed high sensitivity. Detection limits of the method were sufficient to treat all types of plant samples. Conclusions The good agreement between the certified values and the measured values demonstrates the applicability of the ICP-MS for the routine determination of major and trace elements in plant samples provided that the matrix interferences are corrected. In fact, it was established experimentally that matrix effect resulting from moderate amounts (0.2 2%) of mixture components changed analyte signals significantly. Consequently, matrix mismatch occurs between the standard and the unknown solutions, which leads to an erroneous use of calibration graph. Accuracy was degraded. Accurate measurements must be restored by using internal standards, which corrected the losses of energy transfer between the plasma and the sample, and 50% matrix suppression can be eliminated completely. However, the appropriate selection of internal standard was the primary cause of accuracy of analysis, and careful attention must be provided for this choice. Because of improved sensitivity, samples may be often diluted, and matrix effects may be consequently minimized. Acknowledgments This research was supported by the Regional Council of Aquitaine with Agreement A. References Beauchemin, D., J. W. McLaren, and S. S. Berman Study of the effects of concomitant elements in inductively coupled plasma mass spectrometry. Spectrochimica Acta Part B 42: Broekaert, J. A. C State-of-the-art and trends of development in analytical atomic spectrometry with inductively coupled plasmas as radiation and ion sources. Spectrochimica Acta Part B 55: Cai, Y., M. Georgiadis, and J. W. Fourqurean Determination of arsenic in seagrass using inductively coupled plasma mass spectrometry. Spectrochimica Acta Part B 55: Chudinov, E. G., I. I. Ostroukhova, and G. V. Varvanina Acid effects in ICP-AES. Fresenius Journal of Analytical Chemistry 335: Farino, J., J. R. Miller, D. D. Smith, and R. F. Browner Influence of solution uptake rate on signals and interferences in inductively coupled plasma optical emission spectrometry. Analytical Chemistry 59: Fraser, M. M., and D. Beauchemin Effect of concomitant elements on the distribution of ions in inductively coupled plasma mass spectroscopy, part 1: Elemental ions. Spectrochimica Acta Part B 55: Gillson, G., and G. Horlick An atomic fluorescence study of easily ionizable element interferences in the inductively coupled plasma. Spectrochimica Acta Part B 41: Gorsuch, T. T The Destruction of Organic Matter. Oxford, U.K.: Pregamon Press. Gregoire, D. C The effect of easily ionizable concomitant elements on nonspectroscopic interferences in inductively coupled plasma mass spectrometry. Spectrochimica Acta Part B 42:

14 Elemental Determination in Plants 243 Hobbs, S. E., and J. W. Olesik The effect of desolvating droplets and vaporizing particles on ionization and excitation in Ar inductively coupled plasmas Spectrochimica Acta Part B 48: Hoenig, M., H. Docekalova, and H. Baeten Study of matrix interferences in trace element analysis of environmental samples by inductively coupled plasma atomic emission spectrometry with ultrasonic nebulization. Journal of Analytical Atomic Spectrometry 13: Krachler, M., C. Mohl, H. Emons, and W. Shotyk Analytical procedures for the determination of selected trace elements in peat and plant samples by inductively coupled plasma mass spectrometry. Spectrochimica Acta Part B 57: Li, G., Y. Duan, and G. M. Hieftje Space charge effects and ion distribution in plasma source mass spectrometry. Journal of Mass Spectrometry 30: Masson, P Matrix effects during trace element analysis in plant samples by inductively coupled plasma atomic emission spectrometry with axial view configuration and pneumatic nebulizer. Spectrochimica Acta Part B 54: Olesik, J. W., and E. J. Williamsen Easily and noneasily ionizable element matrix effects in inductively coupled plasma optical spectrometry. Applied Spectroscopy 43: Rodushkin, I., T. Ruth, and A. Huhtasaari Comparison of two digestion methods for elemental determinations in plant material by ICP techniques. Analytica Chimica Acta 378: Soltanpour, P. N., G. W. Johnson, S. M. Workman, J. Benton Jones Jr., and R. O. Miller Advances in ICP emission and ICP mass spectrometry. In Advances in Agronomy, ed. D. L. Sparks, vol. 64, pp San Diego, Calif.: Academic Press. Sucharova, J., and I. Suchara, Determination of 36 elements in plant reference materials with different Si contents by inductively coupled plasma mass spectrometry: Comparison of microwave digestions assisted by three types of digestion mixtures. Analytica Chimica Acta 576: Tanner, S. D Space charge in ICP-MS: Calculation and implications. Spectrochimica Acta Part B 47: Thompson, J. J., and R. S. Houk A study of internal standardization in inductively coupled plasma mass spectrometry. Applied Spectroscopy 41: Todoli, J. L., and J. M. Mermet Acid interferences in atomic spectrometry: Analyte signal effects and subsequent reduction. Spectrochimica Acta Part B 54: Tromp, J. W., R. T. Tremblay, J. M. Mermet, and E. D. Salin Matrix interference diagnostics for the automation of inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry 15: Vandecasteele, C., M. Nagels, H. Vanhoe, and R. Dams Suppression of analyte signal in inductively coupled plasma/mass spectrometry and the use of an internal standard. Analytica Chimica Acta 211: Vassilieva, E., H. Docekalova, H. Baeten, S. Vanhentenrijk, and M. Hoenig Revisitation of mineralization modes for arsenic and selenium determinations in environmental samples. Talanta 54: