On-line Removal of Anions for Plant Analysis by Inductively Coupled Plasma Mass Spectrometry AMAURI A. MENEGA� RIO AND MARIA FERNANDA GINE� * Centro de Energia Nuclear na Agricultura, Universidade de Sa�o Paulo, Caixa Postal 96, 13400–970, Piracicaba, S.P., Brazil The multi-element determination of B, Al, Cu, Zn, Mn, Fe, system. Subsequently, the retained species were eluted and sulfate was determined at m/z 48. Cr, Cd, Pb and S in plant digests by ICP-MS with on-line removal of anions is presented.A column with the anionic resin AG1-X8 was placed in a flow system to remove mainly EXPERIMENTAL sulfate and chloride while the solution was flowing to the ICP-MS system for the determination of the other elements. A VG-Plasma Quad PQ-2 ICP-MS system with a concentric Subsequently, the sulfur retained by the column was eluted and nebulizer installed in a water-cooled (10°C) spray chamber determined at m/z 48. The eect of perchlorate on sulfate was used. The experimental conditions are given in Table 1. retention was studied.Interferences on isotopes 65Cu, 64Zn, The ICP-MS system was optimized by continuous nebulization 67Zn and 53Cr were minimized using this approach. The of a multi-element solution (10 mg l-1 B, Mg, Co, In and Pb) analytical range was from 0.2 to 1.0% S for the vegetable covering the analytical range of interest. samples analyzed. The detection limit was 0.02% S (dry basis). The main components of the flow system are a Minipuls 3 Results from reference materials (NIST SRM 1573a Tomato peristaltic pump (Gilson, Worthington, OH, USA), a Model Leaves and ORSTOM palmier and pommier golden) were in 352 time-controlled injector (Micronal, Sa�o Paulo, S.P., Brazil) agreement with certified values at the 95% confidence level.described earlier6 and a resin column connecting the injector and the nebulizer. All the system connections were made of Keywords: Inductively coupled plasma mass spectrometry; polyethylene tubing (0.8 mm id).The tube connecting the resin anion exchange; flow system; matrix removal; plant analysis column to the nebulizer was 5 cm long. The chloride form of the anionic resin AG-1-X8 from Bio- Sulfur is a macronutrient of plants occurring at concentrations Rad Labs. (Richmond, CA, USA) (200–400 mesh) was used. from 0.06 to 1.5% of the dry matter.1 The content of sulfate The resin column was built in a Perspex block by drilling a in samples is a potential interferent in the determination of hole 5 mm wide and 20 mm long.The wetted resin was loaded some elements by ICP-MS, mainly due to the spectral overlap inside the column by pushing with a syringe and both ends from sulfur polyatomic species such as SO2+/ S2+ on 64Zn, were plugged by disks of porous polyethylene fixed with OSO 2H+ on 65Cu and SO+ on 50Cr. Chlorine is also present in ring seals into Perspex disks. Small holes on the disks were plant material at levels up to 6% of the dry matter,1 producing connected to the polyethylene tubing as described in a previous species such as ClOH+, ClO+ and ClO2+, which interfere paper.7 The resin was converted into the nitrate form by with the determination of 52Cr, 53Cr and 67Zn, respectively.pumping 3 ml min-1 of 1.0 M nitric acid through the column Interferences due to Cl and S have been alleviated in a for 1 h. number of ways. For example, Cl interference has been partially Multi-element standard solutions were prepared from SPEX reduced by using a high flow rate of the nebulization gas, but (Metuchen, NJ, USA) stock standard solutions (Multi-element to remove ClO+ and ClO2+interference eectively the addition Plasma Standard ICP-MS 2 and ICP-MS 4).Water purified of 8%nitrogen to the aerosol sample carrier gas was necessary.2 with a Milli-Q system (Millipore, Bedford, MA, USA) was A flow system for on-line removal of interferences for the used throughout.Working standard solutions containing 2.5, determination of V, Mn, Cu, Zn, Cd and Pb in biological 5.0, 25.0, 50.0 and 100.0 mg l-1 of B, Al, Cr, Mn, Fe, Cu, Zn samples by ICP-MS was described by Ebdon et al.3 An and Pb in 0.05 M HClO4 and standard solutions containing iminodiacetate-based resin was used to chelate the cations to 0.00, 20.0, 40.0 and 60.0 mg l-1 of sulfate were prepared. Nitric be separated from the matrix. Later, the same flow system was acid purified by sub-boiling distillation, perchloric and sulfuric proposed using a column with activated alumina (acidic form) acid (Suprapur) were obtained from Merck (Darmstadt, for the separation of anionic forms of As, Cr, Se and V from the biological digests.The determination was achieved after matrix elimination by ICP-MS.4 However, these methods were Table 1 ICP-MS operating conditions restricted to certain elements, thus reducing the multi-element capabilities of ICP-MS. An approach based on the retention Forward power 1350 W of anionic species by a strongly basic anion-exchange resin in Argon gas flow rates— Coolant 13 l min-1 the NO3- form while the analytes were eluted with diluted Auxiliary 0.55 l min-1 nitric acid was described earlier.5 This method allowed the Nebulizer 0.85 l min-1 simultaneous multi-element determination of V, Cr, Cu, Zn, Scan measurements— As and Se in biological and environmental samples.The main Detection mode PC advantage was that analytes in the anionic and cationic forms Range m/z 6–210 were separated from Cl and S.However the separation was Channels per u 19 Dwell time 320 ms performed in columns placed o-line. Sweep 0.5 s In this paper, a flow system with a column of anion-exchange Single ion measurements— resin (AG1-X8) placed on-line with the ICP-MS system is Detection mode PC proposed. The S and Cl species in the plant digests were Dwell time per channel 10.24 ms retained by the resin while the analytes followed to the ICP-MS Journal of Analytical Atomic Spectrometry, June 1997, Vol. 12 (671–674) 671Germany) and 30% hydrogen peroxide (puriss) from Fluka when increasing concentrations of perchloric acid were added to a 100 mg l-1 sulfate solution. The results (Fig. 2) indicate (Buchs, Switzerland). no losses of sulfate on adding perchloric acid up to 0.05 M. Ions at m/z 34 and 48 were monitored. The higher signal at Sample Preparation m/z 34 was probably due to interference from O2+.All signals increased after addition of 0.1 M perchloric acid, indicating Plant samples of dierent origins and reference material from that sulfate passed through the resin. After collecting each National Institute of Standard and Technology (Gaithersburg, euent, the anions were eluted with 1.0 M nitric acid, producing MD, USA) (NIST SRM 1573a Tomato Leaves) and two others transient signals at m/z 48. The smallest transient peak corre- from the Oce de la Recherche Scientifique et Technique sponded to the elution of sulfate after addition of 0.1 M Outre-Mer (ORSTOM) (palmier and pommier golden)8 were perchloric acid, confirming the loss of more than 90% of digested using nitric acid, hydrogen peroxide and perchloric sulfate. The retention of perchlorate from plant digests in acid, following a procedure similar to that described earlier.9 AG-1-X8 was reported earlier.10 The dilution factor was 1000 to leave the solutions in 0.025 M The capability of the resin to remove anions from plant perchloric acid media.Blanks were prepared together with digests was proved by adding increasing concentrations of the samples. sulfate to the NIST-1573a reference sample solution. The results at m/z 64 and 65 for the determination of Zn and Cu, Flow System respectively, are shown in Fig. 3 At m/z 64 the calculated concentration of Zn in the sample was in agreement with the The flow system proposed to perform the removal of anions certified value for Zn of 30.9±0.7 mg g-1, indicating no inter- is presented in Fig. 1 with the injector at the elution (top) and ference from the 0.96% m/m of S in the sample (approximately sampling (bottom) positions. The top design represents the 30 mg l-1 sulfate in solution). Interference occurred with sulfate sample S pumped at 1.4 ml min-1, filling the tubes and going concentrations higher than 100 mg l-1 and increased lto the discarded waste D, while the eluent solution passed up to 1.0 g l-1.The results at m/z 65 showed similar behavior through the column towards the ICP-MS system. On moving to 64Zn. The initial value for Cu was in agreement with the the central part of the injector unit, the configuration shown certified value of 4.70±0.14 mg g-1. On adding sulfate to the at the bottom design is obtained, where the sample flowed sample up to 1.0 g l-1, a linear signal increase up to 8 mg g-1 through the column to the spectrometer for 2 min.of Cu was observed, probably due to 32SO2H+ interference. Simultaneously with the injector movement, the acquisition In both instances the interference was totally removed by using process with a 40 s uptake and two replicates of 25 s each by the proposed procedure. These results showed the eciency of scanning the analyte masses in the ICP-MS system was the resin in removing sulfate at concentrations more than 20 implemented. After the sample, water (W) was passed through times the maximum level found in plant materials.At m/z 67 the column at 2.5 ml min-1 to remove sample residues from the overlap due to the 35ClO2+ and 34SO2H+ species produced the resin. After finishing the multi-element data acquisition process, the injector was moved to the elution position and 1.0 M nitric acid at E was passed through the column at a flow rate of 1.4 ml min-1 to elute the anions. The sulfur species SO+ was determined at m/z 48 using the single ion mode with a 22 s uptake and 8 s of data measurement.At this position the sample could be changed. RESULTS AND DISCUSSION The eciency of sulfate removal in the presence of other anions was determined by analyzing the euents leaving the column Fig. 2 Losses of sulfate from the euents on adding perchloric acid. The average of three values monitored at m/z 34 and 48 produced the upper and lower curves, respectively. The values at m/z 34 were 10 times higher.The sulfate concentration was 100 mg l-1. Fig. 1 Flow diagram of the proposed system. The top and bottom diagrams represent the elution and sampling steps, respectively. The Fig. 3 Eect of sulfate addition to the NIST SRM 1573a sample to assembled three-rectangular design represents the injector port with moving central part. The sample (S) or water (W) flows into the determine Zn at m/z 64 (upper curves) and Cu at m/z 65 (lower curves). [C] indicates the analyte concentration.The dotted and solid lines injector. The peristaltic pump action is represented by the black arrows. E corresponds to the eluent solution, C indicates the resin were obtained with and without the use of the anion-exchange resin, respectively. Means and deviations were obtained with n=3. column and D the discarded waste. 672 Journal of Analytical Atomic Spectrometry, June 1997, Vol. 12Table 2 Results of routine analysis and certified or reported values (mg g-1) NIST SRM 1573a Palmier Pommier golden Tomato Leaves (ORSTOM) (ORSTOM) Element Determined* Certified Determined* Reported Determined* Reported 10B — — 12.04±0.13 14.51±2.5 31.83±0.49 28.35±3.58 27Al — — 234±19 150 195±2 200 50Cr 10.50±0.49 1.99† — — — — 52Cr 1.86±0.07 1.99† — — — — 53Cr 3.59±0.55 1.99† — — — — 55Mn 245±10 246±8 650±35 630 44±1 47 56Fe 318±30 368±7 249±19 200 325±0.4 311 63Cu 4.30±0.23 4.70±0.14 8.31±0.20 8.70 6.96±0.06 8.40 65Cu 4.44±0.34 4.70±0.14 8.60±0.28 8.70 7.05±0.33 8.40 64Zn 29.3±1.0 30.9±0.7 22.8±0.1 23.0 29.9±3.1 30.0 66Zn 28.9±0.7 30.9±0.7 23.1±0.7 23.0 30.6±1.1 30.0 67Zn 32.2±1.66 30.9±0.7 — — — — 68Zn 29.9±0.7 30.9±0.7 21.9±1.1 23.0 31.5±2.1 30.0 114Cd 1.28±0.16 1.52±0.04 — — — — 208Pb — — 2.5±0.3 3.0 11.6±0.8 16.0 48S (%) 0.95±0.03 0.96† 0.23±0.02 0.21±0.03 0.23±0.01 0.34±0.05 Cl (%) — 0.66† — 0.611±0.015 — 0.100±0.007 * Mean±standard deviation (n=3).† Reported values. an initial value of 36.7 mg g-1 of Zn for the reference sample, which was also corrected using the anion-exchange resin.This was not observed at m/z 50 where the interference eect is produced by SO+, ArN+ and ArC+ ions and isobaric species of Ti and V. Even when the anion-exchange resin produced a considerable decrease in the m/z 50 signals, the value obtained for Cr was not in agreement with the reported value of 1.99 mg g-1. At m/z 53 the interferences of ClO+ and ArNH+ were observed in the results presented in Table 2. The initial Cr concentration of 14.9 mg g-1 was reduced to 3.5 mg g-1 when using the resin, indicating good eciency of chloride removal.The reported content of chlorine in this sample was 0.66% m/m. Sulfate from standard solutions was separated and eluted Fig. 5 Calibration curves for sulfate obtained with standard solutions from the anion-exchange resin producing the transient peaks (0.0, 20.0, 40.0 and 60.0 mg l-1 ) . The curves correspond to %, direct at m/z 48 shown in Fig. 4.Calibration curves obtained by nebulization and using the anion-exchange resin at dierent uptake direct nebulization and using the proposed approach showed and measurement times: &, 0 and 60 s; $, 15 and 30 s; and 2, 22 good linearity, as can be observed in Fig. 5. Sulfate determi- and 8 s. Points indicate the means of three values. nation was performed by measuring the dierent areas under the transient peak depicted in Fig. 4. The signal acquisition was programmed using uptake and measurement times of 0 results observed with direct nebulization showed poor sensi- and 60, 15 and 30, and 22 and 8 s, respectively.Sensitivity was tivity. Using the anion-exchange resin, the slope of the curves enhanced for measurements made at the peak maximum. The increased by a factor of 10 when the acquisition procedure was well adjusted to the transient signal. A sample volume of 2.8 ml was consumed per experiment. A limit of detection of 0.6 mg l-1 of sulfate was calculated using the 3s criterion.Routine analysis using the proposed procedure allowed the determination of the analytes in plant samples, as shown in Table 2. The paired t-test at the 95% confidence level was applied to compare the two sets of results (n=32) corresponding to the determined and reported values. The calculated t was 0.85, which was lower than the tabulated value (t=2.042), demonstrating good assessment of accuracy. CONCLUSIONS The flow scheme presented allowed the sequential determination of several analytes free from anion interferences and the total sulfur content in digested plant solutions. Interferences from 64Zn, 67Zn and 65Cu were completely removed, but not those from 50Cr and 53Cr.The proposed approach was very ecient for removing Fig. 4 Transient peaks corresponding to sulfate eluted from the resin. anions commonly found in plant digests. The strongly basic From the baseline to the signal peaks the sulfate concentrations were 0.0, 20, 40 and 60 mg l-1.anion-exchange resin used was robust in removing up to 30% Journal of Analytical Atomic Spectrometry, June 1997, Vol. 12 6735 Goossens J., and Dams R., J. Anal. At. Spectrom., 1992, 7, 1167. m/m S, indicating the possibility of eliminating S in the analysis 6 Bergamin Fo., H., Reis, B. F., Jacintho, A. O., and Zagatto, of materials with high sulfur contents by ICP-MS. E. A. G., Anal. Chim. Acta, 1980, 117, 81. 7 Reis, B. F., Gine�, M. F., Santos Filha, M. M., and Baccan, N., Grateful acknowledgements are made to FAPESP (Fundac�a� o J. Braz. Chem. Soc., 1992, 3, 80. de Amparo a` Pesquisa do Estado de Sa�o Paulo) for financial 8 Pinta, M., Van Schouwenberg, A., Bonvalet, M., Lachica, M., and support and to Dr. Victor Vitorello for language improvement. Herman, P., in Proceedings of the 4th International Colloquium on the Cf Plant Nutrition, Rijksuniversiteit, Ghent, 1976, vol. II, p. 41. REFERENCES 9 Jarvis, I., in Handbook of Inductively Coupled Plasma Mass 1 Martin Prevel, P., Plant Analysis as a Guide to the Nutrient Spectrometry, ed. Jarvis, K. M., Gray, A. L., and Houk, R. S., Requirements of T emperate and T ropical Crops, Lavoisier, New Blackie, Glasgow, 1992. York, 1987. 10 Gomes Neto, J. A., Bergamin Fo., H., Sartini, R. P., and Zagatto, 2 Laborda, F., Baxter, M. J., Crews, H. M., and Dennis, J., J. Anal. E. A. G., Anal. Chim. Acta, 1995, 306, 343. At. Spectrom., 1994, 9, 727. 3 Ebdon, L., Fisher A. S., Worsfold, P. J., Crews, H., and Baxter, Paper 7/00444C M., J. Anal. At. Spectrom., 1993, 8, 691. Received January 20, 1997 4 Ebdon, L., Fisher A. S., andWorsfold, P. J., J. Anal. At. Spectrom., 1994, 9, 611. AcceptedMarch 18, 1997 674 Journal of Analytical Atomic Spectrometry, June 1997, Vol. 12