On the use of line intensity ratios and power adjustments to control matrix eVects in inductively coupled plasma optical emission spectrometry E. H. van Veen* and M. T. C. de Loos-Vollebregt Laboratory of Materials Science, Delft University of Technology, Rotterdamseweg 137, 2628 AL Delft, The Netherlands, E-mail: eric.vanveen@stm.tudelft.nl Received 14th October 1998, Accepted 16th February 1999 In inductively coupled plasma optical emission spectrometry, matrix eVects can be substantially reduced by applying robust operating conditions, i.e.a high rf power level and a low nebulizer gas flow. However, dissimilar line intensity changes are still observed, in particular with varying salt matrices. Calcium and, to a lesser extent, Mg induce stronger eVects than Na and K. In 0.1, 0.3 and 1.0% Ca matrices, signal changes for 29 atomic as well as ionic lines have been determined with respect to the Ca-free solution. The changes range from -5 to -30% for the 1.0% Ca matrix.Starting from robust conditions in radial viewing, the rf power has been adjusted until the Cr ionto- atom line ratio measured for the calcium solutions equalled the ratio determined during the calibration (0% Ca). By this power adjustment, the changes are within ±4%. No matrix eVects originating from the sample introduction system are observed, probably due to the high acid concentrations used. The practical application of power adjustments is illustrated with results for certified sediment samples and with multiple line analysis for qualitative and semiquantitative analysis. The approach is an attractive alternative to matrix matching or standard additions.Internal standardization based on one atomic and one ionic line of the same element is indicated as another possibility. When analysing series of samples in inductively coupled plasma The Mg II/I line ratio has also been used as a fast diagnostic optical emission spectrometry (ICP-OES), the sample matrices tool in the control of ICP systems,10 and in qualitative and may vary in acid and/or salt composition.Variations in acid semiquantitative analysis.11–13 In this kind of analysis, the composition are not expected to be large, as fixed amounts assessment of the presence of an element is based on the are usually applied for digestion or storage of samples. The detection of the prominent emission lines and their relative main components may vary appreciably, such as the transition line intensities compared with predetermined values of the elements in steel or the alkali and alkaline earth elements in pure element solution.To allow the comparison of relative sediment digestions. For proper calibration, one has to con- line intensities, the plasma conditions on analysis should be sider the sample matrix composition, and methods such as close to the conditions at which the reference values were matrix matching, internal standardization, standard addition, determined.In verification, the Mg II/I line ratio should be optimization of ICP operating conditions and mathematical reproduced at its high value indicating robust and identical correction can be applied.1 With matrix matching, the known conditions. Otherwise, it was suggested12 that the rf power amount of acid and the expected average amount of salt (if level should be adjusted slightly in order to reproduce the Mg necessary) are added to all calibration solutions.II/I line ratio. According to the insights obtained by Mermet’s group,2–9 Present-day ICP-OES instrumentation14–17 includes an matrix eVects have their origin in diVerent parts of the ICP echelle grating in combination with a cross disperser to obtain instrument, viz. in the plasma itself and in the sample introduc- a two-dimensional image of the emission spectrum. Charge tion system. It has been shown that the plasma can be made transfer device detectors are then applied to generate the robust with respect to atomization, ionization and excitation digital representation of the full spectrum.This instrumenby appropriate selection of the operating conditions.2–4 The tation has allowed the development of semiquantitative survey matrix eVects can be substantially reduced by using operating analysis, where the full sample spectrum is compared with a conditions that lead to an eYcient energy transfer between the set of pure element spectra.18,19 Based on multicomponent plasma and the sample.With a high rf power level (1200 W), analysis, all elements and all of their lines present in the a low nebulizer gas flow (0.6 l min-1) and a wide injector spectrum are processed simultaneously. As about 70 elements inner diameter (2 mm), all ionic lines having an energy sum can be detected by ICP-OES, the set of spectra should be between 8 and 16 eV are suppressed by more or less the same determined in a once-only calibration using pure element amount in the presence of a matrix as compared to the matrixsolutions. The solutions contain 2% HNO3 as the common free situation.The suppression is quite insensitive to significant matrix and the spectra include a large amount of atomic and changes in the salt or acid matrix composition, and may ionic lines with widely diVerent energy sums. This predeter- originate from changes in the aerosol transport in the sample mined set of spectra is intended not only to calibrate sample introduction system.5–7 It can be compensated for by using signals over the lifetime of the instrument, but also to calibrate matrix matched standards or internal standardization.8 Robust sample signals in divergent matrices.In order to reproduce plasma conditions are characterized by the high intensity ratio the line patterns at a later time or in a diVerent matrix (HCl, (>8) of the Mg II 280.270 nm line with respect to the Mg I Na, K and Ca), rf power adjustments have successfully been 285.213 nm line.9 Hence, the Mg II/I line ratio for standard applied19 using a matrix matched monitor solution containing and sample solutions can be measured to confirm the robustness of the analysis with respect to matrix eVects.Ba, Cd, Cr, Cu and Ga and employing the line ratio of the J. Anal. At. Spectrom., 1999, 14, 831–838 831Table 1 Operating conditions of the Perkin-Elmer Optima 3000 DV Cr II 267.716 nm and Cr I 357.869 nm lines as the sensitive spectrometer criterion for appropriate adjustment.Thompson et al.20 found that matrix-induced changes in Nominal rf power/W 1300 excitation conditions in the plasma and a change in the rf Plasma gas flow/l min-1 15 power supplied to the plasma produced a similar eVect. This Auxiliary gas flow/l min-1 0.5 Nebulizer gas flow/l min-1 0.6 similarity was used to compensate for matrix eVects as large Sample uptake rate/ml min-1 1.0 as 30%.To all test solutions with Al, Ca, K, Mg, P and S as Nebulizer Cross-flow the matrix elements, the same Be amount was added and the Diameter of injector tube/mm 2.0 rf power was automatically tuned proportional to the diVerence Viewing Radial between the observed and the reference Be II intensity at View distance from coil/mm 4 313 nm. Budic¡ and Hudnik21 also increased the power level Resolution Normal Read time Auto by a small amount to correct for KCl and H3PO4 matrix Minimum and maximum read time/s 2 eVects which were found to correlate with the excitation energy of the ionic lines. Mermet7 reported that, when applying robust conditions, matrix eVects can be minimized to almost the same extent, ing 5% HNO3 and 0, 0.1, 0.3 or 1.0% Ca. Suprapure regardless of the elements, line characteristics and radial or CaCl2.4H2O was used.axial viewing mode. However, close inspection of the available The sediments BCR145, BCR277, BCR280, BCR320 and data4,6,22 shows that the signal suppression still varies over Maas were digested by dissolving about 2 g in 100 ml of 16% the analytes or, even worse, over diVerent ion lines of the aqua regia.The Maas sediment is distributed by the Institute for same analyte. In the work of Brenner et al.,23 matrix eVects InlandWaterManagement andWastewater Treatment, Lelystad, owing to 1 g l-1 Ca or Na were observed to be relatively The Netherlands. Calibration was performed using 10 mg l-1 small, but suppressions for most of the analytes were dissimilar.Merck ICP multielement standard IV in 16% aqua regia. Such changes with matrix composition point at changes in plasma conditions. The main objective of this paper is to Results and discussion further investigate the dissimilar intensity changes of emission lines in ICP spectra. Starting from robust conditions, it will Robust conditions be shown to what extent line intensities can be reproduced, how rf power adjustments can help and how ion-to-atom line Robust plasma conditions are applied as the reference conditions in the present study.As in the work of Mermet’s ratios can serve as the criterion. As solutions containing Ca exhibit a large matrix eVect as compared to solutions contain- group, the nebulizer flow was set as low as 0.6 l min-1 and the standard alumina injector tube was mounted. Among the ing Na, K or Mg, the Ca matrix is used as the worst case example. The approach of power adjustments is illustrated available injectors, this tube has the largest inner diameter of 2.0 mm.The rf power was set to 1300 W. Although a more with several certified sediment samples. Herewith, a simple solution to an old and persistent problem of Zn analysis in robust plasma can be obtained at the software controlled maximum power level of 1500 W, the lower setting was used sediments is presented. In addition, it is shown how semiquantitative, multiple line analysis can benefit from power to allow power adjustments when nebulizing heavy matrices.The optimum distance from the rf coil was determined by adjustments. nebulizing a test solution containing Mg and Cr. Intensities were measured for the Mg 280.270 nm and the Cr 267.716 nm ion lines and for the Mg 285.213 nm and the Cr 357.869 nm Experimental atom lines. The normalized line intensities as a function of ICP emission spectrometer and settings distance are displayed in Fig. 1. The atom lines attain their maximum intensity around 4 mm, whereas the ion line intensit- This study was carried out with a Perkin-Elmer (Norwalk, ies continue to increase at smaller distances.Fig. 2 shows the CT, USA) Optima 3000 DV system. The sample introduction normalized ion-to-atom line ratios for Mg and Cr. In order system consisted of a cross-flow nebulizer and a double-pass not to measure too close to the coil, the 4 mm setting was Scott-type spray chamber. According to the procedure selected as the proper distance.This optimum distance of a described below, robust plasma conditions were realized by few millimetres from the coil was also selected in other work.3,4 applying 1300 W rf power and 0.6 l min-1 nebulizer gas flow. As can be seen in Fig. 1, the Mg and Cr ion line intensities The injector inner diameter was 2.0 mm. Most data were show the same behaviour as a function of distance from the obtained in the radial viewing mode at 4 mm distance from coil.The Mg and Cr atom line intensities, however, behave the load coil. diVerently: there is almost no change in the Cr I intensity. The spectrometer consists of an echelle grating and separate This is reflected in the ion-to-atom line ratios in Fig. 2: the Cr cross-dispersers for the UV and visible channels. Both optical II/I ratio has the largest relative increase with decreasing channels end up in separate segmented charge coupled device distance. The same is true for the Cr II/I ratio with increasing (SCD) detectors.The system measures simultaneously 6% of rf power level, as displayed in Fig. 3 together with a large set the continuous ICP spectrum from 167 to 782 nm on 201 of relative line ratios. This example was determined for a 0.1% subarrays. The subarrays cover one to four prominent Ca matrix relative to the matrix-free situation (at 1300 W). analytical lines for each detectable element. By using the multielement standard and subarrays available All pertinent operating conditions are summarized in on the Perkin-Elmer Optima 3000 DV system, the change in Table 1. the relative line ratios as a function of rf power was determined for the combinations: Cd 214/228, Cd 226/228, Cr 205/357, Standards and samples Cr 267/357, Cu 224/324, Cu 224/327, In 230/325, Mg 280/285, Ni 221/232, Ni 231/232, Pb 220/283, Tl 190/276, Zn 202/213 Test solutions of 1 mg l-1 Mg and 20 mg l-1 Cr were prepared in 5% HNO3 and in 1000 mg l-1 Na, K, Mg or Ca using and Zn 206/213. From Fig. 3, the change is similarly large for the Cr 205/357, Cr 267/357 and Pb 220/283 combinations, Merck (Darmstadt, Germany) single element standards. To study the Ca matrix eVect, 10 mg l-1 dilutions of the Merck smallest for the Mg 280/285, Ni 231/232 and Tl 190/276 combinations, and in between for the two Zn combinations. ICP multielement standard IV were made in solutions contain- 832 J. Anal. At. Spectrom., 1999, 14, 831–838Fig. 1 Ionic and atomic line intensities for Mg and Cr as a function of the distance from the rf coil, normalized to the intensities at 1 mm distance. Rf power is 1300 W. 1, Mg II 280.270 nm; %, Mg I 285.213 nm; ×, Cr II 267.716 nm; *, Cr I 357.869 nm. Fig. 2 Ion-to-atom line ratios for Mg and Cr as a function of the distance from the rf coil, normalized to the ratios at 1 mm distance. Rf power is 1300 W. 1, Mg II/I; ×, Cr II/I. In an earlier study,19 the Cr 267/357 ratio was found to DV instrument and by making the reasonable assumption that the ICP continuum background intensities are the same over monitor, on average, the eYciency of atomization and ionization for the full set of elements detectable in ICP-OES. Owing a wavelength range of only 5 nm, our correction value equals 1.34.This results in an Mg II/I ratio of 10.7, confirming that to this fact and to its strong dependence on the rf power, the Cr 267/357 line ratio will be used in the present study when the ICP operates under robust conditions.It is clear that robustness can also be characterized using correcting matrix eVects by power adjustments. Robustness, nevertheless, has been characterized in general the Cr II/I ratio. However, eYciency corrections cannot easily be made in order to report values which can be compared by the Mg II/I ratio, which should be larger than 8.7 As in the Optima 3000 DV system the actual rf power applied to from instrument to instrument, because of the large diVerence between the wavelengths of the two chromium lines.the ICP can diVer from the nominal setting by ±50 W, the ratio was determined over several days for the operating conditions given in Table 1. The obtained average of 14.4 Matrix eVects from Na, K, Mg and Ca should be corrected for diVerences in the eYciencies of the echelle grating and the SCD detector. For one specific Optima Under robust conditions, changes in the plasma conditions are expected to be small when the matrix concentration is 3000 DV instrument, a correction factor of 1.85 was reported.24 Although the authors did not explain how they derived this significantly modified. With an acidic matrix, changes in the amount of acid cause only minor changes in ionic line intensit- value, the correction factor has been used by others for their instruments.6,25–27 By measuring the background intensities in ies.2 Elements such as Na, K, Mg and Ca, however, are notorious for their matrix eVects and, even under robust the Mg 280 nm and Mg 285 nm subarrays of our Optima 3000 J.Anal. At. Spectrom., 1999, 14, 831–838 833Table 3 Lines (nm) used in the determination of line intensity ratios Al I 237 Cr II 205 Ga I 294 Pb II 220 Al I 396 Cr II 267 Ga I 417 Pb I 283 Ba II 455 Cr I 357 In II 230 Zn II 202 Cd II 214 Cu II 224 In I 325 Zn II 206 Cd II 226 Cu I 324 Mn II 257 Zn I 213 Cd I 228 Cu I 327 Ni II 221 Co II 228 Fe II 238 Ni II 231 Co II 230 Fe II 259 Ni I 232 including 5% HNO3 at 1300 W.Fig. 4 shows the line intensity ratios with and without the presence of 1% Ca as a function of the energy sum of the 29 lines. For the present Optima 3000 DV instrument, the observed change in the ratio is more or less linear with the energy sum and suggests that a change should be made to one of the plasma parameters. Here, we will adjust the rf power level to investigate to what extent the eVects owing to diVerent Ca matrices can be corrected for with the Cr II/I ratio as the criterion.Although the feedback electronics define the ultimate power level when switching on Fig. 3 Ion-to-atom line ratios with and without the presence of 0.1% the plasma (±50 W with respect to the nominal value), the Ca as a function of the rf power level. Distance from coil is 4 mm. power level is stable after the warming-up time and can be The dotted line indicates a relative line ratio equal to 1.tuned to any desired level in a reproducible way.18 The ratios of the line intensities measured in the matrix and conditions, dissimilar suppressive eVects for various ionic lines in 5% HNO3 vs. the rf power are displayed in Fig. 5(a) for up to tens of per cent have been observed.4,6,22,23 As a the 0.1% Ca matrix and in Fig. 5(b) for the 1.0% Ca matrix. consequence, the suppressive eVect may not (only) be due to In Fig. 5(a), it can be seen that, at 1300 W, some lines are the sample introduction system,5–7 but may be due to some suppressed by 10% while other lines are not suppressed at all.further change in plasma conditions. By increasing the power, the spread in suppression decreases, In order to clearly observe the remaining matrix eVect under attains a minimum, the suppression changes into an enhancerobust conditions, the alkali and alkaline earth elements were ment and the spread in enhancement increases. The minimum investigated to see which produced the strongest eVect.Mg spread occurs at the rf power level at which the line intensity and Cr line intensities were measured in solutions containing ratios for the Cr II 267 nm and Cr I 357 nm lines are equal. 1000 mg l-1 of Na, K, Mg or Ca. Table 2 shows that the Or, in other words, at the optimum power setting, the Cr II/I plasma is fully robust with respect to the Na and K matrices. ratio in the matrix equals the reference value. The same For the Mg and Ca matrices, the Cr ion line is suppressed, behaviour is observed for the two other matrices, where, with whereas the atom line is not, and the Cr II/I ratio indicates a increasing amount of Ca, the initial spread in the suppressions change in the plasma conditions.The Ca matrix shows the and the suppressions themselves increase and the optimum largest eVect, as also observed by others,19,23 and was selected power level occurs at a higher setting, as is shown for the to further investigate the dissimilar line intensity changes. 1.0% Ca matrix in Fig. 5(b). The Mg intensities varied over the respective matrices, owing Table 4 lists the averaged suppression, i.e. the matrix eVect, to the small contamination in the suprapure salts and of course and the extreme values for the 29 lines measured in the to the presence of Mg itself as the matrix. Therefore, for Mg, diVerent Ca matrices. The values are given at 1300 W and at only the Mg II/I ratio is reported in Table 2.The Mg II/I the optimum rf power level for which the reference Cr II/I ratio suggests that the plasma is still robust for the Ca matrix. ratio is reproduced. At the optimum level, not only is the Its low value of 8.8 in the Mg matrix reflects another problem spread in the line intensity ratios at its minimum, but also the with the use of the Mg II/I ratio, especially when analysing matrix eVect has been removed. environmental samples. Owing to the high content and its The removal of the matrix eVect by power adjustments strong emission, the intensity of the Mg II line is out of the shows that the signal suppression mainly has its origin in the linear dynamic range.plasma conditions and that the sample introduction system hardly generates a suppressive eVect. The fact that all the The eVect of rf power adjustments solutions contained a similarly high acid level of 5% HNO3 may assist in the similar aerosol transport of the test samples As the reference, intensities were measured from a 10 mg l-1 or in hiding the eVect resulting from the sample introduction multielement standard in 5% HNO3 under the robust consystem.Even up to 1% of Ca, no matrix eVect is present, but ditions at an rf power level of 1300 W. Table 3 lists the 29 the spread in the line intensity ratios increases. Therefore, lines that were used. Then, the intensities were measured from although one may adjust the power level to compensate for the same multielement standard in 0.1, 0.3 and 1.0% Ca the Ca matrix eVect, matrix matching may be preferred when the salt content of the sample diVers widely from the salt Table 2 Chromium intensities (cps) and Cr and Mg ion-to-atom line content of the standards.ratios (not corrected for spectrometer eYciency) for diVerent salt In the set of 29 lines, the smallest eVects were observed for matrices of 1000 mg l-1 Cr I 357 nm, Al I 396 nm and Ga I 417 nm, whereas Zn II 202 nm, Zn II 206 nm and Cd II 214 nm showed the largest Line No salt Na K Mg Ca eVects.Obviously, the magnitude of the eVect corresponds Cr II 267 63200 62600 63800 60500 60200 with the atomic/ionic character of the line and its energy sum. Cr I 357 34500 34300 35000 34100 34600 The lines for Li, Na, Mg and K were used to determine the Cr II/I 1.83 1.83 1.82 1.77 1.74 line intensity ratios for the 0.1% Ca solution with respect to Mg II/I 12.0 12.0 12.0 8.8 11.7 the standard, but could not be used for the 0.3% and 1% Ca 834 J.Anal. At. Spectrom., 1999, 14, 831–838Fig. 4 Line intensity ratios with and without the presence of 1% Ca as a function of the energy sum of the 29 lines listed in Table 3. The dotted line indicates an intensity ratio equal to 1. Some lines discussed in the text have been specified. solutions owing to the presence of these elements as contami- Analysis of certified sediment samples nation in the suprapure calcium salt. In all of our measurements, the Mg II/I ratio was reproduced at a 10–50W higher In environmental analysis, samples such as soils and sediments are usually digested in 16% aqua regia and calibration stan- power as compared to the optimum power level based on the Cr II/I ratio (see Fig. 3 for an illustration). dards are accordingly matrix matched. Samples may vary in Fig. 5 Line intensity ratios for the 29 lines listed in Table 3. Intensities were measured from the 10 mg l-1 multielement standard in (a) 0.1% Ca and (b) 1.0% Ca in 5% HNO3.The reference intensities were measured from the same multielement standard in 5% HNO3 at 1300 W. Solid line: ratio for Cr II 267 nm; broken line: ratio for Cr I 357 nm. J. Anal. At. Spectrom., 1999, 14, 831–838 835Table 4 Averaged matrix eVect (%) and its extreme values (%) for the results in the same three values for the Zn content which equal 29 lines listed in Table 3, at 1300W and at the optimum rf power the reference value.The Cr results are also clearly improved. level (W) for which the reference Cr II/I ratio is reproduced in All line intensities increase on power adjustment, but the diVerent Ca matrices enhancement is stronger for the lines with a higher energy sum. When using the Mg II/I ratio instead of the Cr II/I ratio, At 1300 W At optimum power the rf power had to be increased even further. The Mg ratio Matrix Extreme Optimum Matrix Extreme is low at 1380W owing to the 200 mg l-1 Mg present in the eVect values power eVect values BCR277 solution.At this concentration level, the Mg II signal already suVers from non-linearity, whereas the reference signals 0.1% Ca -5 -10; 0 1350 -1 -3; 2 were determined from the 10 mg l-1 calibration standard. 0.3% Ca -7 -15; 0 1380 0 -3; 3 The eVect of rf power adjustments was also measured in the 1.0% Ca -17 -30; -5 1460 -2 -5; 4 axial viewing mode. The reference Cr II/I ratio in this mode diVers from the value in the radial mode, and was determined salt content between several hundreds to several thousands to be 0.835 in the multielement standard.It is not possible to of mg l-1 of Na, K,Mg and Ca. Typically, the ICP is supposed reproduce the Cr II/I ratio: in BCR277, values of 0.641 and to be robust with respect to this range in matrix composition 0.744 were measured at 1300 and 1500 W, respectively. On the and no severe matrix eVects are expected. However, based on contrary, the Zn 202/213 ratio was reproduced at 1500 W and the observations in the previous section, diVerent changes in the Zn 206/213 ratio at 1450 W.As the intensity of the Mg II intensity for various lines could be expected. line is beyond the range for BCR277, the Mg II/I ratio cannot In Table 5, the results are listed for the six elements of be determined. It is concluded that the rf power adjustments interest in the estuarine sediment BCR277, a reference material cannot minimize the spread in line intensity ratios as well as which is applied as a quality control sample in Dutch labora- in the radial viewing mode, although the robust conditions tories. External calibration matched for 16% aqua regia was can minimize the suppressions to the same extent as for radial used.For Cd and Pb, only one analyte line has been measured, viewing.7,22–24 In a very recent paper,28 however, observations as we do not intend to further complicate the present discussion similar to ours have been reported when comparing ionic lineby using prominent lines suVering from spectral interference based internal standardization in axial and radial viewing by Fe.In particular, the results for the three Zn lines measured modes to compensate for sodium eVects on accuracy. at 1300 W are remarkable: each line yields a diVerent content, By measuring the Cr II/I ratios for diVerent known salt and no result equals the indicative reference value. contents in radial mode and by adjusting the rf power level To investigate whether the salt content is responsible for for the reproduction of the reference ratio, the power adjustthese results, matrix matching was applied.Matrix matching ment as a function of the Cr II/I ratio can be determined to is possible for BCR277 because of the known salt content. control the matrix eVects. In a series of sediment samples with When about 1100 mg l-1 Ca and 200 mg l-1 Na, K and Mg unknown salt content (in particular, the amount of Ca and were added to the calibration standard, the results shown in Mg), all samples can be checked for their Cr II/I ratios.To Table 5 were obtained. The results for the three Zn lines are deviate ratios, new rf power levels are estimated from the the same and equal to the reference value. The results for Cr function and the corresponding samples must be rerun. This also improve. As outlined in the previous sections, robust idea is illustrated with four reference samples in Table 6: the conditions combined with acid matrix matching are not sewage sludge BCR145, the lake sediment BCR280 and the suYcient for this kind of analysis and the suppressions will be river sediments BCR320 and Maas.The intensities of Cr and due mainly to the Ca and Mg content. Zn at their prominent lines were measured. Calibration was In the aqua regia matched calibration standard and in the performed with the 10 mg l-1 multielement standard in 16% BCR277 solution, the Cr II/I ratio was determined.The two aqua regia at 1300 W. The samples which were digested in values are diVerent and the rf power requires an increase of 16% aqua regia were run under the same conditions. Based 80 W for the BCR277 solution to reproduce the reference on the measured Cr II/I ratios, the samples were rerun at the ratio. As can be seen from Table 5, this power adjustment optimum power. As can be seen from Table 6, the applied power levels are closely correlated to the Ca (plus Mg) Table 5 The concentrations found (mg g-1) for diVerent elements and concentrations specified for the reference materials. The results lines (nm) in the estuarine sediment BCR277.The indicative values for all samples clearly improve at all selected lines. In particular per element are included for the BCR145 and Maas samples, large power adjustments were made resulting in a much closer agreement for the Cr Concentration found 205, Cr 206 and Cr 267 lines with respect to the Cr 357 line as well as much less variation over the Zn lines.The results Power/W 1300 1300 1380 averaged over the lines are closer to the indicative values. External calibration at Intensities were also measured for the other lines listed in 1300 W matched for Acids Acids+salts Acids Table 5. The power adjustments induced a clear enhancement Element Energy and improvement for Ni and Pb in BCR145 and Maas, but line sum/eV Indicative only minor changes for the other sediments and elements.When analysing the series of sediment samples under robust Cd I 228 5.42 10.2 11.2 11.2 10.8 Cr II 205 12.8 130 140 142 145.6 conditions, several other actions may be taken.1 Matrix match- Cr II 206 12.8 125 132 136 ing, however, is not practical, as BCR280 and BCR320 require Cr II 267 12.9 130 138 140 a diVerent matrix composition of the standards than BCR145, Cr I 357 3.46 134 144 138 BCR277 and Maas. Standard additions are not attractive in Cu I 324 3.82 84 87 88 97.2 multielement analysis.In the present example, one has to spike Cu I 327 3.79 89 94 92 six diVerent elements in all the samples. When analysing Ni II 231 14.0 33 41 37 34.9 Ni I 232 5.34 35 46 37 unknown samples, an additional complication occurs, as, for Pb II 220 14.8 134 161 146 137.5 matrix matching, one has to know in advance the salt content Zn II 202 15.5 476 558 548 557 and, for standard additions, one should have at least an Zn II 206 15.4 489 553 562 idea about the analyte concentrations in order to add the Zn I 213 5.80 512 551 556 appropriate amounts of spikes. 836 J. Anal. At. Spectrom., 1999, 14, 831–838Table 6 The concentrations (mg g-1) found and indicative for several which the patterns of the lines are experimentally assessed for reference sediments. The external calibration was at 1300 W, matrix the actual ICP and spectrometer. When the library has been matched for 16% aqua regia. Samples were measured at 1300W and built, the samples must be measured under the same plasma at the optimum power level for which the Cr II/I ratio was reproduced.conditions. Salt matrix (mg l-1) is given for the measured solution The daily variations in the plasma parameters and the Concentration found sample matrix induce changes in the plasma conditions and, hence, in the line intensity ratios. As the strength of multiple At optimum Salt line procedures is said to be the independence of a prior At 1300W power Indicative matrix knowledge of the sample composition, matrix matching of standards is not feasible and large errors may occur if the BCR145 1420 W Na, 40 plasma conditions are not fully under control.39 As demon- Cr II 205 71 82 85.2±16.3 K, 80 Cr II 206 72 78 Mg, 350 strated in this work, the combination of robust ICP conditions Cr II 267 75 85 Ca, 2100 and rf power adjustments based on the Cr ion-to-atom line Cr I 357 79 85 ratio is an adequate alternative.If Cr happens to be absent in Zn II 202 2470 2760 2772±209 the unknown sample, other ion-to-atom ratios, such as those Zn II 206 2570 2870 for Mg or Pb, will perform almost as well or the sample Zn I 213 2650 2840 should be spiked with Cr. BCR280 1310 W Na, 350 Cr II 205 68 71 76±5 K, 540 Cr II 206 68 71 Mg, 320 Conclusions Cr II 267 70 73 Ca, 330 Cr I 357 71 73 The application of robust conditions is not suYcient to deal Zn II 202 263 275 290±16 with matrix eVects.Dissimilar line intensity changes are Zn II 206 275 286 observed in various salt matrices. Starting from the robust Zn I 213 278 290 BCR320 1330 W Na, 400 conditions, at which a reference value for the Cr II 267.716 nm Cr II 205 59 62 70.1±7.7 K, 490 to Cr I 357.869 nm line ratio is determined, rf power adjust- Cr II 206 60 62 Mg, 400 ments are made for samples with matrices up to 1% Ca to Cr II 267 58 62 Ca, 430 reproduce the Cr II/I ratio. At the optimum rf power level, Cr I 357 60 63 not only is the dissimilarity of the line intensities at a minimum, Zn II 202 107 123 124.4±5.4 but also the matrix eVect has been removed.For the 1% Ca Zn II 206 116 133 Zn I 213 119 131 matrix, no matrix eVect is observed from the sample Maas 1360 W Na, 10 introduction system. Cr II 205 153 164 169.2±13.2 K, 40 As compared to the Mg II/I ratio, the Cr II/I ratio varies Cr II 206 154 163 Mg, 190 more strongly with the coil distance and the rf power level, Cr II 267 153 164 Ca, 1080 making this ratio a better criterion for rf power adjustments.Cr I 357 160 160 The Mg II/I ratio is an appropriate indicator of the robustness Zn II 202 2520 2810 2740±135 Zn II 206 2590 2840 of the plasma, and a procedure to correct the measured ratio Zn I 213 2650 2820 for eYciency diVerences in the spectrometer is presented. The Ca matrix is used as the worst case example, as this matrix shows larger and more dissimilar suppressions for the 29 atomic and ionic emission lines studied as compared to Mg As the emission lines (also those of the same element) and, in particular, to Na and K matrices.experience diVerent suppressions, the use of one internal The practical application of power adjustments is illustrated standard, such as the Sc II 361.384 nm line23 or the Ar I with several certified sediment samples. The rf power adjust- 794.818 nm line,29 cannot compensate for the diVerent ments correspond to the Ca (and Mg) content of the sample responses of the lines to various concentrations of Ca.solutions. Results for several analytes and for diVerent lines Simulation of the change in energy transfer due to matrix of the same analyte improve. For instance, all three prominent mismatching reveals an almost continuous change in line Zn lines for BCR277 give identical results which equal the intensities as a function of the energy sum.8 A more or less reference value. The power adjustment approach does not linear change has indeed been observed by us, as illustrated in work properly in axial viewing, but is an attractive alternative Fig. 4. This indicates that it should be possible to describe this in radial viewing to the application of matrix matching, change by means of two or three lines covering the actual standard additions or internal standards. It is argued that energy sum range of the lines. It should even be possible to multiple line analysis in qualitative and semiquantitative use one atomic and one ionic line of the same element, e.g.analysis can benefit from power adjustments. In, Pd or Rh, on the Optima 3000 DV instrument. In view of the above discussion, rf power tuning based on the Cr II/I ratio is an attractive alternative to matrix matching, References standard additions or internal standardization. 1 D. A. Sadler, F. Sun, S. E. Howe and D. Littlejohn, Mikrochim. Acta, 1997, 126, 301. Multiple line analysis 2 A. Ferna�ndez, M.Murillo, N. Carrio�n and J.-M.Mermet, J. Anal. At. Spectrom., 1994, 9, 217. In qualitative and semiquantitative analysis, the determination 3 I. Novotny, J. C. Farinas, W. Jia-liang, E. Poussel and of whether an element is present is made from the observation J.-M.Mermet, Spectrochim. Acta, Part B, 1996, 51, 1517. of one or more of the most prominent lines of that element. 4 X. Romero, E. Poussel and J.-M. Mermet, Spectrochim. Acta, Part B, 1997, 52, 495. If the lines exist in the sample spectrum, the relative line 5 M.Carre�, K. Lebas, M. Marichy, M. Mermet, E. Poussel and intensities may be checked to confirm the element pres- J.-M.Mermet, Spectrochim. 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