JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 727 Reduction of Polyatomic interferences in inductively Coupled Plasma Mass Spectrometry by Selection of Instrumental Parameters and Using an Argon-Nitrogen Plasma Effect on Multi-element Analyses* Francisco LabordaJ Malcolm J. Baxter Helen M. Crews$ and John Dennis Ministry of Agriculture Fisheries and Food CSL Food Science Laboratory Norwich Research Park Colney Norwich Norfolk UK NR4 7UQ The effect of instrumental parameters and argon-nitrogen plasmas on polyatomic ion formation has been studied in order to reduce their magnitude in routine multi-element analysis without losing detection capability. Special emphasis was placed on the chlorine based polyatomic interferences on V Cr Zn As and Se. A significant reduction in signals from polyatomic ions was attained by using a high aerosol carrier gas flow rate (0.955 I min-') instead of the default flow rate (0.755 I min-') or by adding nitrogen (8%) to the aerosol carrier flow.The ArClf interference produced by 0.05% CI (the maximum concentration expected in digested food stuff samples) was effectively removed by both methods and CIO' and C102+ by addition of nitrogen. Detection limits for elements along the mass range (from Li to U) were on average 2-3 times higher with the mixed gas plasma. This slight degradation of detection limits was not judged to be detrimental to multi- element determinations in five reference materials when the results from using an argon-only plasma (with low and high aerosol carrier flow rates) were compared with the results from the argon-nitrogen plasma.Keywords Inductively coupled plasma mass spectrometry; polyatomic interferences; argon-nitrogen plasma; multi-element analysis Polyatomic ions cause spectroscopic interferences in induc- tively coupled plasma mass spectrometry (ICP-MS) by over- lapping with analytes at the m/z value of interest. These ions are formed by combination of two or more atomic species from precursors in the plasma gas entrained atmospheric gases water added reagents and the sample matrix.' Polyatomic ions from the first three sources cited above are inherent to aqueous ICP-MS systems and the number of significant interfering species is small.2 The most important sources of polyatomic precursors come from the reagents used in the sample preparation mainly acids and the sample matrix itself.Interferences produced by acid used in the digestion or acidification of samples are widely known3 and can be avoided by appropriate selection of the acids. When the precursor is present in the sample matrix its interfering effect has been eliminated in some cases by separating the analyte from the interfering matrix c~mponent,'"~ although more general approaches are based on the control of the ICP-MS system itself. Polyatomic ions containing 0 and/or H can be reduced by reducing the amount of water vapour reaching the plasma using cooled spray chambers- or other desolvation systems.' Optimization of instrumental parameters involved in the con- trol of the plasma discharge (r.f. power and gas flow rate) has been applied to reduce formation of refractory oxides.'0,'' Mixed gas plasmas have been used with different aims in ICP-MS.Oxygen12 has been used mainly to assist in the introduction of organic solvents whilst heliurn,l3 hydrogen14 and methane15 have been used to improve analyte sensitivity. 0xygenl~9~~ and methane25 have been used to reduce polyatomic interferences. Many of these aspects have been reviewed recently.' In the present work two different approaches to overcome polyatomic interferences were studied selection of instrumental parameters in argon-only plasmas and use of argon-nitrogen * Presented at the XXVIII Colloquium Spectroscopicum Internationale (CSI) Post-Symposium 5th Surrey Conference on Plasma Source Mass Spectrometry Durham UK 4-6 July 1993.7 On leave from the Department of Analytical Chemistry University of Zaragoza Spain. .I To whom correspondence should be addressed. plasmas by adding nitrogen to the aerosol carrier gas flow. Special attention was paid to chlorine-based and argon dimer polyatomic interferences. The final aim of this work was to improve the determination of As and Se when part of a routine multi-element analysis procedure for foods and biological samples. Experimental Instrumentation A VG PQ1 inductively coupled plasma mass spectrometer (VG Elemental Winsford Cheshire UK) was used. The sample introduction system consisted of a fixed cross-flow nebulizer and a water cooled double-pass Scott-type spray chamber. Nitrogen was added to the aerosol carrier gas using the second mass-flow controller of the instrument.Standard instrumental and measurement parameters are shown in Table 1. Table 1 Default instrumental ICP-MS conditions Instrumental parameters R.f. power Reflected power Argon gas flow Outer Intermediate Aerosol carrier Sample uptake rate Spray chamber temperature Sample cone Skimmer Measurement parameters Measuring mode Range Number of channels Number of scan sweeps Dwell time Points per peak Collector type 1350 W € 7 w 14 1 min-l 0.5 1 min-l 0.755 1 min-' 0.70 ml min-' 10 "C 1.00 mm Nicone 0.75 mm Nicone Scanning 6.02-239.54 m/z 2048 100 320 ps 5 Pulse728 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 Reagents and Reference Materials Nitric acid (Aristar) and single-element standards (Spectrosol) were obtained from Merck (Poole Dorset UK). Multi-element standards (SPEX ICPMS1 -2 -3 and -4) were obtained from Instruments SA (Stanmore Middlesex UK).Hydrochloric acid (PrimaR) was obtained from FSA Lab. Supplies (Loughborough UK). Purified water (demineralized Milli-Q 18.2 Ma) was used throughout. Four certified materials covering a broad range of chlorine contents were analysed Oyster Tissue Standard Reference Material (SRM) 1566a and Peach Leaves SRM 1547 [National Institute of Standards and Technology (NIST) Gaithersburg MD USA] Lobster Hepatopancreas TORT-1 (National Research Council of Canada Ottawa Ontario Canada) Mussel Tissue MAM2/TM (International Atomic Energy Agency Monaco). Additionally a non-certified reference mate- rial Mixed Diet RM 8431a (NIST) was also analysed.Sample Preparation Digestions of reference materials were performed in stainless- steel pressure decomposition vessels (50 ml capacity). The poly(tetrafluoroethy1ene) (PTFE) liners were cleaned with nitric acid in a microwave oven (CEM 81D) (CEM Microwave Technology Buckingham UK) prior to use. The liners were first rinsed well under running tap water then nitric acid (2.0ml) added and the closed liners subjected to microwaves for 15 min at 25% power. The hot acid was washed out with running tap water and the acid cleaning procedure repeated for a further 20min. Finally the inner surfaces of the liners were rinsed thoroughly with purified water and the liners closed prior to use. For each reference material three samples of approximately 0.500 g were placed in cleaned liners and to one was added an appropriate amount of analyte as spike.Four blanks and one spiked blank were also included in the batch. A similar volume of purified water was added to the unspiked samples and blanks. Nitric acid ( 5 ml) was added to all of the liners. Overnight digestions (6 h) were performed in an air-circulating oven (Gallenkamp Loughborough UK Oven BS) at 150°C after which the vessels were completely cooled at -20 "C (in a temperature controlled cold room) for 30min. The digests were made up to 10.0ml with purified water and stored pending further dilutions. Procedure Experiments to study the effect of instrumental parameters on polyatomic species and analytes were performed using solu- tions containing either 5% (v/v) nitric acid or similar solutions fortified with 0.05% (m/v) chloride as hydrochloric acid.Indium (25 ng ml-l) was added as the internal standard. The selection of the chloride concentration was based on the maximum amount of the element expected in food sample digests. Multi-element solutions of 25 ng ml-I were used except for studies of oxide and doubly charged ions for which 250 ng ml-I solutions were used. Detection limits (LODs) and background equivalent concentrations (BECs) were measured at analyte concentrations of 25 ng ml-I (250 ng ml-' for Se) in 5% (v/v) nitric acid and 5% (v/v) nitric acid plus 0.05% (m/v) chloride matrices. Ion-lens potentials were re-optimized for each of the instrumental conditions studied using ll'In (5% nitric acid). Digested samples were diluted a further ten-fold keeping the nitric acid content at 5%.Internal standardization with In and Bi (50 ng ml-') was used. Analyses were undertaken with three different sets of instrumental conditions. These were (i) default conditions (Table 1); (ii) as ( i ) but with high aerosol carrier gas flow rate; and (iii) as (i) but with nitrogen added to the plasma. Results were corrected in each case for blanks and recoveries. Spikes consisted of the maximum amounts expected of each of the analytes as well as of major components (Na Mg P K and Ca) in the samples. Results and Discussion Effect of Instrumental Parameters on Analyte Intensity The effects of aerosol carrier gas flow rate r.f. power and spray chamber temperature on the "'In signal for an all-argon plasma are shown in Fig.l(a) and (b). For each parameter three different values were tested. These values were chosen to represent the range of values which could be used under normal operating conditions. The optimum value for each parameter was selected by choosing those which after optimiz- ation of the ion-lens potentials gave the maximum response on '151n. A spray chamber temperature of 10°C was selected because although the lowest value tested 1 "C gave a compar- able response it sometimes gave rise to condensation on the cooled surfaces. These optimum conditions corresponded to the default conditions given in Table 1 which are used for the day-to-day running of the instrument in this laboratory. The detection limits using these optimum conditions (ng rnl-l based on 30 of the blank) obtained for "V (O.l) 7 5 A ~ (0.4) 77Se (3) and 78Se (19) compare well with those reported by Hill st al,23 51V (0.96) 75As (3.45) 77Se (15) and 78Se (14) who used simplex optimized conditions for an all-argon plasma.Instrumental conditions which give the maximum elemental response in routine multi-element analyses are a compromise between analyte sensitivity and the possible polyatomic oxide and doubly charged ion interferences." In spite of or because of these compromise conditions several elements remain difficult to analyse by ICP-MS owing to polyatomic inter- ferences. Table 2 shows the isotopes and the interfering poly- atomic species studied in the present work. The larger background signal produced by polyatomic ions generally implies an increase in background noise at the m/z value of interest and hence a decrease in the detection capability.26 ," 0.40 0.50 0.60 0.70 0.80 0.90 1.00 0 10 L I I I I 0.50 0.60 0.70 0.80 0.90 1.00 Flow rate/l min-' Fig.1 Effect of aerosol carrier gas flow rate r.f.power and spray chamber temperature on '151n+ signal (a) spray chamber temperature maintained at 10°C; A 1350; B 1200 and C 1100 W (b) r.f. power maintained at 1350 W; A 1; B 10; and C 25 "CJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 729 Table 2 Polyatomic ions and interfering isotopes of elements studied in the present work Element m/z v 51 Cr 52 53 Mn 55 Polyatomic ion Zn 64 67 68 As 75 Se 74 76 77 78 Furthermore if the polyatomic ions are formed from precursors in the sample matrix for example chloride species a positive systematic error will result unless matrix-matched standards are used or the polyatomic ion signal is reduced. Effect of Instrumental Parameters on Polyatomic Ions In order to reduce polyatomic ion intensities the influence of aerosol carrier gas flow rate r.f.power and spray chamber temperature was studied. The effect of aerosol carrier gas flow rate and r.f. power on the signals of ArAr' ArCl' C10' and ClO,+ are shown in Fig. 2. Chlorine polyatomic species [Fig. 2(b) and (c)] showed a progressive signal decrease with increasing aerosol carrier gas flow rate and decreasing r.f. power. In Fig.2(u) it can be seen that ArAr+ shows a maximum with respect to aerosol carrier gas flow rate which moved to lower flow rates and signal intensity when the r.f.power was reduced although low levels of this polyatomic species were also obtained at the highest flow rate studied. These findings are in agreement with previous work carried out under similar instrumental conditions'' and illustrate that the formation mechanism of ArAr' is different to that of chlorine species. The influence of water loading on the plasma chemistry in ICP-MS has been pointed out by several workers with a reduction in refractory oxide and doubly charged ions being reported when cooled spray chambers were used as a way of reducing the aerosol water content.68 Hutton and Eaton6 have also reported a reduction of ArAr' when reducing the spray chamber temperature this effect being related to three-body collision processes in which oxygen could have a dominant role.The effect of spray chamber temperature on the signals of ArAr' ArCl' and C10+ is shown in Fig. 3. Whereas an increase in the ArAr' signal was observed when the spray chamber temperature was increased the opposite behaviour was observed for the chlorine polyatomic species. The behav- iour of the C10+ signal is difficult to explain because more oxygen would be expected to enter the plasma in the form of water vapour when the spray chamber temperature was increased. However both species containing C1 decreased when the spray chamber temperature increased suggesting that either another C1 species (e.g. containing H which should also increase with increased water loading) which was not moni- tored could have increased or that C1 species are genuinely reduced at increased spray-chamber temperatures.Of the instrumental parameters studied the biggest effect on polyatomic species was observed for aerosol carrier gas flow rate. The most significant reduction of approximately three orders of magnitude was obtained for ArC1+ [Fig. 2(b)] t 20 x lo3 A I t 0 v 1x102 U 10 1 I I I I I I x 1 O 5 I x lo4 1 10 ' 1 I I I 1 0.40 0.50 0.60 0.70 0.80 0.90 1.00 Flow rate/l min-' Fig.2 Effect of aerosol carrier gas flow rate and r.f. power on (a) 36Ar40Ar+ signal (A 1350 W and B 1100 W); (b) 40Ar35C1+ signal (A 1350 W and B 11OOW); chloride concentration 0.05%; and (c) 35C1160+ signal (A 1350W and B 11OOW) and 35Cl'60160+ signal (C 1350 W and D 1100 W); chloride concentration 0.05%.Spray chamber temperature 10 "C throughout whilst for the other polyatomic species studied intensities of around five times lower were obtained. In the case of ArAr' a reduction in signal intensity was also obtained by using spray chamber temperatures close to 0 "C (Fig. 3). Influence of Addition of Nitrogen to the Aerosol Carrier Gas Flow Nitrogen-argon plasmas have been reported to reduce the formation of polyatomic species.'6*18.20 M ontaser and Van HovenZ7 have suggested that the introduction of nitrogen into the aerosol carrier gas flow reduces the temperature of the gas in the axial channel because some of the energy is used to dissociate the diatomic injector gas. Houk et aLZ8 found that the ionization temperature in the central channel is reduced when nitrogen is added to the outer gas.This temperature drop could be responsible for both the reduction of the ionized polyatomic species which require more energy to ionize than atomic species,' as well as for the reported reductions in analyte sensitivities when nitrogen is introduced in the aerosol carrier gas.16 Evans and Ebdoni7 have argued that the reduction of ArAr' ArCl+ and ClCl+ in the presence of nitrogen in the aerosol carrier gas flow can be explained by the competitive formation of ArN' and/or ClN'. The effect of concentration of nitrogen in the aerosol carrier gas is shown in Fig. 4 for different aerosol carrier gas flow rates on the signals of ArAr+ [Fig. 4(u)] ArC1+ [Fig. 4(b)] and ClO' [Fig. 4(c)]. Overall a decrease in polyatomic ion730 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 4 I - 3 v) m C 3 0 4- 9 2 51 X > (0 C (5 ; 7 j 1 I I I I I I 1 0 5 10 15 20 25 30 TemperaturePC Fig.3 Effect of spray chamber temperature on polyatomic ion inten- sities aerosol carrier gas flow rate 0.755 1 min-'; r.f. power 1350 W. A 40A?5Cl+; B 35C1160+; and C 36Ar40Ar+ 1 x lo-' 1 x 1 0 ~ I x 10-2 t- ---4 I 10-3 I l X l O 2 ~ 1 \'" B I 1 x lo-' 1 0 2 4 6 8 1 0 [Nitrogen] (% v/v) Fig. 4 Effect of nitrogen concentration and total aerosol carrier gas flow rate (A 0.655; B 0.755 and C 0.855 1 min-l) on (a) 36Ar40Arf; (b) 40Ar35Cl+; and (c) 35Cl'60+ signals intensities was observed when both the nitrogen concentration and total aerosol carrier gas flow rates were increased. Analyte intensities were similarly reduced.Lower aerosol carrier gas flow rates were not studied because they produced high background signals across the whole m/z range. This increase was probably related t o an increase in the number of photons reaching the detector when nitrogen is introduced into the plasma.18 The background signal was reduced by four orders of magnitude for ArCl' whereas for C10' and ArAr' reductions of around two orders of magnitude were obtained. When nitrogen is introduced into the plasma the formation of NO' is increased," competing with other processes in the plasma involving oxygen. The effect on ArAr+ of adding nitrogen (8% aerosol carrier gas flow rate 0.755 1 min-l) at different spra;y chamber temperatures as a way of varying the amount of oxygen available from water reaching the plasma is shown in Fig.5. In the absence of nitrogen the formation of ArAr+ is influenced by the spray chamber temperature whereas it is independent of this variable in the presence of nitrogen. A similar effect was observed for ArO+ formation [Fig. 5(b)]. These results lend support to the role of oxygen6 in the formation process of ArAr'. Optimization of Signal-to-background and Signal-to-noise Ratios In the experiments reported above aerosol carrier gas flow rate spray chamber temperature and additions of nitrogen showed the most significant effects on the reduction of polya- tomic species. In order to improve the detection limits of the isotopes subjected to interferences a reduction of background signals must give an increase not only in signal-to-background ratios (SBR) but also of signal-to-noise ratios (SNR) since the SNR directly affects the detection limits.The SNR and SBR values for Se and Ar isotopes affected by ArAr+ and ArCl' interferences are shown in Table 3. The reduction of ArAr ' intensity observed when the spray chamber temperature was reduced from 10 to 1 "C produced a parallel improvement in the SBR although no significant improve- ments were observed in the SNR for 76Se and 78Se. The use of a high aerosol carrier gas flow rate produced a significant improvement for both the SBR and the SNR for the four isotopes. With respect to the addition of nitrogen a maximum reduction of the polyatomic signal at a nitrogen concentration of 8% is shown in Fig. 4(4 and (c) with a further reduction of ArAr' and ArCl+ when the aerosol carrier gas flow rate was increased from 0.755 to 0.855 1 min-'.On the other hand 2.5 2.0 "c - 1.5 + L 2 1.0 0.5 0 5.0 4.0 + 5 3.0 + 52 Q 2.0 1 .o I t B ,x-X x-x-x t L I I I I 1 I TemperaturePC 0 5 10 15 20 25 30 Fig.5 Influence of plasma type (A Ar and B Ar-N,) on the effect of spray chamber temperature on (a) ArAr+ and (b) ArO' signalsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 73 1 Table3 SBR and SNR values for As and Se isotopes affected by polyatomic interferences Aerosol carrier gas flow rate/l min-l 0.755 0.755 0.955 0.755 0.855 Spray chamber temperature/"C 10 1 10 10 10 Aerosol nitrogen (%) ~~~ ~~ ~ Isotope Value 0 0 Matrix 5% HN03:- 76Se SBR* 0.12 0.32 SNR* 5.44 6.90 78Se SBR 1.36 3.55 SNR 39.51 42.28 Matrix 5% H N 0 3 f 0.05% C l - "As SBR 1.07 SNR 44.03 77As SBR 0.16 SNR 6.62 0 0.78 7.55 10.84 56.15 130.82 382.53 24.38 40.90 8 0.33 12.43 5.54 86.57 217.11 816.08 16.48 79.00 8 2.21 11.58 20.74 74.16 389.37 626.88 32.67 65.50 * Analyte concentration 250 ng ml-I.n = 10. it can be seen from Table 3 that when nitrogen was added the best improvement in the SBR was obtained at the highest flow rate while the SNR values were higher at the default flow rate of 0.755 1 min-l. In both cases the data obtained for the SBR and SNR using the nitrogen-argon plasma were better than those obtained using the argon-only plasma at high aerosol carrier gas flow rate. Selection of Alternative Operating Conditions to Reduce Polyatomic Interferences In addition to the default conditions (Table l) two sets of instrumental conditions that favoured the formation of low levels of polyatomic species were selected.These sets were (i) default conditions but with a higher aerosol carrier gas flow rate (0.955 1 min-l); and (ii) default conditions plus the addition of nitrogen (8%) to the aerosol flow. The main drawbacks of using high aerosol carrier gas flow rates reside in the increase in the formation of oxide ions of refractory elements as well as of doubly charged ions of elements of low second ionization The effect of the three sets of instrumental conditions on the formation 1 XI02 S B A + 5 0 2 + I x l Y 10 1 lo-' in-2 * .I 400 500 600 700 800 900 Oxide bond strength/kJ mol-' Fig. 6 Influence of selected instrumental conditions on oxide forma- tion as a function of metal-oxide bond strength A default conditions; B default conditions with high aerosol carrier gas flow rate; and C default conditions with 8% nitrogen in the aerosol flow of the oxides of refractory elements is shown in Fig. 6.At the default aerosol carrier gas flow rate (graph A Fig. 6 ) percent- ages of MO+:M+ ranged from 0.03 to 1.5%. These values increased to more than 20% for the most refractory elements when the high aerosol carrier gas flow rate was used (graph B Fig. 6). The oxide levels were reduced in the presence of nitrogen to a maximum value of 0.6% probably owing to the competitive formation of NO+.18 The influence of same set of instrumental conditions (A default conditions; B default con- ditions with high aerosol carrier gas flow rate; and C default conditions with 8% nitrogen) was also investigated with respect to the formation of doubly charged ions as a function of the Table 4 Detection limits ( 3 4 for selected elements in an argon plasma at standard and high aerosol carrier gas flow rates and nitrogen-argon plasma.Matrix 5% HNOJ Aerosol carrier gas flow rate/l min-' 0.755 0.955 0.755 0.755 0.955 0.755 Aerosol nitrogen (YO) ~~ 0 0 8 0 0 8 Element m/z Solution LOD/ng ml-'* Sample LOD/pg g-'*t Li Be B Mg A1 V Cr Mn Fe c o Ni c u Zn As Se Br Y Mo Cd Sn La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hg Pb Th U 7 0.8 9 0.1 11 0.6 24 0.3 27 0.2 51 0.1 52 0.3 53 0.3 55 0.07 57 7 59 0.05 60 0.2 63 0.1 65 0.3 64 0.2 66 0.2 68 0.6 75 0.4 74 37 76 120 77 3 78 19 82 2 81 0.3 89 0.02 95 0.1 111 0.2 120 0.2 139 0.02 140 0.04 141 0.01 146 0.1 152 0.06 153 0.02 157 0.09 159 0.01 162 0.05 165 0.01 166 0.03 169 0.01 174 0.02 175 0.01 200 0.3 202 0.3 206 0.1 207 0.07 208 0.03 232 0.02 238 0.01 0.7 0.2 1 0.5 0.4 0.4 0.1 1 0.08 17 0.06 0.1 0.2 0.2 0.4 0.7 1 0.5 86 94 10 17 4 0.4 0.07 0.2 0.6 0.5 0.07 0.09 0.05 0.2 0.1 0.1 0.2 0.03 0.1 0.05 0.2 0.05 0.1 0.04 0.8 0.8 0.2 0.2 0.09 0.07 0.02 20 2 6 0.6 1.1 0.2 0.2 0.5 0.7 23 0.1 3 0.4 0.6 0.5 0.3 5 0.3 49 66 4 9 32 1.5 0.04 0.1 0.2 0.2 0.03 0.04 0.03 0.1 0.1 0.06 0.2 0.02 0.05 0.03 0.05 0.03 0.1 0.01 1 1 0.3 0.5 0.2 0.02 0.02 0.2 0.02 0.1 0.06 0.04 0.02 0.05 0.05 0.01 1 0.01 0.04 0.02 0.04 0.06 0.05 0.1 0.07 7 24 0.7 4 0.5 0.07 0.005 0.03 0.04 0.04 0.005 0.008 0.003 0.02 0.01 0.004 0.02 0.002 0.01 0.002 0.006 0.002 0.005 0.002 0.05 0.06 0.02 0.01 0.007 0.003 0.002 0.2 0.05 0.2 0.1 0.08 0.08 0.02 0.3 0.02 3 0.01 0.02 0.03 0.09 0.05 0.01 0.2 0.1 17 19 2 3 0.8 0.08 0.01 0.03 0.1 0.1 0.01 0.01 0.009 0.03 0.02 0.02 0.03 0.008 0.02 0.01 0.03 0.01 0.02 0.008 0.2 0.2 0.03 0.05 0.02 0.01 0.004 4 0.4 1.2 0.1 0.2 0.04 0.04 0.1 0.1 4 0.03 0.6 0.08 0.1 0.06 0.8 1 0.06 10 13 0.9 2 6 0.3 0.007 0.02 0.03 0.06 0.006 0.008 0.005 0.03 0.02 0.01 0.04 0.004 0.01 0.005 0.01 0.006 0.02 0.004 0.2 0.2 0.05 0.09 0.04 0.004 0.006 * n = 10.f Dilution 1 + 200.732 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 Table 5 BEC in ng ml-' produced by 0.05% chloride (m/v) on selected isotopes affected by polyatomic interferences. Argon plasma at standard and high aerosol carrier gas flow rate and nitrogen- argon plasma Aerosol carrier gas flow rate/l min-' 0.755 0.955 0.755 Aerosol nitrogen (%) Element m/z V 51 52 Cr 53 Zn 67 68 As 75 Se 74 76 0 292.4 4.0 741.9 20.3 1.7 249.3 22.4 360.6 0 47.1 1 .o 128.0 18.3 1.8 3.3 7.0 86.5 8 4.0 0.1 2.5 0.7 0.2 0.2 - 2.8* 10.3 second ionization potential.The mean values for percentages of M2+:M+ for each of the instrumental conditions were for A 0.17 (ranging from 0.004 to 0.9%0); for B 1.04 (ranging from approximately 0.02 up to 4%); and for C 3.13 (ranging from about 0.1 up to 10-20%). The increase in doubly charged ions in the presence of nitrogen can be also explained by the presence of NO+. It has been suggested that NO+ is involved in charge transfer ioniz- ation mechanisms.'* On the other hand the introduction of nitrogen into the plasma implies the formation of significant amounts of nitrogen polyatomic species which can produce additional interference problems.The most important potential interferences from polyatomic species of nitrogen for the elements studied in this work are included in Table 3.3 Trace amounts of Kr (interfering on s2Se) as well as Xe were detected in the nitrogen supply. 77 1548.2 14.6 0.7 Effects of Operating Conditions on the Limits of Detection and 78 28.0 5.4 1.8 Background Equivalent Concentrations * Negative value due to low sensitivity of the isotope and noisy The LODs in 5% nitric acid for elements across the m/z range using the three sets of operating conditions are shown in Table 4. The LODs for solutions were calculated as the concen- signal.Table 6 Determination of selected elements in certified reference material Mussel Tissue MAM2/TM using an argon plasma at standard and high aerosol carrier gas flow rate and a nitrogen-argon plasma. Reference chlorine content 8.71% Element B A1 V Cr Mn Fe c o Ni c u Zn As Se Y Mo Cd Sn La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hg Pb Th U m/z 11 27 51 52 53 55 57 59 60 65 67 68 75 77 78 82 89 95 111 120 139 140 141 146 152 153 157 159 162 165 166 169 174 175 202 208 232 238 Aerosol carrier gas flow rate/l min-' ~ ~~ 0.755 0.955 0.755 Aerosol nitrogen (%) 0 40.3 f 0.8 155+2 11.0t-0.6 1.62 f 0.03 49.5 + 2.9 64.8 f 0.2 312+7 0.905 k 0.024 1.32 & 0.03 10.0 f 0.1 156k 1 149+2 32.4 + 1.6 172+ 12 30.7 -f 0.4 2.56 f 0.05 0.181 kO.011 0.968 f 0.036 1.44f0.03 0.677 & 0.086 0.145 f 0.001 0.204 f 0.003 0.027 k 0,001 0.107 & 0.002 0.021 f 0.004 0.007 & 0.001 0.022 + 0.004 0.004 f 0.000 0.027 f 0.003 0.005 k 0,001 0.014+0.001 0.002 f o.Oo0 0.007 & 0.003 0.002 f 0.001 0.934 & 0.056 2.16 f 0.02 0.030 f 0.000 0.188+0.002 0 Measured content/pg g- '* 42.7 f 1.6 160+4 2.28 + 0.00 1.38+0.03 4.06k0.11 62.9 f 0.6 326f9 0.873 & 0.027 8.05 k 0.25 166 f 2 165 & 3 14.0 & 0.1 3.80 k 0.02 12.6 k 0.6 2.67 + 0.20 0.171 f0.004 0.592 f 0.066 1.66 f 0.03 0.619f0.038 0.130 & 0.087 0.190 f 0.003 0.033 f 0.OQ3 0.088 f 0.0 14 < LOD < LOD < LOD < LOD 0.035 f 0.007 < LOD < LOD < LOD < LOD < LOD 1.14 f 0.02% 2.09 f 0.01 0.03 1 + 0.0015 0.233 k 0.003 1.27 f 0.03 8 Certified content/pg g-' 40.3 f0.6 165+ 1 1.61 fO.OO 1.35f0.05 1.60f0.01 67.8 kO.1 327 f 0.4 0.898 k 0.005 1.29 f 0.05 7.54 & 0.15 159f 1 154fO 13.1f0.1 2.63 kO.10 3.59 f 0.19 < LOD 0.189 f 0.003 0.698 k 0.01 5 1.40 f 0.03 0.582 f0.014 0.146 k 0.007 0.196 f 0.008 0.03 1 & 0.003 0.093 k0.019 < LOD < LOD < LOD < LOD 0.029 f 0.006 0.005 f 0.001 0.022 + 0.006 < LOD < LOD 0.004 f 0.000 0.862 f0.149 2.13 & 0.08 0.03 5 & 0.001 0.190 -t 0.007 - 1.25 (0.95-1.62) - 67.1 (60.7-75.3) 256.2 (229.2-268.2) 0.88 (0.75-1.07) 7.96 (7.53-8.44) 156.5 (152.8-166.7) 12.8 (11.8-14.4) 2.27 (1.70-2.56) - - - - 1.32 (1.16-1.54) - - 0.95 (0.85-1.06) 1.92 (1.53-2.50)f * Average f standard deviation of duplicates.t Reference value.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL.9 73 3 Table 7 Determination of selected elements in certified reference material Lobster Hepatopancreas TORT-1 using an argon plasma at standard and high aerosol carrier gas flow rate and a nitrogen-argon plasma. Certified chlorine content 5.58% Element B A1 V Cr Mn Fe c o Ni c u Zn As Se Y Mo Cd Sn La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hg Pb Th U m/z 11 27 51 52 53 55 57 59 60 65 67 68 75 77 78 82 89 95 111 120 139 140 141 146 152 153 157 159 162 165 166 169 174 175 202 208 232 238 Aerosol carrier gas flow rate/l min-l 0.755 0.955 0.755 Aerosol nitrogen (%) 0 5.41 40.15 27.2 4 0.5 7.17 k 1.74 2.81 k 0.25 46.0 f 12.3 20.9 4 0.3 2121f 1 0.461 f 0.002 3.49 1f 0.54 409 4 3 1554 1 149-t-0 38.3 f 1.9 86.0f13.5 22.2 f 12.6 6.96 f 0.02 1.57 f0.03 1.33 f0.06 26.0 4 0.1 0.108 4 0.008 5.15 f 0.01 4.39 4 0.04 0.627 'r_ 0.002 2.54 & 0.03 0.333 4 0.003 0.069 0.005 0.371f0.011 0.041 & 0.002 0.190 f 0.002 0.036 & 0.001 0.086 4 0.001 0.008 'r_ 0.001 0.040 & 0.003 0.005 & 0.000 0.247 k 0.021 9.25 k0.41 0.007 & 0.000 0.098 f 0.006 0 Measured contentlpg g-'* 5.56 f 0.22 27.2 f 0.3 1.74 f 0.15 2.36 f 0.24 4.14f 1.69 20.4 k 0.3 201 k 4 0.45 1 -t 0.008 3.29 & 0.53 399 f 1 169F 1 163f3 27.7 f 0.0 8.14k0.32 11.4 f 0.5 5.73 -t 0.58 1.48 k 0.02 1.20-tO.08 29.7 f 0.2 0.130 f 0.01 6 3.94 f 0.04 3.47 f 0.053 0.5 52 h 0.000 2.13 f 0.03 0.311 fO.010 0.063 & 0.000 0.426 -t 0.022 0.047 rt 0.001 0.208 k 0.021 0.040 3_ 0.001 0.08 1 f 0.002 0.012 f 0.000 0.039 f 0.003 0.010 f 0.001 0.378 & 0.029 8.93 +_ 0.39 < LOD 0.1 14 & 0.009 8 5.82 f 0.06 29.3 & 0.6 1.44 & 0.00 2.66 & 0.35 2.64 & 0.43 21.7 f 0.4 222 f 2 0.480 f 0.014 3.55 f 0.66 409 & 1 160& 1 157+1 27.8 k 0.1 7.40 k 0.04 7.44f0.35 < LOD 1.66 f 0.01 1.29k0.06 26.3 & 0.4 0.1 35 & 0.013 5.11 k0.08 4.32 k 0.01 0.637 fO.010 2.51 kO.01 0.344 & 0.009 0.079 f 0.008 0.423 f 0.016 0.040 & 0.004 0.179 & 0.009 0.030 & 0.002 0.097 k 0.006 0.01 1 f 0.002 0.042 & 0.002 0.005 f 0.001 0.196 & 0.023 9.58 +_ 0.3 1 0.009 f 0.002 0.09 1 f 0.002 Certified content/pg g-' - 1.4f0.3 2.4 f 0.6 23.4 f 1.0 186f11 0.42 f 0.05 2.3 f 0.3 439 & 22 177 f 10 24.6 f 2.2 6.88 & 0.47 - - - 1.5h0.3 26.3 f 2.1 0.139 fO.011 __ - 0.330 f 0.060 10.4 f 2.0 - * Average f standard deviation of duplicates.tration of analyte that produced a signal equal to three times the standard deviation of the blank (ten measurements) and the LODs for the samples were calculated on basis of three standard deviations of the blank and corrected for the sample dilution (1 + 200) assuming a sample mass of 0.5 g. Although some increases in LODs were obtained when the alternative sets of operating conditions were used instead of the default conditions these were not significant except for isotopes affected by either nitrogen polyatomic interferences ("Mn and 68Zn) or by isobaric overlap from nitrogen contami- nants (82Se). Lithium Be B and Ni showed higher detection limits in the presence of nitrogen owing to the increase in the background intensities at their m/z values. In the case of Ni the higher background levels could have been due to the increased cone wear observed when nitrogen was used.Variations in residual Ni have also been observed with respect to spray chamber temperature by Hutton and Eatona6 In this work for argon-only plasmas relative standard deviations (RSDs n=10) of l-6% and 14% were obtained at default and high aerosol carrier gas flow rates respectively. Values between 10 and 20% were obtained for 74Se 76Se and 57Fe in both cases. For argon-nitrogen plasma conditions RSDs of 2-10% were measured for most elements 10-16% for "B 74Se 82Se and 'O0Hg with higher values for 7Li and 57Fe. The presence of chloride in the matrix contributes to the signals for isotopes affected by a chlorine polyatomic inter- ference. The magnitude of these contributions owing to the additional presence of 0.05% (m/v) chloride expressed as BEC of the analyte in the standard matrix (5% nitric acid) is shown in Table 5.An indirect effect of chloride on the argon dimer was also observed on 76Se and 78Se. Proper matrix matching could correct for these interferences. The LODs measured in the presence of chloride increased up to 35 times for isotopes affected by chlorine polyatomic interferences. Significant reductions of BECs were obtained for all these isotopes by using nitrogen. For As and Se an argon-only plasma and default conditions with increased aerosol carrier gas flow rate produced considerable reductions in the BECs. However the addition of nitrogen reduced the BECs even further.Multi-element Analysis of Reference Materials Reference materials covering a broad range of chlorine concen- trations (up to 8.71% for Mussel Tissue) were selected. Multi- element analyses were performed using the three sets of operating conditions. The results obtained are summarized in Tables 6-10. When nitrogen was added to the plasma agree- ment with the certified values was observed for approximately734 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 Table 8 Determination of selected elements in certified reference material Oyster Tissue SRM 1566a using an argon plasma at standard and high aerosol carrier gas flow rate and a nitrogen-argon plasma. Certified chlorine content 0.829% Aerosol carrier gas flow rate/l min-' 0.755 0.955 0.755 Aerosol nitrogen (YO) Certified content/pg 8-l Element B A1 V Cr Mn Fe c o Ni c u Zn As Se Y Mo Cd Sn La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hg Pb Th U 11 27 51 52 53 55 57 59 60 65 67 68 75 77 78 82 89 95 111 120 139 140 141 146 152 153 157 159 162 165 166 169 174 175 202 208 232 238 0 9.01 k 0.43 119+1 5.51 k0.50 1.39 k0.19 4.46 & 1.53 11.4 k 0.1 477 k 3 0.538 & 0.014 1.85 k 0.02 63.2 f 0.0 798 f 6 760 k 5 15.3f0.5 12.9 f 3.6 9.21 & 11.69 2.49 f 0.30 0.370f0.001 0.411 k0.183 4.15 '.0.03 2.25 f0.12 0.234 & 0.007 0.312+0.002 0.54 k 0.002 0.236 f 0.001 0.045 0.003 0.01 1 f 0.001 0.052 f 0.001 0.009 f 0.001 0.053 f 0.000 0.01 1 -1 0.002 0.033 f 0.003 0.005 5 0.001 0.029 k 0.001 0.006 & 0.002 < LOD 0.350 f 0.004 0.033 & 0.002 0.122f0.004 0 Measured content/pg g-l* 9.13k0.15 122f0 4.26 & 0.05 1.32 k 0.02 1.54k0.22 11.5 f 0.0 462 k 0 0.558 -10.007 1.94f0.00 67.9 f 0.7 935f4 943 k 2 13.3 k0.4 3.07 k 0.52 7.44 & 4.1 1 1.82k0.09 0.348 f 0.012 0.180 f 0.020 4.58 k0.23 2.20 k 0.1 3 0.164 f 0.005 0.259 f 0.005 0.045 k 0.006 0.205 k 0.836 0.052 k 0.1000 <LOT) 0.054 f 0.000 0.01 1 f 0.000 0.041 f 0.003 0.01 1 f 0.001 <LOT) < LOD 0.036 k 0.013 < Lor) < LOD 0.327 & 0.008 0.030 f 0.001 0.137 fO.002 8 9.06 f 0.21 127f2 4.67 k 0.06 1.33 k 0.00 1.40&0.12 11.2 k 0.1 477 k 8 0.547 k 0.000 1.94 kO.10 60.0 0.7 755 f 9 742 k 9 13.3 k0.4 1.84k0.07 2.33 0.60 < LOD 0.392 f 0.001 0.219 f 0.030 4.12k0.15 2.20 0.09 0.223 f 0.013 0.325 f 0.032 0.060 k 0.001 0.224 & 0.02 1 0.044 & 0.001 < LOD 0.069 f 0.003 0.1008 f 0.001 0.063 f 0.01 5 0.012 f 0.003 0.032$0.015 0.006 f 0.001 0.034 + 0.002 0.007 f 0.001 < LOD 0.372 k 0.045 0.035 f 0.002 0.1 14 k 0.006 - 202.5 f 12.5 4.68 f0.15 1.43 k 0.46 12.3 k 1.5 539 & 15 0.57 If 0.1 1 2.25 k 0.44 66.3 k4.3 830 57 14.0f 1.2 2.21 f 0.24 - - - 4.15k0.38 0.37 0.47 - - 0.067 0.017 - 0.0077 - 0.0642 k 0.0067 0.371 f 0.014 0.047 0.132 f0.012 * Average standard deviation of duplicates.7 Reference value. 70% of the isotopes affected by interference from chloride. The remaining 30% are either slightly above or below the certified values with no obvious trend for particular isotopes. The polyatomic interferences from nitrogen listed in Table 2 did not show any effect on the accuracy of the results for the isotopes affected except for 68Zn (Table 9). For the remaining elements good agreement with the certified values was gener- ally obtained for all three sets of operating conditions.Exceptions were a low value for A1 in Oyster Tissue under all three sets of operating conditions which is probably due to the presence of insoluble silicates of this element in the digest and on some occasions poor agreement with the certified values for the isotopes 78Se and "Se (Tables 6-10) 67Zn 95Mo and 208Pb (Table 9) 55Mn (Table 7 and 9) and 57Fe (Tables 6 7 8 and 10). Lower results were observed for some of the rare earth elements when the high aerosol carrier gas flow rate was used as shown for example by the results for Li and Ce from NIST Oyster Tissue and Peach Leaves SRMs.The recoveries of added analyte for rare earth elements ranged from 110 to 140% in spiked blanks and up to 200% in spiked samples when high aerosol carrier gas flow rate conditions were used. This is indicative of a severe matrix effect dependent upon the aerosol carrier gas flow rate and directly proportional to the oxide bond strength of the element. Preliminary results are summarized in Fig. 7 for the effect of increasing the amount of some matrix components (Mg Ca and Na) on the signals of Ce' CeO' and Ce2' when the aerosol carrier gas flow rate was 0.955 1 min-I using "'In as an internal standard (ideally the internal standard should have been an element with a similar oxide bond strength to the analyte). Under these conditions no significant effect was observed with respect to the addition of Mg but for Ca and especially for Na a significant increase of the effect on the signals for Ce' and decrease on CeO' was observed.Levels of doubly charged ions did not change significantly. These results suggest the inhibition of the oxide formation and the enhancement of the analyte signal owing to the competitive formation of the oxides of major cations of the matrix especi- ally Na (oxide bond strength Na > Ca > Mg > K). Similar effects were observed for the remainder of the rare earth elements in the presence of high concentrations of NaJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 73 5 Table 9 Determination of selected elements in certified reference material Peach Leaves SRM 1547 using an argon plasma at standard and high aerosol carrier gas flow rate and a nitrogen-argon plasma.Certified chlorine content 0.036% Aerosol carrier gas flow rate/l min-l ~~ 0.755 0.955 0.755 Aerosol nitrogen (%) Element B A1 v Cr Mn Fe c o Ni c u Zn As Se Y Mo Cd Sn La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Pb Th U Hg mlz 11 27 51 52 53 55 57 59 60 65 67 68 75 77 78 82 89 95 111 120 139 140 141 146 152 153 157 159 162 165 166 169 174 175 202 208 232 238 0 26.4 f 1.3 199 _+ 3 0.425 f 0.060 0.854 f 0.004 1.11 f0.16 89.4 f 1.3 264 f 2 0.082 f 0.003 0.733 & 0.008 3.92 f 0.12 19.6 f 0.1 18.3f0.4 < LOD < LOD < LOD < LOD 2.99 f 0.03 0.141 f0.034 0.032 f 0.014 0.066 f 0.004 9.12 f 0.02 11.6f0.1 1.81 f0.15 6.94 f 0.03 1.15f0.00 0.220 f 0.003 l .l l ~ O . 0 0 0.121 fO.000 0.566 f 0.008 0.087 f 0.002 0.226 2 0.002 0.025 2 0.001 0.133 f 0.001 0.019 f 0.002 < LOD 0.829 f 0.003 0.045 f 0.002 0.010 f 0.000 0 Measured content/pg g-l* 28.0 & 0.8 195 1- 3 0.319&0.014 0.774 -t 0.009 0.766 2 0.020 90.3 -t 1.0 269 & 1 0.086 & 0.006 0.854 & 0.024 3.75 20.10 25.3 f 1.8 23.8 f 2.0 < LOD < LOD <LOD < LOD 2.86 f 0.26 0.075 f 0.014 0.057 f 0.038 0.088 f 0.010 5.29 & 0.03 5.92 f 0.05 1.57 k0.09 4.78 f 0.10 1.14 k 0.01 0.182f0.013 1.18 fO.01 0.125 k 0.002 0.608 f 0.004 0.100f0.004 0.238 f 0.022 0.027 & 0.003 0.147 & 0.002 0.017 2 0.006 < LOD 0.796 & 0.01 1 0.043 f 0.005 0.016 k 0.003 8 28.0 f 0.2 219+1 0.354 f 0.012 0.885 f 0.005 0.865 f 0.025 96.3 f 0.9 280 f 3 0.078 f 0.008 0.707 rt 0.057 3.64 f 0.17 32.6 f 0.6 29.1 f 0.3 < LOD < LOD < LOD < LOD 3.20 f 0.01 0.051 f 0.003 0.020 f 0.005 0.071 f 0.000 10.4 f 0.2 12.7f0.1 1.91 fO.OO 7.83 f0.14 1.11 fO.OO 0.206 f 0.01 1 1.03 f 0.3 0.108 & 0.006 0.493 & 0.023 0.093 f 0.006 0.245 f 0.005 0.028 f 0.002 0.132f0.002 0.020 5 0.001 < LOD 0.855 f 0.026 0.044 & 0.003 0.01 1 f 0.001 Certified content/pg g-' 29f2 249 f 8 0.37 f 0.03 17 98-t-3 2207 - 0.07t 0.69 _+ 0.09 3.7 f 0.4 17.9 & 0.4 0.060 2 0.018 0.120 f 0.009 - - - 0.2t 0.060 f 0.008 0.03 < 0.027 9 t 107 77 I t 0.lt - 0.177 - - - - - - - 0.03 1 f 0.007 0.87 f0.03 0.05-l 0.015t * Average Ifi standard deviation of duplicates.t Reference value. (500pgml-'). The Na concentrations in the digests of the reference materials were not measured.130 110 - m S 0 v1 a m a M .- 2 90 Y - 70 50 m Ce' a CeO+ a Ce2+ 0 10 50 0 100 200 0 200 500 Mg Ca Na Concentration/mg I-' Fig.7 Effect of adding Mg Ca and Na on Ce' CeO' and Ce2+ signals at high aerosol carrier gas flow rate (0.955 1 min-l) Conclusions The results reported were obtained from one batch of the reference materials digested in duplicate in nitric acid using steel pressure decomposition vessels. Under normal default operating conditions when the instrument had been optimized for maximum signal response the addition of 8% nitrogen to the aerosol carrier gas flow improved the measurement of 51V 53Cr 67Zn and 68Zn (except for NIST SRM 1547 Peach Leaves) 75As and 77Se in five reference materials. Of the other 28 elements which were determined in five reference materials those which had certified or reference values were with few exceptions in good agreement with these values when meas- ured under default conditions or under default conditions with either nitrogen added to the aerosol carrier gas flow or with a higher aerosol carrier gas flow rate.The values for some rare earth elements when a high aerosol carrier gas flow rate was used were exceptions. The increase in detection limits when nitrogen was added was not so great as to cause any problems with the multi-element determinations.736 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY JUNE 1994 VOL. 9 Table 10 Determination of selected elements in reference material Mixed Diet SRM 8431a using an argon plasma at standard and high aerosol carrier gas flow rate and a nitrogen-argon plasma Aerosol carrier gas flow rate/l min-' 0.755 0.955 Aerosol nitrogen (Yo) 0.755 Recommended content/ Pg g-' Element B A1 v Cr Mn Fe c o Ni c u Zn As Se Y Mo Cd La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Hg Pb Th U m/z 11 27 51 52 53 55 57 59 60 65 67 68 75 77 78 82 89 95 111 139 140 141 146 152 153 157 159 162 165 166 169 174 175 202 208 232 238 0 3.94 f 0.52 4.82 f 0.05 0.333 k0.192 0.161 fO.041 1.07f0.65 7.86 f 0.02 41.4f 3.2 0.043 f 0.001 0.659 0.060 3.43 f 0.57 15.1 f0.8 14.8 f 0.1 1.85f0.31 4.52 f 1.87 1.22 & 1.56 0.432 & 0.182 0.005 & 0.001 0.305 f 0.028 0.034 f 0.01 8 0.010 f 0.000 0.009 f 0.002 < LOD < LOD < LOD < LOD < LOD < LOD < LOD 0.002 & 0.001 < LOD < LOD < LOD < LOD < LOD 0.136 f 0.044 < LOD 0.006 f 0.001 0 Measured content/pg g-'* 4.01 & 0.28 4.82 & 0.046 0.102+0.028 0.108 k 0.004 0.273 & 0.070 7.86 k 0.02 40.3 & 0.3 0.043 2 0.001 0.659 & 0.060 3.43 40.57 17.6f0.7 16.3k0.2 0.976 4 0.016 0.3 19 _+ 0.358 4.63 f 0.33 0.492 & 0.262 0.003 & 0.002 0.305 f 0.028 0.034 f 0.018 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD -= LOD < LOD 0.136 & 0.044 < LOD < LOD 8 4.14 f 0.38 5.18 k 0.09 0.053 f 0.001 0.091 -t 0.002 0.090 f 0,023 7.86 f 0.14 41.8 rt 0.6 0.046 f 0.002 0.716 f0.020 3.23 f 0.02 15.1 f0.2 15.0f0.3 0.992 f 0.028 0.23 1 f 0.042 0.908 f 0.089 < LOD 0.004 f 0.001 0.298 k 0.040 0.041 f 0.004 0.01 3 f 0.002 0.010f0.001 < LOD < LOD < LOD < LOD < LOD < LOD <LOD < LOD < LOD < LOD < LOD < LOD < LOD 0.170 f 0.024 < LOD < LOD - 4.39 f 1.07 0.102 k0.006 8.12 f0.31 37.0 f 2.6 0.038 f 0.008 0.644 f 0.1 51 3.36f0.33 17.0 f 0.6 0.924 & 0.344 0.242 f 0.030 - - - - 0.288 k 0.029 0.042 f 0.01 1 * Average f standard deviation of duplicates.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 References Evans E. 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Montaser A. and Fassel V. A. Appl. Spectrosc. 1983 5 425. Paper 3105893 J Received September 30 1993 Accepted February 14 1994