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Improvement of selenium determination in water by inductively coupled plasma mass spectrometry through use of organic compounds as matrix modifiers1
SAB 1742
Spectrochimica Acta Part B 52 (1997) 1825–1838
Improvement of selenium determination in water by inductively coupled plasma mass spectrometry through use of organic compounds as matrix modifiers1 I. Llorente, M. Go´mez, C. Ca´mara* Departemento Quı´mica Analı´tica, Facultad de Quı´mica, Universidad Complutense, 28040 Madrid, Spain Received 30 October 1996; accepted 12 May 1997
Abstract The effect of organic modifiers such as monofunctional alcohols, polyalcohols and organic acids on selenium response intensity and on polyatomic interferences by inductively coupled plasma–mass spectrometry (ICP–MS) has been evaluated and the possible mechanisms discussed. It has been proved that the addition of small amounts ( , 4%) of methanol, ethanol, sugar, ethylene glycol and tartaric acid, in combination with instrumental adjustments, significantly improves the Se/polyatomic interference signal ratio. The 40 Ar37 Cl + interference is significantly alleviated by addition of 4% lactic acid or 4% glycerol, the Se/ArCl ratio with the latter being even higher than in the absence of chloride. Se sensitivity and detection limits are improved about two-to six-fold by addition of the appropriate modifier and selecting the optimum working conditions. The proposed methods for Se determination in environmental samples, even those containing high chloride concentrations, are very promising and have been applied to some certified samples. q 1997 Elsevier Science B.V. Keywords: Selenium; ICP–MS; Organic modifiers; Waters; Polyatomic ion interferences
1. Introduction The determination of selenium in the environment and in human food is important because it is an essential micronutrient [1], but it is toxic in high concentrations [2]. Se plays a key part in biological antioxidation defence mechanisms via its incorporation in active enzymes [3]. Se determination at ultratrace level requires analytical techniques with very low detection limits such as inductively coupled plasma–mass spectrometry (ICP–MS) [4], hydride generation–atomic * Corresponding author. 1 Presented at the First Mediterranean Basin Conference on Analytical Chemistry, November 1995, Co´rdoba, Spain.
absorption spectrometry (HG–AAS) [5,6] or sample preconcentration before analysis [7]. Although the ICP–MS technique exhibits very good analytical performance for ultratrace determination because of its high sensitivity, accuracy and multielement capabilities, the high ionisation potential of selenium (9.8 eV) causes poor selenium ionisation efficiency in the plasma (about 30%) and so determinations have a relatively low sensitivity. In addition, the combination of argon atoms and light elements means that polyatomic species of total mass equal to that of the various Se isotopes are generated in the plasma, and the resolution capacity of the quadrupole is insufficient to resolve atomic and molecular species having the same nominal mass. Table 1 shows the main polyatomic and atomic
0584-8547/97/$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 7 )0 0 06 7 -0
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Table 1 Abundance of Se isotopes and their main interferences Se isotope
% Abundance
74 76
0.96 9.02
77
7.50
78
23.61
80
49.82
82
8.84
Interference 38
36
Ar Ar Ar36 Ar Ge 40 Ar37 Cl 38 Ar2 H 40 Ar38 Ar Kr 40 Ar2 Kr 32 16 S O3 40 Ar2 H2 Kr 34 16 S O3 40
% Abundance – 0.67 7.8 24.5 0.67 0.13 0.35 99.2 2.25 – 0.02 11.6 –
interferences in Se determination and their approximate abundance [8]. Since 40 Ar2+ , a high abundant species in the plasma, overlaps the most abundant 80 Se + isotope, other less abundant isotopes such as 77 Se, 78 Se or 82 Se have to be used in spite of additional possible interferences from 40 Ar37 Cl + and 38 Ar2 H + , 40 Ar38 Cl + and 40 Ar2 H2+ , respectively. As can be seen, the presence of significant amounts of chloride and sulphur results in the formation of 40 Ar37 Cl + and 13 16 + S O3 , respectively, hindering Se determination at masses 77 and 82, respectively. On the other hand, the low abundance of 40 Ar38 Cl + , 38 Ar2 H + and 40 Ar2 H2+ species in the plasma allows selenium determination at 77, 78 and 82 masses if its concentration is not too low and if the argon is not contaminated by krypton. Blank correction does not present any problem for monoatomic interferences such as Kr + or Ge +, but there may be for polyatomic interferences because their formation depends on the plasma and the measurement conditions as well as on the sample matrix. Some authors claimed that the use of organic compounds could lead to an important improvement, depending on the compound, of some of the following factors: transport efficiency in sample introduction or efficiency of nebulization, desolvation and ionisation of the analyte in the plasma, which provide a significant improvement on the Se sensitivity of selenium determination [9–13]. On the other hand, the addition of organic compounds such as 10% 2-propanol [11] and 4% ethanol [9] caused a relevant reduction of the interference
attributed to the formation of polyatomic ions such as 40 Ar37 Cl + , 40 Ar38 Cl + and 40 Ar2 H2+ . Alternatively, the addition of small amounts of a molecular gas like nitrogen, oxygen [11] or methane [14] to the nebulizer gas flow results in an important reduction of the polyatomic ions ArCl + and Ar +2, whereas methane reduces by 10 to 100 times the 40 Ar37 Cl + response [14], probably due to competitive formation of carbides, nitrides and oxides with Ar and/or Cl. This interference decrease could be attributed to the fact that molecular gases increase the kinetic temperature of the plasma, thereby leading to greater decomposition of polyatomic species [11]. In view of these precedents, and considering the inherent difficulties associated with the use of mixture of gases, we carried out a systematic evaluation of the improvement of the analytical performance for Se determination from both points of view, sensitivity and polyatomic interferences alleviation, by using different organic compounds (monofunctional alcohols, polyalcohols and organic acids). Optimal instrumental conditions will be evaluated for each family of the compounds tested. We consider that an improvement in both sensitivity and selectivity in the Se determination in water would be of significant interest in environmental chemistry due to the fact that Se concentration in water and other matrices is rather low and relatively high amounts of chloride are usually present.
2. Experimental 2.1. Instrumentation An ICP mass spectrometer (Fisons Instruments, Eclipse) equipped with a Fassel torch, a Gilson Miniplus-2 peristaltic pump, a Mainhard-type concentric glass nebulizer and a doublepass Scott-type spray chamber with surrounding water jacket at 78C with a recirculating refrigeration–heating system was used. The operating conditions are summarised in Table 2. A Perkin–Elmer 2380 AAS equipped with a deuterium background corrector using an air– acetylene flame (12–2 ml min −1) and an impact bead was employed. The instrument settings were: absorption line, 196.9 nm; monochromator spectral
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Table 2 ICP–MS operating conditions and acquisition parameters for pneumatic nebulization and flow injection Plasma R.F. Power
Solution uptake rate
Forward: 1150 and 1350 W Reflected , 5 W Coolant: 14 l min −1 Nebulizer: variable Auxiliary: 0.7 L min −1 1.0 ml min −1
Mass spectrometer Sampling cone Skimmer Analyser vacuum
Nickel, 1.0 mm orifice Nickel, 0.4 mm orifice 8.3 × 10 −5 mbar
Gas flow-rates
Acquisition parameters Measurement mode Dwell time Points per peak D.A.C. Injection volume Integration mode
Pneumatic nebulization peak jumping (77 Se, 78 Se, 82 Se) 2000 ms 21 3 – peak area
band pass, 0.7 nm; burner position and sample uptake were adjusted to a maximum signal; integration time, 2 s. A six-way injection valve (Omnifit), polytetrafluoroethylene (PTFE) tubes (0.5 mm i.d.) and connectors (Omnifit) were employed to construct the manifold for flow injection ICP–MS. 2.2. Reagents All matrix modifiers and reagents used were of analytical-reagent grade. Stock standard Se(IV) solution of 1000 mg l −1 was prepared by dissolving Na 2SeO 3 (Aldrich, 99%) in water. Working standard solutions of Se were prepared daily by appropriate dilution of the stock standard solution. Pure water from a Milli-Q system (Millipore) was used throughout for preparing sample solutions. Acids used for sample preparation were suprapure grade from Merck. 2.3. Operating procedures Different amounts of the matrix modifiers evaluated were directly added to each 2% HNO 3 Se standard solution to give concentrations within the 1–4% range. Organic modifier concentrations higher
Flow injection peak jumping (77 Se, 78 Se, 82 Se) 2000 ms 199 0 250 ml peak height
than 4% were not tested to avoid dirtying the sample cones. When the original matrix modifier was a liquid, the experiments were performed by direct pneumatic nebulization, and when solid the final Se solutions were introduced into the ICP–MS by a flow injection (FI) system to avoid possible dirtying and/or blocking of the sample cones and significant drift of the signal, owing to total dissolve solids (t.d.s.) higher than those recommended in ICP–MS. As it is known FI–ICP– MS allows the maximum t.d.s. in the sample to be 10 times higher than in continuous introduction. 2% HNO 3 was used as a carrier solution for flow injection and for continuous sample introduction. HNO 3 was selected instead of HCl or any other acid because in this medium the formation of polyatomic molecules (40 Ar37 Cl + , 32 S 16 O3+ and 34 S 16 O3+ ), which could interfere in Se determination, are lower than in any other acid. The signal for each of the three Se isotopes investigated (77, 78 and 82) was systematically recorded to evaluate the influence of the nebulizer gas pressure and forward power operating conditions, first with a 100 mg l −1 Se standard solution, which will be taken as the reference signal (100%), and then for the 100 mg l −1 Se solution spiked with each concentration of the organic modifier tested.
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The determination of Se by FI–ICP–MS produces a transient signal instead of the steady-state signal obtained by pneumatic nebulization. In our case only one unique isotope (78 Se) per injection, instead of three registered by pneumatic nebulization, was evaluated in each case because of the limitation of the ICP–MS used.
3. Results and discussion 3.1. Effect of organic modifiers on the selenium signal It has been previously reported that the introduction of small amounts of some organic solvents into the plasma modifies the physicochemical properties of the solution nebulized with regard to the aqueous sample, decreasing its viscosity and contributing to an overall smaller droplet size, which favours nebulization efficiency and improves desolvation of the aerosol in the plasma, resulting in an improved signal [15]. The effect of different organic compounds like monofunctional alcohols (methanol, ethanol, 1-propanol and 1-butanol), polyalcohols (ethylene glycol, glycerol, glucose and sugar) and organic acids (acetic, lactic, tartaric, citric and gluconic) on Se isotope (77, 78, 82) signals and on polyatomic interferences Ar +2, Ar 2H +, Ar 2H +2 and ArCl + at m/z 78, 77, 82 and 77, respectively, was evaluated. These organic compounds have been selected as representative of their families, having differing effects on the viscosity of the final solutions and therefore on nebulization and transport efficiency to the plasma. A standard solution of 100 mg l −1 Se in 2% HNO 3 was spiked with the above-mentioned modifiers in the 1–4% range, and the signal for the three Se isotopes studied was recorded as well as for their blanks. As already reported by Goossens et al. [9], the introduction of small quantities of organic solvents in a plasma results in the spatial shift of the zone of maximum ion density [16], so the zone of ion extraction from the plasma needed to be optimised again. Thus for each modifier concentration and RF power tested (1150 and 1350 W), it was necessary to optimise the instrumental conditions of nebulizer gas flow-rate, potential applied to the lens and ion sampling from the plasma, in order to achieve the maximum Se signal. Fig. 1 shows the effect of monofunctional alcohol
Fig. 1. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different monofunctional alcohols.
concentration on the 78 Se net signal (total less blank signal) at two different powers. The results are expressed as net signal enhancement, considering 100% the signal for a solution containing 100 mg l −1 of Se in 2% HNO 3 and in the absence of modifier. The shapes of the curves for the three Se isotopes (77, 78, 82) studied are similar in each case tested. This behaviour was similar for all the modifiers subsequently tested, so only the results for 78 Se are included as the most representative Se isotope. The curves obtained always have a maximum, but at a different concentration for each modifier. Methanol and ethanol are the modifiers which provide the highest increase in sensitivity at 1350 and 1150 W, respectively. The optimum concentration of methanol was 3% at 1350 W power and 2% at 1150 W and the
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increases in Se signal were about 5 and 3 times, respectively. The 2% ethanol modifier improved the analytical signal by about 4.5 and 3.5 times at 1350 and 1150 W, respectively. The optimum nebulizer gas flow-rates in 78 Se response for different monofunctional alcohol concentrations at 1150 and 1350 W are shown in Fig. 2. It is observed that as the modifier concentration increases, the maximum of the 78 Se signal shifts to lower values of nebulizer gas flow-rate. This effect is more notable at 1150 than at 1350 W. At 1150 W the nebulizer gas flow rate decreases with increasing length of the aliphatic chain of the alcohol, but it remains constant at 1350 W. This behaviour could suggest readier sample nebulization by increasing
Fig. 2. Effect of concentration of different monofunctional alcohols on the optimum nebulizer gas flow-rate in 78 Se determination by ICP–MS at 1150 and 1350 W.
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concentration and length of the aliphatic chain of the monofunctional alcohol, particularly at 1150 W. The fact that higher power requires higher nebulization flow rate to achieve the maximum Se net signal could be attributed to the shorter residence time of sample into the plasma and to a decrease of polyatomic ion formation as was observed by Evans and Ebdon [11] for Ar +2 and ArCl +. These results are in good agreement with those obtained by Mun˜oz et al. [13], but in the present study some more data such as the effect of nebulization flow rate has been evaluated and more alcohols tested at two different powers. Goossens et al. [9] have also reported an enhancement of the signal of about three times by using 4% ethanol. To check if the behaviour and signal increase observed for monofunctional alcohols depend on
Fig. 3. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different polyalcohols.
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carbon content and/or on –OH groups, a similar study was performed with polyalcohols containing several –OH groups. Fig. 3 shows that, in the modifier concentration range tested, the shapes of the curves obtained for the polyalcohols ethylene glycol and glycerol differ from those of their homologous monofunctional alcohols ethanol and propanol respectively, and a clear maximum is not reached. This could be attributed to a higher content of –OH groups in the molecules. Higher concentrations of modifier were not tested to avoid depositing carbon at the sample cones. At 1150 W, the modifiers glucose and sugar (introduced by FI) improved the signal notably and a plateau was obtained for a modifier concentration of 3%, in contrast with the linear increase in signal with increasing modifier concentration at 1350 W. The optimum improvement of the signal achieved with polyalcohol modifiers was obtained with ethylene glycol (4 times at both powers) and sugar (4.3 times at 1350 W). The influence of organic acid compounds on the net 78 Se signal is illustrated in Fig. 4. All the compounds tested significantly improved the Se response, but none of them gave a plateau at 1350 W. The optimum improvement of the signal achieved with organic acid modifiers was obtained with lactic and citric acid, which resulted in 2.5 and 2.1 times signal improvement at 1350 W, respectively. At 1150 W the maximum signal increase was about 4.2 times using 4% tartaric acid. The optimum nebulizer gas flow-rate achieved with increasing modifier concentration was not as critical for polyalcohols and organic acids as for monofunctional alcohols. Therefore, these compounds do not affect the nebulization and transport efficiency of the sample as much as the monofunctional alcohols do. As we can see the shapes of the curves of Figs 3 and 4, corresponding to the presence of polyalcohols and organic acids, are more similar than those obtained with monofunctional alcohols (Fig. 1). This finding could be attributed more to the different content of the functional groups than to the carbon content. However, in all cases a relevant increase of the analytical signal (ranging from two to five times) is achieved which could be attributed to a mixture of several factors, namely improvement of the transport efficiency as well as the desolvation, atomisation and ionisation process in the plasma. The changes in these
Fig. 4. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different organic acids.
parameters could vary from one group to another of the organic substances tested. However, we think that the predominant effect could be attributed to an improvement in the ionisation process. Introduction of these organic compounds into the plasma generates a high population of C + and/or carbon-containing polyatomic ions such as CO + and COH +. As carbon has a higher ionisation potential (11.26 eV) than Se (9.8 eV) the degree of ionisation of Se would then be improved by electron transfer from Se to the carbon or carbon-containing polyatomic ions. Hence, the addition of organic compounds would modify the ionisation equilibrium over a limited range of energy, changing the population of excited stated by collisions with C or carbon radicals.
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3.2. Effect of organic modifiers on sample transport efficiency To evaluate whether the extension of the signal increase observed for Se isotopes in ICP–MS in the presence of organic modifiers is caused by improved sample nebulization transport to the plasma and/or improved Se ionisation due to the presence of –C and –OH functional groups, the influence of these compounds on Se determination by AAS in an air– acetylene flame, where the sample is subjected to a similar nebulization and transport process, was evaluated. Due to the different mechanisms involved in the absorption process (atoms in the ground state) and in ICP–MS (atomic ions), comparison of the results provided by these two techniques may help to evaluate whether the signal increase is due to an improvement in the nebulization–transport of the sample and/or in the ionisation mechanism of the Se atoms in the plasma. Modifier amounts in the 2–20% range were spiked into a 4 mg l −1 standard Se solution and the instrumental conditions were optimised for any concentration of modifier tested in a reducer air–acetylene flame. Table 3 shows the optimum modifier concentrations and the signal increase in Se determination by AAS. The results are expressed as signal percentage considering 100% as the signal in the absence of modifier. Table 3 Optimum modifier concentration and signal increase in Se determination by AAS Modifier
S × 10 3/N m −1
None Methanol Ethanol Propanol Butanol Ethylene glycol Glycerol Glucose Sugar Acetic acetic Lactic acid Tartaric acid Citric acid S: Surface tension.
– 22.6 22.7 23.8 24.5 – – – – – – – –
% Modifier – 20 20 20 20 4 2 2 4 20 10 2 2
% Signal 100 205 196 196 180 131 121 98 90 133 121 98 102
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All the modifiers tested increase the Se signal from 1.2–2 times, except for glucose, sugar, citric and tartaric acids (originally solids) which do not increase the Se signal. Monofunctional alcohols gave the highest improvement, it being a maximum for 20% methanol (two times). However, this increase is lower than those observed in ICP–MS (five and three times at 1350 and 1150 W, respectively). The improvement in the signal slightly decreases with increasing the length of the aliphatic chain and then the surface tension of the spiked monofunctional alcohol. Thus the improvement in Se determination by ICP–MS in the presence of organic compounds, liquid in origin, is not only attributed to better nebulization–transport efficiency, but also to ready Se ionisation in the plasma. However, the signal increase observed in ICP–MS with modifiers, solid in origin, is only attributed to improved Se ionisation in the plasma. 3.3. Interference study To ascertain the effect of the organic modifiers tested on interference in Se determination by ICP– MS due to different polyatomic ions, the signals for Se (77, 78, 82), Ar 2H (77), Ar 2H 2 (82) and Ar 2 (78) were recorded versus nebulizer gas flow-rate for 100 mg l −1 Se standard solution in 2% HNO 3 and 2% ethanol (modifier which provided an important signal improvement in the previous section) at 1150 and 1350 W. Fig. 5 shows that nebulizer gas flow-rate plays an important role in the signal intensity of the three Se isotopes, but its effect is specially notable for polyatomic interferences. It can be observed (Fig. 5) that there is a significant decrease in the signal from polyatomic interferences with increasing nebulizer gas flow-rate. When ethanol is added, the maximum of all Se isotopes signals increases and shifts to lower nebulizer gas flow-rates. However, the decrease in intensity with increasing argon nebulization flow rate due to polyatomic interferences is much more pronounced than the decrease in the Se signal. This behaviour could be justified by two mechanisms, one affecting the interferences and the other the Se signal. We think that the decrease of the interference level with increasing flow rate, previously observed by other authors with methanol [13], ethanol [9], and 2-propanol [11], is due to a decrease of sample residence time into the plasma. This effect is
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Fig. 5. Effect of 2% ethanol and nebulizer gas flow-rate on Se signal and polyatomic interferences: (a) 1150 W; (b) 1350 W.
more marked for 38 Ar2 H and 40 Ar2 H2 affecting 77 Se and 82 Se isotopes, respectively. However, two different zones in the figures could be observed, the first at low nebulizer gas flow-rate up to about 1 ml min −1, in which the addition of ethanol decreases the interference level, and the second in which the interference level decreasing is more marked without modifier by increasing the nebulizer gas flow rate. Hence, the predominant effect at flow rate lower than 1 ml min −1 could be the competition of formation of the carbon-containing compounds ions such as ArC +, CCl +, CO + and COH + with the polyatomic interferences evaluated, the residence time at higher
flow rate being the most marked effect (more noticeable at 1150 W than at 1350 W). The increase of the Se signal in the presence of ethanol and its shift to lower nebulizer gas flow-rate could be attributed to the improvement of sample transport, desolvation and ionisation into the plasma. At 1350 W (Fig. 5(a)) the maximum Se signal occurs at higher nebulizer gas flow-rates than when 1150 W is used (Fig. 5(b)), both in the presence and absence of modifier. In ICP–MS it is important to have not only a sufficiently high signal for the analyte ion to be quantified, but also a high ion signal/background ratio. Therefore,
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the Se signal/background ratio was measured for the three Se isotopes and for the optimum concentration of the organic modifier obtained in the above studies, to establish the optimum conditions giving the highest Se signal with low polyatomic interferences. Fig. 6 shows the increase in the Se signal/background ratio (DS/B) for a 100 mg l −1 Se standard solution in 2% HNO 3 spiked with the modifiers. For the monofunctional alcohols tested at 1150 W, the highest ratio is obtained for 2% ethanol, which gives 26- and 20-fold improvements for 82 Se and 77 Se, respectively. Therefore, Ar 2H (77) and Ar 2H 2 (82) are more strongly affected by this modifier than Ar 2 (78)
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interference. At 1350 W the results obtained are more variable. The highest increase in 78 Se is obtained for 3% methanol (50-fold) and in 82 Se for 2% ethanol (80-fold), and there is a decrease in 77 Se, except for ethanol. For polyalcohols at 1150 W, 4% glycerol increases the ratio for 78 Se (Ar 2) 10-fold, but decreases it for 77 Se (Ar 2H) and 82 Se (Ar 2H 2). Glucose and sugar modifiers at 3% do not produce practically any variation and ethylene glycol at 4% worsens the results. At 1350 W 4% glycerol produces the optimum improvement for all isotopes (15–30-fold). For the family of organic acids at 1150 W, lactic
Fig. 6. Increase in Se the signal/background ratio for 100 mg l −1 Se spiked with modifiers: (a) 1150 W; (b) 1350 W.
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acid at 4% produces the optimum signal-to-noise ratio: 65-, 55- and 15-fold increases for Se 77, 82 and 78, respectively, and the increase is negligible for other acids. At 1350 W 4% acetic and 4% lactic acids improve the ratio 25–35-fold for Se 77 and 82, respectively, but for 78 Se it decreases. A polyatomic interference that strongly affects 77 Se determination in samples with high chloride content (sea water, urine, etc.), and even annuls the analytical signal, is that of 40 Ar37 Cl + . The possibility of decreases or elimination of this interference by the addition of organic modifier was evaluated by measuring the signal of 100 mg l −1 Se standard solution spiked with 2000 mg l −1 chloride as HCl and 2%
ethanol. Fig. 7 shows the effect of this modifier and nebulizer gas flow-rate on the 77 Se + and ArCl + signal at the two powers. There is a dramatic increase in background in the presence of chloride, which is much more marked than those previously obtained + + for 38 Ar2 H . Therefore, the 38 Ar2 H contribution was not considered in this study. However, the 77 Se + =ArCl + ratio at 1.0 and 1.2 l min −1 nebulizer gas flow-rates at 1150 and 1350 W, respectively, in samples spiked with 2% ethanol increases considerable with respect to that obtained without modifier. The different behaviour of Ar +2, Ar 2H +2 and Ar 2H + (Fig. 5) from that of ArCl + (Fig. 7) indicates that their
Fig. 7. Effect of 2% ethanol and nebulizer gas flow-rate on 77 Se and 40 Ar37 Cl + signals at 1150 and 1350 W.
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Fig. 8. Increase in the 77 Se signal/background ratio for Se + Cl + modifier with respect to 77 Se signal/background ratio for Se + Cl at 1150 and 1350 W.
mechanisms of formation and/or ionisation are different. Similar experiments were performed for the rest of the most suitable modifiers tested previously. Fig. 8 and Fig. 9 illustrate the increase in 77 Se + =ArCl + ratio caused by a 100 mg l −1 Se standard solution spiked with 2000 mg l −1 chloride as HCl and modifier with respect to the Se signal/background ratio for a solution of selenium with chloride and a Se solution without chloride at 1150 and 1350 W, respectively. The addition of any modifier significantly increased the 77 Se + =ArCl + signal ratio at the two powers tested (Fig. 8). This increase is higher (20–30 − fold) for 4% glycerol and 4% lactic acid. The 77 Se + =ArCl + signal
ratios in the presence of modifier are in some cases higher than those obtained for Se solution without chloride, especially 4% glycerol (Fig. 9). There was a significant increase in the 82 Se background when 2000 mg l −1 of chloride as HCl was spiked into the 100 mg l −1 Se standard solution. Surprisingly, this high background was practically eliminated by addition of 2% ethanol. This was possibly due to contamination of the HCl acid used with bromide, and consequent formation in the plasma of the 81 BrH + (m/z = 82) polyatomic interference, which was finally controlled by this organic solvent and instrumental adjustments. The relevant alleviation of the evaluated
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Fig. 9. Increase of the 77 Se signal/background ratio for Se + Cl + modifier with respect to 77 Se signal/background ratio at 1150 and 1350 W.
interferences can be explained as suggested by Evans and Ebdon [11], by the reduction of the ionisation temperature and increase of the kinetic temperature in the plasma by the organic modifiers, which results in an important suppression of their ionisation and a relevant increase of their breakdown, respectively. These results obtained also agree with those previously reported by Evans and Ebdon [11] and Goossens et al. [9], who report that the addition of small amounts of organic modifiers do not completely eliminate the spectral interferences, but shift their optimum formation with respect to Se and, hence, there is no total elimination of the formation of polyatomic species. The addition of small amounts of
organic modifiers and a judicious choice of operating conditions permit an accurate and precise Se determination at non-ideal gas flow-rate. It can be concluded that the use of 4% lactic acid as modifier at 1150 W in combination with an optimum nebulizer gas-flow rate and plasma sampling depth allows analytical determination of any Se isotope, even in the presence of high concentrations of chloride. 40 Ar37 Cl + interference is significantly alleviated by addition of 4% glycerol and 4% lactic acid. 3.4. Analytical performance To evaluate the analytical performance of the
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Table 4 Percentage of increase in selenium calibration slope by addition of modifiers with respect to Se determination in the absence of modifiers Modifier
1150 W 77
None 3% Methanol 2% Ethanol 4% Ethylene glycol 4% Glycerol 4% Sugar 4% Acetic acid 4% Lactic acid 4% Tartaric acid
Se
100 288 192 280 252 – 390 507 –
1350 W 78
82
Se
100 302 200 246 246 193 407 550 430
77
Se
100 302 213 271 248 – 390 540 –
proposed organic matrix modifiers, several Se standard solutions in the range 0–100 and 0–200 mg l −1 for 1150 and 1350 W, respectively, were spiked with the selected modifiers and the analytical signal was measured (in triplicate), as well as their blanks. The net calibration graphs were linear with correlation coefficients of 0.999 or 0.9999 in all cases. Table 4 shows the increase in the calibration slope upon addition of modifiers with respect to Se in the absence of modifier (taken as 100%). For the modifiers solid in origin (tartaric acid and sugar), which were introduced by an FI system (one isotope only is measured in each run), only 78 Se was determined, since it is the most important from the point of view of analytical determination. Marked increases in the calibration slopes were obtained for methanol at 1350 W, acetic acid at both powers and tartaric acid at 1150 W. The best results were achieved for lactic acid (507–590%) at both powers. There was significant reduction in the detection
Se
100 334 224 231 225 – 480 570 –
78
82
Se
100 488 263 273 238 389 480 590 –
Se
100 429 225 230 225 – 477 570 –
limits for all Se isotopes studied and modifiers selected within the 1.5–4.0-fold range, which was accompanied by a reduction in relative standard deviation (R.S.D.). Examples of this reduction are 2% methanol at 1150 W with detection limits (IUPAC [17]) of 0.4, 0.2 and 0.3 mg l −1 Se and R.S.D. values for five determinations of a 50 mg l −1 Se standard solution of 1.0, 0.4 and 0.3% for Se 77, 78 and 82, respectively, and 4% lactic acid at 1350 W with detection limits of 0.3 mg l −1 for all isotopes and R.S.D. of 3.1, 2.1 and 2.8% for Se 77, 78 and 82, respectively. 3.5. Application The selected modifiers were employed for Se determination by using the three isotopes (77, 78 and 82) at 1150 and 1350 W in a water sample with certified Se content (35.50 6 5.14 mg l −1) from A.P.G. (Analytical Products Group, Inc.), 2730 Washington Blvd. Belpre, OH 45714. The second sample was a BCR
Table 5 Determination of selenium (mg l −1) in certified water samples in the presence of modifiers Power/W a
1150 1150 a 1150 a 1350 a 1350 a 1150 b
Modifier 4% ethylene glycol 3% tartaric acid 4% sugar 3% methanol 4% sugar 4% lactic acid
x¯ 6 j, n = 3. Certified value: 35.50 6 5.14 mg l −1 Se. b Certified value: 80 6 5 mg l −1 Se. a
77
Se 33.12 6 0.37 – – 37.20 6 0.10 – 83.13 6 3.13
78
Se 33.22 6 0.87 37.01 6 0.70 35.35 6 0.69 36.31 6 0.59 34.91 6 2.75 –
82
Se 33.65 6 0.25 – – 37.90 6 1.11 – –
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(European Community Bureau of Reference) future candidate reference material with an established Se content of 80 6 5 mg l −1 and whose chloride content is 2000 mg l −1 as NaCl. This sample was analysed employing 4% lactic acid as matrix modifier, which was one of the best-performing modifiers for determining 77 Se + at low power in samples with high chloride content, decreasing the ArCl + interference. As shown in the results listed in Table 5, which were obtained by direct determination from the corresponding calibration curves in the presence of the respective modifiers, in all cases there is a good correlation between the results obtained and the certified values. This agreement validates the proposed methods for Se determination by ICP–MS.
4. Conclusions The addition of small amounts ( , 4%) of certain organic modifiers, such as monofunctional alcohols, polyalcohols and organic acids, in combination with nebulizer gas flow-rate and sampling depth adjustment, increases the sensitivity of Se determination by ICP–MS from two to five times, with methanol, ethanol, sugar, ethylene glycol and tartaric acid giving the highest increases. This significant improvement could be attributed to an increase in the efficiency of Se ion formation by electron transfer from Se to the carbon or carboncontaining polyatomic ions formed, and in the case of monofunctional alcohols, also to an improvement in the transport efficiency and desolvation of the sample in the plasma. Monofunctional alcohols differ from polyfunctional alcohols and organic acids in their effect on net signal increase, which could be attributed more to the different content of functional groups than to the carbon content. Lactic acid at 1150 W gives the highest Se signal/ background ratio from the three Se isotopes studied. 40 Ar37 Cl + interference is significantly alleviated by addition of 4% glycerol and 4% lactic acid. The important alleviation of the ArCl, Ar 2H 2, Ar 2H and 40 Ar38 Ar interferences could be explained by a competitive formation with the polyatomic ions
ArC +, CCl +, CO + and COH +. Another possibility is that these organic modifiers reduce the ionisation temperature and increase the kinetic temperature in the plasma resulting in an important suppression of their ionisation and in an increase of their breakdown, respectively. These conditions enable the accurate determination of Se in environmental and biological samples, as shown by the excellent agreement between the certified values and the results achieved for water samples of different nature.
Acknowledgements The authors thank the Direccio´n General de Investigacio´n Cientifica y Te´cnica (project PB950366-C02-01) for financial support.
References [1] S.C. Apte, A.G. Howard, Mar. Chem. 20 (1986) 119. [2] S.E. Raptis, G. Kaiser, G. To¨lg, Fresenius Z. Anal. Chem. 316 (1983) 105. [3] Y. Shibata, M. Morita, K. Fuwa, Adv. Biophys. 28 (1992) 31. [4] G. Tyler, Spectrosc. Europe 7/1 (1995) 14. [5] M.G. Cobo, M.A. Palacios, C. Ca´mara, Anal. Chim. Acta 238 (1993) 386. [6] M. Janghorbani, B.T.G. Ting, Anal. Chem. 61 (1989) 701. [7] A. Larraya, M.G. Cobo-Ferna´ndez, M.A. Palacios, C. Ca´mara, Fresenius J. Anal. Chem. 350 (1994) 667. [8] S.H. Tan, G. Horlick, Appl. Spectrosc. 4 (1986) 445. [9] J. Goossens, F. Vanhacke, L. Moens, R. Dams, Anal. Chim. Acta 280 (1993) 137. [10] E.H. Evans, L. Ebdon, J. Anal. Atom. Spectrom. 4 (1989) 299. [11] E.H. Evans, L. Ebdon, J. Anal. Atom. Spectrom. 5 (1990) 425. [12] P. Allain, L. Jaunault, Y. Mauras, J.M. Mermet, T. Delaporte, Anal. Chem. 63 (14) (1991) 1497. [13] R. Mun˜oz, C.R. Que´tel, F.X. Donard, J. Anal. Atom. Spectrom. 10 (1995) 865. [14] S.J. Hill, M.J. Ford, L.J. Ebdon, Anal. At. Spectrom. 7 (1992) 1157. [15] S. C. K. Shum, S.K. Jhonson, H. Pang, R.S. Honk, Appl. Spectrosc. 47 (1993) 575. [16] F. Vanhaecke, R. Dams, C. Vandecasted, J. Anal. At. Spectrom. 8 (1993) 433. [17] G.L. Lousand, J. D. Winefodner, Anal. Chem. 55 (7) (1983) 713A.
Spectrochimica Acta Part B 52 (1997) 1825–1838
Improvement of selenium determination in water by inductively coupled plasma mass spectrometry through use of organic compounds as matrix modifiers1 I. Llorente, M. Go´mez, C. Ca´mara* Departemento Quı´mica Analı´tica, Facultad de Quı´mica, Universidad Complutense, 28040 Madrid, Spain Received 30 October 1996; accepted 12 May 1997
Abstract The effect of organic modifiers such as monofunctional alcohols, polyalcohols and organic acids on selenium response intensity and on polyatomic interferences by inductively coupled plasma–mass spectrometry (ICP–MS) has been evaluated and the possible mechanisms discussed. It has been proved that the addition of small amounts ( , 4%) of methanol, ethanol, sugar, ethylene glycol and tartaric acid, in combination with instrumental adjustments, significantly improves the Se/polyatomic interference signal ratio. The 40 Ar37 Cl + interference is significantly alleviated by addition of 4% lactic acid or 4% glycerol, the Se/ArCl ratio with the latter being even higher than in the absence of chloride. Se sensitivity and detection limits are improved about two-to six-fold by addition of the appropriate modifier and selecting the optimum working conditions. The proposed methods for Se determination in environmental samples, even those containing high chloride concentrations, are very promising and have been applied to some certified samples. q 1997 Elsevier Science B.V. Keywords: Selenium; ICP–MS; Organic modifiers; Waters; Polyatomic ion interferences
1. Introduction The determination of selenium in the environment and in human food is important because it is an essential micronutrient [1], but it is toxic in high concentrations [2]. Se plays a key part in biological antioxidation defence mechanisms via its incorporation in active enzymes [3]. Se determination at ultratrace level requires analytical techniques with very low detection limits such as inductively coupled plasma–mass spectrometry (ICP–MS) [4], hydride generation–atomic * Corresponding author. 1 Presented at the First Mediterranean Basin Conference on Analytical Chemistry, November 1995, Co´rdoba, Spain.
absorption spectrometry (HG–AAS) [5,6] or sample preconcentration before analysis [7]. Although the ICP–MS technique exhibits very good analytical performance for ultratrace determination because of its high sensitivity, accuracy and multielement capabilities, the high ionisation potential of selenium (9.8 eV) causes poor selenium ionisation efficiency in the plasma (about 30%) and so determinations have a relatively low sensitivity. In addition, the combination of argon atoms and light elements means that polyatomic species of total mass equal to that of the various Se isotopes are generated in the plasma, and the resolution capacity of the quadrupole is insufficient to resolve atomic and molecular species having the same nominal mass. Table 1 shows the main polyatomic and atomic
0584-8547/97/$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved PII S 0 58 4- 8 54 7 (9 7 )0 0 06 7 -0
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Table 1 Abundance of Se isotopes and their main interferences Se isotope
% Abundance
74 76
0.96 9.02
77
7.50
78
23.61
80
49.82
82
8.84
Interference 38
36
Ar Ar Ar36 Ar Ge 40 Ar37 Cl 38 Ar2 H 40 Ar38 Ar Kr 40 Ar2 Kr 32 16 S O3 40 Ar2 H2 Kr 34 16 S O3 40
% Abundance – 0.67 7.8 24.5 0.67 0.13 0.35 99.2 2.25 – 0.02 11.6 –
interferences in Se determination and their approximate abundance [8]. Since 40 Ar2+ , a high abundant species in the plasma, overlaps the most abundant 80 Se + isotope, other less abundant isotopes such as 77 Se, 78 Se or 82 Se have to be used in spite of additional possible interferences from 40 Ar37 Cl + and 38 Ar2 H + , 40 Ar38 Cl + and 40 Ar2 H2+ , respectively. As can be seen, the presence of significant amounts of chloride and sulphur results in the formation of 40 Ar37 Cl + and 13 16 + S O3 , respectively, hindering Se determination at masses 77 and 82, respectively. On the other hand, the low abundance of 40 Ar38 Cl + , 38 Ar2 H + and 40 Ar2 H2+ species in the plasma allows selenium determination at 77, 78 and 82 masses if its concentration is not too low and if the argon is not contaminated by krypton. Blank correction does not present any problem for monoatomic interferences such as Kr + or Ge +, but there may be for polyatomic interferences because their formation depends on the plasma and the measurement conditions as well as on the sample matrix. Some authors claimed that the use of organic compounds could lead to an important improvement, depending on the compound, of some of the following factors: transport efficiency in sample introduction or efficiency of nebulization, desolvation and ionisation of the analyte in the plasma, which provide a significant improvement on the Se sensitivity of selenium determination [9–13]. On the other hand, the addition of organic compounds such as 10% 2-propanol [11] and 4% ethanol [9] caused a relevant reduction of the interference
attributed to the formation of polyatomic ions such as 40 Ar37 Cl + , 40 Ar38 Cl + and 40 Ar2 H2+ . Alternatively, the addition of small amounts of a molecular gas like nitrogen, oxygen [11] or methane [14] to the nebulizer gas flow results in an important reduction of the polyatomic ions ArCl + and Ar +2, whereas methane reduces by 10 to 100 times the 40 Ar37 Cl + response [14], probably due to competitive formation of carbides, nitrides and oxides with Ar and/or Cl. This interference decrease could be attributed to the fact that molecular gases increase the kinetic temperature of the plasma, thereby leading to greater decomposition of polyatomic species [11]. In view of these precedents, and considering the inherent difficulties associated with the use of mixture of gases, we carried out a systematic evaluation of the improvement of the analytical performance for Se determination from both points of view, sensitivity and polyatomic interferences alleviation, by using different organic compounds (monofunctional alcohols, polyalcohols and organic acids). Optimal instrumental conditions will be evaluated for each family of the compounds tested. We consider that an improvement in both sensitivity and selectivity in the Se determination in water would be of significant interest in environmental chemistry due to the fact that Se concentration in water and other matrices is rather low and relatively high amounts of chloride are usually present.
2. Experimental 2.1. Instrumentation An ICP mass spectrometer (Fisons Instruments, Eclipse) equipped with a Fassel torch, a Gilson Miniplus-2 peristaltic pump, a Mainhard-type concentric glass nebulizer and a doublepass Scott-type spray chamber with surrounding water jacket at 78C with a recirculating refrigeration–heating system was used. The operating conditions are summarised in Table 2. A Perkin–Elmer 2380 AAS equipped with a deuterium background corrector using an air– acetylene flame (12–2 ml min −1) and an impact bead was employed. The instrument settings were: absorption line, 196.9 nm; monochromator spectral
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Table 2 ICP–MS operating conditions and acquisition parameters for pneumatic nebulization and flow injection Plasma R.F. Power
Solution uptake rate
Forward: 1150 and 1350 W Reflected , 5 W Coolant: 14 l min −1 Nebulizer: variable Auxiliary: 0.7 L min −1 1.0 ml min −1
Mass spectrometer Sampling cone Skimmer Analyser vacuum
Nickel, 1.0 mm orifice Nickel, 0.4 mm orifice 8.3 × 10 −5 mbar
Gas flow-rates
Acquisition parameters Measurement mode Dwell time Points per peak D.A.C. Injection volume Integration mode
Pneumatic nebulization peak jumping (77 Se, 78 Se, 82 Se) 2000 ms 21 3 – peak area
band pass, 0.7 nm; burner position and sample uptake were adjusted to a maximum signal; integration time, 2 s. A six-way injection valve (Omnifit), polytetrafluoroethylene (PTFE) tubes (0.5 mm i.d.) and connectors (Omnifit) were employed to construct the manifold for flow injection ICP–MS. 2.2. Reagents All matrix modifiers and reagents used were of analytical-reagent grade. Stock standard Se(IV) solution of 1000 mg l −1 was prepared by dissolving Na 2SeO 3 (Aldrich, 99%) in water. Working standard solutions of Se were prepared daily by appropriate dilution of the stock standard solution. Pure water from a Milli-Q system (Millipore) was used throughout for preparing sample solutions. Acids used for sample preparation were suprapure grade from Merck. 2.3. Operating procedures Different amounts of the matrix modifiers evaluated were directly added to each 2% HNO 3 Se standard solution to give concentrations within the 1–4% range. Organic modifier concentrations higher
Flow injection peak jumping (77 Se, 78 Se, 82 Se) 2000 ms 199 0 250 ml peak height
than 4% were not tested to avoid dirtying the sample cones. When the original matrix modifier was a liquid, the experiments were performed by direct pneumatic nebulization, and when solid the final Se solutions were introduced into the ICP–MS by a flow injection (FI) system to avoid possible dirtying and/or blocking of the sample cones and significant drift of the signal, owing to total dissolve solids (t.d.s.) higher than those recommended in ICP–MS. As it is known FI–ICP– MS allows the maximum t.d.s. in the sample to be 10 times higher than in continuous introduction. 2% HNO 3 was used as a carrier solution for flow injection and for continuous sample introduction. HNO 3 was selected instead of HCl or any other acid because in this medium the formation of polyatomic molecules (40 Ar37 Cl + , 32 S 16 O3+ and 34 S 16 O3+ ), which could interfere in Se determination, are lower than in any other acid. The signal for each of the three Se isotopes investigated (77, 78 and 82) was systematically recorded to evaluate the influence of the nebulizer gas pressure and forward power operating conditions, first with a 100 mg l −1 Se standard solution, which will be taken as the reference signal (100%), and then for the 100 mg l −1 Se solution spiked with each concentration of the organic modifier tested.
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The determination of Se by FI–ICP–MS produces a transient signal instead of the steady-state signal obtained by pneumatic nebulization. In our case only one unique isotope (78 Se) per injection, instead of three registered by pneumatic nebulization, was evaluated in each case because of the limitation of the ICP–MS used.
3. Results and discussion 3.1. Effect of organic modifiers on the selenium signal It has been previously reported that the introduction of small amounts of some organic solvents into the plasma modifies the physicochemical properties of the solution nebulized with regard to the aqueous sample, decreasing its viscosity and contributing to an overall smaller droplet size, which favours nebulization efficiency and improves desolvation of the aerosol in the plasma, resulting in an improved signal [15]. The effect of different organic compounds like monofunctional alcohols (methanol, ethanol, 1-propanol and 1-butanol), polyalcohols (ethylene glycol, glycerol, glucose and sugar) and organic acids (acetic, lactic, tartaric, citric and gluconic) on Se isotope (77, 78, 82) signals and on polyatomic interferences Ar +2, Ar 2H +, Ar 2H +2 and ArCl + at m/z 78, 77, 82 and 77, respectively, was evaluated. These organic compounds have been selected as representative of their families, having differing effects on the viscosity of the final solutions and therefore on nebulization and transport efficiency to the plasma. A standard solution of 100 mg l −1 Se in 2% HNO 3 was spiked with the above-mentioned modifiers in the 1–4% range, and the signal for the three Se isotopes studied was recorded as well as for their blanks. As already reported by Goossens et al. [9], the introduction of small quantities of organic solvents in a plasma results in the spatial shift of the zone of maximum ion density [16], so the zone of ion extraction from the plasma needed to be optimised again. Thus for each modifier concentration and RF power tested (1150 and 1350 W), it was necessary to optimise the instrumental conditions of nebulizer gas flow-rate, potential applied to the lens and ion sampling from the plasma, in order to achieve the maximum Se signal. Fig. 1 shows the effect of monofunctional alcohol
Fig. 1. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different monofunctional alcohols.
concentration on the 78 Se net signal (total less blank signal) at two different powers. The results are expressed as net signal enhancement, considering 100% the signal for a solution containing 100 mg l −1 of Se in 2% HNO 3 and in the absence of modifier. The shapes of the curves for the three Se isotopes (77, 78, 82) studied are similar in each case tested. This behaviour was similar for all the modifiers subsequently tested, so only the results for 78 Se are included as the most representative Se isotope. The curves obtained always have a maximum, but at a different concentration for each modifier. Methanol and ethanol are the modifiers which provide the highest increase in sensitivity at 1350 and 1150 W, respectively. The optimum concentration of methanol was 3% at 1350 W power and 2% at 1150 W and the
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increases in Se signal were about 5 and 3 times, respectively. The 2% ethanol modifier improved the analytical signal by about 4.5 and 3.5 times at 1350 and 1150 W, respectively. The optimum nebulizer gas flow-rates in 78 Se response for different monofunctional alcohol concentrations at 1150 and 1350 W are shown in Fig. 2. It is observed that as the modifier concentration increases, the maximum of the 78 Se signal shifts to lower values of nebulizer gas flow-rate. This effect is more notable at 1150 than at 1350 W. At 1150 W the nebulizer gas flow rate decreases with increasing length of the aliphatic chain of the alcohol, but it remains constant at 1350 W. This behaviour could suggest readier sample nebulization by increasing
Fig. 2. Effect of concentration of different monofunctional alcohols on the optimum nebulizer gas flow-rate in 78 Se determination by ICP–MS at 1150 and 1350 W.
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concentration and length of the aliphatic chain of the monofunctional alcohol, particularly at 1150 W. The fact that higher power requires higher nebulization flow rate to achieve the maximum Se net signal could be attributed to the shorter residence time of sample into the plasma and to a decrease of polyatomic ion formation as was observed by Evans and Ebdon [11] for Ar +2 and ArCl +. These results are in good agreement with those obtained by Mun˜oz et al. [13], but in the present study some more data such as the effect of nebulization flow rate has been evaluated and more alcohols tested at two different powers. Goossens et al. [9] have also reported an enhancement of the signal of about three times by using 4% ethanol. To check if the behaviour and signal increase observed for monofunctional alcohols depend on
Fig. 3. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different polyalcohols.
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carbon content and/or on –OH groups, a similar study was performed with polyalcohols containing several –OH groups. Fig. 3 shows that, in the modifier concentration range tested, the shapes of the curves obtained for the polyalcohols ethylene glycol and glycerol differ from those of their homologous monofunctional alcohols ethanol and propanol respectively, and a clear maximum is not reached. This could be attributed to a higher content of –OH groups in the molecules. Higher concentrations of modifier were not tested to avoid depositing carbon at the sample cones. At 1150 W, the modifiers glucose and sugar (introduced by FI) improved the signal notably and a plateau was obtained for a modifier concentration of 3%, in contrast with the linear increase in signal with increasing modifier concentration at 1350 W. The optimum improvement of the signal achieved with polyalcohol modifiers was obtained with ethylene glycol (4 times at both powers) and sugar (4.3 times at 1350 W). The influence of organic acid compounds on the net 78 Se signal is illustrated in Fig. 4. All the compounds tested significantly improved the Se response, but none of them gave a plateau at 1350 W. The optimum improvement of the signal achieved with organic acid modifiers was obtained with lactic and citric acid, which resulted in 2.5 and 2.1 times signal improvement at 1350 W, respectively. At 1150 W the maximum signal increase was about 4.2 times using 4% tartaric acid. The optimum nebulizer gas flow-rate achieved with increasing modifier concentration was not as critical for polyalcohols and organic acids as for monofunctional alcohols. Therefore, these compounds do not affect the nebulization and transport efficiency of the sample as much as the monofunctional alcohols do. As we can see the shapes of the curves of Figs 3 and 4, corresponding to the presence of polyalcohols and organic acids, are more similar than those obtained with monofunctional alcohols (Fig. 1). This finding could be attributed more to the different content of the functional groups than to the carbon content. However, in all cases a relevant increase of the analytical signal (ranging from two to five times) is achieved which could be attributed to a mixture of several factors, namely improvement of the transport efficiency as well as the desolvation, atomisation and ionisation process in the plasma. The changes in these
Fig. 4. Percentage of 78 Se signal enhancement at 1150 and 1350 W in the presence of different organic acids.
parameters could vary from one group to another of the organic substances tested. However, we think that the predominant effect could be attributed to an improvement in the ionisation process. Introduction of these organic compounds into the plasma generates a high population of C + and/or carbon-containing polyatomic ions such as CO + and COH +. As carbon has a higher ionisation potential (11.26 eV) than Se (9.8 eV) the degree of ionisation of Se would then be improved by electron transfer from Se to the carbon or carbon-containing polyatomic ions. Hence, the addition of organic compounds would modify the ionisation equilibrium over a limited range of energy, changing the population of excited stated by collisions with C or carbon radicals.
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3.2. Effect of organic modifiers on sample transport efficiency To evaluate whether the extension of the signal increase observed for Se isotopes in ICP–MS in the presence of organic modifiers is caused by improved sample nebulization transport to the plasma and/or improved Se ionisation due to the presence of –C and –OH functional groups, the influence of these compounds on Se determination by AAS in an air– acetylene flame, where the sample is subjected to a similar nebulization and transport process, was evaluated. Due to the different mechanisms involved in the absorption process (atoms in the ground state) and in ICP–MS (atomic ions), comparison of the results provided by these two techniques may help to evaluate whether the signal increase is due to an improvement in the nebulization–transport of the sample and/or in the ionisation mechanism of the Se atoms in the plasma. Modifier amounts in the 2–20% range were spiked into a 4 mg l −1 standard Se solution and the instrumental conditions were optimised for any concentration of modifier tested in a reducer air–acetylene flame. Table 3 shows the optimum modifier concentrations and the signal increase in Se determination by AAS. The results are expressed as signal percentage considering 100% as the signal in the absence of modifier. Table 3 Optimum modifier concentration and signal increase in Se determination by AAS Modifier
S × 10 3/N m −1
None Methanol Ethanol Propanol Butanol Ethylene glycol Glycerol Glucose Sugar Acetic acetic Lactic acid Tartaric acid Citric acid S: Surface tension.
– 22.6 22.7 23.8 24.5 – – – – – – – –
% Modifier – 20 20 20 20 4 2 2 4 20 10 2 2
% Signal 100 205 196 196 180 131 121 98 90 133 121 98 102
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All the modifiers tested increase the Se signal from 1.2–2 times, except for glucose, sugar, citric and tartaric acids (originally solids) which do not increase the Se signal. Monofunctional alcohols gave the highest improvement, it being a maximum for 20% methanol (two times). However, this increase is lower than those observed in ICP–MS (five and three times at 1350 and 1150 W, respectively). The improvement in the signal slightly decreases with increasing the length of the aliphatic chain and then the surface tension of the spiked monofunctional alcohol. Thus the improvement in Se determination by ICP–MS in the presence of organic compounds, liquid in origin, is not only attributed to better nebulization–transport efficiency, but also to ready Se ionisation in the plasma. However, the signal increase observed in ICP–MS with modifiers, solid in origin, is only attributed to improved Se ionisation in the plasma. 3.3. Interference study To ascertain the effect of the organic modifiers tested on interference in Se determination by ICP– MS due to different polyatomic ions, the signals for Se (77, 78, 82), Ar 2H (77), Ar 2H 2 (82) and Ar 2 (78) were recorded versus nebulizer gas flow-rate for 100 mg l −1 Se standard solution in 2% HNO 3 and 2% ethanol (modifier which provided an important signal improvement in the previous section) at 1150 and 1350 W. Fig. 5 shows that nebulizer gas flow-rate plays an important role in the signal intensity of the three Se isotopes, but its effect is specially notable for polyatomic interferences. It can be observed (Fig. 5) that there is a significant decrease in the signal from polyatomic interferences with increasing nebulizer gas flow-rate. When ethanol is added, the maximum of all Se isotopes signals increases and shifts to lower nebulizer gas flow-rates. However, the decrease in intensity with increasing argon nebulization flow rate due to polyatomic interferences is much more pronounced than the decrease in the Se signal. This behaviour could be justified by two mechanisms, one affecting the interferences and the other the Se signal. We think that the decrease of the interference level with increasing flow rate, previously observed by other authors with methanol [13], ethanol [9], and 2-propanol [11], is due to a decrease of sample residence time into the plasma. This effect is
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Fig. 5. Effect of 2% ethanol and nebulizer gas flow-rate on Se signal and polyatomic interferences: (a) 1150 W; (b) 1350 W.
more marked for 38 Ar2 H and 40 Ar2 H2 affecting 77 Se and 82 Se isotopes, respectively. However, two different zones in the figures could be observed, the first at low nebulizer gas flow-rate up to about 1 ml min −1, in which the addition of ethanol decreases the interference level, and the second in which the interference level decreasing is more marked without modifier by increasing the nebulizer gas flow rate. Hence, the predominant effect at flow rate lower than 1 ml min −1 could be the competition of formation of the carbon-containing compounds ions such as ArC +, CCl +, CO + and COH + with the polyatomic interferences evaluated, the residence time at higher
flow rate being the most marked effect (more noticeable at 1150 W than at 1350 W). The increase of the Se signal in the presence of ethanol and its shift to lower nebulizer gas flow-rate could be attributed to the improvement of sample transport, desolvation and ionisation into the plasma. At 1350 W (Fig. 5(a)) the maximum Se signal occurs at higher nebulizer gas flow-rates than when 1150 W is used (Fig. 5(b)), both in the presence and absence of modifier. In ICP–MS it is important to have not only a sufficiently high signal for the analyte ion to be quantified, but also a high ion signal/background ratio. Therefore,
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the Se signal/background ratio was measured for the three Se isotopes and for the optimum concentration of the organic modifier obtained in the above studies, to establish the optimum conditions giving the highest Se signal with low polyatomic interferences. Fig. 6 shows the increase in the Se signal/background ratio (DS/B) for a 100 mg l −1 Se standard solution in 2% HNO 3 spiked with the modifiers. For the monofunctional alcohols tested at 1150 W, the highest ratio is obtained for 2% ethanol, which gives 26- and 20-fold improvements for 82 Se and 77 Se, respectively. Therefore, Ar 2H (77) and Ar 2H 2 (82) are more strongly affected by this modifier than Ar 2 (78)
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interference. At 1350 W the results obtained are more variable. The highest increase in 78 Se is obtained for 3% methanol (50-fold) and in 82 Se for 2% ethanol (80-fold), and there is a decrease in 77 Se, except for ethanol. For polyalcohols at 1150 W, 4% glycerol increases the ratio for 78 Se (Ar 2) 10-fold, but decreases it for 77 Se (Ar 2H) and 82 Se (Ar 2H 2). Glucose and sugar modifiers at 3% do not produce practically any variation and ethylene glycol at 4% worsens the results. At 1350 W 4% glycerol produces the optimum improvement for all isotopes (15–30-fold). For the family of organic acids at 1150 W, lactic
Fig. 6. Increase in Se the signal/background ratio for 100 mg l −1 Se spiked with modifiers: (a) 1150 W; (b) 1350 W.
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acid at 4% produces the optimum signal-to-noise ratio: 65-, 55- and 15-fold increases for Se 77, 82 and 78, respectively, and the increase is negligible for other acids. At 1350 W 4% acetic and 4% lactic acids improve the ratio 25–35-fold for Se 77 and 82, respectively, but for 78 Se it decreases. A polyatomic interference that strongly affects 77 Se determination in samples with high chloride content (sea water, urine, etc.), and even annuls the analytical signal, is that of 40 Ar37 Cl + . The possibility of decreases or elimination of this interference by the addition of organic modifier was evaluated by measuring the signal of 100 mg l −1 Se standard solution spiked with 2000 mg l −1 chloride as HCl and 2%
ethanol. Fig. 7 shows the effect of this modifier and nebulizer gas flow-rate on the 77 Se + and ArCl + signal at the two powers. There is a dramatic increase in background in the presence of chloride, which is much more marked than those previously obtained + + for 38 Ar2 H . Therefore, the 38 Ar2 H contribution was not considered in this study. However, the 77 Se + =ArCl + ratio at 1.0 and 1.2 l min −1 nebulizer gas flow-rates at 1150 and 1350 W, respectively, in samples spiked with 2% ethanol increases considerable with respect to that obtained without modifier. The different behaviour of Ar +2, Ar 2H +2 and Ar 2H + (Fig. 5) from that of ArCl + (Fig. 7) indicates that their
Fig. 7. Effect of 2% ethanol and nebulizer gas flow-rate on 77 Se and 40 Ar37 Cl + signals at 1150 and 1350 W.
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Fig. 8. Increase in the 77 Se signal/background ratio for Se + Cl + modifier with respect to 77 Se signal/background ratio for Se + Cl at 1150 and 1350 W.
mechanisms of formation and/or ionisation are different. Similar experiments were performed for the rest of the most suitable modifiers tested previously. Fig. 8 and Fig. 9 illustrate the increase in 77 Se + =ArCl + ratio caused by a 100 mg l −1 Se standard solution spiked with 2000 mg l −1 chloride as HCl and modifier with respect to the Se signal/background ratio for a solution of selenium with chloride and a Se solution without chloride at 1150 and 1350 W, respectively. The addition of any modifier significantly increased the 77 Se + =ArCl + signal ratio at the two powers tested (Fig. 8). This increase is higher (20–30 − fold) for 4% glycerol and 4% lactic acid. The 77 Se + =ArCl + signal
ratios in the presence of modifier are in some cases higher than those obtained for Se solution without chloride, especially 4% glycerol (Fig. 9). There was a significant increase in the 82 Se background when 2000 mg l −1 of chloride as HCl was spiked into the 100 mg l −1 Se standard solution. Surprisingly, this high background was practically eliminated by addition of 2% ethanol. This was possibly due to contamination of the HCl acid used with bromide, and consequent formation in the plasma of the 81 BrH + (m/z = 82) polyatomic interference, which was finally controlled by this organic solvent and instrumental adjustments. The relevant alleviation of the evaluated
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Fig. 9. Increase of the 77 Se signal/background ratio for Se + Cl + modifier with respect to 77 Se signal/background ratio at 1150 and 1350 W.
interferences can be explained as suggested by Evans and Ebdon [11], by the reduction of the ionisation temperature and increase of the kinetic temperature in the plasma by the organic modifiers, which results in an important suppression of their ionisation and a relevant increase of their breakdown, respectively. These results obtained also agree with those previously reported by Evans and Ebdon [11] and Goossens et al. [9], who report that the addition of small amounts of organic modifiers do not completely eliminate the spectral interferences, but shift their optimum formation with respect to Se and, hence, there is no total elimination of the formation of polyatomic species. The addition of small amounts of
organic modifiers and a judicious choice of operating conditions permit an accurate and precise Se determination at non-ideal gas flow-rate. It can be concluded that the use of 4% lactic acid as modifier at 1150 W in combination with an optimum nebulizer gas-flow rate and plasma sampling depth allows analytical determination of any Se isotope, even in the presence of high concentrations of chloride. 40 Ar37 Cl + interference is significantly alleviated by addition of 4% glycerol and 4% lactic acid. 3.4. Analytical performance To evaluate the analytical performance of the
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Table 4 Percentage of increase in selenium calibration slope by addition of modifiers with respect to Se determination in the absence of modifiers Modifier
1150 W 77
None 3% Methanol 2% Ethanol 4% Ethylene glycol 4% Glycerol 4% Sugar 4% Acetic acid 4% Lactic acid 4% Tartaric acid
Se
100 288 192 280 252 – 390 507 –
1350 W 78
82
Se
100 302 200 246 246 193 407 550 430
77
Se
100 302 213 271 248 – 390 540 –
proposed organic matrix modifiers, several Se standard solutions in the range 0–100 and 0–200 mg l −1 for 1150 and 1350 W, respectively, were spiked with the selected modifiers and the analytical signal was measured (in triplicate), as well as their blanks. The net calibration graphs were linear with correlation coefficients of 0.999 or 0.9999 in all cases. Table 4 shows the increase in the calibration slope upon addition of modifiers with respect to Se in the absence of modifier (taken as 100%). For the modifiers solid in origin (tartaric acid and sugar), which were introduced by an FI system (one isotope only is measured in each run), only 78 Se was determined, since it is the most important from the point of view of analytical determination. Marked increases in the calibration slopes were obtained for methanol at 1350 W, acetic acid at both powers and tartaric acid at 1150 W. The best results were achieved for lactic acid (507–590%) at both powers. There was significant reduction in the detection
Se
100 334 224 231 225 – 480 570 –
78
82
Se
100 488 263 273 238 389 480 590 –
Se
100 429 225 230 225 – 477 570 –
limits for all Se isotopes studied and modifiers selected within the 1.5–4.0-fold range, which was accompanied by a reduction in relative standard deviation (R.S.D.). Examples of this reduction are 2% methanol at 1150 W with detection limits (IUPAC [17]) of 0.4, 0.2 and 0.3 mg l −1 Se and R.S.D. values for five determinations of a 50 mg l −1 Se standard solution of 1.0, 0.4 and 0.3% for Se 77, 78 and 82, respectively, and 4% lactic acid at 1350 W with detection limits of 0.3 mg l −1 for all isotopes and R.S.D. of 3.1, 2.1 and 2.8% for Se 77, 78 and 82, respectively. 3.5. Application The selected modifiers were employed for Se determination by using the three isotopes (77, 78 and 82) at 1150 and 1350 W in a water sample with certified Se content (35.50 6 5.14 mg l −1) from A.P.G. (Analytical Products Group, Inc.), 2730 Washington Blvd. Belpre, OH 45714. The second sample was a BCR
Table 5 Determination of selenium (mg l −1) in certified water samples in the presence of modifiers Power/W a
1150 1150 a 1150 a 1350 a 1350 a 1150 b
Modifier 4% ethylene glycol 3% tartaric acid 4% sugar 3% methanol 4% sugar 4% lactic acid
x¯ 6 j, n = 3. Certified value: 35.50 6 5.14 mg l −1 Se. b Certified value: 80 6 5 mg l −1 Se. a
77
Se 33.12 6 0.37 – – 37.20 6 0.10 – 83.13 6 3.13
78
Se 33.22 6 0.87 37.01 6 0.70 35.35 6 0.69 36.31 6 0.59 34.91 6 2.75 –
82
Se 33.65 6 0.25 – – 37.90 6 1.11 – –
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(European Community Bureau of Reference) future candidate reference material with an established Se content of 80 6 5 mg l −1 and whose chloride content is 2000 mg l −1 as NaCl. This sample was analysed employing 4% lactic acid as matrix modifier, which was one of the best-performing modifiers for determining 77 Se + at low power in samples with high chloride content, decreasing the ArCl + interference. As shown in the results listed in Table 5, which were obtained by direct determination from the corresponding calibration curves in the presence of the respective modifiers, in all cases there is a good correlation between the results obtained and the certified values. This agreement validates the proposed methods for Se determination by ICP–MS.
4. Conclusions The addition of small amounts ( , 4%) of certain organic modifiers, such as monofunctional alcohols, polyalcohols and organic acids, in combination with nebulizer gas flow-rate and sampling depth adjustment, increases the sensitivity of Se determination by ICP–MS from two to five times, with methanol, ethanol, sugar, ethylene glycol and tartaric acid giving the highest increases. This significant improvement could be attributed to an increase in the efficiency of Se ion formation by electron transfer from Se to the carbon or carboncontaining polyatomic ions formed, and in the case of monofunctional alcohols, also to an improvement in the transport efficiency and desolvation of the sample in the plasma. Monofunctional alcohols differ from polyfunctional alcohols and organic acids in their effect on net signal increase, which could be attributed more to the different content of functional groups than to the carbon content. Lactic acid at 1150 W gives the highest Se signal/ background ratio from the three Se isotopes studied. 40 Ar37 Cl + interference is significantly alleviated by addition of 4% glycerol and 4% lactic acid. The important alleviation of the ArCl, Ar 2H 2, Ar 2H and 40 Ar38 Ar interferences could be explained by a competitive formation with the polyatomic ions
ArC +, CCl +, CO + and COH +. Another possibility is that these organic modifiers reduce the ionisation temperature and increase the kinetic temperature in the plasma resulting in an important suppression of their ionisation and in an increase of their breakdown, respectively. These conditions enable the accurate determination of Se in environmental and biological samples, as shown by the excellent agreement between the certified values and the results achieved for water samples of different nature.
Acknowledgements The authors thank the Direccio´n General de Investigacio´n Cientifica y Te´cnica (project PB950366-C02-01) for financial support.
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