Determination of arsenic, selenium and lead by electrothermal ...

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Determination of arsenic, selenium and lead by electrothermal vaporization inductively coupled plasma mass spectrometry using iridium-coated graphite tubes Dirce Pozebon, Valderi L. Dressler and Adilson J. Curtius*

Published on 01 January 1998. Downloaded on 20/09/2014 17:17:31.

Departamento de Quı´mica da Universidade Federal de Santa Catarina, 88040–900 Floriano´polis, S.C., Brazil

Iridium is proposed as a chemical modifier for the determination of Pb, As and Se by electrothermal vaporization inductively coupled plasma mass spectrometry. Iridium can be pipetted into the tube as a solution prior to the analytical or sample solution or the tube can be pre-treated with an Ir solution followed by the application of a heating program, forming an Ir coating. The latter procedure is preferred, since the modifier can be cleaned in the tube. More than 100 measurement cycles can be performed with the same Ir coating, if resloping of the analytical curve is carried out for As and Se. For Pb, there is no need for resloping. A pyrolysis temperature up to 1300 °C can be used. The Ir-coated tube was successfully applied to the determination of Pb, As and Se in several certified reference materials, viz., water, estuarine water and urine. For the urine sample, the analyte additions method was used, whereas for the water samples, external calibration was applied. Keywords: Inductively coupled plasma mass spectrometry; electrothermal vaporization; iridium coated graphite tube; chemical modifier; urine; water Chemical modifiers are extensively used in electrothermal atomic absorption spectrometry (ETAAS) in order to allow the separation of the analyte from the matrix inside the graphite tube, during the heating program. The most frequently used chemical modifiers are palladium nitrate, magnesium nitrate, ammonium phosphates, nickel nitrate, EDTA and ascorbic acid.1,2 They have also been used in electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS), not only to promote the separation of the matrix, but also to act as carriers of the vapor containing the analyte, from the vaporizer to the plasma. Diluted sea-water and sodium chloride have also been used for this purpose in ETV-ICP-MS.3,4 Iridium, in spite of having the same characteristics as Pd, has not been used to the same extent. It has been used for the determination of Se, As, Bi, Ag, Te and Sb1,2 by ETAAS, but has not been used in ETV-ICP-MS. According to the classification of the modifiers given by Tsalev et al.,1 Ir belongs to the group of elements (Ru, Pd, Rh, Ir) that can be reduced to the elemental form during the pyrolysis step. Since these modifiers are very effective, only a few micrograms are commonly used. For ETV-ICP-MS, even smaller amounts are used in order to avoid signal suppression due to space charge effects. The effectiveness of these modifiers is attributed to the fact that they are readily reduced at low temperatures (400–800 °C), probably forming a stable intermetallic solid solution with the analyte during the pyrolysis stage.5 More recently, Ir has been used to coat graphite tubes, forming a permanent layer, for the determination of Cd, Pb and Se by ETAAS,6 allowing pyrolysis temperatures of 800, 1200 and 1400 °C, respectively. The same tube could be used for 700 temperature cycles. Iridium-coated tubes have also

been used for the determination of Pb, As, Sb, Se and Sn in a flow system coupled to ETAAS,7,8 where the hydrides of these elements were produced and retained in the tube before atomization. One advantage of the pre-treatment of the tube surface with the modifier is that it can be cleaned before the determinations, reducing the blank signals.1 These pre-treated tubes can also be conveniently used, when on-line separation of the matrix and preconcentration of the analytes are performed using a column filled with a sorbent. The addition of the modifier to the sample before passing the solution through the column may saturate the column if the modifier is also retained. The use of a pre-treated tube also increases the sample throughput because the drying time of the furnace temperature program can be shortened if the modifier is not pipetted into the tube together with the sample. In addition, it is not always possible to add the modifier to the sample, owing to chemical incompatibility. The tube can be coated by two procedures: by pipetting an Ir solution into the graphite tube, followed by application of a heating program and repeating the procedure until the desired mass of Ir is obtained, or by sputtering.6,7 In this work, the simultaneous determination of As, Se and Pb by ETV-ICP-MS using either Ir-coated graphite tubes or the addition of an Ir solution into the tube, as a modifier, was studied. The use of this modifier was also compared with the procedure that uses a Pd solution as a modifier without tube pre-treatment. EXPERIMENTAL An HGA 600 graphite furnace equipped with an AS-60 autosampler, both from Perkin-Elmer, coupled to a Perkin Elmer SCIEX Elan 6000 ICP mass spectrometer, was used for all measurements. Argon of 99.996% purity from White Martins, Brazil, was used.The instrument optimization was performed using conventional pneumatic nebulization, except for the carrier gas flow rate, which was optimized using the ETVICP-MS coupling. The instrumental conditions are shown in Table 1. During the vaporization, an Ar flow rate of 0.1 l min−1,

Table 1

Plasma conditions and data acquisition parameters

Rf power Argon flow rate: coolant auxiliary carrier Sampler and skimmer Measurement Dwell time Resolution m/z per measurement cycle Sweeps per reading Readings per replicate Auto lens

1000 W 15 l min−1 1.2 l min−1 0.90 l min−1 Pt Peak area 50 ms 0.7 u (10% at peak height) 3 1 70 On

Journal of Analytical Atomic Spectrometry, January 1998, Vol. 13 (7–11)

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shown in Table 2, was added to the nebulizer gas flow rate of 0.9 l min−1. A pyrocoated graphite tube with an integrated pyrolytic graphite platform (Perkin-Elmer, Part No. B-3001264) was coated by pipetting 60 ml of a 1000 mg ml−1 Ir solution onto the platform and submitting the tube to the temperature program shown in Table 2. This procedure was repeated five times. For subsequent measurements, the same temperature program was used, except that the pyrolysis and vaporization temperatures were 1200 and 2300 °C, respectively, and the cleaning step was omitted to prolong the lifetime of the Ir coating. A solution containing 1% m/v Pd for ETAAS from Merck and a 0.1% m/v Ir solution, obtained by diluting a Titrisol ampoule from Merck, were used as stock solutions for the modifiers. A 0.4% m/v sodium chloride solution, used as carrier, was prepared from Suprapur sodium chloride (Merck). Before use, the Pd solution was diluted to 0.02% m/v and the Ir solution to 0.005 or 0.01% m/v. The analyte multi-element stock solution, containing 10 mg ml−1 of Pb and 100 mg ml−1 of As and Se, was prepared using Spex products ( lead nitrate, arsenic trioxide and selenium pellets). For the dilutions, Milli-Q water with a resistivity of 18.2 MV cm−1 containing 1% v/v nitric acid (Suprapur, Merck) was used. The following CRMs were analysed: SRM 1643d Water and

Table 2 Furnace temperature program Step

Temperature/°C

Ramp/s

Hold/s

Gas flow rate/ ml min−1

1 2 3 4 5 6 7

90 150 1000–1200 20 2300 2500 20

5 20–40 20 5 1 5 2

5 10 5 10 10 2 15

300 300 300 100 100* 100 100

* Read in this step.

SRM 2670 Freeze-Dried Urine, both from the National Institute of Standards and Technology (NIST), and SLRS 3 Riverine Water from the National Research Council of Canada (NRCC). The SRM 2670, after reconstitution, according to the instructions given in the certificate, was diluted 1+9, while the SRM 1643d was diluted 1+19. The riverine water was not diluted. External calibration was used for the analysis of the water samples, using analytical solutions containing 0.05–1.0 ng ml−1 of Pb and 0.5–10.0 ng ml−1 of As and Se. For the urine sample, the standard additions method was used. When the Ir-coated tube was used, 10 or 20 ml of the analytical or sample solution were pipetted into the tube and the program shown in Table 2 was run. When using a conventional graphite tube, without the Ir coating, 10 or 20 ml of the modifier solution containing Ir or Pd were pipetted into the tube and steps 1 and 2 were run.Then, 10 or 20 ml of the analytical or sample solution were pipetted into the tube, and the full program was run. RESULTS AND DISCUSSION The pyrolysis temperature curves for Pb, As and Se, using Ir as a coating of the graphite tube (Ir-coated tube) or as a solution added to the tube before the analytical or sample solution, are shown in Fig. 1. The curves obtained with sodium chloride as carrier and without a modifier are also shown in Fig. 1, where the signals were corrected for the blank values.The advantage of the Ir-coated tube is clear, particularly for Pb [Fig. 1(a)]. Much higher pyrolysis temperatures can be used with the Ir-coated tube in comparison with the other situations [Fig. 1(a) and (b)]. In addition, with this tube, the net intensities are higher than with Ir added as a solution. Not shown in Fig. 1, the blank value for the Ir-coated tube is much smaller than that for the Ir solution, showing that the modifier, as a coating, can easily be cleaned. In Fig. 1(b), it is shown that sodium chloride is a good carrier, as found previously by other workers;3,4,9 however, it does not act as an efficient modifier for Pb, since for pyrolysis temperatures higher than 600 °C the drop in the signal intensities is very pronounced. Also for As

Fig. 1 Pyrolysis temperature curves for 40 pg of As and Pb and 80 pg of Se. (a): A, 208Pb+Ir-coated; B, 208Pb+1 mg Ir; (b): A, 208Pb+4 mg NaCl; B, 208Pb; (c): A, 75As+Ir-coated; B, 77Se+Ir-coated; C, 77Se+1 mg Ir; D, 77Se+2 mg Ir; E, 75As+2 mg Ir; F, 75As+1 mg Ir; (d): A, 75As+4 mg NaCl; B, 77Se; C, 77Se+4 mg NaCl; D, 75As.

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Journal of Analytical Atomic Spectrometry, January 1998, Vol. 13

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and Se, as shown in Fig. 1(c) and (d), the sensitivities are higher for the Ir-coated tube. It is possible that the amount of added Ir, as a solution, is not sufficient to stabilize the analytes, since increasing the mass of added Ir also increases the signal intensities, as shown in Fig. 1. Sodium chloride also acts as a modifier for As and Se [Fig. 1(d)] but less efficiently than Ir. Two contradictory effects of sodium chloride should be considered: it can act as a signal suppressor and as a modifier/ carrier. This dual function might explain the pyrolysis temperature curve for Se in the presence of this agent: as the pyrolysis temperature is increased above 600 °C, the signal also increases because less Na is present during the vaporization, and consequently less suppression occurs. As the pyrolysis temperature is increased above 700 °C, more Na is lost and the carrier function is also hindered. In conclusion, using the Ir-coated tube, a pyrolysis temperature up to 1300 °C for Pb, As and Se can be used, improving the matrix separation. Without a modifier, severe sensitivity loss occurs for pyrolysis temperatures higher than 300 °C.

Fig. 2 Sequential readings of 4 pg of Pb and 10 pg of As and Se; (a)–(c): 75As; (d)–( f ): 77Se; (g)–(i): 208Pb; (a), (d) and ( g): Ir-coated tube; (b), (e) and (h): analyte+0.5 mg of Ir; (c), ( f ) and (i ): analyte+0.5 mg of Pd.

For Se, the signal at mass 77 was measured, in spite of the low natural abundance of the 77Se isotope, because this mass is less prone10 to interferences from Kr and polyatomic ions of Ar. This isotope, and also 75As, are subject to interference by 40Ar37Cl and 40Ar35Cl, respectively; however, Cl is eliminated in the vaporizer below 500 °C, which is a distinct advantage of this sample introduction system. For further measurements, a pyrolysis temperature of 1200 °C was used. In order to check the precision, 20 measurements of aliquots containing 4 pg of Pb and 10 pg of As and Se were performed, using new tubes, and the results are shown in Fig. 2. Three different tubes were used: an Ir-coated tube, a tube where Ir was added as a solution and a tube where Pd was added as a solution, according to the procedures described above. The signal intensities of the first ten measurements were corrected for the blank reading prior to the measurements, and the last ten measurements were corrected for the blank reading after the 20th measurement (Table 3). As shown in Fig. 2(a)–(c) and Table 4, the sensitivity for As is better when the Ir-coated tube is used. Better precision for As could be obtained by considering the blank value after every five measurements or by resloping the analytical curve more frequently, as will be suggested later. In any case, the three situations lead to similar precison for As, as shown in Table 4, with a slightly better relative standard deviation (RSD) when Pd is added as a solution. Selenium behaves similarly in the three situations [Fig. 2(d)–( f )], the sensitivity being better only in the first seven measurements for the Ir-coated tube. The relatively high RSD for Se can be attributed to the low counts of the Se intensity due to the low natural abundance (7.58%) of the measured isotope and to its lower ionization degree11 (33%, in comparison with 97% for Pb in an Ar plasma at 7500 K and electron density of 1×1015 cm−3). The best condition for Pb is the Ir-coated tube [Fig. 2( g)], which leads to better precison, better sensitivity and much lower blank values (Table 3). The relatively high RSDs, for some situations shown in Table 4, can be attributed to high blank values, for example for Pb using Pd as modifier, and to low counts, for example for 77Se. To verify the lifetime of the Ir coating, consecutive measurements of aliquots containing 40 pg of the analyte were per-

Table 3 Blank intensity (10 ml of 1% v/v HNO ) with the different modifiers: (n=3)* 3 Blank signal/counts 75As Ir, 0.005%

208Pb

Pd, 0.005%

Ircoated

Ir, 0.005%

Pd, 0.005%

Ircoated

Ir, 0.005%

Pd, 0.005%

6759 5505 10 449 (207) (419) (1633) Last 3740 3460 8363 (438) (402) (371) * Values in parentheses are the standard deviations.

2290 (250) 1970 (210)

1946 (40) 1419 (90)

1915 (373) 1633 (79)

12 917 (1470) 5600 (565)

36 654 (1915) 40 499 (1419)

54 331 (2274) 63 048 (7494)

Reading

Ircoated

77Se

First

Table 4 Relative standard deviations (4 pg of Pb, and 10 pg of As and Se) with the different modifiers. The intensity was blank-corrected Relative standard deviation (RSD) (%) 75As Reading 1st–5th 1st–10th 10th–20th 1st–20th

77Se

208Pb

Ircoated

Ir, 0.005%

Pd, 0.005%

Ircoated

Ir, 0.005%

Pd, 0.005%

Ircoated

Ir, 0.005%

Pd, 0.005%

7.1 17.9 7.9 15.2

11.9 15.8 6.8 12.0

7.0 9.5 8.6 7.9

16.9 28.2 6.8 30.7

18.7 30.4 10.1 27.9

15.8 22.9 12.6 29.2

4.2 16.0 4.9 4.9

18.8 19.1 15.8 21.8

13.8 13.2 32.3 33.9


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6

( a)

208

Pb

4

2

75

As

10 intensity

77

0


formed and the results are shown in Fig. 3. About 120 measurements can be carried out for Pb, without a significant loss of the signal intensity. For As and Se, there is a loss of about 15% of the signal intensity after the 30th measurement. This can be compensated for by resloping the analytical curve after every 20 measurements. Also, the Ir coating can be re-formed easily, which could be done after every 100 cycles, for example. Around the 100th temperature cycle, the As and Se intensities are around 50% of those at the beginning of the experiment, whereas, for Pb, the signal decreases by only 13% after 140 reading cycles. The transient signals of the analytes obtained with the Ir-coated tube are shown in Fig. 4(a) and (b). The signals for the analytical and sample solutions are very similar, suggesting that the matrix was conveniently eliminated before the vaporization. Analytical curves, showing good linearity, were obtained for the Ir-coated tube. The correlation coefficients (r2) and the detection limits, 3s/S, where s is the standard deviation of ten measurements of the blank and S is the slope of the calibration curve, are shown in Table 5, using external calibration or the analyte additions method for the urine sample diluted 1+9. The detection limits for the urine sample are significantly higher, particularly because of the dilution factor. For this sample, external calibration, even using matrix matching with sodium chloride, was not successful, probably because of other constituents of the sample that may act as carriers. The detection limit for Pb in the urine sample is worse than that for As, because of the more severe effect of Na on this element,

Table 5 Detection limits (DL) and linear correlation coefficients (r2). Graphite tube coated with Ir r2 Isotope 75As 77Se 208Pb

DL/ng ml−1

Analyte additions

External calibration

Analyte additions

External calibration

0.9992 0.9991 0.9900

0.9990 0.9998 0.9996

0.18 0.88 0.32

0.034 0.25 0.017

2

1

3

4

5

( b)

5

Fig. 3 Stability of the signal for 40 pg of the analyte using an Ir-coated tube.

Se

20 75

As

10 8 6 208

Pb

4

77

Se

2

0

1

2

3

4

5

6

7

8

Time/s Fig. 4 Transient signals for: (a) 40 pg of the analytes and (b) the urine sample diluted 1+9.

probably owing to its condensation during the vapor transport.9 According to Park and Hall,12 pyrolysis temperatures higher than 1700 °C are required to remove Na from the graphite tube completely, although Na loss can be observed at lower temperatures. In addition, other species also present in the urine may cause signal suppression. Good results, using the Ir-coated tubes, in comparison with the certified or recommended concentration values, for the analysed reference materials, were obtained, as shown in Table 6. The good results with the Ir-coated tubes, particularly for the urine sample, were obtained because a higher pyrolysis temperature was possible and also because the modifier was purified in situ, producing lower blank signals. If the tube is not coated, but the Ir is added as a solution, then as shown in Fig. 1(a) a much lower pyrolysis temperature for Pb should be used otherwise the sensitivity will be very low. In conclusion, Ir is a good chemical modifier for As, Se and Pb in ETV-ICP-MS. The Ir coating of the tube can easily be performed and the impurities of the modifier can easily be removed. In addition, pyrolysis temperatures as high as 1300 °C are tolerated, allowing the simultaneous determination of a

Table 6 Analysis of CRMs, n=9: (graphite tube coated with Ir) Certified/ng ml−1

Isotope 75As 77Se 208Pb

Found/ng ml−1

SLRS 3 Riverine Water

SRM 1643d Water

SRM 2670 Freeze-Dried Urine

SLRS 3 Riverine Water

SRM 1643d Water

SRM 2670 Freeze-Dried Urine

0.72±0.09 — 0.068*

56.02±0.73 11.43±0.17 18.15±0.64

60* 30±8 10*

0.73 (0.08)† — 0.079 (0.009)

56.20 (0.63) 11.67 (0.65) 16.3 (0.86)

67.26 (3.4) 30.54 (0.93) 13 (1.3)

* Recommended value (not certified). † Values in parentheses are the standard deviations.

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larger number of elements in different samples of interest such as plant and animal tissues. Iridium seems to be an advantageous alternative to Pd, particularly when a pre-treated tube is preferred or Pd is the analyte. It is not possible to form a permanent Pd coating on the graphite tube because of its relative volatility. The authors thank Financiadora de Estudos e Projetos (FINEP). D. P. and V. L. D. have doctorate scholarships from Conselho Nacional de Pesquisas e Desenvolvimento Tecnolo´gico (CNPq). The iridium solution was donated by Dr. G. Schlemmer (Bodenseewerk Perkin-Elmer, Germany). REFERENCES

Paper 7/06837I Received September 22, 1997 Accepted November 4, 1997


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4 Gre´goire, D. C., Al-Maawali, S., and Chakrabarti, C. L., Spectrochim. Acta, Part B, 1992, 47, 1123. 5 Yang, P.-Y., Ni, Z.-M., Zhuang, Z.-X., Xu, F.-C., and Jiang, A.-B., J. Anal. At. Spectrom., 1992, 7, 515. 6 Rademeyer, C. J., Radziuk, B., Romanova, N., Skaingset, N. P., Skogstad, A., and Thomassen, Y., J. Anal. At. Spectrom., 1995, 10, 739. 7 Haug, H. O., Spectrochim. Acta, Part B, 1996, 51, 1425. 8 Tsalev, D. L., D’Ulivo, A., Lampugnani, L., Di Marco, M., and Zamboni, R., J. Anal. At. Spectrom., 1996, 11, 979. 9 Ediger, R. D., and Beres, S. A., Spectrochim. Acta, Part B, 1992, 47, 907. 10 Vaughan, M. A., and Horlick, G., Appl. Spectrosc., 1986, 40, 434. 11 Jarvis, K. E., Gray, A. L., and Houk, R. S., Handbook of Inductively Coupled Plasma Mass Spectrometry, Blackie, New York, 1992, p. 380. 12 Park, C. J., and Hall, G. E. H., J. Anal. At. Spectrom., 1987, 2, 473.


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