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Use of a refractor plate for automatic background correction in electrothermal vaporization inductively coupled plasma atomic emission spectrometry
058‘%?547/91 53.00+ .ua 0 1990 Pergamon Press pk.
Spectrochrmrco Aclo Vol. 46. No 1, pp. 99-102. 1991
Printed m GreatBritain.
TECHNICAL NOTE
Use of a refractor plate for automatic background correction in electrothermal vaporization inductively coupled plasma atomic emission spectrometry (Received 6 July 1990; accepted 19 September 1990)
1.
INTR~~UC~~N
THE USE
of electrothermal vaporization (ETV) for sample introduction in inductively coupled plasma atomic emission spectrometry (ICP-AES) has been reported by many authors [l-8]. The most attractive features of this technique are: only a small sample volume is required (S-50 pl), high sensitivity is obtained because of the high transport efficiency [P-11, 131, and samples containing high concentrations of salts can be analysed [12]. One of the disadvantages is the difficulty in making an accurate background correction. Most of the time, determinations were made without a background correction, although many authors have indicated the importance of appropriate background correction in work involving transient signals [U-18]. During the heating process of the ETV unit, the nebulizer gas (Ar) expands rapidly thus causing a change of the background [13, 181. One approach to correct for the changing background as a function of time is to measure the intensity at the peak maximum as a function of time and to integrate the obtained signal. Afterwards the background is determined, using a blank, over the same integration period and subtracted [18]. However, this does not take into account the possible presence of interfering elements from the sample which may influence the background and it assumes that the background changes in the same way for blank and sample measurements [14, 171. Therefore it is better to use an automatic background correction by means of a wavelength modulation system. The difficulty is to measure the background quasi-simultaneously with the peak, so that evolutions in background level can be followed and corrected for. This paper describes a wavelength modulation method for background correction for ETV-ICP-AES, based on the method described by SNELLEMAN[19]. The system was implemented on an existing commercial spectrometer, the Jobin Yvon VHR 1000. A computer program was developed for the automatization of the device and for data handling. The system is somewhat similar to the one described by BAXTER et al. [20], and OITAWAY et al. [21] for electrothermal atomisation atomic emission spectrometry (ETA-AES) [20,21] and to the one described by O’HAVER et al. [22] for simultaneous multi-element atomic absorption with a continuum source.
2.
EXPERIMENTAL
2.1. Instrumentation An existing Jobin Yvon VHR 1000 monochromator was modified. A refractor plate was inserted behind the entrance slit to obtain automatic background correction by wavelength modulation, as described in detail in Ref. [23]. By oscillating the plate at a high frequency (l-40 Hz) a scan can be made over a certain wavelength interval. The plate is oscillated using an optical scanner Model G325D and controlled by an AT computer implemented by an A660 driver amplifier card delivered with the scanner. For this work the commercial P.S. Analytical 60.800 ETV unit was used, but the technique described here can be used as well for other types of sample introduction yielding a transient signal (e.g. flow injection [2], laser ablation [24], thermospray ICP-AES [25]). 2.2. Description of the wavelength scanning technique The technique was first tested and optimized on the ICP with a pneumatic nebulizer (Meinhard TR-50-C3) as sample introduction system [26]. which yields a continuous signal. The movement 99
100
Technical note
of the refractor plate is synchronized with the detection system. A scan consists of 10 to 160 steps, to be set at the beginning of the measurement together with the scan frequency. The signal obtained by the photomultiplier (Hamamatsu R106UH). during one step is sent to an AD converter and stored into the memory of the computer. After the first scan, the following ones are added to the former so that the obtained spectrum is the summation of many scans (20-500, depending on program parameters) and can be interpreted and visualized on a computer screen. The program allows it to select, with a pair of cursors, the borders of the peak. Also regions to calculate the background on the left- and the right-hand side of the peak, can be chosen by means of two pairs of cursors. The background to be subtracted is calculated by linear interpolation between the mean left- and right-hand background. This has been described in detail by VANDECASTEELE et al. [27]. When using ETV as a sample introduction system a transient emission signal is obtained. In this case it is necessary to visualize the time resolved emission signal, so that the arrival and, even more importantly, the wash-out of the analyte can be followed. Therefore every scan is stored in a different range of addresses. After choosing the most intense peak and setting the cursors for the calculation of the background, as described above (Fig. la-d), all the other scans are automatically corrected in the same way. The corrected peak areas are then integrated and the time resolved emission signal is visualized on the computer monitor (Fig. 2). 2.3. Operating conditions The ICP used, was a Plasma Therm ICP 2500. The operating conditions ETV units are listed in Table 1.
3.
for the ICP and the
RESULTSAND DISCUSSION
As quoted earlier, the wavelength modulation technique was first optimized with a pneumatic nebuliser. No degradation of resolution was found by inserting the quartz plate nor any loss of stability or sensitivity. Therefore one might conclude that the insertion of the refractor plate had no effect on the quality of the monochromator. Of course the advantages of the system become more valuable when measuring transient signals. Since the expanding gas, during the
Channel
161
Channel H3
P
B %
Channel
M
I&I
iII
d)
L Channel
‘a
Fig. 1. Background changes caused by an interfering element. Spectrum of Pb (220.353 nm) in an Al matrix (220.462 nm and 220.467 nm) obtained resp. 3.2 s (a). 4.3 s (b). 6.0 s (c). 7.4 s (d) after the start of pyrolysis II.
101
Technical note lntearated
peak area8 (Thouaandd
I3
a
10
12
14
lime (8) Fig. 2. Integration of the temporally resolved peak areas. Influence of interfering elements on the background changes. Ic Background corrected peak areas; + uncorrected peak areas.
Table 1. Operating conditions for ICP and ETV ICP
Forward power (kW) Reflected power (W) Outer gas flow (I/min) Intermediate gas flow (l/min) Observation height (mm)
I <5 I5 3.75 I7
ETV
Nebuliser gas flow (I/min) Make Up gas flow (I/min) Cooling gas flow (He) (I/min) Heating program: Evaporate Pyrolysis I Pyrolysis II Vaporise
I.5 0.2 1.5
Sample volume (111) Measuring time (s)
30s
30 s 5s 5s
I.5 4.5 12.5 12.5
v v V V
((((-
70°C) 200°C) 2000°C) 2000°C)
20 lo-30
heating process, causes a pressure pulse, the background signal changes [13, 181. This can be corrected for with a blank measurement [18]. However, in the case that an interfering element is present (e.g. as a matrix element), a blank measurement cannot be used to correct for the background changes. This occurs, for instance. when Pb (7 mg/l) is measured in a 6 g/l Al matrix, as in many environmental samples. DARKE er al. [28] investigated this problem and found that Al only vaporises above 14OO”C, so that a separation based on a temperature program is possible. If, however, the analyte vaporization coincides with that of the matrix, a background correction procedure is necessary [14]. To avoid difficult temperature programming, we applied the wavelength modulation technique in the situation of Pb in an Al matrix. As can be seen in Fig. la, first only Pb (220.353 nm) is measured, since this is the more volatile element. Then 3.2 s after the start of the pyrolysis II step, the corrected Pb peak area amounts to 6615 units, while the background is only 688. In Fig. lb, which shows the spectrum obtained 1.1 s later than that in Fig. la, Al (220.462 and 220.467 nm) starts interfering with the Pb peak. At this point the emission signal of Pb has reached its maximum: the corrected peak area amounts to 7704 and the background has increased to 3043. Another 0.72 s later (Fig. lc), the AI peak has reached its maximum while the Pb peak is already much smaller. It is important to mention that the corrected Pb peak area is now reduced to 3417 while the background still amounts to 2970. Finally, in the wavelength spectrum taken 7.4 s after the start of the pyrolysis II step (Fig. Id) the Pb signal has disappeared, while the background still is equivalent to 1043. This experiment illustrates that, in the considered situation, when peak heights or peak areas are used without background correction, large errors can be made. This is illustrated in Fig. 2,
102
Technical note
where both the corrected and uncorrected peak areas are shown as a function of time. Integration of the temporally resolved signal gives for the uncorrected signal 81975 unitss and 47487 units.s for the corrected signal. This means that errors of more than 70% can be made when no background correction is applied.
4.
CONCLUSION
When using sample introduction systems yielding transient signals, a background correction, becomes necessary when interfering elements are present. The system described here, based on wavelength modulation using a refractor plate, is ideally suited for this purpose.
5.
SUMMARY
A method, based on wavelength modulation using a refractor plate is described for an accurate background correction in electrothermal vaporization inductively coupled plasma emission spectrometry (ETV-ICP-AES). The importance of such a system is illustrated for situations where interferences occur. Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, University of Gent, Proeftuinstraat 86, B-9000 Gent, Belgium
P. VERREPT C. VANDECASTEELE* G. WINDELS
R. DAMS
REFERENCES Matusiewicz, J. Anal. AI. Spectrom. 1, 171 (1986). F. Browner and A. W. Boom. Anal. Chem. 56, 787A (1984). Kumamaru, Y. Okamoto and H. Matsuo, Appl. Spcctrosc. 41. 918 (1987). Matusiewicz, F. L. Fricke and R. M. Barnes, J. Anal. At. Spectrom. 1. 203 (1986). J. Sneddon and F. Bet-Pera. Trends Anul. Chem. 5, 110 (19R6). ;z; R. M. Barnes and P. Fodor, Spectrochim. ACM 388, 1191 (1983). [71 J. R. Dean and R. C. Snook, J. Anal. At. Spectrom. 1, 461 (1986). PI D. Klaus, H. Berndt. J. A. C. Broekaert, G. Schaldach and G. Tolg. J. Anal. AI. Spectrom. 3. 1105 (1988). 191A. M. Gunn, D. L. Millard and G. F. Kirkbright, Analysr 103. 1066 (197X). IlW D. L. Millard, H. C. Shan and G. F. Kirkbright. Analysr 105. 502 (1980). S. M. Schmertmann, S. E. Long and R. F. Browner, J. Anal. AL Spectrom. 2. 687 (1987). t::; C. Ng Kin and J. A. Caruso, Anal. Chem. 55. 1513 (1983). iI31 C. Ng Kin and J. A. Caruso, Appl. Specrrosc. 39. 719 (1985). 1141G. Crabi, P. Cavalli. M. Achilli, G. Rossi and N. Ometto, AI. Specrrom. 3. XI (1982). [W R. K. Skogerboe, P. J. Lamothe, G. J. Bastiaans, S. J. Frecland and G. N. Coleman. Appl. Specfrosc. 30, 495 (1976). [161 J. A. C. Broekaert, W.-D. Hagenah, K. Laqua. F. Leis and D. Stiiwer. Specrrochim. AC& 41B. 1357 H. ;:; R. [31 T. [41 H.
(1986).
1171C. C. Wohlers, R. G. Schleicher, M. A. Sainz and D. D. Nygaard. ICP Information Newsleft. 12. Abstract 231, 190 (1986).
iI81 J. Alvarado. P. Cavalli, N. Omenetto.
G. Rossi and J. M. Ottaway. J. AM/. At. Spectrom. 2. 357
(1987).
1191W. Snelleman. T. C. Rains, K. W. Yee. H. D. Cook and 0. Menis. Anul. Chem. 42. 394 (1970). WI D. C. Baxter, I. S. Duncan, D. Littlejohn. J. Marshall, J. M. Ottaway. G. S. Fell and 0. Y. Ataman, J. Anal. At. Spectrom.
1, 29 (1986).
PII J. M. Ottaway, L. Bezur and J. Marshall. Analysr 105. 1130 (1980). P21 T. C. O’Haver. Anal. C/rem. 51, 91A (1979). v31 E. Courtijn, P. Verrept, C. Vandecasteele, G. Windels and R. Dams, to be published. M. Thompson, J. E. Goulter and F. Sieper. Analysl 106. 32 (1981).
t::; K. A. Vermeiren, P. D. P. Taylor, R. Dams. J. Anal. At. Spec/rom. 3. 571 (198X). WI P. Verrept and C. Vandecasteele. unpublished results. C. Vandecasteele,
G. Windels, B. Desmet. A. De Ruck and R. Dams, AMI/W 113. I691 (1988).
;i’x; S. A. Darke, C. J. Pickford and J. F. Tyson, Anul. Proc. 26. 379 (1989).
* Research Director of the Belgian National Fund for Scientific Research.
Spectrochrmrco Aclo Vol. 46. No 1, pp. 99-102. 1991
Printed m GreatBritain.
TECHNICAL NOTE
Use of a refractor plate for automatic background correction in electrothermal vaporization inductively coupled plasma atomic emission spectrometry (Received 6 July 1990; accepted 19 September 1990)
1.
INTR~~UC~~N
THE USE
of electrothermal vaporization (ETV) for sample introduction in inductively coupled plasma atomic emission spectrometry (ICP-AES) has been reported by many authors [l-8]. The most attractive features of this technique are: only a small sample volume is required (S-50 pl), high sensitivity is obtained because of the high transport efficiency [P-11, 131, and samples containing high concentrations of salts can be analysed [12]. One of the disadvantages is the difficulty in making an accurate background correction. Most of the time, determinations were made without a background correction, although many authors have indicated the importance of appropriate background correction in work involving transient signals [U-18]. During the heating process of the ETV unit, the nebulizer gas (Ar) expands rapidly thus causing a change of the background [13, 181. One approach to correct for the changing background as a function of time is to measure the intensity at the peak maximum as a function of time and to integrate the obtained signal. Afterwards the background is determined, using a blank, over the same integration period and subtracted [18]. However, this does not take into account the possible presence of interfering elements from the sample which may influence the background and it assumes that the background changes in the same way for blank and sample measurements [14, 171. Therefore it is better to use an automatic background correction by means of a wavelength modulation system. The difficulty is to measure the background quasi-simultaneously with the peak, so that evolutions in background level can be followed and corrected for. This paper describes a wavelength modulation method for background correction for ETV-ICP-AES, based on the method described by SNELLEMAN[19]. The system was implemented on an existing commercial spectrometer, the Jobin Yvon VHR 1000. A computer program was developed for the automatization of the device and for data handling. The system is somewhat similar to the one described by BAXTER et al. [20], and OITAWAY et al. [21] for electrothermal atomisation atomic emission spectrometry (ETA-AES) [20,21] and to the one described by O’HAVER et al. [22] for simultaneous multi-element atomic absorption with a continuum source.
2.
EXPERIMENTAL
2.1. Instrumentation An existing Jobin Yvon VHR 1000 monochromator was modified. A refractor plate was inserted behind the entrance slit to obtain automatic background correction by wavelength modulation, as described in detail in Ref. [23]. By oscillating the plate at a high frequency (l-40 Hz) a scan can be made over a certain wavelength interval. The plate is oscillated using an optical scanner Model G325D and controlled by an AT computer implemented by an A660 driver amplifier card delivered with the scanner. For this work the commercial P.S. Analytical 60.800 ETV unit was used, but the technique described here can be used as well for other types of sample introduction yielding a transient signal (e.g. flow injection [2], laser ablation [24], thermospray ICP-AES [25]). 2.2. Description of the wavelength scanning technique The technique was first tested and optimized on the ICP with a pneumatic nebulizer (Meinhard TR-50-C3) as sample introduction system [26]. which yields a continuous signal. The movement 99
100
Technical note
of the refractor plate is synchronized with the detection system. A scan consists of 10 to 160 steps, to be set at the beginning of the measurement together with the scan frequency. The signal obtained by the photomultiplier (Hamamatsu R106UH). during one step is sent to an AD converter and stored into the memory of the computer. After the first scan, the following ones are added to the former so that the obtained spectrum is the summation of many scans (20-500, depending on program parameters) and can be interpreted and visualized on a computer screen. The program allows it to select, with a pair of cursors, the borders of the peak. Also regions to calculate the background on the left- and the right-hand side of the peak, can be chosen by means of two pairs of cursors. The background to be subtracted is calculated by linear interpolation between the mean left- and right-hand background. This has been described in detail by VANDECASTEELE et al. [27]. When using ETV as a sample introduction system a transient emission signal is obtained. In this case it is necessary to visualize the time resolved emission signal, so that the arrival and, even more importantly, the wash-out of the analyte can be followed. Therefore every scan is stored in a different range of addresses. After choosing the most intense peak and setting the cursors for the calculation of the background, as described above (Fig. la-d), all the other scans are automatically corrected in the same way. The corrected peak areas are then integrated and the time resolved emission signal is visualized on the computer monitor (Fig. 2). 2.3. Operating conditions The ICP used, was a Plasma Therm ICP 2500. The operating conditions ETV units are listed in Table 1.
3.
for the ICP and the
RESULTSAND DISCUSSION
As quoted earlier, the wavelength modulation technique was first optimized with a pneumatic nebuliser. No degradation of resolution was found by inserting the quartz plate nor any loss of stability or sensitivity. Therefore one might conclude that the insertion of the refractor plate had no effect on the quality of the monochromator. Of course the advantages of the system become more valuable when measuring transient signals. Since the expanding gas, during the
Channel
161
Channel H3
P
B %
Channel
M
I&I
iII
d)
L Channel
‘a
Fig. 1. Background changes caused by an interfering element. Spectrum of Pb (220.353 nm) in an Al matrix (220.462 nm and 220.467 nm) obtained resp. 3.2 s (a). 4.3 s (b). 6.0 s (c). 7.4 s (d) after the start of pyrolysis II.
101
Technical note lntearated
peak area8 (Thouaandd
I3
a
10
12
14
lime (8) Fig. 2. Integration of the temporally resolved peak areas. Influence of interfering elements on the background changes. Ic Background corrected peak areas; + uncorrected peak areas.
Table 1. Operating conditions for ICP and ETV ICP
Forward power (kW) Reflected power (W) Outer gas flow (I/min) Intermediate gas flow (l/min) Observation height (mm)
I <5 I5 3.75 I7
ETV
Nebuliser gas flow (I/min) Make Up gas flow (I/min) Cooling gas flow (He) (I/min) Heating program: Evaporate Pyrolysis I Pyrolysis II Vaporise
I.5 0.2 1.5
Sample volume (111) Measuring time (s)
30s
30 s 5s 5s
I.5 4.5 12.5 12.5
v v V V
((((-
70°C) 200°C) 2000°C) 2000°C)
20 lo-30
heating process, causes a pressure pulse, the background signal changes [13, 181. This can be corrected for with a blank measurement [18]. However, in the case that an interfering element is present (e.g. as a matrix element), a blank measurement cannot be used to correct for the background changes. This occurs, for instance. when Pb (7 mg/l) is measured in a 6 g/l Al matrix, as in many environmental samples. DARKE er al. [28] investigated this problem and found that Al only vaporises above 14OO”C, so that a separation based on a temperature program is possible. If, however, the analyte vaporization coincides with that of the matrix, a background correction procedure is necessary [14]. To avoid difficult temperature programming, we applied the wavelength modulation technique in the situation of Pb in an Al matrix. As can be seen in Fig. la, first only Pb (220.353 nm) is measured, since this is the more volatile element. Then 3.2 s after the start of the pyrolysis II step, the corrected Pb peak area amounts to 6615 units, while the background is only 688. In Fig. lb, which shows the spectrum obtained 1.1 s later than that in Fig. la, Al (220.462 and 220.467 nm) starts interfering with the Pb peak. At this point the emission signal of Pb has reached its maximum: the corrected peak area amounts to 7704 and the background has increased to 3043. Another 0.72 s later (Fig. lc), the AI peak has reached its maximum while the Pb peak is already much smaller. It is important to mention that the corrected Pb peak area is now reduced to 3417 while the background still amounts to 2970. Finally, in the wavelength spectrum taken 7.4 s after the start of the pyrolysis II step (Fig. Id) the Pb signal has disappeared, while the background still is equivalent to 1043. This experiment illustrates that, in the considered situation, when peak heights or peak areas are used without background correction, large errors can be made. This is illustrated in Fig. 2,
102
Technical note
where both the corrected and uncorrected peak areas are shown as a function of time. Integration of the temporally resolved signal gives for the uncorrected signal 81975 unitss and 47487 units.s for the corrected signal. This means that errors of more than 70% can be made when no background correction is applied.
4.
CONCLUSION
When using sample introduction systems yielding transient signals, a background correction, becomes necessary when interfering elements are present. The system described here, based on wavelength modulation using a refractor plate, is ideally suited for this purpose.
5.
SUMMARY
A method, based on wavelength modulation using a refractor plate is described for an accurate background correction in electrothermal vaporization inductively coupled plasma emission spectrometry (ETV-ICP-AES). The importance of such a system is illustrated for situations where interferences occur. Laboratory of Analytical Chemistry, Institute for Nuclear Sciences, University of Gent, Proeftuinstraat 86, B-9000 Gent, Belgium
P. VERREPT C. VANDECASTEELE* G. WINDELS
R. DAMS
REFERENCES Matusiewicz, J. Anal. AI. Spectrom. 1, 171 (1986). F. Browner and A. W. Boom. Anal. Chem. 56, 787A (1984). Kumamaru, Y. Okamoto and H. Matsuo, Appl. Spcctrosc. 41. 918 (1987). Matusiewicz, F. L. Fricke and R. M. Barnes, J. Anal. At. Spectrom. 1. 203 (1986). J. Sneddon and F. Bet-Pera. Trends Anul. Chem. 5, 110 (19R6). ;z; R. M. Barnes and P. Fodor, Spectrochim. ACM 388, 1191 (1983). [71 J. R. Dean and R. C. Snook, J. Anal. At. Spectrom. 1, 461 (1986). PI D. Klaus, H. Berndt. J. A. C. Broekaert, G. Schaldach and G. Tolg. J. Anal. AI. Spectrom. 3. 1105 (1988). 191A. M. Gunn, D. L. Millard and G. F. Kirkbright, Analysr 103. 1066 (197X). IlW D. L. Millard, H. C. Shan and G. F. Kirkbright. Analysr 105. 502 (1980). S. M. Schmertmann, S. E. Long and R. F. Browner, J. Anal. AL Spectrom. 2. 687 (1987). t::; C. Ng Kin and J. A. Caruso, Anal. Chem. 55. 1513 (1983). iI31 C. Ng Kin and J. A. Caruso, Appl. Specrrosc. 39. 719 (1985). 1141G. Crabi, P. Cavalli. M. Achilli, G. Rossi and N. Ometto, AI. Specrrom. 3. XI (1982). [W R. K. Skogerboe, P. J. Lamothe, G. J. Bastiaans, S. J. Frecland and G. N. Coleman. Appl. Specfrosc. 30, 495 (1976). [161 J. A. C. Broekaert, W.-D. Hagenah, K. Laqua. F. Leis and D. Stiiwer. Specrrochim. AC& 41B. 1357 H. ;:; R. [31 T. [41 H.
(1986).
1171C. C. Wohlers, R. G. Schleicher, M. A. Sainz and D. D. Nygaard. ICP Information Newsleft. 12. Abstract 231, 190 (1986).
iI81 J. Alvarado. P. Cavalli, N. Omenetto.
G. Rossi and J. M. Ottaway. J. AM/. At. Spectrom. 2. 357
(1987).
1191W. Snelleman. T. C. Rains, K. W. Yee. H. D. Cook and 0. Menis. Anul. Chem. 42. 394 (1970). WI D. C. Baxter, I. S. Duncan, D. Littlejohn. J. Marshall, J. M. Ottaway. G. S. Fell and 0. Y. Ataman, J. Anal. At. Spectrom.
1, 29 (1986).
PII J. M. Ottaway, L. Bezur and J. Marshall. Analysr 105. 1130 (1980). P21 T. C. O’Haver. Anal. C/rem. 51, 91A (1979). v31 E. Courtijn, P. Verrept, C. Vandecasteele, G. Windels and R. Dams, to be published. M. Thompson, J. E. Goulter and F. Sieper. Analysl 106. 32 (1981).
t::; K. A. Vermeiren, P. D. P. Taylor, R. Dams. J. Anal. At. Spec/rom. 3. 571 (198X). WI P. Verrept and C. Vandecasteele. unpublished results. C. Vandecasteele,
G. Windels, B. Desmet. A. De Ruck and R. Dams, AMI/W 113. I691 (1988).
;i’x; S. A. Darke, C. J. Pickford and J. F. Tyson, Anul. Proc. 26. 379 (1989).
* Research Director of the Belgian National Fund for Scientific Research.