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Modifications of the roughing pump system of an Elan 500 ICP-MS for analysing complex matrices
Spefrrochtmrca Acto, Vol 47B.No.4,pp.%5-S90, 1992 Pnnted in Great Britain.
LABORATORY NOTE Modifications of the roughing pump system of an Elan 500 ICP-MS for analysing complex matrices (Received 1 May 1991; accepted 13 September 1991)
INTRODUCTION originating from process research provide, in most cases, very complex matrices for ICP-MS analytical work. Analysis needs to be performed, for instance, on highly concentrated inorganic solutions and on different kinds of organics. Studying the ICP-MS literature results in the following relevant references. For analysis of inorganic solutions, a number of references describe separation/preconcentration procedures [l-4] to prevent the problems associated with the introduction of highly concentrated solids solutions. Flow injection is also used as a sample introduction system for ICP-MS [S,6]. Continuous nebulization is used for different analytical problems [7-lo], but the applications described are limited to solutions of a few per cent of dissolved solids. The solid deposition on samplers and skimmers was studied by means of the pressure change in the interface and ion optics region [ll]. The analysis of geological samples by slurry nebuli~tion ICP-MS is also reported [12]. Here, some modifications to the standard ICP-MS operating conditions were necessary. Solid sample introduction into an ICP-MS is extensively studied in Refs [H-16]. The analysis of organic solutions is carried out after destruction of the organic matrix [17] and by continuous nebulization of the organic solutions while oxygen is added to avoid problems related to carbon deposition on the sampler [16-191. For crud: oil, an extraction procedure is described [20]. The applications of ICP-MS in the petroleum industry are presented in Ref. [6]. The author concludes that the analysis of organics by ICP-MS is still in its infancy and up to now limited to the “easy ones”. The ICP-MS inst~ment was used for the determination of platinum, palladium and germanium in highly concentrated phosphoric acid (25% m/v) and ammonium nitrate (12% m/v) by means of a standard addition flow injection technique [21]. A comparable matrix (phosphoric acid plus sodium nitrate) was examined also by FI-ICP-MS for 50 elements. The outstanding properties of ICP-MS serving as a chromatographic detector were used during the determination of tellurium compounds by LC-ICP-MS 1221. Natural gas condensate, a very complex organic matrix, was analysed for the presence of trace element concentrations. All these applications, which are rather extreme compared to the applications reported in the literature, put an extra burden on the quality and mainten~ce of the instrument. Due to the matrices analysed, the instrument frequently suffered from vacuum problems, caused by one of the solenoid valves. This article describes the modifications installed in the DSM Research instrument necessary when analysing complex matrices with a Perkin-Elmer Sciex Elan 500. SAMPLES
VACUUM PROBLEMSCAUSED BY COMPLEXMATRICES The set-up of an Elan 500 ICP-MS is outlined in Fig. 1. The vacuum in the first vacuum stage was maintained by an Edwards E2Ml8 roughing pump (Edwards, High vacuum, Manor Royal, Crawley, West Sussex RHO 2LW, U.K.). This pump was connected to the interface region when solenoid valve 1 is opened and valve 2 is closed. The high vacuum in the mass spectrometer was maintained by means of a CT1 cryogenic pump (CD-cryogenics, Kelvin Park, Waltham, MA, U.S.A.). During two to ‘three weeks’ operation, an argon crust builds up on the cryoshells that surround the mass spectrometer in the main recipient. This argon crust causes a less effective energy transport and a decreasing pump capacity. Because of this, the vacuum cannot be maintained sufficiently and the instrument needs to be ‘cryocleaned’. During cryoclean, solenoid valve 2 was opened, valve 1 was closed and the cryogenic pump was stopped. The inst~ment “warms up” and the solid argon starts to evaporate. It was removed from the mass spectrometer by means of the Edwards E2M18 pump.
586
Laboratory
note
solenoid valve 1
Edwards E2 Ml8 Roughing pump
Fig. 1. Schematic set-up of a Perkin-Elmer
Sciex Elan 500 ICP-MS.
After a few hours, when all the vapours were removed, the vacuum passed a trip level at which valve 2 was closed and the cryogenic pump took over. After approximately 4-5 hr, the vacuum level necessary for analysis was reached. Cryocleaning was carried out under computer control, for convenience always at night, so that the instrument can return to the ‘READYstate at daytime. After analysing the complex matrices described in the Introduction, the instrument frequently did not reach the ‘READY’-state after performing a cryoclean. Each time, solenoid valve 1 was the cause of the problem because of a leaking ‘0’~ring seal inside the valve. While pumping down, this leak prevented the cryogenic pump from taking over. Inspection of the valve always showed the “analytical history” of the instrument. In Fig. 2, an Edwards solenoid valve is shown, opened for maintenance. After analysing solutions with high concentrations of inorganic material, deposits could be found, both on the viton ‘O’-ring, Fig. 3, and the valve seat. So, in spite of using flow injection as the sample introduction technique, deposits could not be avoided. After analysing only organics for a prolonged period, no inorganic salts had accumulated in the valve, but still every two or three weeks the valve caused vacuum problems. As a result of the addition of oxygen to the plasma, ozone and oxygen radicals were formed. Most probably, these compounds damaged the O-ring in such a way that it looked as if small cuts had been made (Fig. 4). It clearly can be seen that this damage is only present in one half of the O-ring, in the half most exposed to the gas originating from the plasma. In summary, both the addition of excess oxygen and the analysis of liquids with high dissolved solid contents, made it necessary to change the O-ring and clean the solenoid valve 1 every two or three weeks. Every time this maintenance had to be carried out, the instrument was down for at least one day.
Fig. 2. Edwards solenoid valve opened for maintenance.
587
Fig. 3. Close-up of the viton O-ring inside the Edwards solenoid valve, afteianalysing with high concentrations of dissolved solids.
solutions
Fig. 4. Damage to the viton O-ring after analysing organics during two weeks.
HARDWARE MODIFICATIONS The Elan 500 ICP-MS has been modified as outlined schematically in Fig. 5. In this set-up there are two separate roughing pump lines: line 1 serves the interface region; and line 2 is intended for cryoclean operation only. As a result of this, solenoid valve 1 could be removed.
Roughing
pump 2
Fig. 5. Schematic set-up of the modified Perkin-Elmer
Roughing
pump1
Sciex Elan 500 ICP-MS.
Laboratory note
588
Fig. 6. Overview of all;the modifications installed in the Elan 500 ICP-MS.
In Fig. 6, all modifications
Fig. 7. Solenoid valve 2 mounted at approximately 10 cm distance from the mass spectrometer body.
are shown. Both Edwards E2M18 roughing pumps were installed at the back of the instrument and connected to the first vacuum stage and mass spectrometer by means of addit,ional NW 20125 flexible piping or elbows and flanges, respectively (Edwards, Crawley, U.K.). In the original instrument set-up, solenoid valve 2 was mounted directly on the mass spectrometer body. At this place, it is very difficult to reach for maintenance. For this reason, a home-made connecting-pipe (length 9 cm) was installed between valve 2 and the mass spectrometer body, (Fig. 7). To ensure complete leak tightness, this’pipe was connected to the spectrometer body by means of a home-made ring, completely encapsulating the connecting flange. The thermocouple originally used in the first vacuum stage (TC 2) was placed in line 2, between the Edwards pump and the solenoid valve. This thermocouple was used to check the performance of roughing pump 2 when the solenoid valve was closed. TC 2 also provided a “second opinion” on the vacuum level in the mass spectrometer during cryoclean, normally monitored by TC 1. In order to continuously monitor the pressure in line 1, an Edwards BarocelTM Type 600 pressure sensor, equipped with an Edwards Type 1500 digital pressure display and power supply, was installed and connected to line 1 by means of an NW 25120 T-piece and an NW 25110 reducing piece (Edwards, Crawley, U.K.). This pressure sensor had a range of O-10 torr. The pressure in line 1 was the result of the diameter of the sampler orifice, the resistance of line 1 and the pumping capacity of roughing pump 1. Because of the excellent accuracy (0.15% of reading) and precision (2 0.001 torr) of the BarocelTM pressure sensor, a continuous performance status for the first vacuum stage was provided. As can be seen in Fig. 8, the pressure equals approximately 6.1 torr one hour after instrument start-up. Experiments showed that after a lhr warm-up, the pressure was constant for more than 8 hr within a -C 0.025 torr limit. Even the optimization of the nebulizer argon flow 10 min after start-up, resulting in a change in argon flow by approximately 0.1 l/min, is clearly visible.
589
Laboratory note
I
4
3
2 Time (hours)
1
I
1
0
Fig. 8. Pressure in the first vacuum stage of the Elan 500 ICP-MS, starting from the ignition of the plasma (0 hr). ELECTRICALMODIFICATIONS As described above, the roughing pump serves two tasks: (1) Initial pumpdown and cryoclean of the main recipient containing during ‘stage 1’ and ‘stage 2’ until the cryogenic pump takes over; (2) Maintaining state.
an intermediate
the mass spectrometer
vacuum in the interface during sample analysis in the “READY”-
All operational functions in the instrument were controlled by a number of relays on a large printed circuit board also containing the unregulated power supplies. Relay K4 controls between “READY” and pumpdown, as far as the roughing pump was concerned. To separate these operations, the contact of relay K4 that activates the roughing pump relay K3 at “READY”-time, was disconnected. Another, parallel contact of K4 was used to control a new relay, identical to K3, which itself controls the new roughing pump which pumps the intermediate vacuum only. In practice, these modifications consist of simply cutting a track on the relay board and mounting a new relay on a small bracket screwed to the supporting frame on the relay board. Some additional wiring completes the modification. For convenience, the electrical supply outlets for both roughing pumps were mounted on the rear switch panel of the Elan. They were clearly marked with the same number as the vacuum line to which the corresponding pump was connected. Care has to be taken to derive the power for these pumps from the proper point in the 220 V circuit, i.e. directly from the 20 A circuit breaker. In addition, relay K7, controlling the removed solenoid valve 1, can be removed from the relay board.
CONCLUSIONS The original Perkin-Elmer Sciex Elan 500 was installed at DSM Research in July 1988, with the intention to use it for the complete spectrum of analytical requirements coming up in an industrial research laboratory. The somewhat extreme applications put an extra burden on the performance and maintenance of the instrument. This led to the modifications described in this article. After more than a year of operation of this modified instrument, not a single maintenance had to be carried out on the remaining solenoid valve. Maintenance on the first vacuum stage was limited to changing the oil in roughing pump 1 once a month and cleaning the interface region once every three or four weeks. So, although the modified Elan 500 was used for the extreme applications indicated in the Introduction, performance and maintenance of this instrument were within the expectations and specifications of both user and manufacturer.
SUMMARY When a Perkin-Elmer Sciex Elan 500 ICP-MS instrument is used for extreme applications like analysing samples with very high solid content, the solenoid valve built into the first vacuum
590
Laboratory note
stage causes vacuum problems resulting in an at least one day’s instrument downtime every two or three weeks. Modifications of the roughing pump system resulted in stable and unattended operation of the first vacuum stage for more than a year. This article describes these modifications, which can be carried out in every Elan 250 or 500 ICP-MS. DSM Research
H. KLINKENBERG* ZICHENG PENG~ T. BEEREN
FA-AE
P.O. Box 18 6160 MD Geleen The Netherlands Pet-kin-Elmer Nederland B. V. P.O. Box 490 2800 AL Gouda The Netherlands
K. FLACH
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
M.R. Plan& J.S. Fritz, F. G. Smith and R. S. Houk, Analyl. Chem. 61, 149 (1989). A. Makishima, I. Inamoto and K. Chiba, Appf. Specwosc. 44, 91 (1990). Y. Nakamura and T. Fukuda, Bunseki Kagaku 39, T17 (1990). M. R. Plan& Energy Res. Abstr. 14(9), Abstr. no. 17911 (1989). R. C. Hutton and A. N. Eaton, J. Anafyt. Atom. Specrrom. 3, 547 (1988). A. van Heuzen, Dissertation, Inductively coupled plasma-mass spectrometry: a spectrum, The University of Amsterdam, Holland. C. J. Pickford and R. M. Brown, Spectrochim. Acta 41B, 183 (1986). T. J. Brotherton, W. L. Shen and J.A. Caruso, J. Analyt. Atom. Spectrom. 4, 39 (1989). J. G. Williams and A. L. Gray, Analyt. Proc. 25, 385 (1988). S. A. Wood and D. M. M. Vlassopoulos, Analytica chim. Acta 229, 227 (1990). D. J. Douglas and L. A. Kerr, J. Analyt. Atom. Spectrom. 3, 749 (1988). K. E. Jarvis and J. G. Williams, Chem. Geol. 77, 53 (1989). L. Blain, E. D. Salin and D. W. Boomer, J. Analyt. Atom. Spectrom. 4, 721 (1989). V. Karanassios and G. Horlick, Spectrochim. Acta 44B, 1345 (1989). V. Karanassios and G. Horlick, Specrrochim. Acta 4lB, 1361 (1989). V. Karanassios and G. Horlick, Spectrochim. Acta 44B, 1387 (1989). A. Boorn, J.E. Fulford and W. Wegscheider, Mikrochim. Actu 2, 171 (1985). R. C. Hutton, J. Analyr. Atom. Spectrom. 1, 259 (1986). D. Hausler, Spectrochim. Actu 42B, 63 (1987). H. M. Al Swaidan, Analyt. Letr. 21, 1487 (1988). Z. Peng, H. Klinkenberg, A. Beeren and W. Van Borm, Spectrochim. Acra 46B, 1051 (1991). H. Klinkenberg, S. van der Wal, J. Frusch, L. Terwint and A. Beeren, Atom. Spectrosc. 11, 198 (1990).
*Author to whom correspondence should be addressed. tPresent address: Dept Earth and Space Science, University of Science and Technology of China, Hefei, Anhui, People’s Republic of China.
LABORATORY NOTE Modifications of the roughing pump system of an Elan 500 ICP-MS for analysing complex matrices (Received 1 May 1991; accepted 13 September 1991)
INTRODUCTION originating from process research provide, in most cases, very complex matrices for ICP-MS analytical work. Analysis needs to be performed, for instance, on highly concentrated inorganic solutions and on different kinds of organics. Studying the ICP-MS literature results in the following relevant references. For analysis of inorganic solutions, a number of references describe separation/preconcentration procedures [l-4] to prevent the problems associated with the introduction of highly concentrated solids solutions. Flow injection is also used as a sample introduction system for ICP-MS [S,6]. Continuous nebulization is used for different analytical problems [7-lo], but the applications described are limited to solutions of a few per cent of dissolved solids. The solid deposition on samplers and skimmers was studied by means of the pressure change in the interface and ion optics region [ll]. The analysis of geological samples by slurry nebuli~tion ICP-MS is also reported [12]. Here, some modifications to the standard ICP-MS operating conditions were necessary. Solid sample introduction into an ICP-MS is extensively studied in Refs [H-16]. The analysis of organic solutions is carried out after destruction of the organic matrix [17] and by continuous nebulization of the organic solutions while oxygen is added to avoid problems related to carbon deposition on the sampler [16-191. For crud: oil, an extraction procedure is described [20]. The applications of ICP-MS in the petroleum industry are presented in Ref. [6]. The author concludes that the analysis of organics by ICP-MS is still in its infancy and up to now limited to the “easy ones”. The ICP-MS inst~ment was used for the determination of platinum, palladium and germanium in highly concentrated phosphoric acid (25% m/v) and ammonium nitrate (12% m/v) by means of a standard addition flow injection technique [21]. A comparable matrix (phosphoric acid plus sodium nitrate) was examined also by FI-ICP-MS for 50 elements. The outstanding properties of ICP-MS serving as a chromatographic detector were used during the determination of tellurium compounds by LC-ICP-MS 1221. Natural gas condensate, a very complex organic matrix, was analysed for the presence of trace element concentrations. All these applications, which are rather extreme compared to the applications reported in the literature, put an extra burden on the quality and mainten~ce of the instrument. Due to the matrices analysed, the instrument frequently suffered from vacuum problems, caused by one of the solenoid valves. This article describes the modifications installed in the DSM Research instrument necessary when analysing complex matrices with a Perkin-Elmer Sciex Elan 500. SAMPLES
VACUUM PROBLEMSCAUSED BY COMPLEXMATRICES The set-up of an Elan 500 ICP-MS is outlined in Fig. 1. The vacuum in the first vacuum stage was maintained by an Edwards E2Ml8 roughing pump (Edwards, High vacuum, Manor Royal, Crawley, West Sussex RHO 2LW, U.K.). This pump was connected to the interface region when solenoid valve 1 is opened and valve 2 is closed. The high vacuum in the mass spectrometer was maintained by means of a CT1 cryogenic pump (CD-cryogenics, Kelvin Park, Waltham, MA, U.S.A.). During two to ‘three weeks’ operation, an argon crust builds up on the cryoshells that surround the mass spectrometer in the main recipient. This argon crust causes a less effective energy transport and a decreasing pump capacity. Because of this, the vacuum cannot be maintained sufficiently and the instrument needs to be ‘cryocleaned’. During cryoclean, solenoid valve 2 was opened, valve 1 was closed and the cryogenic pump was stopped. The inst~ment “warms up” and the solid argon starts to evaporate. It was removed from the mass spectrometer by means of the Edwards E2M18 pump.
586
Laboratory
note
solenoid valve 1
Edwards E2 Ml8 Roughing pump
Fig. 1. Schematic set-up of a Perkin-Elmer
Sciex Elan 500 ICP-MS.
After a few hours, when all the vapours were removed, the vacuum passed a trip level at which valve 2 was closed and the cryogenic pump took over. After approximately 4-5 hr, the vacuum level necessary for analysis was reached. Cryocleaning was carried out under computer control, for convenience always at night, so that the instrument can return to the ‘READYstate at daytime. After analysing the complex matrices described in the Introduction, the instrument frequently did not reach the ‘READY’-state after performing a cryoclean. Each time, solenoid valve 1 was the cause of the problem because of a leaking ‘0’~ring seal inside the valve. While pumping down, this leak prevented the cryogenic pump from taking over. Inspection of the valve always showed the “analytical history” of the instrument. In Fig. 2, an Edwards solenoid valve is shown, opened for maintenance. After analysing solutions with high concentrations of inorganic material, deposits could be found, both on the viton ‘O’-ring, Fig. 3, and the valve seat. So, in spite of using flow injection as the sample introduction technique, deposits could not be avoided. After analysing only organics for a prolonged period, no inorganic salts had accumulated in the valve, but still every two or three weeks the valve caused vacuum problems. As a result of the addition of oxygen to the plasma, ozone and oxygen radicals were formed. Most probably, these compounds damaged the O-ring in such a way that it looked as if small cuts had been made (Fig. 4). It clearly can be seen that this damage is only present in one half of the O-ring, in the half most exposed to the gas originating from the plasma. In summary, both the addition of excess oxygen and the analysis of liquids with high dissolved solid contents, made it necessary to change the O-ring and clean the solenoid valve 1 every two or three weeks. Every time this maintenance had to be carried out, the instrument was down for at least one day.
Fig. 2. Edwards solenoid valve opened for maintenance.
587
Fig. 3. Close-up of the viton O-ring inside the Edwards solenoid valve, afteianalysing with high concentrations of dissolved solids.
solutions
Fig. 4. Damage to the viton O-ring after analysing organics during two weeks.
HARDWARE MODIFICATIONS The Elan 500 ICP-MS has been modified as outlined schematically in Fig. 5. In this set-up there are two separate roughing pump lines: line 1 serves the interface region; and line 2 is intended for cryoclean operation only. As a result of this, solenoid valve 1 could be removed.
Roughing
pump 2
Fig. 5. Schematic set-up of the modified Perkin-Elmer
Roughing
pump1
Sciex Elan 500 ICP-MS.
Laboratory note
588
Fig. 6. Overview of all;the modifications installed in the Elan 500 ICP-MS.
In Fig. 6, all modifications
Fig. 7. Solenoid valve 2 mounted at approximately 10 cm distance from the mass spectrometer body.
are shown. Both Edwards E2M18 roughing pumps were installed at the back of the instrument and connected to the first vacuum stage and mass spectrometer by means of addit,ional NW 20125 flexible piping or elbows and flanges, respectively (Edwards, Crawley, U.K.). In the original instrument set-up, solenoid valve 2 was mounted directly on the mass spectrometer body. At this place, it is very difficult to reach for maintenance. For this reason, a home-made connecting-pipe (length 9 cm) was installed between valve 2 and the mass spectrometer body, (Fig. 7). To ensure complete leak tightness, this’pipe was connected to the spectrometer body by means of a home-made ring, completely encapsulating the connecting flange. The thermocouple originally used in the first vacuum stage (TC 2) was placed in line 2, between the Edwards pump and the solenoid valve. This thermocouple was used to check the performance of roughing pump 2 when the solenoid valve was closed. TC 2 also provided a “second opinion” on the vacuum level in the mass spectrometer during cryoclean, normally monitored by TC 1. In order to continuously monitor the pressure in line 1, an Edwards BarocelTM Type 600 pressure sensor, equipped with an Edwards Type 1500 digital pressure display and power supply, was installed and connected to line 1 by means of an NW 25120 T-piece and an NW 25110 reducing piece (Edwards, Crawley, U.K.). This pressure sensor had a range of O-10 torr. The pressure in line 1 was the result of the diameter of the sampler orifice, the resistance of line 1 and the pumping capacity of roughing pump 1. Because of the excellent accuracy (0.15% of reading) and precision (2 0.001 torr) of the BarocelTM pressure sensor, a continuous performance status for the first vacuum stage was provided. As can be seen in Fig. 8, the pressure equals approximately 6.1 torr one hour after instrument start-up. Experiments showed that after a lhr warm-up, the pressure was constant for more than 8 hr within a -C 0.025 torr limit. Even the optimization of the nebulizer argon flow 10 min after start-up, resulting in a change in argon flow by approximately 0.1 l/min, is clearly visible.
589
Laboratory note
I
4
3
2 Time (hours)
1
I
1
0
Fig. 8. Pressure in the first vacuum stage of the Elan 500 ICP-MS, starting from the ignition of the plasma (0 hr). ELECTRICALMODIFICATIONS As described above, the roughing pump serves two tasks: (1) Initial pumpdown and cryoclean of the main recipient containing during ‘stage 1’ and ‘stage 2’ until the cryogenic pump takes over; (2) Maintaining state.
an intermediate
the mass spectrometer
vacuum in the interface during sample analysis in the “READY”-
All operational functions in the instrument were controlled by a number of relays on a large printed circuit board also containing the unregulated power supplies. Relay K4 controls between “READY” and pumpdown, as far as the roughing pump was concerned. To separate these operations, the contact of relay K4 that activates the roughing pump relay K3 at “READY”-time, was disconnected. Another, parallel contact of K4 was used to control a new relay, identical to K3, which itself controls the new roughing pump which pumps the intermediate vacuum only. In practice, these modifications consist of simply cutting a track on the relay board and mounting a new relay on a small bracket screwed to the supporting frame on the relay board. Some additional wiring completes the modification. For convenience, the electrical supply outlets for both roughing pumps were mounted on the rear switch panel of the Elan. They were clearly marked with the same number as the vacuum line to which the corresponding pump was connected. Care has to be taken to derive the power for these pumps from the proper point in the 220 V circuit, i.e. directly from the 20 A circuit breaker. In addition, relay K7, controlling the removed solenoid valve 1, can be removed from the relay board.
CONCLUSIONS The original Perkin-Elmer Sciex Elan 500 was installed at DSM Research in July 1988, with the intention to use it for the complete spectrum of analytical requirements coming up in an industrial research laboratory. The somewhat extreme applications put an extra burden on the performance and maintenance of the instrument. This led to the modifications described in this article. After more than a year of operation of this modified instrument, not a single maintenance had to be carried out on the remaining solenoid valve. Maintenance on the first vacuum stage was limited to changing the oil in roughing pump 1 once a month and cleaning the interface region once every three or four weeks. So, although the modified Elan 500 was used for the extreme applications indicated in the Introduction, performance and maintenance of this instrument were within the expectations and specifications of both user and manufacturer.
SUMMARY When a Perkin-Elmer Sciex Elan 500 ICP-MS instrument is used for extreme applications like analysing samples with very high solid content, the solenoid valve built into the first vacuum
590
Laboratory note
stage causes vacuum problems resulting in an at least one day’s instrument downtime every two or three weeks. Modifications of the roughing pump system resulted in stable and unattended operation of the first vacuum stage for more than a year. This article describes these modifications, which can be carried out in every Elan 250 or 500 ICP-MS. DSM Research
H. KLINKENBERG* ZICHENG PENG~ T. BEEREN
FA-AE
P.O. Box 18 6160 MD Geleen The Netherlands Pet-kin-Elmer Nederland B. V. P.O. Box 490 2800 AL Gouda The Netherlands
K. FLACH
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
M.R. Plan& J.S. Fritz, F. G. Smith and R. S. Houk, Analyl. Chem. 61, 149 (1989). A. Makishima, I. Inamoto and K. Chiba, Appf. Specwosc. 44, 91 (1990). Y. Nakamura and T. Fukuda, Bunseki Kagaku 39, T17 (1990). M. R. Plan& Energy Res. Abstr. 14(9), Abstr. no. 17911 (1989). R. C. Hutton and A. N. Eaton, J. Anafyt. Atom. Specrrom. 3, 547 (1988). A. van Heuzen, Dissertation, Inductively coupled plasma-mass spectrometry: a spectrum, The University of Amsterdam, Holland. C. J. Pickford and R. M. Brown, Spectrochim. Acta 41B, 183 (1986). T. J. Brotherton, W. L. Shen and J.A. Caruso, J. Analyt. Atom. Spectrom. 4, 39 (1989). J. G. Williams and A. L. Gray, Analyt. Proc. 25, 385 (1988). S. A. Wood and D. M. M. Vlassopoulos, Analytica chim. Acta 229, 227 (1990). D. J. Douglas and L. A. Kerr, J. Analyt. Atom. Spectrom. 3, 749 (1988). K. E. Jarvis and J. G. Williams, Chem. Geol. 77, 53 (1989). L. Blain, E. D. Salin and D. W. Boomer, J. Analyt. Atom. Spectrom. 4, 721 (1989). V. Karanassios and G. Horlick, Spectrochim. Acta 44B, 1345 (1989). V. Karanassios and G. Horlick, Specrrochim. Acta 4lB, 1361 (1989). V. Karanassios and G. Horlick, Spectrochim. Acta 44B, 1387 (1989). A. Boorn, J.E. Fulford and W. Wegscheider, Mikrochim. Actu 2, 171 (1985). R. C. Hutton, J. Analyr. Atom. Spectrom. 1, 259 (1986). D. Hausler, Spectrochim. Actu 42B, 63 (1987). H. M. Al Swaidan, Analyt. Letr. 21, 1487 (1988). Z. Peng, H. Klinkenberg, A. Beeren and W. Van Borm, Spectrochim. Acra 46B, 1051 (1991). H. Klinkenberg, S. van der Wal, J. Frusch, L. Terwint and A. Beeren, Atom. Spectrosc. 11, 198 (1990).
*Author to whom correspondence should be addressed. tPresent address: Dept Earth and Space Science, University of Science and Technology of China, Hefei, Anhui, People’s Republic of China.