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Liquid cooling of a torch for microwave induced plasma spectrometry
Spechochimica Acq Vol. 48B. No. 4. pp. 515-519, hinted in Great Bntain.
0584-8547/93 $6.00 + .oo @ 1993 Pergamon Press Ltd
1993
Liquid cooling of a torch for microwave
induced plasma spectrometry
HENRYK MATUSIEWICZ* Politechnika Poznanska, Department of Analytical Chemistry, 60-965 Poznad, Poland
and RALPH E. STURGEON Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario KlA 0R9, Canada (Received
28 August
1992; accepted
8 December
1992)
Abstract-A
simple torch design for use with a microwave induced plasma (MIP) is described that can be internally cooled by synthetic or hydrocarbon based fluids. The MIP is generated within a liquid coolantcooled concentric quartz discharge torch, energized within a novel microwave plasma cavity. Erosion of the discharge tube by the plasma, assessed by measurement of the emission intensity of ablated silicon, is essentially eliminated. The emission spectra of the liquid-cooled plasmas are compared to those sustained in uncooled discharge tubes.
1. INTR~DUC~ON THE ATMOSPHERICpressure
microwave induced plasma (MIP) is becoming an important and powerful excitation source for atomic emission analysis of metals and many nonmetals. A crucial component of the MIP is the torch, which functions to both contain the plasma and permit mixing of sample streams with make-up gas where necessary. When elevated microwave powers are employed, the choice of discharge tube material becomes very important, The ideal discharge tube should contain the plasma without melting, degradation or cracking, allow easy centering of the plasma, not introduce contaminants into the discharge and enjoy a long lifetime. Quartz discharge tubes, often used at lower power levels, melt at forward powers of around 200 W, depending on the tube size and flow rate of plasma gas. Aluminium oxide is superior to quartz for use as a discharge tube but cracks easily when rapidly heated and is rarely usable for more than a few such thermal cycles. Yittria-stabilized zirconium oxide is superior to alumina and quartz with respect to thermal stability in that it does not crack upon rapid heating and is reusable many times. However, both A1203 and ZrOz suffer from the drawback of containing several contaminants. Other discharge tube materials, such as boron nitride, are also superior to quartz, but it is difficult to sustain lower power plasmas in such tubes. The need to develop a plasma torch for rigorous analytical use requires a different approach to reach this goal. A clear alternative to the foregoing is the cooled MIPtorch that uses either water or gases (air, argon, nitrogen) as external coolant to enhance the range of practical operating conditions. In an effort to minimize the problems associated with etching and melting of the interior wall of quartz plasma torches, QUIMBY and SULLIVAN [l] designed a watercooled capillary discharge tube (not a plasma torch) and reported prolonged tube life. Water-cooled capillary plasma torches described by ALVAREZ BOLAINEZ et al. [2] and SINGet al. [3] are similar in design to Quimby and Sullivan’s in that a thin sheet of water is used to cool the capillary tube containing the plasma. However, the capillary discharge tube and water-cooling jacket were made considerably longer to completely contain the surface-wave launched Ar plasma. In all these attempts, water-cooled torches have been developed specifically for * Author to whom correspondence should be addressed. 515
H. MATUSIEWICZ and R. E. STURGEON
516
I.D. 1.0 O.D. 3.0
cooling fluid
-I--Ii 13
A-_
4
Fig. 1. Schematic diagram of the fluid-cooled torch (all dimensions are in mm).
MIPS used as gas chromatographic detectors. However, these torches have serious drawbacks resulting from the complexity of the design and the difficulty involved in their extremely precise fabrication. Clearly, advantages would be gained, in general, for many different applications if the torch design could be simplified, thereby extending its capabilities for use beyond those of gas chromatography. It should be stressed that fluids such as certain Freons, oils, hydrocarbons and silicones can be used as coolants for the inside surface of discharge tubes. Recently, papers describing the use of commercially available dimethyl polysiloxane [4] and hydraulic fluid [5] as liquid coolants for high-power plasma tubes excited in microwave structures, such as resonant cavities, have been published. These stable, non-toxic and non-flammable liquids have much lower microwave absorption coefficients at 2.45 GI-Iz than water and can be used over a wide temperature range (up to 260°C). For the preliminary investigation reported here, an easily constructed liquid-cooled quartz torch was evaluated for use in a highly efficient microwave cavity [6]. Several non-aqueous cooling fluids were investigated.
2. EXPERIMENTAL 2.1. Instrumentation
The plasma system consisted of a 700 W, 2.45 GHz stabilized generator, Model MPC-Ola (Enterprise for Implementation of Scientific and Technological Progress, Plazmatronika Ltd, Wroclaw, Poland) coupled to a TE 1o1rectangular cavity (Plazmatronika, Poland). The microwave cavity and generator used have been previously described [6]. For this study, only a plasma torch capable of being fluid-cooled was employed. This system was operated without modification and it should be noted that any reference to power refers to that measured at the generator, which is the actual power coupled to the plasma. The microwave cavity assembly was mounted on a pedestal, which could be moved both vertically and horizontally, so as to adjust the observation position. The gas flow rates were controlled and monitored with a mass flow controller Model 5850 (Brooks Instrument Division, Hatfield, U.S.A.) with digital read-out. Argon and helium were of high-purity grade. Analyte emission signals, obtained in the single-element mode, were recorded using output from the photomultiplier tube (PMT), which was processed through a laboratory-built picoammeter-filter network, digitized with 1Zbit A/D (300 Hz) and processed in Turbo Pascal (version 4-0, Borland International) using in-house software and an IBM AT. 2.2. Fluid-cooled plasma torch The design of the torch used in this study is shown in Fig. 1. The torch is made entirely from transparent quartz to avoid elongated quartz-glass connections. A larger diameter quartz tube acts as a jacket around the inner quartz discharge tube. The outer tube had a 13 mm o.d. and 4 mm i.d.; the inner tube a 3 mm o.d. and a 1 mm i.d. The two tubes were positioned concentrically. The plasma support gas was introduced through the inner tube while an outer
Torch cooling in MIPS
517
gas flow was introduced approximately perpendicular to the nebulizer flow. The outer gas flow rate was controlled separately from the nebulizer gas flow rate. For obvious reasons, the inlet point of the fluid supply cannot be located within or on top of the cavity. Both the inlet and outlet ports of the cooling fluid must be located at the far end of the jacket and in order to force the fluid toward the hot plasma region inside the cavity, where cooling is most required, an additional quartz capillary tube was placed inside the cooling jacket. As the inlet and outlet ports are on opposite sides of the two perpendicular partitioning walls, cooling fluid is forced to pass the plasma region. The outer diameter of the jacket must be less than the inner diameter of the cavity (13.5 mm). For cooled torch operation, the cooling fluid was peristaltically pumped from a reservoir through a 14-turn, 5 cm o.d. glass coil (2 mm i.d.) and on to the torch at a flow rate of 150 ml/min. The coil served as a radiative cooler although a more efficient heat exchanger could have been constructed by simply immersing it into a temperature controlled water bath. Because the plasma containment torch was almost the same diameter as the microwave plasma cavity, the latter also served as the torch holder. The position of the tip of the sample delivery tube was approximately level with the lower inner base of the cavity. A 1:l axial image of the plasma was focused onto the entrance slit of a computer controlled echelle spectrometer described by BERMANand MCLAREN[7]. Entrance and exit slit widths were set at 50 pm with a height of 0.5 mm. The position of observation was selected at the center of the plasma. 2.3. Coolant fluids Commercial fluids were used as coolant media in these studies. Dimethyl polysiloxane (Syltherm XLT heat transfer liquid) was obtained from Dow Coming Canada Inc. (Mississauga, Canada). MIL-H-5606F (AeroShell Fluid 41) hydraulic fluid was supplied by Harcros Chemicals Canada Inc. (Toronto, Canada). Automotive transmission fluid was purchased from a local dealer. 2.4. Procedures The plasma was allowed to equilibrate for 10 min prior to use. Silicon emission was monitored from both the cooled and uncooled torch to provide an indication of the extent of plasma contact with the walls. Measurement of the Si I 288.2 nm line was made under typical operating conditions (i.e. forward power 140 W, argon and helium flow rate 0.6 Vmin). The same data collection system and viewing geometries were used for both measurements. The excitation temperature of a liquid-cooled dry He plasma (forward power 140 W) was calculated from the slope of a Boltzmann plot prepared by using the He plasma gas as the thermometric species. The instrumentation, procedure and data for the spectroscopic lines used have been previously published [8]. Wavelength scans covering 200-700 nm were also undertaken for both the cooled and uncooled torch configurations.
3. RESULTS AND DISCUSSION 3.1. Operation of fluid-cooled torch The plasma is easily ignited within either the uncooled or cooled torch by momentarily inserting a short length of tungsten wire mounted on an appropriate insulated handle. Not only is minimal retuning required when converting from an uncooled to cooled mode of operation, but also when altering either the applied power or plasma gas flow rates. After some period of time, the temperature of the cooling fluid increased.
However, boiling of fluid at the wall of the coolant jacket was never observed under these operating conditions. Fluid-cooling reduced erosion of the interior wall of the discharge tube. With an uncooled, thick-walled tube in the TElol system, the wall becomes visibly etched within minutes of operation and, depending on the power level, melting occurs. Cooling the discharge tube eliminates these problems. 3.2. Emission characteristics A recent investigation into the vaporization of silicon from capillary quartz walls at the point of contact with the microwave plasma showed that Si emission occurs owing to the erosion of the discharge tube [9]. In the present study, Si I emission at the 288.2 nm line was monitored during operation of both argon and helium plasmas contained in the uncooled and cooled torch.
518
H. MATUSIEWICZ and R. E. STURGEON
The intensity of Si I emission from the He plasma was approximately 6900-fold greater than that in the Ar plasma under identical conditions of flow rate, forward power and viewing geometry. This may reflect either an increase in the amount of material ablated from the torch wall in a He plasma and/or the increased excitational capabilities of this plasma (measured excitation temperature 3000 ? 200 K). Compared to their uncooled counterparts, meaurements in a torch cooled with Syltherm revealed an 11200-fold reduction of Si I intensity whereas for the Ar plasma only a 1Cfold decrease was observed. It may thus be concluded that the He plasma is in more direct contact with the quartz tube wall, thereby (thermally) eroding more Si into the plasma than in the case of Ar. Cooled discharge tubes showed no visible signs of etching after 6 h that the plasma was in operation at the completion of this study, indicative of low quartz tube surface temperature. No significant difference was noted in the performance of the three cooling fluids examined, although the lower viscosity of Syltherm allowed this material to be more easily pumped through the narrow orifices of the torch. Wavelength scans covering the range 200-700 nm revealed intense activity below 300 nm. In this region, Si I lines as well as weak SiO bands (main system A CX %+) were observed in an uncooled He plasma. These systems completely disappeared with torch cooling. An obvious significant benefit arises for the use of cooled plasmas for atomic emission spectrometric analysis of elements whose primary emission lines lie below 300 nm. Additionally, SING et al. [3] have reported decreased background continuum in the near-IR for water-cooled torch operation and, although not specifically tested, the same can be assumed to hold true here.
4. CONCLUSION The fluid-cooled quartz torch described in this study is easily fabricated and can be used with a conventional MIP-system without modification of the cavity and/or nebulizer. Additionally, the lower operating temperature sustained with the fluidcooled torch no longer necessitates that it be constructed of quartz. Lower melting temperature borosilicate glass, which is much easier to work with, probably may be conveniently substituted. Although all of the fluids evaluated in this study show negligible absorption of microwave energy, automatic transmission fluid is particularly attractive because of its low cost and wide availability. In practice, the cooling fluid may be used in conjunction with a temperature controlled bath to enhance heat loss and aid the maintenance of the discharge tube temperature, which may be an important variable for the introduction of gas or liquid chromatography column effluents or for solution nebulization [6]. The fluid-cooled torch enhances discharge tube lifetime and significantly decreases erosion/ablation of the quartz tube surface, leading to a background emission spectrum that is comparatively free of Si lines and molecular SiO bands in the UV-VIS and a decreased continuum in the near-IR [3]. A minor disadvantage that may arise in the use of these fluids is the deposition of a carbonaceous coating onto the interior cooling jacket if the torch is inadvertently overheated in the absence of coolant flow. Further studies will be required to assess the analytical performance and future experiments of this type are planned. Acknowledgements-Financial support by the State Committee for Scientific Research (KBN), Poland, Grant No. PB618/P3/92/02 Application of Microwave Techniques to Analytical Chemistry, is gratefully acknowledged. H. MATUSIEWICZthanks the NRCC for financial support while in Canada. The authors are grateful to Dow Coming and Shell Canada for kindly supplying the cooling fluids.
REFERENCES [l] B. D. Quimby and J. J. Sullivan, Anal. Chem. 62, 1027 (1990). [2] R. M. Alvarez Bolainez, M. P. Dziewatkoski and C. B. Boss, Anal. Chem. 64, 541 (1992).
Torch cooling in MIPS [3] [4] [5] [6] [7] [8] [9]
R. L. L. H. S. R. R.
L. A. Sing, C. Lauzon, K. C. Tran and J. Hubert, Appf. Specrrosc. 46, 430 (1992). A. Schlie, R. D. Rathge and E. A. Dunkle, Rev. Sci. Instrum. 62, 381 (1991). A. Schlie, Rev. Sci. Instrum. 62, 542 (1991). Matusiewicz, Spectrochim. Acta 47B, 1221 (1992). S. Berman and J. W. McLaren, Appi. Spectrosc. 32, 372 (1978). E. Sturgeon, S. N. Willie and V. T. Luong, Specfrochim. Acru 46B, 1021 (1991). F. Wandro and H. B. Friedrich, Anal. Chem. 56, 2727 (1984).
519
0584-8547/93 $6.00 + .oo @ 1993 Pergamon Press Ltd
1993
Liquid cooling of a torch for microwave
induced plasma spectrometry
HENRYK MATUSIEWICZ* Politechnika Poznanska, Department of Analytical Chemistry, 60-965 Poznad, Poland
and RALPH E. STURGEON Institute for Environmental Chemistry, National Research Council of Canada, Ottawa, Ontario KlA 0R9, Canada (Received
28 August
1992; accepted
8 December
1992)
Abstract-A
simple torch design for use with a microwave induced plasma (MIP) is described that can be internally cooled by synthetic or hydrocarbon based fluids. The MIP is generated within a liquid coolantcooled concentric quartz discharge torch, energized within a novel microwave plasma cavity. Erosion of the discharge tube by the plasma, assessed by measurement of the emission intensity of ablated silicon, is essentially eliminated. The emission spectra of the liquid-cooled plasmas are compared to those sustained in uncooled discharge tubes.
1. INTR~DUC~ON THE ATMOSPHERICpressure
microwave induced plasma (MIP) is becoming an important and powerful excitation source for atomic emission analysis of metals and many nonmetals. A crucial component of the MIP is the torch, which functions to both contain the plasma and permit mixing of sample streams with make-up gas where necessary. When elevated microwave powers are employed, the choice of discharge tube material becomes very important, The ideal discharge tube should contain the plasma without melting, degradation or cracking, allow easy centering of the plasma, not introduce contaminants into the discharge and enjoy a long lifetime. Quartz discharge tubes, often used at lower power levels, melt at forward powers of around 200 W, depending on the tube size and flow rate of plasma gas. Aluminium oxide is superior to quartz for use as a discharge tube but cracks easily when rapidly heated and is rarely usable for more than a few such thermal cycles. Yittria-stabilized zirconium oxide is superior to alumina and quartz with respect to thermal stability in that it does not crack upon rapid heating and is reusable many times. However, both A1203 and ZrOz suffer from the drawback of containing several contaminants. Other discharge tube materials, such as boron nitride, are also superior to quartz, but it is difficult to sustain lower power plasmas in such tubes. The need to develop a plasma torch for rigorous analytical use requires a different approach to reach this goal. A clear alternative to the foregoing is the cooled MIPtorch that uses either water or gases (air, argon, nitrogen) as external coolant to enhance the range of practical operating conditions. In an effort to minimize the problems associated with etching and melting of the interior wall of quartz plasma torches, QUIMBY and SULLIVAN [l] designed a watercooled capillary discharge tube (not a plasma torch) and reported prolonged tube life. Water-cooled capillary plasma torches described by ALVAREZ BOLAINEZ et al. [2] and SINGet al. [3] are similar in design to Quimby and Sullivan’s in that a thin sheet of water is used to cool the capillary tube containing the plasma. However, the capillary discharge tube and water-cooling jacket were made considerably longer to completely contain the surface-wave launched Ar plasma. In all these attempts, water-cooled torches have been developed specifically for * Author to whom correspondence should be addressed. 515
H. MATUSIEWICZ and R. E. STURGEON
516
I.D. 1.0 O.D. 3.0
cooling fluid
-I--Ii 13
A-_
4
Fig. 1. Schematic diagram of the fluid-cooled torch (all dimensions are in mm).
MIPS used as gas chromatographic detectors. However, these torches have serious drawbacks resulting from the complexity of the design and the difficulty involved in their extremely precise fabrication. Clearly, advantages would be gained, in general, for many different applications if the torch design could be simplified, thereby extending its capabilities for use beyond those of gas chromatography. It should be stressed that fluids such as certain Freons, oils, hydrocarbons and silicones can be used as coolants for the inside surface of discharge tubes. Recently, papers describing the use of commercially available dimethyl polysiloxane [4] and hydraulic fluid [5] as liquid coolants for high-power plasma tubes excited in microwave structures, such as resonant cavities, have been published. These stable, non-toxic and non-flammable liquids have much lower microwave absorption coefficients at 2.45 GI-Iz than water and can be used over a wide temperature range (up to 260°C). For the preliminary investigation reported here, an easily constructed liquid-cooled quartz torch was evaluated for use in a highly efficient microwave cavity [6]. Several non-aqueous cooling fluids were investigated.
2. EXPERIMENTAL 2.1. Instrumentation
The plasma system consisted of a 700 W, 2.45 GHz stabilized generator, Model MPC-Ola (Enterprise for Implementation of Scientific and Technological Progress, Plazmatronika Ltd, Wroclaw, Poland) coupled to a TE 1o1rectangular cavity (Plazmatronika, Poland). The microwave cavity and generator used have been previously described [6]. For this study, only a plasma torch capable of being fluid-cooled was employed. This system was operated without modification and it should be noted that any reference to power refers to that measured at the generator, which is the actual power coupled to the plasma. The microwave cavity assembly was mounted on a pedestal, which could be moved both vertically and horizontally, so as to adjust the observation position. The gas flow rates were controlled and monitored with a mass flow controller Model 5850 (Brooks Instrument Division, Hatfield, U.S.A.) with digital read-out. Argon and helium were of high-purity grade. Analyte emission signals, obtained in the single-element mode, were recorded using output from the photomultiplier tube (PMT), which was processed through a laboratory-built picoammeter-filter network, digitized with 1Zbit A/D (300 Hz) and processed in Turbo Pascal (version 4-0, Borland International) using in-house software and an IBM AT. 2.2. Fluid-cooled plasma torch The design of the torch used in this study is shown in Fig. 1. The torch is made entirely from transparent quartz to avoid elongated quartz-glass connections. A larger diameter quartz tube acts as a jacket around the inner quartz discharge tube. The outer tube had a 13 mm o.d. and 4 mm i.d.; the inner tube a 3 mm o.d. and a 1 mm i.d. The two tubes were positioned concentrically. The plasma support gas was introduced through the inner tube while an outer
Torch cooling in MIPS
517
gas flow was introduced approximately perpendicular to the nebulizer flow. The outer gas flow rate was controlled separately from the nebulizer gas flow rate. For obvious reasons, the inlet point of the fluid supply cannot be located within or on top of the cavity. Both the inlet and outlet ports of the cooling fluid must be located at the far end of the jacket and in order to force the fluid toward the hot plasma region inside the cavity, where cooling is most required, an additional quartz capillary tube was placed inside the cooling jacket. As the inlet and outlet ports are on opposite sides of the two perpendicular partitioning walls, cooling fluid is forced to pass the plasma region. The outer diameter of the jacket must be less than the inner diameter of the cavity (13.5 mm). For cooled torch operation, the cooling fluid was peristaltically pumped from a reservoir through a 14-turn, 5 cm o.d. glass coil (2 mm i.d.) and on to the torch at a flow rate of 150 ml/min. The coil served as a radiative cooler although a more efficient heat exchanger could have been constructed by simply immersing it into a temperature controlled water bath. Because the plasma containment torch was almost the same diameter as the microwave plasma cavity, the latter also served as the torch holder. The position of the tip of the sample delivery tube was approximately level with the lower inner base of the cavity. A 1:l axial image of the plasma was focused onto the entrance slit of a computer controlled echelle spectrometer described by BERMANand MCLAREN[7]. Entrance and exit slit widths were set at 50 pm with a height of 0.5 mm. The position of observation was selected at the center of the plasma. 2.3. Coolant fluids Commercial fluids were used as coolant media in these studies. Dimethyl polysiloxane (Syltherm XLT heat transfer liquid) was obtained from Dow Coming Canada Inc. (Mississauga, Canada). MIL-H-5606F (AeroShell Fluid 41) hydraulic fluid was supplied by Harcros Chemicals Canada Inc. (Toronto, Canada). Automotive transmission fluid was purchased from a local dealer. 2.4. Procedures The plasma was allowed to equilibrate for 10 min prior to use. Silicon emission was monitored from both the cooled and uncooled torch to provide an indication of the extent of plasma contact with the walls. Measurement of the Si I 288.2 nm line was made under typical operating conditions (i.e. forward power 140 W, argon and helium flow rate 0.6 Vmin). The same data collection system and viewing geometries were used for both measurements. The excitation temperature of a liquid-cooled dry He plasma (forward power 140 W) was calculated from the slope of a Boltzmann plot prepared by using the He plasma gas as the thermometric species. The instrumentation, procedure and data for the spectroscopic lines used have been previously published [8]. Wavelength scans covering 200-700 nm were also undertaken for both the cooled and uncooled torch configurations.
3. RESULTS AND DISCUSSION 3.1. Operation of fluid-cooled torch The plasma is easily ignited within either the uncooled or cooled torch by momentarily inserting a short length of tungsten wire mounted on an appropriate insulated handle. Not only is minimal retuning required when converting from an uncooled to cooled mode of operation, but also when altering either the applied power or plasma gas flow rates. After some period of time, the temperature of the cooling fluid increased.
However, boiling of fluid at the wall of the coolant jacket was never observed under these operating conditions. Fluid-cooling reduced erosion of the interior wall of the discharge tube. With an uncooled, thick-walled tube in the TElol system, the wall becomes visibly etched within minutes of operation and, depending on the power level, melting occurs. Cooling the discharge tube eliminates these problems. 3.2. Emission characteristics A recent investigation into the vaporization of silicon from capillary quartz walls at the point of contact with the microwave plasma showed that Si emission occurs owing to the erosion of the discharge tube [9]. In the present study, Si I emission at the 288.2 nm line was monitored during operation of both argon and helium plasmas contained in the uncooled and cooled torch.
518
H. MATUSIEWICZ and R. E. STURGEON
The intensity of Si I emission from the He plasma was approximately 6900-fold greater than that in the Ar plasma under identical conditions of flow rate, forward power and viewing geometry. This may reflect either an increase in the amount of material ablated from the torch wall in a He plasma and/or the increased excitational capabilities of this plasma (measured excitation temperature 3000 ? 200 K). Compared to their uncooled counterparts, meaurements in a torch cooled with Syltherm revealed an 11200-fold reduction of Si I intensity whereas for the Ar plasma only a 1Cfold decrease was observed. It may thus be concluded that the He plasma is in more direct contact with the quartz tube wall, thereby (thermally) eroding more Si into the plasma than in the case of Ar. Cooled discharge tubes showed no visible signs of etching after 6 h that the plasma was in operation at the completion of this study, indicative of low quartz tube surface temperature. No significant difference was noted in the performance of the three cooling fluids examined, although the lower viscosity of Syltherm allowed this material to be more easily pumped through the narrow orifices of the torch. Wavelength scans covering the range 200-700 nm revealed intense activity below 300 nm. In this region, Si I lines as well as weak SiO bands (main system A CX %+) were observed in an uncooled He plasma. These systems completely disappeared with torch cooling. An obvious significant benefit arises for the use of cooled plasmas for atomic emission spectrometric analysis of elements whose primary emission lines lie below 300 nm. Additionally, SING et al. [3] have reported decreased background continuum in the near-IR for water-cooled torch operation and, although not specifically tested, the same can be assumed to hold true here.
4. CONCLUSION The fluid-cooled quartz torch described in this study is easily fabricated and can be used with a conventional MIP-system without modification of the cavity and/or nebulizer. Additionally, the lower operating temperature sustained with the fluidcooled torch no longer necessitates that it be constructed of quartz. Lower melting temperature borosilicate glass, which is much easier to work with, probably may be conveniently substituted. Although all of the fluids evaluated in this study show negligible absorption of microwave energy, automatic transmission fluid is particularly attractive because of its low cost and wide availability. In practice, the cooling fluid may be used in conjunction with a temperature controlled bath to enhance heat loss and aid the maintenance of the discharge tube temperature, which may be an important variable for the introduction of gas or liquid chromatography column effluents or for solution nebulization [6]. The fluid-cooled torch enhances discharge tube lifetime and significantly decreases erosion/ablation of the quartz tube surface, leading to a background emission spectrum that is comparatively free of Si lines and molecular SiO bands in the UV-VIS and a decreased continuum in the near-IR [3]. A minor disadvantage that may arise in the use of these fluids is the deposition of a carbonaceous coating onto the interior cooling jacket if the torch is inadvertently overheated in the absence of coolant flow. Further studies will be required to assess the analytical performance and future experiments of this type are planned. Acknowledgements-Financial support by the State Committee for Scientific Research (KBN), Poland, Grant No. PB618/P3/92/02 Application of Microwave Techniques to Analytical Chemistry, is gratefully acknowledged. H. MATUSIEWICZthanks the NRCC for financial support while in Canada. The authors are grateful to Dow Coming and Shell Canada for kindly supplying the cooling fluids.
REFERENCES [l] B. D. Quimby and J. J. Sullivan, Anal. Chem. 62, 1027 (1990). [2] R. M. Alvarez Bolainez, M. P. Dziewatkoski and C. B. Boss, Anal. Chem. 64, 541 (1992).
Torch cooling in MIPS [3] [4] [5] [6] [7] [8] [9]
R. L. L. H. S. R. R.
L. A. Sing, C. Lauzon, K. C. Tran and J. Hubert, Appf. Specrrosc. 46, 430 (1992). A. Schlie, R. D. Rathge and E. A. Dunkle, Rev. Sci. Instrum. 62, 381 (1991). A. Schlie, Rev. Sci. Instrum. 62, 542 (1991). Matusiewicz, Spectrochim. Acta 47B, 1221 (1992). S. Berman and J. W. McLaren, Appi. Spectrosc. 32, 372 (1978). E. Sturgeon, S. N. Willie and V. T. Luong, Specfrochim. Acru 46B, 1021 (1991). F. Wandro and H. B. Friedrich, Anal. Chem. 56, 2727 (1984).
519