- Email: [email protected]
Supersonic jet expansions in analytical spectroscopy
58
trends in analytical chemistry, vol. 3, no.*.?, I984
Supersonic
jet expansions spectroscopy
in analytical
Supersonic jet expansions allow gas phase molecules to be cooled to very low temperatures without condensation. At these temperatures, electronic absorption and emission spectra consist of sharp lines rather than broad bands. The narrow bandwidth character of the s ctra provides a highly selective method for the detection op” individual compounds. Murray V. Johnston Boulder, CO, USA The detection of individual compounds at minute levels in complex samples presents the greatest challenge to analytical chemistry. These situations usually require elaborate procedures which incorporate both separation and detection steps. If satisfactory resolution is not achieved in the chromatographic separation, then additional selectivity is necessary in the detection process so that the overall resolving power is sufficient. The development of new, highly selective detection methods is important not only to supplement existing chromatographic techniques, but also to replace them when direct, real time analyses are required. In the past, selective detection methods based upon UV-visible absorption or emission spectroscopy have been largely confined to atomic spectroscopy since most molecules do not exhibit narrow band spectra at elevated temperatures. The broad bands encountered in molecular spectroscopy are primarily the result of hot bands, transitions originating from thermally populated vibrational and rotational levels. The traditional approach to achieve selectivity in molecular spectroscopy has been to cool the sample to a low temperature in a solid ‘host’ matrix to reduce spectral congestion arising from hot bands. This technique, however, is limited by a number of solid state broadening mechanisms which cannot be eliminated. Recently, supersonic jet spectroscopy1*2 has developed into a powerful tool for studying low temperature molecular spectra. A supersonic jet produces cold, isolated, gas phase molecules which are free of solid state broadening effects. Thus, the full potential of molecular selectivity can be exploited. This possibility suggests that molecular spectra can act as ‘fingerprints’ of individual compounds (even isomers) in a chemical analysis in much the same way that atomic spectra act as fingerprints of specific elements.
Supersonic jet expansions
0165-9936/84/$02.00.
Reservoir PO, To) 7
Vacuum
+
Chamber
Pl)
Theory
A supersonic free jet is pressure gas is expanded vacuum through a small orifice, molecular motion
represented by a Maxwellian distribution at the equilibrium reservoir temperature. If the reservoir pressure is high enough, then the molecular mean free path between collisions is much smaller than the orifice diameter. Of all the molecules in the vicinity of the jet orifice, only those which have achieved, through collisions, a large velocity component in the axial direction are able to escape, Thus, the mass flow of the gas outside the orifice is directed. This principle is illustrated in Fig. 1. The directed flow constitutes a supersonic jet since the expansion proceeds faster than the local speed of sound in the vacuum chamber. Conversion of random motion into a directed mass flow requires energy. To the extent that heat conduction in the chamber can be ignored, this energy is provided through a substantial decrease in the internal energy of the gas, hence the cooling effect of the jet expansion. The lowest temperatures are achieved in expansions of the noble gases since they have no vibrational or rotational modes and exhibit near ideal behavior. Now, let’s consider what happens when a small amount of a polyatomic species is ‘seeded’ into a supersonic jet expansion of helium. Virtually all of the collisions the polyatomic molecule experiences will be
established when a high from a reservoir into a orilice’-3. Behind the jet is random and can be
Fig. I. Conversion of random motion into a direct massjlow in a supersonicjet when the reservoir pressure (PO) is much greater than the background pressure (PI) of the vacuum chamber and the mean free path between collisions is less than the orz$ice diameter. Arrows indicate the velocity vectors of the gas particles. @
1984 Elswicr
Science
Publishers
B.V.
59
trcnak irf analytical chemishy, vol. 3, 7w. 2, 1984
with helium atoms if the polyatomic concentration is low enough. In the initial region of the jet, most collisions serve to accelerate the polyatomic to the velocity of the carrier gas (i.e. helium). Once similar velocities are established, only low energy collisions between carrier and seeded gas molecules are possible since both are moving in approximately the same direction at the same speed. These collisions are indicative of an extremely low effective translational temperature and their result is to equilibrate the various degrees of freedom of the vibrationally and rotationally ‘hot’ polyatomic molecule. In other words, the effective vibrational and rotational temperatures are decreased at the expense of a slight increase in the translational temperature. As the expansion proceeds further downstream from the orifice, the gas density falls off rapidly. Eventually, molecules become separated enough that no more collisions are possible until the shock wave arising from supersonic flow is reached. (The shock wave turbulence is usually referred to as the Mach disc.) In the ‘free flow’ region before the Mach disc, an ideal spectroscopic medium is established consisting of low temperature, isolated molecules. Rotational temperatures on the order of 0. l-l OK are achieved in supersonic jet expansions. Vibrational temperatures are much higher, typically 20-150K. This disparity arises from the smaller collisional cross section for vibrational depopulation. Even so, the overall impact of incomplete vibrational cooling is minimized by a number of factors. First, low rotational temperatures mean that the rotational contours of vibrational bands are small. Thus, sharp vibronic bands are obtained. Second, the energies of most vibrational modes are large (> 200 cm-l) and therefore thermal populations at the equilibrium reservoir temperature are small. Third, low frequency vibrational modes which are most likely to have *significant thermal populations also tend to have the greatest collisional cross sections. For example, the 250 cm-’ stretching mode of 12 is cooled to below 50K, while the 750 cm-’ bending mode of NO2 is cooled to only 150K under the same conditions2. In principle, electronic absorption spectra can act as a highly sensitive probe of molecular structure. Relative placement of functional groups around an aromatic ring system produces shifts of the order of 10 to 300 cm-‘. These changes, too small to allow selective detection at room temperature, are readily distinguished at the temperatures achieved in a supersonicjet. Cooling in a supersonic jet expansion is a nonequilibrium process. Molecules are cooled to very low temperatures, but condensation is largely inhibited by the absence of collisions in the free flow region. In practice, the ‘rate’ of cooling must be carefully controlled so that low temperatures are achieved without extensive condensation. If the gas cools too slowly, then molecules enter the free flow region before their internal modes are sufficiently depopulated. If the gas cools too quickly, then collisions between cold
seeded molecules and cold carrier gas atoms form large numbers of clusters.’ The formation of clusters is particularly troublesome because their spectral transitions can overlap or obscure the free molecule spectrum. The rate of cooling in a supersonic jet expansion is determined by the collision frequency which is dependent upon the reservoir backing pressure (PO)and or&e diameter (0). Cooling is the result of two body collisions and therefore is proportional to the value paD. Clustering requires at least a three body collision and is proportional topo2 “D where n is the number carrier gas atoms in the cluster. Given these relationships, it is apparent that the number of clusters can be reduced while retaining an acceptable degree of cooling by decreasing the backing pressure and increasing the orifice diameter by the same amount. Under these conditions,paD is constant and po’“D is reduced. The practical limit to this manipulation is the gas throughput of the vacuum system used since thejet orifice throughput increases as pOLF.
To a large extent, jet expansion characteristics are determined by the physical properties of the carrier gas. In theory, all gases should behave equally well since the cooling process is a purely kinetic phenomenon. In practice, the temperature attained in a jet expansion is limited by the formation of clusters between carrier gas atoms or molecules. Ifintramolecular attraction is small, then cluster formation is minimized and the translational temperature is low. The noble gases represent the best choice of carrier gas since they exhibit near ideal behavior. Of these, the lowest translational temperatures are achieved in expansions of helium. Since the collisional crosssection for rotational depopulation is large, the rotational temperature of a molecule seeded in an expansion is usually close to the translational temperature attained. Vibrational temperatures, however, can vary widely. It is found that the heavier noble gases (Ar, Kr, Xe) provide more effective vibrational cooling than helium or neon4. This result can be’ rationalized on the basis of the velocity slip effect. Helium expansions have a high velocity due to the small atomic weight of the carrier gas. Typical seeded molecules are much heavier and therefore travel at slower speeds. The initial collisions just outside the jet orifice serve only to accelerate the seeded molecule to the velocity of helium. Subsequent collisions cool the molecule. In an argon expansion, the carrier gas velocity is much lower due to the larger atomic weight. Fewer collisions are required to accelerate the seeded molecule and therefore a larger number of collisions are available for cooling. Since the collisional cross section for vibrational depopulation is small, increasing the number of collisions in the cooling process can have a significant impact upon the vibrational temperature reached. Lower vibrational temperatures are achieved in expansions of argon, even though the final rotational temperatures achieved are greater than those in expansions of helium.
60
trmdr in analytical chem+y,
For a particular application, the carrier gas composition, backing pressure, and orifice diameter can be varied for optimum performance. The experimental parameters chosen will reflect a balance among the different processes involved.
multiphoton ionization increases rapidly relative to fluorescence. In the limit of high power, every molecule interacting resonantly with the laser can be ionized5s6. Both methods enjoy two-dimensional selectivity, i.e. discrimination in both the excitation/ionization and detection steps. For non-fluorescent materials, multiphoton ionization is the method of choice.
Detection methods
The gas densities encountered in the free flow region of the jet expansion are usually too small to be detected by normal absorption spectroscopy. For analytical applications, more sensitive methods, such as laser induced fluorescence or resonance enhanced multiphoton ionization, are required. Laser based detection methods are ideally suited for supersonicjet expansions since the inherent selectivity of low temperature molecular spectroscopy is effectively utilized by the high power, narrow bandwidth character of laser emission. In fluorescence spectroscopy, selective excitation and detection is accomplished by tuning the laser to an absorption transition of the molecule of interest and then isolating a specific fluorescence line with a high resolution monochromator. In resonance enhanced multiphoton ionization spectroscopy, one photon is absorbed at the laser wavelength to reach an excited electronic level. Subsequent absorption of one or more additional photons provides enough energy for ionization and fragmentation. A mass spectrometer is then used to monitor specific ion fragments. Either method is applicable to the detection of fluorescent materials. The relative amount of fluorescence v. multiphoton ionization is dependent upon laser power. As the laser power is increased, the probability for 1B
t
31.6
Prospects for chemical analysis Although supersonic jet expansions are now widely used for fundamental spectroscopic studies, analytical applications have only begun to appear’-“. The pioneering work in this area was done by Small and co-workers who demonstrated the applicability of supersonic jet expansions with fluorescence detection to the direct analysis of organic pollutants’**. Fig. 2 is a fluorescence excitation spectrum for a mixture of naphthalene, cr-methylnaphthalene and P-methylnaphthalene’. This spectrum was obtained by setting the monochromator at a wavelength where all three compounds fluoresce. The transition line-widths observed are limited by the laser band-width. It can be seen that the excitation transitions of these compounds in a supersonic jet environment are easily distinguished from one another and can act as fingerprints of each. Furthermore, the spectrum demonstrates the utility of supersonic jet spectroscopy in the selective detection of isomers. Quantitative aspects of molecular detection in supersonic jet. expansions have been considered by several groups*-‘“. Both laser induced fluorescence’ and multiphoton ionization”’ have been found to give similar detection limits, partial pressures behind the jet orifice on the order of 10m6 to lo-’ torr. For a total backing pressure of 1 atm, these values correspond to
bOl
I 1
31.8
I
L
r
1
1
32’.0 D
32.2 [CM-’
vol. 3, no, 2, 1984
Xl0
32.4
------t
32.6
-31
Fig. 2. Fluorescence excitation spectrum of a mixed sample of naphthalene, cz-methylnaphthnlene, and /3-methylnaphthalene in a supersonicjet. N(O”), (Y(O’), and p(O’) denote th respective electronic origin bands of thefirst excited singlet states. (Reprinted with permission from Ref. 7. Copyright 1982 by the American Chemical Society.)
tmds in mal$ical
61
chcmir~, vol. 3, tw. 2,lW
less than 1 p.p.b. in the expansion gas. Small and Hayes* have demonstrated the direct analysis of naphthalene and the isomers of methylnaphthalene in a crude oil sample. The sample was injected into a gas chromatograph interfaced to a supersonic jet expansion. The chromatograph served not as a separation device but only as a quantitative injection method. Detection limits on the order of lo-* g were reported, although at least a four order of magnitude improvement could be obtained through rather straightforward changes in instrumentation. These preliminary studies have only begun to explore the analytical potential of supersonic jet expansions. Many related applications can be envisioned. The supersonic jet can act as a highly selective detector for capillary gas chromatography where the total effluent is passed through the jet orifice. If the jet is used to interface a chromatograph to a mass spectrometer, resonance enhanced multiphoton ionization could add an extra dimension of selectivity for difficult analyses which cannot be accomplished by normal GC-MS alone. Since low temperature absorption spectra of isomers are distinguishable in principle, the analysis of isomeric mixtures should be possible even without complete chemical separation. The selectivity available through supersonic jet spectroscopy may allow time consuming sample preparation steps to be eliminated and the rapid, direct analysis of volatile samples to be accomplished on a routine basis. Finally, supersonic jet expansions may find applications to the analysis of nonvolatile materials as well. A supercritical fluid could be used to dissolve a nonvolatile compound and the resulting mixture sent through the jet orifice. Here, the supercritical fluid which is really a pressurized gas heated above its critical temperature acts both as a
Modern
solvent and as the carrier gas in the cooling process. Alternatively, laser desorption from a solid surface could be performed just behind the jet orifice. Desorbed neutrals would then be swept into the expansion and cooled. The above ideas represent only a partial list of the potential applications of supersonic jet expansions. The inherent selectivity ofjet spectroscopy holds great promise for the development of new, powerful analytical methods and more vigorous research can be expected in the future.
Acknowledgements Preparation of this manuscript was supported by the National Science Foundation under grant number CHE-8308049.
References 1 Levy, D. H. (1981) ScietlGe214, 263 2 Levy, D. H., Wharton, L. and Smalley, R. E. (1977) in &mica1 and Biochemical Ap~1ication.sof Lasers, Vol. II (Moore, C. B., ed.), pp. 1-41, Academic Press, New York 3 Hayes, J. M. and Small, G. J. (1983) Anal. Gem. 55, 565A 4 Amirav, A., Even, U. and Jortner, J. (1980) Chcm. Phys. 51,31 5 Dietz, T. G., Duncan, M. A., Liverman, M. G. and Smalley, R. E. (1980) C&m. Pfp. Z.&t. 70, 246 6 Antonov, V. S., Letkhov, V. S. and Shibanov, A. N. (1981) Opt. Commun. 38, 1982 7 Warren, J. A., Hayes, J. M. and Small, G. J. (1982) Anal. Chem. 54, 138 8 Hayes, J. M. and Small, G. J. (1982) Anal. Chem. 54, 1202 9 Amirav, A., Even, U. and Jortner, J. (1982) Anal. Chem. 54,1666 10 Lubman, D. M. and Kronick, M. N. (1982) Anal. Chem. 54,660 Murray V. Johnston received his Ph.D. in Analytical Chemistryfrom the University of Wisconsin, Madison in 19&I and is currently an Assistant Professor of Chemishy in the Department of Chemishy and CIRES (Co-operative Institute for Research in Environmental Studies) at the University of Colorado, Campus Box 215, Boulder, CO 8y)309, USA. His research interests include th applications of lasers and supersonix jet expansions in analytical chemistry.
ion separation techniques analysis: ion chromatography isotachophoresis
in inorganic and
In recent years the traditional chemical approaches to separation and analysis of highly ionic compounds have been re laced by two new accurate and versatile methods: Yon chromatography and isotachophoresis. J. Vialle Vernalson, France Over the past 10 years, various methods ofHPLC have been developed which allow separation and accurate quantification of ionizable organic compounds. However, these methods are often not suitable for the separation and routine analysis of highly dissociated compounds (e.g. strong acids and bases). Such compounds can be separated on conventional 0165-9936/64/50’2.00.
ion-exchange resins, but these materials are not convenient for use with HPLC due to swelling and slow mass transfer rates. In the past the determination of these ionic compounds has involved laborious and time consuming methods, which were not always very selective nor sensitive, such as wet chemical analysis, extraction, precipitation, gravimetry, complexometric titration, etc., or open-bed ion exchange chromatography. However, with the growth of environmental @ 1984 Ekvicr Scicncc Publiakrs B.V.
trends in analytical chemistry, vol. 3, no.*.?, I984
Supersonic
jet expansions spectroscopy
in analytical
Supersonic jet expansions allow gas phase molecules to be cooled to very low temperatures without condensation. At these temperatures, electronic absorption and emission spectra consist of sharp lines rather than broad bands. The narrow bandwidth character of the s ctra provides a highly selective method for the detection op” individual compounds. Murray V. Johnston Boulder, CO, USA The detection of individual compounds at minute levels in complex samples presents the greatest challenge to analytical chemistry. These situations usually require elaborate procedures which incorporate both separation and detection steps. If satisfactory resolution is not achieved in the chromatographic separation, then additional selectivity is necessary in the detection process so that the overall resolving power is sufficient. The development of new, highly selective detection methods is important not only to supplement existing chromatographic techniques, but also to replace them when direct, real time analyses are required. In the past, selective detection methods based upon UV-visible absorption or emission spectroscopy have been largely confined to atomic spectroscopy since most molecules do not exhibit narrow band spectra at elevated temperatures. The broad bands encountered in molecular spectroscopy are primarily the result of hot bands, transitions originating from thermally populated vibrational and rotational levels. The traditional approach to achieve selectivity in molecular spectroscopy has been to cool the sample to a low temperature in a solid ‘host’ matrix to reduce spectral congestion arising from hot bands. This technique, however, is limited by a number of solid state broadening mechanisms which cannot be eliminated. Recently, supersonic jet spectroscopy1*2 has developed into a powerful tool for studying low temperature molecular spectra. A supersonic jet produces cold, isolated, gas phase molecules which are free of solid state broadening effects. Thus, the full potential of molecular selectivity can be exploited. This possibility suggests that molecular spectra can act as ‘fingerprints’ of individual compounds (even isomers) in a chemical analysis in much the same way that atomic spectra act as fingerprints of specific elements.
Supersonic jet expansions
0165-9936/84/$02.00.
Reservoir PO, To) 7
Vacuum
+
Chamber
Pl)
Theory
A supersonic free jet is pressure gas is expanded vacuum through a small orifice, molecular motion
represented by a Maxwellian distribution at the equilibrium reservoir temperature. If the reservoir pressure is high enough, then the molecular mean free path between collisions is much smaller than the orifice diameter. Of all the molecules in the vicinity of the jet orifice, only those which have achieved, through collisions, a large velocity component in the axial direction are able to escape, Thus, the mass flow of the gas outside the orifice is directed. This principle is illustrated in Fig. 1. The directed flow constitutes a supersonic jet since the expansion proceeds faster than the local speed of sound in the vacuum chamber. Conversion of random motion into a directed mass flow requires energy. To the extent that heat conduction in the chamber can be ignored, this energy is provided through a substantial decrease in the internal energy of the gas, hence the cooling effect of the jet expansion. The lowest temperatures are achieved in expansions of the noble gases since they have no vibrational or rotational modes and exhibit near ideal behavior. Now, let’s consider what happens when a small amount of a polyatomic species is ‘seeded’ into a supersonic jet expansion of helium. Virtually all of the collisions the polyatomic molecule experiences will be
established when a high from a reservoir into a orilice’-3. Behind the jet is random and can be
Fig. I. Conversion of random motion into a direct massjlow in a supersonicjet when the reservoir pressure (PO) is much greater than the background pressure (PI) of the vacuum chamber and the mean free path between collisions is less than the orz$ice diameter. Arrows indicate the velocity vectors of the gas particles. @
1984 Elswicr
Science
Publishers
B.V.
59
trcnak irf analytical chemishy, vol. 3, 7w. 2, 1984
with helium atoms if the polyatomic concentration is low enough. In the initial region of the jet, most collisions serve to accelerate the polyatomic to the velocity of the carrier gas (i.e. helium). Once similar velocities are established, only low energy collisions between carrier and seeded gas molecules are possible since both are moving in approximately the same direction at the same speed. These collisions are indicative of an extremely low effective translational temperature and their result is to equilibrate the various degrees of freedom of the vibrationally and rotationally ‘hot’ polyatomic molecule. In other words, the effective vibrational and rotational temperatures are decreased at the expense of a slight increase in the translational temperature. As the expansion proceeds further downstream from the orifice, the gas density falls off rapidly. Eventually, molecules become separated enough that no more collisions are possible until the shock wave arising from supersonic flow is reached. (The shock wave turbulence is usually referred to as the Mach disc.) In the ‘free flow’ region before the Mach disc, an ideal spectroscopic medium is established consisting of low temperature, isolated molecules. Rotational temperatures on the order of 0. l-l OK are achieved in supersonic jet expansions. Vibrational temperatures are much higher, typically 20-150K. This disparity arises from the smaller collisional cross section for vibrational depopulation. Even so, the overall impact of incomplete vibrational cooling is minimized by a number of factors. First, low rotational temperatures mean that the rotational contours of vibrational bands are small. Thus, sharp vibronic bands are obtained. Second, the energies of most vibrational modes are large (> 200 cm-l) and therefore thermal populations at the equilibrium reservoir temperature are small. Third, low frequency vibrational modes which are most likely to have *significant thermal populations also tend to have the greatest collisional cross sections. For example, the 250 cm-’ stretching mode of 12 is cooled to below 50K, while the 750 cm-’ bending mode of NO2 is cooled to only 150K under the same conditions2. In principle, electronic absorption spectra can act as a highly sensitive probe of molecular structure. Relative placement of functional groups around an aromatic ring system produces shifts of the order of 10 to 300 cm-‘. These changes, too small to allow selective detection at room temperature, are readily distinguished at the temperatures achieved in a supersonicjet. Cooling in a supersonic jet expansion is a nonequilibrium process. Molecules are cooled to very low temperatures, but condensation is largely inhibited by the absence of collisions in the free flow region. In practice, the ‘rate’ of cooling must be carefully controlled so that low temperatures are achieved without extensive condensation. If the gas cools too slowly, then molecules enter the free flow region before their internal modes are sufficiently depopulated. If the gas cools too quickly, then collisions between cold
seeded molecules and cold carrier gas atoms form large numbers of clusters.’ The formation of clusters is particularly troublesome because their spectral transitions can overlap or obscure the free molecule spectrum. The rate of cooling in a supersonic jet expansion is determined by the collision frequency which is dependent upon the reservoir backing pressure (PO)and or&e diameter (0). Cooling is the result of two body collisions and therefore is proportional to the value paD. Clustering requires at least a three body collision and is proportional topo2 “D where n is the number carrier gas atoms in the cluster. Given these relationships, it is apparent that the number of clusters can be reduced while retaining an acceptable degree of cooling by decreasing the backing pressure and increasing the orifice diameter by the same amount. Under these conditions,paD is constant and po’“D is reduced. The practical limit to this manipulation is the gas throughput of the vacuum system used since thejet orifice throughput increases as pOLF.
To a large extent, jet expansion characteristics are determined by the physical properties of the carrier gas. In theory, all gases should behave equally well since the cooling process is a purely kinetic phenomenon. In practice, the temperature attained in a jet expansion is limited by the formation of clusters between carrier gas atoms or molecules. Ifintramolecular attraction is small, then cluster formation is minimized and the translational temperature is low. The noble gases represent the best choice of carrier gas since they exhibit near ideal behavior. Of these, the lowest translational temperatures are achieved in expansions of helium. Since the collisional crosssection for rotational depopulation is large, the rotational temperature of a molecule seeded in an expansion is usually close to the translational temperature attained. Vibrational temperatures, however, can vary widely. It is found that the heavier noble gases (Ar, Kr, Xe) provide more effective vibrational cooling than helium or neon4. This result can be’ rationalized on the basis of the velocity slip effect. Helium expansions have a high velocity due to the small atomic weight of the carrier gas. Typical seeded molecules are much heavier and therefore travel at slower speeds. The initial collisions just outside the jet orifice serve only to accelerate the seeded molecule to the velocity of helium. Subsequent collisions cool the molecule. In an argon expansion, the carrier gas velocity is much lower due to the larger atomic weight. Fewer collisions are required to accelerate the seeded molecule and therefore a larger number of collisions are available for cooling. Since the collisional cross section for vibrational depopulation is small, increasing the number of collisions in the cooling process can have a significant impact upon the vibrational temperature reached. Lower vibrational temperatures are achieved in expansions of argon, even though the final rotational temperatures achieved are greater than those in expansions of helium.
60
trmdr in analytical chem+y,
For a particular application, the carrier gas composition, backing pressure, and orifice diameter can be varied for optimum performance. The experimental parameters chosen will reflect a balance among the different processes involved.
multiphoton ionization increases rapidly relative to fluorescence. In the limit of high power, every molecule interacting resonantly with the laser can be ionized5s6. Both methods enjoy two-dimensional selectivity, i.e. discrimination in both the excitation/ionization and detection steps. For non-fluorescent materials, multiphoton ionization is the method of choice.
Detection methods
The gas densities encountered in the free flow region of the jet expansion are usually too small to be detected by normal absorption spectroscopy. For analytical applications, more sensitive methods, such as laser induced fluorescence or resonance enhanced multiphoton ionization, are required. Laser based detection methods are ideally suited for supersonicjet expansions since the inherent selectivity of low temperature molecular spectroscopy is effectively utilized by the high power, narrow bandwidth character of laser emission. In fluorescence spectroscopy, selective excitation and detection is accomplished by tuning the laser to an absorption transition of the molecule of interest and then isolating a specific fluorescence line with a high resolution monochromator. In resonance enhanced multiphoton ionization spectroscopy, one photon is absorbed at the laser wavelength to reach an excited electronic level. Subsequent absorption of one or more additional photons provides enough energy for ionization and fragmentation. A mass spectrometer is then used to monitor specific ion fragments. Either method is applicable to the detection of fluorescent materials. The relative amount of fluorescence v. multiphoton ionization is dependent upon laser power. As the laser power is increased, the probability for 1B
t
31.6
Prospects for chemical analysis Although supersonic jet expansions are now widely used for fundamental spectroscopic studies, analytical applications have only begun to appear’-“. The pioneering work in this area was done by Small and co-workers who demonstrated the applicability of supersonic jet expansions with fluorescence detection to the direct analysis of organic pollutants’**. Fig. 2 is a fluorescence excitation spectrum for a mixture of naphthalene, cr-methylnaphthalene and P-methylnaphthalene’. This spectrum was obtained by setting the monochromator at a wavelength where all three compounds fluoresce. The transition line-widths observed are limited by the laser band-width. It can be seen that the excitation transitions of these compounds in a supersonic jet environment are easily distinguished from one another and can act as fingerprints of each. Furthermore, the spectrum demonstrates the utility of supersonic jet spectroscopy in the selective detection of isomers. Quantitative aspects of molecular detection in supersonic jet. expansions have been considered by several groups*-‘“. Both laser induced fluorescence’ and multiphoton ionization”’ have been found to give similar detection limits, partial pressures behind the jet orifice on the order of 10m6 to lo-’ torr. For a total backing pressure of 1 atm, these values correspond to
bOl
I 1
31.8
I
L
r
1
1
32’.0 D
32.2 [CM-’
vol. 3, no, 2, 1984
Xl0
32.4
------t
32.6
-31
Fig. 2. Fluorescence excitation spectrum of a mixed sample of naphthalene, cz-methylnaphthnlene, and /3-methylnaphthalene in a supersonicjet. N(O”), (Y(O’), and p(O’) denote th respective electronic origin bands of thefirst excited singlet states. (Reprinted with permission from Ref. 7. Copyright 1982 by the American Chemical Society.)
tmds in mal$ical
61
chcmir~, vol. 3, tw. 2,lW
less than 1 p.p.b. in the expansion gas. Small and Hayes* have demonstrated the direct analysis of naphthalene and the isomers of methylnaphthalene in a crude oil sample. The sample was injected into a gas chromatograph interfaced to a supersonic jet expansion. The chromatograph served not as a separation device but only as a quantitative injection method. Detection limits on the order of lo-* g were reported, although at least a four order of magnitude improvement could be obtained through rather straightforward changes in instrumentation. These preliminary studies have only begun to explore the analytical potential of supersonic jet expansions. Many related applications can be envisioned. The supersonic jet can act as a highly selective detector for capillary gas chromatography where the total effluent is passed through the jet orifice. If the jet is used to interface a chromatograph to a mass spectrometer, resonance enhanced multiphoton ionization could add an extra dimension of selectivity for difficult analyses which cannot be accomplished by normal GC-MS alone. Since low temperature absorption spectra of isomers are distinguishable in principle, the analysis of isomeric mixtures should be possible even without complete chemical separation. The selectivity available through supersonic jet spectroscopy may allow time consuming sample preparation steps to be eliminated and the rapid, direct analysis of volatile samples to be accomplished on a routine basis. Finally, supersonic jet expansions may find applications to the analysis of nonvolatile materials as well. A supercritical fluid could be used to dissolve a nonvolatile compound and the resulting mixture sent through the jet orifice. Here, the supercritical fluid which is really a pressurized gas heated above its critical temperature acts both as a
Modern
solvent and as the carrier gas in the cooling process. Alternatively, laser desorption from a solid surface could be performed just behind the jet orifice. Desorbed neutrals would then be swept into the expansion and cooled. The above ideas represent only a partial list of the potential applications of supersonic jet expansions. The inherent selectivity ofjet spectroscopy holds great promise for the development of new, powerful analytical methods and more vigorous research can be expected in the future.
Acknowledgements Preparation of this manuscript was supported by the National Science Foundation under grant number CHE-8308049.
References 1 Levy, D. H. (1981) ScietlGe214, 263 2 Levy, D. H., Wharton, L. and Smalley, R. E. (1977) in &mica1 and Biochemical Ap~1ication.sof Lasers, Vol. II (Moore, C. B., ed.), pp. 1-41, Academic Press, New York 3 Hayes, J. M. and Small, G. J. (1983) Anal. Gem. 55, 565A 4 Amirav, A., Even, U. and Jortner, J. (1980) Chcm. Phys. 51,31 5 Dietz, T. G., Duncan, M. A., Liverman, M. G. and Smalley, R. E. (1980) C&m. Pfp. Z.&t. 70, 246 6 Antonov, V. S., Letkhov, V. S. and Shibanov, A. N. (1981) Opt. Commun. 38, 1982 7 Warren, J. A., Hayes, J. M. and Small, G. J. (1982) Anal. Chem. 54, 138 8 Hayes, J. M. and Small, G. J. (1982) Anal. Chem. 54, 1202 9 Amirav, A., Even, U. and Jortner, J. (1982) Anal. Chem. 54,1666 10 Lubman, D. M. and Kronick, M. N. (1982) Anal. Chem. 54,660 Murray V. Johnston received his Ph.D. in Analytical Chemistryfrom the University of Wisconsin, Madison in 19&I and is currently an Assistant Professor of Chemishy in the Department of Chemishy and CIRES (Co-operative Institute for Research in Environmental Studies) at the University of Colorado, Campus Box 215, Boulder, CO 8y)309, USA. His research interests include th applications of lasers and supersonix jet expansions in analytical chemistry.
ion separation techniques analysis: ion chromatography isotachophoresis
in inorganic and
In recent years the traditional chemical approaches to separation and analysis of highly ionic compounds have been re laced by two new accurate and versatile methods: Yon chromatography and isotachophoresis. J. Vialle Vernalson, France Over the past 10 years, various methods ofHPLC have been developed which allow separation and accurate quantification of ionizable organic compounds. However, these methods are often not suitable for the separation and routine analysis of highly dissociated compounds (e.g. strong acids and bases). Such compounds can be separated on conventional 0165-9936/64/50’2.00.
ion-exchange resins, but these materials are not convenient for use with HPLC due to swelling and slow mass transfer rates. In the past the determination of these ionic compounds has involved laborious and time consuming methods, which were not always very selective nor sensitive, such as wet chemical analysis, extraction, precipitation, gravimetry, complexometric titration, etc., or open-bed ion exchange chromatography. However, with the growth of environmental @ 1984 Ekvicr Scicncc Publiakrs B.V.