chromatographic interfaces for supersonic jet spectroscopy

94 trends in analytical chemistry, vol. 7, no. 3,1988.

with an example representing soft drink consump- tionl.

Here, a short SAS program macro is presented for performing a basic correspondence analysis. The program is written in PROC MATRIX for the VWCMS operating system. PROC IDPLOT3’4 is used to plot the row and column points with descrip- tive labels.

This basic program can be used to handle the many variants of correspondence analysis, including multiple correspondence analysis5 and generalized correspondence analysi8. The user will need to pre- pare the input data accordingly. For details on inter- pretation of the output, see ref. 7.

References 1 D. L. Hoffman and G. R. Franke, J. Marketing Res., 23 (Au-

gust) (1986) 213-227. 2 Technical Report: P-135 (The MATRIX Procedure: Lan-

guage and Applications), SAS Institute Inc., Cary, NC, May 6,1985.

3 W. F. Kuhfeld, Psychometrika, 51 (March) (1986) 155-161. 4 IDPLOT and QPRINT Procedures (Preliminary Documenta-

tion), SAS Institute Inc., Cary, NC, May 6,1985. 5 M. Greenacre, Theory and Applications of Correspondence

Analysis, Academic Press, London, 1984. 6 P. G. M. van der Heijden and J. de Leeuw, Psychometrika, 50

(December) (1985) 429-447.

7 M. Greenacre and T. Hastie, J. Am. Stat. Assoc., 82 (398) (1987) 437-447.

D. L. Hoffman is Associate Professor of Marketing at the Gra- duate School of Business, Columbia University, New York, NY 10027, U.S.A. T. P. Novak is Associate Research Director at Young & Rubi- cam, New York, NY10017, U.S.A.

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Information on chemical applications, software and math- ematical tools for improving information content should be sent to:

TrAC Computer Corner, D. L. Massart, M.-P. Derde, Vrije Universiteit Brussel, Fakulteit der Geneeskunde en der Farmacie, Farmaceutisch Scheikunde, Laarbeeklaan 103, B-1090 Brussels, Belgium.

1

trends

Chromatographic interfaces for supersonic jet spectroscopy Murray V. Johnston Boulder, CO, U.S.A.

Until recently, applications of supersonic jet spectroscopy to chemical analysis have been plagued by relatively poor detection limits and the lack of suitable intet$aces to standard chromatographic techniques. Supersonic jet nozzles based upon new technologies have been developed which permit low picogram detection limits to be obtained and provide convenient interfaces for capillary gas chromatography. Ex- tension of these methods to liquid and supercritical fluid chromatography may also be possible.

Supersonic jet spectroscopy is a widely used method for studying the low temperature spectroscopy of molecules. The sample of interest is coexpanded with a monoatomic gas, usually helium or argon,

01659936/88/$03.00.

from a high-pressure region through a small orifice into a vacuum. Isentropic cooling during the expansion efficiently depopulates thermally excited rotational levels producing an effective rotational temperature of ~10 K. Under these conditions, molecular absorption and emission spectra are extremely sharp and can act as fingerprints of specific compounds. The narrow bandwidth character of jet- cooled spectra necessitates the use of laser-based detection methods for maximum sensitivity. Although a variety of methods have been used to study jet- cooled molecules, most analytical applications have involved laser-induced fluorescence. At the time of our previous review on jet spectroscopy in this jour- nal’, this method had been applied only to packed- column gas chromatography (GC) and the best detection limits reported were in the 14-60 ng range.

OElsevier Science Publishers B .V

trends inhnalytical chemistry, vol. 7, no. 3,198s 95

Since that time, new supersonic nozzles have been developed which permit much lower detection limits to be achieved and provide effective chromatographic interfaces.

Capillary GC interfaces

Interface considerations Three major problems are associated with inter-

facing supersonic jet spectroscopy to capillary GC. First, there are the intrinsic properties of a jet expansion. Since the gas must expand for cooling to occur, the molecular density and hence detection sensitivity must be decreased when a sample is cooled in a supersonic jet. This dilution effect can be substantial. At a typical probe distance of 50 nozzle diameters downstream from the orifice, the sample density is reduced by ca. lo4 from its value prior to expansion. Furthermore, the terminal molecular velocity in a jet expansion is quite large, on the order of 104-lo5 cm/s depending upon the expansion gas. Analyte molecules quickly traverse the laser interaction region and are pumped away. Therefore, the duty factor for detection is reduced relative to methods where the analyte residence time in the detector is comparable to the chromatographic peak width.

The second problem concerns the flow-rate mismatch between capillary GC and the flow-rate re- quired to sustain a jet expansion. A continuous expansion of argon at 250 Torr through a 200~pm orifice into a vacuum requires a gas flow-rate of 120 ml/min corrected to STP. This value is between 10 and 100 times greater than the gas flow exiting a capillary gas chromatograph. If this difference is overcome by simply mixing the chromatographic effluent with makeup gas, the analyte is diluted and sensitivity is reduced. In principle, jet spectroscopy could be performed with capillary GC flow-rates by using a pinhole restrictor on the order of 10 pm diameter at the column outlet. This approach suffers from orifice clogging problems, inefficient cooling (cooling scales with orifice diameter), and an unacceptably short distance between the orifice and Mach disc which disrupts supersonic flow.

The third problem is the duty factor encountered when pulsed light sources such as Excimer or Nd:YAG pumped dye lasers are used to detect ana- lytes in a continuous jet expansion. For a pulse rate of 10 Hz, a pulse width of several nanoseconds, a beam diameter of 1 mm, and a molecular velocity in the jet of 5.104 cm/s, only 2.10-’ of the sample tra- versing the excitation volume is actually interro- gated by the light source. The remaining sample is lost. Given these constraints, it is not surprising that early work in analytical jet spectroscopy employed

packed columns and achieved only moderate detection limits2-‘.

Types of interfaces There are essentially two approaches to interfac-

ing capillary GC to supersonic jet spectroscopy depending upon which of the above problems is tar- geted. The first and most widely discussed approach is to modulate the expansion with a pulsed valve so that gas flows through the orifice in short, intense pulses. The gas pulse can be timed to coincide with the laser pulse so that the effective duty factor for molecular detection is increased. In addition, the flow-rate mismatch between capillary GC and jet spectroscopy is overcome since the time averaged gas flow through the orifice is reduced relative to a continuous jet expansion. A high-temperature, low- dead-volume pulsed valve has been reported in which the GC effluent feeds directly into the valve5,6. Although this method is applicable to capillary GC, it is unclear how the pulsed gas flow affects chromatographic resolution and whether or not a true enhancement of signal-to-noise is obtained relative to a continuous jet expansion having the same analyte mass flow-rate. A potential disadvantage is that the GC effluent must be diluted with argon to achieve good cooling.

Pepich and co-workers738 have described an ele- gant variation of this approach in which a continuous gas flow from the capillary GC (helium carrier) is combined with a pulsed makeup gas flow (argon). The two gases mix in an antechamber and then expand through a small orifice. This method over- comes the duty factor problem by concentrating most of the continuous gas flow from the GC into the makeup gas pulses while maintaining good chromatographic resolution. A 7-fold enhancement in signal-to-noise is obtained over a conventional jet expansion having the same analyte mass flow-rate.

The second approach to interfacing capillary GC to jet spectroscopy has been adopted by our group and involves modifying the fundamental expansion properties so that the sample dilution effect of a jet expansion is reduced’,“. In our method, the GC effluent is entrained in a concentric sheath gas flow and expanded through a tapered nozzle. The sheath gas provides the necessary gas flow to sustain the jet expansion and focuses the sample stream from the chromatograph to achieve lower detection limits.

Sheath flow focused jet expansions The principle of sheath flow focusing in supersonic

jet spectroscopy is illustrated in Fig. 1. The sample stream from the chromatographic column is entrained in a concentric sheath gas flow. Laminar flow

96

Capillary

Fig. 1. A schematic of the sheath flow nozzle for capillary GC-jet-cooled fluorescence spectroscopy. (Reprinted with per- mission from ref. 8, Copyright 1987 by the American Chemical Society.)

is quickly established at the point of injection since the nozzle is designed to keep the sample stream linear velocity at or below the effective sheath gas velocity. The sample stream is focused in the tapered nozzle by the sheath gas to an area less than 20pm in diameter at the orifice. A slight compression occurs as the sample and sheath gases approach the orifice resulting in an increase of the effective molecular density by a factor of two at the orifice. Tight focusing of the sample stream is not observed in the supersonic jet downstream from the orifice. However, the central core of the jet is enriched with sample by a factor of 20 over a conventional jet expansion where the sheath and sample streams are thoroughly mixed prior to expansion. These results along with the ob- servation that cooling is dependent upon the sheath gas flow characteristics indicate that substantial but incomplete mixing of the sheath and sample streams occurs downstream from the orifice.

In practice, the range of orifice diameters that can be used is limited by several competing phenome- na”. Large orifices may have Reynolds numbers large enough to give turbulent flow immediately up- stream from the orifice which destroys the focusing effect. Small orifices have the advantage of provid- ing good cooling with a low total gas throughput into the vacuum chamber. Therefore, a smaller pumping system can be used. However, small orifices also give severe spectral broadening with all but the low- est analyte mass flows in the sample stream which se- verely limits the useful working range. The origin of this spectral broadening is unknown but it is believed to arise from incomplete mixing between the sheath and sample streams in the expansion which reduces the overall cooling efficiency.

trends in analytical chemistry, vol. 7, nb. 3, 1~88

Sheath flow focused jet expansions are normally run with a 200~pm diameter orifice and 250-400 Torr argon as the sheath gas. Since the sample stream constitutes roughly 1% of the total gas flow through the orifice and mixing between sample and sheath occurs in the expansion, cooling is deter- mined by the sheath gas flow characteristics’“. Therefore, argon is chosen as the sheath gas since it provides good cooling at modest stagnation pressures. The optimum temperatures we have been able to achieve in sheath flow focused jet expansions are ca. 5 K rotationally and 100-200 K vibrationally for 100-400 cm-’ vibrational modes.

Analytical applications Sheath flow focusing provides well over an order

of magnitude enhancement of sensitivity for jet spectroscopy. With laser-induced fluorescence, sensitivity can be further increased by replacing the fluorescence monochromator with an interference filter and using an ellipsoidal reflector to collect and focus the fluorescence radiation at the photomultiplier. This arrangement is shown in Fig. 2. Excitation transitions of jet-cooled molecules are usually narrow (cu. 5 cm-’ or less at half-height) so that dispersed fluorescence detection through a monochromator is rarely needed to augment the selectivity of laser excitation. Furthermore, rejection of excitation scatter is not as crucial as with condensed phase measure- ments since relatively little scatter is induced by the low pressure gas. Fig. 3 shows the signal-to-noise ra- tios we have been able to achieve with a properly op- timized system”. The estimated detection limit for naphthalene at a signal-to-noise ratio of 3 is 8 pg.

An example of the selectivity of jet cooled fluorescence spectroscopy is given in Fig. 4 for the detection of naphthalene in unleaded gasoline’. Fig. 4A is a chromatogram of unleaded gasoline obtained with a flame ionization detector. Fig. 4B is the same chromatogram using laser-induced fluorescence where the laser is tuned to a narrowband excitation transi- tion of naphthalene. In this case, naphthalene alone is detected. The difference in retention times shown in Fig. 4A and B is due primarily to the differing

Nozzle I Reflect-or

!

Laser Beam

Fig. 2. Ellipsoidal reflector system for jet spectroscopy.

trends 61 analytical chemistry, vol. 7, no. 3, 1988 97

7) -3

.-I

i

J c -

A

J-l 4-l

Y

u-l

E J c

H

n

6

i

Time Time

Fig. 3. Capillary GC-jet-cooled fluorescence chromatograms (A) 40 pg and (B) IS pg naphthalene injected; laser tuned 308.12 nm.

of to

pressures at the column outlet. Although the selectivity of jet spectroscopy is apparent from Fig. 4, it is not obvious that this method can be used to quanti- tate trace constituents in complex samples or that it provides any advantage over existing analytical techniques. Quantitation of trace constituents by jet cooled fluorescence has been demonstrated by Hayes and Small* (naphthalene and methylnaphtha- lenes in crude oil) and by Pepich et ~1.~ (mono- methylanthracenes in a complex environmental sample). In the former study, the quantitative results

A

8 IS 24

tme(mm)

8 IS 24 tlme(mn)

Fig. 4. Unleaded gasoline chromatograms, temperature pro- grammed from WC to 150°C at 4”Clmin; (*) denotes naphthalene; (A) flame ionization detector, (B) jet-cooled fluorescence detection with laser tuned to 308.12 nm. (Reprinted with permis- sion from ref. 8, Copyright 1987 by the American Chemical So- ciety.)

were equivalent to those obtaind by gas chromatography-mass spectrometry (GC-MS). In the lat- ter study, quantitation of individual methylanthra- cene isomers was not possible by GC-MS under the chromatographic conditions employed. Herein lies the major advantage of supersonic jet spectroscopy for chemical analysis. The spectral resolution of jet- cooled fluorescence significantly reduces the de- mands placed upon sample cleanup and chromatographic steps of an analysis. This advantage is par- ticularly well suited to the resolution of geometric isomers which have similar chromatographic retention indices and similar mass spectra.

With the advent of improved GC interfaces and lower detection limits, supersonic jet spectroscopy has become a viable analytical technique. However, several advances must be made if it is to be widely used. First, a suitable database of jet cooled excitation and emission transitions must be developed. Surprisingly few compounds of analytical relevance have been studied by jet spectroscopy despite the widespread use of this method for fundamental spec- troscopic investigations. Second, a movement away from rather exotic excitation sources (Excimer and Nd:YAG pumped frequency doubled dye lasers) to more convenient sources is needed to offset the cost and complexity associated with laser-based detection methods. Simple, low cost nitrogen and flash- lamp pumped dye lasers are available which provide sufficient pulse energies for fluorescence spectroscopy in the near ultraviolet and visible regions. These sources could be used to detect substituted polynuclear aromatic hydrocarbons larger than naphthalene. A potential disadvantage of laser-induced fluorescence is that the laser can be tuned to detect only one or a small number of compounds during each chromatographic run. High power broadband lamp sources could be combined with multiplex methods such as Fourier or Hadamard transform spectroscopies to provide sufficient resolution in the excitation spectrum while retaining high light throughput. In addition, the short data acquisi- tion times inherent to these methods could allow complete excitation spectra to be obtained on the chromatographic timescale and facilitate multicom- ponent determinations. In terms of sensitivity, broadband excitation with multichannel fluorescence detection using a photodiode array is an at- tractive alternative since it yields a multiplex advantage while Fourier and Hadamard transform spectroscopies are subject to a multiplex disadvantage in the UV-VIS region. However, this approach suffers from the problem that fluorescence spectra of jet- cooled molecules tend to be broader and more con- gested than the corresponding excitation spectra12.

98

This phenomenon arises from intramolecular vibrational energy redistribution in the excited electronic state. As a result, the ultimate selectivity of jet- cooled fluorescence spectroscopy will not be as great as with excitation spectroscopy.

Liquid and supercritical fluid jet expansions Extension of the above methods to liquid chroma-

tography (LC) and supercritical fluid chromatography (SFC) requires that two additional problems be overcome. First, the chromatographic effluent must be vaporized without decomposition or precipitation. This problem is similar to that encountered in coupling LC or SFC to MS. Second, direct jet expansions of many vaporized eluents commonly used in LC or SFC are not expected to achieve uni- formly low rotational temperatures. Polyatomic gases contain internal degrees of freedom that can inhibit rotational cooling relative to monoatomic gases when expanded. As a result, the cooling achieved strongly depends upon the identity of the liquid or supercritical fluid carrier.

The cooling problem for large molecular eluents can be reduced by mixing the vaporized effluent with an excess of argon prior to expansion. This principle was first demonstrated by Ishibashi and co-work- erst3,14 and later by our grou sion of the sheath flow nozzle p5

using a modified ver- . When used for liquid

or supercritical fluid jet expansions, the nozzle shown in Fig. 1 is modified by inserting a short restricting capillary at the end of the sample inlet capillary.,The restricting capillary dimensions are chosen to maintain a desired pressure in the sample line at the liquid flow-rate used. This design is similar to those developed for SFC-MS. The sample stream is vaporized in a concentric argon flow and expanded through the jet orifice. Since the vaporized sample stream linear velocity is orders of magnitude greater than the effective sheath gas velocity, laminar flow is not established and focusing does not occur. There- fore, detection limits using liquid or supercritical fluid sample introduction will be poorer than with capillary GC. Good cooling is obtained independent of the identity of the eluent as long as argon constitutes more than 90% of the total gas flow through the jet orifice.

Research in this area has only begun. Laser-induced fluorescence studies of jet-cooled molecules injected through continuous liquid and supercritical fluid inlets have been carried out only on naphtha- lene15, perylene13, and substituted anthracenes14. The ability of these inlets to vaporize truly non-vola- tile compounds remains unclear. Future devel- opments will probably mirror related advances in LC-MS and SFC-MS.

trendsin analyticalchemistry, vol. 7, no: 3, I988

Acknowledgements Preparation of this manuscript was supported by

grants from the National Science Foundation (CHE8614097) and National Institutes of Health (GM34457).

References 1 2 3

4

5

6

7

8

9

10

11

12

13

14

15

M. V. Johnston, Trends Anal. Chem., 3 (1984) 58. J. M. Hayes and G. J. Small, Anal. Chem., 54 (1982) 1204. T. Imasaka, T. Shigezumi and N. Ishibashi, Analyst (Lon- don), 109 (1984) 277. T. Imasaka, H. Fukuoka, T. Hayashi and N. Ishibashi, Anal. Chim. Actu, 156 (1984) 111. T. Imasaka, T. Okamura and N. Ishibashi, Anal. Chem., 58 (1986) 2152. T. Imasaka, K. Tashiro and N. Ishibashi, Anal. Chem., 58 (1986) 3244. B. V. Pepich, J. B. Callis, J. D. S. Danielson and M. Gouter- man, Rev. Sci. Instrum., 57 (1986) 878. B. V. Pepich, J. B. Callis, D. H. Burns, M. Gouterman and D. A. Kalman, Anal. Chem., 58 (1986) 2825. S. W. Stiller and M. V. Johnston, Anal. Chem., 59 (1987) 567. S. W. Stiller and M. V. Johnston, Appl. Spectros., 41 (No. 8) (1987) in press. S. W. Stiller, Ph.D. Dissertation, University of Colorado, Boulder, 1987. See for example: W. P. Lambert, P. M. Felker and A. H. Ze- wail, J. Chem. Phys., 81 (1984) 2209. H. Fukuoka, T. Imasaka and N. Ishibashi, Anal. Chem., 58 (1986) 375. T. Imasaka, N. Yamaga and N. Ishibashi, Anal. Chem., 59 (1987) 419. B. D. Anderson and M. V. Johnston, Appl. Spectros., 41 (No. 8) (1987) in press.

Murray V. Johnston is at the Department of Chemistry and Bio- chemistry, Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309-021.5, U.S.A.

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chromatographic interfaces for supersonic jet spectroscopy

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