Eric Sevy, Chemistry

Introduction

Supercritical fluid chromatography coupled with supersonic jet spectroscopy has been shown to be an effective analytical technique in the analysis of complex samples. This highly selective technique derives its power from the combination of a powerful separation technique and an discriminating spectroscopic identification technique. Using them in tandem an investigator can analyze complex mixtures for specific compounds. Unfortunately, till now the compounds that could be analyzed were limited in their size, by the requirements necessary to couple the two methods together. The work reported here is the first step to opening up this analysis to larger molecular weight samples.

In all previous work done in our lab using this combined technique was done using a sheath flow nozzle design. The purpose of this work was to design, construct, and test an extended sheath flow nozzle for supercritical fluid chromatography-supersonic jet spectroscopy; a new nozzle design that would facilitate the introduction of larger molecular weight samples for analysis with this highly selective technique. Then perform initial testing to determine the effectiveness of this modification.

Experimental

The experiments described in this work are, with the exception of the nozzle design which is discussed in the results and discussion section of this report, are the same as those previously performed in our laboratory; therefore, I will only give a brief outline of the experimental procedure and refer the reader to previous work. l-4

Sample line pressures of up to 150 atm could be obtained with a PerkinElmer syringe pump. The samples were introduce into the flow stream of the pure carrier ( supercritical C02) using an injection valve. An injection split line was also used with these experiments.

The vacuum chamber and pumping systems are the same as discussed elsewhere.2 Background pressure in the chamber was maintained in the 10·5 region while the supercritical fluid was flowing. A Lambda Pnysik excimer pumped dye laser system was used as the excitation source, and was perpendicular to the jet axis. The laser induced fluorescence was collected and collimated into a 1/3- m monochromator and analyzed with the aid of photo diode array and a personal computer.

Results and Discussion

The first step in the development of extended sheath flow nozzle involved theoretical calculations. The length of the nozzle must be long enough to ensure laminar flow, one of the requirements of the sheath flow design. This increase in length is necessary since a higher pressure sheath gas is being used to provide back pressure restriction. This back pressure restriction eliminates the need for capillary restriction and allows for capillary openings wide enough for the passage of larger molecules.

The nozzle itself is very similar to that used in other experiments performed in our lab4, except that the antechamber of the nozzle is longer, roughly six inches. The other difference between this nozzle and those previously used is the addition of a centering ring. A stainless steel capillary with a fused silica capillary inside is placed in the center of the nozzle. Through the capillary the sample is passed through the nozzle and into the laser beam. These two capillaries must remain in the center of the nozzle to facilitate laminar flow out of the nozzle. If the capillary becomes off center, molecules corning out of the capillary may not be swept out of the nozzle and become condensed on the inside. This would ruin separation and limit detection. The centering ring was made by drilling six small holes around the nozzle, a distance of n/3 radians apart. Through these holes we feed a small gauge wire in a clover pattern with the capillary in the center. The excess wire was then wrapped tight around the nozzle. The rest of the setup was as previously done in our lab.1

Spectra of perylene were analyzed in a repeat of the experiments previously run with the sheath flow nozzle.1 These experiments were repeated to be certain that we could obtain at least as good results with this design as with the previous one. The tips of the fused silica capillaries were restricted and low flow rates and gas pressures were used to ensure that the sheath flow could be done with the new design. Various orifice sizes were used ranging from 200J.L to 25J.L. In each case we determined that the extended sheath flow nozzle gave as good results as the design previous used.

This result is expected, since the length of the nozzle is only required to be the minimum necessary to produce the laminar flow of the sheath gas. The one thing that could have caused problems is if the capillary slipped off center. It seems however that the centering ring designed was sufficient to keep the capillary in the center of the nozzle and reduce its disruption effects on the gas flow.

Conclusions and Future Work

The extended sheath flow nozzle produces perylene spectra that are comparable with those obtain using the regular sheath flow nozzle under the same conditions used in the sheath flow nozzle experiments. The work that remains·to·be-doneincludes-measurements.at incremental increases of pressure of the sheath gas. This increase in pressure along with the opening of the SF capillary restrictor will allow us to determine if the back pressure restriction will be sufficient to keep the carrier above the critical point, while still allowing the passage of analyte through the nozzle into the jet expansion. Once these parameters have been optimized, the analysis of higher molecular weight samples can be performed.

References

1. Sin, C.H. Linford, M.R. and Goates, S.R. Anal. Chem. 64(2), 233 (1992).
2. Sevy, E.T. Honors Thesis, Brigham Young University 1994.
3. Simons, J.K. Sin, C.H. Zabriskie, N.A. Lee, M.L. and Goates, S.R. J. Microcol. Sep. 1: 207(1989).
4. Goates, S.R. Sin, C.H. Simons J.K. Markides, K.E. and Lee, M.L. J. Microcol. Sep. 1:207 (1989).