Sensitivity improvement in infrared detection for supercritical fluid chromatography

Abstract

Dramatlc Improvements in the sensltlvity of Fourier transform infrared detectors for supercritical fluid chromatography are explained In terms of optical and chromatographic parameters. By use of methyl palmitate as a model compound, minimum ldentlkation HmHs of 10 ng of component delivered to the column using carbon dioxide mobile phase are reported. Detectlon limits such as these are achieved by balancing optical throughput of a 600 q internal diameter flow ceU against the effect of moMle phase absorption with a 5inm path length. Results from a study of the detector cell temperature indicate an improvement In sensJtivity can also be attributed to concentration of the chromatographic peak when using cooler detector cell temperatures.Supercritical fluid chromatography (SFC) is attracting a great deal of interest as a powerful new technique in the field of chromatographic analysis. One of the primary areas of current research in SFC is the development of full range spectral detectors such as Fourier transform infrared (FT-IR) spectroscopy. This is an important development because FT-IR detectors with SFC can provide full infrared spectral information for identification of chromatographic peaks.Recently, there has been a dramatic improvement in the performance of FT-IR flow cell detectors with SFC. While this improvement has generated considerable interest, there has been little explanation of how these improved detection limits were obtained or the factors affecting detection limits. This paper addresses these concerns and reports results from experiments that study the optical and chromatographic parameters involved in the SFC/FT-IR experiment. By use of model compounds, the identification capability and chromatographic performance of the flow cell type of SFC/FT-IR interface are defined.

Applications of Supercritical Fluids in Industrial Analysis

“…The cell is a cylinder with windows of calcium fluoride, zinc selenide or zinc sulphide and the restrictor is connected to the outlet (Shafer and Griffiths, 1983;Olesik et al, 1984;Hughes and Fasching, 1985;Johnson et al, 1985;French and Novotny, 1986;Morin et al, 1986;Wieboldt et al, 1988;. The advantage of an on-line system is that chromatograms based on the total IR absorbance can be reconstructed, using the Gram-Schmidt algorithm.…”

Xenon as a mobile phase in supercritical fluid chromatography

“…Use of xenon results in reduced chromatographic resolution loss, compared with CO,, from the effects of peak compression [36] in the FTIR cell. As xenon passes from the SFC column to the cooler FTIR cell, the density change is smaller since xenon remains above its critical temperature.…”

Hyphenated Methods for Identification After Chromatographic Separation

“…Typical flow cells are cylindrical in shape and polished internally to maximize reflection with infrared transparent endwindows made from calcium fluoride, zinc selenide or some similar material [128,132,133]. The cells are positioned between the column and restrictor or backpressure regulator and must be capable of withstanding the operating pressure and temperature required to maintain the separation conditions (or different conditions if these are selected to improve sample detectability).…”