Off-line supercritical fluid chromatography/plasma desorption mass spectrometry for simple mixture characterization

Abstract

Supercritical fluid chromatography/mass spectrometry (SFC/MS) is a powerful tool for the characterization of less volatile, complex mixtures. Unfortunately, on-line SFC/MS instrumentation is relatively expensive, and instrument time is limited. Here we describe an investigation of off-line SFC/plasma desorption mass spectrometry (PDMS) for the characterization of relatively simple mixtures. As an example of the potential of this approach, PD mass spectra of individual oligomers from a mixture of propoxylated dimethylpyrazoles are presented. The mixture was separated by capillary SFC with ultraviolet (UV) absorbance detection, individual peaks were collected on PDMS foils, and the foils were subjected to PDMS analysis. A variety of PDMS foils were compared, and NaI-pretreated, nitrocellulose-coated, aluminized-mylar foils provided the best results for the mixtures investigated in this work. Thirteen nanograms of an ethoxylated standard provided a signal-to-noise ratio of 3. The reproducibility of the overall method, as implemented here with manual collection, was relatively poor. The relative standard deviation of the PDMS response for an alkyl-phenol ethoxylate standard was 26%. Off-line PDMS characterization of a non-UV-absorbing compound, sucrose octamyristate, is also demonstrated. The retention time of the compound of interest is determined during an SFC separation using flame ionization detection. During a second separation, the detector flame is extinguished and the PDMS foil is held above the detector at the appropriate retention time. While limited in its ability to resolve closely eluting components, the off-line approach does provide greater flexibility and potentially greater mass range than conventional on-line SFC/MS. This approach also has the potential to provide the majority of SFC users, who do not have on-line SFC/MS capabilities, with a relatively straightforward method for obtaining mass spectral information on important peaks in an SFC separation.