Capillary column supercritical fluid chromatography/mass spectrometry of polycyclic aromatic compounds using atmospheric pressure chemical ionization

Abstract

Capillary column supercritical fluid chromatography (SCF) was combined with atmospheric pressure chemical ionization (APCI) mass spectrometry through a heated pneumatic-nebulizer interface, originally developed for liquid chromatographylmass spectrometry (LCIMS), and subsequently modified for use with supercritical fluid chromatography/mass spectrometry (SFC/MS). A high pressure syringe pump was used to pass SFC-grade carbon dioxide, used as the mobile phase, through a capillary SFC column which was contained in an oven, maintained at 100 "C. The eluent passed through the heated pneumatic-nebulizer interface into the ionization region of the mass spectrometer with the aid of the flow of nebulizing gas. The system was optimized using benz[a]anthracene, and then applied to analyze a standard mixture of polycyclic aromatic compounds (PACs) as well as complex mixtures of PACs obtained by the fractionation of a pond sediment contaminated by coke-oven residues. A detection limit of 40 pg was established for chrysene.The mass spectrometer is the ideal detector for chromatography due to its inherent specificity, selectivity, and sensitivity.'-3 The direct interfacing of supercritical fluid chromatography (SFC) with mass spectrometry may provide significant advantages over liquid chromatography/mass spectrometry (LC/MS) because of the relative ease of mobile phase evaporation and removal in SFC. Potential application areas for SFC/MS are in the analysis of high molecular weight compounds that are of limited volatility, and of thermally labile compounds for which gas chromatography (GC) is either impossible or impracticable.' The operation of an SFC/MS interface requires that the mobile phase undergo a pressure reduction prior to ionization within the mass spectrometer. For capillary SFCIMS, the sensitivity obtained using electron ionization (EI) is one or two orders of magnitude less than that obtained using chemical ionization (CI);4 in any event, mass spectra that contain ions resulting from both EI and CI are often obtained due to the high C 0 2 pressures in the EI ion s o~r c e .~ Most EI ion sources are relatively gas-tight and are designed to operate at low flow rates or with gases that produce spectra similar to EI spectra (i.e., charge-transfer from He as in GUMS). By using a more open source design to minimize the pressure in the ionization volume, CI spectral contributions in capillary SFC/MS can be rninimi~ed.~ Detection limits of 20 pg have been reported for the capillary SFC/MS analysis of pyrene.'For the development of an effective SFC/MS interface, three requirements have to be fulfilled. First, the gas flow rates generated by volatilization of the mobile phase have to be handled by the interface (this is obviously easier for capillary SFC/MS than for packed + NRCC # 34889.

Supercritical Fluid Chromatography and Extraction

“…The versatility of atmospheric pressure ionization (API) has led to an increase in its use. Thomas et al used API for open-tubular SFC/MS of polycyclic aromatic compounds (127), while Matsumoto used SFC/API-MS for a more fundamental purpose, the estimation of hydrogen ion affinities (128). Arpino and Haas reviewed progress and future prospects (focused on API) in SFC/MS interfacing (129).…”

Packed Column Supercritical Fluid Chromatography/Mass Spectrometry for High-Throughput Analysis

“…The use of SFC should expand as knowledge of its advantages accumulates, especially in high-throughput applications. , Since it is often of interest to separate multicomponent mixtures in a minimal amount of time, the technique is finding increased use in industry for applications in which liquid chromatography separations are considered too slow. Similarly, when mass analysis of the analytes is necessary, SFC/MS will begin to replace LC/MS. − …”

“…Providing sufficient restriction at the API source is the critical point in maintaining a supercritical fluid prior to the source. Both ESI and APCI have been employed in numerous SFC/MS experiments. − ,,,,− High flow rates typical of packed column SFC are amenable to ESI-MS even when appreciable levels of organic modifiers are used . However, response factors are dependent on the concentration of modifier in solution as ESI depends on proton transfer in solution.…”

Packed Column Supercritical Fluid Chromatography/Mass Spectrometry for High-Throughput Analysis. Part 2

“…In recent years, supercritical fluid chromatography (SFC) has been exploited as an alternative to high-performance liquid chromatography (HPLC) because of its superior selectivity and speed. Similarly, SFC/MS has been demonstrated by several groups as a viable replacement for LC/MS in the analysis of small molecules. − Its applications to industries requiring high sample throughput become obvious as its characteristics are explored.…”

“…Similar flow rates in LC/MS often result in prohibitively high back-pressure for most columns, loss of coherent mass spectral signal due to incomplete vaporization of the solvent, and overloading of the mass spectrometer vacuum system. The efficiency of positive APCI with SFC/MS can be enhanced by the introduction of methanol, water, and other gas-phase proton-generating solvents to the mobile-phase modifier without adversely affecting SFC chromatography. ,,− We describe a system not only capable of rapid change between SFC/MS and LC/MS modes of operation, but also between autosamplers which inject from individual vials or multiple 96-well plates.…”