SFC-MS interfaces

Table 7.36 SFC-MS interfaces, ionisation techniques and mass analysers...

The most common interfaces for MS coupling to cSFC and pSFC are given in Table 7.37. There is no universal, ideal SFC-MS interface. Mobile-phase-eliminating and direct-coupling interfaces are compared in Table 7.38. 

Table 7.38 Comparison of mobile-phase-elimination and direct-coupling SFC-MS interfaces...

Satisfactory performance of the SFE-SFC-HRMS instrumentation (resolution 1200) was only possible after optimisation (temperatures, restrictor and quartz tube positions, flow characteristics and sample transfer conditions). Mass spectra obtained for Irganox 1010/1076/1330 and Irgafos 168/P-EPQ by SFC-HRMS were identical with those obtained by use of DIP [431]. However, the sensitivity of the SFE-SFC-MS interface is low (at best 4 % of that obtained with sample introduction via DIP). An enormous amount of sample is lost in all parts of the coupling system (SFE, SFC and... 

SFC-MS coupling has repeatedly been reviewed [3, 397,398,413,417,418,433-437] and cSFC-MS detection [438] and SFC-DFS [439] were considered specifically. SFC-MS interface technology was recently reviewed by Niessen [440]. 

There has been a >30-year effort to develop robust SFC-MS interfaces (9-17). SFC-MS has become a viable technology. The inherent advantages of SFC over HPLC, make SFC-MS superior to HPLC-MS (18,19). 

Earlier implementation of SFC-MS followed the evolution of both HPLC-MS and GC-MS interfaces [11,21,23-26], As the API interfaces of HPLC-MS became mainstream analytical techniques in recent decades, they were also quickly employed for SFC-MS [21,23,26-37], The atmospheric pressure chemical ionization (APCI) [27,33] and electrospray ionization (ESI) [36,37] sources are the most popular API interfaces for SFC-MS systems and allow for direct introduction of the effluent to the inlet of the mass spectrometer (Table 9.1). In some cases, the commercial API sources used for HPLC-MS system were proven to be applicable to the SFC-MS system with no modification [11,21,38-41], However, some modification in the SFC-MS interface may be desired for SFC to achieve stable operation and enhanced ionization [22], The ideal interfaces for SFC-MS would provide uniform pulse free flow, maintain chromatographic integrity, and ionize a wide range of analytes. 

Figure 9.10. Schematic diagram of a packed column SFC-MS interface with electrospray ionization. This interface features a liquid pressure-regulating flow for pressure control to just before the expansion region. (From ref. [75] Elsevier).

SFC-MS Interface Tip Temperature. Figure 3 shows the effect of increasing MS interface probe-tip temperature on the separation of the oligomers in the surfactant Triton X-100. At probe tip temperatures below 300 0, the restrictor consistently... 

Decomposition is probably occurring in the source or in the interface. Additional experiments, in which both the source and SFC-MS interface temperatures are varied, will be required to establish the site of decomposition. It is clear, however, that the application of SFC-MS to thermally-labile materials will ultimately be limited, not by the SFC, but by the interface-detector combination. 

The molecular beam approach, however, does not provide a practical SFC-MS interface for routine analytical applications. The... 

The SFC/MS interface is a direct-introduction system with careful temperature control up to the very tip of the capillary [27]. The end of the capillary contains a frit or a diaphragm that keeps the phase supercritical until it leaves the capillary and enters the ion source. Otherwise, the dissolved analyte would clog the end of the capillary. Since the gas load is rather high, chemical ionization is normally used with capillary SFC, although El is possible. When packed columns are used, momentum separators can reduce the pressure in the ion source, making electron impact possible. Limitations of the method with respect to classes of compounds are mainly due to SFC itself, not to the interface or the MS. 

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