SFC-MS systems

The chromatographic and mass spectrometric choices facing the analyst in coupling SFC and MS successfully, namely injection method column type of flow restrictor and mass spectrometer ionisation method and type of vacuum system, have been described [398]. In SFC-MS coupling, the restrictor plays a major role, as the expansion behaviour to a large extent determines the overall performance of the SFC-MS system and defines the range of applications. 

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. 

The inherent lower viscosity of the SFC mobile phase relative to HPLC yields significant advantages for chiral separation applications.Indeed, preparative SFC is experiencing an increase in utilization in the pharmaceutical industry for chiral purification for a wide range of scales. Now, mass-directed purification technology has been incorporated with SFC for both chiral and achiral purification applications,and commercial prep-SFC/ MS systems are currently in development. 

The mass spectra of mixtures are often too complex to be interpreted unambiguously, thus favouring the separation of the components of mixtures before examination by mass spectrometry. Nevertheless, direct polymer/additive mixture analysis has been reported [22,23], which is greatly aided by tandem MS. Coupling of mass spectrometry and a flowing liquid stream involves vaporisation and solvent stripping before introduction of the solute into an ion source for gas-phase ionisation (Section 1.33.2). Widespread LC-MS interfaces are thermospray (TSP), continuous-flow fast atom bombardment (CF-FAB), electrospray (ESP), etc. Also, supercritical fluids have been linked to mass spectrometry (SFE-MS, SFC-MS). A mass spectrometer may have more than one inlet (total inlet systems). 

Section 6.4 deals with other EI-MS analyses of samples, i.e. analyses using direct introduction methods (reservoir or reference inlet system and direct insertion probe). Applications of hyphenated electron impact mass-spectrometric techniques for poly-mer/additive analysis are described elsewhere GC-MS (Section 7.3.1.2), LC-PB-MS (Section 7.3.3.2), SFC-MS (Section 13.2.2) and TLC-MS (Section 7.3.5.4). 

Various transport type interfaces, such as SFC-MB-MS and SFC-PB-MS, have been developed. The particle-beam interface eliminates most of the mobile phase using a two-stage momentum separator with the moving-belt interface, the column effluent is deposited on a belt, which is heated to evaporate the mobile phase. These interfaces allow the chromatograph and the mass spectrometer to operate independently. By depositing the analyte on a belt, the flow-rate and composition of the mobile phase can be altered without regard to a deterioration in the system s performance within practical limits. Both El and Cl spectra can be obtained. Moving-belt SFE-SFC-MS" has been described. 

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 provide attractive advantages for the analytical chemist. Many commercially available interfaces originally designed for LC-MS require minimal modification to effectively couple an SFC to API-MS systems. Another advantage of such interfaces is the self-volatilizing effect provided by CO2 in nebuliza-tion of the mobile-phase solutes. 

One of the earliest reports of SFC interfaced with APCI was by Huang et al. [121]. The authors used a pin-hole restrictor to maintain supercritical fluid conditions in a packed-column (pcSFC) system. Results for a mixture of five corticosteoids were described with an injection of 25 ng of each of the components. The system was also amenable for capillary SFC/MS applications with minimum modification. Sadoun and Virelizier [122] reported an SFC interface with ESI in which a two-pump SFC and a packed column were used with the outlet directly interfaced to an ESI source of a quadrupole mass spectrometer. Also, 1-30% (v/v) of polar organic modifier (Me0H-H20 95 5) was added to CO2 mobile phase to help elute polar organic compounds. The setup was shown to allow analysis of polar organic compounds that were difficult to analyze with earlier implementations of SFC-MS with a chemical ionization interface. A recent review article is available on pcSFC-MS [123]. 

Capillary electrophoresis (CE) is a powerful separation technique. It is especially useful for separation of ionic compounds and chiral mixtures. Mass spectrometry has been coupled with CE to provide a powerful platform for separation and detection of complex mixtures such as combinatorial libraries. However, the full potential of CE in the application of routine analysis of samples has yet to be realized. This is in part due to perceived difficulty in the use of the CE technique compared to the more mature techniques of HPLC and even SFC. Dunayevskiy et al. [136] analyzed a library of 171 theoretically disubstituted xanthene derivatives with a CE/ESI-MS system. The method allowed the purity and makeup of the library to be determined 160 of the expected compounds were found to be present, and 12 side products were also detected in the mixture. Due to the ability of CE to separate analytes on the basis of charge, most of the xanthene derivatives could be resolved by simple CE-MS procedures even though 124 of the 171 theoretical compounds were isobaric with at least one other molecule in the mixture. Any remaining unresolved peaks were resolved by MS/MS experiments. The method shows promise for the analysis of small combinatorial libraries with fewer than 1000 components. Boutin et al. [137] used CE-MS along with NMR and MS/MS to characterize combinatorial peptide libraries that contain 3 variable positions. The CE-MS method was used to provide a rapid and routine method for initial assessment of the construction of the library. Simms et al. [138] developed a micellar electrokinetic chromatography method for the analysis of combinatorial libraries with an open-tube capillary and UV detection. The quick analysis time of the method made it suitable for the analysis of combinatorial library samples. CE-MS was also used in the analysis... 

The distillation eliminates virtually all significant contaminants. The vapor phase will not transport colloidal rust from water-contaminated steel cylinders. Greases and oils are not volatile at the temperature of the distillation. Small residuals of nitrogen, oxygen, and even water at the 5-10-ppm level usually do not interfere with analysis or purification. In fact, after a break-in period, the gas delivery system in our laboratory provides significantly lower background noise in SFC-MS than cylinders filled with SFC-grade CO2. 

In the gas phase, the proton transfer reaction occurs when the proton affinity (PA) of the neutral solvent molecules is greater than that of the donors. Ion source chemistry is of fundamental importance in the SFC-APCI system. The evaporation process to remove the solvent and to leave the ionized analytes in the gas phase plays an important role in all API interfaces. For HPLC-MS, higher mobile-phase flow rates normally result in appreciable negative impact on the ionization efficiency of the analytes. However, the tolerable input of mobile-phase flow rate into the APCI... 

Phospholipids are commonly found in mammalian blood and animal tissue as structure components in membranes and have been identified to be a major source of ion suppression [38,71], Due to their hydrophilicity, phospholipids do not tend to be retained on RP HPLC. SFC may be an excellent solution if significant ion suppression of the analyte due to the co-elution of phospholipids is observed in the RP HPLC system. In addition, ion suppression from DMSO solvent was found to be a greater problem with the SFC-MS method than with the standard LC-MS method [21],... 

SFC-APCI-MS using methanol-water-modified carbon dioxide as the mobile phase has been applied to the analysis of PACs. A direct fluid introduction interface with flow splitting removed two thirds of the SEC effluent transferred to the APCI-MS system. PACs were separated on a Cjg packed column at 45°C and LCDs were similar to those obtained with HPLC-UV. SFC-APCI-MS-MS using microbore packed columns applied to the analysis of PACs in coal tar was able to provide structural information for isomer differentiation. LODs in the low ng range were obtained by SIM. 

PBI was evaluated for packed-column SFC-MS [91]. Fundamental studies on the effects of various operational parameters were reported and the sensitivity of the system was evaluated. 

The purification of combinatorial libraries on a Berger system is deseribed by Farrell et al. at Pfizer for their parallel solution-phase syntheses. The overall process employs as well analytical SFC in combination with mass spectrometry and nitrogen chemiluminescence deteetion off-line of the preparative-scale SFC systems. Pre-purification analytical SFC/ MS/CLND allows the triage of samples for purification, and an in-house software package analyzes data for predicted quality based on an evaluation of UV and MS data for the potential of co-eluting peaks during purification. This same software package selects a collection time window for purification, which is necessary to limit the number of fractions per sample. This system accommodates the purification of samples up to 50 mg... 

The basic concepts have already been presented in this review. However, the study of single compounds does not require an interface between the chromatographic unit and the MS. The analysis of mixtures by MS relates not only to GC/MS or HPLC/MS . The popularity of GC/MS systems can be examined by the nuihber of relevant publications. These were less than 100 in 1968, rose to a peak of 2000 papers in 1979, dropped to 1500-1750 yearly until 1988, when they rose again to 2000. The use of MSD for HPLC (also called LC/MS) also rose from a few publications in 1975 to a few hundred in 1988. A method, less used for hydrocarbon analysis, SFC (Super-critical Fluid Chromatography)/MS, was initiated only in 1985 and is gaining interest slowly. These data are given by Evershed . Obviously it will be impossible to review all these developments in the use of MSD ", even for the analysis of alkanes and cycloalkanes. 

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