SFC-MS methods

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],... 

Mass Spectrometric Methods for Capillary SFC-MS. A significant advantage associated with capillary SFC-MS methods, and in contrast to all mechanical (e.g., moving ribbon) HPLC-MS interfaces, results from the flexibility in selection of ionization methods. Although initial studies were conducted using chemical ionization, and it remains the method of choice for most applications, the DFI process is also compatible with electron impact ionization (37). 

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). 

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. 

Table 7.40 summarises the general characteristics of on-line SFC-MS. The method is potentially most useful for thermally labile and involatile compounds that are unsuitable for GC-MS. Because the MS instrument is the main source of information, the reproducibility of the retention and the separation selectivity are much less important than for other SFC applications. As a result, mass spectroscopists do not feel restrained by the limits on reproducibility, which slowed the uptake of SFC by chromatographers. Method development should not be underestimated. Practical problems are associated with interfacing and the effect of the expanding... 

For sample identification, especially for complex mixtures, SFC-MS coupling gives better results than DIP. In addition to the mass-spectral data, when using this method the retention times can also be used for identification. The great disadvantage of DIP is the incomplete peak separation, because separation can be achieved only by the use of different evaporation temperatures. An important difference to GC-MS... 

The application range of cSFC-DFI-MS (Table 7.41) appears to be restricted either to the analysis of low-MW substances or to problems related to high-MW samples where low detection limits are not needed [124,444,445], The analysis of surfactants [446] by SFC-MS is frequently performed to demonstrate the feasibility of newly developed interface technology for practical applications. A rugged cSFC-MS method has been developed for the analysis of ethoxylated alcohols (AEs), which are non-ionic surfactants incorporated into a wide variety of industrial and consumer products [447]. cSFC-DFT-DFS was used for the analysis of low-MW, thermally unstable peroxides, and the higher-MW surfactants Triton X-100 and... 

SFC-based methods still need to show their potential, in spite of past great promise. pSFC-APCI-MS is a powerful method for identification of polymer additives, provided that a library of mass spectra of polymer additives using this technique is available. SFC-MS appears less performing than originally announced nevertheless, SFE-SFC-EIMS is an interesting niche approach to additive analysis. On the other hand, we notice the lack of real breakthrough in SFE-SFC-FTIR. 

Hoke et al. [60] demonstrated fast analysis of ultratrace drugs in biomatrixes using packed column SFC tandem mass spectrometry (SFC/MS/MS). Compared to an LC-MS/MS method, the SFC separation provided a roughly threefold reduction in... 

Hoke et al. [47] recently did a detailed comparison of SFC-MS-MS, EFLC-MS-MS, and HPLC-MS-MS (hexane/2-propanol/trifluoroacetic acid) conditions for thebioanalytical determination ofR and S ketoprofen in human plasma. The optimum chromatographic conditions included 55% methanol/45% CO2 (EFL conditions) with a Chiralpak AD column. The performance parameters (specificity, linearity, sensitivity, accuracy, precision, and ruggedness) for SEC, EELC, and HPLC were found to be comparable. However, the optimized EELC conditions provided the analysis in one-third the amount of time for the LC-MS-MS conditions, which is 10-fold faster than an LC-UV method [48,49],... 

Fig. 4b shows a MALDI-MS spectrum containing only linear diols which eluted at 100-150 s out of the column. Fig. 4c a cyclosiloxane fraction. Thus MALDI-MS may assist HPLC in developing separation methods it enables characterizing cyclic and linear components in a qualitative manner. The short time required and its easy handling with MALDI-MS is noteworthy in comparison with SFC-MS techniques. 

LC/MS), GC or LC/MS/MS, LC/DAD/MS, LC/NMR, LC/DAD/MS/NMR, SFC/MS, or CE/MS and various other combinations are extremely useful and can shorten the time needed for characterization of impurities. These methods have been discussed at length (Chapters 11-14 in this book also see references 1 and 2). 

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... 

These results show for the first time that it is possible to generate and detect high-mass ions (> 1500 Da) after their parent compounds have been introduced into a mass spectrometer by SFC. Both of these examples, PDMS and silylated oligosaccharides, are mixtures of relatively volatile compounds. We have not succeeded in observing ions related to less volatile materials, like sucrose octaoleate, that require high SFC elution pressures (370 atm). Future SFC-MS work will examine the applicability of this technique to high mass species that are less volatile. The combination of this unique separations method and a high-mass quadrupole mass spectrometer will be a powerful analytical tool. 

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