Mass spectral techniques chemical ionization

A number of recent investigations have shown that mass spectrometry (MS) is a rapid and effective method for the identification of triacylglycerol species of milk fat that are compositionally different (Myher et al., 1988, 1993 Laakso and Kallio, 1993 Spanos et al., 1995 Laakso and Manninen, 1997 Mottram and Evershed, 2001 Kalo et al., 2004). In fact, a range of mass spectral techniques, such as electron ionization, fast atom bombardment, chemical ionization, atmospheric pressure chemical ionization and electrospray MS, have been used to study triacylglycerols. The later three are soft ionizing techniques, which retain substantial amounts of the molecular ion, rather than fragmenting the molecule into a number of parts. These methods have allowed the determination of... 

Chemical ionization using chiral reagent gases (e.g. l-amino-2-propanol or 2-amino-1-propanol) has been shown to induce distinctive behaviour between enantiomers. Enantiomers could also be differentiated via S 2 reactivity with chiral reagents in ion-molecule reactions. The resulting diastereoisomeric products were distinguished by tandem mass spectral techniques (MS/MS). 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

A subsequent study of A. cavernosa from Thailand by California workers revealed two additional F-type kalihinols in addition to kalihinol-X (109) and -Y (110) [45]. The structures of kalihinol-I (121) and -J (122) were secured by low resolution chemical ionization mass spectral and NMR data. Both 1H-1H COSY and 13C NMR techniques were used extensively. Furthermore, hydrolysis of compound 109 to kalihinol-J (122) confirmed the assignments. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Both positive and negative ion mass spectrometry are useful for fluorine compounds and often are complimentary. Fluorocarbons rarely show an M+ peak in positive ion El spectra but often exhibit intense M in negative ion spectra. Chemical ionization techniques routinely offered in inexpensive mass spectrometer systems greatly enhance the abihty to observe molecular ions in both positive and negative ion spectra. Most mass spectral data on simple main group... 

A possible solution to the above problems would be the triple-dimensional analysis by using GC x GC coupled to TOFMS. Mass spectrometric techniques improve component identification and sensitivity, especially for the limited spectral fragmentation produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a unique identity for overlapping components in the chromatogram makes identification much easier. Thus MS is the most recognized spectroscopic tool for identification of GC X GC-separated components. However, quadru-pole conventional mass spectrometers are unable to reach the resolution levels required for such separations. Only TOFMS possess the necessary speed of spectral acquisition to give more than 50 spectra/sec. This area of recent development is one of the most important and promising methods to improve the analysis of essential oil components. 

Various ionization techniques applied in association with Py-GC/MS are reported in literature (see e.g. [12]). However, the most common ionization method is electron impact with the detection of positive ions (EI+). The chemical ionization (Cl) is sometimes used, but Cl spectra interpretation is difficult because of the lack of fragmentation and because the reproducibility in Cl is affected by the experimental conditions in which the spectra are generated. However, Cl spectra provide valuable information regarding the molecular mass of the analyte, and this can be very useful in combination with EI+ spectral information. 

Compilation of spectral catalogs for the soft ionization technit]ues— chemical ionization, FAB, and API—has not occurred, for at least two reasons. First, the exact nature of a soft ionization spectrum depends upon instrument and sampling conditions. Second, for many compounds analyzed by the soft ionization techniques, the spectrum consists principally of a molecular ion. A compilation of soft ionization spectra, in its ideal form, would be a compilation of molecular masses. Movements promoting the compilation of MS-MS product ion spectra have surfaced several times. As we have seen from some of the discussions above, these too depend upon exact operating conditions. Lack of an acceptable standard set of conditions has hampered everyone s efforts to make such a project happen. 

Mass spectrometry (MS) has not been applied extensively to the study of naturally occurring xanthones, but the mass spectral data provide valuable information about the structure elucidation of xanthones. As well as electron impact MS, which is a routine technique for the structure elucidation of xanthones, recently developed soft ionization techniques, such as desorption-chemical ionization MS (D/CI-MS) and fast atom bombardment MS (FAB-MS), are of great interest for the analysis of glycosides. Molecular ion peaks can be observed without derivatization. Tandem MS/MS can be extensively employed in directly characterizing constituents of complex mixtures. Recently, xanthone profiles of H. perforatum cell cultures were identified by HPLC-MS/MS analysis [106]. 

Numerous additional advances in GC/MS occurred throughout the last decade. Mass spectrometers have now become almost routine, reliable instruments. Improvements in design of both sector and quadrupole instruments is today reflected in greater spectral resolution and sensitivity parameters. Versatility of the GC/MS combined instruments has been dramatically improved by better interfacing techniques and an increased use of capillary columns. The chemical ionization methods have become important for work with relatively unstable molecules there is a significant rationale for their increasing use in biochemical research. 

A target may resist hydrolysis or chemical degradation, or the degradation products may not yield useful information. It is also common that insufficient material exists for proper analysis. In these cases, an alternative degradation technique is available that uses the ionizing electron beam of a mass spectrometer. The ionization pathways available from electron impact in the mass spectmm are bond fission processes that occur by known and predictable pathways. Indeed, each pathway usually follows analogous chemical reaction pathways in a retro-synthetic manner. It therefore follows that an examination of mass spectral ionization patterns can give clues for suitable disconnections and a synthetic tree. 

Surface Characterization. Most modem techniques for the characterization of surfaces have been developed since 1970 (74,75). Surface techniques allow for both qualitative and quantitative characterization of trace levels of molecular species (see Surface AND INTERFACE ANALYSIS). Most recently an extension of surface analysis utilizing laser ionization has been introduced (76). In surface analysis by laser ionization (sah), a probe beam, composed of ions, electrons, or laser light, is directed to the surface under examination to remove a sample of material. An untuned, high intensity laser passes dose to, but paralld and above the surface. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. A highlight of this technique is the use of efficient, nonresonant, and therefore nonselective photoionization by pulsed imtuned laser radiation. The commercial availabiUty of intense laser radiation makes this technique viable. The mass spectrometer, not the laser, performs the chemical differentiation. 

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