Chemical ionization methods

Electron impact ionization, also known as particle beam ionization, has been applied to the online determination of steroids such as hydrocortisone, cortisone, prednisolone and prednisone. Polymer additives such as NC-4, Irga-nox 1076,1-octadecanol and Naugard -XL were identified and quantitated online by electron impact and, separately, by atmospheric pressure chemical ionization methods.78... 

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. 

Separation selectivity can also be altered by the type of ion that is formed for the compound of interest. Cation or anion adducts can have a significant effect on the separation. In the positive ion mode, most ions formed from electrospray or any chemical ionization methods are protonated cations. However, if Na+ is added to the electrospray solution, sodiated adducts may be formed as the primary response ion. For example, the protonated ions of two isomers, methyl-P-glucopyranoside and methyl-a-glucopyranoside, cannot be baseline separated in DTIMS, but the sodiated adducts of the same isomers can be baseline separated. Figure 8.9 demonstrates separation selectivity induced by cation adduction. These IMS spectra are of the methyl-p-D-galactopyranoside and the methyl-a-D-galactopyranoside isomers adducted with cobalt acetate, silver, and lead acetate. The cobalt acetate adducts had a separation factor of 1.02, the silver adducts had a separation factor of 1.05, and the lead acetate adducts had a separation factor of 1.07. 

It may at first appear at variance with the rest of the handbook to discuss derivatization for fast atom/ ion bombardment mass spectrometry, where chromatography may be only incidentally rather than directly involved. Nevertheless, it seems appropriate to include this aspect because these techniques are complementary to the electron impact and chemical ionization methods used with gas chromatography—mass spectrometry (GC-MS) as described in Chapter 14, and thus extend the coverage to include derivatization methods for the majority of mass spectrometric techniques. 

Nassar, A.E. Varshney, N. Getek, T. Cheng, L. Quantitative analysis of hydrocortisone in human urine using a high-performance liquid chromatographic-tandem mass spectrometric-atmospheric-pressure chemical ionization method. J. Chromatogr. Sci. 2001, 39 (2), 59-64. 

Chemical ionization methods have been used with methyl ethers of methyl glycosides and have indicated that anomers with equatorial methoxy-groups decompose faster than those with axial groups. The same method applied to permethylated disaccharides allows better distinction between isomers than does the electron-impact method. A permethylated amino-sugar trisaccharide derivative has been studied as have permethylated dialdose anhydrides. ... 

Chemical Ionization The absence of a prominent molecular ion in El mass spectra may favor the use of chemical ionization methods coupled with GC separations. The spectra resulting from different chemical ionization modes, reagent gases, and mass analyzers can vary significantly. Positive ion chemical ionization (PICI) of TATP has been reported using methane, isobutene, and ammonia. 

The other method involves direct ionization of the molecule to the ion energy of interest. This can be done optically by photoelectron-photoion coincidence (described below), or by charge exchange, which is a form of chemical ionization. Common chemical ionization methods include charge transfer and proton transfer. 

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