Thermal desorption, analysis normalization

APCI is a relatively soft ionization technique. In fact, only few fragment ions are normally recorded. Nevertheless, analyte decomposition may occur due to heating. It can be used in the analysis of polar and low polar analytes with molecular weights up to 1500 Da [29] as long as the proton affinity of the analytes is higher than that of the solvents. On the other hand, ionization techniques such as ESI and matrix-assisted laser desorption/ionization (MALDI) are mainly used in the analyses of polar, less volatile, and thermally labile analytes, for example, large biomolecules. These two ionization techniques are discussed in the following (Sections 2.4 and 2.6). 

In 1970, Vastola, Mumma, and Pirone obtained a mass spectrum of sodium hexylsulfonate (Na C HijSOj) using a pulsed laser.Their spectra recorded canonized salts and dimers Na2 X and Naj Xj (where X = hexylsulfonate), but none of the decomposition products normally observed when these compounds were thermally vaporized for analysis by electron impact. This was a significant achievement since organic salts were deemed to be the compoxmds most intractable to mass spectral analysis. Because they observed molecular organic ions formed directly by the laser, their report is perhaps the first example of laser desorption. 

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