Atmospheric-pressure chemical ionization disadvantages

El and Cl methods can be used if the compound to be studied is sufficiently volatile and stable to be vaporized intact. However, only 20% of the organics found in surface water are volatile enough to be amenable to GC-EI-MS or GC-CI-MS. Today, there are a variety of other ionization techniques available electrospray ionization (ESI), atmospheric pressure chemical ionization, matrix-assisted laser desorption ionization, and fast atom bombardment. Each of these has its advantages and disadvantages. A simple guideline to the most likely optimum ionization technique for a given class of substance is given in Table 1. 

Nearly all available ionization techniques of MS have already been applied successfully to Upids, and their individual advantages and disadvantages discussed [25]. Nowadays, the overwhelming majority of lipidomic investigations makes use of electrospray ionization (ESI) MS [26], and useful protocols have been developed in this respect [27, 28]. In comparison to ESl-MS, Upid studies based on MALDl [29] or atmospheric-pressure chemical ionization (APCI) [30] are much less abun-danL and a major aim of this chapter will be to emphasize the benefits of MALDI-MS. 

The introduction of liquid samples into the mass spectrometer has proved especially difficult. However, the advent of electrospray ionisation (ESI) and atmospheric pressure chemical ionisation (APCI) have overcome this problem and revolutionized the interfacing of HPLC with mass spectrometry. This success has pointed the way toward a major change in the mass spectrometric analysis of vitamin D, its analogs, and their metabolites. A disadvantage at the present time is that ionization using ESI and APCI is less efficient than the EI(+) used in GC-MS, but the immediate advantage is that it obviates the need for derivatiza-tion. 

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