The Choice of Electrospray or APCI

The vast majority of LC-MS analyses currently in use employ either electrospray ionization or APCI. In the previous example, electrospray ionization was employed because of the highly polar nature of the analytes but, as discussed above in Sections 4.7 and 4.8, this and APCI are, to a large extent, complementary, with APCI being used for low- to medium-polarity analytes and electrospray for medium- to high-polarity analytes. There are many compounds, therefore, for which the best ionization technique is not immediately obvious and their relative merits must be investigated. 

Why was a volatile ion-pairing reagent used in this work  

An involatile ion-pairing reagent would be deposited in the electrospray interface and lead to a reduction in performance. Some interfaces have been specifically designed to minimize this by removing the line-of-sight between the spray and the entrance to the mass spectrometer, and are thus more tolerant to involatile buffers. The performance of the interface will be improved by the use of volatile alternatives. 

In this study, the effect of mobile-phase flow rate, or more accurately, the rate of flow of liquid into the LC-MS interface, was not considered but as has been pointed out earlier in Sections 4.7 and 4.8, this is of great importance. In particular, it determines whether electrospray ionization functions as a concentration-or mass-flow-sensitive detector and may have a significant effect on the overall sensitivity obtained. Both of these are of great importance when considering the development of a quantitative analytical method. 

In general terms, electrospray ionization is considered to be concentration-sensitive at low flow rates and mass-flow-sensitive at high flow rates, while APCI is considered to be mass-flow-sensitive. Low and high are both subjective terms and require investigation as part of method validation. 

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