Two Sources are Better Than One – More Choices for Sample Ionization with ESI and APCI

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Introduction
Two compounds were submitted for molecular weight deter-mination using the 1200L LC/MS. Electrospray Ionization (ESI) is generally the ionization method of choice for compounds analyzed with LC/MS systems. ESI works well for a wide variety of analytes. However, there are instances where particular analytes do not ionize well with ESI. In these cases, a second ionization method, Atmospheric Pressure Chemical Ionization (APCI), may often give the needed sensitivity.

While both positive ESI and APCI can result in a charged analyte, the process of ionization is very different. With ESI, protonation of the analyte occurs in liquid phase. With APCI, a protonated solvent molecule donates a proton to the analyte in the gas phase.

In general, APCI is better suited for ionization of non-polar, volatile low molecular weight molecules while ESI is better suited for more polar, non-volatile molecules.

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