Why acids in HPLC & LC/MS Applications

Recently, I received an “Ask the Doctor” email from a reader asking why formic acid was specified as an additive for the mobile phase in an HPLC method he was using. Formic or trifluoroacetic acid at 0.1% concentrations are common, especially for LC-MS work. There are a number of reasons for adding an acid at low concentration to the mobile phase. Let’s look at two of these: the influence on the column and the sample. 
Silica acts as the support material for most reversed-phase columns. As we saw in earlier HPLC Solutions discussions (#34 and 35), the silica surface can vary depending on its source and how it is treated. However, we can consider the silica surface as somewhat acidic, with pKa values in the 4.8 region often cited for low-purity, type-A silicas, and higher pKa values for the high-purity, type-B silicas in more common use today. Ionized silanol groups interact through ion-exchange with ionized bases, and are a major factor in peak tailing for basic solutes. One way to minimize this problem is to suppress the ionization of the silanol groups. Thus, when the mobile phase pH is <≈3, the silica surface silanols are mostly suppressed, so cation-exchange interactions are minimal.

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