Simple means to alleviate sensitivity loss by trifluoroacetic acid (TFA) mobile phases in the hydrophilic interaction chromatography–electrospray tandem mass spectrometric (HILIC–ESI/MS/MS) bioanalysis of basic compounds

Trifluoroacetic acid (TFA) is a commonly used additive in HPLC and LC–MS analysis of basic compounds. It is also routinely added to aqueous–organic mobile phases utilized in the hydrophilic interaction chromatography–electrospray tandem mass spectrometry (HILIC–ESI/MS/MS) technique used in our laboratories for bioanalysis. However, TFA is known to suppress the ESI signals of analytes due to its ability to form gas-phase ion pairs with positively-charged analyte ions. The most common method to overcome this problem involves the post-column addition of a mixture of propionic acid and isopropanol. However the post-column addition setup requires additional pumps and is not desirable for continuous analysis of large amounts of samples. In this paper we present a simple yet very effective means of minimizing the negative effect of TFA in bioanalysis by direct addition of 0.5% acetic acid or 1% propionic acid to mobile phases containing either 0.025 or 0.05% TFA. A factor of two- to five-fold signal enhancement was achieved for eight basic compounds studied. Furthermore, chromatography integrity was maintained even with the addition of acetic acid and propionic acid to existing TFA mobile phases. This method has been successfully applied to the HILIC–ESI/MS/MS high-throughput analysis of extracted biological samples to support pre-clinical and clinical studies.