Ion spray mass spectrometric detection for liquid chromatography: A concentration- or a mass-flow-sensitive device?

As the mass spectrometer becomes more accepted as a detector for HPLC, its characteristics should become better understood by those performing routine LC-MS experiments. In particular, the ion current response for quantitative analysis studies involving significant dynamic range in concentration for target analytes must be determined as well as other factors that affect MS response. This work describes the concentration-sensitive response for the ion spray (pneumatically-assisted electrospray) LC-MS interface from the chromatographer's perspective. A comparison of LC-MS ion current response in the isocratic mode resulting from studies of a synthetic mixture containing alkyl benzoates is presented. LC-MS total ion current chromatograms from three different column sizes (1 mm I.D., 2.1 mm I.D. and 4.6 mm I.D.) with and without a post-column split, and high-flow ion spray LC-MS without a post-column split illustrates that the former behaves as a concentration-sensitive detector whereas the latter behaves as a mass-flow-sensitive detector. The flexibility of ion spray to high-flow applications allows the use of HPLC eluent flow ranging from 0.001-2.0 ml/min. The use of solvent—buffer post-column addition also allows optimization for improved analyte ion current response.