Impact of Salt and Urine Matrices on Electrokinetic Stacking Coupled to Paper Spray Mass Spectrometry

Relative to liquid chromatography coupled to electrospray ionization (ESI), paper spray ionization (PSI) can more rapidly ionize analytes from complex matrices with less sample preparation and less expensive instrumentation. However, matrix effects can lead to ion suppression and poorer sensitivity. To overcome these problems, we combined on-paper faradaic ion concentration polarization (f-ICP), a form of electrokinetic stacking, with PSI to preconcentrate analytes directly on the spray substrate prior to ionization. Electrokinetic stacking also enables desalting because of the separation of analyte molecules from small ions like sodium. 3D-printed cartridges containing chemically modified PTFE papers were utilized with a high voltage isolated power supply to perform on-paper stacking from the same device as paper spray. In this study, ion suppression was compared for paper spray with and without electrokinetic stacking in the presence of five salts and artificial urine at varying concentrations. The signal enhancement achieved for f-ICP/PSI relative to normal PSI was also quantified. Finally, the effect of salt type and concentration on stacking time was assessed. Increasing salt concentrations generally results in longer stacking times and increased currents, indicative of increasing quantities of charge being desalted. In the presence of single salt-containing matrices, f-ICP/PSI frequently decreased ion suppression and gave a signal enhancement of 2 to >200× for small molecule drugs compared to unstacked paper spray (uPSI). In artificial urine, a more complex matrix with various salts and metabolites, f-ICP/PSI did not reliably decrease matrix effects due to creatinine and other matrix components costacking with the analytes. Nevertheless, f-ICP/PSI still resulted in signal enhancements of 8 to >50× relative to uPSI due to analyte preconcentration from electrokinetic stacking. Overall, this research demonstrated that f-ICP/PSI results in significant sensitivity improvements relative to uPSI.