Reduction of signal suppression effects in ESI-MS using a nanosplitting device

Electrospray ionization mass spectrometry is a valuable tool in the identification and quantification of drug metabolites in biological fluids. However, there are many instances where matrix components present in these fluids interfere with analyte detection and prevent the acquisition of accurate or complete results. In some instances, the matrix can suppress ionization to such an extent that analytes are completely undetectable by MS. In this work, we investigate how ionization and ion-transfer efficiencies are affected by drastically reducing the flow into the MS. A postcolumn concentric flow-splitting device was constructed to allow the measurement of analyte signal and ionization suppression across a range of flow rates (0.1-200 muL/min). Using this device, the effects of flow rate on signal intensity and ionization suppression were measured in analytical experiments that included flow injection analysis MS, postcolumn addition LC-MS, and on-line LC-MS analysis of metabolites generated from rat liver microsomes. The device used to deliver 0.1 muL/min flows is referred to as a nanosplitter because it achieved high split ratios (2000: 1), producing flow rates comparable to those observed in nanoelectrospray. The nanosplitter maintained chromatographic integrity with high fidelity and allowed the direct comparison of analyte signal across a range of flow rates (0.1-200 muL/min). A significant improvement in concentration and mass sensitivity as well as a reduction in signal suppression is observed when the performance at 200 versus 0.1 muL/min flow rate is compared. Using this specially designed concentric splitting device, the advantages of ultralow flow ESI were easily exploited for applications employing large bore chromatography.