Direct Analysis of Liquid Samples by Desorption Electrospray Ionization-Mass Spectrometry (DESI-MS)

Desorption electrospray ionization-mass spectrometry (DESI-MS) was evaluated for the direct analysis of liquid samples. Several interesting results were found. First, in contrast to the previous DEST analysis of dried solid samples that was limited to Proteins with MW <= 25 kDa (Anal. Chem. 2007, 79, 3514), bovine serum albumin (BSA, 66 kDa) was successfully ionized from solutions by DEST with observation of corresponding Multiply charged ions. Second, direct DEST analysis of protein tryptic digest solutions without chromatographic separation, sample clean-up, and the sample drying step was demonstrated, providing reasonably good sequence coverage of 52%, to 97%. Third, direct analysis of biofluids such as an undiluted Urine sample without sample pretreatment is possible, emphasizing the high tolerance of DEST with salt. These results suggest that a charged droplet pick-LIP mechanism is responsible for desorption and ionization of liquid samples by DESI. Also, unlike in electrospray ionization (EST), inhibition of electrochemical reduction in the negative ion mode was observed for liquid sample DESI. In addition, reactive DEST can be performed with ion/ion reactions of Zn(II) complexes for the selective binding of phosphoserine in the presence of serine. DEST experiment can also be carried Out directly to liquid samples flowing out of a pumped syringe needle tip, allowing rapid analysis. Furthermore, on-line coupling of electrochemical cell with DESI-MS was demonstrated, in which perylene radical cations generated in the cell were successfully transferred to the gas-phase for MS detection by DESI. This study extended the scope of DESI-MS applications, which could have potentials in bioanalytical and forensic analysis. (J Am Soc Mass Spectrom 2009, 20, 10-19) (C) 2009 Publislied by Elsevier Inc.on behalf of American Society for Mass Spectrometry