Improving the Microdialysis Procedure for Electrospray Ionization Mass Spectrometry of Biological Samples

Abstract

Significant advances in the area of microdialysis which allowed more effective handling of small volumes (microliters) of samples, more efficient desalting and enhanced mass spectrometric detection sensitivity are described. The previously reported on-line coupling of microdialysis with electrospray ionization (ESI) mass spectrometry has been found to be highly effective; however, direct coupling requires relatively high sample flow rates (∽2 μl min-1) to obtain a stable ESI current compared with the flow rates of newer ESI sources (e.g. ‘microspray,’ 10–100 nl min-1). To circumvent this major limitation imposed by the dimensions of currently available materials, the microdialysis procedure was modified to an off-line mode in order to avoid excessive sample consumption. A more than tenfold decrease in sample consumption was achieved using the off-line mode vs the on-line mode, which resulted in a similar quality spectrum. In addition, several other aspects of the microdialysis approach were altered to improve its performance further: (i) an increase in dialysis temperature was found to increase the desalting efficiency greatly and therefore improve the spectrum quality; (ii) the addition of piperidine and imidazole to the dialysis buffer solution resulted in a reduction of charge states and a further increase in detection sensitivity for DNA and (iii) use of low concentrations (0–2.5 mM NH4OAc) of dialysis buffer shifted the DNA negative ions to higher charge states and produced a nearly tenfold increase in detection sensitivity and a slightly decreased desalting efficiency. Protocols for desalting different samples using microdialysis are discussed. © 1997 by John Wiley & Sons, Ltd.