Atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) is known to suffer from poor ion transfer efficiencies as compared to conventional vacuum MALDI (vMALDI). To mitigate these issues, a new AP-MALDI ion source utilizing a coaxial gas flow was developed. Nitrogen, helium, and sulfur hexafluoride were tested for their abilities as ion carriers for a standard peptide and small drug molecules. Nitrogen showed the best ion transport efficiency, with sensitivity gains of up to 1900% and 20% for a peptide standard when the target plate voltage was either continuous or pulsed, respectively. The addition of carrier gas not only entrained the ions efficiently but also deflected background species and declustered analyte-matrix adducts, resulting in higher absolute analyte signal intensities and greater signal-to-noise (S/N) ratios. With the increased sensitivity of pneumatically assisted (PA) AP-MALDI, the limits of detection of angiotensin I were 20 or 3 fmols for continuous or pulsed target plate voltage, respectively. For analyzing low-mass analytes, it was found that very low gas flow rates (0.3-0.6 l min(-1)) were preferable owing to increased fragmentation at higher gas flows. The analyte lability, type of gas, and nature of the extraction field between the target plate and mass spectrometer inlet were observed to be the most important factors affecting the performance of the in-line PA-AP-MALDI ion source.
Copyright 2010 John Wiley & Sons, Ltd.