2016 Volume 64 Issue 6 Pages 237-243
Atmospheric pressure matrix-assisted laser desorption/ionization using an infrared laser (AP-IR-MALDI) was compared for a solution of a peptide, angiotensin II, in water with 0.1% trifluoroacetic acid (TFA) at wavelength bands of 3 and 6 μm. The absorption coefficient of the liquid phase water at the 3 μm wavelength band was about 5 times larger than that for the 6 μm wavelength band, while the peak ion signal intensities for [M+H]+ of angiotensin II were almost the same at both wavelength bands. The maximum peak ion signal intensity was obtained at laser fluences of 0.28 and 0.36 J/cm2 for the wavelengths of 2.88 and 6.00 μm, respectively, and it was slightly higher at the 6.00 μm wavelength. To understand such characteristics of the ion signal intensity, the ablation depth of water was estimated using the blow-off model. The results showed that the dependences of the ion signal intensity on wavelength and fluence could qualitatively explained with the blow-off model. A difference was observed between the threshold fluence for ion detection in the experiment and that of ablation in the blow-off model. The effect of the stress wave produced by the short pulse laser irradiation was not taken into account in the case of the blow-off model, and this simplification could explain the difference in the threshold fluences. The ionization of angiotensin II dissolved in a 0.1 M Tris–HCl buffer solution was examined with electrospray ionization (ESI) and AP-IR-MALDI. In the case of ESI, numerous peaks originated in clusters of Tris–HCl were detected over a wide m/z range. On the other hand, mass spectra obtained using AP-IR-MALDI were similar to those obtained in the case of water containing 0.1% TFA. When the peaks for analyte ions overlap those originated in the buffer solution, AP-IR-MALDI would be useful as a novel interface for online liquid chromatography/mass spectrometry.
