Fundamental studies of MALDI with an orthogonal TOF mass spectrometer
The interaction between the matrix and analyte molecules are studied with a high resolution MALDI imaging technique in an orthogonal-injection TOF mass spectrometer. The analyte incorporation and distribution patterns have been clearly demonstrated. Purified protein analytes were found to be homogeneously incorporated in large single crystals of DHB and sinapinic acid matrices, with no evidence for preferred crystal faces. Segregation of some species was observed and appeared to correlate with analyte hydrophobicity, and to a lesser extent analyte mass or mobility. The influence of incident laser parameters on sensitivity in MALDI has been investigated using orthogonal-injection time-of-flight (TOF) instruments. A qualitative comparison was first made between the beam profiles obtained with a N2 laser and a Nd:YAG laser using 2-m long optical fibers. The N2 laser gives better sensitivity, consistent with a more uniform fluence distribution and therefore better coverage of the N2 laser profile. Most of the difference disappears when a 30-m long fiber is used or when the fibers are twisted during irradiation to smooth out the fluence distribution. In more systematic measurements, the total integrated ion yield from a single spot (a measure of sensitivity) was found to increase rapidly with fluence to a maximum, and then saturate or decrease slightly. For a fluence near threshold, the integrated yield has a steep (cubic) dependence on the spot size, but the yield saturates at higher fluence for smaller spots. The area dependence is much weaker (close to linear) for fluence values above saturation, with the result that the highest integrated yields per unit area are obtained with the smallest spot sizes. Finally the detection properties of the MCP detector were studied with a hybrid MCP and CuBe venetian blind converter detector. The measurements show that the detection efficiency of the MCP drops with the increasing of ion mass and the decreasing of the ion energy. The secondary electron emission coefficient of the MCP shows a linear dependence on mass and a power law dependence on velocity (~ 3.2). No clear velocity threshold is observed for secondary electron emission.
MALDI, Mass spectrometer
Qiao, Hui (2008). The effect of laser profiles, fluence, and spot size on sensitivity in orthogonal-injection MALDI TOF mass spectrometery, Rapid Commun. Mass Spectrom. 22:2279-2790.