The effect of laser profile, fluence, and spot size on sensitivity in orthogonal-injection matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

The influence of incident laser parameters on sensitivity in matrix-assisted laser desorption/ionization (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 N(2) laser and a Nd:YAG laser using 2-m long optical fibers. The N(2) laser gives better sensitivity, consistent with a more uniform fluence distribution and therefore better coverage of the N(2) 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. Thus, the optimum sensitivity is achieved at high fluence. 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. The results have particular relevance for imaging MALDI, where sensitivity and spatial resolution are important figures of merit.