Sensitivity in MALDI MS with small spot sizes
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In MALDI, for laser fluences below the saturation point the ion yield per shot follows a cubic dependence on the irradiated area, leading to a conclusion that smaller spots produce overall less ions and therefore are less viable. However, Qiao et al. showed that by decreasing the laser spot size it is possible to raise the saturation point, and thus increase the ion yield per unit area, also known as sensitivity. Here we explore laser spots below 10 micrometer diameter to determine whether they offer any practical advantage. We show that sensitivity is greater for a flat-top 3-4 micrometer spot than for a 10 micrometer spot. The sensitivity is greater for a Gaussian-like 3-5 micrometer spot than for flat-top 5-25 micrometer spots. We also report for the first time sensitivity versus theoretical fluence profile for a Gaussian-like beam focused to a spot of 3-5 micrometer.