Hysteresis effects observed in MALDI using oriented, protein-doped matrix crystals.

Irradiation effects on large crystals of sinapinic acid doped with apomyoglobin were studied. These crystals were oriented in specific directions with respect to the direction of laser incidence. The crystal faces were initially subjected to repeated laser irradiation at the energy necessary to produce an observable ion signal, resulting in a gradual disappearance of the matrix and protein ion-signal. One or two high-energy laser shots, on the same spot, induce a renewed ion emission with near threshold-fluence shots. Multiple sequences of high/low-energy shots could be carried out with very reproducible ion emission intensity cycling. This hysteresis effect was observed on all the studied crystal faces. The influence of the time delay between the cycles of the high/low-energy illumination was studied. All of the results were consistent with the creation of a perturbed region directly below the desorbed volume, along the path of the incident beam during low-energy irradiation, which could be removed or altered by a high-fluence laser pulse. (C) 1997 Elsevier Science B.V.