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Delayed Extraction and Laser Desorption: Time-lag Focusing and Beyond

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ASMS, 1996

Steven Colby, Scientific Instrument Services, 1027 Old York Road, Ringoes, NJ 08551

In the two years since Robert Brown's demonstration that delayed extraction can dramatically improve the mass resolution of MALDI-TOFMS, considerable advances have been made in the field. Yet a number of questions still remain regarding the best approach toward identifying optimal conditions. Is the simple algorithm defined by Wiley and McLaren, sufficient for calculating instrument conditions? If so, why was substantial improvement in resolution not observed by the many expert researchers who combined delayed extraction with MALDI before the work by Dr. Brown? We examine the assumptions and "short cuts" employed by Wiley and McLaren in their calculations and demonstrate how some of these restrict the use of their equation to gas phase ionization. We then show how their approach may be modified to generate an expression more applicable to desorption from a surface. This generalized version of Wiley and McLaren's time-lag focusing equation may be used to identify a good value for the time delay between desorption and ionization. However, it is not guaranteed to give the optimum result. We examine conditions where poor results are to be expected. The equation is also limited because it generates a value for only one of a number of critical instrumental parameters. In order to optimize resolution it is necessary to go "beyond time-lag focusing" and briefly examine methods for optimizing other variables. We conclude that non-optimum values of these parameters were the primary factors that limited the resoltion obtained with time delayed extraction before the ground breaking work of Robert Brown.