Design considerations for high speed quantitative mass spectrometry with MALDI ionization

Abstract

A MALDI ion source on a triple quadrupole mass spectrometer constructed for the purpose of obtaining high speed quantitative measurements on drugs and other low molecular weight compounds is described. Particular attention is given to the ion generation and transport phenomena that affect analysis speed, throughput, and practical instrument robustness. In this regard parameters that affect desorption speed, beam spreading, ion flight times, sensitivity, signal-to-noise, ion fragmentation, sample carry-over, and instrument contamination are examined and experimental results are provided. MALDI and electrospray sensitivity is compared, to provide a practical frame of reference. . One interesting configuration is MALDI on a triple quadrupole (QqQ) designed for the express purpose of acquiring rapid quantitative measurements on drug entities and other low molecular weight compounds dried and arrayed on microtiter plates. The concept is analogous to plate readers used in high throughput screening albeit these optical based systems derive their speed from parallel measurements. In the case of a mass spectrometer the analysis is inherently serial in nature but could be operated at rates that would approach the throughputs of the optical systems. Assessments of the general utility of MALDI TOF for small molecule quantitative and qualitative analyses have been reported [2] and reviewed [3]. Studies of the practicality of the triple quadrupole approach for various applications have begun to appear in the literature using prototype triple quadrupole instruments [4,5], which have examined in detail the quantitative figures of merit of the technique in biological samples, emphasizing the importance of the use of internal standards. In a sister publication to this work [4], issues important to quantitation and general analytical utility such as chemical diversity, background produced by blank measurements, and s/n are covered in detail. The focus of this work are the characteristics of the instrumentation.The reasons why a triple quadrupole is a logical choice for a MALDI based high speed quantitation tool will be elaborated on in this paper but can be summarized as follows. MALDI on a triple quadruple decouples the synchronization of the desorption event with the mass analysis, similar to orthogonal MALDI TOF, but quite distinct from traditional low-pressure MALDI TOF. The mass spectrometer performance is thus independent from the sample morphology. This allows target preparation to be a noncritical issue so inhomogeneities in the sample, as would be expected in any high throughput methodology, will have no effect on the mass spectrometer mass resolution or accuracy. High-pressure operation of the source and the preanalyzer RF optics is readily accommodated in a QqQ. Collisional cooling of the ion beam is achieved at higher pressures, which reduces ion fragmentation and allows a broader range of laser energy to be utilized. With low-pressure operation on TOF instruments the laser energy must be kept low, near the ionization ...

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