At present, MALDI has seen very little application in the quantitative analysis of small molecules and their metabolites. The reasons for the limited use of MALDI are based on its limitations: many drug molecules have molecular weights well below 1000 Da, where there is significant potential for interferences with fluctuating signals from the organic matrix used for desorption and ionization. Also, MALDI signals are notoriously irreproducible due to non-homogeneous co-crystallization of analyte and matrix. Moreover, hyphenated LC-MALDI is predominantly conducted with off-line spotting devices, as no commercial on-line interfaces are available. Nevertheless, MALDI has intrinsic advantages over electrospray ionization (ESI), which would be invaluable for quantitative analysis. Most importantly, MALDI can achieve a high sample throughput, which makes it attractive to analytical laboratories to boost productivity and efficiency. Furthermore, in comparison to ESI, MALDI is not as susceptible to ion suppression from salts or buffers. It has been demonstrated that MALDI can be used for extremely rapid and quantitative analysis of low-molecular weight pharmaceutical drugs.1 In particular, the combination of MALDI with triple quadrupole MS (MALDI-QqQ) has shown great promise in drug analyses that are currently dominated by ESI methods.

For the present work, two specific advantages of MALDI-QqQ are important: first, the availability of dedicated scan modes (precursor ion and neutral loss scans), for selective class-specific screening routines. Second, the considerably higher duty cycle of QqQ in multiple reaction monitoring (MRM) mode compared to, for example, quadrupole TOF, for a significant sensitivity increase for quantitative measurements. MRM also provides a convenient way of circumventing interfering MALDI background signals in the low mass range. These advantages are further enhanced by using a high frequency (kHz) laser for ion generation instead of the more common N2 lasers with repetition rates usually well below 100 Hz. A kHz laser generates a semi-continuous ion beam, which is ideal for mass analysis in beam-type instruments such as QqQ. Statistically meaningful results with several thousand laser shots per sample can be obtained in only a few seconds, the averaging of which dramatically improves the overall precision. The aim of the present work was to exploit the specific advantages of MALDI and QqQ for detection and quantitation of small molecules. The selected compounds were a group of spirolide toxins. They represent a novel class of bioactive imines, initially discovered in shellfish samples from Nova Scotia. Their polyketide structures include cyclic imine and spiro-linked tricyclic ether moieties. A comprehensive study in phytoplankton samples was carried out, including screening for structural analogs, quantitation of concentrations and preliminary structural characterization, based on low-energy CID spectra. We are aware that a MALDI assay would immediately be scrutinized for its ability to compete with established ESI methods for toxins. Therefore, we compared and cross-correlated the results with data from ESI LC/MS/MS analyses. The spirolides’ mass spectra, obtained under ESI in triple quadrupole, ion trap as well as infrared multiphoton photodissociation FTICR MS have been previously studied.2,3 Several product ions and neutral losses are common for known and unknown spirolide compounds. This consistency provided an opportunity for screening experiments of biogenetically-linked spirolide species. Precursor ion and neutral loss scans by MALDI-QqQ were achieved using crude extracts with no chromatographic separation prior to analysis. Therefore, the opportunity exists for very high-throughput screening of biological extracts with little or no sample preparation.