To perform matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS), researchers combine the sample with a molar excess of matrix—usually an aromatic organic acid that absorbs light energy at the laser's wavelength—and spot this mixture onto a metal target plate. When the focused light pulse strikes the target, it rapidly excites the matrix, inducing ionization and vaporization of both the matrix and the sample molecules in the surface layer. The desorbed ions are accelerated in a vacuum through an electric field into the TOF mass analyzer, where they travel toward the detector at a constant velocity characteristic of their mass-to-charge ratio (m/z). Some instruments are designed with a reflectron, an ion "mirror" that extends the length of the flight path and increases resolution. The m/z normally represents the mass of the ion, assuming each ion has a single charge. This "soft" ionization process produces very little fragmentation and enables scientists to study molecules with high molecular weights of 500 to 300,000 daltons.
MALDI can produce either positive or negative ions, and different classes of compounds produce different types of ions. "Biomolecules ionize differently," explains Dominic Gostick, MALDI Technical Specialist for Micromass' Life Sciences Group. "Peptides ionize well as positive ions but oligonucleotides don't analyze well in positive mode." He adds, "Positive-ion detection mode was originally designed for the high-throughput protein analysis arena, but expanding the functionality of the current MALDI-TOF system adds flexibility, and for people working outside core high-throughput proteomics, it opens up the world to other areas of analysis," including polysaccharide and oligonucleotide analyses.
But the new, extended M@LDI has other uses as well. These instruments assist scientists in genotyping applications by directly analyzing DNA and RNA, and by sequencing single nucleotide polymorphisms (SNPs). Researchers can also use the systems to monitor therapeutic pharmaceuticals and abused drugs in biological samples. Finally, in the chemical and petroleum industries, scientists can employ negative-ion detection to characterize certain synthetic polymers.
The M@LDI family of instruments includes the M@LDI L, a compact linear instrument for intact protein, DNA, and bacterial fingerprint analyses; the M@LDI R, a reflectron system refined for monoisotopic peptide mass fingerprinting (mPMF); the M@LDI LR, a dual detector machine for mPMF and intact protein analyses; and the M@LDI HT, a high-throughput system featuring Micromass' MagLift™ robotic autochanger for unattended analysis of up to 50 sample target plates containing 96 or 384 samples each. Scientists can operate both the linear and reflectron instruments in negative-ion mode through simple software selection.
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