IP-MALDI

Typically, MALDl has been performed under low pressure conditions (ca 10 Torr). In recent years, intermediate pressure (IP, 10 to 1 Torr) [10,12-14] and atmospheric pressure (AP) MALDl [15,16] systems have been developed to analyze more labile molecules and for tissue analysis [10]. Atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDl) offers the ability to couple a MALDl source to most commercial instruments, including quadrupole ion traps, because the source uses existing ion optics and vacuum transfer lines of commercial instruments to introduce the ions generated by MALDL A brief discussion of IP-MALDI with ion trap technology is presented, but the primary focus is on the instrumentation aspect of ion trap technology, in particular linear ion trap technology, for direct tissue analysis and imaging MS studies. 

Figure 11.17. Schematic representations of external MALDI-FTMS systems. (A) vacuum MALDI-FTMS system where ions are generated in the external source and transferred to the analyzer cell via an RF-only multipole ion guide. (B) IP-MALDI-FTMS system. Argon is introduced in a short burst via a pulsed valve to briefly elevate the pressure within the source region. Ions are first stored in the source hexapole and undergo collisional cooling before transfer to the analyzer cell via an RF-only ion guide.

Haff LA, Smirnov IP. Single-nucleotide polymorphism identification assays using a thermostable DNA polymerase and delayed extraction MALDI-TOF mass spectrometry. Genome Res 1997 7 378-388. 

Recombinant wild type hIL-ip, the K138C mutant and the K138C, R4A, L6A triple mutant (mutant 1) were isolated from the soluble fraction of E. coli lysates by ammonium sulfate fractionation and hydrophobic interaction chromatography. The purified proteins were characterized by SDS-PAGE, western blots, N-terminal sequence, size exclusion chromatography (SEC), isoelectric focusing (lEF), matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), and electrospray mass spectrometry (ESMS). 

Arnold, R.J., Karty, J.A., Ellington, A.D., Reilly, IP. (1999) Monitoring the growth of a bacteria culture by MALDI-MS of whole cells. Analytical Chemistry, 71,1990-1996. 

Dourges, M.A., Charleux, B., Vairon, IP., Blais, J.C., Bolbach, G, Tabet, IC. (1999) MALDI-TOF mass spectrometry analysis of TEMPO-capped polystyrene. Macromolecules, 32, 2495-2502. 

Fig. 5.4. MALDI-IM 2D plot from the cerebrum of a cocaine-injected rat (20 mg of free base/kg), 20 min after ip injection with DFIB matrix. Note the mobility separation of cocaine from matrix interference ions.

The most intuitive ionization pathway in UV MALDI might be direct matrix photoionization. The ionization potentials (IPs) of free, isolated matrix molecules have been found to lie... 

More important for UV-MALDI might be IP reductions due to matrix-analyte interactions. Matrix-analyte interactions can be stronger than those between matrix molecules, and there is no a priori reason to expect low two-photon efficiencies for such complexes. Kinsel and colleagues have reported several experimental and theoretical studies of this effect in clusters. In one example, strongly reduced IPs for DHB-proline complexes were found, down to 7eV. ° In addition, fragmentation of some complexes after ionization produced protonated analytes.This is an efficient two-photon process that is probably active for certain matrix-analyte combinations. How often it contributes to MALDI is not yet clear, and this cannot be easily predicted in advance. 

Figure 5.8. Positive-mode MALDI spectra versus matrix-analyte mole ratio (DCTB matrix) for an equimolar five-component mixture. A, M-T data ionization potential (IP), 6.04 eV (CAS number 124729-98-2) B, TTB IP, 6.28 eV (76185-65-4) C, NPB IP, 6.45eV (123847-85-8) D, rubrene IP, 6.50eV (104751-29-9) E, D2NA IP, 7.06 eV (122648-99-1). The molar mixing ratios of matrix to analyte arc indicated for each spectrum. TTiese analytes are observed exclusively as radical cations, and they exhibit matrix and analyte suppression effects analogous to those known from proton or cation transfer secondary reactions. Low ionization potential (IP) analytes suppress high IP analytes and matrix. (Adapted from Ref. 32.)...

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