Matrix-assisted laser desorption/ionization surface preparation

Kemptner J (2009) Evaluation of matrix-assisted laser desorption/ionization (MALDI) preparation techniques for surface characterization of intact Fusarium spores by MALDI linear time-of-flight mass spectrometry. Rapid Commun Mass Spectrom 23 877-884. doi 10.1002/... 

In addition to gel electrophoresis, proteins can also be characterized using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Developed by Karas etal. (1987), this system uses a laser to irradiate intact cells, which ionizes surface cell proteins that are then characterized. The patterns of proteins yield a profile unique to the microorganism and are therefore used for identification purposes. MALDI-TOF-MS offers the advantage of rapid results with minimal sample preparation and reagents. However, the cost of equipment makes these methods impractical for wineries. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Surface-assisted laser desorption/ionization (SALDI). SALDI is a matrix-free approach for the analysis of low-mass molecules. In this innovative approach, the sample solution is placed directly onto a solid surface prepared by depositing an active material, such as powdered graphite, active carbon, carbon nanotubes, or silica sol-gel, onto a suitable substrate (e.g., A1 foil or Cu tape) and bombarded with a laser beam [47,48,60]. 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

ELDI is that samples can be presented to the inlet nozzle directly from the outside as compared to ESI or MALDI that require additional sanple preparation [46], ELDI has also been applied to the analysis of peptides and proteins [47] even from biological media [48], to detect chemicals on different surfaces [49], and of course, for conpound identification on TLC plates [50]. It has been found that addition of a matrix is beneficial for the laser desorption part of the method. This gave rise to matrix-assisted laser desorption electrospray ionization (MALDESI)... 

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