Thin MALDI

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

From the analytical point of view, reactions were efficiendy monitored using a combination of FTIR (Fourier transform infrared), TLC (thin-layer chromatography), and MALDI-TOF mass spectroscopy analysis of crudes resulting from the cleavage of small resin samples. 

The AE blend Brij 35 with the general formula CnH2n+i 0(CH2CH20)mH was analysed by MALDI MS prior to use for biochemistry research. Separation results of thin-layer (TLC) and RP-LC of these surfactants were compared [30]. Brij 35, as a mixture of Ci2 and C14 homologues (to = 15-39), was detected qualitatively as [M + Na]+ and [M + K]+ ions and quantitatively after TLC and RP-LC separation. 

Note The vast majority of MALDI instruments use UV nitrogen lasers (337 nm, 3 ns). IR-MALDI has been restricted to applications where its deeper penetration offers advantages, e.g., for the direct desorption of analytes from sodium dodecyl sulfate (SDS) gels or thin layer chromatographic (TLC) plates. 

Guittard, J. Hronowski, X.L. Costello, C.E. Direct MALDI Mass Spectrometric Analysis of Glycosphingolipids on Thin Layer Chromatographic Plates and Transfer Membranes. Rapid Commun. Mass Spectrom. 1999,13, 1838-1849. 

In 1988, Tanaka et al. first reported the use of ultrafine metal powder in protein analysis. Since then, many inorganic materials, including graphite particles, fine metal or metal oxide powder, silver thin-film substrates or particles,and silica gel, have been used in the MALDI-TOF analysis of low-mass molecules. 

It has been demonstrated that ILMs are suitable for qualitative and quantitative analyses of low-molecular weight compounds of biological interest, for example, carbohydrates, vitamins and amino acids [38], and glycolipids [40]. ILMs were further used for fhe direct analysis of alkaloids, anesthetics and antibiotics, separated by thin-layer chromatography (TLC) [46]. For this purpose, the ILM was spotted onto the fractions on the TLC-plates and the complete plate was measured in MALDI MS without the need for additional pretreatment of the TLC-samples. The mass deviation inherently caused by the inhomogeneous surface of fhe TLC-plafe was balanced by using the... 

Santos, L. S. et al.. Fast screening of low molecular weight compounds by thin-layer chromatography and "on-spot" MALDI-TOF mass spectrometry. Anal. Chem., 76, 2144, 2004. 

For a few tissue samples, the matrix deposition can be performed manually. A matrix solution is sprayed onto the tissue section with a hand-held thin layer chromatography (TLC) sprayer or an artist airbrush. The reproducibility of manual matrix deposition is an issue. When the manual sprayer is used, the MALDI target plate with the tissue section is held vertically about 15-25 cm from the sprayer nozzle. It is recommended to spray multiple coats of matrix across the tissue section and each coating cycle consists of passing the sprayer two to hve times across the tissue section and allowing the tissue to dry for about 1-5 min. This process is usually repeated between 10 and 20 cycles. 

Another example of a Pc-based 1-D polymer is that reported by Armstrong and co-workers [158], They prepared a Pc with eight styrene-type polymerizable sites at the end of the peripheral substituents. This molecule forms highly ordered, rod-like aggregates at the air-water interface that can be transferred onto solid supports. Irradiation of the thin films affords polymerization between the olefin moieties of adjacent molecules by photostimulated [2 + 2] cycloaddition. The rod-like Pc macromolecules were conveniently studied by matrix-assisted laser desorp-tion/ionization (MALDI-TOF) spectrometry and atomic force microscopy (ATM), the latter showing rods with lengths up to 290 nm. 

Currently developed for many applications (Stoeckli et al. 1999 Stoeckli et al. 2001, 2002,2003 Chaurand et al. 2004), MALDI MSI is achieved by rastering sequentially the surface of a defined area while acquiring a mass spectrum from every location (see Figure 2). Atypical sample preparation for MSI involves the fixation of the sample, for example, tissue section, on a MALDI plate and the application of the matrix solution over the latter, either as a thin layer or as a spot pattern, to get co-crystallization of analytes with matrix while solvents evaporate. Once dried, the sample is introduced in the mass spectrometer, where, for each defined image position, short UV laser pulses are fired onto the surface to generate ions. Those are analyzed by the TOF instrument and a mass spectmm is acquired. 

MALDI-MS was performed using a Kratos Kompact MALDI III mass spectrometer fitted with a standard 337 nm nitrogen laser, and operated in the linear mode at an accelerating voltage of 20 kV. Two sample preparation methods were used (1) for collected peptides, 0.3 pL aliquots of sample and matrix (a-cyano-4-hydroxy cinnamic acid, Biomolecular Separations, Inc.) were mixed on the probe slide and allowed to air dry or (2) for unfiactionated digests, a thin polycrystalline film was prepared according to (11) (with modifications for use on a probe slide (12)), matrix and sample aliquots were mixed (usually 0.3 pL each) on this surface and prior to drying, rinsed twice with 2 pL of deionized water. 

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