Matrices in UV-MALDI

Nicotinic add (NA) was the first organic compound to be successfully employed as a matrix in UV-MALDI of peptides and proteins [12-14]. Ever since, better matrices have been sought, the following now being widespread in use (Fig. 11.8) picolinic acid (PA) [67], 3-hydroxypicolinic acid (HPA) [68], 3-aminopicolinic add (3-APA) [69], and 6-aza-2-thiothymine (ATT) [70,71] for oligonucleotides 

Note It is common to use acronyms rather than compound names for matrices. However, these are not always consistently used, e.g., a-CHC, 4-HCCA, CHCA, and CCA all refer to a-cyano-4-hydroxycinnamic acid. Others may be easily confused, e.g., nicotinic acid (NA) and 9-nitroanthracene (9-NA). 

Example The choice of a matrix and optimized conditions of sample preparation have substantial influence on the analytical value of MALDI spectra. Even when employing standard matrices such as CHCA or DHB, significant improvements can be achieved, e.g., by appropriate mixing of the two matrices to analyze ribonuclease B (Fig. 11.9) [76]. 

For a first approach to a new analytical problem, it is recommended to try a matrix from this collection those highlighted by a frame represent the most frequently used matrices. In general, highly polar analytes work better with highly polar matrices, and nonpolar analytes are preferably combined with nonpolar matrices. In unfortunate cases, only one specific analyte-matrix combination might yield useful MALDI spectra. 

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Ion yields in UV-MALDI mass spectrometry as a function of excitation laser wavelength and optical and physico-chemical properties of classical and novel halogen-substituted MALDI matrices. Anal. Chem., 84, 6567-5575. 

The role of the matrix in MALDI is analogous to that in FAB (Chap. 9.3.1). Different from FAB, MALDI matrices are generally crystalline solids of low vapor pressure in order not to be volatalized in the ion source vacuum. While basically any liquid can serve as a FAB matrix, the matrix in MALDI has to absorb light of the wavelength which is intended to be used for the experiment. [63] In UV-... 

Common UV-MALDI matrices are listed in Table 1.2 with the class of compounds with which they are used. The matrices used with IR lasers, such as urea, caboxylic acids, alcohols and even water, are often closer to the natural solutions than the highly aromatic UV-MALDI matrices. In addition, there are many more potential matrices for IR-MALDI owing to the strong absorption of molecular compounds at IR wavelengths, even if the correlation between ion formation and matrix absorption in IR-MALDI is not clear [36],... 

To summarize briefly, the most important aspect regarding successful MALDI MS is the embedding of the analyte with the most appropriate matrix for optimum ionization performance. Therefore, the careful selection of the matrix is extremely important using UV MALDI (the majority of commercially available MALDI devices are equipped with a UV laser), we recommend 2,5-dihydroxybenzoic acid (DHB) for positive ion detection and 9-aminoacridine (9-AA) for negative ions. A survey of further useful MALDI matrices is available in Reference 22. Matrices are often specialized for different experimental issues and the combined use of DHB and 9-AA is often quite helpful because... 

Although a UV laser is commonly used to des-orb/ionise matrix species in the vacuum of the mass spectrometer also IR lasers may be employed in the desorption process. IR MALDI normally uses tuneable IR (from 1.5 to 4.0 /rm), Q-switched Er-YAG (2.94 /rm, t = 90 ns) or Er-YSGG (2.79 /rm, r = 90 ns) lasers. Smaller absorption coefficients of usable matrices in IR as compared to UV result in an at least tenfold increase in ablated material per laser exposure. With IR lasers localised thermal desorption or laser assisted pyrolysis occurs. The desorbed neutrals expand into a small ionisation chamber where they may be post-ionised by electron impact or photoionisation. The dependence of... 

Perhaps the most insightful aspect of the pneumatic assistance model is the recognition that some matrices can thermally decompose to low-molecular-weight species that create significant overpressures at MALDI melt temperatures. In particular, many UV MALDI matrices are carboxylic acids that are known to decompose at moderate temperatures. Consistent with this, laser-induced CO2 emission from such matrices in vacuo has been found to rise with increasing laser pulse energy. " ... 

Many UV matrices are not thermally labile, so the model is limited in potential application. In IR MALDI, however, it could be more generally relevant. Entrapped residual low-molecular-weight solvent might be directly vaporized by the laser. However, this would mean that IR MALDI is highly dependent on sample preparation in order to obtain the optimum amount and distribution of trapped solvent. It seems unlikely that this is consistent with experiment, but needs to be further explored. 

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