Matrix adducts

Example The Bunte salt [ 3( 2)158-803] Na yields a very useful negative-ion FAB spectrum from NBA matrix (Fig. 9.11). NBA forms [Ma-H] and Ma " ions. The salt anion contributes the base peak at m/z 337.3. [Ch-2A] m/z 697.5, and [2C-I-3A] m/z 1057.6, cluster ions are observed in addition, their isotopic patterns being in good agreement with theoretical expectation. It is noteworthy that the matrix adduct at m/z 513.4 is a negative radical ion. 

The ions have little excess energy and show little propensity to fragment. For this reason, the method is fairly useful for mixtures. The mass accuracy is low when used with a TOF-MS, but very high resolution can be obtained with a FT-MS. As with other matrix-assisted methods, MALDI suffers from background interference from the matrix material, which is further exacerbated by matrix adduction. Thus, the assignment of a molecular ion of an unknown compound can be uncertain. 

The accuracy of protein mass measurement depends on a number of factors because many proteins are sufficiently massive that resolution is not achieved from additional species such as salts or matrix adducts. Under these conditions, such species cause peak broadening and result in a small shift in measured mass to higher values. Post-translational modifications such as glycosylation can often be resolved for the smaller proteins such as ribonuclease B (Figure 3) with a single glycosylation site, but larger compounds with multiple modifications often produce only broad peaks. 

Finally, the intermediate peaks may arise from matrix adducts to the polymer. Matrix adducts of polystyrene polymers of molecular mass 3900 and 7000 of the form MatrixAgPolymer and MatrixAgAgPolymer have been found (21). Figure 3 shows the entire spectrum, and Figure 4 shows these adduct peaks in a polystyrene of molecular mass of about 7000 used in the NIST Interlaboratoiy Comparison (49,54). 

Fig. 4. Detail of MALDI-tof-ms of polystyrene pol3rmer shown in Fignre 3. The smaller peaks between the main peaks have been shown (21) to be the result of matrix adducts. This is an example where these intermediate peaks are not from polymer molecules terminated with other end groups or from ra-mers charged with different salts.

These sputtering techniques suffer from two main drawbacks, discrimination and irreproducibility. Highly surface-active species may dominate the spectra as a result of their suppression of other components of the sample at the surface of the matrix. This discrimination may result in poor sensitivity for certain species and makes quantitation difficult (see section 12.6.2). Ionisation of the matrix gives rise to chemical noise, and the spectra may be further complicated by the formation of sample-matrix adduct ions. These effects prevent the use of libraries in the interpretation of FAB and LSIMS spectra. 

Typically ionization is performed at the pressure of the DTIM (e.g., 1-10 torr), which results in moderate pressure MALDI. Thus some collisional cooling typically takes place after ionization and can result in matrix-adducted and cluster species. These can be dissociated prior to DTIM by performing injected ion experiments. Furthermore, matrix optimization may be required. One effect of moderate pressure MALDI that we have observed is that higher ion currents can be achieved at slightly lower matrix-to-analyte ratios (i.e., 1,000-100 1) than those used in high-vacuum MALDI (i.e., 10,000-1,000 1). 

Positive-ion MALDI mass spectrum of PE species is characterized by the presence of a specific fragment ion corresponding to the loss of the phosphoethanolamine head group [93]. PE species can also be ionized in the negative-ion mode with a lower sensitivity in comparison to that in the positive-ion mode. Negative-ion MALDI mass spectra of PE species are usually dominated by the matrix adducts of PE. 

In the MALDI process, mostly singly charged ions are formed, although these ions can be accompanied by some doubly and, occasionally, triply charged ions. In addition, noncovalent adducts, such as dimers, trimers, etc., of proteins and/or matrix adducts of certain analytes can also be observed. This is well illustrated in... 

Figure 6.12. The first exploration of MALDI at elevated pressure showed reduced fragmentation of myoglobin and increased matrix adduction. (Reproduced from Ref. 121, with permission.)...

Figures 6.14 and 6.15, however, clearly demonstrate one of the limiting factors of high-pressure MALDI as well. As the desorbed ions are stabilized, the number of noncovalent matrix adducts increases. The prevailing picture of MALDI is that ions are blasted from the (usually) crystalline matrix in chunks that shake themselves apart because of residual...

The abihty to desorb and cool ions even from such a hot matrix as a-cyano-4-hydroxycinnamic acid suggests that another approach may also work. While only a half dozen matrix preparation methods are routinely used in MALDl, all of these techniques have been developed initially for MALDl-TOF instruments. Since MALDl-TOFMS requires very flat desorption surfaces for the best resolving power, many potential matrices have not been explored. However, systems where the external ion source is decoupled from the mass analyzer, as is the case with MALDI-QTOF, MALDI-QIT, and MALDl-FTMS instruments, do not have this limitation. Indeed, some of the best signals from MALDl-FTMS systems are generated from large, irregular DHB crystals. Thus, there is a high probability that exploration of matrices with the decoupled MALDl sources will identify new matrices with favorable properties, such as improved ion abundance or reduced matrix adduction. 

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