Negative ion MALDI

MALDI matrix spectra are characterized by strong molecular and/or quasi-molecular ion signals accompanied by series of matrix (Ma) cluster ions and some more abundant fragment ions. [32] In positive-ion MALDI, [Man-hH]" cluster ions predominate, while [Ma -H] cluster ions are preferably formed in negative-ion MALDI. The principal ion series may be accompanied by [Ma H-alkali] ions and some fragments of minor intensity, e.g., [Ma H-H-H20]. In particular with aprotic matrices, radical ions may predominate. In addition, a continuous background is... 

Fig. 10.7. Negative-ion MALDI spectra of the oligonucleotide pd[T]g (a) 5 pmol, (b) same as (a) after addition of 5-10 cation exchange beads to the sample preparation. Adapted from Ref [24] by permission. John Wiley Sons, 1992.

CCK-33 with trypsin 45 as well as by the negative-ion MALDI-TOF-MS spectra of CCK-12 that show intense [M-H]- peaks indicating a stabilized sulfate ester moiety even in the gas phase (see Analytical Characterization). 44149 ... 

Tire MALDI method is especially useful for complex mixtures of peptides and can be utilized in peptide sequencing. The technique is also appropriate for studying mixtures of glycoproteins. Negative-ion MALDI can be applied to oligonucleotide mixtures. Further improvements in resolution in both MALDI... 

Fig. 21.2 Negative ion MALDI-TOF mass spectra of l-palmitoyl-2-oleoyl-sn-phosphatidyl-ethanolamine (a), l-palmitoyl-2-oleoyl-sn-phosphatidylserine (b) and 1,2-dipalmitoyl-sn-phosphatidylinositol-3-phosphate (c). In all cases a 0.5 M solution of DHB in methanol was used as matrix and the sample solutions (0.5 mg/ml) diluted 1 1 (v/v) with the matrix. All peaks are marked according to their m/z ratio and DHB matrix peaks are marked with asterisks. Please note the considerable intensity of the cluster ions between DHB and the different analytes. Please also note that the contribution of matrix peaks is much higher than in the positive ion spectra...

Five triplets of deprotonated molecules at m/z ranging from 908.6 to 1750.9, separately by a multiple of 162 was detected in the negative ion MALDI-TOF mass spectrum, suggested the presence of glycan chains containing 0, 1, 2, 4 or 5 hexose residues [74]. The lipid portion was characterized as a ceramide composed of C-18 phytosphingosine N-aeylated by hydroxy C24 fatty acid (80%), C24 (15%) or 2,3-hydroxy lignoceric acids (5%). 

When the determination of the composition is not required, the assignment of mass spectral peaks can be directly carried out to establish what kind of oligomers is present. In a straightforward application, Vitalini et al. recorded the negative ion MALDI-TOF mass spectrum of a copolymer containing units of bisphenol (referred to as A) and units of a derivatized fullerene (referred to as B). The mass spectrum (see Figure 10.28) covers a wide mass range (1600-14000 Da). The peaks are due to cyclic ions of the type A B where n extends from 1 to 15. 

MALDI has also been used to analyze several molecular forms of phospholipids (GPC, GPE, GPS, GPI, GPG, phosphatidic acid, and cardiolipin) [29]. In the positive-ion mode, all phospholipids yield signals due to the [M - - H]+, [M -h Na]+, and [M - - K]+ ions. In-source fragmentation and PSD provide useful structure information [30]. With the exception of GPC, all phospholipids can also be analyzed in the negative-ion MALDI mode, where the molecular ion region is relatively noise-free and only the [M — H] signal is produced. 

The oligonucleotide 5 -snake venom phosphodiesterase. At what m/z values will the 5 -sequence ladders be seen in the negative-ion MALDI mass spectrum ... 

Figure 7.7 Negative-ion MALDI-TOF mass spectra of an organic extract from hen egg yolk, (a) Trace recorded in the presence of DHB (b) Trace recorded in the presence of 9-AA. Peaks are labeled according to their...

Figure 7. Positive- (upper) and negative- (lower) ion MALDI mass spectra of the oligosaccharides obtained from birch wood xylan by mild acid hydrolysis. In positive ion mode, both neutral and acidic oligosaccharides are detected, whereas acidic oligosaccharides only are detected by negative-ion MALDI-MS. Reproduced from Reference 6. Copyright 2001 American Chemical Society.

Fuchs, B., Bischoff, A., Suss, R., Teuber, K., Schurenberg, M., Suckau, D. and Schiller, J. (2009) Phosphatidylcholines and -ethanolamines can be easily mistaken in phospholipid mixtures A negative ion MALDI-TOF MS study with 9-aminoacridine as matrix and egg yolk as selected example. Anal. Bioanal. Chem. 395, 2479-2487. 

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. 

Modifications such as phosphorylation are chemically simpler and are more easily identified by MS, although the addition of 80 Da can be due to either a phosphate (HPO3) or a sulfate (SO3). Serine and threonine phosphates are acid sensitive and may not survive LC/MS whereas tyrosine phosphates are generally more robust. Sulfate is also labile and may not be observed in positive ion MALDI but it usually can be identified by ESI or negative ion MALDI. The... 

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