Matrix-assisted laser impurities

Experimental considerations Sample preparation and data evaluation are similar to membrane osmometry. Since there is no lower cut-off as in membrane osmometry, the method is very sensitive to low molar mass impurities like residual solvent and monomers. As a consequence, the method is more suitable for oligomers and short polymers with molar masses up to (M)n 50kg/mol. Today, vapour pressure osmometry faces strong competition from mass spectrometry techniques such as matrix-assisted laser desorption ionisation mass spectrometry (MALDI-MS) [20,21]. Nevertheless, vapour pressure osmometry still has advantages in cases where fragmentation issues or molar mass-dependent desorption and ionization probabilities come into play. 

Chromatographic methods, such as reverse phase HPLC (RP-HPLC) and size exclusion chromatography, were used for identity and purity control, and also for quantitative measurement. The major impurity, which can be detected by RP-HPLC after the two first purification steps (Fig. 3), was isolated and analysed by mass spectrometiy. The mass measured by Matrix Assisted Laser Desorption Ionisation Time Of Flight (MALDI-TOF) mass spectrometry for this impurity was around twice the mass of the monomer (78265 Da), and was interpreted to be a dimer of BBG2Na. 

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry is the best method to find evidence of minor impurities. So whereas the MALDI-TOF spectra of G1-Allyl and G2-Allyl show the high purity of the samples, for the next-bigger branched compound G3-Allyl additional peaks were detected, probably corresponding to a defect structure. All MALDI-TOF analyses of the allyl-substituted dendrimers Gl-G3-Allyl afforded the [M+Na] or [M-i-H] ion, or both (Table 1). 

Shaler, T.A., Wickham, J.N., Sannes, K.A., Wu, K.J., and Becker, C.H. (1996) Effect of impurities on the matrix-assisted laser desorption mass spectra of single-stranded oligodeoxynucleotides. Anal. Chem.,... 

Since the development of modern ionization techniques (i.e. matrix assisted laser desorption ionization, MALDI [13-15] and electrospray [16-18]), mass spectrometry (MS) has proven to be one of the foremost tools for the characterization of numerous dendrimers. Gel permeation chromatography (GPC) has been successfully and widely appKed to the characterization of the molecular weight and polydispersity of these dendrimer molecules [19]. Furthermore, a great deal of information about the structural impurities and defects in dendrimers has... 

Mowthorpe,S., Clench, M.R., Cricelius, A., Richards, D.S., Parr, V, Tetler, L.W (1999) Matrix-assisted laser desorption/ionisation time-of-flight thin layer chromatography mass spectrometry—a rapid method for impurity testing. Rapid Communications in Mass Spectrometry RCM, 13, 264-270. 

FIGURE 10.4 Matrix-assisted laser desorption mass spectra of (a) peptide fragments produced by endoproteinase Lys-C digestion of melittin, (b) Endo Lys-C fragments isolated by immunopre-cipitation with mab 83144, (c) chymotrypsin fra ents of melittin, and (d) chymotrypsin fragments of melittin immunoprecipitated with antimelittin antibody mab 83144. Peaks labeled with an asterisk are dynorphan A,.,3 added as internal mass calibrant, and peaks labeled II, 12, and I are impurities. Reprinted with permission from reference 10. 

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