Molecular-weight assignment with

Molecular weight assignments for such resins will be dependent on the method used for measurement. Each manufacturer has his own relative scale. In practice, absolute molecular weight determination is not important. The method used must discriminate between resins with sufficient resolution to allow prediction of resin performance in the applications of interest. 

Using method-dependent assigned values for external quality control means having many different target values for the same analyte in the same control serum a very impractical and, from a theoretical point of view, very unsatisfactory procedure which generates many different results for a substance of known molecular weight and with a defined number of molecules. 

Figure 1 LC/ESI-MS analysis of oxidized PtdCho in oxidized LDL. Total positive ion current profile of oxidized LDL (A) single ion plots of representative PtdCho oxidation products. Peak identification is as given in figure. LDL was oxidized by incubation with 5pmoll CiS04 in 0.1 moll phosphate buffered saline (PBS) for 12h at 37°C. The total lipid extract of the oxidized LDL was dissolved in chloroform/methanol (2 1, v/v) and 20 pi of the sample containing 10 pg lipid was analyzed. Structural assignment for aldehydes and hydroperoxides are according to reference standards. Ions 832 and 830 were identified on the basis of retention time and molecular weight. Reproduced with permission of publisher from Ravandi A, Kuksis A, and Shaikh NA (2000) Arteriosclerosis, Thrombosis, and Vascular Biology 20. 467-477.

The original molecular weight assignments (24) for the enzymes from B. subtilis were probably incorrect the ornithine transcarbamylase has now been shown to be a trimer of M, 140,000 and the corresponding enzymes from Saccharomyces cerevisiae which interact in a similar manner, to be discussed below, are both trimers with about the same molecular weight 26). 

The molecular ion,, provides the most valuable information in the mass spectrum its mass and elemental composition show the molecular boundaries into which the structural fragments indicated in the mass spectrum must be fitted. Unfortunately, for some types of compounds the molecular ion is not sufliciently stable to be found in appreciable abundance in the El spectrum. An increasingly large proportion of mass-spectrometry facilities also have a soft ionization technique such as chemical ionization or fast-atom bombardment (Cl or FAB, Chapter 6) available. Such data should be used for molecular-weight assignment wherever possible. However, even with evidence from soft ionization, the unknown spectrum should still be examined as described in this chapter, since this should lead to useful structure information as well as verification of the M" assignment. 

The high-molecular weight was assigned to the PMMA grafted to the copolymer chains and the low-molecular weight to the PMMA initiated by the MMA radical (II). However, only one molecular weight distribution peak was observed for the PMMA initiated by the latter system, i.e., in combination with BP, which implies that only aminomethyl radicals are capable of initiating the polymerization. 

The ionization techniques most widely used for LC-MS, however, are termed soft ionization in that they produce primarily molecular species with little fragmentation. It is unlikely that the molecular weight alone will allow a structural assignment to be made and it is therefore desirable to be able to generate structural information from such techniques. There are two ways in which this may be done, one of which, the so-called cone-voltage or in-source fragmentation, is associated specifically with the ionization techniques of electiospray and APCl and is discussed later in Section 4.7.4. The other, termed mass spectrometry-mass spectrometry (MS-MS) or tandem mass spectrometry, is applicable to all forms of ionization, provided that appropriate hardware is available, and is described here. 

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