Of high-molecular-weight samples

III. F.A.B.-M.S. OF High-Molecular-Weight Samples 1. Definition of High Mass... 

The most recent methods use desorption ionization of high molecular weight samples, up to 100 000 dalton, by UV laser irradiation of a matrix surface. 

Polymers comprising the disubstituted biphenyls also exhibit unusual solution and solubility properties. For example, highly concentrated solutions of high molecular weight samples of all three classes of polymer show no evidence of lyotropism. As the concentration is increased the viscosity increases up to the limit of solubility at which point the polymer precipates instead of forming a biphasic solution. The solution properties of several of these polymers have been examined in great detail, and in every example the criteria for lytropism, e. g., persistence length, solubility, etc., has been met or exceeded. 

Small particle size resins provide higher resolution, as demonstrated in Fig. 4.41. Low molecular weight polystyrene standards are better separated on a GIOOOHxl column packed with 5 /u,m resin than a GlOOOHg column packed with 10 /Ltm resin when compared in the same analysis time. Therefore, smaller particle size resins generally attain a better required resolution in a shorter time. In this context, SuperH columns are best, and Hhr and Hxl columns are second best. Most analyses have been carried out on these three series of H type columns. However, the performance of columns packed with smaller particle size resins is susceptible to some experimental conditions such as the sample concentration of solution, injection volume, and detector cell volume. They must be kept as low as possible to obtain the maximum resolution. Chain scissions of polymer molecules are also easier to occur in columns packed with smaller particle size resins. The flow rate should be kept low in order to prevent this problem, particularly in the analyses of high molecular weight polymers. 

The dissolution of high molecular weight analytes increases the viscosity of the samples. Samples with a high viscosity can exhibit viscous fingering on the column top. Because this phenomenon can impair resolution, it is recommended to keep the sample concentration below certain limits. These limits are shown in Table 11.5. 

In contrast to this result, polymer samples taken from processing without a micro mixer displayed a small but significant fraction of high-molecular weight polymer with a mass > 10 [126]. Here, in some cases, heavy precipitation occurred, resulting even in plugging of the static-mixer internals of the tube reactors [125]. 

Figure 18 Effect of branching on the secondary nucleation and linear growth rates from the work of Lambert and Phillips [59]. (a) Low molecular weight samples. Sample S-5 exhibits only Regime II behavior as explained in the text, (b) High molecular weight samples. Reprinted with permission from Lambert and Phillips [59]. Copyright 1996, Elsevier.

The rate of polymerization may be dramatically accelerated upon addition of a bulky Lewis acid. For example, addition of (184) to a sample of living PMMA generated by irradiation of (181)/MMA causes an increase in polymerization rate by a factor of >45,000.444 The dualcomponent systems (181)/(184), and (181)/(185), have been used to prepare monodisperse, ultra-high-molecular-weight samples of PMMA (Mn > 106, Mw/Mn= 1.2).445... 

The DEP ends with a filament wire onto which a drop of sample is deposited. After evaporation to dryness, the probe is introduced into the source of the mass spectrometer and is rapidly heated to a temperature that can reach 1000°C. This probe is ideal for the study of high molecular weight or polymeric components. It is mostly dedicated to the analysis of samples in the liquid state. Although a small solid fragment of matter may be placed on the filament, this critical operation may lead to the loss of the sample, especially if it is particularly small. To avoid such a difficult handling, the sample may be ground and homogenised in a mini-mortar and then made into suspension with a few drops of appropriate solvent (Scalarone et al., 2003). 

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