Van der Hoff

Figure 10.13 GC clrromatogram obtained after on-line LC-GC(ECD) of a human milk sample analysed for PCBs (attenuation X 64). Peak identification is as follows (1) PCB 28 (2) PCB 118 (3) PCB 153 (4) PCB 138 (5) PCB 180 (6) PCB 170 (7) PCB 207. Reprinted from Journal of High Resolution Chromatography, 20, G. R. van der Hoff et al, Determination of organochlorine compounds in fatty matiices application of normal-phase LC clean-up coupled on-line to GC/ECD , pp. 222-226, 1997, with permission from Wiley-VCH.

G. R. van der Hoff, R. A. Baumann, P. van Zoonen and U. A. Th. Brinkman, Determination of organocWoiine compounds in fatty matrices application of normal-phase PC clean-up coupled on-line to GC/ECD , 7. High Resolut. Chromatogr. 20 222-226(1997). 

E. A. Hogendoom, G. R. van der Hoff and P. van Zoonen, Automated sample clean-up and fractionation of organochlorine pesticides and polychlorinated biphenyls in human milk using NP-HPEC with column-switching , J. High Resolut. Chromatogr. 12 784-789 (1989). 

B. M. E. Van Der Hoff, Polymerization and Polycon-densation Process, Advances in Chemistry Series, American Chemical Society, Washington D.C. (1962). 

Elegant work by Van der Hoff [60] seems to suggest that it is the latter, or more correctly there is a Gaussian distribution about the midpoint of a chain. However, as to whether the main chains are primarily broken by ultrasonic action is still open to question since it is possible that the main chain scissions are secondary effects due to chemical reactions initiated by unstable intermediates, such as free radicals or ions, produced by sonication. For example McKay [61] has shown that hydroxyl radicals, generated by the oxidation of Fe by H2O2, are the cause of chain scission in polyacrylamide molecules in aqueous solution. 

Pond water Micro liquid-liquid extraction of 1.5 mL water with 1.5 mL methyl t-butyl ether. 500 pL of extract slowly introduced into GC pre-column with solvent venting. cap. GC/FPD 0.02 pg/L (ppb) 102 (5% RSD) at 0.50 pg/l level van der Hoff et al. 1993... 

On the other hand, a number of qualified investigations on the emulsion polymerization initiated by chemical catalysts and on the experimental verification of the quantitative theory of Smith and Ewart were performed by van der Hoff (14) and Gerrens et ah (4,6). The high experimental standard of these papers in determining the kinetic data is rarely met by papers on the gamma-induced emulsion polymerization. 

Van der Hoff,B.M.E. The stress-strain relation of swollen rubbers. Polymer (London) 6,397-399(1965). 

Although there have been several references on the theory of emulsion polymerization kinetics, it is surprising that its commercial importance and multiphase kinetics have not generated more interest. Smith and Ewart (46) predicted a constant rate of polymerization per particle, based on initiation in the water phase and three ranges of radical concentration per particle. Van der Hoff (55) confirmed this for concentrations... 

The idea of particle inhomogeneity was supported experimentally by Williams [149], However, his representation of growth is more complicated. In phase II, the monomer concentration in the particle decreases with conversion, while the rate remains constant. The particle has a core with a relatively high polymer content surrounded by a monomer-rich layer (see Fig. 16). Polymerization occurs at the polymer—monomer interface. Under these conditions, monomer concentration at the interface remains constant, even when its amount in the particle decreases. Napper presented the idea of an exactly opposite composition of the monomer—polymer particle [150], The core should be enriched in monomer and surrounded by a layer with a higher polymer content. Van den Hul and van der Hoff found most growing ends of macromolecules at the particle surface [151], which supports Napper s model. 

The possibility for nucleaiion by mixed-micelle formation (process G) has also been proposed (van der Hoff. I960), and quite recently experimental work has investigated this mechanism (Munro et ol., 1979 Chen and Phrma, 1980). Evidence has been found indicating assodiation of the sur ce active oligomers in systems both with and without emulsifiers (below the CMC). No quantitative treatment was given by these authors. It may be questioned i ether there is any real difference between the self-nucleation and the mixed-micelle models. When emulsifier is absent, there is clearly a... 

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