Molecular weight gain

Porin channels are impHcated in the transport of cephalosporins because ceds deficient in porins are much more impermeable than are ceds that are rich in porins. The porins appear to function as a molecular sieve, adowing molecules of relatively low molecular weight to gain access to the periplasmic space by passive diffusion. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to permeabdity. However, such a relationship is not clearly defined for Pseudomonas aeruginosa where additional barriers may be involved (139,144,146). 

Low Molecular Weight Flocculants - Contrary to the trend to increase molecular weight, which has resulted in the commercial availability of ultra high molecular weight (HMW) flocculants, there are a number of applications where low molecular weight (LMW) flocculants are gaining considerable credibility. 

Applications. Many applications have been proposed for polyphosphazenes, particularly the non-cyclic polymers of high molecular weight, but those with the most desirable properties are extremely expensive and costs will have to drop considerably before they gain widespread use (cf. silicones, p. 365). The cheapest compounds are the chloro series... 

At its simplest, mass spectrometry (MS) is a technique for measuring the mass, and therefore the molecular weight (MW), of a molecule. In addition, it s often possible to gain structural information about a molecule by measuring the masses of the fragments produced when molecules are broken apart. 

Once the molecular weight of a particular species has been determined and its significance ascribed, it is not always necessary to continue to acquire mass spectra over the full mass range to provide the required analytical information. Indeed, as discussed earlier in Section 3.5.2.1, there are often significant benefits to be gained by only monitoring a relatively small number of ions generated by an analyte. 

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