Molecular mass limit

Quantitative analysis of multicomponent additive packages in polymers is difficult subject matter, as evidenced by results of round-robins [110,118,119]. Sample inhomogeneity is often greater than the error in analysis. In procedures entailing extraction/chromatography, the main uncertainty lies in the extraction stage. Chromatographic methods have become a ubiquitous part of quantitative chemical analysis. Dissolution procedures (without precipitation) lead to the most reliable quantitative results, provided that total dissolution can be achieved follow-up SEC-GC is molecular mass-limited by the requirements of GC. Of the various solid-state procedures (Table 10.27), only TG, SHS, and eventually Py, lead to easily obtainable accurate quantitation. 

A method to overcome this molecular mass limitation was developed by two groups independently, led by Tanaka and Hillenkamp. The two groups essentially applied the same principle which was to combine the analyte with a matrix to facilitate desorption/ionization without inducing fragmentation. 

Sample preparation Filter (Millipore Ultrafree-MC, 10000 molecular mass limit) 250 p,L serum while centrifuging at 17000 g for 1.5 h, ii ject a 50 p-L aliquot of the clear ultrafiltrate. 

Ultrafiltration (UF), a pressure-driven separation process on the basis of molecular size using a membrane. Suspended solids and solutes of high molecular mass are retained. UF is used for the concentration of protein solutions and for the separation of molecules with significant mass differences. The nature of the membrane determines between compounds which permeate and those which are retained. UF membranes are classified according to the molecular mass limit ( molecular weight... 

Analytes must be nonpolar Molecular mass limited to -600 Da depending on structure... 

Kassalainen, G.E. Williams, S.K.R. Lowering the molecular mass limit of thermal field-flow fractionation for polymer separations. J. Chromatogr. A, 2003, 988, 285-295. 

Molecular Mass Limit for Trace Quantitation by Mass Spectrometry... 

Although there has been some controversy concerning the processes involved in field ionization mass spectrometry, the general principles appear to be understood. Firstly, the ionization process itself produces little excess of vibrational and rotational energy in the ions, and, consequently, fragmentation is limited or nonexistent. This ionization process is one of the mild or soft methods available for producing excellent molecular mass information. The initially formed ions are either simple radical cations or radical anions (M ). 

Until 1981, mass spectrometry was limited, generally, to the analysis of volatile, relatively low-molecular-mass samples and was difficult to apply to nonvolatile peptides and proteins without first cutting them chemically into smaller volatile segments. During the past decade, the situation has changed radically with the advent of new ionization techniques and the development of tandem mass spectrometry. Now, the mass spectrometer has a well-deserved place in any laboratory interested in the analysis of peptides and proteins. 

Different samples exhibit different levels of response to FAB, and, with a mixture of components, it is feasible that not all will be detected. In some cases, the minor components of a sample appear more prominently in the mass spectrum than the major ones. Despite these limitations, FAB is in widespread use and is an excellent technique for determining the molecular masses of peptides up to 10,000 Daltons, with an accuracy of 0.5 Da. 

There is no theoretical upper limit on m/z that can be examined, and TOF mass spectrometry is useful for substances having very high molecular mass. In practice, the current upper limit is about 350,000. Unfortunately, ions even of the same m/z value do have a spread of velocities after acceleration, so the resolution achievable with TOF is not very high because bunches of ions of one m/z value overlap those at the next m/z value. 

The resolution required in any analytical SEC procedure, e.g., to detect sample impurities, is primarily based on the nature of the sample components with respect to their shape, the relative size differences of species contained in the sample, and the minimal size difference to be resolved. These sample attributes, in addition to the range of sizes to be examined, determine the required selectivity. Earlier work has shown that the limit of resolvability in SEC of molecules [i.e., the ability to completely resolve solutes of different sizes as a function of (1) plate number, (2) different solute shapes, and (3) media pore volumes] ranges from close to 20% for the molecular mass difference required to resolve spherical solutes down to near a 10% difference in molecular mass required for the separation of rod-shaped molecules (Hagel, 1993). To approach these limits, a SEC medium and a system with appropriate selectivity and efficiency must be employed. 

Homologous Polymer Blends A subclass of polymer blends limited to mixtures of chemically identical polymers differing in molecular mass. 

Since 1 is a monomer with low activity, copolymers 2 obtained at any stage of the copolymerization process, irrespective of the monomer ratio in the initial mixture, always contain a smaller amount of monomeric units of 1 than that in the corresponding monomer mixture. 1 being prone to enter the chain-transfer reaction, the increase of its content in the initial monomer mixture reduces substantially the reaction rate and decreases the molecular mass of the copolymers. It was found that copolymers 2 which contain 2—8% of monomeric units of 1 and are suitable for obtaining fibres must have a molecular mass between 45 000 and 50000. Such copolymers can be obtained with a AN 1 ratio in the initial mixture between 95 5 and 85 15. Concentrated solutions of copolymers, especially those with a molecular mass smaller than the above limit, are characterized by a very low stability which is a substantial shortcoming of these copolymers. 

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