Spectroscopy, molecular weight from mass

So far it has not been possible to determine the size and mass of the human RISC complex in vivo. The literature states molecular weights from 160 kDa to 2MDa, depending on the experimental procedure applied in vitro [33,34], Fluorescence correlation spectroscopy offers the possibility to investigate the diffusion properties of fluorescing molecules, and therefore to deduce their molecular weight via the Stokes-Einstein Relation [35],... 

Boduszynski et al. (70,71), using field ionization mass spectroscopy (FIMS), obtained average molecular weights from 873 to 1231 for the Corbett fractions, with asphaltenes actually the smallest molecules. The VPO value for asphaltenes was over 4000. The values obtained for polar aromatics was 1020 by FIMS and over 1400 by VPO. Results for naphthene aromatics and saturates were quite close by the two methods. It should be realized that the designation of asphaltenes is arbitrary, depending on the precipitating solvent (72,73). Propane precipitates most of the polar aromatics, and pentane asphaltenes can be nearly twice the heptane asphaltenes. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

Unfortunately, the method is not straightforward. It is necessary to consider a number of other factors, such as preferential depletion of a species from the surface of the sample, reaction of the gas with the cell itself, and the effusion of the gas which does not always follow ideal behaviour. Recently much work has been done to improve accuracy through better understanding of the method itself, and much early work pre-1960 is now considered to be unreliable (Komarek 1972). The method also relies on knowledge of the molecular weight of the vapour species and there may be more than one species involved. To this end the method is now often linked with mass spectroscopy (Komarek 1972, Kubaschewski et al. 1993) so that composition of the gas is better understood. 

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