Molecular abundance

Solving the chemical kinetic equations and comparison with the observed molecular abundance. 

Molecular abundances of carbon chain molecules in selected sources (Winnewisser and Walmsley 1979)... 

Eukaryotic homology, gene-orientated clusters of transcript sequences, conserved protein domain, markers and mapping, population study (PopSet), expression and molecular abundance profiles (GEO), and GEO data sets and cancer chromosomes. 

The result of these calculations shows that even in normal HI clouds with I M and no strong extinction appreciable molecular abundances can exist. The predicted values are ... 

Hara et al. (1980) employed GC/MS for the analysis of trace amounts of mixed halogenated compounds in the blood and tissue of humans. Identification and quantitative analysis of various compounds was achieved by monitoring the mass fragments for selectively molecular, abundant or characteristic ions for each compound. Thus the monitoring ion (m/z) for quantification of 1,1 -dichloroethane was set at 83 [(M) -CH3]. A lower detection limit of 10 to 20 pg per sample was achieved. 

The recent ai roach of large comets such as IP/Halley, C71996 B2 Hyakutake, and C/1995 Ol Hale-Bopp to the Earth provided a good opportunity to investigate the detailed composition of cometary ices by various methods such as mass spectrometry, infrared spectroscopy, and radio emission. The composition of interstellar ices is compared with that of the cometary ices in Table 9.3. It is striking that cometary and interstellar ices have quite comparable relative molecular abundances. 

MOLECULAR ABUNDANCES OF CERTAIN GASES IN COMETS (continued) >... 

The chemistry changes drastically when interstellar UV radiation can penetrate the more tenuous outer regions of the CSE, and photochemistry starts to control molecular abundances. Neutral molecule plus radical reactions and neutral molecule plus ion ( ion-molecule ) reactions are important for the... 

One of the main problems to overcome in order to realise quantitative proteomics is that mass spectrometry is not strictly a quantitative technique with respect to molecular abundance. One approach to overcome this problem is to perform comparative experiments in parallel where one sample is labelled with an isotope (such as 0 or H) and the other is unlabelled. Mass differences between labelled and unlabelled samples may be observed in a mass spectrometer and the relative ratios quantified (Figure 9.21). Obviously this method of quantification relies on equivalent mass expression by both samples in spite of the isotopic labelling of the one and not the other. In other words, in spite of the labelling, both samples should behave identically chemically. The simplest way to effect labelling is to use isotope-coded affinity tags (ICATs). According to this technique, two identical populations of proteins... 

At present, there is a renewed interest in high-energy states which is stimulated by general advances in space research. This is producing a wealth of raw spectral data which is leading to a close interaction between observers, astrophysical modelers, and molecular physicists and chemists. In diagnosing astrophysical observations, the availability of reliable theoretical transition probabilities is essential, since they can allow the determination of molecular abundances, electron temperature and densities, among other properties. 

Figure 5 Mass spectra and chemical structures of secobarbital (A), Hs-secobarbital (B), and C4-secobarbital (C) (All as methyl-derivatives). (Reproduced with permission from Chang W-T, Smith J, and Liu RH (2002) Isotopic analogues as internal standards for quantitative GC/MS analysis - molecular abundance and retention time difference as interference factors. Journal of Forensic Sciences 47 873-881.)...

Chang WT, Smith J, and Liu RH (2002) Isotopic analogs as internal standards for quantitative GC/MS analysis -molecular abundance and retention time difference as interference factors. Journal of Forensic Science 47 873-881. 

ABSTRACT. The current status of models of diffuse interstellar clouds is reviewed. A detmled comparison of recent gas-phase steady-state models shows that both the physical conditions and the molecular abundances in diffuse clouds are still not fully understood. Alternative mechanisms are discussed and observational tests which may discriminate between the various models are suggested. Recent developments regarding the velocity structure of diffuse clouds are mentioned. Similarities and differences between the chemistries in diffuse clouds and those in translucent and high latitude clouds are pointed out. 

Although the steady-state models can most likely account for many of the observed features of diffuse clouds, additional shocked layers appear necessary to reproduce the measured abundances and part of the H2 rotational excitation. It is still not known how ubiquitous shocks are in diffuse clouds, and to what extent they contribute to the formation of other molecules. It would be of interest to find a diffuse cloud which does not show any shock signatures, but still has substantial molecular abundances. Grain surface production of molecules other than H2 in the classical diffuse clouds is evidently very inefficient, although large molecules or small grains may play a role in the ionization balance. 

For selected dissodative recomtnnation reactions involving protonated molecular ions AH" ", HLl/2 include more neutral product channels than MN, who on occasion only include the A + H channel. Including only this one channel means that protonation of the neutral species A by any of the abundant ions H3" , HCO" , or HoO is, because of 100% recycling, not a destruction step and therefne leads to enhanced abundances for A. The question of the proper neutral branching ratios for dissociative recombination reactions is a vexing one theoretical ideas on this subject are discussed by Bates and Herbst in this volume and their in lications for calculated molecular abundances in this article in section 4. However, the astrochemical community eagerly awaits some definitive experimental studies. 

---