Molecular weight, retention volatiles

Effect of molecular weight of volatiles on their retention during drying. 

Extracts collected from reproductive females and boars contained on average more compounds than those from non-reproductive females (106, 125 and 61 respectively). There were a range of volatile compounds although there were many at a high molecular weight. A quantitative analysis of the compounds present in more than 50% of the animals per type revealed a high similarity between the compounds found in reproductive females and boars. Two major compounds of all extracts have a retention time of 8.45-8.51 (RI = 800) and 14.93-14.95 (RI = 992). 

Due to the volatility of some of the compounds present in food, it is very important to utilize cryogenic cooling when the sample is introduced onto the GC column. This helps to prevent the loss of low-molecular weight volatiles and also tends to focus volatiles on the initial portion of the column, thus allowing for improved separation and quantification. The use of a film thickness of 1.0 mm will also aid in the retention of the aforementioned compounds. In the static headspace procedure, the 4-min pressurization step is also crucial, in that equal pressures between the sample vials and the GC must be attained to ensure reproducible sample injections. Forboth the static and SPME procedures, heating the samples for 30 min prior to injection is important to ensure proper equilibration between the sample and the head-space. 

The present section and Section IV.A.4 complement each other. The GC of metal chelates derived from monothio-/3-diketones, /3-diketones, -dithioketones and /3-keto-enamines was reviewed. The discussion includes column phenomena, analytical improvements and limitations and various applications . A study of the GC behavior of Group 13 /9-diketonates 37 shows that when the chelate contains a Mef group its volatility is enhanced, while a Ph group tends to decrease it. When R and R are alkyl groups the retention time of the chelates increases with the molecular weight, but it does not depend on the volatility however, the retention time of the chelates containing a Mef group is almost inversely correlated with the volatility . ... 

Chromatograms obtained while heating the three PE-catalyst samples show catalyst-dependent differences in volatile product distributions. Figure 2.3 shows the gas chromatograms obtained at the temperatures corresponding to the maximum volatile product evolution rates for each PE-catalyst sample. Figure 2.3 clearly shows that relative hydrocarbon product yields depended on which catalyst was employed. For the PE-HZSM-5 sample, many isomeric hydrocarbons were detected, most of which were low molecular weight substances with short retention times. Volatile product diversity is less evident... 

For volatile compounds the specificity of the mass fragments in combination with their retention time will generally be sufficient. Their volatility corresponds to a low molecular weight, limiting the number of possible false positive results there are not many low molecular weight compounds with the same retention time on a GC-column and similar mass spectra. 

HPLC, GC/MS, and sometimes GC techniques require the derivatization of aldehydes. This is highly recommended for Ci to C4 aldehydes. The advantages are that (1) retention time is shifted (delayed), so that peaks do not coelute with the solvents (2) volatile compounds are stabilized, so the loss due to evaporation is reduced and (3) when free aldehydes are analyzed by GC/MS, electron-impact ionization produces ions the same as those that would result from the corresponding alcohols therefore, the mass selective detector cannot distinguish between a low-molecular-weight aldehyde and its alcohol. This drawback is overcome by converting the aldehyde into its derivative. There are several compounds that can be used to derivatize an aldehyde. Some of these are 2,4-dinitrophenylhydrazine, semi-carbazine, thiosemicarbazine, oximes, and the... 

HALS-12 106990-43-6 Chimassorb 119 High-molecular-weight, monomeric iow volatility/ migration color retention in pigmented POs... 

Lycopodium Alkaloids. Gerard and MacLean (1986) reported the analysis of Lycopodium species by GC-MS. Most of the alkaloids previously reported to occur in the species examined could also be found using the GC system. One of the exceptions was lycodiflexine, which might not be sufficiently volatile since its molecular weight (562) is well above that of the other components of the extract. Besides the alkaloids reported, other minor alkaloids could be detected by this method. The retention index of 15 alkaloids is included. 

On-column injection is another admission mode, which prevents component discrimination due to volatility or molecular weight. The sample is applied directly into the column head. This might be seen as an ideal injection, but with a large number of samples there is the risk of severe column contamination. This is caused by non-volatile sample components which deposit inside the column and eventually clog it. Split/splitless injection ports include a liner just before the column, to act as a filter for these interfering substances. In the case of on-column injection, a pre-column, or retention gap, which is a short capillary with no timer coating, is used for this purpose. 

The volatility of the series of n-alkanes (HC) depends on their molecular weight which in turns the chain length. They are nonpolar, their interaction with the GC column are the same, their retentions on the GC solely depend upon the molecular weight. Therefore, it is useful to use as references for gas chromatographic system. 

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