GLC—Gas Liquid Chromatography

In this form of chromatography retention can readily be expressed in thermodynamic terms. The definition equation for the capacity factor (k) is 

If we assume very dilute solutions (as is usually the case in chromatography), we can write for the concentration in the stationary phase 

Because this ratio is independent of the concentration of the solute, it equals the ratio of the average concentrations, and hence psRTVs RT ns 

From eqn.(3.6) we conclude that there are two solute-dependent factors that affect retention. In the first place, this is the vapour pressure of the pure solute. p° is a strong function of the temperature (see below) and therefore, temperature may be used as a parameter to influence retention. However, the vapour pressure is a pure component property and it cannot be changed at will. Differences in the vapour pressure of two solutes (or differences in variation of vapour pressure with temperature) may or may not provide us with a means to achieve separation. When the vapour pressure is not sufficiently different, we need to create differences in the second solute-dependent factor. 

This is the activity coefficient of the solute in the stationary phase (y ). The value of y is determined by molecular interactions between the solute and the stationary phase. Therefore, (chemically) different stationary phases will lead to different values for y. This explains the availability of many different stationary phases for GC, many of which show different selectivity (see section 2.3.2). 

Gas liquid chromatography (GLC) has found only limited application in lichen chemistry because most lichen substances are very polar and have low volatility. To overcome this problem, more volatile derivatives are used. Nishikawa et al. (1973) analyzed the low molecular weight carbohydrates of eight lichen species as their acetyl, trifluoroacetyl and trimethylsilyl derivatives by GLC under the following conditions Hj, 

50mlN2/min acetyl derivatives 1.5% SE 30 on chromosorb W, 60-80 mesh, 200 x 0.4 cm column, temp, progr trifluoroacetyl derivatives 1% XF-1105 on Chromosorb W trimethylsilyl derivatives 1.5% OV-17 on Shimalite. The relative retention times of the derivatives are given in Table 16. 

Acetylation by reaction of 2 mg of the sample with 1 ml of acetic anhydride and 1 ml of pyridine at room temperature for 12 h. 

Trifluoroacetylation by reaction of Img of the sample with 0.1ml N,N-dimethylformamide and 0.2 ml trifluoroacetic anhydride at room temperature for 10 min and direct injection. Trimethylsilylation 10 mg of the compound was treated in a small stoppered vial with 1ml of anhydrous pyridine (dried with KOH pellets), 0.2 hexamethyldisilazane and 0.1ml trim-ethylchlorosilane. The mixture was shaken vigorously for about 1 min and then kept at room 

Gaskell et al. (1973) analyzed the hydrocarbons from three lichens by GLC and Furuya et al. (1966) some lichen anthraquinones. Ikekawa et al. (1965) reported on the GLC of zeorin and other triterpenes. Zeorin has a retention time of 4.96 min (1.5% SE-30 on Gas Chrom P, 80-lOOmesh, 150 x 4mm, 225°C, 80mlN2/min). Shibata et al. (1965) found that free zeorin gave two peaks, the minor one corresponding to zeorinine. The relative retention times for the acetates of the sterols isolated from Pseud-evernia furfuracea are summarized in Table 17 (Wojciechowski et al. 1973). 

Examples of the quantitative determination of MA using GLC have been reported. In Table 1.6, information concerning stationary phases and solid supports has been given. The key to quantitative determination by this method is calibration. Given that, the method is fast and accurate. In a number of patents involving production, GLC has been used for MA determination in the product stream. In Table 1.6, details about the origin of MA are included. This may help the choice of column since the product distributions are different 

Chromosorb W Firebrick 3 1 Chromosorb W-TND-TSM Chromosorb AW-DMCS Chromosorb AW-DMCS Chromosorb W Celite 

The importance of gas-liquid chromatography lies in its powerful separating ability. Columns can have the equivalent of up to 1,000,000 theoretical plates. Compare this with an ordinary distillation, which normally operates in the range of 5 to 10 theoretical plates. What this means in practice is that a gas chromatograph can separate the odor components of coffee into over 400 compounds  

The gas stream, containing the separated compounds, then is passed through a detector the simplest type is one that measures the difference in thermal conductivity between the sample plus He or Nj gas and a reference He or Nj gas. The detector signal is monitored continuously by a recorder, which plots the components as a function of time, resulting in Gaussian-shaped peaks. 

The elapsed time between the injection and the center of a peak is called the retention time of that compound. Note that although a gas chromatograph separates compounds, it DOES NOT IDENTIFY them. However, the retention time may be used as corroborating evidence in the identification of a compound by injecting a known sample under identical conditions and observing identical retention times. The unknown-known comparison then should be repeated on another column of different polarity to substantiate the previous agreement of retention times. 

Failure to use columns of different polarity to identify compounds can be embarrassing. DDT was blamed for many problems caused by PCB s by some early investigators, who failed to use at least two columns. 

The concept of HETP was described in Chapter 3, p. 26. The same concept applies here except that HETP for a distillation differs from an HETP for a chromatographic separation, because in a distillation, the entire column is in use at the same time, whereas in a chromatographic column, only a small portion is being used at any one time. 

The first two groups have been collectively termed as Gas Chromatography .Its phenomenal growth at almost logarithmic pace may be attributed to its unparalleled potential in resolving components of a complex mixture. Gas chromatography fundamentally is a separation technique that not only essentially provides prima facie indentification of a compound but also caters for quantitative estimation after due calibration. 

Gas chromatography makes use, as the stationary phase, a glass or metal column fdled either with a powdered adsorbent or a non-volatile liquid coated on a non-adsorbent powder. The mobile-phase consists of an inert-gas loaded with the vapourised mixture of solutes flowing through the stationary phase at a suitable temperature. In the course of the passage of the vapour of the sample through the column, separation of the components of the sample occurs in two ways, namely  

Martin and Synge in 1952, became the Nobel Laureates for their excellent, innovative research work on the development of partition chromatography. 

however, pertinent to mention here that GLC has a much greater application in the field of pharmaceutical analysis which extends over to most organic constituents that have a measurable vapour present at the temperature employed. 

The overall cost of equipment is comparatively low and its life is generally long. 

As the name implies, a gas phase (moving phase) and a liquid phase (stationary phase) are involved in gas-liquid chromatography. Gas-liquid chromatography is usually employed for the separation of volatile compounds. The device used for this is called a gas chromatograph. Usually a syringe is used to inject the sample into the device, and the sample injected is readily turned 

The maintenance of the correct temperature of the column is extremely important for an efficient separation of the components. The choice of the stationary liquid phase is also important, and so is the maintenance of an optimum rate of flow of the sample through the column. The latter can be maintained by controlling the flow of the carrier gas. This technique is highly sensitive. Hence, it can be used to detect compounds even if very little of the compound is present (as low as in the order of micrograms). 

Technological developments in the field of identification of compounds using infra red and NMR techniques have revolutionized the ease with which we can analyze and identify even the most complex structures in organic chemistry. For the MCAT, we should be familiar with the techniques and the basic principles that are used in this field. 

Infrared spectroscopy is a widely used tool to identify and analyze compounds. As you know, the infrared radiation is part of the electromagnetic spectrum. The IR has wavelengths between those of the visible light and the microwaves. From the analytical point of view, only a portion of the entire infrared range is actually useful for the spectroscopic analysis. 

For example, the carbonyl groiq) ( C=0) shows absorbance close to 1700 cm in the IR spectrum. The peaks around this value indicate the presence of carbonyl group in the compound that is analyzed. By knowing the IR values, we can predict the structure of a compound. Our discussion from the MCAT point of view will be limited to some of the common and important IR peaks. 

This method suffered from sensitivity problems initially as the bile-acid molecules lack a chromophore, but did offer the distinct advantage that conjugated bile acids could be determined without hydrolysis. The sensitivity issue was addressed by use of fluorescent derivatives such as dimethoxycoumarin esters with a C18 reverse phase column and were able to resolve endogenous mixtures of bile acids. The combination of hplc and mass-spectroscopy detection has further improved the sensitivity along with providing specific identification, important as the resolution of bile acids by hplc is not as good as capillary column glc.  

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Gas-liquid chromatography (GLC) finds many applications outside the chemistry laboratory. If you ve ever had an emissions test on the exhaust system of your car, GLC was almost certainly the analytical method used. Pollutants such as carbon monoxide and unbumed hydrocarbons appear as peaks on a graph such as that shown in Figure 1.7. A computer determines the areas under these peaks, which are proportional to the concentrations of pollutants, and prints out a series of numbers that tells the inspector whether your car passed or failed the test. Many of the techniques used to test people lor drugs (marijuana, cocaine, and others) or alcohol also make use of gas-liquid chromatography. 

Quinoxaline (161) with 1,3,6-trioxanegavethecyclic acetal, 2-(l,3,5-trioxan-2-yl)-quinoxaline (162) (Bu 02H, FeS04, MeCN, reflux, 5 h crude), and thence 2-quinoxalinecarbaldehyde (163) [5% H2SO4, reflux, briefly 35% overall, by gas-liquid chromatography (glc)] 1,4-dioxane has been used in a some-... 

Hewlett-Packard Model 6890 equipped with a nitrogen-phosphorus flame ionization detector Capillary column for gas-liquid chromatography (GLC), DB-1, 0.53-mm i.d. x 15 m, l-pm film thickness (J W Scientific)... 

The extracted fractions were esterified with either BF3-MeOH reagent or diazomethane and analyzed by GLC. Gas liquid chromatography (GLC) was conducted with a Perkin-Elmer Sigma 3 equipped with flame ionization detector. Separations were obtained on a Hewlett Packard 12 m x 0.2 mm i.d. capillary column coated with methyl silicon fluid (OV-101). The temperature was maintained at 80°C for 2 min then programmed from 80 to 220°C at 8°C/min. The injector temperature was 250°C. Mass spectra were obtained on a Hewlett Packard model 5995 GC-MS mass spectrometer, equipped with a 15 m fused silica capillary column coated with 5% phenyl methyl silicone fluid. Spectra were obtained for major peaks in the sample and compared with a library of spectra of authentic compounds. 

As mentioned before, there are two common types of GC gas-liquid chromatography (GLC) and gas-solid chromatography (GSC), depending on the physical state of the stationary phase. GSC is seldom used. In GLC the analyte is partitioned between the mobile phase (gas) and a liquid phase, which is retained on an inert solid support. The liquid phase should ideally possess a low volatility (so that it does not volatilise with the analyte), be thermally stable and chemically inert, and have favourable solvent characteristics. 

Spiroaziridinium compounds have also been synthesized under photochemical conditions. For example, the photolysis of piperidine 83 in acetonitrile resulted in 3-azoniaspirooctane 85 (Equation 19) <1997JOC6903>. Its presence was detected by H NMR spectroscopy and mass spectrometry. This azoniaspiro species was thought to be transient and went on to form N-substituted piperidines which were qualitatively identified by gas liquid chromatography (GLC). 

The reactions of Eq. (2) could not be carried to completion in a reactive olefin such as 1-hexene. After several increments of MeCl2SiH had been added to the mixture, succeeding increments were mostly consumed to form /i-C6H13MeSiCl2. When this product appeared in the solution, gas-liquid chromatography (GLC) showed that hexene in the solution was no longer only 1-hexene but a mixture of isomers which contained mostly 2- and-3-hexene, in both cis and trans conformations. 

Gas-Liquid Chromatography. In gas-liquid chromatography (GLC) the stationary phase is a liquid. GLC capillary columns are coated internally with a liquid (WCOT columns) stationary phase. As discussed above, in GC the interaction of the sample molecules with the mobile phase is very weak. Therefore, the primary means of creating differential adsorption is through the choice of the particular liquid stationary phase to be used. The basic principle is that analytes selectively interact with stationary phases of similar chemical nature. For example, a mixture of nonpolar components of the same chemical type, such as hydrocarbons in most petroleum fractions, often separates well on a column with a nonpolar stationary phase, while samples with polar or polarizable compounds often resolve well on the more polar and/or polarizable stationary phases. Reference 7 is a metabolomics example of capillary GC-MS. 

Gas chromatography (GC) can also be referred to as vapor-phase chromatography (VPC) and even gas—liquid chromatography (GLC). Usually the technique, the instrument, and the chart recording of the data are all called GC ... 

Gas lasers, 14 681-696 carbon dioxide, 14 693-696 excimer lasers, 14 691-693 helium-neon, 14 681-683 ion lasers, 14 683-688 molecular nitrogen, 14 688-691 Gas lift electrolyte circulation, 9 621 Gas-liquid base stocks, 15 217 Gas-liquid chromatography (glc), 6 374 analysis of sugars via, 23 476 silylation for, 22 692, 697 Gas-liquid contactor, reciprocating jet,... 

Norbornadiene (from Koch-Light Ltd.) of purity greater than 99% was purified by distillation through a Normatron automatic still at a reflux ratio of 25 1. The fraction (60 vol%) boiling at 89.3 °C/998 mbar was collected [18] bp 89.5 °C/1013 mbar). Analysis by gas liquid chromatography (GLC) (Pye Unicam - with OVO-1 columns), showed one... 

EtN02 was fractionally distilled and the last 15%, which was free from impurities detectable by gas liquid chromatography (GLC), was stirred with baked Na2S04 in a reservoir attached to the vacuum line. 

Since the introduction of Gas-Liquid-Chromatography (GLC) (see Part V, Chapter 29) as an essential analytical tool, it has been judiciously exploited as an useful alternative means for not only determining water content in pharmaceutical chemicals but also limiting specific volatile substances present in them. It may be expatiated with the help of the following examples ... 

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