Column gas chromatography

many compounds may be deterinined by gas chromatography, but there are limitatioiis. They must be volatile and stable at the temperature employed, typically from 50 to 3Q0°G. GG is useful for  

If compounds are not volatile or stable, often they can be derivatized to make them amenable to analysis by GG. GC cannot be used for salts nor macromolecules, but these can be determined by HPLC, one of its major uses. 

The two types of columns used in GC are packed columns and capillary columns. Packed colunms were the first type and were used for many years. Capillary columns are more commonly used today, but packed columns are stiU used for applications that do not require high resolution or when increased capacity is needed. 

Packed columns can be used with large sample sizes and are convenient to use. 

The solid support for a liquid phase should have a high specific surface area that is chemically inert but wettable by the liquid phase. It must be thermally stable and available in uniform sizes. The most commonly used supports are prepared from diatomaceous earth, a spongy siliceous material. They are sold under many different trade names. Chromosorb W is diatomaceous earth that has been heated with an alkaline flux to decrease its acidity it is light iu color. Chromosorb P is crushed firebrick that is much more acidic than Chromosorb W, and it tends to react with polar solutes, especially those with basic functional groups. 

---

In general, the longer a chromatographic column, the better will be the separation of mixture components. In modem gas chromatography, columns are usually made from quartz and tend to be very long (coiled), often 10-50 m, and narrow (0.1-1.0 mm, internal diameter) — hence their common name of capillary columns. The stationary phase is coated very thinly on the whole length of the inside wall of the capillary column. Typically, the mobile gas phase flows over the stationary phase in the column at a rate of about 1-2 ml/min. 

At such a rate of scanning, it is even possible to examine eluants from capillary GC (gas chromatography) columns during GC/MS operations. 

Application of the Design Equations to Packed Liquid Chromatography Columns and Open Tubular Gas Chromatography Columns... 

Deuterium exchange of conjugated enones and dienones on pretreated gas chromatography columns has been found useful for the characterization of these compounds by combined gas chromatography-mass spectrometry. ... 

Z. Fiu and J. B. Phillips, Farge-volume sample introduction into narrow-hore gas chromatography columns using thermal desorption modulation and signal averaging , /. Microcolumn Sep. 2 33-40 (1990). 

This ether wash may be combined with the main neutral fraction and distilled to obtain 29-30 g. (33-34%) of 2-(dichloro-methylene)bicyclo[3.3.0]octane, b.p. 53-56° (01 mm.), n25d 1.5179-1.5182 (pure by gas chromatography) (column as in Note 8, 125°, retention time 4 minutes). 

Using the 3 mm. by 2 m. gas chromatography column described above, a mixture of stereoisomers of 2-allyl-5-methylcyclohexanone [Cyclohexanone, 5-methyl-2-(2-propenyl)-], prepared by allylation of the enamine of 3-methylcyclohexanone,7a showed peaks at retention times of 8.4 minutes (more stable isomer) and 9.6 minutes. A mixture of the two isomeric 2-allyl-3-methylcyclohexanones and the two isomeric 2-allyl-5-methylcyclohexanones clearly exhibited four distinct peaks on gas chromatography. 

Enolizable hydrogens can be replaced by deuterium (and 0 by 0) by passage of a sample through a deuterated (or 0 containing) gas chromatography column. There are many enol-keto intereonversions and acidifications of enolate ions to the keto forms listed in Organic Syntheses. No attempt is made to list them here. 

When the desired hydrogen uptake had been achieved, the vessel was opened, catalyst separated by filtration, and the reaction solution analysed by chiral gas chromatography (column Cydex B, 50 m, SGE Ltd). Analysis gave conversion and enantiomeric excess Enantiomeric excess is defined as IR - SI /(R+S). 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

In the commonly used pulse technique introduced by Kokes et al. C2), a spike of reagents is introduced into a carrier gas flowing over the catalyst and after that over a gas chromatography column which is used to determine the conversion of reagents to products. In this method, the concentrations of reagents in the catalyst bed are not defined, and kinetic measurements relating reaction rates to concentrations are impractical. 

Gas ionization detectors are widely used in radiochemistry and X-ray spectrometry. They are simple and robust in construction and may be employed as static or flow detectors. Flow studies have received attention in the interfacing of radioactive detectors with gas chromatographs. A radio-gas chromatograph (Figure 10.9) uses a gas flow proportional counter to monitor the effluent from the gas chromatography column. To achieve... 

Other combinations are available. For example, liquid chromatographs connected to mass spectrometers (known as liquid chromatography-mass spectrometry [LC-MS]) are fairly common. Almost any combination of two instruments that can be thought of has been built. In addition, two of the same instruments can be connected so that the output from one is fed directly into the other for further separation and analysis. Examples include two mass spectrometers in an MS-MS arrangement and two different gas chromatography columns connected in a series, known as GC-GC. To keep up with these advances, one needs to have a working knowledge of the fundamental principles involved in the techniques and of the abbreviations used for the various instrumentation methods. 

The retention time of a certain component on a particular 2.0-m gas chromatography column is 3.1 min. The width at the base for the peak is 0.39 min. How many theoretical plates are in this column and what is the height equivalent to a theoretical plate ... 

Comparative polarity studies have also been performed by absorption of ionic liquids on to gas chromatography columns followed by the elution of various compounds and lead to the conclusion that the polarity of [bmim][BF4] is similar to that of lower alcohols [18]. 

The course of the reaction can conveniently be followed by gas chromatography. A sample of the reaction mixture is withdrawn at intervals, neutralized with solid sodium bicarbonate, dried over magnesium sulfate, and injected directly into a gas chromatography column consisting of 15% phenyldiethanola-mine succinate (PDEAS) substrate coated on 60/80 mesh, acid-washed fire brick contained in a i in. by 5 ft. spiral-shaped copper... 

The product may be analyzed by use of a gas chromatography column packed with either LAC-728 (diethylene glycol succinate) or Carbowax 20M suspended on Chromosorb P. Using a 2.5-m. LAC-728 column heated to 100°, the submitters found retention times of 9.4 and 13.0 minutes for cyclohexyl methyl ketone and cyclohexyldimethylcarbinol. Less than 1% of the carbinol by-product was present. 

The various fractions of the forerun were analyzed employing a gas chromatography column packed with silicone gum, No. XE-60, suspended on Chromosorb P and heated to 248°. The components found (with the retention times indicated) were benzyl bromide (9.0 minutes), 2-methylcyclohexanone (5.3 minutes), and, in some cases, bibenzyl (22.6 minutes). The bibenzyl, formed by reaction of the benzyl bromide with the excess methyllithium, was identified from the infrared spectrum of a sample collected from the gas chromatograph. 

The progress of this reaction may be followed by quenching aliquots of the reaction solution in acidic aqueous ammonium sulfate followed by extraction with ether and analysis of the ethereal extract by gas chromatography. With 1.2-m. gas chromatography column packed with silicone fluid. No. 710, on Chromosorb P and heated to 215°, the retention times of naphthalene and 1-bromonaphthalene were 1.9 minutes and 6.7 minutes, respectively. The submitters employed a 30-cm. gas chromatography column packed with Porpak P for this analysis. 

On a 2-m- gas chromatography column packed with 10% Apiezon L and heated to 100°, the retention times for N,N-dimethylcyclohexylamine and cyclohexanol are 15 and 4 minutes, respectively. The composition of this forerun is 80-85% of the amine and 20-15% of the alcohol. 

On a gas chromatography column, packed with Apiezon M suspended on Chromosorb P and heated to 70°, the product exhibits a single peak with a retention time of 23.2 minutes. The sample exhibits infrared absorption (CCI4 solution) at 3100, 3030, 2985, 1440, 1340, and 1235 cm. with n.m.r. singlets (CCI4 solution) at 3.45 (CHaBr) and 0.90 p.p.m. (cyclopropyl CHj). The mass spectrum of the sample has abundant fragment peaks at m/e 149, 147, 67, 41, and 39. 

An example of a piece of equipment that would need to be validated is a new gas chromatography column that a vendor is touting as especially useful for the work. For the validation study, the column is installed in the instrument and the procedure is executed, perhaps repeatedly on all types of possible samples, so that the analyst can be certain that, again, the quality of the work meets expectations. 

The product was analyzed with a 2-m. gas chromatography column packed with Carbowax 20M suspended on... 

Materials. The gases used (methane, ethane, ethylene, propane, propylene, n-butane, 2-methylpropane, and the butenes) were at least 99% pure (Cambrian Chemicals, Ltd.). The purity of each gas was tested by gas chromatography (columns of molecular sieve 5A, silica gel, or Porapak Q). 

Batch slurry reactions were carried out in liquid phase in a stirred glass vessel with condenser. Catalyst was added to a preheated solution containing aromatic reactant (35ml, Aldrich) and iso-butyric anhydride (16ml, Aldrich) in a 3 1 molar ratio. Samples of the reaction mixture were removed from the reaction mixture after various reaction times, filtered and analysed by gas chromatography (column DB-5, 30m, He carrier gas, FID detector) to determine reaction progress. Product identification was made by comparison with appropriate reference materials. 

The aroma of foods is caused by volatile compounds which are perceived by the human nose. Many studies (reviews in [1, 2]) have indicated that only a small fraction of the hundreds of volatiles occurring in a food sample contribute to its aroma. To detect these compounds, a method proposed by Fuller et al. [3] is used. In this procedure, which is designated gas chromatography-olfactometry (GC-O), the effluent from a gas chromatography column is sniffed by an expert who marks in the chromatogram each position at which an odour impression is perceived. 

Different capillary columns are available for organic acid separation and analysis. In our laboratory, the gas chromatography column in all GC-MS applications is crosslinked 5% phenyl (poly)methyl silicone, 25 m internal diameter 0.20 mm stationary phase film thickness 0.33 pm (Agilent HP-5, DB-5, or equivalent). Several instrument configurations are commercially available, which allow for positive identification of compounds by their mass spectra obtained in the electron impact ionization mode. A commercially available bench-top GC-MS system with autosampler (Agilent 6890/5973, or equivalent) is suitable. Software for data analysis is available and recommended. The use of a computer library of mass spectra for comparison and visualization of the printed spectra is required for definitive identification and interpretation of each patient specimen. 

Urine (5 ml) urine spiked with 0.2% (v/v) isopropylamine is placed in a screw-capped 15-ml vial [28]. Pelleted potassium hydroxide (3 g) is added before sealing the vial with an airtight polytetrafluoroethylene-lined septum cap. Potassium hydroxide raises the pH of the sample to ensure that the amines are present as volatile bases. The vial is heated in an aluminium block at 90 C for 20 min. While still in this block, 2 ml head-space gas is withdrawn through the septum with a disposable syringe and injected immediately on the gas chromatography column. The operating temperatures of the column, injector port and detector unit are 70 C isothermal, 150 C and 200 C, respectively, with nitrogen carrier gas at 60 ml/min. This allows quantification of TMA and other amines. TMA N-oxide is measured after quantitative reduction into TMA. For this, titanous chloride (30%, w/v 0.2 ml) is added to 2 ml urine in a screw-capped vial and incubated for 30 min at room temperature. The sample is then diluted ten-fold with distilled water and analysed as described above. The result represents the sum of TMA and TMA N-oxide present in the sample. 

Historically, silica gel, which is thermally stable and insensitive to oxygen, was one of the first compounds used as a stationary phase for gas chromatography columns. Today, solid phases are made with more elaborate materials. The efficiency of graphite-based columns is very high (see Fig. 2.9 below). 

Materials. Cyclohexene, obtained by dehydration of reagent grade cyclohexanol (3), was heated at reflux over sodium metal, fractionated on a 60-cm. Helix packed column, stored over sodium, and filtered just before use. No impurity was found by gas chromatography (column, TCP and Si-550 carrier gas, helium). Propylene (Neriki Research Grade) used showed no impurity by gas chromatography (column, active carbon and acetonylacetone). 

Stereospecific Epoxidation of 2-Butene. The hydroperoxide epoxidation reaction is stereospecific. Pure cis- and trans-2-butene were epoxi-dized separately by cumene hydroperoxide. The cis olefin gave exclusively cis epoxide, and the trans olefin gave exclusively trans epoxide. In both cases, the epoxide was the sole product formed from the olefin. They can be distinguished easily by their different retention times on a gas chromatography column of 20% diisodecyl phthalate on Chromosorb W(60-80 mesh). They were also identified by comparing their infrared spectra with authentic samples. 

---