Sampling and gas-chromatographic analysis

In virtually all water resources there are gases dissolved in the water. Carbon dioxide is almost always present in more or less considerable quantities (see Chapter 1 and Section 3.2). The content of carbon dioxide is particularly significant in groundwater at greater depths or in subterranean water and may reach concentrations of up to several g/1. 

Dissolved nitrogen may also frequently be detected in the latter type of water. Other substances which may be identified in subterranean waters in the form of trace amounts of dissolved gases are hydrogen, methane, carbon monoxide, inert gases (including radon 222), hydrogen sulphide and hydrocarbons. 

Oxygen virtually never occurs in subterranean waters of this type. If it is 

Following pressure equalization, the gases from the water collect at the 

The importance of correct transport of the gas or water samples to the analytical laboratory cannot be stressed enough. The samples are best transported under water, and in a quantity of water sufficiently large to 

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The high pressure, liquid-phase hydrogenation of 3-methyl crotonaldehyde was carried out in a well-stirred batch autoclave under 4 MPa Ha (Air Liquide, 99.995% purity) pressure using 0.1 mol of 3-methyl crotonaldehyde (UAL) (Merck) and 0.6 g catalyst. Isopropanol (37.5 cc) was used as a solvent. The catalyst was activated by stirring under 4 MPa Ha pressure at 373K for two hours prior to introduction of the unsaturated aldehyde UAL reactant at the same temperature. The reaction products were monitored by repetitive sampling and gas chromatographic analysis. Since this was a batch reaction, data are reported as selectivity vs. conversion. Time of reaction to reach about 30% conversion was close to 60 minutes for Ru/NaY and 150 minutes for Ru/KY. 

Colenutt BA, Davies DN. 1980. The sampling and gas chromatographic analysis of organic vapours in landfill sites. Int J Environ Anal Chem 7 223-229. 

Bruner F, Bertoni G, Crescentini G. 1978. Critical evaluation of sampling and gas chromatographic analysis of halocarbons and other organic air pollutants. J Chromatogr 167 399-407. 

For et P 0.05, l.e., z-1.645, an Important conclusion emerges the detection limit does not exist for m > 1/z - 0.61. This may be academic, however, since so large a slope Is unlikely for any reasonable analytical method. A slope of 10%, however, would result In D/C - 2.39. To Illustrate, let us take the blank standard deviation for the measurement of toluene In air, by a fully specified method of sampling and gas chromatographic analysis, to be 0.21 pg/L. The critical level for detection decisions, assuming normality, would then be 1.645 (0.21) - 0.34 ftg/L. The corresponding detection limit would be 2.39(0.34,) - 0.83 ms/L. 

Occasionally a colorless solution was obtained at approximately — 50°. In most eases the reaction mixture contained a small quantity of fine suspended black powder (presumably copper metal). Examination of the reaction mixture at this stage by hydrolysis of a sample at —35° and gas chromatographic analysis demonstrated the presence of approximately 10% of 2,4-hexadienes [predominantly the (E),(E)-isomer],... 

Analytical methods for determining disulfoton in environmental samples are reported in Table 6-2. The steps included in the methods are solvent extraction, purification and fractionation, and gas chromatographic analysis. Other analytical techniques, including capillary gas chromatography with mass selective detection (Stan 1989), high-performance liquid chromatography with either mass spectrometric (MS) or MS-MS detection (Betowski and Jones 1988), have been used to determine disulfoton in environmental samples. 

EXTRACTIONS. Simple partition between two phases can add another valuable piece of information about the sample. A gas chromatographic analysis before and after extraction indicates the character of the components present. For example, carboxylic acids are readily separated from phenolic compounds by extracting a nonaqueous solution of the sample with dilute aqueous sodium bicarbonate. The carboxylic acids are almost completely transferred to the aqueous phase, whereas the phenolic constituents remain in the organic layer. Additional information on extractants for specific classes can be found in most organic analysis textbooks or inferred from solubility tables. 

Elemental Sulfur. In 1942, Chatterjee (44) reported the presence of elemental sulfur in weathered Indian coal but not in fresh samples. He suggested that, during weathering, pyrite is first oxidized to ferrous and ferric sulfates, and that then ferric sulfate oxidizes pyrite to elemental sulfur. The presence of elemental sulfur in U.S. coals was confirmed recently by Richard et al. (45) and White and Lee (46). Duran et al. (47) used extraction and gas chromatographic analysis to determine elemental sulfur in a suite of U.S. coals. They found that elemental sulfur (0.03-0.17%) is present in coal that has been exposed to the atmosphere, but is absent in pristine samples that have been processed and sealed under a nitrogen atmosphere. These data support Chatteijee s discovery that elemental sulfur in coal is a weathering product. 

Harrison et al. [33] studied the factors governing the extraction and gas chromatographic analysis of PAHs in water. Factors such as initial concentration, presence of suspended solids and prolonged storage of the samples affected considerably extraction efficiencies. It is recommended that water samples should be collected directly into the extraction vessel and that analysis should be carried out as soon as possible after extraction. 

The sampling loops were replaced by two stainless steel U-tubes of 1.5- and 20-cc. capacity. The expansion bomb is a 1.7-liter stainless steel cylinder. The trap between the helium supply and the Beckman valve is 1/4-inch stainless steel tubing. A null detector is used to measure pressures in the inlet system. Samples are obtained in 10-ml. stainless steel cylinders fitted with a Vg-inch stainless steel Hoke valve with a V-stem and Teflon packing. When the sample is liquid, it is entirely vaporized into the 1.7-liter expansion bomb, and a gaseous sample is taken for infrared, near infrared, and gas chromatographic analysis. 

Nitrobenzene is determined in environmental samples by collection, extraction with an organic solvent and gas chromatographic analysis (EPA 1982a, 1982b NIOSH 1984). Flame ionization detection or mass spectrometry may be used for detection. 

Berezkina et al. [126] proposed a reaction method for determining trace amounts of ammonia, based on pre-concentration of ammonia with dilute sulphuric acid, oxidation of ammonia in alkaline solution with potassium hypobromite and gas chromatographic analysis of the nitrogen evolved. The detection limit is 5 10 g of ammonia in a concentrated solution (ca. 10 % in a 500-ml sample). 

A bubble formed at the interface of the two polyetherimide filinis. A special sampling technique was employed and gas chromatographic analysis of the bubble performed. The sampling technique consisted of placing the coated specimen into a flask which was connected to a gas chromatograph equipped with a thermal conductivity detector. 

Here n is the number phenolic units in the chain. The methylene bridges are characterised by C-NMR spectroscopic analysis. The characteristics peaks for o, o, o, p and p, p methylene carbons are 30,35 and 40 ppm. The free phenol is characterised from peaks at 115 ppm and 120 ppm. The free phenol content of a resin sample is determined by dissolving the sample in acetone and gas chromatographic analysis of the solution using m-cresol as an internal standard. The -OH and -CO functional groups are characterised by Fourier transform infra red spectroscopy (FT-IR) from their characteristic peaks at 3350-3500 cm and 1730 cm", respectively [7]. 

A procedural blank of the HPLC system involves elution of the HPLC column with the solvent(s) to be used for subsequent sample separations, concentration of the solvent fractions to be collected and injection into the GC. A procedural blank of the HPLC system in combination with sample concentration by, e.g., liquid-solid adsorption on XAD-2 resin (see Chapter 18), involves extraction of a clean resin column, concentration of the extract, separation with the HPLC system using the proper combination of solvents, concentration of the eluates to about 50 pL (the volume is checked with a 100 pL syringe) and gas chromatographic analysis of 2pL injections with ECD (organochlorines) or FlD-MS (PAH, alkanes) detection. 

Kampe and McMahon used the Westoo procedure to determine methylmercury in fish. Their procedure involved the partitioning of methylmercury chloride in benzene and gas chromatographic analysis with electron capture detection. Down to 0.02ppm of methylmercury chloride can be detected in a lOg sample. 

Chamber B, the pH adjusted to about 3 with acid, and the sample extracted with diethyl ether. Sufficient sample is removed for gas chromatographic analysis, e.g., 1 pi. The aqueous phase is made alkaline and the sample reextracted with the same diethyl ether and gas chromatographic analysis repeated. Finally, the sample is made neutral and saturated with sodium bisulfite and reextracted. The ethereal phase is reanalyzed. In this case three different analyses can be made from the same sample in a short period of time and subjected to GC-MS and organoleptic analysis. 

Two examples from the analysis of water samples illustrate how a separation and preconcentration can be accomplished simultaneously. In the gas chromatographic analysis for organophosphorous pesticides in environmental waters, the analytes in a 1000-mL sample may be separated from their aqueous matrix by a solid-phase extraction using 15 mb of ethyl acetate. After the extraction, the analytes are present in the ethyl acetate at a concentration that is 67 times greater than that in... 

Precision The precision of a gas chromatographic analysis includes contributions from sampling, sample preparation, and the instrument. The relative standard deviation due to the gas chromatographic portion of the analysis is typically 1-5%, although it can be significantly higher. The principal limitations to precision are detector noise and the reproducibility of injection volumes. In quantitative work, the use of an internal standard compensates for any variability in injection volumes. 

Monobasic acids are determined by gas chromatographic analysis of the free acids dibasic acids usually are derivatized by one of several methods prior to chromatographing (176,177). Methyl esters are prepared by treatment of the sample with BF.—methanol, H2SO4—methanol, or tetramethylammonium hydroxide. Gas chromatographic analysis of silylation products also has been used extensively. Liquid chromatographic analysis of free acids or of derivatives also has been used (178). More sophisticated hplc methods have been developed recentiy to meet the needs for trace analyses ia the environment, ia biological fluids, and other sources (179,180). Mass spectral identification of both dibasic and monobasic acids usually is done on gas chromatographicaHy resolved derivatives. 

Bromine ttifluoride is commercially available at a minimum purity of 98% (108). Free Br2 is maintained at less than 2%. Other minor impurities are HF and BrF. Free Br2 content estimates are based on color, with material containing less than 0.5% Br2 having a straw color, and ca 2% Br2 an amber-red color. Fluoride content can be obtained by controlled hydrolysis of a sample and standard analysis for fluorine content. Bromine ttifluoride is too high boiling and reactive for gas chromatographic analysis. It is shipped as a Hquid in steel cylinders in quantities of 91 kg or less. The cylinders are fitted with either a valve or plug to faciUtate insertion of a dip tube. Bromine ttifluoride is classified as an oxidizer and poison by DOT. 

The compound 2-amino-2, 6 -propionoxylidide was synthesized by saturating with gaseous ammonia at room temperature a suspension of 50 g (0.195 mol) of 2-bromo-2, 6 -propion-oxylidide in a mixture of 500 ml of 95% alcohol and 400 ml of concentrated aqueous ammonia. The saturation was carried out under mechanical stirring. After 25 hours the mixture was resaturated with ammonia gas. The stirring at room temperature was continued for a total period of 116 hours, and a sample was taken at that time. Gas chromatographic analysis indicated that about 95% of the bromo compound had been converted to the desired product. 

Gas chromatographic analysis showed this material to be at least 99% pure. The melting point of methyl 4-acetoxvbenzoate has been reported8 as 81-81.6°. In both their runs, the checkers obtained a distilled product which melted from 60° to 74°, resolidified at 74°, and then remolted at 78-79°. A sample recrystallized from hexane showed the same behavior. 

The purity of the product is greater than 99% as determined by gas chromatographic analysis using a 6-m. column of 30% Carbowax 20M on 60-80 Chromosorb W. The major impurity (<1%) was shown to be 3-heptanol by comparison of gas chromatographic retention times and mass spectral fragmentation patterns with those of an authentic sample. 

The checkers found that gas chromatographic analysis of one sample using a 305 cm. by 0.3 cm. column packed with 10% SF-96 on Chromosorb P operated at 70° with a 60 ml./minute helium carrier gas flow rate gave five minor impurity peaks, two at shorter retention times, and three at longer retention times. None of these impurities was present in greater than 1.1% total impurities wrere 3%. 

Ambrus A, Visi W, Zakar F, et al. 1981. General method for determination of pesticide residues in samples of plant origin, soil, and water. III. Gas chromatographic analysis and confirmation. J Assoc Off Anal Chem 64 749-768. 

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