Direct injection method

Slower introduction of the dinitrogen tetroxide, by either dropwise addition of an ether solution or entrainment in a slow stream of nitrogen, gave similar results. The direct injection method was found to be easiest. The checkers distilled dinitrogen tetroxide at atmospheric pressure (b.p. 21°) prior to use. 

Of course, to be able to use the direct injection method of sample introduction, the analyte or the polymer system must be soluble in a solvent. Other methods of sample introduction need to be considered in order to eliminate the involatile material from the chromatographic separation. These have become extremely effective in the analysis of matrices such as polymers. 

The volatile fraction as defined by the various wet oxidation methods and most of the direct injection methods would be that fraction removed by acidification and purging with inert gas at room temperature. In the freeze-drying method of Gordon and Sutcliffe [29] the volatile fraction is that fraction lost by sublimation in vacuo. There have been no actual determinations of these losses, and for the most part Skopintsev s numbers were accepted as valid for all of these methods, largely because they are the only numbers available. 

Table 11.3 presents some typical results obtained by this procedure on seawater samples. MacKinnon [92] concluded that since the volatile organic carbon contents of normal (i.e., unpolluted) seawaters are small, the effect of complete or partial loss of volatile organic components during the determination of total organic carbon in most ocean areas (except highly reducing environments) with either the wet and direct injection methods or dry oxidation methods should be small (about 5%), and within the precision of these methods. 

For the red wines (82-84), which were injected directly into the HPLC without sample preparation, a ternary-gradient system using aqueous acetic acid (1% and 5% or 6%), and acidified acetonitrile (acetonitrile-acetic acid-water, 30 5 6) was used for cinnamic acid derivatives, catechins, flavonols, flavonol glycosides, and proanthocyanidins. Due to the large number of peaks, the gradient was extended to 150 min for the resolution of many peaks of important phenolics. This direct injection method was able to separate phenolic acids and esters, catechins, proanthocyanidins, flavonols, flavonol glycosides, and other compounds (such as tyrosol, and rrans-resveratrol) in wine in a single analysis. However, use of acetic acid did not permit the detector (PDA) to be used to record the UV spectra of phenolics below 240 nm (84). 

The following table provides data on the common salts used for salting out in chromatographic headspace analysis, as applied to direct injection methods and to solid phase microextraction.1 2 Data are provided for the most commonly available salts, although others are possible. Sodium citrate, for example, occurs as the dihydrate and the pentahydrate. The pentahydrate is not as stable as the dihydrate, however, and dries out on exposure to air, forming cakes. Potassium carbonate occurs as the dihydrate, trihydrate, and sesquihydrate however, data are provided only for the anhydrous material. The solubility is provided as the number of grams that can dissolve in 100 ml of water at the indicated temperature. The vapor enhancement cited is the degree of increase of the concentration of vapor over the solution of a 2% (mass/mass) ethanol solution in water at 60°C.3... 

Good reproducibility has been reported for capillary supercritical fluid chromatography using a direct injection method without a split restrictor. This method (Fig. 1.2(b)) utilises a rapidly rotating internal-loop injector (Valeo Inst. Switzerland) which remains in-line with the column for only a short period of time. This then gives a reproducible method of injecting a small fraction of the loop into the column. For this method to be reproducible the valve must be able to switch very rapidly to put a small slug of sample into the column. To attain this a method called timed-split injection was developed (Lee Scientific). For timed split to operate it is essential that helium is used to... 

Results for individual volatile acid concentrations in raw sewage determined by the direct injection procedure of Narkis and Henfield-Furie [578] and by that of Standard Methods [579], The results were also expressed as acetic acid for comparison with the collective total amount of organic acids determined by the Standard Method [579], The total amount of organic acids determined according to the Standard Method is higher than that found by the Narkis and Henfield-Furie [578] method. On average between 85 and 98% of the organic acids determined by the Standard Methods procedure were found to be volatile acids by the direct injection method. 

Gas Chromatographic Methods. Gas chromatographic methods may be used for measuring volatile oxidation products. Static headspace, dynamic headspace, or direct injection methods may be employed. Specific aldehydes may be measured as indicators for oxidative stability of oils and fats. Thus, propanal is an and as indicator for stability of omega-3 fatty acids, whereas hexanal is best for following the oxidative stability of omega-6 fatty acids. 

Integral Mass Spectra of LC-GPC Subfractions. The heavy oil from coal liquids consists of numerous components with wide differences in boiling point. The size of molecular ions changes progressively with the GC retention time or with the residence of the sample in the ion source for the direct injection method. Mass spectra were measured repeatedly at short interval times. The sum of these spectra represents the whole... 

Halothane in blood and tissues can be analyzed by gas chromatography after extraction into heptane (36, 43, 61-63). This method takes more time than the direct injection methods mentioned below, but it is not necessary to clean the injection port frequently, and column life may be extended. 

A direct injection method was proposed for phenolic acid extracts from plant tissue or soil, based on CZE at pH higher than the pK of the acids. Tetradecyltrimethylammonium bromide was added to reverse the electroosmotic flow. LOD was 1-7 p.M for eight phenolic acids at pH 7.20. ... 

A. Tangerman, Highly sensitive gas chromatographic analysis of ethanol in whole blood, serum, urine, and fecal supernatants by the direct injection method, Clin. Chem., 43, 1003-1009 (1997). 

Gas chromatographic (GC) methods have been used for determining volatile oxidation products. Static headspace, dynamic headspace or direct injection methods are the three commonly used approaches. These methods were compared in an analysis of volatile compounds in an oxidized soybean oil. It was found that each method produced significantly different GC profiles (Frankel 1985). The dynamic headspace and direct injection methods gave similar results, but the static headspace is more sensitive to low molecular weight compounds. Lee and co-workers (1995) developed a dynamic headspace procedure for isolating and analyzing the volatiles from oxidized soybean oil, and equations were derived from theoretical considerations that allowed the actual concentration of each flavor component to be calculated. 

This direct injection method was abandoned because such small volumes of concentrated, viscous silicate solutions were difficult to measure and dilute reproducibly into the molybdic acid. Also samples of the corresponding silicic acids were needed for the extractions into THF and for ultrafiltration studies ... 

Milk, being a two-phase substance, also creates some analytical difficulties for direct injection methods and therefore headspace techniques have been used to overcome these and to determine the odor components that have a bearing upon milk quality in processing and storage. 

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