Interfering substances

The most common interfering substance, especially with alcohols of low mole cular weight, is water this may result in an inaccurate interpretation of the test if applied alone. Most of the water may usually be removed by shaking with a little anhydrous calcium sulphate,. though dry ethers (and also the saturated aliphatic and the simple aromatic hydrocarbons) do not react with sodium, many other classes of organic compounds do. Thus ... 

Ozone can be analyzed by titrimetry, direct and colorimetric spectrometry, amperometry, oxidation—reduction potential (ORP), chemiluminescence, calorimetry, thermal conductivity, and isothermal pressure change on decomposition. The last three methods ate not frequently employed. Proper measurement of ozone in water requites an awareness of its reactivity, instabiUty, volatility, and the potential effect of interfering substances. To eliminate interferences, ozone sometimes is sparged out of solution by using an inert gas for analysis in the gas phase or on reabsorption in a clean solution. Historically, the most common analytical procedure has been the iodometric method in which gaseous ozone is absorbed by aqueous KI. 

When the sample contains <0.1% selenium or if interfering substances are present, selenium may be preconcentrated by distillation from a bromine—hydrobromic acid mixture ... 

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Spectrophotometric deterrnination at 550 nm is relatively insensitive and is useful for the deterrnination of vitamin B 2 in high potency products such as premixes. Thin-layer chromatography and open-column chromatography have been appHed to both the direct assay of cobalamins and to the fractionation and removal of interfering substances from sample extracts prior to microbiological or radioassay. Atomic absorption spectrophotometry of cobalt has been proposed for the deterrnination of vitamin B 2 in dry feeds. Chemical methods based on the estimation of cyanide or the presence of 5,6-dimethylben2irnida2ole in the vitamin B 2 molecule have not been widely used. 

Analytical and Test Methods. Aqueous titration with 1 AiNaOH remains the official method for assaying citric acid (39,40). Although not citrate-specific, the procedure is satisfactory in the absence of interfering substances. Low concentrations of citric acid can be deterrnined by a... 

Water vapor is a serious interfering substance in this technique. A moisture trap such as a drying agent or a water vapor condenser is required to remove water vapor from the air to be analyzed. 

If there is reason to think that interfering substances may be present, it is advisable to sample them to determine whether their concentrations are sufficiently high to actually constitute an interference. It is very important to establish that an instrument responds properly to the substance it is designed to sample. This is generally done by calibration procedures with standard concentrations of the substance of interest. 

More sensitive detection methods and more objective recording methods (e g the employment of scanners) are constantly been striven for m order to overcome this illusion It IS for this reason too that fluorescent methods have been introduced to an increasing extent on account of their higher detection sensitivity This allows an appreciable reduction in the amount of sample applied, so that possible interfering substances are also present m smaller quantibes This increases the quality of the chromatographic separation and the subsequent m situ analysis... 

Trace enrichment and sample clean-up are probably the most important applications of LC-LC separation methods. The interest in these LC-LC techniques has increased rapidly in recent years, particularly in environmental analysis and clean-up and/or trace analysis in biological matrices which demands accurate determinations of compounds at very low concentration levels present in complex matrices (12-24). Both sample clean-up and trace enrichment are frequently employed in the same LC-LC scheme of course, if the concentration of the analytes of interest are Sufficient for detection then only the removal of interfering substances by sample clean-up is necessary for analysis. 

Both ammonium and potassium thiocyanates are usually available as deliquescent solids the analytical-grade products are, however, free from chlorides and other interfering substances. An approximately 0.1M solution is, therefore, first prepared, and this is standardised by titration against standard 0.1 JVf silver nitrate. 

The reagent is employed for the determination of ammonia in very dilute ammonia solutions and in water. In the presence of interfering substances, it is best to separate the ammonia first by distillation under suitable conditions. The method is also applicable to the determination of nitrates and nitrites these are reduced in alkaline solution by Devarda s alloy to ammonia, which is removed by distillation. The procedure is applicable to concentrations of ammonia as low as 0.1 mgL-1. 

Treat the colourless solution (ca 15mL), free from interfering substances and about 1M in sulphuric acid, with 1 mL of 30 per cent hypophosphorous acid solution and 1 mL of 10 per cent aqueous potassium iodide solution. Dilute to 25 mL and match the yellow colour produced against standards containing the same concentrations of sulphuric acid and hypophosphorous acid. Alternatively, measure the absorbance at or near 460 nm or with a blue filter. 

When large quantities of interfering substances are present, it is usually best to proceed in either of the following ways (1) remove the iron by precipitation with a slight excess of ammonia solution, and dissolve the precipitate in dilute hydrochloric acid (2) extract the iron(III) thiocyanate three times either with pure diethyl ether or, better, with a mixture of pentan-l-ol and pure diethyl ether (5 2) and employ the organic layer for the colour comparison. 

With flame emission spectroscopy, there is greater likelihood of spectral interferences when the line emission of the element to be determined and those due to interfering substances are of similar wavelength, than with atomic absorption spectroscopy. Obviously some of such interferences may be eliminated by improved resolution of the instrument, e.g. by use of a prism rather than a filter, but in certain cases it may be necessary to select other, non-interfering, lines for the determination. In some cases it may even be necessary to separate the element to be determined from interfering elements by a separation process such as ion exchange or solvent extraction (see Chapters 6, 7). 

Extraction of the analyte or of the interfering element(s) is an obvious method of overcoming the effect of interferences . It is frequently sufficient to perform a simple solvent extraction to remove the major portion of an interfering substance so that, at the concentration at which it then exists in the solution, the interference becomes negligible. If necessary, repeated solvent extraction will reduce the effect of the interference even further and, equally, a quantitative solvent extraction procedure may be carried out so as to isolate the substance to be determined from interfering substances. 

Complete resolution was not achieved due to the carryover of interfering substances which frequently occurs when separating the components of biological samples. The column carried a reverse phase, but as the mobile phase contained low concentrations of lauryl sulfate, some would have adsorbed on the surface of the stationary phase and significantly modified its interacting properties. The retention mechanism is likely to have involved both ionic interactions with the adsorbed ion exchanger together with dispersive interactions with any exposed areas of the reverse phase. 

This, on the one hand, reduces the detection Umit so that less sample has to be applied and, thus, the amounts of interfering substances are reduced. On the other hand, the linearity of the calibration curves can also be increased and, hence, fewer standards need to be applied and scanned in routine quantitative investigations so that more tracks are made available for sample separations. However, the introduction of a large molecular group can lead to the equalization of the chromatographic properties. 

The methodology selected for the laboratory of Neonatology needs to have as its prime objective, accuracy, that Is spec-ificity for what it is analyzing, this automatically eliminates many of the commercial automatic systems which are In common use, and which do not process the specimen adequately to remove interfering substances before analysis. The methodology discussed below is selected as being applicable to the type of specimen that is supplied to the analyst in the laboratory of Neonatology. 

Greater specificity since the rate of the reaction usually is not affected by interfering substances commonly found in serum, and since each sample serves as its own blank. 

End-point methods are often not based on kinetic-ally optimum conditions. However, an end-point method is often the only convenient one available. In this case, the method should have been validated by showing that the catalysis of the substrate follows well defined kinetics, rate of reaction is proportional to enzyme concentration, blanks and interfering substances are corrected for, and that appropriate standards are available. 

Presence of interfering substances 4.2 Tests for bactericidal activity... 

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