Colorimetric procedures, detection

This method was developed as a second independent method to complement the usual colorimetric procedure in the determination of a certified concentration of dissolved silica in a seawater reference material. Ion exclusion affords a separation of the dissolved silica not only from major seawater cations but also from potentially interfering anions. The detection unit limit, conservatively estimated at 2.3 ng/g Si (0.08. im), is superior to that achievable by direct analysis using inductively coupled plasma emission spectrometry. 

GLC, atomic absorption and mass spectrophotometry, enzymatic, and specific colorimetric procedures seem to be the likely candidates for routine use in the future. Automation will certainly be common. GLC is now used to detect imitation muscat wines (127). Characteristic flavor byproducts of yeasts may be detected and measured. Multiple correlation of the amounts of the more influential major and minor constituents with wine quality is the goal of such research. A simple apparatus for the simultaneous determination of the redox potential (two platinum electrodes), pH, specific conductivity, oxygen, and carbon dioxide (ion-specific electrode) has been devised (128). Molecular oxygen in wines has been determined by several procedures—polarography (129) and GLC being the latest. 

Another sensitive colorimetric procedure is that of Mackenzie et al (1967), which utilizes the dye Rhodamine B to form benzene-soluble complexes with fatty acids. Nakai et al. (1970) developed a rapid, simple method for screening rancid milk based on the foregoing procedure. The test is said to detect rancid milk with an ADV above 1.2. Like the copper or cobalt soap method, the Rhodamine B reagent is also limited to the longer-chain fatty acids. Kason et al (1972) used the method employing Rhodamine 6G of Chakrabarty et al (1969) to investigate... 

Perhaps the most exacting TLC studies were carried out by Simon et al43. phenytoin was extracted frcm plasma, concentrated by evaporation, and subjected to TLC on silica gel plates. After the phenytoin spot was scraped off, the colorimetric procedure of Dill et al4 was applied directly to the scrapings. Phenytoin levels of 5 ug/ml or more could be recognized by visual inspection of the plates under ultraviolet light, and serum levels of 1 ug/ml were detectable with the colorimetric procedure. 

The analysis of PCR products is usually carried out by electrophoresis, and the desired product is identified by length. This is not necessarily the best approach for large scale routine analysis. Alternative sequence-specific procedures are available in which the product is hybridised to labelled oligonucleotides that can be detected by fluorimetric antibody-coupled colorimetric procedures. 

Garleb, K.A., Bourquin, L.D. and Fakey, G.C. 1991. Galacturonate in pectic substances from fruits and vegetables comparison of anion-exchange HPLC with pulsed amperometric detection to standard colorimetric procedure, J. Food Sci., 56 423 -26. 

The improvement of UV-visible spectrophotometers has led to a renewal in the colorimetric procedures, actually absorptiometric, with the possibility of simultaneous metallic compounds determination. Based on the reaction between a ligand and some metallic elements, these complexometric procedures use absorptiometry for final detection. The characteristics of these methods are also improved with regard to classical colorimetric ones, but they often need some pretreatment steps for preconcentration or interferences removal. 

Unfortunately, at this point, interpreting results for the electrode measurement of salicylate in blood samples is complicated by the fact that a large fraction of the total salicylate is bound to proteins (22, 23). The Sn(TPP)Cl2 based membrane electrode detects "free" salicylate while the conventional colorimetric procedure (i.e. the Trinder method (21) or variations thereof) measures total salicylate concentration (free plus bound). The former is the physiologically active form of the compound (24). Thus, the salicylate selective electrode could provide a new analytical tool for scientists who are examining the pharmacological effects of aspirin and other... 

Oxidant residuals in the form of total residual chlorine were first reported by the New York State Department of Environmental Conservation in rainfall and lake waters of the Western Adirondacks of New York State in 1976-77. Chlorine residuals analyzed by the orthotoline colorimetric procedure developed by Palin (1957) were detected in rainfall at Boonville and Stillwater, New York and in Echo Lake, Hinckley Reservoir and Stillwater Reservoir (Figure 2. Total residual chlorine in rainfall near Boonville, New York... 

Hunkdr s (1938) procedure for oxidation of methyl alcohol to formaldehyde by bromine in the presence of sugar is capable of detecting only 5% of the alcohol. A colorimetric procedure for methyl alcohol in vinegar was developed by Piccoli (1933). Jauker (1937) tested various procedures and used a semiquantitative method said to be sensitive to 0.0001%. 

Flanzy and Banos (1938) also isolated isopropyl alcohol from the fusel oil of a wine. They calculated that wines contain at least 6.6 mg. per liter. Metra et al, (1938), however, were unable to detect this alcohol in samples from wines or from the heads using a colorimetric procedure reputedly sensitive to 0.01% of this alcohol. They concluded that isopropyl alcohol may be present only in traces, if at all. In the fraction of the heads boiling at 27.8° to 30° C. (82° to 86° F.) only sec-butyl alcohol was found in a wine distillate by Durodie and Roelens (1942) and isopropyl alcohol was not detected. 

This monograph gives both the color reactions which the authors have found suitable during their long practice, and a series of reactions taken from the literature. The procedures are formulated so that the reaction could be carried out with pure compound. It is not excluded that many analysts will be compelled to adjust the procedures to suit their own conditions. We have compiled a very large number of color reactions, as it is often necessary to detect even a common substance in a medium which does not allow the use of any of the common reagents. Very often references to colorimetric applications are also given. This is because in very delicate or controversial cases it is best to carry out a quantitative procedure, i.e., a colorimetric procedure (but without the colorimetric evaluation). 

It is important to be able to determine which chromatography column fractions contain polysaccharides and, specifically, which fractions contain hexosyl, pentosyl, or uronosyl residues. It is also important to detect in the fractions the presence of proteins and the presence of the specific amino acid characteristic of wall proteins, hydroxyproline. The detection of these substances is carried out by facile and sensitive colorimetric procedures. Although these reactions are not discussed here, the most frequently used colorimetric assays in our laboratory are the anthrone assay for detection of hexosyl residues 50), the orcinol assay for detecting pentosyl residues 50), the m-hydroxy-diphenyl assay for detection of uronosyl residues 35), the Lowry assay for detection of proteins 90), and the Kivirikko and Liesmaa assay for the detection of hydroxyprolyl residues 76). 

The earliest methods for detection and assay of sialic acids were colorimetric procedures that capitalized on the more or less specific chromogens derived from these carbohydrates. Over the years a variety of colorimetric agents have been employed orcinol (Klenk and Langer-beins, 1941), diphenylamine (Ayala et aL, 1951), hydrochloric acid... 

Analytical methods A variety of procedures have been reported for the measurement of ureides, including those based on the detection of urea and, in the case of ALN and ALA, on the selective formation and subsequent determination of glyoxalate. Methods for measuring substituted ureas include enzymatic or chemical degradation to form urea followed by the urease-catalyzed conversion of urea into carbon dioxide and ammonium with subsequent analysis (e.g., Nessler reaction, dichloroindophenol method) of the ammonium or colorimetric procedures such as the diacetylmonoxime reaction (which detects the carbamyl group—CONH2). The use of specific enzymes confers a degree of specificity to the former procedures. The latter procedure, however, detects ureides as a class of reactive compounds and does not differentiate between various ureido compounds. 

Transposed laboratory methods - This first group is historically the most important as the first developments were carried out in that way. All colorimetric systems using automatic sampling feeding a fast reaction/detection line (for example, with a flow-injection procedure) have been developed from classical procedures, first to increase the analytical rate in laboratories before being transposed for off-line measurement. 

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