Colorimetry detection limits

Simultaneous determination of both cations and anions in acid rain has been achieved using a portable conductimetric ion-exclusion cation-exchange chromatographic analyzer.14 This system utilized the poly(meth-ylmethacrylate)-based weak acid cation exchange resin TSK-Gel OA-PAK-A, (Tosoh , Tokyo, Japan) with an eluent of tartaric acid-methanol-water. All of the desired species, 3 anions and 5 cations, were separated in less than 30 minutes detection limits were on the order of 10 ppb. Simultaneous determination of nitrate, phosphate, and ammonium ions in wastewater has been reported utilizing isocratic IEC followed by sequential flow injection analysis.9 The ammonium cations were detected by colorimetry, while the anions were measured by conductivity. These determinations could be done with a single injection and the run time was under 9 minutes. 

Spectrophotometry (or colorimetry) has been used to measure chlorine dioxide in water using indicators that change colors when oxidized by chlorine dioxide. Spectrophotometric analyzers determine the concentration of chlorine dioxide by measuring the optical absorbance of the indicator in the sample solution. The absorbance is proportional to the concentration of the chlorine dioxide in water. Indicators used for this technique include jV,jV-diethyl-p-phenylenediamine, chlorophenol red, and methylene blue (APHA 1998 Fletcher and Hemming 1985 Quentel et al. 1994 Sweetin et al. 1996). For example, chlorophenol red selectively reacts with chlorine dioxide at pH 7 with a detection limit of 0.12 mg/L. The interferences from chlorine may be reduced by the addition of oxalic acid, sodium cyclamate, or thioacetamide (Sweetin et al. 1996). 

In addition, some metals may be determined by other methods, including ion-selective electrode, ion chromatography, electrophoresis, neutron activation analysis, redox titration, and gravimetry. Atomic absorption or emission spectrophotometry is the method of choice, because it is rapid, convenient, and gives the low detection levels as required in the environmental analysis. Although colorimetry methods can give accurate results, they are time consuming and a detection limit below 10 pg/L is difficult to achieve for most metals. 

The choice of analytical method is obviously largely dependent upon the availability of instrumentation. There are four techniques, however, which are used far more widely in the analysis of lead in environmental samples than other methods they are XRF, ASV, colorimetry with dithizone and AAS. Because of its versatility, ease of use and the low capital cost of equipment, atomic absorption is by far the most commonly used technique. Comments will be restricted to these four more important methods. Detection limits based upon experience, rather than manufacturer s literature are cited in Table 8.1. Obviously these may be an important determinant of the techniques selected for low-level work. 

The sensitivity of hybridization assays is a function of the detection limit of the label, some of which are not only as sensitive as radiolabelled assays, but are much faster in exposure times to X-ray film. There are four broad categories of detection for non-isotopic labels. These are bi-oluminescence, colorimetry, electrochemiluminescence, and fluorescence. 

Greater range of detection systems to which the desorbed gas can be subjected (e.g. chromatography, infra-red and ultraviolet spectroscopy, colorimetry) Limitations Certain resins undergo degradation even below 250°C Test sample may be thermally unstable Not all compounds readily desorb ... 

Aliphatic amines have been determined by a number of methods. Batley et al. [290] extracted the amines into chloroform as ion-association complexes with chromate, then determined the chromium in the complex colorimetri-cally with diphenylcarbazide. The chromium might also be determined, with fewer steps, by atomic absorption. With the colorimetric method, the limit of detection of a commercial tertiary amine mixture was 15ppb. The sensitivity was extended to 0.2 ppb by extracting into organic solvent the complex formed by the amine and Eosin Yellow. The concentration of the complex was measured fluorometrically. Gas chromatography, with the separations taking place on a modified carbon black column, was used by Di Corcia and Samperi [291] to measure aliphatic amines. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

Apart from 5 to 8% of all existing molecules that possess natural fluorescence, many others can be made to fluoresce by chemical modification or by association with a fluorescence molecule. By chemical reaction, a fluorophore reagent can be incorporated into the analyte (7-hydroxycoumarines can be used to this effect). This is called fluorescence derivatisation, a process analogous to that used in colorimetry. In HPLC, amines can be fluorescently labelled, permitting very low limits of detection to be achieved — in the order of attomoles (10-18 mol). 

Historically, analysis for selenium has been difficult, partly because environmental concentrations are naturally low. Indeed, selenium analysis still remains problematic for many laboratories at concentrations below 0.01 mg a relatively high concentration in many environments (Steinhoff et al., 1999). Hence, selenium has often been omitted from multi-element geochemical surveys despite its importance (Darnley et al., 1995). Analytical methods with limits of detection of <0.01 mgL include colorimetry, total reflectance-XRF, HG-AFS, gas chromatography... 

A number of EA exhibit a strong fluorescence upon irradiation. This feature is extensively used for the detection of EA after their separation by HPLC or TLC. Without any preceding separation, the fluorescence of ergot derivatives can also be used for the development of some fluorimetric assays (Gyenes and Bayer, 1961 Hooper et al., 1974). The limitation of all direct methods of EA determination including colorimetry, spectrophotometry or fluorimetry is the lack of specificity regarding related alkaloids or their own degradation products. Consequently, the majority of these methods has now been abandoned, but the specific features or reactions of EA are further employed for their detection by more sophisticated HPLC and TLC methods. 

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