Spot Test Limitations

There are chemicals that can be used as indicators that are sensitive to low concentrations of lead and also reasonably selective. Based on field experience, there is considerable confidence in a sodium sulfide spot test procedure for testing for lead in paint. On completing further studies being supported by HUD and the EPA, more definitive statements about chemical spot field tests should be possible. 

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Figures 1 and 2 show examples of the types of results obtained. In the spot test (Fig. 1), a small amount of the chemical to be tested is placed in the center of the dish, the chemical diffused out into the agar, and revertant colonies appear as a cloud around the central spot. This method is somewhat limited in sensitivity(3) but is extremely rapid. Normally, one tests individual dose levels of a chemical in the more sensitive plate test, and quantitative dose-response curves can be generated as shown in Figure 2. The initial portion of these curves is almost always linear, and most mutagens are detected at nanogram dose levels.

Li or a Li compound in the flame gives a bright crimson color due to its emission of670.8 nm photons produced by the short-lived species LiOH. This is the property that allows for the spectrophotometric determination of Li by atomic absorption spectroscopy (AAS) down to 20 ppb. Inductively-coupled plasma emission spectroscopy (ICPAES), inductively-coupled plasma mass spectroscopy (ICPMS), and ion chromatography (IC) improve this limit to about 0.1 ppb. A spot test for detection of Li down to 2 ppm is provided by basic KIO4 plus FeCl3. 

Analysis. Na or a Na compound in the flame gives a bright yellow color due to its emission of 589.0 and 589.6 nm photons. This is the property that allows for the spectrophotometric determination of Na by emission or absorption flame or plasma spectroscopy. AAS and ETAAS are capable down to 10 ppb. ICPAES extends this to 1 ppb, and a limit of detection of 0.1 ppb can be obtained by ICPMS and IC. A spot test for 250 ppm or more can... 

Analysis. K or a K compound in the flame gives a reddish-violet (lilac) color. This is due to its emission of404.4,404.7, 766.5, and 769.9 nm photons. These photon lines allow for the determination of K by AA (limit of detection = 20 ppb), ETAAS (1 ppb), and ICPAES (0.1 ppb). This latter limit is also attainable by ICPMS and IC. Spots tests with sodium hexanitritocobaltate(III) and dipicrylamine both detect 100 ppm of K. 

Analysis. The colorimetric method for In is capable of a detection limit of 20 ppb. Indium or an In compound in the flame gives an indigo blue color (451.1 nm). This photon line allows for the spectrophotometric determination ofinby AAS (atomic absorption flame spectroscopy). The method is sensitive to about 300 ppb. With ETAAS, this limit drops to 10 ppb, as it does with ICPAES. ICPMS drops the limit to 0.01 ppb. Alizarin detects In, as well as Al, but the reaction with Al can be masked by addition of F to a spot test. The limit of detection is about 1 ppm. 

Analysis. Colorimetric analysis of Pb with dithizone can be performed down to 200 ppb, with ETAAS and ICPAES going down to 1 ppb, and ICPMS to 0.1 ppb. Spot testing to a detection limit of almost 1 ppm can be carried out using dithizone and KCN. 

The spot-test technique, for which the reaction is particularly well adapted, is as follows. Place a drop of the nearly neutral or faintly acid test solution on a spot plate, introduce a minute crystal of potassium xanthate, followed by 2 drops of 2m hydrochloric acid. An intense red-violet colouration is obtained. Sensitivity 0-04 pg Mo. Concentration limit 1 in 250,000. 

The spot-test technique is as follows. Mix a drop of 15-20 per cent sulphuric acid and a drop of the test solution either on a spot plate or in a porcelain micro crucible. After a few minutes add 1 drop of 3 per cent hydrogen peroxide solution and then another drop, if necessary. A red to pink colouration appears. Sensitivity 2 5 xg V. Concentration limit 1 in 20,000. 

Atomic absorption spectrometry with flame (AA-F) or electrothermal atomization furnace (AA-ETA), inductively coupled plasma-emission spectroscopy (ICP-ES), inductively coupled plasma-mass spectrometry (ICP-MS), and high-performance liquid chromatography-mass spectrometry (LC-MS) are state-of-the-art analytical techniques used to measure metals in biological fluids. They are specific and sensitive and provide the cfinical laboratory with the capability to measure a broad array of metals at clinically significant concentrations. For example, ICP-MS is used to measure several metals simultaneously. Photometric assays are also available but require large volumes of sample and have limited analytical performance. Spot tests are also... 

Paper chromatography was another popular method of studying organic water constituents (Shapiro, 1957 Packham, 1964 Midwood and Felbeck, 1968) and also was of limited value in characterizing humic substances and colored organic acids, because the complex mixtures often produced broad diffuse bands and streaks. Some of the spot-tests and chromatograms of Shapiro (1957) demonstrated the presence of colorless acids and suggested the presence of phenols and enols. 

Using a new method, the catalytic kinetic method of analysis, Beklemishev et al. (1997) measured the concentrations of manganese in tap and river water. Their analytical method relies on an indicator reaction that is catalyzed by Mn(Il) (the oxidation of 3,3, 5,5 -tetramethylbenzidine [TMB] by potassium periodate [KIO4]) and is carried out on the surface of a paper-based sorbent. The advantages of this new technique are that it has a much lower detection limit (0.005 pg/L) than do established methods and is transportable, allowing it to be used for rapid tests in the field (i.e., spot tests and similar procedures). 

Spot tests for the in-field detection of mancozeb in water use copper (II) chloride-acetic acid as a coloring reagent. A cupric salt is formed which is soluble in chloroform extractant to produce a red-brown solution this method has a detection limit of 15 pg (Rathore et al. 1996). 

Before dealing with the apparatus required for spot test reactions, it is necessary to define clearly the various terms which are employed to express the sensitivity of a test. The limit of identification is the smallest amount recognizable, and is usually expressed in micrograms (pg), one microgram being one-thousandth part of a milligram or one-millionth part of a gram ... 

These qualitative methods of analysis are dependent upon characteristic chemical reactions of herbicides to produce colored compounds or metal complexes (see Table 5). The detection limit of these spot tests ranges from micrograms to milligrams. 

In this text, the limits of identification and the dilution limits which can be attained by the respective spot tests are given immediately after the outline of the various procedures. A tabular summary (Chapter 6) presents the identification limits of a large number of the more important spot reactions. If the drop volume, which ordinarily is around 0.05 ml, is taken into account, the dilution limits corresponding to the identification limits can be readily computed. 

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