Detection turbidimetric method

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

Physical methods for endpoint detection have been suggested. Hellsten [226] proposed an instrumental turbidimetric method to determine the endpoint, which does not need indicators. Since chloroform is emulsified by the anionic surfactant, changes in the optical density can be followed by a colorimeter thus detecting the endpoint when the emulsion breaks. Another turbidimetric method based on commercially available automatic titrators has also been proposed [227],... 

Although extended reaction time was also used by Selsted and Martinez (1980) in their turbidimetric method, they did not stress the importance of the kinetic order of cell lysis. McKenzie and White (1986) showed that, while the reaction is biphasic, simple kinetics are obeyed over a sufficiently long period that this property can be exploited in the determination. The kinetic order must be considered in the quantitative treatment of the results. The limit of detection was found to be 100 pg ml of reaction mixture, equivalent in the case of bovine milk lysozyme to 6 ng ml of skim milk, for a 50-/xl sample. 

Limit of Detection. Detection limits of approximately 10p,g/mL (lOmg/L) have been attained with routine nephelometric and turbidimetric methods this limit has been lowered by 10 to 100 fold using antibodies adsorbed onto latex particles. RID methods are able to detect down to a minimum of 10 to 20p.g/mL the detection limits have been lowered even further by using special low-level plates (Calbiochem-Behring Corp., La Jolla, Cahf.). Methods using labeled antibodies (e.g., RIA, EIA, chemiluminescence) are very sensitive and can measure levels as low as nanograms per milliliter level. 

Multicommutation also allows implementation of procedures for assessment of accuracy. For example, the same analyte can be determined by two different methods with the same flow network. In this way, a multicommutated flow manifold was proposed for chloride determination in natural waters with different sample matrices based on the spectrophotometric procedure using mercury(II) thiocyanate and the turbidimetric method using silver nitrate [19]. Moreover, in-line addition/recovery tests were implemented for every assayed sample in order to detect matrix effects. 

Fig. 3. Schematic of optical detection methods, (a) absorbance or colorimetric (b) turbidimetric and (c) nephelometric.

Nephelometric methods in general are more sensitive than turbidimetric assays and have an average lower limit of detection of 0.1 to 10 mg/L for a serum protein. Lower detection limits are obtained in fluids, such as cerebrospinal fluid and urine, because of their lower lipid and protein concentrations, which results in a better signal-to-noise ratio. In addition, for low molecular weight proteins (e.g., myoglobin, MW 17,800), assay detection Emits can be lowered using a latex-enhanced procedure based on antibody-coated latex beads. ... 

The alternative method to turbidimetric detection used for measuring solubility in early discovery is to quantify the aqueous supernatant directly via UV absorbance [13, 20, 21]. Typically, DMSO stock solution is added to aqueous buffer such that the final DMSO composition is kept to a minimum (5% or less) and the resulting precipitate is removed by filtration. A UV plate reader is then used to determine the aqueous solubility by comparing the filtrate absorbance against that of a calibration solution prepared in an identical solvent. It is important to match the sample and calibration solutions to prevent solvochromic effects. Care must also be taken in the selection of the filter plate since nonspecific binding of compound can occur with some filter materials leading to erroneously low solubility values [22], Like nephelometry, the plate-based UV detection approach is amenable to automation. 

The microbiological method using Lactobacillus phntarum, originally described by Snell and Wright (49), has been widely used for the determination of niacin in urine, serum, blood, foods, and tissues (16, 48,50,51,52). This organism is able to detect minute amounts of niacin (1-30 ng/mL) using a turbidimetric or titrimetric assay. 

Spectroscopic, luminescence, turbidimetric, and electrochemical methods of detection have been combined with SIA for the successful determination of amino acids, sugars, and trace elements in matrices such as meats, vegetables, breads, wines, juices, and milks. Many of these methodologies required sample pretreatment and whilst most performed this in an offline manner there have been some reports of online sample cleanup. Microwave assisted digestion was performed in-line for the determination of phosphorous, calcium, magnesium, and iron in slurried foodstuffs, wine, milk, and soft drinks whilst gaseous diffusion allowed interference removal for the determination of urea in milk. 

Zattoni, A. Loti Piccolontini, E. Torsi, G. Reschighan, P. Turbidimetric detection method in flow-assisted separation of dispersed samples. Anal. Chem. 2003, 75, 6469-6477. 

Standard deviations were higher when induction times were measured by microscopy. Significant differences between methods were detected (P < 0.01). Induction times determined by microscopy were not equivalent to those measured by laser turbidimetry. Crystals must reach a minimum size of 0.2 pm to be detected by microscopy, whereas the laser turbidimetric technique is able to detect smaller nuclei [5]. 

A turbidimetric end point has been found to make the two-phase titration easily automated (26). The indicator is omitted, and the titration is followed by monitoring the turbidity of the aqueous phase, which suddenly increases at the end point. The titration is, in other respects, similar to the standard method, although methanol is added to the system to optimize end point detection. Hendry and Hockings found that the organic phase could be omitted altogether, as described below under one-phase titration (27). Pinazo and Domingo performed a similar titration with only a small amount of chloroform finely dispersed by stirring. In this case, the end point corresponds to an inflection point on the turbidity curve (28). 

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