Stripping analysis, automation

Eskilsson et al. [868] have described equipment for automated determination of traces of cobalt and nickel by potentiometric stripping analysis, which used a freshly prepared mercury film on a glassy carbon support as the working... 

The portable instrumentation and low power demands of stripping analysis satisfy many of the requirements for on-site and in situ measurements of trace metals. Stripping-based automated flow analyzers were developed for continuous on-line monitoring of trace metals since the mid-1970s [16,17]. These flow systems involve an electrochemical flow detector based on a wall-jet or thin-layer configuration along with a mercury-coated working electrode, and downstream reference and counter electrodes. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

L.B.O. dos Santos, J.C. Masini, Square wave adsorptive cathodic stripping voltammetry automated by sequential injection analysis potentialities and limitations exemplified by the determination of methyl parathion in water samples. Anal. Chim. Acta 606 (2008) 209—216. 

Potentiometric stripping analysis (PSA) is another commonly used technique in water analysis. This technique can usually be applied directly to the analysis of water samples without previous treatment, and it is virtually free from interferences of dissolved oxygen. Both, PSA and ASV techniques are based on the same principle the anal) e is first deposited on the electrode surface while the solution is stirred, and then stripped back to the solution in the measurement step [14,22,196]. The ASV technique works on a film electrode (electrochemically deposited mercury or gold on a glassy carbon support). One advantage of PSA is that it requires simpler equipment than ASV, and can compete with nonelectroanalytical techniques in terms of price, and possibility of automation [247-249]. This method has been applied to determine metals in tap water and rainwater samples [250-253], coupled with FIA to determine copper in natural waters [254,255], etc. In addition, portable PSA instruments have also been developed, and demonstrated to be useful for metals determination in aquatic samples [256-259]. 

Sequential injection analysis (SIA, Fig. 4.) is other flow methodology that has been coupled to on-line stripping analysis. The heart of SIA manifold is the multipart selection valve solutions are aspired and transported as zones using a bidirectional pump. SIA advantages are the low consumption of sample and reagents, the flexibility and the pwtential for automated sample manipulation (Ivaska Kubiak, 1997). The sample volumes used in SIA are smaller than those employed for continuous flow systems and FIA, the amoimt of analyte deposited is lower, thus yielding a decreased signal. 

Cool on-column injection is used for trace analysis. Ah. of the sample is introduced without vaporization by inserting the needle of the syringe at a place where the column has been previously stripped of hquid phase. The injection temperature must be at or below the boiling point of the solvent carrying the sample. Injection must be rapid and no more than a very few, usuahy no more than two, microliters may be injected. Cool on-column injection is the most accurate and reproducible injection technique for capihary chromatography, but it is the most difficult to automate. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

The first work on pKa determination by zone electrophoresis using paper strips was described by Waldron-Edward in 1965 (15). Also, Kiso et al. in 1968 showed the relationship between pH, mobility, and p/C, using a hyperbolic tangent function (16). Unfortunately, these methods had not been widely accepted because of the manual operation and lower reproducibility of the paper electrophoresis format. The automated capillary electrophoresis (CE) instrument allows rapid and accurate pKa determination. Beckers et al. showed that thermodynamic pATt, (pATf) and absolute ionic mobility values of several monovalent weak acids were determined accurately using effective mobility and activity at two pH points (17). Cai et al. reported pKa values of two monovalent weak bases and p-aminobenzoic acid (18). Cleveland et al. established the thermodynamic pKa determination method using nonlinear regression analysis for monovalent compounds (19). We derived the general equation and applied it to multivalent compounds (20). Until then, there were many reports on pKa determination by CE for cephalosporins (21), sulfonated azo-dyes (22), ropinirole and its impurities (23), cyto-kinins (24), and so on. 

The Dipex extractant shown in Scheme 9.1, as actinide-Resin, has not been used in an automated separation scheme. With two phosphoryl groups, this extractant is very effective at retaining actinides, but it is difficult to recover them by elution. For laboratory analysis, schemes have been developed to strip the extractant from the resin after capturing the actinides.151... 

Maxwell, T.J. and Smyth, WF. (1996) A study of the stripping voltammetric behavior of selected metal chelates and its application to automated analysis of river waters. 

Examples of Automated Sequence Analysis. Peptide samples for C-terminal sequencing were covalently attached to carboxylic acid modified polyethylene prior to sequence analysis and proteins were non-covalently applied to Zitex strips. 

BIO. Boulton, A. A., The automated analysis of absorbent and fluorescent substances separated on paper strips. Methods Biochem. Anal. 16, 327-393 (1968). 

Flow-injection analysis is also well-suited for the automation of anodic stripping voltammetry. Metals can be plated from the sample solution as it passes over the electrode. Stripping is then carried out in the deox-ygenated carrier stream (15, 34). The sample itself does not have to be deox-ygenated. Detection limits of 3 nM have been reported for lead by this technique (34). 

Froom P, Bieganiec B, Ehrenrich Z, Barak M. Stability of common analytes in urine refrigerated for 24 h before automated analysis by test strips. Clin Chem 2000 46 1384-6. 

Clearly, the preferred method to use for the analysis of phosgene depends upon the particular application to hand. For routine use in the laboratory, for monitoring the ambient air, impregnated paper strips and Dra ger tubes can be recommended as both reliable and easy to use. On the plant, automatic methods for continuous analysis would be appropriate, and one of the electrical or automated spectroscopic techniques would be suitable. For accurate measurement of very low (p.p.b.) concentrations however, a gas chromatographic procedure using one of the special detectors is most suitable. 

Among the devices that completely automated a biochemical analysis by microfluidic integration into one miniature piece of hardware, the test strips became the first devices that obtained a remarkable market share and still remain one of the few microfluidic systems which is sold in high numbers. 

Volatiles are usually analyzed by the use of purge and trap (which is an EPA-approved technique ), stripping, and headspace (HS) analysis. These methods either require expensive instmmentation or are not sufficiently sensitive. Nonvolatiles are analyzed primarily by means of LEE (also an EPA-approved technique) and SPE. These methods are generally time consuming, difficult to automate, and use expensive high-purity toxic organic solvents. 

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