Reagent carry-over

Metal alkyls can be prepared in a simple manner from the main group halides (X = Cl, Br, I) and the appropriate alkyl Grignard reagent (RMgl) or the alkyllithium salt (RLi), as shown for the cadmium alkyls (Equation (2)).13 The elimination of impurities from the precursor source is of great importance, as any remaining impurities are invariably carried over into the growing semiconductor layers. Incorporation of impurities, even at levels as low as 1015 free carriers per cubic centimeter (one part in ca. 107), can drastically affect the electronic properties of the... 

These analysers resemble hquid and gas chromatographs, although their background is markedly different. They differ from discrete automatic analysers (DAA) in various respects, namely the fashion in which samples are transported and mixed with diluents and reagents, the manner in which carry-over between samples and reagents is avoided and the type of detection used. 

Fig. 14 a. Schematic arrangement for Flow Injection Analysis for a simple spectrophotometric determination, b A typical experimental output from such a system, showing lack of carry over even with large samples. (Metal ion Bismuth Reagent Pyrocatechol violet, 5 x 10 3 M pH 2-4 Sample size 200 pi Analysis rate 80 hr-1)... 

U, Np and Pu at 6 M nitric acid concentration, then reduce the acidity to slightly below 0.1 M and extract the remaining actinides, mainly Am and Cm, at this lower acidity. After the removal of Pa, U, Np and Pu it was found that the excess nitric acid could easily be removed by extraction with a 50% TBP (tri-n-butyl phosphate) solution in Nysolvin, thus avoiding some of the difficulties, such as slow reaction rates and crud formation, encountered when using formic acid for the destruction of the excess nitric acid. The CTH process can thus basically be divided into three fairly independent extraction cycles, see Figure 2. The order in which the two extractants are used is such that any minor carry-over of reagent from an earlier cycle to the next will not impair the operation of any cycle. 

Make several reagent blank determinations, using only the chromium trioxide solution in the above procedure. The ratio of the sodium hydroxide titration (Vb) to the sodium thiosulfate titration (7b), corrected for variation in normalities, will give the acidity-to-oxidizing ratio, V b/Kb = K, for the chromium trioxide carried over in the distillation. The factor K should be constant for all determinations. 

Alternatively, place a drop of the test solution (or a few milligrams of the test solid) in the reaction bulb of Fig. 11.56, add 2 thin pieces zinc foil and 2-3 drops dilute sulphuric acid. Place a small circle of quantitative filter paper (or drop-reaction paper) moistened with acetate-benzidine acetate reagent across the funnel. The paper is coloured blue by the hydrocyanic acid carried over with the hydrogen. 

Within the last few years, there have been several instances in which independent brick liners have experienced problems in resisting the effects of wet flue gases. In certain instances the problems have been due to actual deterioration of mortar and/or brick which were subjected to chemical attack by certain constituents of either the flue gas itself or carry over" reagents from the flue gas desulfurization system. In general, the commonly used silicate mortars for chimneys are quite resistant to a wide range of acids and actually thrive in a wet acid environment. However, certain acids, such as hydrofluoric acid, and most... 

Digitally controlled autosamplers that incorporate a loop injector are available commercially. These sophisticated devices are extremely precise and can be programmed for continuous and automated operation. In addition, the sample loop is flushed automatically with the mobile phase between samples to prevent sample carry over. The ability to inject multiple aliquots from a single sample vial or to add reagents from designated vials to derivatize the analyte just before injection are additional features of many autosamplers. 

Perchloric or sulphuric acid is usually used in the distillation. HCIO4 is more suitable since it neither complexes nor precipitates metal ions. However, perchloric acid must not be used in the presence of organic substances for fear of explosion. When HF is distilled from sulphuric acid medium, minute amounts of H2SO4, carried over to the distillate, can interfere in the determination of fluoride by complexing Zr or Th if these are used as reagents. The sample volume can be reduced by evaporation from a platinum crucible, after the sample has been made slightly alkaline. 

One of the main requirements of using GC-MS for the determination of metals is the preparation of a suitable metal chelate, which should have adequate thermal stability and should also be volatile. Further, the metal chelate should not introduce any memory effect or carry-over effect when injecting sequential samples of different isotopic compositions. We have been able to identify suitable metal chelates for the toxic metals Pb, Hg, Cd and As. Another requirement for the determination of toxic metals in biological samples is the development of a suitable digestion procedure, without introducing any contaminants from the reagents employed. 

If the damaged strand is carried over into a new reaction vessel, it is prevented from functioning as a template for amplification. Effective sterilization requires the use of these reagents at concentrations that are tailored to the length and sequence of the target and the level of amplification (Table 3). 

To summarise, microwave sample preparation reduces blank contribution from environmental exposure, reagent use and losses from evaporation, and in addition, reduces sample preparation times. It also offers a more reproducible method for duplicating metals determination, both within a laboratory and between laboratories (Lamble Hill 1998). But, in the case of contaminated soil sample digestions, due to surface effects of PTFE, a microwave-cleaning step is highly recommended between each bath of samples in order to avoid any memory effects (carry over contamination) from previous digested samples. 

Carry-over effects can also be classified according to which Ingredients should not be mixed. Thus, in principle, there can be cross-contamination between samples, reagents or both (Table 3.2). 

In order to avoid carry-over as far as possible in multi-determinations with batch analysers, the determinative sequence chosen should be that which results in the smallest effects possible from the preceding reagents. 

Automatic air-segmented analysers are characterized by the use of one or several liquid streams (diluents, washing solutions, reagents) where the sequentially aspirated samples are introduced and spaced by means of air bubbles aimed at avoiding the undesirable carry-over [10-12]. 

Carry-over arising from the contact between samples, species and reagents should be kept below detectable levels. Alternatively, a computer can be used to compensate for mutual contamination. A logical and dual solution to this problem involves minimizing contact between surfaces by using disposable pipettes, cuvettes, etc., and carefully washing and drying re-usable labware between samples. 

The coil length depends primarily on the speed of reaction and the sample plus reagent flow-rates, and varies considerably in different applications. However, extremely long reaction coils create excessive impedance, as well as carry-over problems in the... 

---