Analytical reagents digestive

Instead of concentrated nitric acid, analytical-reagent grade 100 volume hydrogen peroxide may be used to effect decomposition of the sample. After each small addition of peroxide, most of the water formed should be driven off so that the digest does not become diluted and the condensate should be drawn off as it accumulates. 

Having as necessary, dried, homogenized or comminuted the samples, they must now be digested in a suitable reagent to extract elements in a suitable form for chemical analysis. In many organizations we have reached the point where the analyses pass from the hands of the person who took the sample to those of the analytical chemist. In the author s experience, however, it must be emphasized that to ensure best-quality results the whole procedure from, for example, statistically sampling a sediment to the final chemical analysis, should be handled by the same person. 

The TRAACS 800+ shown in Fig. 2.18 consists of one or more analytical consoles, a random-access sampler and personal computer (PC). The analytical console is fitted with two peristaltic pumps which can each handle up to 10 pump tubes. The analytical manifolds are located above the pumps and utihze glass parts 1 mm in diameter, in order to keep reagent consumption low. Heating baths, UV digesters and dialysers can be built into the manifold. 

Usually, samples are presented for analysis as liquids. Thus, solid samples must be dissolved. Analytical or ultra-high-purity grade reagents must be used for dissolution to prevent contamination at trace levels. Certain volatile metals (e.g. cadmium, lead and zinc) may be lost when dry ashing, and volatile chlorides (e.g. arsenic and chromium) lost upon wet digestion. It is particularly easy to lose mercury during sample preparation. Appropriate steps must be taken in the choice of method of dissolution, acids and conditions (e.g. whether to use reflux conditions) to prevent such losses. 

The laboratory sample is usually dissolved for analysis. It is important to dissolve the entire sample, or else we cannot be sure that all of the analyte was dissolved. If the sample does not dissolve under mild conditions, acid digestion or fusion may be used. Organic material may be destroyed by combustion (also called dry ashing) or wet ashing (oxidation with liquid reagents) to place inorganic elements in suitable form for analysis. 

Type of sample Determinand Dry ash acid digestion reagent Analytical finish Reference... 

In a further method, sample digests are prepared according to method 1(c) of the Analytical Methods Committee [ 100] using precautions described subsequently [101]. The resulting 100 ml of digest, which is in normally 5% v/v sulfuric acid, should not be colourless and should contain any suspended solids. At the same time, prepare two reagent blanks from the volume of acid used in sample oxidation. 

Speed. For applications where a great number of analyses must be performed on a routine basis, speed is perhaps the most important criterion for a system. Analytical procedures which often take more time than the actual analysis itself include such sample pretreatment procedures as dissolution and digestion. If these pretreatment steps could be kept to a minimum, analysis time would be cut down considerably. Also, mimimum pretreatment reduces the chances of sample contamination by the reagents used, or the loss of volatile elements during pretreatment. Thus the ideal system would require no sample pretreatment at all real systems should strive for minimum sample pretreatment. 

The results confirmed that the ancillary materials used in an analytical procedures might contribute to blank values more than reagents themselves do, especially glassware and microwave digestion systems. Although the latter have been extensively employed to shorten the time required for sample dissolution, some problems have been ascribed to this procedure, as reported by Lima et al. [8],... 

Teflon beakers or test tubes on a hot plate [25-27], Closed digestion systems produce pressures above atmospheric which provide higher temperatures and facilitate the complete oxidation of the samples digestion time and reagent consumption can be also reduced in this way. In addition, loss of volatile elements is minimized and the rate of digestion is increased, yielding low analytical blanks. 

Note 7. Using freshly prepared acetyl bromide solution made up in analytical grade acetic acid, the absorbances at 280 nm of the digested samples were constant after standing at 20°C for periods up to 30 min. This is contrary to the findings of van Zyl (1978) and Cho (1984), who observed that when freshly prepared reagents were used, the absorbances were not stable over this period. 

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