Pretreatment techniques irradiation

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

Table 7.1 summarises the different techniques of final determination used pretreatment techniques were based on digestion with combination of acids in pressurised or atmospheric mode, programmed dry ashing, combustion and irradiation with thermal neutrons. A detailed description of the methods is given in the certification report [1]. 

The techniques used by the participating laboratories (see section 8.1.4) are summarised in Table 8.2. Pretreatment techniques included dilution, addition of HF (for Al complexa-tion), irradiation with thermal neutrons (for NAA), addition of internal standards (for ICPMS), addition of buffer (for titration) etc. 

Many pretreatment techniques are used in practice (Table 8.2). The normal physical method used to improve the adhesive strength of the coating to the substrate is to slightly roughen the surface by solvent treatment, abrasion, or blasting. Some plastics (e.g., polyolefins) require special pretreatment methods processes that modify the surface molecular layers of the plastic to increase their polarity have proved suitable (e.g., flaming, immersion in an oxidizing acid, immersion in a benzophenone solution with UV irradiation, corona treatment, plasma treatment). 

Photochemical operations offer several routes of hydroxyl radical formation by UV irradiation. The formation of hydroxyl radicals by irradiation of samples doped with hydrogen peroxide or ozone is the state-of-the-art in water treatment. Two comprehensive reviews cover the historical development of the UV photo-oxidation technique as a pretreatment step in the inorganic analysis of natural waters, its principles and the equipment available, and its principal applications in the analytical field.3,4 They include tables summarizing the elements determined, the analytical techniques used, and the sample matrices studied. 

Analytical determinations of heavy metals in the atmospheric particulate matter are performed using INAA and ETA-AAS. INAA is a multi-elemental and nondestructive technique. Using such a technique, it is possible to analyze the sample without any kind of pretreatment, which avoids the possibility of additional contamination and, at the same time, allows for a high degree of sensitivity. However, there are some heavy metals of a great interest, such as Cd, Ni and Pb, that cannot be determined with sufficient precision due to spectral interference or are insensitive to the neutron irradiation (e.g., Pb). These are analyzed by means of the ETA-AAS technique. 

Each laboratory which participated in the certification made a minimum of five independent replicate determinations. The techniques used by the participating laboratories are summarised in Table 8.20 they were based on different pretreatment procedures (e.g. complexation/extraction for AAS, UV irradiation for ASV) and detection techniques (e g. ETAAS, ICPAES, ICPMS, DPASV). 

Two techniques were selected for certification, namely cold vapour atomic absorption spectrometry (CVAAS) and cold vapour atomic emission spectrometry (CVAES) applied after a variety of sample pretreatment, e.g. oxidation with BrCl or Kmn04, reduction with SnCl2, UV-irradiation, gold preconcentration etc. 

Neutron activation analysis has the advantage that virtually no pretreatment of tissue samples is necessary before irradiation. Operations after irradiation will not be affected by the problem of contamination since only the radioactive isotopic form of manganese is measured. Thus many studies of tissues have used this technique (for example, Cotzias et al., 1968 Miyata et al., 1983). 

However, most cosmetic samples require a pretreatment like complete acid digestion or leaching of the analytes, which may or may not be relatively difficult depending on how efficiently the analytes are extracted from the matrix. Microwave energy permits rapid heating of samples, which considerably reduces pretreatment time. For example, heavy metals have been determined in some cosmetics (lipsticks, powders) using different atomic spectroscopic techniques after acid treatment assisted by micro-wave irradiation. 

A survey of recent literature shows the decreasing importance of TLC as an analytical technique in the vitamin D field. For the separation of photoisomers of irradiated provitamins D, TLC has been almost completely superseded by HPLC. Likewise, TLC has lost its impact as a purification step in the complex sample pretreatment schemes previously used to separate vitamin D from interfering sterols and fat-soluble vitamins present in oils, foods, and multivitamin preparations. The technique still has some potential for monitoring the purity of radiolabeled vitamin D derivatives. 

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