Counting after combustion

Combustion methods are applied with very low radioactivities. HjO, COj and S02 can be determined either directly in the gas phase [243] or after trapping in liquid scintillation fluids. The enumeration of commercially available apparatuses for this purpose is beyond the scope of this review (see Refs. 244-250). 

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Arukwe et al. [15] dosed Atlantic salmon with radioactive labeled NP. Fish were frozen immediately until gall bladder, skin, kidney, gill, liver, muscle, fat, remaining carcass and viscera were sampled and analysed. Tissue radioactivity was analysed by liquid scintillation counting after combustion of aliquots in an oxidiser apparatus. Metabolites (biliary and urinary) were separated by radio-HPLC. 

After combustion of the sample and carriers in an oxygen stream, reducing conditions are achieved by a flow of carbon monoxide over the sample ash. Arsenic, zinc, cadmium, and any remaining selenium and mercury are reduced to elemental form. When the sample is heated to 1150°C in a slow carbon monoxide stream in a quartz tube in a furnace, recovery of all five elements in the liquid nitrogen trap is complete in 30 min. The recovery trap is washed with nitric acid to dissolve all the metals, and the radioactivity of a nitric acid solution of the products is counted with a Ge(Li) detector. 

Three aliquots from each (processed) sample were used for radioactivity determination. The aliquots from the urine samples were measured directly in the liquid scintillation counting procedure after addition of the scintillator Roth-rotiszint eco plus (Roth, Karlsruhe, Germany). The other matrix aliquots taken up on Combusto Cones (Canberra-Packard), weighed, dried at room temperature, were combusted in a Tricarb combuster (Canberra-Packard GmbH, Model 307) and the 14CC>2 formed was absorbed by Carbo-Sorb (Canberra-Packard). The subsequent radioactivity measurements were carried out by the liquid scintillation counting procedure in a ((-spectrometer (Canberra-Packard 2500 TR) after addition of the scintillator Permafluor E+ (Canberra-Packard). 

After solubilisation, the sample is added to a sdntillation cocktail suitable for aqueous solutions. Solubilisation of tissues often produces chemiluminescent materials which can distort measurements of radioactivity. To reduce or remove this problem, it is recommended that samples should be kept in the dark before counting. An expensive alternative to scintillation counting for tissue samples, and one which is only justified when handling highly repetitive samples, is combustion of the sample in an appropriate furnace the 14C and 3H labelled material is converted respectively into C02 and H20 which are collected in suitable wash bottles. 32P label is converted into phosphate which remains in the ash following combustion. 

Determination of Radioactivity. All samples were counted in a Nuclear Chicago Isocap 300 liquid scintillation counter equipped with a Teletype computerized for direct calculation of disintegrations per minute. Fifty microliters of blood were directly counted for radioactivity after solubilization (1 mL of 1 N NaOH). After incubation at room temperature for 15 min the sample was decolorized by adding 200 fxL of hydrogen peroxide and incubating at 80°C for 30 min. The processed samples were mixed with 100 fxL of 80% acetic acid and 15 mL of Insta-gel (Packard), and were counted. Approximately 60-100 mg of tissue were solubilized following the same method as for blood. From the collected urine an aliquot (2 mL) was counted directly with 15 mL Insta-gel. The feces were dried overnight at room temperature, and a 60-100-mg aliquot was combusted (12) and counted for radioactivity. 

Each animal was sacrificed by captive bolt after the appropriate withdrawal interval (4, 6, 14 and 28 days after the 2nd dose) and processed as in an abattoir. The entire liver, kidneys and udder were excised and 1-2 kg samples of muscle from both the flank and the udder diaphragm and 1-2 kg samples of fat from the abdominal area were collected. Each organ and tissue was minced and processed three times through a commercial meat grinder to prepare respective homogenate samples. Sub-samples (200-300 mg) were prepared in triplicate for total residue analysis. Total radioactivity concentrations, expressed as pirlimycin free base equivalents, were determined by direct liquid scintillation counting (liquids) or combustion analysis (solids) following standard techniques. 

Cereals, food CRMs I Combust in quartz tube in train with fiir-nace/02, purify, trap and irradiate I2 in reactor [DA] Count gamma rays after thermal neutron irradiation of separated I2 in pneumatic facility of reactor [SEP/CONC-RNAA] [DA-SEP/ CONC-RNAA] Norman and Iyengar(1994)... 

In case of materials labelled with carbon-14 and/or tritium (Fig.l), after the additicn of a small amount of tungstic oxide to retain phosphoms and promote combustion, the sanple is burnt in a stream of oxygen. Halogens and sulfur are retained by silver wool, vhile the combustion products including HTO and carbon dioxide are led with the oj en stream caito a column of partially dehydrated alumina kept at 20CAi . The water is retained cn the column quantitatively, meamhile the carbon dioxide is transferred into an absorbent suitable for counting. The tritium (HTO) stored ten 5)orarily... 

Combustion stops automatically after the disappearance of the metal phase, within a few minutes. It is followed by flushing of the circuit for 3 to 5 minutes. Counting of the soda asbestos trap begins about 10 to 15 minutes after the end of irradiation. Activity measurement is prolonged for 30 to 40 minutes, for a final check of the type of radioisotope detected. 

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