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solids appear to berticeably hygroscopic). Weight-stable~quot residue s should then be alpha and/or beta counted until at least 10.000 total count s have been accumulated. A single set of reference standards prepared in thi s way can be used for each counting instrument for separate graph preparation s and can be stored for reverification whenever needed. 7.2 Tran sfer to a beaker an aliquot of water sample of a volume tha t contains no more than 100 mg (for alpha only or alpha and beta determination) o r 200 mg (for beta only determination) of total water solids. Evaporate th e aliquot to near dryness on a hot plate. If water samples are known or suspecte d to contain chloride salts. those chloride salts should be converted to nitrate salts before the sample residue is transferred to a stainless steel planche t (chlor ides will attack stainless steel and increase the sample solids. and n o correction can be made for those added solids). Chloride salts can be converte d to nitrate salts by adding 5- mL portions of 16 N HNO , to the sample residue and evaporating to near dryness. (Two treatments are usually sufficient.) Add 10 mL 1 N HN 0, to the beaker and swirl to dissolve the residue. Quantitativel y transfer the aliquot concentrate in small portions (not more than 5 mL at a time ) to a tared planchet. evaporating each portion to dryness. 7.3 Dry the sample residue in a drying oven at 105 °C for at least 1 hr cool in a desiccator. weigh, and count. Store the sample residue in a des~ccato r until ready for counting. 7.4 Some types of water-dissolved solids, when converted to nitrate salts are quite by groscopic even after being dried at 105 °C for 1 hr. When suc h hygrosco pic salts are present with samples that are put into an automati c counting system, those samples gain weight while they are waiting to be counted and inaccurate counting data result. When there is evidence of hygroscopic salt s in sample counting planchets. it is recommended that they be flamed to a dull re d heat with a Meeker burner for a few minutes to convert the nitrate salts t o oxides before weighing and counting. (It is possible to have a loss of cesium during the flaming of the samples.) 7.5 Count for alpha and beta activity at their respective voltag e plateaus If the sample is to be recounted for reverification. store it in a desiccator. NOTE: As long as counting chambers are capable of handling the same siz e planchet alpha and beta activities can be determined at thei r respective voltage plateaus in the designated countin g instruments. Keep t he planchet in the desiccator until ready to cou nt because vapors from moist residue can damage detector an d window and can cause erratic measurements. If the gas-flo w internal proportional counter does not discriminate for the highe r energy alpha pulses at the beta plateau, the alpha activity must be subtracted from the beta plus alpha act~v~ty. This i s particularly important for samples with high alpha activity 9310 - 5 CD-ROM Revision 0 Date September 1986