A multicommutated flow system was designed and evaluated for the determination of selenium by hydride generation atomic absorption spectrometry (HG-AAS). It was applied to the determination of total selenium in samples of cow's milk (fluid and powder) and infant formulae. Linearity was satisfactory in the range up to 27.5 mg L À1 (h ¼ 0.082 C + 0.0033, h ¼ peak-height, absorbance, C ¼ concentration in mg L À1 , r 2 ¼ 0.999). Detection (3s) and quantification (10s) limits in solution were LD ¼ 0.08 mg L À1 and LQ ¼ 0.27 mg L À1 , corresponding to LD ¼ 3.2 mg kg À1 and LQ ¼ 10.8 mg kg À1 in solid samples, and to LD ¼ 0.8 mg L À1 , LQ ¼ 2.7 mg L À1 in fluid milk samples. Trueness was verified by analysis (n ¼ 5) of two reference materials (NIST 1549, Non-fat Milk Powder and NIST 1846 Infant Formula). At the 95% significance level, results were statistically equivalent to the certified values. Instrumental precision (s r (%), n ¼ 5) was in the range 1.4% to 11.7%, analytical precision (s r (%), n ¼ 5) being 4.2 and 9.3% respectively for the determination of the above mentioned reference materials. The sampling frequency of the system was 160 hour À1 .
A multicommutated flow system was designed and evaluated for the determination of total arsenic and selenium by Hydride Generation Atomic Absorption Spectrometry (HG-AAS). It was applied to the determination of arsenic and selenium in samples of natural and drinking water. Detection limits were 0.46 and 0.08 μg l(-1) for arsenic and selenium, respectively; sampling frequency was 120 samples h(-1) for arsenic and 160 samples h(-1) for selenium. Linear ranges found were 1.54-10 μg l(-1) (R = 0.999) for arsenic and 0.27-27 μg l(-1) (R = 0.999) for selenium. Accuracy was evaluated by spiking various water samples and using a reference material. Recoveries were in the range 95-116%. Analytical precision (s ( r ) (%), n = 10) was 6% for both elements. Compared with the Standard Methods, APHA, 3114B manual method, the system consumes at least 10 times less sample per determination, and the quantities of acid and reducing agent used are significantly lower with a reduction in the generation of pollutants and waste. As an additional advantage, the system is very fast, efficient and environmentally friendly for monitoring total arsenic and selenium levels in waters.
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