Table 3.

Results for Cr, Cu, Fe, Mn, Ni and Zn from the analysis of freshwater standard reference material (SRM 1643e)

Element	Certified value (µg L−1)	Found (µg L−1)a	Relative error (%)
Cr	19.9±0.23	18.6±0.3	−7
Cu	22.20±0.31	20.8±0.4	−6
Fe	95.7±1.4	90.2±1.6	−6
Mn	38.02±0.44	35.6±0.5	−6
Ni	60.89±0.67	58.4±0.8	−4
Zn	76.5±2.1	72.7±2.2	−5

^aMean of three determinations at 95 % confidence level $(\overline{x}\pm ts/\sqrt{N})$.