Internal standardization in graphite furnace atomic absorption spectrometry: Comparative use of As and Ge to minimize matrix effects on Se determination in milk

Arsenic and germanium have been evaluated as internal standards to minimize matrix effects on the direct determination of selenium in milk by graphite furnace atomic absorption spectrometry (GFAAS) using tubes with integrated platform, pre-treated with W together with Pd as chemical modifier. The efficiency of As and Ge as internal standards for 25 μg L− 1 Se plus 500 μg L− 1 As or Ge in diluted (1 + 9 v/v) milk plus 1.0% (v/v) HNO3 was evaluated by means of correlation graphs plotted from the normalized absorbance signals (n = 20) of internal standard (axis y) versus analyte (axis x). The equations that describe the linear regression were: AAs = − 0.004 ± 0.019 + 1.02 ± 0.019 ASe (r = 0.9967 ± 0.005); AGe = − 0.017 ± 0.015 + 1.01 ± 0.015 ASe (r = 0.9978 ± 0.004). Samples and reference solutions were automatically spiked with 500 μg L− 1 Ge or As and 1.0% (v/v) HNO3 by the autosampler. For 20 μL of aqueous standard solutions, analytical curves in the 5.00-40.0 μg L− 1 Se range were established using the ratio of Se absorbance to internal standard absorbance (ASe / AIS) versus analyte concentration, and good linear correlations were obtained. The characteristic mass was 40 pg Se. Limits of detection were 0.55 and 0.40 μg L− 1 with As and Ge as the internal standard, respectively. Relative standard deviations (RSD) for a sample containing 25 μg L− 1 Se were 1.2% and 1.0% (n = 12) using As and Ge, respectively. The RSD without internal standardization was about 6%. The accuracy of the proposed method was evaluated by an addition-recovery experiment and all recovered values were in the 99-105% range with IS and in the 70-80% range without IS. Using Ge as the internal standard, results of analysis of standard reference materials were in agreement with certified values at a 95% confidence level. The selenium concentration for 10 analyzed milk samples varied from 5.0 to 20 μg L− 1.