The %recoveries of the spikes was determined by spiking 2 lg L1 to 8 lg L1 Cr(VI) standard to samples since reference materials of tea with certified values of Cr(VI) are not currently on market.
The recoveries of Cr(VI) was in the range between 98 ± 5.0% and 101 ± 11% and therefore the results were acceptable.
The application of the standard addition method of calibration in the analysis of a tea variety with a noticeable content of chromium lead to 10.0 ± 0.5 lg g1 of Cr(VI) and this value is in a good agreement with 10.3 ± 0.7 lg g1 obtained by external calibration
(Fig. 2).
These results are also evidence of absence of matrix interference on results of analysis with external calibration.
The total chromium determination in tea was validated by theanalysis of certified reference material of tea (NCS ZC73351) with a good agreement of trueness between the found 0.81 ± 0.09 and certified value 0.80 ± 0.10 lg g1.
The repeatability and intermediate precision determined by analysing NCSZC73315 under the same experimental conditions present values in the range of 0.60–9.50% and 0.10–7.45%, respectively.