17
From comparison of the spectra of the Ni-standard, the synthetic- as well as the human urine it became clear, that synthetic urine (recipe) was unsuitable for method development and calibration. The recipe spectrum was alike that of the nickel standard (Figure 1), while the set of 3D-spectra for the human urine gave evidence for the presence of molecular absorptions (Figure 3, TOP PANEL) that were identified as phosphates and silicates using the in-built line-library. Those molecular absorptions arise from matrix constituents of human urine samples and directly overlap with the 232.0nm Ni-AA-line. Direct insight into such interferences simplifies required spectral correction routines and further benefits lower limits of analytical determination (sensitivity).
Figure 5. Set of 3D-spectra for a nickel-free sodium silicate collected on contrAA®
700 for the wavelength range of the 232.0nm Ni-AA-line.
Figure 3. Set of 3D-spectra for the Ni-AA-line at 232.0nm as- collected on contrAA®
700 during the atomisation of human urine
sample S 02 (top). Same set of 3D-spectra after spectral background correction of PO and SiO molecular absorptions (bottom).
In other words, using recipe as calibration standard for the screening of human urine samples means that spectral interferences due to PO and SiO molecular absorptions systematically falsify results, i.e. their contributions (absorbance) are unaccounted for in the calibration standards. Moreover, using certified human urine samples as calibration standard will yield similar problems as matrix constituents may differ substantially in any two urine samples.
For all three urine samples the nickel content determined by electrothermal HR-CS AAS were found to be in excellent agreement with certified nickel values; yet only when using the spectral background correction routines described above (Table 1). That is, mean nickel contents for S 01, S 02 as well as S 03 were well within certified ranges when the background was properly corrected. On the contrary, when spectral interferences from PO and SiO constituents of human urine samples were unaccounted for in the analysis, nickel contents for S 01 and S 02 were over-estimated by about 300% and 200%, respectively.
Figure 4. Set of 3D-spectra for a nickel-free sodium dihydrogenphosphate collected on contrAA® wavelength range of the 232.0nm Ni-AA-line.
700 for the
In order to develop a reliable screening method, the best approach to deal with spectral interferences is to correct/subtract false contributions using suitable correction spectra. In this study, sets of correction spectra were obtained from ultra-pure sodium dihydrogenphosphate (Figure 4) and sodium silicate (Figure 5) both of which were nickel-free.
These standards were chosen, because they produce the same spectral interferences found in the spectrum of human urine samples (Figure 3, TOP PANEL). Using these correction spectra, a software-driven scaled subtraction routine developed by Analytik Jena can eliminate spectral interferences for human urine samples as evident in Figure 3, BOTTOM PANEL. Close inspection of both sets of 3D-spectra in Figure 3 reveals that spectral contributions of the matrix constituents at 232.0 nm (Ni-AA-line) have been removed entirely.
It is worth noticing that the analytical routine developed at Currenta GmbH & Co OHG only involves the measurement of one aqueous blank and the urine sample as well as a 25 ppb spike for both of them. Considering the high accuracy and reproducibility of such approach it further safeguards the economic viability of a successful screening method.
References
[1] Angerer, J.; Heinrich-Ramm, R.; Begerow, J. Analytiker-Taschenbuch (1991) 10, 397-433.
[2] Bundesgesundheitsblatt-Gesundheitsforschung-Gesundheitsschutz 44 (2001) 12, 1243-1248.
The potential of this powerful background correction routine is best demonstrated when evaluating the results for the analysis of three human urine samples of known nickel contents (previously determined in a round robin test) denominated S 01, S 02 and S 03 (see Table 1). Nickel contents from 4.5 to 36.7 ppb were chosen to account for the wide variety of human urine samples to be submitted to this screening routine.
Table 1. Comparison of nickel contents for three human urine samples analysed by electrothermal HR-CS AAS on contrAA® with and without spectral background correction.
700
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