25 The better way of Background Correction
With the introduction of the High-Resolution Continuum Source AAS (HR-CS AAS) in the contrAA®
instrument series in 2004 a new method of Background Correction became
available. With a light source emitting a continuous spectrum, a high-resolution Echelle spectrometer and a CCD array detector, these instruments routinely display sample absorbance spectra – for the first time in AAS.
The spectral background is corrected directly and simultaneously in the spectrum by automatically fixing the baseline at Zero Absorbance. At the same time fluctuations of the lamp intensity are corrected the same way – double-beam functionality in a single-beam instrument.
The sample spectrum contains valuable information, in the simplest case it proves that the background is corrected properly and there are no spectral interferences. It gives the user the confidence that the analyte element is measured correctly (Figure 3).
As mentioned before, structured background and spectral interferences can often not be corrected using the classical BGC techniques. In fact, with these techniques an interference can easily remain undetected if the results are not carefully examined. Here the High- Resolution Continuum Source AAS instruments and their specific software are clearly ahead.
The high spectral resolution avoids almost any interference in the first place. To be sure, the spectrum of each sample is displayed and can be printed in the report. So even in the rare case of spectral interferences they can be detected, identified and corrected easily. The typical spectral structures that might appear in AAS are stored in a reference library in the operating software for identification. Then it is enough to record a matrix spectrum of the interfering substance and implement it in the method. Now the interference will automatically be subtracted from each sample spectrum (Figure 5), always ensuring correct results.
Figure 3. Background corrected, interference-free Absorbance spectrum
Besides giving the analyst trust in his results, the spectrum contains much more useful information. Additional lines in the spectrum give qualitative information about other elements present in the sample (Figure 4), and can be quantified simultaneously if required.
Figure 5. (a) Absorbance spectrum of Zn 213.857 nm in a matrix containing Nitrate and Fe; (b) Nitrate matrix spectrum; (c) Fe 213.859 nm matrix spectrum; (d) Corrected Zn spectrum after subtraction of Nitrate and Fe spectra
Summary
Traditional methods of background correction have limited capabilities, particularly in case of structured background or matrix wavelengths directly overlapping with the analyte wavelength. Often enough, correction errors remain undetected or unidentified because of the limited information provided by the instrument. As a result, erroneous analysis values are produced and complicated method development becomes necessary to minimise the error.
HR-CS AAS in the contrAA® series has clear advantages because of its revolutionary
optical design. All corrections can be done simultaneously, directly in the sample spectrum. Thanks to the spectrum display any interference, including the one between Ni and Fe mentioned above, is obvious at first glance and can be corrected with a few mouse-clicks. In addition to this, it is the ideal instrument for multi element analysis. The continuous light source makes lamp exchange a matter of the past, the possibility of simultaneous evaluation of multiple wavelengths further increases the sample throughput. Even non-metals like the halogens or sulphur can be analysed – a completely new application field for an AAS! The 10 years approved HR-CS AAS technology can drastically increase the productivity and the quality of results of any analytical lab.
Figure 4. Vanadium analysis reveals additional elements Read, Share and Comment on this Article, visit:
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