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Mass Spectrometry & Spectroscopy


Background Correction In AAS – Can You Really Trust It As Much As You Do?


Oliver Büttel, Product Manager Optical Spectroscopy, Analytik Jena AG, Konrad-Zuse Str. 1, 07745 Jena


Background Correction (BGC) is an essential part of any spectrometric analysis. Typically a non-specifi c signal, the spectral background, is superimposed on the analyte-specifi c signal and needs to be excluded to obtain the net analyte signal. Different techniques have been developed to distinguish specifi c from non-specifi c signals. In ICP OES with an array detector for example, the spectral background can be calculated from the emission spectrum of a sample by evaluating the baseline on both sides of the analyte emission line.


In traditional Atomic Absorption Spectrometry however, sample spectra cannot be used for this purpose because of the line emission profi le of the light source used and the detection system, which make it impossible to record wavelength-resolved signals. For this reason the spectral background must be measured in a separate measurement. Practically, total (specifi c and non-specifi c) and background (non-specifi c only) absorbance are measured alternatingly and automatically subtracted from each other.


Traditionally three different methods are used in AAS: Deuterium-, Zeeman- and Self-Reversal- BGC. In all three cases the instrument software only shows numeric values for Atomic Absorption (AA) and Background (BG). As we will see, these numeric values are sometimes not suffi cient because no information is available where they actually come from, and the user must have a certain level of trust in these values without being able to verify them.


It should also be mentioned that all three BGC methods increase the noise level of the signal to a certain extent because of the differential measurement of AA and BG. The fact that AA and BG are not measured exactly at the same time is an additional error source, particularly in Graphite Furnace AAS with its fast-changing signals. Each of the traditional BGC methods has its typical correction limitations and additional disadvantages.


Traditional Correction Methods


Deuterium Background Correction Nowadays commonly used in Flame and low-cost Graphite Furnace AAS instruments, Deuterium-BGC is the oldest one of the traditional methods. It uses the continuous radiation of a Deuterium lamp to measure the average background absorption over the slit width monitored. An inherent limitation of this method is the Deuterium lamp itself, which only emits in the UV-range. At wavelengths higher than about 350nm Deuterium-BGC cannot be used. In addition to this, any noise or drift of the Deuterium lamp will directly be added to the AAS signal.


As this method can only measure the average background over the spectral range monitored, it only works correctly if the baseline is horizontal. In case of a structured baseline, for example if molecule absorption bands or other atomic absorption lines are present, it will calculate a false value. The Graphite Furnace signal shown in Figure 1 shows an over-correction leading to a negative AA signal for Arsenic in presence of Phosphate molecule absorption bands.


Zeeman Background Correction


Born in the 1980s, Zeeman-BGC uses a strong magnetic fi eld to make the specifi c Atomic Absorption ‘invisible’ to the spectrometer. This way the Background alone can be measured exactly on the analyte wavelength using the hollow cathode lamp of the analyte element. Zeeman-BGC is almost exclusively used in Graphite Furnace AAS, where the correction performance of the Deuterium method is often not suffi cient.


Zeeman-BGC shows a better correction performance than Deuterium-BGC because it measures the atomic and background absorbance on exactly the same wavelength and with the same light source. Thus it can handle structured background in a much better way, given that the background does not change in the magnetic fi eld. This fundamental requirement is fulfi lled by most background structures.


However, certain molecule bands and all atomic lines show the Zeeman effect, i.e. a spectral background of molecular or atomic absorption by matrix constituents can be changed by the magnetic fi eld. These will not be corrected accurately if they are close to the analyte wavelength. Some examples have been described in the literature, and a particular one many AAS users may face is the interference on the primary Nickel line by Iron (Figure 2). The absorption spectrum shows the interference of a high Iron signal on the relatively small Nickel line. The Nickel result measured with Zeeman-BGC will be falsely high with increasing Iron concentrations. This case will occur whenever traces of Nickel are determined next to a high Iron content.


Figure 2. Interference of Fe on Ni-line (left), False Ni-signal produced by high Fe-concentrations Self-Reversal Background Correction


Self-Reversal-BGC (SR-BGC) is the least common one of the three techniques described here. It is also often referred to as Smith-Hieftje-BGC, according to the names of its inventors. A self-reversal condition is created in the analyte HCL by applying a high-current pulse, during which the background is measured.


Figure1. Over-correction of BG (blue) leads to negative AA-signal (red)


SR-BGC has not been very successful commercially, currently only one major instrument manufacturer uses this technology. SR-BGC has certain disadvantages that originate in the function principle. The high current applied to the light sources used reduces their lifetime and requires suitable lamps. Self reversal in the lamp is not complete, resulting in sensitivity being reduced by up to 70% and non-linear calibration curves. Background showing molecular or atomic absorption structures close to the analyte wavelength cannot be corrected. Additionally, the lamp needs a certain relaxation time after a high-current pulse, which limits the frequency of alternating AA- and BG-measurements and thus the capability of following the fast signals in Graphite Furnace AAS.


INTERNATIONAL LABMATE - APRIL 2014


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