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Atomic Absorption Spectrometry (AAS) Data Analysis
Data analysis in atomic absorption spectrometry (AAS) is a multi-step process, with the user having to select the correct method and wavelengths in order to obtain optimal results. Background correction is also a key component of successful AAS data analysis. There are two typical correction methods that are widely used with AAS technology: deuterium and Zeeman background correction.
Deuterium background correction
Deuterium background correction is the oldest and still most commonly used technique, particularly in flame AAS. In this technique, a separate source (deuterium lamp) with broad emission is used to measure background absorption over the entire width of the exit slit of the spectrometer. The use of a separate lamp makes this technique the least accurate one because it cannot correct for any structured background. It also cannot be used at wavelengths above 320 nm because the emission intensity of the deuterium lamp is very weak beyond that wavelength.
Zeeman background correction
With Zeeman background correction, an alternating magnetic field is applied at the atomizer (graphite furnace) to split the absorption line into three components: the π component, which remains at the same position as the original absorption line, and two σ components, which are moved to higher and lower wavelengths.
Total absorption is first measured without the magnetic field turned on, after which the background absorption is measured with the magnetic field turned on. The π component must be removed during this set of measurements so that the σ components do not overlap with the emission profile of the lamp. In this way, only the background absorption is measured. This step is often performed using a polarizer.
The advantage of using such a process is that the total and background absorption are measured within the same emission profile of the same lamp, so any kind of background (including fine structure background) is corrected. This process cannot be used if the molecule responsible for the background is also affected by the magnetic field. A more powerful spectrometer is needed for such background correction, complete with its own increased power supply (to operate the magnet that splits the absorption line).
Figure 1. A signal that is corrected for Zeeman background. Click image to enlarge
There are other methods available in AAS data analysis and optimization, including using an internal standard, and the standard addition method.
Internal standards
In analytical chemistry, an internal standard is a chemical substance that is added at a constant amount to the samples, the blank, and the calibration standards during chemical analysis. This substance can then be used for calibration by plotting the ratio of the analyte signal to the internal standard signal as a function of the analyte concentration in the standards. This is performed in order to correct for the loss of analyte during sample preparation or intake.
The internal standard is a compound that is very similar, but not identical to, the chemical species of interest in the samples. As such, the effects of sample preparation should be the same for both the internal standard and the chemical species of interest (relative to the concentration of each species).
Figure 2. A software platform setup for different data analysis methods. Click image to enlarge
Standard addition
Standard addition is frequently used in instrument-mediated chemical analysis, including atomic absorption spectrometry. Standard addition is a quantitative analysis approach in which the standard is added directly to aliquots of analyzed sample. This method is used in situations where sample matrix also contributes to the analytical signal, a phenomenon known as matrix effect. This effect makes it impossible to compare the analytical signal output between sample and standard using the traditional calibration curve approach.
Atomic absorption spectrometry software
Not only is correct instrumentation required for accurate and precise AAS measurement, but also software. In fact, the right software not only facilitates instrument control, it also enables powerful data acquisition, manipulation, and interpretation.
Software wizards often make instrument and data handling tasks much simpler. They also help any user regardless of expertise level. Some software wizards, such as the Thermo Scientific SOLAAR wizard shown in figure 3, are integrated into the instrument.
Most software wizards are designed to simplify specific tasks or procedures and provide detailed step-by-step instructions for required actions. Some wizards even perform calculations and prompt the user on which workflow steps to take next.
Figure 3. The Thermo Scientific SOLAAR software step-by-step wizard page. Click image to enlarge
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