Description

Utilizing a high-resolution monochromatic is essential when measuring AAS with a continuum radiation source. In order to preserve the linearity and sensitivity of the calibration graph, the resolution needs to be at least equal to or better than the half-width of an atomic absorption line, which is approximately 2 pm. The US teams of O'Haver and Harnly were the first to conduct research using High-Resolution (HR) CS AAS. They also created the one-and-only simultaneous multi-element spectrometer for this method.

Atomic Absorption Lines and their Narrow Width

However, the breakthrough came when the Becker-Ross group built a spectrometer specifically for HR-CS AAS in Berlin, Germany. Analyst Jena, based in Jena, Germany, introduced the first commercial HR-CS AAS equipment at the turn of the 21st century. It was designed by Becker-Ross and Florek. For high resolution, these spectrometers make use of a small double monochromatic, a prism pre-monochromator and an echelle grating monochromator. The detector is a linear Charge-Coupled Device (CCD) array with 200 pixels. There is no exit slit on the second monochromatic; consequently, at high resolution, the spectral environment on both sides of the analytical line is visible. Since only 3-5 pixels are typically used to measure atomic absorption, the remaining pixels can be used for correction. One of these corrections is for lamp flicker noise, which is not related to wavelength and results in very low noise levels in measurements; background absorption corrections are the other corrections, which will be discussed later. Spectral overlap is rare due to the small number of atomic absorption lines and their narrow width (a few pm) in comparison to atomic emission lines; molecular absorption, on the other hand, is much broader, making it more likely that some molecular absorption bands will overlap with an atomic line. There are only a few examples that are known to occur. Flame gases or un-dissociated molecules of the sample's concomitant elements could cause this kind of absorption. We need to be able to tell the difference between the spectra of di-atom molecules, which have a lot of fine structure, and those of larger molecules, usually tri-atom molecules, which don't have as much structure. Scattering of the primary radiation at particles generated during the atomization stage, when the matrix could not be sufficiently removed during the pyrolysis stage, is another source of background absorption, particularly in ET AAS.

Magnetic Field and Background Absorption

Both molecular absorption and radiation scattering can lead to artificially high absorption as well as an incorrect calculation of the analytics concentration or mass in the sample. Background absorption can be corrected using a variety of methods, but LS AAS and HR-CS AAS differ significantly in this regard. Instrumental techniques can only be used to correct background absorption in LS AAS, and all of them are based on two consecutive measurements: total absorption (atomic plus background) and background absorption alone. The net atomic absorption is determined by the difference between the two measurements. Background-corrected signals always have a lower signal-to-noise ratio than uncorrected signals because of this and the addition of additional components to the spectrometer. In addition, it is important to note that in LS AAS, there is no way to correct for the extremely rare circumstance in which two atomic lines directly overlap. The oldest and still most widely used method, especially for flame AAS, is this one. In this instance, the background absorption across the entire width of the spectrometer's exit slit is measured using a separate source with broad emission a deuterium lamp. This method is the least accurate because it requires a separate lamp and cannot adjust for a structured background. Additionally, it cannot be utilized at wavelengths greater than approximately 320 nm due to the extremely weak deuterium lamp emission. Due to the better alignment of the former lamp's image with that of the analyte HCL, the utilization of deuterium HCL is preferable to that of an arc lamp. Based on the line-broadening and self-reversal of HCL emission lines when a high current is applied, this method is named after its creators. Background absorption is measured following the application of a high-current pulse with the profile of the self-reversed line, which has little emission at the original wavelength but strong emission on both sides of the analytical line. Total absorption is measured with normal lamp current, i.e., with a narrow emission line. This method has the advantage of utilizing only one radiation source the technique can only be used with relatively volatile elements because only those exhibit sufficient self-reversal to avoid a significant loss of sensitivity, and the high-current pulses shorten lamp life. Because background is not measured at the same wavelength as total absorption, the method is not suitable for correcting structured background. This is another issue. At the atomizer (graphite furnace), an alternating magnetic field is used to divide the absorption line into three parts. The first part stays in the same place as the original absorption line, and the two other parts are moved to higher and lower wavelengths, respectively. Total absorption is measured without the magnetic field, and background absorption is measured with the magnetic field on. In this instance, the component must be removed, such as with a polarizer, and the components do not overlap with the lamp's emission profile, so only background absorption is measured. This method has the advantage of measuring total and background absorption with the same lamp's emission profile. This means that any kind of background, including background with fine structure, can be accurately corrected, unless the background molecule is also affected by the magnetic field and using a chopper as a polarizer makes the signal to noise ratio smaller. The disadvantages include the spectrometer's increased complexity and the power supply required to run the powerful magnet needed to split the absorption line. Total absorption is measured without magnetic field and background absorption with the magnetic field on. The π component has to be removed in this case, e.g. using a polarizer, and the σ components do not overlap with the emission profile of the lamp, so that only the background absorption is measured. The advantages of this technique are that total and background absorption are measured with the same emission profile of the same lamp, so that any kind of background, including background with fine structure can be corrected accurately, unless the molecule responsible for the background is also affected by the magnetic field and using a chopper as a polarizer reduces the signal to noise ratio. While the disadvantages are the increased complexity of the spectrometer and power supply needed for running the powerful magnet needed to split the absorption line.