“Tell me and I’ll forget : show me and I may remember; involve me and i’ll understand”
— Chinese Proverb
Before you move to background correction it is necessary to understand what is background absorption. The main reason for background absorption is presence of undissociated molecules of matrix that have broadband absorption spectra and tiny solid particles in the flame which may scatter light over a wide wavelength region. When this type of non-specific adsorption overlaps the atomic absorption wavelength of the analyte the ground state absorption is cut. The problem is overcome by measuring and subtracting the background absorption from the total of measured absorption to determine true atomic absorption component
A number of background correction approaches have been proposed but we shall limit our discussion to two main approaches which have found widespread application in commercial instruments
Continuuum Deuterium Source Background Correction
Continuum background correction measures and compensates for any background component present in atomic absorption measurements
The broadband contimuum source emits light over a broad spectrum of wavelengths instead of specific line.Spectral interferences which are caused by atoms with absorption lines very close to the analyte absorption line or by fine structure in the molecular absorption profile can result in either positive or negative errors in the measurement of the concentration of the analyte. Fortunately spectral interferences are rare in flame atomic absorption which is a distinct advantage over emission techniques
A common inexpensive technique for background correction in flame Atomic Absorption Spectroscopy is deuterium background correction. The correction is effective over the wavelength range of 180nm -420nm. Background level becomes significant at lower wavelength range.
The cathode lamp and the deuterium lamp are sequentially pulsed with a chopper or electronically with delay of about 2ms. When hollow cathode lamp is on and deuterium lamp off total absorbance (AA + BG) is measured. When the HCL is off and the deuterium lamp on the continuum energy recorded is (BG). The atomic signal is automatically calculated by subtracting background from total absorbance.
Limitations of Continuum D2 background correction :
- The D2 lamp has a finite lifetime and requires periodic replacement
- The D2 lamp and HCL light may not view the same portion of atom cloud in the flame due to time lag. This could become significant at high background levels.
- Proper alignment of both light sources is required for right background correction
- Correction is limited to the specific wavelength range of the two lamps
- Intensities of both lamps if not similar will result in errors
Zeeman Background Correction
Zeeman Background Correction is used mainly in graphite furnace atomic absorption systems. When an atom is placed in a magnetic field and its absorption of observed in polarised light, the normal single line is split into three components – б-, π and б +displaced symmetrically about the normal position
Free atoms show Zeeman splitting in a magnetic field but molecules, liquid droplets or solid particles show no Zeeman splitting and so advantage can be taken of polarised light. The π component is linearly polarised parallel to the magnetic field while the б components are circularly polarised perpendicular to the magnetic field. A polariser is positioned in the optical system to remove the π components of the transmitted radiation. This affords background measurement at the exact analyte wavelength when magnetic field is applied. Since the background is measured at the analyte wavelength and not averaged as in D2 system structural molecular background and spectral interferences are easily corrected.
In AC modulated Zeeman system the combined atomic and background absorption is measured while magnetic field is off. When the magnetic field is on, the detector measures only the background absorption as the π component is not detected. The difference between the two is the Zeeman corrected atomic absorption signal
The magnetic field may be applied to the graphite tube with either transversally( perpendicular) or longitudinal ( parallel) to the optical axis. A major advantage of longitudinal Zeeman background correction is that the polariser is not needed to eliminate the π component, thereby providing higher light throughput by eliminating polarizer absorption
Advantages of Zeeman background correction :
- Corrects high levels of background
- Corrects at exact analytical absorption line
- Requires only a single standard light source. Alignment problems of multiple light sources are not encountered
Limitations of Zeeman background correction :
- More expensive than continuum background correction
- Loss in sensitivity for some elements due to splitting of б and π components which may overlap
So far you have been introduced to the basic concepts and principles of Atomic Absorption Spectorscopy. The next module is a set of questions which you may face at time of interview when you apply for a suitable opening in a laboratory equipped with an Atomic Absorption Spectrometer.