Background Correction

When obtaining absorption spectra, there are frequently other signals detected by the detector from things other than the sample. Unfortunately, interferences from a variety of places are inevitable and will influence the absorbing species and the radiation reaching the detector. While it does not mean that the resulting spectrum is not the spectrum of the sample, it will result in a loss in spectral detail, such as broadening of peaks and peaks in places other than where the sample absorbs. This could influence quantitative and qualitative work. Spectral deviations from background interferences can be significant at times and slight at others, but with the proper application of background correction techniques, these deviations can be minimized while enhancing the signal from the analyte.

One technique that should be done for every analysis is to obtain the signal of a blank; a substance prepared in the same manner as the analyte, except it contains no analyte. You can calibrate the instrument you are using to be set to a signal of zero for the signal of the blank.  You can also subtract the signal produced by the blank from all the signals obtained throughout the analysis, generating “corrected” values. The corrected values can then be used for further data manipulation such as generating calibration curves to solve for unknown concentrations (shown below). Subtracting the blank signal, from the absorbance value, will correct for some of the interferences resulting from the matrix of the solution.

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Example: solving for an unknown concentration of analyte using the linear regression equation from the calibration curve generated from known standards

Instrumental settings can also be manipulated to counter matrix effects. Adjusting fuel-to-oxidant ratios in flames, or using a different oxidant, can reduce certain matrix interferences. If incomplete combustion is causing organic substances within the matrix to produce a signal, increasing the temperature of the flame can ensure complete combustion, reducing or eliminating the effects of organics.

Another way to think about Background correction is taking the fluctuation (Δ) of a blank reading and the fluctuation of a signal reading (Δ) then average the Δ between these variances to cancel out the noise. To increase the clarity of both the blank and the signal, the analyte should be run through the instrument multiple times to get more readings. More readings, means more values to average out and will result in a clearer absorption spectra.

 

Background correction

Background correction: This diagram shows the benefits of using background correction by taking an average of the data points from the noise and creating a clean line graph easier to analyze.

Deuterium Continuum Source

In this method of background correction, the signal from a continuum source, such a deuterium lamp, is subtracted from the signal from a line source, such as the analyte’s hallow cathode lamp. A chopper alternates the radiation passing through the instrument between the deuterium continuum and the analyte source.

The signal from the continuum source indicates when radiation is attenuated by something other than the analyte. When the continuum source is passed through the atomizer, the atoms of interest only absorb the  resonance wavelength. Because this is a very small fraction of the total radiation, the effect of the analyte on the continuum signal is negligible.  Aside from the analyte, the continuum source can be attenuated by scattering or broadband absorption. By subtracting the signal from the continuum source from the analyte’s line source, results in an analyte signal that is corrected for these attenuations. This method does, however, have flaws. Some systems “over” or “under” correct. The signal-to-noise ratio often decreases with the addition of another source because it cannot correct for background specific to the incident radiation’s interaction with the system. It is also limited in its wavelength range, since deuterium is an ultraviolet source.

A chopper alternates between sources in this deuterium source background correction instrument

A chopper alternates between sources in this deuterium source background correction instrument

Both the beam from the deuterium lamp and the beam from the hollow-cathode lamp, hit the chopper, which is constantly spinning. This produces alternating pulses of radiation from the deuterium source and the hollow cathode lamp. When the beam from the deuterium lamp passes through the atomizer, and the beam from the hollow cathode lamp is reflected off the mirror. Conversely, when the beam of the hollow cathode passes through the atomizer, the beam of the deuterium lamp is reflected off of the mirror.

 

Zeeman Effect

In a Zeeman Effect instrument, a magnetic field is applied to atoms, splitting the electronic energy levels. This causes multiple absorption lines to be present, and the sum of these absorption lines is equal to the original line that formed them. In it, the absorption line is split into two components: the pi component, which is present at the original wavelength, and sigma component which is both negatively and positively shifted so that two absorption lines are present. One is present at the right of the pi line and the other is present at left of the pi line.

Absorption by the pi and sigma lines corresponds to different components of polarized light. Polarized light waves are light waves in which the vibrations are occurring in the same plane.

Pi lines correspond to radiation that polarized parallel to the magnetic field, while σ lines correspond to radiation polarized perpendicular to the field.

In a DC Zeeman correction system, radiation from a hallow cathode lamp passes through a rotating polarizer to a furnace surrounded by a permanent magnet.  When the radiation from the HCL passes through the rotating polarizer two forms of polarized light are produced: light that is polarized parallel to the field, and light that is polarized perpendicular to the field. When light is polarized parallel to the field, both atomic and background absorption occurs, and when light is polarized perpendicular to the field, only background absorption occurs. The background absorption signal is subtracted from the combined background and atomic absorption signal to obtain a net atomic absorption signal.

In an AC Zeeman correction system, a stationary polarizer is present in front of the source, and an electromagnet is present around the furnace. When the magnetic field is off, both the atomic and the background absorption occurs. When the magnetic field is on, only background absorption occurs.  Again, the background signal is subtracted from the combined atomic and background signal to obtain a net atomic absorption signal.

The Zeeman Effect is preferred over most other forms of background correction, as it tends to be more accurate. AC Zeeman systems tend to be more expensive than DC Zeeman systems, but they are more sensitive and have larger linear working ranges.

 

Zeeman effect

 For DC Zeeman correction systems, the polarizer rotates to produce polarized light that is both parallel and perpendicular to the magnetic field produced by a permanent magnet around the furnace. In AC Zeeman correction systems, the polarizer is stationary and an electromagnet is present around the furnace.

Two-line Correction

In two-line corrections, a reference line from the source is used to measure background effects from the matrix of the sample.  Even though hollow-cathode lamps are specific to an element of interest, it still produces numerous emission lines.  Some of these emission lines are the resonance lines for the element of interest, but nonresonance  lines are present as well. One the nonresonance lines can be used as a reference line. Such lines can result from emission of a gas in the cathode, such as neon. The nonresonance  line will lie close to the sample line, but will not absorbed by the sample. Any absorbance measurements from this source are then used to correct the absorbance of the sample.

Source Self-Reversal

Also called Smith-Hieftje background correction, this method of background correction involves the alternation of low and high current operation of hollow-cathode lamp source radiation. When a low current is operating, the total absorbance is obtained, which includes the absorbance of the atoms of interest and everything else in the sample (the matrix). During high current operation, a large amount of nonexcited atoms are produced, which absorb radiation produced from excited species.  Production of the free atoms also produces broadened emission lines as a result of the Doppler effect. As a result, emission lines that are slightly higher and slightly lower than the resonance wavelength absorbed by the free atoms (see blue high current signal below) . These emission lines are used to measure the background signal that is subtracted from the low current signal.

Graph of high current and low current absorbances in a source self-reversal.

Graph of high current and low current absorbances in a source self-reversal.