Question

In atomic absorption spectrometry, why is "modulation" of the line source necessary for accurate results and how is modulation achieved?

Answer 1

The function of the chopper is to chop the light leaving the source so that when the incident beam hits the chopper at the solid surface, the beam will be blocked and detector will only read the emitted signal from the flame. As the chopper rotates and the beam emerges to the detector, the detector signal will be the sum of the transmitted signal plus that emitted from the flame. The signal processor will be able to subtract the first signal from the second one, thus excluding the signal from emission in flames. This can be represented by the following equations:

Signal 1 (Blocked Beam) = P_e

Signal 2 (Transmitted Beam) = P + P_e

Overall Difference Signal = (P + P_e) - P_e = P (Corrected Signal)

This correction method for background emission in flames is called Source Modulation.

It should be recognized that addition of extra components to an instrument will decrease the signal to noise ratio and addition of a moving component is usually regarded as a disadvantage due to higher need for maintenance.

Another procedure which can overcome the emission from flames is to use a modulated power supply that will give fluctuating intensities at some frequency (say for example pulsed radiation at a specific frequency). The emission from flames is a continuous signal but that from the source is modulated. Now if we use a high pass RC filter, only the fluctuating signal will be measured as signal while the DC signal will be considered zero as it can not pass through the electronic filter. The high pass RC filter is a device which uses a resistor and a capacitor the impedance of which is inversely proportional to the frequency of the modulated signal. Therefore, only high frequencies will have low impedance and can pass through the capacitor while signals of low frequencies will suffer very high resistance and will not be able to go through the capacitor.