Spectroscopic analysis has seen growth in a number of application oriented domains such as UV – Visible, Infrared, Raman, Atomic Absorption Spectroscopy, etc.
Design of spectroscopic systems is based on the fundamental principle of light absorption by absorbing species-the Beer Lambert law. Over the years basic design has been based on single beam or double beam optics with the latter gaining prominence due to its distinct advantages. Advances in electronics and detection systems have contributed further to the popularity of double beam systems. In the present article the discussion will be limited to Atomic Absorption Spectroscopy systems.
It is important to understand the optical layout of both single beam and double beam systems before you begin to appreciate the advantages of one over the other.
Single Beam Systems
Single Beam Schematic Diagram
The light source comprising of a hollow cathode lamp emits sharp atomic line of the element whose determination is required. The light is modulated (switched on and off) rapidly by means of a rotating chopper located between the light source and the flame. Modulation can also be achieved by pulsing the power (switched on and off rapidly) to the light source. Modulation serves to differentiate the light coming from the source lamp from the emission from the flame. The modulated light is led to the flame where ground state atoms of the element of interest are present and after absorption is led to the monochromator which isolates the wavelength of interest which is then led to the detector.
Advantages of Single Beam Systems
- Single beam instruments are less expensive
- High energy throughput due to non-splitting of source beam results in high sensitivity of detection
Instability due to lack of compensation for disturbances like electronic circuit fluctuations, voltage fluctuations, mechanical component’s instability or drift in energy of light sources. Such drifts result in abnormal fluctuations in the results.
Double Beam Systems
Double Beam AAS Schematic Diagram
The light beam from the source is split into sample beam and reference beam by the mechanical chopper. The reference beam monitors the lamp energy whereas the sample beam reflects sample absorption. The observed absorbance measurement is the ratio of the sample and reference beams which are recombined before moving to the monochromator. This arrangement compensates the effects due to drift in lamp intensity, electronic and mechanical fluctuations which affect both the sample and reference beams equally.
Advantages of Double Beam Systems
- Modern improvements in optics permit high level of automation and offer the same or even better level of detection as compared to earlier single beam systems. Instability factors due to lamp drift, stray light, voltage fluctuations do not affect the measurement in real-time.
- Little or no lamp warm up time is required. This not only improves throughput of results but also conserves lamp life
We have seen that the double beam system design features far outweigh the advantages offered by single beam systems. The cost factor is more than offset by the advantages offered by modern double beam systems and therefore these have become the preferred choice.
The topic is open for discussion and you may offer your valued comments.