Tuneable Diode Laser Absorption Spectroscopy (TDLAS) is a well established technique for measuring molecule concentration (species), using calibrated wide dynamic range detection. By using very narrow laser line widths, additional information such as temperature and pressure of the species can also be determined. LASER COMPONENTS offer a choice of spectroscopic system configurations.
The highest sensitivities, such as those needed in the detection of trace elements in the parts per billion (ppb) range, are achieved through a method called derivative spectroscopy. Although this method is not new, it is used in too few laboratories! Why? Perhaps it is still an unknown technique? In brief, then, the derivative principle uses a laser diode controller, a lock-in amplifier and a computer. A modulation current with amplitudes in the % range is added to the direct current of the laser diode. This is achieved using the LASER COMPONENTS’ L5830 controller. No external frequency is necessary. Perhaps one reason that the derivative method is not prevalent is the fact that most of the laser diode controllers do not have integrated frequency generation and the derivative method cannot, therefore, be inferred. Using a reference, phase sensitive detection permits the measurement signal to be read out on via the computer. The profile of the absorption line can be recorded with a series of different currents, preferably with small increments, applied to the laser diode. When using a Taylor series, a relationship between the detected signal and the mathematical derivatives produces the absorption lines.
