RECENT DEVELOPMENTS IN SECOND DERIVATIVE SPECTROMETRIC ANALYTICAL INSTRUMENTATION

Abstract

A derivative spectrometer contains a means for producing wavelength modulation in time such that radiation exiting the spectrometer is of wavelength $\lambda =\lambda_{0}+ a \sin (wt)$, where ``a” is constant setting the amplitude of modulation. Radiation to be analyzed by a derivative spectrometer will produce a harmonic of the fundamental frequency w in a detector output, the amplitude of the nth harmonic being directly related to the amplitude of the nth derivative of intensity with respect to wavelength. Instrumentation has been developed and manufactured which measures the second derivative of intensity with respect to wavelength. In a laboratory analyzer, $\lambda_{0}$, the center wavelength, can be scanned from 200 to 700 nanometers to obtain extremely sensitive measurements of any trace gas which exhibits molecular band absorption. Also, tuned instruments are available which have a fixed $\lambda_{0}$ and can continuously monitor a particular gas concentration to levels below 10 parts per billion. The same techniques are very successful in analyzing emission spectra for very weak lines. For example, trace nitrogen in argon measurements are continuously monitored by analyzing radiation from an electrically excited discharge within the flowing sample. The specific methods employed to obtain second derivative spectra and details of the applications will provide the body of this paper.