Browse Prior Art Database

Derivative Spectrometer

IP.com Disclosure Number: IPCOM000088925D
Original Publication Date: 1977-Aug-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 63K

Publishing Venue

IBM

Related People

Ewalt, AB: AUTHOR [+2]

Abstract

The quantitative chemical analysis of molecular or atomic mixtures is a common requirement for optical and atomic absorption spectrometry. The key factor in determining accuracy of measurement results is spectral interference at the analytical wavelength of the band peak or line of interest caused by band or line overlap. Similarly, weak line-spectra nested in or on broad band or continuum backgrounds degrade detectivity.

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Derivative Spectrometer

The quantitative chemical analysis of molecular or atomic mixtures is a common requirement for optical and atomic absorption spectrometry. The key factor in determining accuracy of measurement results is spectral interference at the analytical wavelength of the band peak or line of interest caused by band or line overlap. Similarly, weak line-spectra nested in or on broad band or continuum backgrounds degrade detectivity.

A technique which can improve detection limit by more than an order of magnitude is to measure the derivative of the usual transmission curve with respect to wavelength. The derivative output signal is directly proportional to the concentration of absorbers in the sample path. This may be done by rapid repetitive scan, or "dithering", of a narrow wavelength interval with the center wavelength being the line, band peak, or other choice of analytical wavelength. This periodic scan is synchronized with a tuned AC amplifier locked to the fundamental or harmonic multiple of the dither frequency, f(d). The modulated signal produced is the first derivative of intensity with respect to wavelength when the amplifier is locked to f(d). At twice this frequency, or 2f(d), the second derivative is obtained.

The signal-to-noise and signal-to-background ratio is increased by bandpass filter rejection of the low-frequency "flicker noise" (commonly called "1/f noise") and power-line GOH(z) interference noise, associated with the spectrometer light source and electronic components. Thus, noise sources of different frequency than the modulation frequency will not pass the phase-sensitive lock-in amplifier. Similarly, by amplitude modulating light beam intensity by "chopping" the source, DC offset and drift can be eliminated.

As shown in Fig. 1, the spectrometer includes a rotary wheel 1 driven by a stepping motor 2. Mounted on wheel 1 is a CVF (circular variable filter) of the interference type, the CVF being in the shape of a sector of an annulus. The CVF passes light of the same nominal wavelength through any radial section, but passes light of different wavelengths through different angular positions which vary uniformly from one end of the filter to the other. A movable slit member or slit 3 is disposed adjacent wheel 1 and has a slit therein that extends in a radial direction relative to the wheel. Light from a polychromatic light source passes through slit 3 and impinges on wheel 1 so that, when CVF is opposite slit 3, a narrow band of monochromatic light having a nominal wavelength of Lambda(C) passes through the CVF. A sample 6 may be inserted into the light path emerging from the CVF, and a detector 7, such as a photomultiplier tube (PMT), is irradiated by the light passing through the sample so as to produce an output proportional to the intensity of such light. This output is fed to a lock-in ampli...