In gas chromatography a sample of interest is volatilized and injected into a gas chromatography column, typically housed in an oven. A carrier gas flows constantly through the column sweeping the sample along with it. Differential adsorption and desorption of the sample constituents on the partition medium in the column separates the sample into its components. Having been thus separated, the constituents of the sample elute from the column at different times and flow to a detector which continuously measures one or more properties of the gas eluting from the column. A change in the properties being measured relative to the baseline property of the carrier gas signifies that a sample constituent is passing through the detector. This is commonly referred to as a "peak". A recording of the detector signal, which may contain a large number of peaks, is called a chromatogram.
A variety of detectors are available to the chromatographer. The selection of what type of detector to use is a function of a variety of factors including the type(s) of samples being investigated, cost, sensitivity, selectivity and others. Some detectors respond well to a broad variety of sample species while others are useful for only specific types of compounds.
Other type of detector which has gained increasingly widespread attention for use in gas chromatography is the plasma emission detector. In a plasma emmission detector sample from the GC column is introduced into a high temperature atmospheric pressure plasma where the sampled molecules are broken up by action of the thermal energy into atomic species and ionized. As the atoms in the plasma undergo engergy transitions they emit characteristic light spectra which are detected by light sensors, typically photodiodes. Sample identification can be made by monitoring the wavelengths and intensities of the light emitted from the plasma. The plasma emission detector has been shown to be a highly sensitive universal detector.
In the prior art, the use of photodiodes to correct background changes has consisted of using two separate photodetector packages, with one designated as a reference. Typically, the two signals were digitized, stored in a computer, and with appropriate attenuation the reference signal was subtracted to the signal of interest. (See: R.K. Skogerboe et al., A Dynamic Background Correction System for Direct Reading Spectrometry, Appl. Spectrosc. 30, 495-500 (1976); B.E. Weekley et al., A Versatile Electronic Computer for Photoelectric Spectrochemical Analysis, Appl. Spectrosc. 18, 21 (1964).)
Another form of prior art background correction utilizes a single detector in which the wavelength of interest and the background wavelength are alternatively focussed on the detector. The means to accomplish this could be to translate the entrance slit, tilting the grating, or the use of a refractor plate. The resulting modulated signal can be synchronously detected. The result is a signal that is proportional to the difference between the signals at the two wavelengths (signal & background--background). (See: W. Snelleman et al., Flame Emission Spectrometry with Repetitive Optical Scanning in the Derivative Mode, Anal. Chem. 42, 394-398 (1970) and S.A. Estee et al., Microwave-Excited Atmospheric Pressure Helium Plasma Emission Detection Characteristics in Fused Silica Capillary Gas Chromatography, Anal. Chem. 53, 1829-1837 (1981).)
Another form of prior art background correction utilizes a diode array. The wavelength of interest falls on one or more elements of the array, while the background light at adjacent wavelenghts fall on array elements on both sides. The array is then scanned, digitized, and the background sustracted. (See: G. M. Levy et al., A Photodiode Array Based Spectrometer System for Inductively Coupled Plasma-Atomic Emission Spectrometry, Spectrochim Acta 42B, 341-351 (1987).)
Prior art requires using two or more electrometers; two or more signals to be digitized, stored and manipulated to acquire a spectrum free of background fluctuations. However, positioning a reference diode close to the signal of interest or in a position where background fluctuations are representative of changes at the wavelength of interest may be difficult or impossible.
A further disadvantage of prior art devices is the increase in noise due to the additional random noise sources in the readout electronics that follow the diodes. Both wavelength modulation and diode arrays have the additional limitation of signal bandwidth or readout time, which would limit the noise bandwidth that could be compensated for.