In analytical chemistry liquid chromatography (LC), gas chromatography (GC) and supercritical fluid chromatography (SFC) techniques have become important tools in the identification of chemical sample components. The basic principle underlying all chromatographic techniques is the separation of a sample chemical mixture into individual components by transporting the mixture in a moving fluid through a porous retentive media. The moving fluid is referred to as the mobile phase and the retentive media has been referred to as the stationary phase. One of the differences between liquid and gas chromatography is that the mobile phase is either a liquid or a gas, respectively.
In a gas chromatograph, typically, a supply of inert carrier gas (mobile phase) is continually passed as a stream through a heated column containing porous sorptive media (stationary phase). GC columns have also been known to comprise a hollow capillary tube having an inner diameter in the range of few hundred microns. A sample of the subject mixture is injected into the mobile phase stream and passed through the column. As the subject mixture passes through the column, it separates into its various components. Separation is due primarily to differences in the volatility characteristics of each sample component with respect to the temperature in the column. A detector, positioned at the outlet end of the column, detects each of the separated components as they exit the column.
In supercritical fluid chromatography, a fluid heated above the critical point, is used as the mobile phase. Such fluid is passed under pressure through a media, typically a capillary column, which differentially retains sample components. As the pressure of the mobile phase is increased, for example, from about 40 ATM to approximately 400 ATM, the sample being analyzed separates into its various components dependent upon the relative differential solubility of each component with the mobile phase. Since the mobile phase is a gas, detectors used in GC can be utilized a the outlet end of the column to detect the separated components as they exit.
In certain circumstances it is desirable to detect the actual elements or molecules present in each component, as each component exits the column. In such cases spectrochemical analysis is the choice detection technique. Such analysis typically utilizes atomic emission spectrometry detectors or mass spectrometers. Similar to other GC and SFC detectors, atomic emission detectors (AED) and mass spectrometers are typically positioned at the outlet end of the column to detect elements in the chromatographic effluents.
In such devices, a discharge tube is oriented to surround the outlet end of the column. An energy chamber is formed around portion of the discharge tube. A plasma generator is positioned to provide energy to the energy chamber to sustain plasma, for example, a magnetron positioned to provide microwave energy. The microwave energy causes a plasma discharge to be formed in the discharge tube. Chromatographic effluents entering the plasma are energized and separated into excited atoms or molecules. Excited ions escaping from the plasma are available as a source for mass spectrometry detection. As the electrons of the excited atoms or molecules return to their stable state, light is emitted which is unique to an element or molecular bond. In atomic emission detection, the light is separated into characteristic wavelengths and detected. For atomic emission detection, the detection process can also be described as monitoring and plotting as a function of time the atomic emission line for a particular element. The power output of the energy source, i.e., the magnetron, determines the intensity of the atomic emission line versus the concentration of the element in question. U.S. Pat. Nos. 4,654,504--Sullivan et al. and 4,776,690 --Quimby describe and apply such AED devices.
The problem with such prior spectrochemical analysis devices is the ability of the detector to repeat measurements over time, i.e. day to day detector precision. In other words, the measurements made by such prior devices can vary from day to day depending on ambient laboratory conditions. Consequently, a need exists for a spectrochemical device having stable precision.
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