Spectrophotometric measurement of gas and vapour absorption is well known in various applications. An example is analytical chemistry where ultraviolet (UV) spectrometers are combined with gas chromatography (GC) for determining gaseous compounds. Examples of equipment for such use are disclosed in U.S. Pat. No. 4,668,091 and U.S. Pat. No. 6,305,213. Various UV spectrometers or spectrographs as well as gas chromatographs are commercially available.
Ultraviolet radiation has a wavelength shorter than that of visible light but longer than X-rays, in the range 10-400 nm, and its spectrum can be subdivided in a number of ways, for instance, near UV 400-300 nm, middle UV 300-200 nm, far UV 200-122 and vacuum UV 200-100 nm. Ordinary glass is opaque to shorter wavelengths. Windows made of quartz glass are normally used when analyzing wavelengths shorter than around 350 nm. Wavelengths shorter than around 190 nm, i.e. radiation in the far UV or vacuum region, are normally absorbed too strongly also in quartz. In this wavelength region it is possible to use windows and other optical elements made of alkaline earth metal fluorides, such as MgF2. However, such materials are not as resistant to chemicals as quartz and there are a number of applications where these materials are not suitable. This is in particular a problem at elevated temperatures.
UV wavelengths in the far UV or vacuum region is of great interest in chemical analysis since most chemical compounds absorb light in that region. Many compounds that cannot be detected at longer wavelengths may be analyzed using radiation in the far UV region. However, also air, or rather oxygen (O2) and water vapour, absorbs light strongly in this region (in the range below around 190 nm) which means that the analytic equipment must be specially adapted to avoid interference from air. For this purpose there are, for instance, UV spectrographs commercially available that are adapted to be evacuated or to be filled with an inert gas, such as nitrogen (N2) (which absorbs UV only at very short wavelengths).
Equipment for analyzing gases and/or vapours with UV spectroscopy typically includes an elongated heated channel (cell) that accommodates a sample gas during measurement, a UV-source (e.g. a deuterium lamp) positioned at one end the channel and a UV detector (a UV spectrograph) located at the opposite end of the channel. Gas tight and UV-transparent windows, typically made of quartz, are provided at each end of the channel. The channel may be provided with inlets and outlets for leading sample gas, e.g. from a GC, and carrier gas to and from the channel in a continuous manner. UV light emitted by the UV-source passes through the window of the UV-source, through the entrance window of the channel and into and through the channel where part of the light is absorbed in the sample gas. Remaining radiation passes the exit window of the channel and enters the UV spectrograph through a window or slit. The UV spectrograph measures the intensity at different wavelengths of the UV radiation that has passed through the channel and the absorption spectra obtained is used to identify and quantify the compounds present in the sample gas.
To adapt such equipment to measurements of radiation in the far UV region a UV transparent (e.g. evacuated) UV detector can be used and the quartz windows can be replaced by windows made of an alkaline earth metal fluoride. However, such an adaptation works well only in situations where the sample gas does not contain compounds that may decompose the sensitive material of the channel windows. Such degradation of the channel windows is difficult to prevent in many cases, in particular at elevated temperatures, such as when the sample gas is supplied from a heated column of a gas chromatograph, Accordingly, there is a need for improvements in the field of equipment for use in far UV spectroscopy analysis of a gas.
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