Open path spectrometer

An open-path spectrometer is designed according to the features of dispersive and interferometer-based instruments with the exception that the sample is located remotely from the instrument and the light source is either sunlight or laser power. The basic configuration for sampling in the open path design is shown in Fig. 4. 

Applications The differential optical absorption spectrometer has been used to monitor concentrations of gases or intermediate compounds such as SO, NO, O5, HCHO, HNO, CS, NO, and OH in the atmosphere.In atmospheric measurements with open paths of 100 to 1000 m, a detection limit of about 1 ppb can be achieved. In the emission measurements, the path length across the duct or the plume can range in meters. 

A balloon-borne, open path, tunable diode laser spectrometer (21) provides a particularly elegant technique that combines the advantages of in situ and remote sampling. The radiation absorption path is defined by the laser on the balloon gondola and a retro reflector suspended up to 500 m below. Thus, a well-defined parcel of air is analyzed and the effects of sampling inlets are avoided. NO, NO , HN03, N20, and 03 have been measured simultaneously (22). 

P R. Griffiths, R. L. Richardson, D. Qin, and C. Zhu, Open-path atmospheric monitoring with a low-resolution FT-IR spectrometer, in Optical Sensing for Envimnmental and Process Monitoring, O. A. Simpson, Ed., Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 1995, Vol. 2365, p. 274. 

For many applications, there may be some advantage in employing phase modulation 54,85) instead of the usual amphtude modulation. In the latter technique the path of the radiation from the source to the detector is blocked and opened periodically by a chopper (cf. Fig. 20 and Section 4.3). For phase modulation, the chopper is removed from the spectrometer and the fixed mirror of the Michelson interferometer is moved back and forth about its mean position with a certain frequency. In contrast to the interference modulation (see Section 4.2), the amplitude of the mirror motion is small, being a quarter of the wavelength of the light. For the analogue Fourier transform or interference modulation, the amplitude of the mirror has to have many wavelengths in order to achieve a reasonable resolution... 

The most widely used atomiser for hydride generation is the heated quartz T-tube atomiser with a typical diameter of 10 mm and a length of 100—150 mm, making it compatible with the optical path of most AA spectrometers. The quartz tube is electrically heated to 700—1000 °C which permits one to optimise the atomisation temperature for each element. The quartz tube may either have open ends, or these ends are sealed by removable quartz windows, and holes at the extreme ends of the quartz tube provide the gas flow outlets. This set-up increases the residence time of the atoms in the light path and thus improves sensitivity. With continued use the performance of the quartz tube atomiser invariably deteriorates in terms of sensitivity and precision. This is attributed both to devitrification of the inner surface of the quartz tube to a less inert modification, and to contamination of the inner atomiser surface by deposition of small particles and droplets that were not efficiently removed by the gas—liquid phase separator. 

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