Sulfur Selective Detectors

Sulfur is an important element which turns up in many fields usually with deleterious effects; 0.1 ppm of a mercaptan in isopropyl alcohol would, for example, render it totally unfit for perfumery applications.

The flame photometric detector (FPD) monitors the light emitted by a hydrogen-rich 'cold' flame. Under these conditions sulfur (the S2 species) has a band spectrum with a maximum at 394 nm and phosphorus (HPO) has a band spectrum with a maximum at about 526 nm. Since these are band spectra they do not exhibit the very sharp emission lines of atomic spectra and are, therefore, only moderately selective. There are carbon band spectra, for example at 388 nm, that tend to interfere with the sulfur spectrum. The response is approximately equimolar for different sulfur compounds unless oxygen is also present in the molecule. Since it is an S2 species being monitored in the flame the response is approximately proportional to the square root of the concentration. This disadvantage can be catered for by suitable electronics but there is always some doubt about whether the response follows the square root relationship accurately and calibration is essential for reliable quantitative results. Another severe disadvantage of the simple FPD is that co-eluted organic compounds not containing sulfur will 'quench' the sulfur emission and cause a drastic diminution of signal.

In spite of these disadvantages the FPD has proved a popular and important detector especially in the food and petroleum industries. For example, in the former it has been used for the detection of mercap-tans in lager. In the latter it has been shown that a crude oil, so biodegraded that it can no longer be identified by its hydrocarbon fingerprint, can still be recognized by its sulfur fingerprint since the sulfur compounds are much more slowly degraded.

There have been a number of attempts over the years to improve the performance of the FPD. A dualflame version oxidizes the sulfur compounds to SOx in an ordinary oxidizing flame and the products of combustion are then taken to the hydrogen-rich cold flame. This gives a considerable reduction in the quenching effect of co-eluting compounds but at the cost of at least 10-fold loss in sensitivity from about 10~9 gs_1 for the single flame version. Another version, the pulsed FPD, developed by Amirav et al. in 1991, reduced the hydrogen flow rate so that the flame is extinguished and re-ignited about 2-4 times a second. The emission during each period of emission is scanned from the time of ignition and electronically time-gated. Under these conditions it is possible to discriminate on a time basis between carbon emissions taking place at 2-3 ms after ignition and sulfur emissions which take place at about 6 ms. Thus with this design there is both wavelength and time discrimination and the combined effect is a very high selectivity and a sub-picogram per second sensitivity. In spite of these advantages there are few publications on the use of the PFPD to date.

Most versions of the FPD can be used for phosphorus detection by changing the filter from 394 nm for S to 526 nm for P and a number of other elements such as Se, Sn, As and Ge have also been determined at various wavelengths and with varying degrees of selectivity and sensitivity. Simultaneous detection of two elements is also possible.

A different type of sulfur/nitrogen selective detector has been successfully developed in the last few years. In the sulfur chemiluminescence detector (SCD), the sulfur-containing compounds are combusted in an oxidizing flame or in later versions in a miniature ceramic furnace to SO^ which is then reacted with ozone in a low pressure chamber at about 10-15 Torr. Reaction with ozone raises the sulfur oxides to an excited state and as the molecules drop back to the ground state they emit light in the far blue end of the spectrum which is monitored by a photo-multiplier tube after passing through an optical filter. Sub-picogram per second sensitivity is claimed for this detector with a linear response over five orders of magnitude, equimolar response for different sulfur compounds and no quenching effects. By replacing the optical filter for sulfur with one in the red region (610 nm) the detector can be used selectively for nitrogen. This is, in effect, similar to the so-called Thermal Energy AnalyserTM which was produced in the 1970s specifically for the analysis of nitrosamines in food but which never achieved wide popularity.

Figures 1A-D show chromatograms of a gas oil before and after a hydrotreater unit designed to reduce the total sulfur content of the fuel from about 220 ppm to 2 ppm. Figures 1A and B show the before and after FID traces where it is difficult to see any difference and Figures 1C and D show the before and after SCD chromatograms where the difference is clearly apparent.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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