Air

Near real-time results may be particularly beneficial for a system where a centrally located high speed gas chromatograph is connected to a network of sampling lines. The high speed GC analysis of VOCs must be able to detect compounds at and below regulatory levels, such as the threshold limit value (TLV).

Original work utilized a valve loop system for sampling, but the use of mechanical valves may be a problem for an automated system. Valves may require periodic maintenance, or may change the sample through interaction with valve surfaces. In addition, changing the sample size requires either changing the loop or the use of several loop cycles. Ice formation which may plug the collection tube, resulting from water vapour in the air is another area of concern. However, studies to date have not shown this to be a problem.

Figure 1 Complete high speed GC system with direct atmospheric sample collection. T, Cold trap system; V, valves; R, restrictors; P, vacuum pump; I, atmospheric pressure source; C, separation column; G, gas source. Arrows show flow directions during sample collection. (Reproduced with permission from Akard and Sacks, 1994.)

Figure 1 Complete high speed GC system with direct atmospheric sample collection. T, Cold trap system; V, valves; R, restrictors; P, vacuum pump; I, atmospheric pressure source; C, separation column; G, gas source. Arrows show flow directions during sample collection. (Reproduced with permission from Akard and Sacks, 1994.)

An alternative approach to achieve the detection limits required is a technique termed cryointegra-tion. In this method, VOCs from an air sample, usually contained in a sampling loop, are cryofocused in a liquid nitrogen-cooled metal tube. Thus, dilute samples can be preconcentrated prior to high speed analysis. Resistive heating of the metal tube is used to ensure a narrow injection plug compatible with high speed GC requirements. This has been incorporated with a reversed-flow sample collection system. Figure 1 shows the complete high speed GC system for an air monitor using this type of set-up.

The vacuum pump pulls sample and carrier gas through the cryotrap for the sample collection period. This time period can be adjusted to allow for more or less sample to be collected as required. After collection, the sample is introduced on to the separation column by heating the metal cryotrap tube.

The positioning of the sample near the column end of the cryotrap reduces band broadening and the potential for sample decomposition. The decreased bandwidth leads to narrower, taller peaks, which in turn improve the limit of detection (LOD). Although water vapour in the sample condensing in the cryo-trap is not a problem, water vapour from the flame ionization detector (FID) is. For this reason, a gas source attached midway through the separation column has been used to prevent water vapour from the detector reaching the cryotrap.

Figure 2 shows representative chromatograms obtained using gasbag samples. The sampling flow rate is constant and about 0.3 mL min-1 once steady-state flow is achieved. There is, of course, a trade-off between reducing the LOD and loss of real-time data as the sampling time increases.

Tables 1 and 2 show the limit of detection values obtained using the conventional loop system and the reverse-flow collection system respectively. It can be seen that, as the sampling time increases with the cryointegration system, the LOD values approach those obtained using a fixed 1 mL loop. One would expect that further increases in sampling time would reduce the LOD even further. The disadvantage with further increases in sampling time is the loss of realtime information. These LOD values are already below regulatory or guideline concentrations such as the TLV and a further reduction in LOD would seem unnecessary.

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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