Detection Systems

Even the highest resolution capillary columns often have insufficient peak capacity to resolve all components in a typical atmospheric sample. Since selectivity in trapping is not always possible, selectivity in detection is a useful tool in the simplification of atmospheric analysis.

The flame ionization detector (FID) is by far the most commonly used detector in atmospheric analysis by GC since it offers high sensitivity, extremely wide linearity and good reliability. Using well-cleaned fuel gases coupled to low noise electrometer circuitry it is possible to determine amounts down to 1 pg s_1. With a typical sample volume of 1 L, detection limits for individual species may be in the low parts per trillion range (10~12). Calibration can be performed with relative ease but the complexity of samples can make peak identification difficult. Methods for alkene/aromatic analysis utilizing selective response from the photoionization detector (PID) have been proposed although they are not in widespread use.

Mass spectrometry offers obvious solutions to problems of compound identification but the operation of currently available bench-top MS in full scan mode often has insufficient sensitivity for trace level measurements. Spectral information obtained from GC/MS of atmospheric hydrocarbons often leads to highly similar fragmentation patterns and assists little in the identification of isomeric species. Similarly, identification of monoterpene species can only be confirmed through a combination of both spectral information and retention time data.

While MS of hydrocarbon species in clean air is often unsuccessful when operating in full scan mode there is sufficient sensitivity and resolution to allow for detection of long-lived CFC species. Long-term monitoring of these species has been performed by GC-ion trap detection in the worldwide MS-GAGE (global atmospheric gases experiment) network. In single ion mode, femtogram sensitivities can be achieved and this approach has been used for field monitoring of naturally produced trace level iodo-and bromo- compounds.

More commonly used for CFC measurements in the atmosphere is the electron-capture detector (ECD) that offers high sensitivity to certain species plus high selectivity over hydrocarbon compounds. GC-ECD measurements require careful calibration due to the great variation in response to halogenated species, although their high stability allows gas standards to be used over many years. Some halogen-containing species of interest (e.g. CH3Cl, CHF2Cl, CH2Cl2) have a relatively poor ECD response and the use of the oxygen-doped ECD to enhance their response has been successful and is demonstrated in Figure 3. The determination of some nitrogen-containing species is also performed using ECD, notably in the areas of organic nitrate analysis and the determination of PAN. Organic nitrate analysis using ECD is often complicated by the co-elution of halogenated compounds, so often a nitrogen-specific detector such as the chemiluminescence detector is used in parallel.

Detection of CO is generally performed using the reduction gas detector (RGD) where hot HgO reduction with one CO molecule releases one Hg molecule from a catalytic bed. The Hg molecule released is then detected by UV absorption.

The analysis of sulfur compounds in the atmosphere, in particular dimethyl sulfide (DMS), has often been performed using a combination of GC with sulfur selective detection to overcome problems of insufficient chromatographic resolution. Flame photometric detection (FPD) has been used extensively in the past although signal quenching by co-eluting hydrocarbons results in drastically reduced sensitivity. The Hall detector or electrolytic conductivity detector (ELCD) has also been used for atmospheric determinations although it requires regular maintenance making it unattractive for an automated instrument. Emerging methods are now taking advantage of significant advances in bench-top atomic emission detectors (AED). The multielemental nature of the AED offers significant advantages in atmospheric measurements both in terms of sensitivity (sulfur - 2pgs~1), and where concurrent emission line measurements for carbon and hydrogen may provide information on empirical formulae of unknown species. The sulfur chemiluminescence detector, which is a more recent technique that offers extremely high sensitivity and selectivity, may yet find an important role in atmospheric sulfur analysis.

The analysis of oxygenated species in the atmosphere is one of the least studied areas using gas chromatography. It is an area of fundamental significance where species may be present in the atmosphere as direct emissions or as degradation products of olefins. While most aldehydes and ketones may be successfully separated using GC, sample preparation and analyte detection are the key problem areas. Sample preparation is an area where new adsorbents may provide selective concentration of polar species simplifying the chromatogram produced. Detection of oxygenates is a further difficulty due to their often very low response in both ECD and FID. Use of elemental specific detection such as AED offers some potential in atmospheric oxygenate analysis although AED sensitivity for oxygen is only around 100 pgs-1. Detectors such as the helium ionization detector (HID), which produce a nonselective high sensitivity response to these types of compounds, may in future allow on-line measurements of oxygenates with GC, assuming sufficient chromatographic resolution or trapping selectivity can be obtained.

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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