Sample Acquisition

The initial step of sample acquisition is a far from trivial task in atmospheric measurements, and segregates a major grouping of techniques into either in situ or post-acquisition analysis. The ability to store an atmospheric sample is critical to the decision on whether analysis may be performed back in the laboratory or on-site immediately following acquisition. Stable species such as methane, carbon monoxide and chlorinated fluorocarbons may be stored successfully in sample vessels for many weeks without affecting sample integrity, and widespread measurements of these species have therefore been performed. Atmospheric degradation products, such as peroxyacetyl nitrate (PAN, CH3C(O)2ONO2), are so unstable, however, that analysis must be performed immediately. For many other important reactive species such as alkenes, monoterpenes and dimethyl sulphide, the storage of samples has been shown to lead to a degree of analyte losses due to reaction with co-sampled pollutants such as ozone and oxides of nitrogen. To overcome these problems of reaction during storage, many forms of scrubber (e.g. potassium iodide and glycerol to remove ozone) have been tested to remove such species without affecting sample integrity.

Much atmospheric sampling is performed using canister methods, filled either by vacuum release or by pumping sample into the canister using stainless steel or Teflon diaphragm pumps. For any sample to be stored successfully there must be minimal interaction with the canister walls and coating methods such as electropolishing, silica or Teflon coating are currently in use. The preparation and cleaning of canisters require careful attention and high vacuums are often applied together with elevated temperatures for periods of hours or even days. A similar method to canister samples is the use of collapsible Teflon or Tedlar sample bags. These bags may be filled by pump and returned to a laboratory for analysis using very similar procedures to those used with canister

Figure 1 Low molecular weight hydrocarbons including methane determined using an online activated charcoal adsorbent trap in a programmed temperature vaporization injector. Column 50 m, 0.53 mm i.d. Al2O3/NaSO4 PLOT (Chrompack) 10 ^m film thickness. Desorption temperature was at 16°C s~1 from - 20°C to 400°C, and column temperature programmed from 45°C to 200°C. Chromatograms showing (a) the separation of C1-C6 components of Leeds city-centre air, and (b) a blank following desorption. Peaks: 1 = methane, 2 = ethane, 3 = ethene, 4 = propane, 5 = propene, 6 = 2-methylpropane, 7 = ethyne, 8 = n-butane, 9 = trans-2-butene, 10 = 1-butene, 11 = isobutene, 12 = cis-2-butene, 13 = 2-methylbutane, 14 = n-pentane, 15 = 1,3-bu-tadiene, 16 = pentenes, 17 = 2-methylpentane, 18 = 3-methyl-pentane, 19 = n-hexane, 20 = methyl hexanes and hexenes, 21 = heptane, 22 = methylcyclopentane, 23 = benzene, 24 = toluene. Reproduced with permission from Lewis AC and Bartle KD 1996. A simplified method for the determination of atmospheric hydrocarbons. LC-GC International9: 297-304.

samples. While generally of lower unit cost, the bag method often produces a greater degree of sample artefact and analyte losses.

Some low volatility species are less suitable for sampling using canister methods because of the increased possibility of analyte condensation on to the walls of the container. For this reason a second method of sample acquisition is widely used based on use of a solid-phase adsorbent as an analyte trap. The adsorbent used in the trap may be chosen to introduce an element of selectivity to the trapping mechanism, although in practice a trap-all approach is commonly used. Adsorbent traps allow for instrument automation that is not always possible with canister methods and such traps now form the basis of many national monitoring programmes in the urban environment, as well as automated instruments for research in very clean air.

A huge range of adsorbents is currently available from high surface area (>1000m3g~1) carbon-based materials with strong retention characteristics to lower surface area (< 50 m3 g"1) polymers such as Tenax TA. While being relatively low cost compared to sample canisters, care is often required in the cleaning and preparation of sample tubes. Samples may be introduced to the adsorbent tubes either dynamically over short periods of time (typically minutes) or via diffusional sampling over longer periods (typically several days). Carbon-based adsorbents are suitable for a wide range of species ranging from volatile hydrocarbons and CFCs to organic nitrates. Polymeric materials are used mainly for the concentration of low volatility species such as aro-matics and monoterpenes in air, although compounds as large as two and three ring polycyclic aromatics may also be successfully trapped and desorbed from the gas phase.

Sulfur species trapping is often performed via chemisorption on to gold wool traps that provide a stable matrix for the sample to be stored for reasonable periods of time.

The complete retention of all target analytes on the adsorbent bed must be carefully evaluated and calculation of retention volume (often referred to as breakthrough volume) is essential. For sampling very volatile species (for example ethane or ethene) the retention volume for an adsorbent system is often the critical factor in determining system sensitivity. Trapping at sub-ambient temperatures is commonly employed to increase the maximum sample volume, often using liquid nitrogen or carbon dioxide. For operation of any instrument in a field location, the supply of these coolants may be a problem and modern instruments often now employ Peltier coolers.

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.

Get My Free Ebook


Post a comment