Whole-Air Sampling

Gas Samples for Subsequent Laboratory Analysis. Collection of air samples for later analysis in the laboratory is a common technique used for aircraft sampling. Whole air sampling for stable compounds (CO, CO , ha-locarbons, and low-molecular-weight hydrocarbons) is usually accomplished by filling a container to a pressure of about 2 atm (203 kPa) with a metal bellows pump. Alternatively, containers may be evacuated in the laboratory to a low pressure and filled during flight by simply opening a valve at the appropriate time. Containers are typically constructed of stainless steel that has been electropolished or treated in some way to reduce surface activity... 

Collection. Procedures commonly used to collect vapor-phase organic compounds include whole-air, cryogenic, adsorption, absorption, and de-rivatization methods. Whole-air sampling involves the capture of an air parcel in a container. Stainless steel canisters or plastic bags constructed from an inert material, such as Teflon or Tedlar, are most commonly used. Each of... 

This presentation focuses on the vertical distribution of halocarbons obtained by analysis of cryogenically collected whole-air samples. The balloon-borne cryogenic samplers developed and flown by the Max-Planck-Institut fur Aeronomie (MPAE) and the Kem-forschungsanlage Jiilich (KFA) are described by Fabian [17] and Schmidt [18]. Between 1977 and 1993, a total of 33 balloon ascents have been carried out by both institutions, 28 at northern midlatitudes (southern France, 44°N), 3 at high latitudes(Kiruna, 69°N) and 2 in the tropics (Hyderabad, 17,5°N). These stratospheric data are limited to balloon altitudes, i.e. up to about 35 km. Tropospheric data were obtained from balloon samples, samples collected aboard aircraft and at ground level. 

Fig. 6 shows a typical result of single balloon flight of the MPAE cryogenic sampler carried out on 23 June 1987 in southern France (44°N). Vertical profiles of 14 halocarbons are displayed resulting from analyses of 15 whole air samples collected during... 

Figure 6. Vertical profiles of 14 halocarbons as analysed from whole air samples collected on 23 June 1987...

Compared with whole air sampling into Tedlar bags and canisters, active sampling onto sorbent materials is used more widely in these indoor air quality (lAQ) studies. Only a few studies made use of organic vapour monitor passive samplers. Of the sorbent materials used, Tenax is the most frequently employed, possibly because of its virtues, which are mentioned in Sect. 4.2.1. It has been used for the characterisation of aromatics, alkenes, cycloalkanes, aldehydes, ketones, esters, alcohols, terpenes, glycol derivatives and even amines [33,59]. 

FIGURE 16.1 Cryogenic preconcentration/high-resolution gas chromatographic system for the analysis of whole-air samples collected in canisters. 

FIGURE 2.3 Atmospheric abundance of N20 over the last millennium, as determined from ice cores, fim, and whole-air samples (IPCC 2001). Sources of data are indicated, references for which are given in IPCC. The inset contains deseasonalized global averages. 

Whole-air samples, in general, collect a segment of the atmosphere and return it to the lab unchanged for analysis. When sampling with concentration techniques are used, the desired substances are separated from air by absorption in a liquid reaction to form a nonvolatile product, adsorption on solid sorbents or condensation. 

Whole air sample, where a sample is taken and returned unchanged to the laboratory for analysis. This approach is very useful for maintaining sample integrity and where the analyte concentration is sufficiently high to avoid a sample preparation stage before analysis. 

Possible and, for certain carrier materials, already known decomposition reactions have been mentioned. For this reason, another method is favoured by the EPA for air analysis. Electropolished and passivated canisters, called SUMMA canister expressing the highest inertness, of about 2 L capacity, maximum up to 15 L are evacuated for sample collection. The whole air samples collected onsite on opening the canister can be measured several times in the laboratory following EPA methods such as TO-14 or TO-15. Suitable samplers are used, which are connected online with GC-MS. Cryofocussing is used to concentrate the analytes from the volumes collected. If required, the sample can be dried with a semiper-meable membrane (Nafion drier) or by condensation of the water (MCS, see also section 2.1.5.2). Adsorption materials are not used in these processes. 

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