Test atmosphere generation

When insufficient data are available, it may be necessary to generate test atmospheres to determine the physical state of a compound and the collection efficiency of a filter and vapor collector. The estimation of the vapor/particulate ratio may also depend on concentration and sample loading. For example, in a short sampling time, vapor may be efficiently collected on a filter, but longer term sampling may reveal saturation of the filter with vapor and eventual passage of the vapor into a backup bubbler or sorbent tube. 

If the ratio EVC/std is in the range 0.05 to 100-300, then a mixture of particulate and vapor may be present (7). A ratio above this range indicates the presence of vapor alone and below the range, particulate. The reliability of this determination depends on the accuracy of vapor pressure data. In method development and validation studies, it is often necessary to perform special tests with generated test atmospheres at different temperatures and concentrations to demonstrate the physical form of the substance. 

Generation of test atmospheres of organic vapours by the syringe injection technique Generation of test atmospheres of organic vapours by the permeation tube method On-site validation of sampling methods... 

MDHS 3 Generation of test atmospheres of organic vapours by the syringe injection technique... 

Melcher RG, Borders RA, Coyne LB. 1986. Development and validation of personal monitoring methods for low levels of acrylonitrile in workplace atmosphere I. Test atmosphere generation and solvent desorption methods. Am Ind Hyg Assoc J 47 152-157. 

Examples of such procedures are making dilutions (not all components of the mixture may be equally soluble or miscible with the vehicle) and generating either vapors or respirable aerosols (not all the components may have equivalent volatility or surface tension, leading to a test atmosphere that contains only a portion of the components of the mixture). 

A detailed protocol for laboratory validation of sampling and analytical methods for toxic substances in workplace environments is given in Figure 1. The literature was searched and a method of sampling and analysis was selected. The next step was to evaluate and, if necessary, develop an analytical method that was compatible with the sampling medium. If a satisfactory analytical method became available only then did we undertake generation of a test atmosphere. Then samples were collected with the appropriate collection medium. Both capacity and collection efficiency were evaluated. 

Aerosols. Test atmospheres containing aerosols may be conveniently generated by a number of techniques including ... 

Test atmospheres of known concentration of the pesticide in air were dynamically generated at levels of twice, one-half, and at the OSHA standard for the specific pesticide. 

Another factor regarding recovery of the sample is that the complete sampling device should be tested, not just the filter itself. Generation of test atmospheres may be necessary to perform these tests. This became most apparent in the work on thiram where the filter cassette top collected thiram to some degree. The amount varied from 1% to 12% of the total sample. Fortunately, the thiram on the cassette was stable so that cassettes containing filter samples could be stoppered and shipped for analysis. In laboratory analysis, a separate, cassette-rinse, was analyzed based on a 5-ml wash of the cassette top with acetonitrile. 

A Versatile Test Atmosphere Generation and Sampling System... 

The most widely accepted method of evaluating the accuracy and precision of an analytical procedure is to sample known concentrations of contaminants in the atmosphere. Thus an important aspect of analytical method development is the generation of test atmospheres that simulate the conditions (i.e., concentration range, humidity, temperature and interferences) found during the field sampling. 

Several test atmosphere generating systems have been reported (1-6), Basic principles and techniques employed have been discussed in detail by Nelson (7). The methods are generally divided into two catagories static methods and dynamic methods. 

Generation of Test Atmospheres of Toxic Substances for Evaluation of Air Sampling Methods... 

Laskin-Type Nozzle Generator. A third type of atomizer, the Laskin-type nozzle generator, is used to create test atmospheres of particulate or aerosol and vapor mixtures. A pure liquid or melt is used, and no solvent is necessary. Again, this is important for evaluating filter and solid sorbent combination sampling trains. 

Figure 8. Glass generator for particulate/vapor test atmospheres...

We have developed and used a number of generation techniques for producing test atmospheres for the evaluation of methods of collection of ambient air samples. These methods are applicable to many compounds and, with modification, can be extended to cover most materials. In principle, several compounds may be generated simultaneously with these techniques. Testing over an appropriate concentration range is easily accomplished using the dilution system we have designed. 

Figure 4. Dust generation system the Wright dust feeder introduces dust into the air stream at a constant rate to produce test atmospheres.

The affinity that the solvent vapor has for the activated charcoal or the charcoal s adsorptivity is reflected in the "collection" efficiency. Early studies(i> 2) show that for many solvents, collection efficiencies are similar. No generalization is without exception and therefore test atmospheres should be generated where this information is important. The collection efficiency and desorption efficiency, together with the analytical precision and accuracy are incorporated into the total coefficient of variation for the method. Many solvent vapor sampling methods are not this thoroughly documented in the literature because of the difficulty of generating known test atmospheres. In this study both direct injection and flowing of vapor-air mixtures over the charcoal were used for efficiency determinations these values are reported in table 1 and required much time and effort to obtain. 

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