Occupational Saftey and Health Administration (OSHA) and National Institute for Occupational Saftey and Health (NIOSH) are organizations which set the standards for monitoring airborne contaminants. They have published the methods for monitoring many airborne contaminants. These methods are continually being revised and updated as collection techniques improve and/or exposure regulations change.
The most reccomended method for collecting airborne contaminant vapors is buy the use of absorbent tubes. As shown in figure A, below.
There are other methods such as filter cassettes and/or sampling devices for collecting airborne contaminants. For example, in the NIOSH method 1,400 an absorbent tube containing 150 mg of coconut charcoal, devided into 100mg front bead and 50 mg backup bed is recommended for collecting alcohol vapors. The front bed efficiently traps the airborne analyte and the backup bed is monitored to ensure that no analyte has passed through the front bed. Such breakthrough would indicate possible sample loss and therefore would make a sample invalid. This method is not as effective as the airborne tubes which incorperate absorbence and filters. Some examples of these more modern methods are OSHA versatile sampler and OVS tubes. An even more in depth approach is to have an absorbent tube with filter cassettes.
To commence sampling of airborne vapors, the seals at the end of the absorbent tube must be broken, or for a filter cassette the end of the tube must be unplugged. The sampler must be placed in a holder then connect the holder to the inlet of a battery operated personal pump. See figure 4, below
The pump must be adjusted to draw air through the tube at the appropriate flow rate. The standard time set by the Environmental Protection Agency (EPA) is 4 hours. The pump must then be calibrated to work at the desired inflow. When deciding on a flow rate it is important to make sure that saturation does not occur. If the sample is saturated it will be useless to the study. The sampling device should be carried near the breathing zone of persons who are normally exposed to the contaminant, alternatively if monitoring an area, simply place the device in the area to be monitored. The air to be monitored should not pass through any other material before entering the collection device. During the sampling the absorbant tubes should be kept in vertical positions to prevent shift in the absorbant bed. If not kept vertical it might be allowed for vapor to channel through. The temperature and pressure should be monitored throughout the sampling. This should be done to correct the air sampling volume to standard temperature (25 degrees celcius) and pressure (760 mm HG). To establish a baseline (field blank) value open and immediately cap a tube at the sample site. After the recommended volume of air has been drawn through the sample device, remove the device from the holder and cap the ends. The absorbent tubes can be refrigerated for analysis at a later date if necessary.
In the lab, desorbing or extracting the airborne contaminants for the absorbant or filter, use a solvent. It is reccomeended to use a well ventilated hood. During desorbtion avoid exposing the samples to potential contaminants such as laboratory chemicals. It is recommended that if more than one contaminant is being tested, a test should be perform for each one seperatly. This is suggested because of the need to use different solvents in the desorbtion process. It is suggested to use only high purity solvents, for example: spectrograde CS2, during the desorbtion process. The desorbtion solvent should be analyzed for impurities.
The blanks should be dealt with in the same fashion as the contaminated samples. The next step is to remove the absorbant beds and seal the front bed. Place back up beds in seperate vials. Sometimes it is necessary to retain the plugs of the samples to further analyze. Inject the appropriate amount of desorbant solution through septum of the vial. The vials are typically recommended for desorbtion have a screw top with a hole in the center and a teflon coated septum to permit sampling with a syringe. The samples should then be agitated or put through an ultasonic bath, for a specific length of time. This is to completely desorb the analyte. If analysis of the sample cannot be done immediately they should be refrigerated. Changes in the anaylte concentration should be monitored. This can be done by storing freshly prepared standards under the same conditions and analyzing them along with the other samples.
US Geological Survey, National Mapping Information.
US Agricultural Research Service
Go Home
Email toledg@rpi.edu