While these regulations deal with detection of different chemicals, all of them stress a common concern: Detection instruments must be calibrated correctly. This is the only way to safeguard against false positives, false negatives, or incorrect measurements of target chemical compounds. The challenge here is that often instrument calibration is accomplished using surrogate gases. Propane or methane, for example, can be used to calibrate a monitor for up to 188 different hazardous air pollutants (HAPs) using EPA method 25a regulations.
Why don’t we calibrate the monitor using the actual compound we wish to detect? Primarily because many chemical compounds cannot be easily put into a compressed gas cylinder and allow for easy calibration. This leads to another question: “If I am not calibrating my instrument with the same chemical compound that I am testing, can I really expect to receive accurate performance from my detector?” The answer, unfortunately, is no.
As an example, if you are detecting for acetone using an instrument that was calibrated using ethane, instrument response for 100 ppm acetone will be incorrect. The chemicals are from different structure groups, so you cannot measure them accurately calibrating the instrument with the different structures.
Also, you must consider whether or not interference gases may affect you. If you are monitoring for a particular gas but there are other gases present that alarm the monitor, you must know how to distinguish between them. In this case, you must take an interference gas (or combination of gases) to decrease the chances of a false positive. For instance, if you are monitoring for carbon disulfide and there is a coal-burning power plant across the street emitting sulfur dioxide, the monitor may go off. You have to be able to recognize exactly what you are measuring for via calibration and the level of outside interference.
Creating calibration gases for different compounds is a challenge because of the fact that some compounds cannot be made in a gas form, because either their boiling point is too high or their vapor pressure is too low. In this case you may want to try permeation tubes, but they have other, worse restrictions on temperature maintenance and altitude that may make them worthless.
Fortunately, however, most HAPs can be made in calibration gas form; they should always be used to calibrate your instruments whenever possible. Though they may involve greater calibration expense, these mixtures can save your facility a great deal of time and expense, not to mention prevent a possible violation of regulations.
Take, for example, the case of a power plant that burns natural gas and is required by the state to monitor for VOCs. Its personnel did an ambient test using method 25a for a portable GC and failed for the ppm levels of hazardous air pollutant compounds. However, there is a composter in operation just a few blocks away. The power plant personnel speciated the hazardous air pollutant (or VOC) to low level hydrogen sulfide calibration gas. It was determined that it was stronger on a part of the plant’s property where its source plume was not drifting; rather, the prevailing winds and HAP typically came from the composter. As a result, the plant petitioned the state that it should not have to monitor for HAPs based on the results of the test. The power plant won, and its permit was changed.
The key, then, is to determine whether a gas of the specific pollutant or compound for which you wish to monitor is available. Because it is possible to have gas mixtures containing more than 100 different components, be sure to ask your gas supplier for the proper mixture. Also, be sure to determine whether there are interference gases present.
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