April 15, 2019
Measuring Air Contaminants Accurately
By Michael D. Shaw
Governmental activities in air pollution control date back more than 100 years, but efforts to formalize proper measurement procedures began with our EPA, founded in 1970. Not long after that, the agency’s Air Emission Measurement Center set out to promote methods for the measurement of pollutants from smokestacks and other industrial sources.
Likewise, on the ambient air side, National Ambient Air Quality Standards have been established. Accompanying this is a list of designated reference and equivalent methods. These methods have been subjected to extensive evaluation, and are being used throughout the world.
In the realm of occupational health, NIOSH has published its Manual of Analytical Methods, and OSHA details its Sampling and Analytical Methods. For the most part, both of these sources describe laboratory (rather than field instrumentation) methods.
An important element of analytical methods, and one that is not well promoted or understood, is the matter of interferences. An “interference” is an unwanted response to your analytical method by some chemical other than the target analyte. These can be quite detrimental, especially if they are unknown. But even if they are known, documenting them can only go so far.
For example, if you are trying to measure the concentration of hydrogen sulfide (H2S), you may already know that your method shows an interference from other reduced sulfur compounds. Unfortunately, in some important applications for hydrogen sulfide monitoring, such reduced sulfur compounds are likely to be present. It is interesting to know that, say, 3 parts-per-million of methyl mercaptan (a reduced sulfur compound) will register as 1 ppm H2S with your method. However, you typically have no immediate way of discerning if that interferent is present or not–let alone precisely how much there is.
At best, you are simply put on notice that your supposed H2S reading may not be correct. So, what do you do then?
1. You can search for an analytical method that does not exhibit this interference, or at least shows it to a much lesser degree.
2. You can install some sort of filter or scrubber that would selectively remove only your interference from the sample. Purists might wonder if the scrubber could be adding something else to the sample, or suggest that by replacing the interferent with air, you are affecting the mole fraction (a unit of concentration) of the mix.
3. You can redefine the parameters of your analytics, such that you are only looking qualitatively for all reduced sulfur compounds (and that includes H2S). Of course, you would have to bear in mind that your method does not respond equally to all reduced sulfur compounds, and you are not even sure which ones are present.
In practice, the most accepted analytical methods will show the fewest interferences, but virtually none are interference free.
Moreover, interferences can be hidden or forgotten. In the infamous FEMA trailers/formaldehyde matter, the CDC analyzed the air inside hundreds of trailers for formaldehyde. You may recall that these trailers were rushed out to the hurricane-ravaged areas by manufacturers working 24/7 to help the victims. Unfortunately, there is some inherent formaldehyde in certain interior components of the trailers (cabinetry and paneling), that is normally aired out. [The solution to pollution is dilution.]
Alas, this factory process was skipped or shortened in many cases, and the situation was exacerbated by numerous occupants keeping the windows shut once they moved into the trailers. In addition, a goodly percentage of the occupants smoked, and cigarette smoke contains formaldehyde. There is no question that this affected the published results.
So, in this case, the “hidden” interference was the formaldehyde in the cigarette smoke, which affected the results for the chemists searching for formaldehyde inherent to the trailer.
Beware of interferences in chemical analysis!