Is it time to embalm U.S. EPA Method TO-11A??? A multi-blog series on airborne carbonyls, part II.

Why would I dare put forth such a question? Perhaps because formaldehyde is the classic preservative used in embalming solutions, due to its ability to fix tissue by irreversibly connecting a primary amine group in a protein molecule with a nearby nitrogen in a protein or DNA molecule. AND… it also just happens to be one of the three most important volatile organic compounds (VOCs) typically measured at ambient concentrations with 2,4-dinitrophenylhydrazine (DNPH)-coated solid sorbents (i.e., U.S. Environmental Protection Agency’s (EPA) Compendium Method TO-11A). AND… despite formaldehyde’s ubiquitous nature and thereby concurrent role in multiple arenas (environmental impact, human health, atmospheric chemistry, etc…), DNPH-based methods have several significant shortcomings for measuring formaldehyde. A couple of weeks ago, I hopefully whetted your appetite with the promise of elucidating some of these shortcomings, so here we go…

It has been known for the past two decades that ozone - another ubiquitous constituent in the atmosphere - interferes with formaldehyde measurements made using DNPH-coated solid sorbents (Achatz et al., 1999). In an attempt to keep this blog as brief as possible, I will not cover the mechanism behind this; however, Achatz et al., 1999 is an excellent resource. Fortunately, the ozone interference may be reduced or alleviated with the use of upstream copper oxide- (Helmig, 1997) or potassium iodide-coated (Possanzini and DiPalo, 1999) denuders or scrubbers. Even better for you (the end-user) is the fact that these remediation strategies are formally addressed in the U.S. EPA’s Compendium Method TO-11A (U.S. EPA, 1999a); and the major DNPH-coated cartridge vendors offer said scrubbers. Oddly enough, as a result of this it appears that most end-users I have polled are aware of the ozone “situation”.

 

However, a much lesser known difficulty associated with measuring formaldehyde using DNPH-coated solid sorbents is that nitrogen dioxide (NO₂) reacts with DNPH to form 2,4-dinitrophenylazide (DNPA) (Karst et al., 1993). As the figure below illustrates, DNPA and the formaldehyde-DNP-hydrazone have similar chromatographic properties. This may result in a high formaldehyde concentration estimate using DNPH-based methods. It gets worse though… not only is NO₂ a ubiquitous atmospheric pollutant (Hesterberg et al., 2009), but this problem is compounded by the fact that an upstream ozone scrubber (recall the aforementioned ozone situation) can oxidize NO to NO₂ (Tang et al., 2004).

Adopted from Potter and Karst et al., 1996. Chromatogram represents same sample analyzed with dual-wavelength detection at λ = 360 nm (-) and λ = 300 nm (••). Note that FA = formaldehyde and DNPA = dinitrophenylazide (reaction product of DNPH and NO2) coelute, resulting in possible positive biases for formaldehyde concentrations.

There is promise though, as several researchers (Ban-Weiss et al., 2008; Gromping et al., 1993; Gromping and Cammann, 1996) have shown that the reaction between DNPH and NO₂ can in fact be utilized to measure NO₂. Potter and Karst (Potter and Karst, 1996) modified the HPLC gradient and in doing so successfully separated DNPA and the formaldehyde-DNP-hydrazone. In addition, the DNPA and formaldehyde-DNPH-hydrazone interference can also be overcome with dual-wavelength detection or diode array detection, with DNPA’s absorption maximum at 300 nm and formaldehyde-DNPH-hydrazone at 360 nm (Potter and Karst, 1996). It is also important to acknowledge that Potter and Karst (1996) identified 2,4-dinitrochlorobenzene (DNCB) as another potential interfering (albeit to a lesser degree) compound with the formaldehyde-DNP-hydrazone. Potter and Karst (1996) traced the origin of DNCB back to hydrochloric acid and recommended substituting the acidifying agent.

BUT… and as you can see this is a big BUT... Despite the acknowledgement of the NO₂ issue in the peer-reviewed literature (done so back in the mid-90s); and the publication of solutions to the NO₂ interference, I am concerned that these corrective methods are not widely applied. Why… because to the best of my knowledge no standardized method has been modified to reflect these latest advancements. It is also important to note that the magnitude of the NO₂ interference has yet to be quantified over different air sheds.

So, is it time to embalm U.S. EPA Method TO-11A? Probably not the methodology, but at least the current revision from 1999 needs to be embalmed. A prudent path forward may be to update and modify the most commonly used standardized methods to accurately reflect interferences due to NO₂ and outline the best available remediation strategies (i.e., the aforementioned information and references should be included in a simple summary outline within standardized methods). Otherwise, in the presence of NO₂, formaldehyde concentrations produced by DNPH-based methods may continue to be overestimated.

 

References

Achatz, S., Lorinci, G., Hertkorn, N., Gebefugi, I., Kettrup, A., 1999. Disturbance of the determination of aldehydes and ketones: structural elucidation of degradation products derived from the reaction of 2,4-Dinitrophenylhydrazine (DNPH) with ozone. Fresenius Journal of Analytical Chemistry 364, 141-146.

Ban-Weiss, G.A., McLaughlin, J.P., Harley, R.A., Kean, A.J., Grosjean, E., Grosjean, D., 2008. Carbonyl and nitrogen dioxide emissions from gasoline- and diesel-powered motor vehicles. Environmental Science & Technology 42, 3944-3950

Gromping, A.H.J., Karst, U., Cammann, K., 1993. Development of a method for simultaneous determinations of nitrogen-oxides, aldehydes and ketones in air samples. Journal of Chromatography A 653, 341-347.

Gromping, A.H.J., Cammann, K., 1996. Field evaluation and automation of a method for the simultaneous determination of nitrogen oxides, aldehydes and ketones in air. Journal of Automatic Chemistry 18, 121-126.

Helmig, D., 1997. Ozone removal techniques in the sampling of atmospheric volatile organic trace gases. Atmospheric Environment 31, 3635-3651.

Hesterberg, T.W., Bunn,W.B., McClellan, R.O., Hamade, A.K., Long, C.M., Valberg, P.A., 2009. Critical review of the human data on short-term nitrogen dioxide (NO₂) exposures: evidence for NO2 no-effect levels. Critical Reviews in Toxicology 39, 743-781.

Karst, U., Binding, N., Cammann, K., Witting, U., 1993. Interferences of nitrogen dioxide in the determination of aldehydes and ketones by sampling on 2,4-dinitrophenylhydrazine-coated solid sorbent. Fresenius Journal of Analytical Chemistry 345, 48-52.

Possanzini, M., DiPalo, V., 1999. Performance of a 2,4-DNPH coated annular Denuder/HPLC system for formaldehyde monitoring in air. Chromatographia 49, 161-165.

Potter, W., Karst, U., 1996. Identification of chemical interferences in aldehyde and ketone determination using dual-wavelength detection. Analytical Chemistry 68, 3354-3358.

Tang, S., Graham, L., Shen, L., Zhou, X., Lanni, T., 2004. Simultaneous determination of carbonyls and NO2 in exhausts of heavy-duty diesel trucks and transit buses by HPLC following 2,4-dinitrophenylhydrazine cartridge collection. Environmental Science & Technology 38, 5968-5976.

U.S. EPA, 1999a. Determination of Formaldehyde in Ambient Air Using Adsorbent Cartridge Followed by High Performance Liquid Chromatography (HPLC) [Active Sampling Methodology]: Compendium Method TO-11A in Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air. U.S. Environmental Protection Agency, Washington, DC.