Why are my acetaldehyde concentrations off… has someone been drinking? A multi-blog series on airborne carbonyls, part III.

Ask yourself the following questions: (1) Are you using 2,4-dinitrophenylhydrazine (DNPH)-coated solid sorbents (i.e., U.S. Environmental Protection Agency (EPA) Compendium Method TO-11A) to measure ambient levels of airborne acetaldehyde? (2) Are you sampling for long-term (i.e., 24 hr or greater) durations? (3) Have you compared the results you generated with a DNPH-based method and another sampling method or model data? (4) Did you find a discrepancy in the acetaldehyde concentrations amongst methods/models?

If you answered “yes” to all four of these questions, then you may have said “why are my acetaldehyde concentrations off… has someone been drinking?” WHY? For starters… Method TO-11A is clearly titled “Determination of FORMALDEHYDE in Ambient Air Using Adsorbent Cartridge Followed by …” The method is not titled “Determination of ACETALDEHYDE…” OR Determination of CARBONYLS…”, because the method was originally developed in the early 80’s for formaldehyde only, not acetaldehyde and other carbonyls. However, over the past three decades researchers found that the formaldehyde-DNPH chemistry was applicable to other carbonyls, and therefore the method was expanded to include additional carbonyls.

BUT there were some assumptions (you know what happens when we ASS-U-ME) and some inadequate/inappropriate method validations made over the past three decades. This is where you may find yourself asking “has someone been drinking?”, because the oversight I am about to point out to you is gross enough to make you think some booze was involved along the way. Speaking of booze… before we delve into the aforementioned oversights, if your acetaldehyde concentrations have been off, you might also ask if someone has been drinking based on the information found in the following story:

So my “friend” in graduate school just so happened to be doing his Ph.D. dissertation on VOCs, in particular, carbonyl sampling with derivatizing agents like DNPH. Well, one Friday evening he may have indulged a little too much on the libation during a card game with his fellow graduate students. However, despite his irresponsible behavior Friday evening, he was diligently working in the lab early Saturday morning preparing a fresh batch of DNPH cartridges for an upcoming experiment. Well… it turned out that when he went to test the blanks for that batch of cartridges the acetaldehyde concentrations were inordinately high. See… when booze (ethanol) is metabolized, the first metabolite along the metabolic pathway is acetaldehyde. So… apparently my friend consumed enough alcohol Friday night and late enough in the evening, that when he was preparing the cartridges Saturday morning, he was still off-gassing enough acetaldehyde to contaminate his blanks.

Now back to the oversights… Back in 2007, for reasons that I do not have the time nor space to get into, I conducted an exhaustive search that failed to produce any documentation from the body of readily available literature that appropriately evaluated the long-term sampling collection efficiencies (CEs) of DNPH-coated solid sorbent sampling methods for carbonyls other than formaldehyde. So I set out to evaluate the CEs of DNPH-coated solid sorbents for acetaldehyde. I built and used a dynamic atmosphere generation system (Figure 1) to generate controlled atmospheres of formaldehyde and acetaldehyde gas standards with permeation devices (VICI Metronics, WA, USA).

 

Figure 1: Dynamic atmosphere generation system.

With the use of the dynamic atmosphere generation system, the long-term sampling CEs were determined for four different DNPH-coated solid sorbents. The following commercially available DNPH-coated cartridges were used in this study: SUPELCO’s (Bellefonte, PA, USA) LpDNPH Air Monitoring Cartridge (referenced as SUPELCO); Waters (Milford, MA, USA) Sep-Pak DNPH-Silica Cartridge (referenced as WATERS); and Waters Sep-Pak XPoSure Aldehyde Sampler (referenced as XPOSURE). These cartridges were selected based on their ubiquitous citation in the literature. In addition to the commercially available DNPH-coated cartridges, an “in house” cartridge (referenced as HOUSE) was evaluated. The long-term CEs were determined for 24- and 48-hour sampling durations at 30 and 60% RH. The results from these CE experiments (Table 1) indicated that DNPH-coated solid sorbent sampling methods consistently underestimate acetaldehyde concentrations for long-term sampling durations.

 

Table 1: Collection efficiency, ratio of concentration measured to concentration generated in the dynamic atmosphere generation system, reported as mean ± sd, parentheses represent sample number. a Temperature = 30°C b Sample flow rate = 150 mL min-1 c Only determined with WATERS cartridge d Sample flow rate = 75 mL min-1

The aforementioned study and results are discussed at great length in Herrington et al., 2007 (http://pubs.acs.org/doi/abs/10.1021/es061247k) with all the supporting references. In addition, this work has been backed and cited in more recent manuscripts, such as Mason et al., 2011 (http://pubs.acs.org/doi/abs/10.1021/es102500v).

However, the take away message is that assuming (there is that ASS-U-ME again) DNPH-coated solid sorbent methods have 100% acetaldehyde CEs for long-term sampling durations (as many researchers are currently doing) will result in a substantial under-estimation of acetaldehyde concentrations. This is likely to be the case for other carbonyls as well. Evaluations of long-term sampling CEs, using DNPH-coated solid sorbents, need to be extended to other commonly measured carbonyls in future studies. AND… until said studies are conducted, you must have been drinking to think it is safe to continue using DNPH-coated solid sorbent methods for any carbonyl other than formaldehyde.

More importantly, the issues I raised in my last two blogs and in the current blog need to be addressed in the most commonly used standardized methods (i.e., EPA Method TO-11A) to accurately reflect these shortcomings, so that end-users are aware of them. However... the part about my graduate school “friend” can probably be omitted from any such amendments.