Next generation laboratory solutions for VOC air monitoring
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Poster presented at the National Ambient Air Monitoring Conference (NAAMC) in Atlanta, GA (August 11-14, 2014)
Air pollution remains a global concern and there is increasing pressure on analytical laboratories to provide analyses at ever-lower detection limits without compromising overall analysis time. Two sets of compounds are routinely monitored in ambient air using on-line or canister systems;
US EPA method TO-15 describes the methods for monitoring a list of ‘air toxics’ compounds, comprising hazardous air pollutants ranging in volatility from freons to hexachlorobutadiene, in canister air samples. TO-15 encompasses a wide range of polarity and volatility with compounds typically at sub-ppb to low-ppb concentration levels requiring hundreds of millilitres of sample and splitless analysis for quadrupole mass spectrometers.
Ozone precursor compounds, hydrocarbons in the volatility range acetylene to trimethyl benzene, are also routinely monitored and present a different set of challenges; the C2 hydrocarbons present the most significant challenge due to their high volatility and small molecular size and the uniquely powerful combination of sorbent capacity, cold trap dimensions and electrical cooling allows quantitative retention of these compounds without the need for cryogen cooling. More demanding air monitoring applications require not only high sensitivity for C2 ozone precursor hydrocarbons, but simultaneous analysis of oxygenated and polar species such as those found in US EPA method TO-15. Membrane dryers typically employed to remove water from ambient air samples also remove polar and oxygenated species from the sample stream, this presents a significant water management challenge for quadrupole systems for which hundreds of millilitres of sample gas is required to reach limits of detection, and sub-ambient trapping temperatures are required to prevent breakthrough of the most volatile C2 hydrocarbons.
While current systems are able to quantitatively retain such sample volumes, ambient air can contain up to 100% relative humidity (%RH), hundreds of millilitres of which would load several milligrams of water onto the cold trap without membrane drying. This presents the challenge: Is it possible to quantitatively analyse C2 hydrocarbons and oxygenated species in a single analysis, without using liquid cryogen, while effectively managing the humidity that may be expected in ambient air? Overcoming the challenges of simultaneous analysis of very volatile organic compounds (VVOCs) and oxygenated volatile organic compounds (OVOCs) in high humidity atmospheres is demonstrated in this paper.