In December 2017, the Chinese Ministry of Environmental Protection issued a document relating to the Environmental Air Volatile Organic Compounds (VOCs) Monitoring Program for key areas (Environmental Air Monitoring Letter No. ), which requires monitoring of 117 compounds.
What compounds are covered under Letter ?
Letter  requires the monitoring of three main categories of hazardous airborne volatile pollutants:
- Ozone precursors: These are volatile hydrocarbons ranging from acetylene to trimethylbenzene, as listed under the US EPA PAMS scheme.
- ‘Air toxics’: These are listed in US EPA Methods TO-15 and TO-17, and range in volatility from propene to hexachlorobutadiene. Included in the list are polar and non-polar VOCs, as well as a number of halogenated compounds.
- Oxygenated volatile organic compounds (OVOCs): These are a more recent addition to target lists for air monitoring, and include a range of aldehydes and ketones.
What are the challenges of Letter ?
Canisters are required for manual sampling under letter , but monitoring the full list of 117 compounds is particularly challenging using this technique. This is because two subsets of this group of compounds are typically monitored by different techniques: dual-column GC separation with FID detection for ultra-volatiles, and derivatisation followed by HPLC for OVOCs.
In addition, obtaining good peak shape and chromatographic separation for the full set typically requires cryogenic cooling of the GC column, with the associated cost and inconvenience (in addition, many thermal desorption systems also require cryogen).
Complying with Letter 
An efficient approach to monitoring of the 117-compound list is to use an analytical system comprising a canister autosampler with thermal desorber and Deans switch dual-column GC–MS/FID.
Such systems offer optimum responses for the three C2 and two C3 hydrocarbons monitored using FID, as well as confident compound identification and high sensitivity for the remaining compounds monitored using MS. In addition, a water removal device allows samples at 100% relative humidity to be analysed without risking loss of the most volatile species.
Markes’ UNITY–CIA Advantage-xr systems, incorporating a Kori-xr water condenser, are configured with the specific requirements of Letter  in mind, and offer full compliance with its requirements.