The task at hand: It normally takes an ice-cold approach to reliably and comprehensively determine volatile organic compounds (VOCs) in air, including in emissions from a material sample, by thermal desorption(TD)-GC/MS. The rule-of-thumb is: For non-target compound (unknowns) analysis, the cold trap (CIS/PTV inlet) temperature should be kept very low to trap even the very volatile VOCs (VVOCS). However, such an approach requires additional technical equipment and laboratory logistics associated with the use of a cryogen (LN2 or LCO2). The determination of VOCs in air across a wide volatility range can be performed with simpler means when using dynamic focusing (GERSTEL Dynamic Focusing), an innovative approach that enables focusing of VOCs over a wide boiling point range without relying on very low temperatures while fully meeting the requirements of the standard methods US EPA TO-17 or EN DIN ISO 16000-6. 

The solution: A liner filled with a sorbent material inserted into the GERSTEL Cooled Injection System (CIS), PTV-type inlet, serves as the cold trap. The CIS is cooled by the Universal Peltier Cooling System (GERSTEL UPC Plus), an electrically powered module. The UPC Plus is versatile, enabling cooling of both the CIS and the associated thermal desorber (TDU, TD 3.5+ etc.). The ethanol based coolant is circulated, keeping supply and maintenance costs low. The UPC reaches temperatures as low as 10 °C, which is fully sufficient when operating in dynamic focusing mode, which is based on a dynamic overlap of the thermal desorption and trapping stages. An added DHS option that can be used for automated screening of materials for emissions before resorting to more cumbersome and costly chamber tests. 

The trick: The CIS quickly reaches the low starting temperature of 10 °C with the help of the UPC Plus. Instead of focusing and desorbing the analytes in two separate stages, first in the thermal desorption system, and subsequently in the CIS, the stages are overlapped and performed with a time lag. While the medium and high boilers (SVOCs) are captured by the sorbent and only desorbed upon heating, the movement of low-boiling VOCs is only briefly slowed down and they are focused in a sharp band. The effects are evident in the clear and sharp peaks formed in the resulting chromatogram. The run starts with the temperature program of the thermal desorption system. The more volatile VOCs are transferred through the cold trap to the separation column for GC-MS determination with a short delay, while the less volatile fraction of the VOCs along with the SVOCs are transferred with a delay when the trap is heated. 

The system: The instruments used for the analysis were a TD Core system (GERSTEL) combined with a GC-MS system from Agilent Technologies (8890 GC/5977 MSD). TD Core consists of a CIS (PTV-type inlet) used as cold trap, mounted on top of a TD 3.5+ Thermal Desorber (both GERSTEL). The system also includes an autosampler (GERSTEL MultiPurpose Sampler , MPS), for automated sample preparation and sample introduction. Two automated options (Internal Standard and Dry Purge [ISDP/ISDP+]) were used to add gaseous and liquid internal standards to the sample tubes before analysis. All details of the study, the materials and instruments used, as well as the analysis conditions can be found in AppNote 261.  

The Application:  

Extensive tests were performed to prove the performance of the Dynamic Focusing technique for the determination of analytes across the volatility range specified in US EPA TO-17 and EN DIN ISO 16000-6. These include: propylene, 1,3-butadiene, vinyl bromide, acetone, 2-propanol, carbon disulfide, allyl chloride, methyl tert-butyl ether (MTBE), dichloroethylene, n-hexane, vinyl acetate, butanone, ethyl acetate, tetrahydrofuran, cyclohexane, isooctane, n-heptane, 1,4-dioxane, bromodichloromethane, methyl isobutyl ketone, 2-hexanone, dibromochloromethane, bromoform, 4-ethyltoluene and benzyl chloride. Their determination was flawless, as documented by the successful participation in a round robin test (DIN ISO 16000-6 and DIN EN ISO 16017-1); 1,2,4-trimethylbenzene, alpha-pinene, cumene, ethylbenzene, m-xylene, n-butyl acetate, n-octane, benzene and toluene were detected. 

Conclusion:  

In our method development, the focus was on achieving the limits of determination and detection required including proof of performance of dynamic focusing for US EPA Method TO-17 and ISO 16000-6; sample to sample carry-over of analytes was not detected (see AppNote 261 for details). Dynamic focusing can be used successfully to determine a wide range of highly volatile to semi-volatile organic compounds (VVOCs, VOCs and SVOCs), which was demonstrated by the determination of a standardized TO-17 gas phase and associated internal standards. Statistically proven results were achieved for the determination of compounds ranging from propylene (boiling point: -46.6 °C) and 1,3-butadiene (-4.4 °C) through tetrahydrofuran (66 °C) and cyclohexane (80.75 °C) to 4-ethylenetoluene (approx. 16 °C) and benzyl chloride (179 °C) with only very moderate trap cooling used. The valve-free TD Core system without cryogenic cooling options and the associated complexity was used. The successful participation in a round robin test and the analysis of the standard used in it (ISO 16000-6) underlines the suitability of dynamic focusing for the cryogen-free determination of volatile organic compounds over a wide boiling point range and thus also its use in routine laboratory operations. 

Literature reference 

Nünemann L, Hoffmann A, Haferkamp M, Thaxton K (2024). Ambient Air Analysis with Dynamic Focusing. GERSTEL AppNote 261. www.gerstel.com