| dc.description.abstract | Due to the complexity of ambient volatile organic compounds (VOCs), conventional one-dimensional gas chromatography is limited in separation power and peak capacity. The advent of comprehensive two dimensional GC, called GCxGC, has greatly improved peak resolution by employing two columns of different polarities connected with a modulator. Although this GCxGC technique was largely used for liquid samples such as fuels and fragrance, the potential and benefits in VOC analysis at ambient level is tremendous and worth exploring in response to the growing public concern of air pollution in recent years. In this study, we coupled a thermal desorption (TD) pre-concentrator with GCxGC-FID (called TD-GCxGC-FID), ambient-level VOCs from traffic emissions were able to be effectively analyzed.
The modulator is a core component in GCxGC, and thermal modulators are the most widely used owing to the advantages of sensitivity and the ease of optimizing modulation parameters. However, the need of cryogen, such as liquid nitrogen, hinders many users from ownership due to high cost in purchase and maintenance. Instead, flow modulators, which do not need cryogen, has steadily gain popularity. Although commercial flow modulators are available, the flexibility in the flow design is low. In this study, we used two flow splitters (provided by SGE SilflowTM) to configure a flow modulator to improve versatility in modulation, such as flow path, flow rate, and sample filling loop length without adding dead volume.
In this study, we report the optimization process of a self-built TD-GCxGC-FID system by analyzing both standard mixtures of VOCs (both PAMS and NIEA715.15B) and ambient air samples from the aspects of column phase selection, modulation parameters, flow adjustment, and the control parameters of the filling loop and bleeding. The important GCxGC parameters were determined as: column combination = DB-1 (30 mx 0.25 mm x 1.0 um) x Stabilwax (2 m x 0.18 mm x 0.18 um), modulation period (Mp) = 3 s, duty cycle = 0.15, modulation ratio (MR) = 5-7, filling loop = 25 cm x 0.32 mm O.D. (or 25 um in volume) and the bleed column length = 1.6 m x 0.18 mm.
The standard mixture of NIEA 715.15B containing 61 compounds of various polarity were analyzed to ensure the ability to separate target compounds of different functional groups. With the conditions set by the standard mixtures, real ambient samples were then collected by the TedlarTM bags in the streets with VOCs mainly contributed by the traffic source and analyzed by the TD-GCxGC method in the laboratory. VOCs were successfully separated in groups on GCxGC chromatograms with reproducibility of better than 1.4% for all PAMS target compounds and sensitivity in detection limit of 18 ppb for toluene as an example. | en_US |