Gas Chromatography
HIRAC has been fitted with two gas chromatography (GC) based instruments; a commercial GC-FID (gas chromatography coupled with flame ionisation detection) (HP 6890) for the detection of organic compounds and a GC-pHID (gas chromatography coupled with a pulsed helium ionisation detector) for detection of HCHO and glyoxal ((HCO)2).
Picture of the Agilent Technoligies HP6890 GC-FID used in HIRAC.
Picture of the custom built GC-pHID instrument.
GC-FID
A commercial gas chromatography (GC) instrument (Agilent Technologies, HP 6890) with a flame ionisation detector (FID) is coupled to HIRAC for the detection of organic compounds such as alkanes, acetone and methyl nitrite. Different species elute from the column at different times, depending on their affinity to the column, allowing quantification of separate species via integration of peaks where the area of this peak corresponds to the concentration of the substance.
Gas samples are drawn from HIRAC via Teflon tubing into a 5 ml stainless steel sample loop using a diaphragm pump. The GC sampling system has been automated using two solenoid valves with each sampling cycle lasting ~120 s. The time taken for this cycle can be adjusted to include more samples per hour, depending on the elution times of the species being measured. The samples are injected and separated on a column and an FID maintained at 250°C is used for detection. Heating the column increases the speed of the elution but peaks for different compounds may overlap making integration impossible. The duty cycle for typical measurements using the GC-FID is ~20 hr-1 and the sampling rate is ~700 sccm.
The GC-FID is calibrated using known pressures (and therefore concentrations) of the organic species of interest taken from commercially available, high purity gas and liquid samples. Calibrations are carried out within the HIRAC chamber at the conditions of pressure (and temperature) at which experiments were carried out.
GC_pHID
The GC-HID instrument was developed for the in-situ measurement of atmospheric HCHO and a detailed description can be found Hopkins et al., 2003 (Atmospheric Environment, 37, p. 2557 - 2656).
A loop sample of ~6 ml is transferred onto a column (50 m, 0.32 mm id, 100% dimethyl polysiloxane, WCOT column, 5 μm phase thickness, CP-Sil 5CB Chrompack, Netherlands) using helium carrier gas (further purified by a helium purifier HP2, Valco Instruments) and HCHO is refocused at the head of the column with a liquid N2 cold trap. Following elution of untrapped air, the analytes are released, separated in the column, and HCHO is detected using a pulsed helium ionisation detector (HID) (Model D44, VICI AG, Schenkon, Switzerland).
Calibration of this instrument is carried out by producing gaseous HCHO standards using a commercial permeation source (Kintek, Lamarque, TX, USA) based on weight loss from a permeation tube containing polymeric HCHO with a known emission rate. The GC-HID has a detection limit of ~42 ppt with a duty cycle of ~11 hr-1.