The GC-MD is a gas chromatograph with multiple detectors. In the case of the Tacolneston GC-MD (also termed the TAC-MD), a mercuric oxide (HgO) detector measures hydrogen (H2) and carbon monoxide (CO), whilst the electron capture detector (ECD) measures nitrous oxide (N2O) and sulphur hexafluoride (SF6). One sampling module is used with two separate loops for containing fixed volumes of air. The air in these loops is simultaneously injected onto two sets of columns: one set in the GC oven for separating SF6 and N2O and one set in the GC oven of the Peak Performer 1 (PP1) to separate H2 from CO followed by detection using the mercuric oxide detector.
Peak Performer 1
The PP1 is the commercial name for the system optimised to measure H2 and CO. This ultra-trace level gas detection system monitors low (ppb) concentrations of reducing (combustible) gases with accuracies of ±10 ppb and ±1 ppb for H2 and CO respectively. This instrument is fitted with a mercuric oxide bed and, following its reduction, mercury vapour is measured by UV photometry. High instrument sensitivity is attributed to the ease of measurement of mercury vapour at low concentrations, while the instrument shows no response to argon, helium, nitrogen (N2) or oxygen (O2).
Collected gas samples are flowed through a custom made, temperature and pressure controlled sampling module known as the MD (multiple detector), before collection in a 1 ml sample loop. Prior to separation samples are dried using a permeation Nafion drier (Permapure, USA). The nafion dries air samples by purging dry air in the opposite direction to the air sample with a water permeable membrane between them. The water in the air sample is removed across the membrane and vented out into the lab with the counterpurge gas. Gas samples are introduced via electrically actuated gas sampling valves and separated chromatographically within an isothermal mandrel-heated column. Eluted compounds travel directly to the detector which contains a heated bed of mercuric oxide. Following its reduction to mercury vapour (Hg(v)) by reducing gas X as follows,
X + HgO(solid) → XO + Hg(vapour)
the resulting vapour is quantitatively determined by precision UV photometry downstream of the reaction bed. The detector outlet flow passes through a colour-indicating filter for mercury removal before venting.
The PP1 is a commonly used instrument for making continuous long-term measurements of both H2 and CO due to its low consumption of inexpensive carrier gas (zero-air) and the low maintenance required. With continuous use the PP1 mercury lamps only require replacement every 4-6 months and a zero-air generator can be run continuously.
However, a drawback of the PP1 is the detector's non-linear response to increases in concentration of both H2 and CO. For example when the concentration of CO doubles the response of the detector may be 1.8 times that. To correct for non-linearity a high concentration standard, of accurately known concentration, must undergo serial dilutions, each of which are run through the PP1 to produce a calibration curve. A quadratic fit can be made through the calibration curve which is then used to correct data for non-linearity. The plot below shows the differences between data that has not been corrected and has been corrected.
This plot shows how the PP1 detector responds to high concentrations in a non-linear manner, data corrected for this non-linear response is shown in blue with raw uncorrected data shown in red.