Gas chromatography – Discussion

In contrast to results from the CO2 sensor, findings from GC analysis showed that there were no statistically significant differences between CO2 levels and treatments. This could be due to poor resolution from the type of detection available – thermal conductivity detector (TCD). This detector works by using two tubes parallel to one another which are fitted with gas and heating coils (The Linde Group, 2017). Gases are then investigated by measuring the heat loss rate from the heating coils to the gas (The Linde Group, 2017). While TCD is a good, general purpose indicator, flame ionization detector (FID) gives greater resolution and therefore would have been better suited for this study. FID consists of a hydrogen flame and a collector plate. It works by passing the effluent from the GC column through the flame, breaking down organic molecules and returning ions (The Linde Group, 2017). FID is also more practical as it can measure more than one GHG at the same time (Rapson and Dacres, 2014). This would allow for a greater exploration of GHG emissions from soils but on the other hand, samples are destroyed under FID.

Results from GC also showed that there were no differences in the percentage of CO2 taken from samples from both inside and outside the chamber. This suggests that CO2 must have been leaking out of the chamber (either through the vent or USB port for Arduino) and therefore not accumulating enough to demonstrate a difference. Had additional time been available, any openings on the chamber would have been sealed to investigate if this would make a significant difference to CO2 accumulation within the chamber. Rochette and Hutchinson (2005) suggested a range of factors which influence chamber performance including soil and air temperature, humidity, pressure fluctuations, chamber air mixing regime and site sampling. With additional time, investigations into how the above listed factors affect chamber function would have been carried out.

It is also possible that the timing of sampling was too spaced out as samples were taken once every two hours. Collier et al. (2014) suggested that for chambers which are 10-30cm in height, samples should be taken every 20-60 minutes. Rochette et al. (2012) advised a deployment period of 30-40 minutes for chambers with a maximum height of 20cm. According to Rochette and Eriksen-Hamel (2008) chamber height:deployment ratio should be ≥40cm/hr. Therefore, the deployment time of the chamber (4 hours) was far too long to get significant measurements in this case. A deployment time of several hours would also subject gas samples to variation from cofounding variables such as changes in the external environment. Chambers which are deployed for short time intervals are less exposed to these variables and give more reliable results.

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