Characterization of a microelectromechanical systems-based counter-current flame ionization detector

This work is concerned with the influence of different operating parameters on the response of a counter-current micro flame ionization detector (cc-μFID) with low gas consumption for mobile applications. At cc-μFID flow rates (<10 ml/min hydrogen), the response depends mainly on the oxygen flow. At 7.5 ml/min hydrogen flow, highest sensitivity (13.7 mC/gC) is obtained with the smallest flame chamber and nozzle size, moderate sample gas flow (2.0 ml/min), and an oxygen flow above stoichiometry (9.4 ml/min, λ = 2.5). The largest absolute signal is obtained at increased sample gas flow (8.0 ml/min). However, to prevent parting of the micro-flame by the sample gas stream, largest nozzles (smallest outflow velocity) give the best result (4.37 nA). Whereas cc-μFID sensitivity is comparable with conventional FID sensitivity, peak-to-peak noise of 1 pA is relatively large. Therefore, the minimum detectable carbon mass flow of 1.46 × 10−10 gC/s and the minimum detectable methane concentration of 3.43 ppm are larger than typical FID detection limits. μGC–μFID experiments show the difference between premixing the sample with the hydrogen or with the oxygen with respect to sensitivity and response factors. Sensitivity is decreased considerably when the column effluent is added to the oxygen instead of to the hydrogen. For hydrogen premixed samples the response factor to butane can be increased up to 0.81 (methane = 1), whereas for oxygen premixed samples it is maximally 0.31. This smaller sensitivity to oxygen premixed samples and the larger variation of response factors shows the importance of the hydrogen atom during breakdown of organic molecules to single-carbon fragments before ionization.