Absorbance characteristics of a liquid-phase gas sensor based on gas-permeable liquid core waveguides

• Absorbance raises linearly with time after requisite response time.
• Rate of absorbance change gives higher accuracy than instantaneous absorbance.
• Thinner tube wall and limited faster sample flow can speed up response.
• Longer 2400 tube gives higher sensitivity.

The absorbance characteristics and influential factors on these characteristics for a liquid-phase gas sensor, which is based on gas–permeable liquid core waveguides (LCWs), are studied from theoretical and experimental viewpoints in this paper. According to theory, it is predicted that absorbance is proportional to the analyte concentration, sampling time, analyte diffusion coefficient, and geometric factor of this device when the depletion layer of the analyte is ignored. The experimental results are in agreement with the theoretical hypothesis. According to the experimental results, absorbance is time-dependent and increasing linearly over time after the requisite response time with a linear correlation coefficient r2 > 0.999. In the linear region, the rate of absorbance change (RAC) indicates improved linearity with sample concentration and a relative higher sensitivity than instantaneous absorbance does. By using a core liquid that is more affinitive to the analyte, reducing wall thickness and the inner diameter of the tubing, or increasing sample flow rate limitedly, the response time can be decreased and the sensitivity can be increased. However, increasing the LCW length can only enhance sensitivity and has no effect on response time. For liquid phase detection, there is a maximum flow rate, and the absorbance will decrease beyond the stated limit. Under experimental conditions, hexane as the LCW core solvent, a tubing wall thickness of 0.1 mm, a length of 10 cm, and a flow rate of 12 mL min−1, the detection results for the aqueous benzene sample demonstrate a response time of 4 min. Additionally, the standard curve for the RAC versus concentration is RAC = 0.0267 c + 0.0351 (AU min−1), with r2 = 0.9922 within concentrations of 0.5–3.0 mg L−1. The relative error for 0.5 mg L−1 benzene (n = 6) is 7.4 ± 3.7%, and the LOD is 0.04 mg L−1. This research can provide theoretical and practical guides for liquid–phase gas sensor design and development based on a gas-permeable Teflon AF 2400 LCW.

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