[Development and performance testing of a high-sensitivity pulsed flame photometric detector].

Abstract

The conventional flame photometric detector （FPD） is widely used for sulfur-selective gas chromatography. However， the FPD is sensitive to hydrocarbon backgrounds. Its sulfur response also depends strongly on flame stability. These factors limit stable trace sulfur determination in complex matrices. A pulsed flame photometric detector （PFPD） provides a time-resolved approach to this problem. Sulfur emission appears later than most hydrocarbon background emissions after pulsed combustion. A suitable acquisition window can therefore improve sulfur selectivity. In this study， a high-sensitivity single-channel PFPD was designed and tested. The aim was to establish reproducible operating conditions for trace sulfur analysis. The detector consisted of five functional modules： the combustion and ignition module， the gas supply module， the optical collection and detection module， the temperature-control module， and the mechanical support and sealing module. The combustion zone consisted of a vertically mounted quartz chamber. The chamber had an outer diameter of 4 mm. Its inner diameter was 2 mm. Its total length was 17.5 mm. The effective combustion length was 12 mm. The calculated chamber volume was 37.68 mm³. Numerical simulation was used to examine flame propagation and self-extinction. A 12 mm effective length maintained a stable finger-shaped flame front. The flame was self-terminated near the lower limiting orifice at about 1.97 ms. The hydroxyl radical mass fraction decreased close to baseline within 5 ms， whereas the temperature decayed more gradually and approached its initial level at approximately 30 ms. These results supported the chamber design for periodic pulsed combustion. The optical unit used a quartz light guide and a BG-12 band-pass filter. The filter matched the sulfur emission band near 394 nm. A photomultiplier tube （PMT） recorded the time-resolved emission signal. Two electronic pneumatic control （EPC） units were used for gas regulation. One unit controlled hydrogen and air for the ignition chamber. The other unit controlled hydrogen and air for the combustion chamber. This design separated ignition conditions from analytical combustion conditions. It also reduced uncertainty caused by coupled gas adjustment. The optimization started with the ignition chamber. Then the combustion chamber was optimized under the fixed ignition baseline. Finally， detector temperature and PMT voltage were evaluated. Peak area， sensitivity， and signal-to-noise ratio （SNR） were used as indicators. Gas chromatographic tests used a sulfur-specific quartz capillary column. Nitrogen was used as the carrier gas. The gate delay was 6 ms. The gate width was 18 ms. The trigger level was 500 mV. A stable pulsed flame was obtained at an ignition-chamber hydrogen flow of 8.0 mL/min. The corresponding ignition-chamber air flow was 19.0 mL/min. Under this ignition baseline， the combustion chamber was further adjusted. A higher sulfur response was obtained at 5.0 mL/min hydrogen. The corresponding air flow was 3.7 mL/min. Detector temperature also affected the delayed sulfur emission. The response was low at 110-130 ℃. At about 150 ℃， the delayed sulfur signal was higher. Peak area and sulfur sensitivity were also higher at this temperature. Higher temperatures from 170 to 250 ℃ reduced the peak area and sensitivity. PMT voltage affected signal amplitude， SNR， and waveform fidelity. Increasing the voltage from 590 to 710 V improved the SNR. At 730 V， the delayed emission plateau showed peak clipping. This behavior indicated saturation or nonlinear response in the detection chain. Dynamic dilutions of a 10 μmol/mol hydrogen sulfide （H₂S） standard gas were used to evaluate the linear range. A 1.1 μmol/mol H₂S standard gas was used for the quantitative repeatability test and， after 30-fold dynamic dilution， for determination of the sulfur detection limit. The quantitative repeatability， evaluated from the peak areas of seven consecutive injections， was 0.37%. The sulfur detection limit was 4.1×10⁻¹³ g/s， and the linear range spanned approximately two orders of magnitude. The developed PFPD showed stable pulsed combustion behavior. It also provided reliable quantitative response for trace sulfur detection. The dual-EPC design helped clarify the relationship between ignition and analytical combustion. The optimized conditions provide an experimental basis for self-developed PFPD systems.