Differential Fourier Transfomr Infrared Photoacoustic Spectroscopy Study For Atmosphere Contaminants Detection

Air pollutants have adverse health effects to humans,and are significant factors in the Earth’s climate system.The air contaminants monitoring has recently become a hotspot.There is an urgent need for cost-effective devices capable of recognizing and detecting various ambient pollutants.Although there have been competing gas phase contaminants detection methods,nearly none have the wide spectral bandwidth as the method of Fourier transform infrared spectroscopy which is famous for the advantages of Fellgett,Jacquinot and Connes.There is an increasing interest in photoacoustic spectroscopy（PAS）because of its inherent advantage of high sensitivity,low noise level,free of pretreatment and comparatively simple configuration.In this research,a highly selective broadband detection configuration:a step-scan Differential Fourier Transform Infrared Photoacoustic Spectroscopy（DFTIR-PAS）methodology based on a commercial FTIR spectrometer was developed for air contaminants monitoring.This work was devised to overcome spurious signal generated by strongly absorbing background gases which tend to mask overlapping spectrum signal from weakly absorbing target trace gas such as ambient air contaminants.Both single and differential modes FTIR-PAS theories have been developed.The results showed that the amplitude channel of FTIR-PAS signal is proportional to target gas concentration;while the phase channel is corrected with gas molecular relaxation time.It was theoretically demonstrated that the differential mode should be able to suppress the coherent noise and cancel the spurious photoacoustic signal caused by background gas the absorption peaks of which are overlapped with the test gas.After reviewing various kinds of resonators,T-cell was determined to be the most appropriate resonator in view of the low-frequency requirement and space limitations in the sample compartment of the FTIR spectrometer.The basic T-cell geometry consists of three parts:the absorption,buffer,and resonance cylinders.The finite element method was used for T-cell simulation.The simulated and measured behavior of T-cell showed excellent agreement with each other.The resonant frequency of T-cell was 342 Hz,within the chopper modulation frequency range.The quality factor was 83.5,and the cell constant was 2,063 Pa·cm^-1·W.An analytical solution was acquired with a semi-empirical boundary condition,providing a reliable approach for estimating the resonance frequency of T-cell.This cell design provides a practical solution for low-frequency amplitude modulation despite the small-volume sample compartments of today’s FTIR spectrometers.The FTIR-PAS system was set up comprising two identical and small-size T-cells.The incident light was directed into the sample and reference resonator chamber sequentially,the optical square waveform being shaped by a chopper and alternatively transmitted and reflected using a homemade one-side-coated（mirror）chopper blade.This approach effectively maximizes light energy utilization efficiency as it avoids beam splitting.The sample resonator contains a mixture of target gas and other background gases while the reference resonator is filled up with only the background gases.The configuration is flexible for switching between the single and the differential lock-in detection modes providing an easy approach to investigate each cell separately and compare directly with the differential mode.The designated concentrations of the test gas were premixed in the self-made gas assembly.The typical ambient contaminants carbon dioxide and acetylene were chosen for the preferred test gas analysis.All the measurements were conducted under standard temperature and pressure.When the intensity of incident light at 2,349 cm^-1 was 12.6μW,the signal-to-noise ratio（SNR）of the mixture of 5,000 ppmv carbon dioxide and nitrogen for differential mode was 3,491,well above the value 1,515 found for the single mode.This performance lead to a carbon dioxide detection sensitivity of 2 ppmv for the differential mode and 4ppmv for the single mode.The results showed the differential mode is capable of suppressing the coherent noise and improving sensitivity.The linear response between the amplitude signal and carbon dixode concentration for the two modes is a powerful evidence of capability for quantitative analysis of ambient gases using this method.The phase signal of this approach is corrected to the relaxation time of carbon dioxide,which provides an alternative method for gas molecular relaxation time measurements.The acetylene peaks(1302 and 1360 cm^-1)was obviously distinguished by the differential mode when the sample was the mixture of 100 ppmv acetylene and ambient air（the relative humidity was 45%）,however,the above peaks cannot be detected by the single mode.The detection sensitivity and the lower limit of quantification of the differential mode was 5 ppmv and 17 ppmv respectively with 30μW incident light intensity at 1360 cm^-1.The experimental data of the differential mode showed an excellent linear dependence on acetylene.These characteristics demonstrate that the DFTIR-PAS modality is an effective approach for revealing the presence of“hidden”gases,the absorption peaks of which are concealed due to an overlap with other strongly absorbing background gases.To summerize,by virtue of efficient noise level suppression and differential spectra cancellation,the T-cell enhanced step-scan DFTIR-PAS as a spectral deconvolution method was exhibited to be a sensitive,broadband,quantitative spectroscopic technique for ambient trace gas detection in the presence of strongly overlapping overlapping background absorptions.