Study On The Infrared Spectrum Of A Sample With Its Concentration Having Time-dependent During The Scanning Process

As an indispensable tool for modern analytical science,infrared spectroscopy can provide rich material structure information and is widely used in qualitative,quantitative and structural analysis in the fields of chemistry,materials,environment,biomedicine,coal and agriculture.Generally,the sample state is required to be stable during the process of measurement with conventional Fourier transform infrared spectroscopy,which means not changing with time.However,in practical work,we will also encounter unstable samples,such as volatile samples and samples in the chemical reactions.When the concentration of sample change during the process of measurement,the distortion phenomenon signifies the measured spectra cannot represent the actual structure and quantity information of the sample.This paper attempts to simulate the spectrum of sample whose concentration change during the measurement process,establishes the mathematical expression of the infrared spectrum of the concentration-changing system,and discuss the degree of distortion of the sample measurement spectrum under different conditions with the concentration change rules of the zero order reaction and the first order reaction.For volatile samples,the degree of distortion of the sample measurement spectrum can be reduced by reducing the volatilization rate?reducing temperature or exposure area?and by appropriately reducing the scanning time?reducing the number of scans?.The humidity fluctuation in the spectrometer sample chamber has caused the spectral subtraction technique to meet the challenge of deducting the water vapor bands.The spectral features of water vapor will change as the relative humidity of air changes.Namely,if the spectrum of atmospheric air with x% relative humidity is given,we cannot predict and obtain the spectrum of air with y% humidity regardless of how carefully the scaling factor has been chosen.The size and composition ratio of water clusters?H₂O?n in the air change with humidity fluctuation,the infrared spectrum of water vapor is actually the additive effect of the absorption of water molecules in various sizes.The absorption of infrared light by water vapor does not satisfy the Lambert-Beer's law,which is the main reason for the limitation in the elimination of water vapor bands by spectral subtraction.The spectrum of water vapor between relative humidity x% and y% can be matched near perfectly by a combination of x% spectrum and y% spectrum.This provides a new and effective way for the elimination of water vapor interferences,and the high efficiency of the humidity titration method has been demonstrated experimentally.There are various kinds of sample preparation technique for organic liquid samples.In order to ensure the quality of the infrared spectra of different volatile samples and the simplicity of the preparation method,it is particularly important to select an appropriate sample preparation technique.The organic samples with different volatile rates were prepared with different sample preparation techniques and the measured spectra were compared with the standard spectra.The simplest sample preparation technique is recommended for different volatile samples,and a method which is not only simple and quick and low cost but also can ensure the quality of infrared spectra is proposed.This study will provide a meaningful reference for samples with its concentration?quantity?having time-dependent during the scanning process.