Concept And Properties Of Infrared Hybrid Single-beam Spectrum And Its Application For Eliminating Water Vapor Bands And Other Background Interferences

Infrared spectroscopy is one of the classic and well-established techniques. With the development of infrared sampling techniques, the measurements for infrared spectra of many types of samples (i.e., gases, liquids, and bulk and powdered solids in milligram quantities) have become quite rountine. However, the inappropriate sampling and inappropriate measurement method may lead to the distortion of the IR spectrum. If the sample is not continuous, like a bubble in a liquid cell filled with sample, the infrared radiation will pass around the liquid sample instead of through it. The radiation being not modified by the sample will seriously affect absolute and relative intensities. Unfortunately, the reason of this distortion phenomenon still remains to be quantitively clarified. Herein, we introduced a new concept named hybrid single-bem spectrum and developed hybrid spectrum theory, attempt to afford new insights into understanding of the distortion of the IR spectrum caused by the uneven sample. A hybrid single-beam spectrum (φh) has been defined as a mixture of two single-beam spectra (φb1and φb2) of the same sample but of different pathlengths (b1and b2), namely,φh=αφb1+(1-α)φb2, where a (0≤α≤1)is a component factor. The properties of the hybrid spectrum were studied. Under conditions of b2> b1>0.7b2and Amax≤0.869(in the case of solution, c2> c1>0.7c2and Amax<0.869)(Amax is the maximum absorbance of b2sample in the spectral range of interest), all the synthesized hybrid spectra are free from significant distortion regardless of the component factor. Thus, the hybrid single-beam spectrum with any desired intensity can be easily obtained simply by choosing an appropriate component factor. The proposed methodology opens a new way in background elimination. If a qualified background single-beam spectrum with the desired intensity can be obtained, the contributions from solvent or other background components can be ratio out exactly and their bands will be absent from the final transmittance/absorbance spectrum. Because of the nature of the single-beam spectrum, the hybrid spectrum method can be applied to measurements of spectra with any ordinate scale including transmittance, reflectance, absorbance, etc. It has been experimentally demonstrated that the features caused by atmospheric water vapor interferences, solvent interferences and other background interferences can be completely eliminated using the proposed methodology. The hybrid spectrum method is a fast，economic, simple yet high effective in eliminating backgoundinterferences, especially in removal of water vapor.