红外光谱差减技术在扣除背景组份干扰方面得到广泛应用, 但其扣除空气中水汽效果却不尽如人意。 研究了不同湿度的水汽光谱与光谱差减效果的关系, 以探究光谱差减技术在水汽扣除领域局限性的原因。 结果表明: （1）相对湿度改变, 水汽的红外光谱也发生变化, 不管如何小心地选择比例系数f, 从相对湿度x%的水汽光谱Ax%, 也不能完全扣除相对湿度为y%的水汽光谱Ay%, 即fAx%≠Ay%。 （2）相对湿度改变, 水分子团簇(H2O)n的相对组成也会发生变化, 这是导致光谱差减技术局限性的主要原因。 （3）将湿度为x%和y%的两水汽光谱Ax%和Ay%进行线性组合, 则可以高度近似地模拟出介于两者之间的湿度的水汽光谱。 比如用40%水汽谱和30%水汽谱, 可以模拟得到32%或35%或37%的水汽谱。 实验结果表明这是扣除水汽干扰效果更好的路径。 （4）论证了水汽补偿湿度滴定法具有高效性的原因。

Spectral subtraction is a powerful and conventional tool for the elimination of background interferences, but this technique has been suffered from severe limitations in the case of atmospheric water vapor. In order to understand the real reasons of the problems above , IR spectra of water vapor at different humidity have been investigated. The following conclusions have been reached: (1) 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. In general, spectral subtraction is not a good practice to remove water vapor interferences. (2) The relative composition of water clusters (H2O)n changes with the relative humidity, and this is the main reason of spectral subtraction limitation. (3) 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. (4) The high efficiency of the humidity titration method has been demonstrated experimentally.