Near-IR versus Mid-IR: Separability of Three Classes of Organic Compounds

Abstract

Recently there has been a surge of interest in spectroscopic sensors operating in the near-IR, although it is recognized that the mid-IR contains more spectral information. The general question addressed in this paper is, How much specificity is lost in choosing the near-IR over the mid-IR for sensor applications? The example considered is the separability among three classes of organic compounds: alkanes, alcohols, and ketones/aldehydes. We use spectra from two sources: the Hummel polymer library (mid-IR) and the library of Buback and Vögele (near-IR). This is the first paper on class separability to make use of this new near-IR library, available in digital form only since July 1995. Five spectral regions are considered: region 5, 10,500 to 6300 cm⁻¹; region 4, 7200 to 5200 cm⁻¹; region 3, 5500 to 3800 cm⁻¹; region 2, 3900 to 2500 cm⁻¹; and region 1, 2500 to 500 cm⁻¹. Class separability is explored both qualitatively and quantitatively with the use of principal component scatter plots, linear discriminant analysis, Bhattacharyya distances, and other methods. We find that the separability is greatest in region 1 and least in region 2, with the three near-IR regions being intermediate. Furthermore, we find that, in the near-IR, there is sufficient class separability to ensure that organic compounds of one class can be determined in the midst of interference from the other classes.