Quantitative analysis of impurities in ε-caprolactam by Raman spectroscopy

Abstract

Laser Raman spectroscopy was used for the quantitative analysis of the most common impurities contained in ε-caprolactam: adipic acid, sodium sulfate and ammonium sulfate. Calibration graphs were constructed from spectra obtained from pellets with known quantities of ε-caprolactam and each of the impurities. Linear relationships between the caprolactam content and the ratio of the relative intensities at characteristic wavelengths were found for the impurities examined. The calibration lines were defined as: adipic acid–ε-caprolactam I₉₁₅/I₇₄₆=χ^–1_cap 0.62 – 0.58; ammonium sulfate–ε-caprolactam I₉₇₅/I₇₄₆=χ^–1_cap 2.78 – 2.23; and sodium sulfate–ε-caprolactam I₉₉₄/I₇₄₆=χ^–1_cap 4.26 – 4.45. I is the relative Raman intensity at the wavenumber noted by the subscript and χ_cap is the molar fraction of caprolactam. The detection limits are better than 1 mol% for the impurities examined.