指纹区Fingerprint region

官能团区（4000–1500 cm⁻¹）的吸收主要由特定官能团决定，可用于判断有哪些官能团；指纹区的振动涉及整个分子骨架，相互耦合强烈，难以逐一归属，但结构上的微小差异（如同分异构体、顺反异构、取代位置）都会造成明显不同的峰形。即使两种化合物官能团相同，指纹区也不同。

Absorptions in the functional-group region (4000–1500 cm⁻¹) are governed by particular groups and show which are present; vibrations in the fingerprint region involve the whole skeleton and couple strongly, so they are hard to assign individually, yet small structural differences, such as isomers, cis–trans pairs or substitution positions, give clearly different patterns. Two compounds with the same functional groups still differ here.

指纹区中也有一些有用的特征峰：C–O 伸缩 1000–1300 cm⁻¹（醇、醚、酯）；C–Cl 600–800 cm⁻¹；苯环取代类型的 C–H 面外弯曲振动：单取代 690 和 750 cm⁻¹，邻二取代约 750，间二取代约 690 和 780，对二取代约 820 cm⁻¹；反式烯烃 =C–H 面外弯曲约 965 cm⁻¹，顺式约 690 cm⁻¹。

There are useful bands even here: C–O stretches at 1000–1300 cm⁻¹ (alcohols, ethers, esters); C–Cl at 600–800 cm⁻¹; out-of-plane aromatic C–H bends that reveal substitution, at 690 and 750 cm⁻¹ for monosubstituted, about 750 for ortho, about 690 and 780 for meta and about 820 for para; and =C–H out-of-plane bends near 965 cm⁻¹ for trans alkenes and 690 cm⁻¹ for cis.

通过与标准谱图库（如萨德勒谱图库、SDBS 数据库）中的指纹区比对，可确认化合物身份；用于药物真伪鉴别、高分子材料种类识别、法医证物分析（纤维、油漆）、宝石和文物材料鉴定。药典常规定用红外光谱指纹区与对照品比对作为鉴别试验。

Matching the fingerprint region against reference libraries such as Sadtler or SDBS confirms identity; it is used to authenticate drugs, identify polymers, analyse forensic evidence such as fibres and paint, and identify gems and the materials of artworks. Pharmacopoeias often specify comparison of the IR fingerprint with a reference standard as an identity test.