穩定同位素比值質譜儀 (stable isotope ratio mass spectrometer, IRMS) 廣泛被運用於地質、海洋、環境變遷、環境汙染、醫藥及藥檢、農業、生態及鑑識科學等領域，本文將介紹 IRMS 的原理與操作方式，包含雙通道進氣、時序氣體進樣兩種進樣界面，元素分析儀、高溫轉換元素分析儀、氣相層析儀、高效液相層析儀、氣體架及預濃縮進樣等進樣系統，以及離子源、質量分析器及偵測器等部分質譜儀本體之操作原理，並將討論 IRMS 於鑑識科學領域之應用情形。

Isotope ratio mass spectrometer (IRMS) is a powerful analytical instrument applied on many fi elds like geology,oceanography, environmental changes, environmental pollution, medical science, drug testing, agriculture, ecologyand forensic science. We introduce the theory and operational modes of IRMS in this article, including the samplinginterface: dual inlet and continuous fl ow, and the options: EA, TC EA, GC, HPLC, gas bench and preconcentration,and the mass spectrometer: ion source, mass analyzer and detector. We also discuss IRMS applications on forensicscience.