内标法Internal standard

若进样体积、离子化效率或检测器灵敏度发生变化，被测物和内标物的信号会按相同比例变化，二者之比保持稳定。定量公式：A_x/A_is = f·(c_x/c_is)，f 为相对校正因子，由标准溶液求得。在前处理之前加入内标，还能校正萃取、浓缩等步骤中的损失。

If injection volume, ionisation efficiency or detector sensitivity changes, analyte and internal-standard signals change in the same proportion, so their ratio stays stable. Quantification uses A_x/A_is = f·(c_x/c_is), with the relative response factor f found from standards. Adding the internal standard before sample preparation also corrects for losses during extraction and concentration.

内标物应：样品中不存在；化学性质和保留行为与被测物相近；色谱峰与被测物和其他组分完全分离（质谱检测时可共流出但质荷比不同）；纯度高、稳定、不与样品反应。质谱分析中最理想的是稳定同位素标记的被测物（如氘代或 ¹³C 标记），即同位素稀释法，被认为是准确度最高的定量方法之一，用于兴奋剂检测、临床药物监测和环境二噁英分析。

The internal standard should be absent from the sample, chemically and chromatographically similar to the analyte, well resolved from the analyte and other components (or, with MS detection, co-eluting but at a different m/z), pure, stable and unreactive. In mass spectrometry the ideal is a stable-isotope-labelled version of the analyte, deuterated or ¹³C-labelled, in isotope-dilution analysis, regarded as one of the most accurate quantitative methods and used in doping control, therapeutic drug monitoring and dioxin analysis.

内标法广泛用于气相色谱（进样体积小、重现性较差时）、LC-MS/MS 生物样品分析（校正基质效应）、ICP-MS（用铟、铑、铋等校正信号漂移）和定量核磁共振（qNMR，用马来酸、对苯二甲酸二甲酯等作内标测定纯度）。

Internal standards are widely used in gas chromatography, where small injection volumes reproduce poorly, in LC-MS/MS of biological samples to correct matrix effects, in ICP-MS with indium, rhodium or bismuth to correct drift, and in quantitative NMR (qNMR) with standards such as maleic acid or dimethyl terephthalate to determine purity.