检出限Detection limit

常用 3σ 法：LOD = 3s_b/S，LOQ = 10s_b/S，其中 s_b 为空白信号（或低浓度样品）的标准偏差，S 为校准曲线斜率（灵敏度）。也可用信噪比法：信号与基线噪声之比 S/N = 3 时的浓度为检出限，S/N = 10 为定量限。IUPAC 推荐考虑假阳性和假阴性两类错误。

The common 3σ approach gives LOD = 3s_b/S and LOQ = 10s_b/S, with s_b the standard deviation of blank (or low-level) signals and S the calibration slope (sensitivity). Alternatively, the concentration giving a signal-to-noise ratio of 3 is the LOD and of 10 the LOQ. IUPAC recommends accounting for both false positives and false negatives.

检出限是评价分析方法灵敏度的关键指标，决定了方法能否用于痕量和超痕量分析。典型检出限：紫外分光光度法约 10⁻⁶ mol/L；原子吸收光谱 μg/L 级；ICP-MS 可达 ng/L 甚至 pg/L 级；GC-MS（SIM）可达 pg 级。随着检出限不断降低，许多过去“未检出”的污染物被发现，这推动了环境和食品安全标准的修订。

LOD is a key measure of sensitivity and decides whether a method can be used for trace or ultra-trace analysis. Typical values are about 10⁻⁶ mol/L for UV spectrophotometry, μg/L for atomic absorption, ng/L or even pg/L for ICP-MS, and picograms for GC-MS in SIM mode. As detection limits fall, many pollutants once 'not detected' are found, prompting revisions of environmental and food-safety standards.

“未检出”不等于不存在，只表示含量低于方法检出限，报告时应注明检出限数值。方法检出限（考虑整个样品前处理）通常高于仪器检出限。检出限与灵敏度不同：灵敏度是校准曲线的斜率，检出限同时取决于灵敏度和噪声。

'Not detected' does not mean absent, only below the method's detection limit, which should be reported. The method detection limit, including sample preparation, is usually higher than the instrument detection limit. Detection limit differs from sensitivity: sensitivity is the calibration slope, while the detection limit depends on both slope and noise.