小中大按重金属元素来说，
检出限：仪器对待测元素最小浓度有区别于空白的响应值。一般用7次测量空白的标准偏差的3倍来计算。
为什么是7次，我的理解是，7次测量空白是随机不相关事件，出现错误的概率为2的7次方分之一，那么置信区间为（1-2的7次方分之一）即99%。（此做法为单点校正法，仅给出对I类错误的要求）。
那为什么是3倍呢？一般来说，仪器的信噪比大于3，则认为这个浓度能区别于背景.
检测下限，又称定量限：指仪器可以准确测试的最低浓度。一般来说，按检出限10倍来计（经验值）。事实上，这种做法是不严谨的，因为定量限基于方法上才有意义。一般取方法检出限的2倍。我们这边的做法是2*MDL＝LOQ≈10*IDL.（2MDL＝LOQ可参考EN71-3：2013）
给两个原文解释：
Instrument Detection Limit (IDL) is the concentration equivalent to a signal, due to the analyte of interest, which is the smallest signal that can be distinguished from background noise by a particular instrument. The IDL should always be below the method detection limit, and is not used for compliance data reporting, but may be used for statistical data analysis and comparing the attributes
of different instruments. The IDL is similar to the "critical level" and "criterion of detection" as defined in the literature.
Limit of Quantitation (LOQ), or lower limit of quantitation (LOQ), is the level above which quantitative results may be obtained with a specified degree of confidence. The LOQ is mathematically defined as equal to 10 times the standard deviation of the results for a series of replicates used to determine a justifiable limit of detection. Limits of quantitation are matrix, method, and analyte specific.
摘自：Analytical Detection Limit Guidance - April 1996
上述仅个人理解，仅供参考，亦希望版友能指出不对之处，一起讨论学习。
