Advanced Topic 942 of 1102 Analytical, Lab & Spectroscopy
Detection Limit
The lowest concentration reliably distinguished from background.
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In-depth explanation of Detection Limit
The detection limit is the lowest analyte concentration that a method can reliably distinguish from a blank, and it defines the sensitivity of the method. A common definition sets the limit of detection at three times the standard deviation of the blank divided by the slope of the calibration curve, while the limit of quantification is taken as ten times that quantity. Results below the detection limit are reported as not detected, because their signal cannot be separated from noise. The limit depends on instrument noise, the calibration slope and the sample matrix, and should be determined experimentally. It sets the practical lower bound for environmental and clinical analysis.
Going deeper: Detection Limit
Key ideas: LOD = 3 s_blank/m and LOQ = 10 s_blank/m, where m is calibration slope. Estimate s_blank from at least seven replicate blanks or low-level spikes. Between LOD and LOQ a result is detected but cannot be quoted precisely. Concentration units such as ppm (mg/L), ppb (microgram/L) and ppt (ng/L) must be tracked. A method with a steeper slope has a lower detection limit for the same noise.
Common mistakes and connections: Students confuse detection limit with sensitivity, which is the slope, or with the lowest standard they measured. Reporting zero for results below LOD is misleading, so state less than the LOD. Contamination raises the blank and the LOD. Instrument detection limit is lower than method detection limit, which includes sample preparation. Related to calibration, standard deviation and ICP-MS.
Visualization of Detection Limit
With a blank standard deviation of 0.5 counts, a signal three standard deviations above the blank (1.5 counts) is the smallest reliably distinguishable from noise. The interactive lesson shows this as a live animation or chart.
Worked example and use case
An ICP-MS method with a blank standard deviation of 0.5 counts and a slope of 100 counts per ppt has a detection limit of 0.015 ppt.
Frequently asked questions about Detection Limit
What is Detection Limit?
The lowest concentration reliably distinguished from background.
Is Detection Limit a beginner, intermediate or advanced topic?
Detection Limit is a advanced topic in Analytical, Lab & Spectroscopy. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Detection Limit?
Study these earlier topics in Analytical, Lab & Spectroscopy first: Conductometric Titration, Error Analysis, Standard Deviation.
What comes after Detection Limit?
The next topics in Analytical, Lab & Spectroscopy are Sampling and Quality Control.
Can I practise Detection Limit with quizzes?
Yes. The interactive lesson includes a self-check question on Detection Limit, and the Analytical, Lab & Spectroscopy quiz draws on every topic in this area.
Where Detection Limit fits in Analytical, Lab & Spectroscopy
Detection Limit is topic 66 of 99 in Analytical, Lab & Spectroscopy.