In liquid chromatography analysis, various "limits" are involved to describe the sensitivity, quantitative capability, and applicable range of methods. Below are detailed explanations and comparisons of common concepts:

Concept Explanations

Limit of Detection (LOD)

Refers to the lowest concentration of an analyte in a sample that can be reliably detected by an instrument or method, but accurate quantification is not possible at this point. It is usually determined using a signal-to-noise ratio (S/N) ≥ 3:1 as the criterion.

Limit of Quantification (LOQ)

Refers to the lowest concentration at which an analytical method can accurately quantify the target analyte, meeting certain precision and accuracy requirements. It is typically defined with a signal-to-noise ratio ≥ 10:1.

Instrument Detection Limit (IDL)

The lowest detectable concentration measured solely based on instrument performance (e.g., detector sensitivity), without considering factors such as sample pretreatment. It is usually determined using pure standard solutions.

Method Detection Limit (MDL)

The lowest concentration detectable by the entire analytical method (including pretreatment steps), which needs to be verified in actual sample matrices. It is generally higher than the IDL.

Reporting Limit (RL)

The lowest concentration that laboratories or regulations require to be reported, which may be equal to or higher than the LOQ. It is usually set based on practical needs or standards.

Lower Limit of Quantification (LLOQ)

Equivalent to LOQ, referring to the lowest concentration point at which an analytical method can accurately quantify.

Upper Limit of Quantification (ULOQ)

The highest concentration point at which an analytical method can accurately quantify. Exceeding this value may lead to saturation of the response signal or failure of the linear relationship.

Comparison of Various "Limits" in Liquid Chromatography

Key Distinctions

· LOD vs. LOQ: LOD is used to determine "whether a substance exists," while LOQ is used for "accurate quantification."

· IDL vs. MDL: IDL only reflects instrument performance, whereas MDL includes the comprehensive impact of methods such as pretreatment.

· RL vs. LOQ: The reporting limit may be specified by external regulations, while LOQ is an inherent characteristic of the method itself.

By clarifying these "limits," we can more scientifically evaluate the sensitivity, applicable range, and data reliability of analytical methods.