Detailed detection limit

1. Concept of detection limit

The detection limit was 1947, by the German H. Kaiser first proposed the same precision and accuracy as the analytical method. The detection limit is also an important indicator for evaluating the performance of an analytical method. After several years of research, the International Union of Pure and Applied Chemistry (IUPAC) was established in 1975. Kaiser's proposal was formally accepted, and the concept of using the detection limit and the corresponding estimation method were implemented among the relevant sub-members.

The defined detection limit given by IUPAC is defined as the detection limit is the minimum concentration or amount at which a particular method can detect a substance to be tested from a sample within a given confidence level. The so-called "detection" is a specified detection, that is, it is determined that a sample having a higher concentration than a blank exists in the sample. The ACS (American Chemical Society) provides a more concise summary of this definition: the detection limit is an analytical method that reliably detects the lowest concentration of analyte.

2 , the calculation method of the detection limit

For half a century, people have accepted the definition of detection limit more uniformly in terms of concept, but the international analysis community has been controversial on how to correctly or more accurately estimate the detection limit. The special significance is that the detection capability of a given analytical method at a low concentration level can be accurately evaluated. Investigate the performance of an analytical method in the low concentration range, which can be based on different angles or different focuses, such as the ratio of the minimum signal value to the instrument noise; the method is used to determine the average volatility of the blank to estimate the estimate; Quantitative estimation and the like can be performed based on the deviation characteristics of the analytical method calibration curve. The detection limits of the same analytical method obtained from different angles and different focuses may be quite different.

The calculation methods for the detection limit are as follows:

(1) The International Union of Pure and Applied Chemistry (IUPAC) stipulates the detection limit DL as follows:

For various optical analysis methods, the measurable minimum analytical signal x is determined by the following equation:

XL=Xb+KSb

Where: Xb-blank is the average value of the signal measured multiple times;

Sb-blank measured the standard deviation of the signal multiple times:

K - A coefficient determined according to a certain confidence level.

The concentration or amount corresponding to XL-Xb (ie KSb) is the detection limit:

DL=( XL-Xb )/a=KSb/a

Where: the sensitivity of the a-method (ie the slope of the calibration curve).

In order to evaluate Xb and Sb, the number of experiments must be at least 20 times.

In 1975, IUPAC recommended K=3 for spectrochemical analysis. Since the measurement error of the low concentration level may not follow the normal distribution, and the number of determinations of the blank is limited, the confidence level corresponding to K=3 is about 90%. In addition, there are suggestions for taking K as 4, 4.6, 5 and 6.

(2) The Global Environmental Monitoring System Water Monitoring Operation Guide states that when the given confidence level is 95%, the difference between the measured value of the sample and the measured value of the zero-concentration sample is the detection limit (DL). The zero concentration sample here is a sample containing no substance to be tested.

D. L=4.60σwb

Where: σwb - blank parallel determination (intra-assay) standard deviation (repeated measurement more than 20 times).

(3) U.S. Environmental Protection Agency EPA describes the method detection limit as the lowest concentration that can be detected and reported at 99% confidence when the analyte concentration is greater than zero. The method for calculating the detection limit MDL is as follows:

Seven sets of low-concentration spiked samples in each group were determined. The spiked concentrations were generally 1-3 times MDL. The concentrations of the two groups were similar, and the variance was not significantly different by F test; and according to the given analytical method The whole process is processed and measured, and the calculation formula is:

MDL=tν1+ν2,1-α×Spooled

Ν1, ν2 - degrees of freedom;

Tν1+ν2,1-α——degree of freedom is

Student'st value of ν1+ν2 (obtained by table)

1-α is the confidence level

Spooled-synthetic standard deviation

If the measurement is repeated 7 times, the confidence level is 99%, and the value of t can be simplified to MDL=3.143s.

(4), ISO definition of detection limit:

The detection limit of the analytical method refers to the lowest concentration or amount that can be qualitatively detected at a given probability, that is, P=95%, and the significance level is 5%. The ISO method for estimating the limit (VBx for short) is completely different from the above method. It is based on the intercept of the calibration curve, the residual standard deviation, the slope and the linear working range. describe. When a calibration curve is created, the confidence limit of each concentration point on the corresponding curve can be determined. The statistical limit can be determined. The detection limit Xn can be obtained (for specific calculation, please refer to GBl7378-1998 “Ocean Monitoring” The calculation of the limit of detection of seawater quality analysis in the Code). The VBx method generally only needs to calculate the detection limit according to a calibration curve. As the parameters (conditions) of the calibration curve change, the detection limit also changes, so it is only a reference value, and does not represent an analytical method. The best value.

3 , classification of detection limits

(1) Instrument detection limit (IDL): refers to the minimum or minimum concentration of the analyte that the analytical instrument can detect. This concentration is the minimum response signal that can be discerned by the specific instrument from the background noise. Corresponding. The detection limit of the instrument does not take into account the influence of any sample preparation steps. Generally, the detection limit is determined by the solvent blank. Therefore, the value is always lower than the detection limit of the method. Instrument detection limits are generally used for performance comparison of different instruments.

(2) Method Detection Limit (MDL): refers to the signal generated by the analyte can be distinguished from the blank by 99% confidence after all the measurement processes (including sample pretreatment) by an analytical method. The lowest concentration at which the sample was measured. The detection limit of the method is one of the more important parameters in the analysis method, especially the evaluation of an analytical method is of great significance for the detection quality of low concentration samples.

(3), Reliable Quantitation Limit (RQL, Reliable Quantitation Limit): Under the premise that the measurement error can meet the predetermined requirements, the minimum concentration or amount of the substance to be tested can be accurately quantified by a specific method, which is called the lower limit of the method.

The lower limit of measurement reflects the ultimate possibility that the analytical method can accurately quantify the substance to be tested at a low concentration level. Without (or eliminating) systematic errors, it is limited by the precision requirements, the higher the precision requirements of the analytical method, the more the lower limit of detection is higher than the detection limit.

United States. EPA SW-846 (chemical physical analysis method for solid waste) stipulates that 4MDL is the lower limit of quantitation (RQL), that is, the limit of 4 times the detection limit is the lower limit of measurement, and the relative standard deviation of the measured value is about 10%; The lower limit is 10 times MDL. In a report published by IUPAC in 1982, the lower limit of determination was specified, and the concentration value corresponding to 10 times the standard deviation of the blank measurement was used as the lower limit of the analytical method. In China, IUPAC generally recommends that the concentration corresponding to the standard deviation of the 10-fold blank measurement value be used as the lower limit of measurement, and its confidence level is about 90%.

4. Defects in the current detection limit regulations

We can see from the above that the standard deviation of the "blank" is inevitable in the calculation method detected, but there is no clear definition of this blank standard deviation. For the instrument, the standard deviation of this blank The length of the test is very large. If we measure 20 times, after 2 minutes of measurement and 2 hours, the data may vary greatly. But in any case, the detection limit can express the detection performance of the instrument or this analysis method, and the introduction of the detection limit concept is of great significance.

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