Laboratory qualification recognition requires periodic verification of reference materials, and the laboratory should regularly verify the reference materials in order to adopt appropriate methods or measures to minimize and reduce the costs and risks caused by the calibration status failure of reference materials as much as possible. Below, the editor of Bolinda will introduce the method of verifying reference materials during the period.

What is intermediate verification of reference materials

Intermediate verification of standard materials refers to the inspection of the stability of standard materials during their use period, in accordance with the environmental conditions, storage methods, and testing and analysis methods specified on the certificate of standard materials, to ensure the accuracy of the quality values of standard materials.

In ISO/IEC17025, it is proposed that during verification, reference standards, benchmarks, transfers or work, and reference materials (reference materials) should be verified according to the prescribed procedures and schedules to maintain confidence in their calibration status.

Verification objects of reference materials:

The verification objects include certified reference materials, standard samples or quality control samples in the laboratory, as well as the verification of standard material reserve solutions, standard gases, solid reference materials that have been opened and are currently in use. The periodic verification of standard materials and standard reserve solutions prepared from standard materials is also regulated.

Verification frequency of reference materials:

Each parameter of each standard substance in the laboratory should be analyzed in detail, and the frequency of verification should be determined based on its impact on the test results.

1. Frequently used

For some reference materials with a shorter validity period (such as 6 months) and significant impact on the test results, the verification interval should be shortened, and the verification standards should be strictly enforced. In case of suspicious analysis results, it is only necessary to trace back to the data after the last verification to reduce the risk of the laboratory.

2. Not frequently used

Reference materials can be verified before each use.

3. Certified reference materials

For those with good stability, such as unopened certified reference materials, in principle, only the verification method for unopened reference materials should be implemented. As long as technical and economic conditions permit, a valid standard solution can also be compared with the standard solution to see if there is a significant difference between the two solutions.

4. Only for labeling purposes

Recycling standard materials for testing and verifying their uncertainty may be more meaningful than their traceability.

Verification parameters of reference materials:

Verification parameters include type, level, medium, concentration (content), expiration date, batch number, consistency between account and substance, environmental conditions, storage method, testing and analysis method, etc. Conduct inspections selectively based on the types and methods of inspections.

1. Type

The reference materials corresponding to each testing item in the laboratory. The reference materials or standard samples corresponding to the newly added testing items should be included in the scope of verification management in a timely manner.

2. Level

The commonly used standard materials in chemical analysis laboratories generally include national first level standard materials, national second level standard materials, working standard materials, chromatographic purity, superior purity, analytical purity, etc. According to the accuracy requirements of the measurement method or relevant regulations for standard reference materials, verify whether the laboratory has selected the appropriate level of standard materials.

3. Media

Verify whether the medium of the standard substance used meets the requirements of the monitoring method for the standard substance medium.

4. Concentration (Content)

According to the monitoring method requirements, verify whether appropriate reserve solution concentrations (contents) and working standard solutions have been purchased or used, and their selection will directly affect the determination of the effectiveness of standard materials and the selection of verification frequency.

5. Validity period and storage conditions

Check whether the standard materials have expired and whether the storage conditions meet the requirements based on the testing method or standard material certificate.

6. Detection and analysis methods

When there are requirements for testing and analysis methods on the certified standard reference material certificate, it is necessary to verify whether the actual monitoring method used is consistent with the certificate provided.

7. Effectiveness

After excluding the environmental conditions, storage methods, testing and analysis methods and other influencing factors specified on the standard material certificate, effectiveness verification can be conducted.

Verification method for reference materials:

The verification of standard materials, in addition to items such as appearance and storage conditions, is a relatively difficult task to re verify the quantity values. How should laboratories choose effective testing standards to verify standard substances? There are several methods:

1. Benchmark method

In the field of chemical measurement, the benchmark method is a method with the highest stoichiometric characteristics. The benchmark direct method does not need to use the same amount of standard as a reference to measure the value of unknown samples. The following are several potential benchmark measurement methods that have been identified:

(1) Isotope dilution mass spectrometry;

(2) Coulomb method;

(3) Weighing method;

(4) Titration method;

(5) Freezing point reduction method.

The weighing method is widely used in the preparation of solutions and mixed gases, the freezing point reduction method is used as a direct method for determining the purity of materials (the fraction of substance content), and the isotope dilution mass spectrometry method is widely used for material calibration and other high-quality measurements.

2. Verify according to the instructions of certified reference materials

Some standard specifications already provide their calibration methods, such as the calorific value of coal, ash content, and total iron content in iron ore. Due to the unique combustion measurement method for determining the calorific value of coal, the classic gravimetric method for coal ash content, the potassium dichromate titration method for the total iron content of iron ore, and the weighing method or AAS method for copper content of copper ore. So, laboratories can refer to such methods to verify standard materials. Perform high-level validation when resources permit.

Result evaluation of reference materials:

How to evaluate the measured values of reference materials based on experimental results and whether they are within the confidence interval, we must make a reasonable evaluation of their test results based on mathematical statistical theory.

1. Use t-test method to conduct intermediate verification of quantity values

The t-test method is applicable to the comparative test of two sets of data with small sample size and no significant difference in sample variance.

2. En number method

The criteria for the En number method are as follows: if En ≤ 1, the result is considered satisfactory (indicating that the confidence level of the calibration state of the reference material is maintained), otherwise it is considered unsatisfactory.

3. Determine according to the critical value (CD value)

If the standard method used for this detection can provide reliable repeatability standard deviation Sr and reproducibility standard deviation SR, then the critical value (CD value) can be used to determine the verification results of the reference material.

The criteria for determining this method are as follows: if the average value x of n measurements of a reference material under repeated conditions is different from the guaranteed value μ If the absolute value of the difference between 0 is less than the critical value CD, then the measurement result is satisfactory (indicating that the confidence level of the calibration state of the standard material has been maintained), otherwise it is judged as unsatisfactory.

4. Evaluate according to professional standards and methods

When both En and CD values are not available, if the corresponding professional standards specify the allowable deviation of the test results, it can be determined according to the standards.

The criteria for determining this method are as follows: if Z ≤ 1, the verification result is deemed satisfactory (indicating that the confidence level of the calibration state of the reference material has been maintained), otherwise it is deemed unsatisfactory.