FAQs in the use of reference materials
Lu Xiaohua, Li Hongmei
China Academy of Metrology
1、 Is certified reference material CRM?
To answer this question, first understand the meaning of "certified reference material". Certified reference materials refer to "reference materials with one or more characteristic values with uncertainty and traceability obtained by using effective procedures attached with documents issued by authoritative organizations [1]". Therefore, certified reference materials are not necessarily certified reference materials. Certified reference materials should have the following basic characteristics: (1) Traceability and uncertainty statement; (2) The preparation, calibration and accreditation of reference materials comply with the effective procedures given in ISO Guide 34 and Guide 35. China has equivalently transformed the above guidelines into JJF1342 General Requirements for Reference Material Development (Production) Institutions and JJF1343 General Principles and Statistical Principles for Calibration of Reference Materials, which have become the technical basis for the review and issuance of national reference materials.
2、 What elements should be considered when selecting, purchasing and accepting reference materials?
Users should consider the following factors when selecting and purchasing reference materials: (1) Type and setting method of characteristic quantity: some reference materials may only be applicable to a specific method or application in the exclusive field. The value of some reference materials has special provisions, such as the value containing crystal water. Attention should be paid to such descriptions in the certificate to prevent misuse. (2) Characteristic quantity level: the characteristic quantity level of reference materials shall match the level of daily measured samples. (3) Acceptable uncertainty level: the relevant uncertainty level of the characteristic quantity of the reference material shall match the precision and accuracy limit requirements in daily measurement. (4) Matrix and possible interference: When reference materials are used for method validation, quality control and calibration of some measurement methods with serious matrix effects, the matrix should be as close as possible to the matrix of daily measurement samples. (5) Form: Reference materials can be prepared into different forms, such as freeze-dried and frozen samples. Different preparation methods may lead to different behaviors of the same characteristics in reference materials and real samples, resulting in interchangeability problems. Full investigation should be conducted before purchasing. (6) Minimum sampling quantity: as long as the minimum sampling quantity is specified in the reference material certificate, the sampling quantity used for measurement shall not be less than the minimum sampling quantity, so whether the minimum sampling quantity can meet the requirements of the measurement method shall be considered when purchasing. (7) Dosage: The purchase amount of reference materials should be sufficient to meet the application of the entire experimental plan, including sample preparation as required. (8) Stability: before purchasing, the validity period of the purchased batch of reference materials should be confirmed to avoid expiration during use. For purchased reference materials, the transportation conditions of reference materials shall be confirmed according to the certificate after receipt, and then the variety and quantity shall be checked to see if they are consistent with the purchase requirements; Whether the packaging and appearance are normal, and whether the identification is clear and complete; Whether there is a certificate; Whether it is within the validity period of the certificate statement, etc. After verification, it shall be stored immediately according to the storage conditions specified in the certificate. If there is any problem, contact the research and development or sales unit in time.
3、 What are the differences between certified reference materials and other reference materials in use?
First of all, it should be clear that only the certified value or standard value provided by certified reference materials can be used for calibration, assignment of values to other materials and confirmation of measurement accuracy, and the metrological traceability of measurement results can be declared through its use. In our daily measurement, we will also use other types of reference materials, which are mainly used for measurement quality control, measurement precision confirmation, etc. Sometimes, in the absence of certified reference materials, these reference materials have to be used to carry out calibration and other activities, but it is necessary to further evaluate whether these reference materials conform to the characteristics of certified reference materials, or the traceability and measurement uncertainty level can fully meet the traceability and accuracy requirements of measurement results.
4、 How to use the reference value or information value of certified reference materials?
Some additional reference values or information values are sometimes provided in the certificate of certified reference materials. These values usually lack clear metrological traceability, uncertainty information or comprehensive uncertainty evaluation. Therefore, their main purpose is to help users understand the matrix composition of reference materials and judge the applicability of reference materials, Or depending on the reliability, it can be used to carry out measurement quality control.
5、 Must reference materials be used within the validity period?
The validity period of reference materials is determined by the development unit according to the stability research data in order to ensure the reliability of the quality value and uncertainty of reference materials. Therefore, they must be used within the validity period. The unused reference materials may still be stable after the expiration. For some reference materials with large batch and long stability period (such as five years), the development unit will sometimes provide the service of extending the validity period. However, if the stability of the reference materials cannot be guaranteed by the development unit, the user will be responsible for continuing to use the reference materials.
6、 What is the significance of the minimum sampling amount of reference materials?
The minimum sampling amount of reference materials is specified in the uniformity study. The actual sampling amount when using reference materials shall not be less than the minimum sampling amount of reference materials. When using less than the minimum sampling amount of reference materials, the parameters such as the characteristic quantity value and uncertainty of reference materials declared in the certificate may no longer be effective due to the non-uniformity of reference materials.
7、 Must the measured value of reference materials be within the uncertainty?
If the measured value of the reference material by the laboratory exceeds the uncertainty range of the characteristic value of the reference material, it is impossible to directly draw the conclusion that there is a deviation in the measurement result, because the uncertainty U of the reference material only includes the uncertainty related to the fixed value, uniformity and stability of the reference material, It does not involve the random uncertainty U-test related to the user's measurement of reference materials in the laboratory. Therefore, the following general formula should be adopted to judge that there is significant deviation between the measured value of reference materials and the standard value: S represents the standard deviation, n represents the number of repetitions, and t value can be obtained by looking up the table according to n and confidence probability level. In addition, attention should be paid to the use of reference materials after the measurement conditions are stable, otherwise it will lead to wrong conclusions.
8、 Why must reference materials be used and stored according to the conditions specified in the certificate?
The use and storage conditions specified in the certificate of reference materials are necessary to ensure. The storage conditions of reference materials are determined in the stability study, while the use conditions of reference materials, such as temperature, preparation method, drying method, moisture correction method, mixing method, etc., are strictly determined and followed during the calibration process of reference materials, and the use and storage of reference materials not in accordance with the regulations will lead to the invalid quantity of reference materials, The measurement result is deviated from the standard value.
9、 What should be paid attention to during the dilution and use of calibration reference materials?
The calibration reference materials purchased by users are often high in concentration and cannot be used directly. Quality control of these processes should be done well. Attention should be paid to the containers and diluents used in the dilution and preparation of reference materials. When some substances are stored in glass bottles, they are easily affected by degradation or dissolution. In this case, bottles of other materials, such as K, Na and other elemental solution reference materials, should be used for storage in plastic bottles; Some substances stored in plastic bottles will be adsorbed or dissolved, such as mercury, which may be adsorbed in plastic bottles, so it is more reliable to choose glass bottles. In addition to necessary control of blank, different diluents have different stability effects. In principle, the diluent shall be prepared according to the diluent type and concentration specified in the test standard method. Measuring instruments used in the weighing and dilution process of reference materials, such as balance, pipette, volumetric flask, etc., should be properly calibrated or verified to confirm that they meet the accuracy and accuracy requirements. In particular, the micro syringe and pipette gun used in organic analysis have large errors, so attention should be paid to daily calibration.
10、 How to carry out buoyancy correction when weighing reference materials?
In some occasions where accurate weighing of reference materials is required, for example, the certificate specifically states that buoyancy correction formula is required to calculate the mass of samples, the following formula can be used: Taking the reference material of aqueous solution as an example, the influence of buoyancy correction is about one thousandth. Therefore, whether to correct depends on the requirements for measurement accuracy.
11、 How to store and conduct value verification of opened reference materials?
The certificate of reference materials sometimes stipulates "one-time use". These reference materials are generally unstable or have high measurement accuracy. For example, the national first-class solution reference materials packaged in ampoules are prone to change in measurement value beyond the uncertainty range after opening the package, so they should be removed as soon as possible as required, and cannot be retained for repeated use. The intermediate standard stock solution can be prepared once for storage and use. For certified reference materials that can be used for many times, it is very important to ensure the proper preservation and packaging of the reference material packaging unit after opening, and the integrity of the certificate. In some cases, it is necessary to reseal and package the remaining materials according to the certificate requirements. Measures to prevent contamination shall be taken during sampling. How to control the quantity value of reference materials in the use link is often a puzzling problem for users. Specific measures shall be formulated for the control of the measurement value of reference materials according to the following situations: (1) For certified reference materials, as long as they are kept in accordance with the conditions and methods specified in the certificate of reference materials and used within the validity period, the measurement value is guaranteed, and the verification of such reference materials shall be simplified to checking the integrity, validity, storage conditions, etc. of packaging, labels and certificates; If it is a certified reference material that can be used for many times after opening, in addition to the above verification, the laboratory should formulate corresponding documents to standardize its subsequent use and management, regularly check its use, and if necessary, check its value by comparison with another unopened unit or batch of reference material; (2) For internal reference materials or calibration solutions prepared by certified reference materials in the laboratory, due to the lack of sufficient uniformity and stability data, strict quantity control is very necessary, and the methods can be flexibly mastered according to the economic and technical conditions of the laboratory, such as trend checking Verify the accuracy of the quantity value by comparing the quality value of the next batch of standards or between laboratories. In addition, the frequency of verification shall be determined according to the characteristics of reference materials, and reference materials with good stability can be relatively relaxed.
