Purchase and use of reference materials for metrology institutes
First, is a certified reference material a certified reference material (CRM)?
To answer this question, we must first understand "certified reference materialThe meaning of (certified reference material) ". A certified reference material is a" reference material accompanied by a document issued by an authoritative body providing one or more characteristic values with uncertainty and traceability obtained using valid procedures [1] ". Therefore, a certified reference material is not necessarily a certified reference material. Certified reference materials should have the following basic characteristics: (1) traceability and uncertainty statements; (2) the preparation, valuation and identification of reference materials comply with the effective procedures given by ISO Guide 34 and Guide 35. Our country has converted the above guidelines into JJF1342 "General Requirements for the Development (Production) of Reference Substances" and JJF1343 "General Principles and Statistical Principles for the Valuation of Reference Substances", which have become the technical basis for the review and release of national reference materials.
What factors should be considered when selecting, purchasing, and accepting reference materials?
When selecting and purchasing reference materials, users should consider the following elements: (1) Types and valuation methods of characteristic quantities: Some reference materials may only be suitable for a specific method or application in a dedicated field. The value of some reference materials has special provisions, such as the value containing crystal water. Attention should be paid to the description in the certificate to prevent misuse. (2) Characteristic quantity level: The characteristic quantity level of the reference material should match the level of the daily measurement sample. (3) Acceptable uncertainty level: The relevant uncertainty level of the characteristic quantity of the reference material should match the precision and accuracy limit requirements in the daily measurement. (4) Matrix and possible interference: When the reference material is used to carry out method confirmation, quality control, and calibration of some measurement methods with serious matrix effects, the matrix should be as close as possible to the daily measurement sample matrix. (5) Form: The reference material can be prepared into different forms, such as lyophilized and frozen samples. Different preparation methods may lead to differences in the behavior of the same characteristics in the standard material and real samples, resulting in interchangeability problems. Before purchasing, it should be fully investigated. (6) Minimum sampling amount: As long as the minimum sampling amount is specified in the standard material certificate, the sampling amount used for measurement should not be less than the minimum sampling amount. Therefore, the minimum sampling amount should be considered when purchasing whether it can meet the requirements of the measurement method. (7) Dosage: The purchased dosage of the reference material should be sufficient to meet the application in the entire experimental plan, including the spare samples to be considered as needed. (8) Stability: Before purchasing, the validity period of the purchased batch of reference materials should be confirmed to avoid the situation of expiration during use. For the purchased reference material, after receiving it, it should first confirm that the transportation conditions of the reference material meet the requirements against the certificate, and then check whether the variety, quantity, etc. are consistent with the purchase requirements; whether the packaging and appearance are normal, and whether the logo is clear and complete; whether there is a certificate; whether it is within the valid period stated by the certificate, etc. After checking, it should be stored in accordance with the storage conditions specified in the certificate immediately. If there is any problem, it should contact the research and development or sales unit in time. The purchase of reference materials can be selected from the National Standard Products Network.
III. What are the differences in the use of certified reference materials and other reference materials?
First of all, it should be made clear that only the certified value or standard value provided by the certified reference material can be used for calibration, assignment of values to other substances, and confirmation of measurement accuracy, and through its use to declare the metrological traceability of the measurement results. In our daily measurements, other types of reference materials are also used, 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 for calibration and other activities, but it should be further evaluated whether these reference materials meet the characteristics of the certified reference material, or the traceability and uncertainty level of the measurement value can fully meet the traceability and accuracy requirements of the measurement results.
IV. How to use the reference value or information value of the certified reference material?
In the certificate of certified reference materials, some additional reference values or information values are sometimes provided, which usually lack clear metrological traceability, lack of uncertainty information or incomplete uncertainty assessment. Therefore, their main use is to help users understand the matrix composition of the reference material, judge the suitability of the reference material, or use it for measurement quality control depending on the degree of reliability.
Must the reference material be used within the valid period?
The valid period of the reference material is determined by the development unit based on the stability research data in order to ensure the reliability of the quality value and uncertainty of the standard material. Therefore, it must be used within the valid period. The unused reference material may still be stable after expiration. For some reference materials with large batches and long stability periods (such as five years), the development unit may sometimes provide services to extend the valid period. However, if the stability of the reference material cannot be guaranteed by the development unit, the user shall be responsible for continuing to use the reference material.
What is the significance of the minimum sample size of the reference material?
The minimum sample size of the reference substance is specified in the uniformity study. The actual sample size when using the reference substance shall not be less than the minimum sample size of the reference substance. When less than the minimum sample size of the reference substance is used, parameters such as the magnitude of the characteristics of the reference substance and the uncertainty stated in the certificate may no longer be valid due to the non-uniformity of the reference substance.
Must the measurement value of the reference material be within the degree of uncertainty?
If the measured value of the reference material in 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. The reason is that the uncertainty of the reference material U The reference material only includes the uncertainty related to the fixed value, uniformity and stability of the reference material, and does not involve the random uncertainty U test related to the measurement of the reference material by the user in the laboratory. Therefore, the following general formula should be adopted to determine that there is a significant deviation between the measured value of the reference material and the standard value:
S represents the standard deviation, n represents the number of repetitions, and the value of t can be obtained from n and the confidence probability level lookup table. In addition, care should be taken to use the reference material after the measurement conditions are stable, otherwise it will lead to wrong conclusions.
Why must reference materials be used and stored in accordance with the conditions specified in the certificate?
The conditions of use and storage specified in the reference material certificate are necessary to ensure. The storage conditions of the reference material are determined in the stability study, while the conditions of use of the reference material, such as temperature, preparation method, drying method, moisture correction method, mixing method, etc., are strictly determined and followed by the process of setting the value of the reference material. Failure to use and store in accordance with the regulations will cause the amount of the reference material to be no longer valid, and the measurement results will deviate from the standard value.
IX. What should be paid attention to during the dilution and use of calibration reference materials?
The calibration reference materials purchased by users are often in high concentrations 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 standard substances. When some substances are stored in glass bottles, they are easily affected by degradation or dissolution. At this time, bottles of other materials, such as K, Na and other elemental solution standard substances, need to be stored in plastic bottles; some substances will be adsorbed or dissolved when stored in plastic bottles. For example, mercury may be adsorbed in plastic bottles, so it is more reliable to choose glass bottles. In addition to the necessary control of blanks, different diluents have different stabilization effects. In principle, they should be prepared according to the types and concentrations of diluents specified in the detection standard method. The measuring instruments used in the weighing and dilution of reference materials, such as balances, pipettes, volumetric bottles, etc., should be properly calibrated or verified to meet the requirements of precision and accuracy, especially the microinjectors and pipettes used in organic analysis. Large errors should be paid attention to daily calibration.
How to perform buoyancy correction when weighing reference materials?
In some cases where accurate weighing of reference materials is required, such as the certificate specifically stating the need to use a buoyancy correction formula to calculate the quality of the sample, the following formula can be used:
Taking the aqueous solution reference material as an example, the effect of buoyancy correction is about one thousandth, so whether to correct depends on the requirements for measurement accuracy.
How to store and verify the value of the opened reference material?
"One-time use" is sometimes stipulated in the certificate of reference substances. These reference substances are generally unstable or have high magnitude accuracy. For example, the national first-class solution standard substance sub-packed in ampoules is prone to changes in the magnitude beyond the uncertainty range after opening the package. It should be removed as soon as possible as required, and cannot be retained and used repeatedly. It can be prepared as an intermediate standard stock solution for storage and use at one time. For certified reference substances that can be used multiple times, it is very important to ensure proper storage and packaging of the standard substance packaging unit after opening, and the integrity of the certificate. In some cases, it is necessary to re-seal the remaining substances according to the requirements of the certificate. Measures to prevent contamination should be taken when sampling. How to control the quantity of reference materials in the use process is often a problem that users are confused about. The quantity control of reference materials should be formulated in the following situations: (1) For certified reference materials, as long as they are stored in accordance with the conditions and methods specified on the standard substance certificate and used within the valid period, the quantity is guaranteed, and the verification of such reference materials should be simplified to check the integrity, valid period, storage conditions, etc. of packaging, labels and certificates; if it is a certified reference material that can be used multiple 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, compare with another unopened unit or batch of reference materials for quantity verification; (2) For internal reference materials or laboratory calibration solutions prepared by certified reference materials, etc Due to the lack of sufficient uniformity and stability data, strict magnitude control is very necessary. The methods that can be adopted depend on the economic and technical conditions of the laboratory, such as using quality control charts for trend inspection, and verifying the accuracy of the values by comparing the values of the upper and lower batches of reference materials or comparing them between laboratories. In addition, the frequency of verification should be determined according to the characteristics of the reference materials, and the time interval for verification of reference materials with good stability expectations can be relatively relaxed.
