reference material(RM)It is a sufficiently uniform material or substance with one or more characteristic values that have been well determined, and a certified standard substance(CRM)It is a standard substance with a certificate attached, and one or more of its characteristic values are determined by a traceable program, making it traceable to the measurement unit accurately used to represent the characteristic value, and each standard value is accompanied by a given confidence level of uncertainty.

Standard substances are widely used in chemical analysis laboratories for calibrating instruments, evaluating testing methods, or assigning values to materials. The correct use and standardized management of standard substances are of great significance for ensuring the accuracy and traceability of analysis results.

1,Certified reference materials are certified reference materials(CRM)ma?

Firstly, it is important to understand“Certified reference materials(certified reference material)”The meaning. Certified standard substances refer to“Attached by Quan WeiA document issued by an organization that provides standard substances with one or more characteristic values obtained through effective procedures, which have uncertainty and traceability”. Therefore, certified reference materials are not necessarily certified reference materials.

2,What factors should be considered when selecting, purchasing, and accepting standard substances?

1. Types and methods of determining characteristic quantities

Some standard substances may only be applicable to a specific method or exclusive field of application, and the values of certain standard substances have special regulations, such as the value containing crystal water. Attention should be paid to such descriptions in the certificate to prevent misuse.

1. Characteristic level

The characteristic quantity level of standard substances should match the level of daily measurement samples.

1. Acceptable level of uncertainty

The level of uncertainty related to the characteristic quantities of standard substances should match the precision and accuracy limits required in daily measurements.

1. Matrix and possible interference

When standard substances are used for method confirmation, quality control, and calibration of measurement methods with severe matrix effects, the matrix should be as close as possible to the matrix of the daily measurement sample.

1. form

Standard substances can be prepared in different forms, such as freeze-drying and frozen samples. Different preparation methods may lead to differences in the behavior of the same characteristics between standard substances and real samples, resulting in interchangeability issues. Therefore, thorough research should be conducted before purchasing.

1. Minimum Sampling Quantity

As long as the minimum sampling amount is specified in the standard substance certificate, the sampling amount used for measurement should not be less than this minimum sampling amount. Therefore, when selecting, consideration should be given to whether the minimum sampling amount can meet the requirements of the measurement method.

1. dosage

The purchase quantity of standard substances should be sufficient to meet the application requirements of the entire experimental plan, including sample preparation as needed.

1. stability

Before purchasing, it is necessary to confirm the expiration date of the purchased batch of standard substances to avoid the occurrence of expiration during use.

For the purchased standard substances, upon receipt, the transportation conditions of the standard substances should be confirmed against the certificate to meet the requirements, and then the variety, quantity, etc. should be checked to ensure consistency with the purchase requirements; Whether the packaging and appearance are normal; Whether the identification is clear and complete; Whether there is a certificate; Whether it is within the validity period stated in the certificate. After verification, immediately store according to the storage conditions specified in the certificate. If there are any problems, please contact the research and development or sales unit in a timely manner.

3,What are the differences in the use of certified reference materials and other reference materials?

Firstly, it should be clarified that only the certified values or standard values provided by certified reference materials can be used for calibration, assigning values to other substances, and confirming measurement accuracy, and declaring the metrological traceability of measurement results through their use.

In daily measurements, other types of standard substances are also used, which are mainly used for quality control and precision confirmation in measurements. Sometimes, in the absence of certified reference materials, it is necessary to use these reference materials for calibration and other activities. However, it is necessary to further evaluate whether these reference materials meet the characteristics of certified reference materials, or whether the traceability and uncertainty levels of measurement values can fully meet the requirements of traceability and accuracy of measurement results.

4,How to use the reference value or information value of certified reference materials?

In certificates of certified reference materials, sometimes additional reference values or information values are provided, which often lack clear metrological traceability, uncertainty information, or comprehensive uncertainty assessment. Therefore, their main purpose is to help users understand the composition of the reference material matrix, judge the applicability of the reference material, or use it for measurement quality control based on reliability.

5,Do standard substances have to be used within their expiration date?

The validity period of a standard substance is determined by the development unit based on stability research data to ensure the reliability of the quality value and uncertainty of the standard substance. Therefore, it is necessary to use it within the validity period. Standard substances that have not been used after expiration may still be stable, especially for some larger batches with longer stability cycles(as5year)The development unit may sometimes provide services to extend the validity period of the standard substance, but if the stability of the standard substance cannot be guaranteed by the development unit, users who continue to use the standard substance shall bear the responsibility themselves.

6,What is the significance of the minimum sampling size for standard substances?

The minimum sampling size for standard substances is specified in the uniformity study. The actual sampling amount when using standard substances should not be less than the minimum sampling amount of the standard substance. When it is less than the minimum sampling amount of the standard substance, the characteristic values and uncertainties declared in the certificate may no longer be valid due to the non-uniformity of the standard substance.

7,Do the measured values of standard substances have to be within the range of uncertainty?

If the measurement value of the standard substance in the laboratory exceeds the uncertainty range of the characteristic value of the standard substance, it cannot be directly concluded that there is a deviation in the measurement result because of the uncertainty of the standard substanceUThe standard substance only includes uncertainties related to the value, uniformity, and stability of the standard substance, and does not involve random uncertainties related to user measurements of the standard substance in the laboratoryUMeasure. Therefore, the following general formula should be adopted to determine the significant deviation between the measured value of the standard substance and the standard value:

8,Why must standard substances be used and stored according to the conditions specified in the certificate?

The usage and storage conditions specified in the standard substance certificate are necessary conditions to ensure the validity of the standard substance. The storage conditions of standard substances are determined in stability studies, while the usage conditions of standard substances(Such as temperature, preparation method, drying method, moisture correction method, mixing method, etc)If the standard substance is not used and stored according to regulations, it will cause the value of the standard substance to no longer be valid, and the measurement results will deviate from the standard value.

9,What issues should be noted during the dilution and use of calibration reference materials?

The calibration standard substances purchased by users often have high concentrations and cannot be used directly. Quality control should be carried out for these processes. Attention should be paid to the containers and diluents used in the dilution and preparation process of standard substances. When some substances are stored in glass bottles, they are easily affected by degradation or dissolution. In this case, bottles made of other materials should be used, such asK，NaStandard substances for elemental solutions should be stored in plastic bottles; Some substances stored in plastic bottles may undergo adsorption or dissolution, such as mercury in plastic bottles, so choosing glass bottles is more reliable.

The diluent used should not only control the blank as necessary, but also have different stabilizing effects. In principle, it should be prepared according to the diluent variety and concentration specified in the testing standard method. Measuring instruments used in the weighing and dilution process of standard substances(Such as scales, pipettes, volumetric flasks, etc)Proper calibration or verification should be carried out to confirm compliance with accuracy requirements, especially for micro injectors and pipettes used in organic analysis, which have significant errors. Daily calibration should be emphasized.

10,How to store and maintain the opened standard substance?

Sometimes standard substance certificates may specify“Single-use”These standard substances are generally unstable or have high accuracy in measurement, such as the national level yi for ampoule packagingSolution standard substances are prone to changes in quantity beyond the uncertainty range after opening the packaging. They should be removed as soon as possible according to requirements and should not be retained for repeated use. It can be prepared as an intermediate standard reserve solution at once for storage and use.

For certified reference materials that can be used multiple times, it is important to ensure proper storage and packaging of the packaging unit after opening, as well as the integrity of the certificate. In some cases, it is necessary to reseal the remaining materials according to the certificate requirements. Measures should be taken to prevent contamination during sampling.

How to maintain the use of standard substances is often a confusing problem for users. Basic Terminology in International Metrology(VIM)The maintenance of measurement standards is a set of operations necessary to maintain the metrological characteristics of measurement standards within specified limits. Usually includes periodic verification or calibration of predetermined metrological characteristics, storage under appropriate conditions, and careful maintenance and use. As a measurement standard, standard substances should also be regularly maintained and verified to ensure the reliability of their values.

Generally speaking, specific measures can be formulated based on the following situations:

(1)For certified reference materials, as long as they are stored according to the conditions and methods specified on the reference material certificate and used within the validity period, the quantity is guaranteed. The maintenance of such reference materials should be simplified to checking the integrity, validity period, and storage conditions of the packaging, labels, and certificates;

If it is a certified standard substance that can be used multiple times after opening, in addition to the above inspections, the laboratory should develop corresponding documents to regulate its subsequent use and management, regularly verify its usage, and if necessary, compare it with another unopened unit or batch of standard substance for value verification;

(2)For internal standard substances or calibration solutions prepared from certified standard substances in laboratories, strict value verification is necessary due to the lack of sufficient uniformity and stability data. The methods that can be adopted depend on the economic and technical conditions of the laboratory, such as using quality control charts for trend checking, verifying the accuracy of value verification through comparison of quality values between batches of standard substances or between laboratories.

In addition, the frequency of verification should be determined based on the characteristics of the reference material, and reference materials with good stability expectations can have relatively relaxed verification intervals.

Article source: Laboratory instrument analysis