How are Reference Standards periodically verified?

A standard sample (RM) is a sufficiently homogeneous material or substance whose characteristic values have been well determined, and a certified standard sample (CRM) is a standard sample with a certificate, and one or more of its characteristic values are determined by a program that establishes traceability so that it can be traced to the accurate realization of the measurement used to represent the characteristic value, and each standard value is attached to a given level of
uncertainty.

In chemical analysis laboratories, standard samples are widely used to calibrate instruments, evaluate test methods or assign values to materials, and the correct use and standardized management of reference materials are of great significance
to ensure the accuracy and traceability of analysis results.

1.
Is a certificate CRM a CRM?

To answer this question, we must first understand the meaning of
"certified reference material".
A certified Reference Standard is "a Reference Material with a document issued by an authority providing one or more characteristic values with uncertainty and traceability obtained using a valid procedure"
.
Therefore, a certified Reference Standard is not necessarily a Certified Reference Material
.

2.
What factors should be considered when selecting, purchasing, and accepting Reference Materials?

Users should consider the following factors when selecting and purchasing Reference Materials:

1.
Types of characteristic quantities and valuation methods: Some reference materials may only be applicable to a specific method or application in an exclusive field, and the value of some reference materials has special provisions, such as the value of crystalline water, and attention should be paid to such instructions in the certificate to prevent misselection and misuse
.

2.
Characteristic quantity level: The characteristic amount level of the reference material should match
the level of the daily measurement sample.

3.
Acceptable level of uncertainty: The level of uncertainty associated with the characteristic quantity of the reference material should match
the precision and correctness limit requirements in daily measurements.

4.
Substrate and possible interference: When the reference material is used for method confirmation, quality control, and calibration of some measurement methods with serious matrix effects, the substrate should be as close as
possible to the daily measurement sample matrix.

5.
Form: Reference materials 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 the real sample, resulting in interchangeability problems, should be fully investigated
before purchasing.

6.
Minimum sampling amount: As long as the minimum sampling amount is specified in the reference material certificate, the sampling amount used for measurement should not be less than the minimum sampling amount, so the minimum sampling amount should be considered when purchasing
.

7.
Dosage: The purchase amount of reference materials should be sufficient for the application of the entire experimental plan, including sample
preparation considered as needed.

8.
Stability: Before purchasing, the expiration date of the purchased batch of reference materials should be confirmed to avoid expiration when used
.

For the purchased reference materials, after receiving them, they should first confirm that the transportation conditions of the reference materials 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; Whether the logo is clear and complete; with or without a certificate; Whether it is within the validity period of the certificate statement, etc
.
After the check, it is immediately stored
in accordance with the preservation conditions specified in the certificate.
If there is any problem, you should contact
the development or sales in time.

3.
What is the difference between the use of CRMs and other CRMs?

First, it should be clarified that only the identified values or standard values provided by certified reference materials can be used for calibration, assignment of other substances, and confirmation of measurement correctness, and their use to declare the metrological traceability
of measurement results.

Other types of reference materials are also used in routine measurements, and they 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 activities such as calibration, but it should be further assessed whether these reference materials meet the characteristics of certified reference materials or whether traceability and quantitative uncertainty levels adequately meet the traceability and accuracy requirements
of measurement results.

4.
How to use the reference value or information value of the certified reference material?

Certificates for certified reference materials sometimes provide additional reference values or information values, which often lack clear metrological traceability, lack uncertainty information, or incomplete uncertainty assessment, so their main use is to help users understand the composition of the reference material matrix, judge the suitability of reference materials, or use them for measurement quality control
depending on reliability.

5.
Do Reference Standards have to be used within the expiration date?

The expiration date of Reference Standards is determined based on stability study data to ensure the reliability of the quality values and uncertainties of the Reference Materials, so it is important to use
them within the validity period.
For some standard materials with large batches and long stability periods (such as 5 years), the development sometimes provides services to extend the validity period, but in the case that the stability of the reference materials cannot be guaranteed, the user shall be responsible
for continuing to use the reference materials.

6.
What is the significance of the minimum sample size of Reference Materials?

The minimum sample size for reference materials is specified
in the homogeneity study.
The actual sampling amount when using Reference Standards shall not be less than the minimum sampling amount of the Reference Material, and when less than the minimum sampling amount of the Reference Material, parameters such as the characteristic measure value of the Reference Material and the uncertainty declared in the certificate may no longer be valid
due to the inhomogeneity of the Reference Material.

7.
Do the measured values of reference materials have to be within the uncertainty range?

If the laboratory's measurement of the reference material exceeds the uncertainty range of the characteristic value of the reference material, it cannot be directly concluded that there is a deviation in the measurement result, because the uncertainty of the reference material U The reference material only includes the uncertainty related to the value of the reference material, uniformity, and stability, and does not involve the random uncertainty U measurement related to the user's measurement
of the reference material in the laboratory.
Therefore, the following general formula should be used to determine the existence between the reference material measurement and the standard value.

8.
Why is it important to use and preserve Reference Standards in accordance with the conditions specified in the certificate?

The conditions of use and storage specified in the Reference Standard Certificate are necessary to ensure the effectiveness of
Reference Materials.
The storage conditions of reference materials are determined in the stability study, and the use conditions of reference materials (such as temperature, preparation method, drying method, moisture correction method, mixing method, etc.
) are strictly determined and observed by the standard material valuation process, and the use and storage of the reference material will not be used and stored in accordance with the provisions will lead to the value of the reference material is no longer valid, and the measurement results will be deviated
from the standard value.

9.
What problems 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, and quality control
of these processes should be done.
Containers and diluents used in the preparation of reference material dilution require attention
.
When some substances are stored in glass bottles, they are easily affected by degradation or dissolution, so bottles of other materials are used, such as K, Na and other element solution standards, which need to be preserved in plastic bottles; Some substances will be adsorbed or dissolved when stored in plastic bottles, such as mercury may cause adsorption in plastic bottles, so it is more reliable
to choose glass bottles.
In addition to the necessary control of the blank, the stabilizing effect of different diluents is also different, and in principle, it should be prepared according to the diluent variety and concentration specified in the detection standard method
.
The measuring instruments used in the weighing and dilution process of reference materials (such as balances, pipettes, volumetric flasks, etc.
) should be properly calibrated or verified to meet the accuracy requirements, especially the error of microsyringes and pipette guns used in organic analysis is large, and daily calibration
should be paid attention to.

10.
How to store and maintain the Reference Standard after opening?

Reference material certificates sometimes stipulate "single use", these reference materials are generally unstable or have high accuracy of the measurement, such as ampoule bottles of national first-class solution reference materials, after opening the package is prone to change beyond the uncertainty range, should be transferred as soon as possible, can not be retained after repeated use
.
It can be prepared into an intermediate standard stock solution for storage and use
at one time.

For certified Reference Materials, it is important to ensure proper storage and packaging of Reference Standards packaging units after opening, the integrity of the certificate, and in some cases it is necessary to reseal the remaining substances in accordance with the certificate requirements
.
Measures to prevent contamination should be taken when taking samples
.

How to do a good job of maintenance in the use of reference materials is often a confusing problem
for users.
The International General Metrology Basic Terminology (VIM) stipulates that the maintenance of a measurement standard is a set of operations
necessary to keep the metrological properties of the measurement standard within the specified limits.
This typically includes periodic verification or calibration of pre-specified metrological characteristics, storage under suitable conditions, and careful maintenance and use
.
As a measurement standard, reference materials should also be regularly maintained and verified to ensure the reliability
of their values.

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

(1) For certified reference materials, as long as they are kept in accordance with the conditions and methods specified in the reference material certificate and used within the validity period, the measurement value is guaranteed, and the maintenance of such reference materials should be simplified to check the integrity, expiration date, storage conditions, etc.
of packaging, labels and certificates; If it is a certified reference material that can be used many times after opening, in addition to the above inspection, the laboratory should formulate corresponding documents to standardize its subsequent use and management, regularly verify its use, and if necessary, verify the measurement value by comparing with another unopened unit or batch of reference material;

(2) For internal reference materials or calibration solutions prepared by certified standard materials in the laboratory, etc.
, because there is no sufficient uniformity and stability data, strict verification of the measurement value is very necessary, and the way can be taken to flexibly grasp the economic and technical conditions of the laboratory, such as the use of quality control charts for trend checking, through the comparison of the quality values of the upper and lower batches of standards or the comparison between laboratories to verify the accuracy of the value of
the quantity.

In addition, the frequency of verification is based on the characteristics of reference materials, and reference materials with good stability expectations can relatively relax the time interval
of verification.