I. Main uses
1. Application in Evaluating and Verifying New Methods of Environmental Monitoring
At present, standard samples are widely used to evaluate the accuracy and precision of new technologies and methods in the world, because this evaluation method is more convenient and reliable. To develop or revise an analytical method, one or more samples with good characteristics are needed to evaluate the performance characteristics of the method. It is obviously most appropriate to select suitable standard samples at this time. For example, in 1971, the US Environmental Protection Agency (EPA) recommended a monitoring method for NO2 in ambient air (Jacobs-Hochheisev method), which is a colorimetric method based on diazotization. An important assumption of this method is that the conversion of NO2 is a constant (about 35%) in all concentration ranges. The reason why EPA recommended this method was that it had been used as a health research and had good precision. Unfortunately, there was no suitable NO2 standard sample at that time, so the accuracy of the method could not be verified. When four standard samples of NO2 osmosis tubes were developed by NBS (now NIST) in 1973, EPA reassessessed the accuracy of the method and found that the conversion of NO2 was not a constant, but decreased with the increase of NO2 concentration. After confirming that the method has large systematic errors through standard samples, EPA cancelled the original recommendation and re-studied and recommended three new NO2 monitoring methods.
2. Application in Quality Management of Testing and Analysis Laboratory
Standard sample is a powerful tool for environmental testing and analysis laboratory to carry out quality management. Selecting standard samples as control charts can periodically check the laboratory measurement system to determine the accuracy and precision of the measurement. The control limit of the control chart can be determined according to the uncertainty provided by the standard sample and the actual testing ability of the laboratory. In daily monitoring activities, the standard sample and the unknown sample are measured under the same conditions. If the results of the standard sample are outside the control limit, the measurement system is out of control. At this time, the measurement results of the unknown sample should be invalid. The laboratory should immediately find out the reasons, take measures to correct them, and re-determine the standard sample until the standard is reached. Only when the measured value of the sample falls within the control limit can the unknown sample be determined again. In a word, it is a practical method to ensure the quality of laboratory test data to determine a suitable amount of standard samples in the process of detection and analysis.
3. Application in Laboratory Capability Verification
Through the detection of standard samples, we can not only verify the personal ability of the personnel engaged in monitoring and analysis, but also verify the testing ability of the laboratory. The former is certificated on-the-job assessment, and the latter is laboratory quality control assessment. The application of standard samples for proficiency verification has been widely used in laboratory accreditation, metrology certification and related supervision and evaluation.
4. Application in Instrument Calibration and Detection
Since most modern instrumental analysis methods are based on the principle of relative measurement, standard samples are generally required for calibration. Although the laboratory can also prepare standard series with high purity reagents, it is more convenient to use standard samples. For environmental samples with relatively complex matrix, the selection of environmental standard samples roughly equivalent to the sample composition can effectively eliminate the matrix effect and improve the measurement accuracy significantly. At the same time, it has become a common understanding to use standard sample to detect the analytical characteristics of measuring instruments, such as linearity, reproducibility, stability and so on.
5. Application of Traceability in Establishing Environmental Monitoring System
Once the characteristic value of the standard sample is determined, it is stored in the standard sample and is not limited by time and space in a certain range. It can be used as a reference for comparison and transmission. The comparability of laboratory test and analysis data can be effectively improved by using national standard samples which have been established for traceability to carry out inter-laboratory value transfer and comparison. National Environmental Standard Samples (NESA) are the basic tools for the transmission and traceability of environmental monitoring system in China.
6. Application in Arbitration Analysis of Environmental Pollution Monitoring
In practical environmental monitoring work, there is often a serious inconsistency between the monitoring results of different laboratories, which often occurs between polluters and polluted persons, between environmental law enforcement departments and polluted enterprises, and sometimes between environmental monitoring and analysis laboratories representing different administrative regions. If the controversial environmental samples are analyzed at the same time, the environmental standard samples with similar matrix are detected, and the analysis data of the controversial environmental samples are determined according to the analysis results of the standard samples, the controversy can be easily solved, because the quantities of the national standard samples have been fully identified and can not be questioned.
2. Correct Use
The correct use of standard samples includes correct selection, correct use (to prevent misuse) and precautions for use.
1. Priority should be given to the use of certified standard samples approved by the State. The use of certified standard samples or self-made samples instead of standard samples will result in excessive waste of financial, material and human resources, let alone the accuracy, comparability and traceability of measurement values.
2. Selection of standard samples should be as close as possible to the expected monitoring and analysis samples, which mainly include matrix, morphology, etc.