1.1 Quality assurance for sample collection and transportation circulation
The collection method of soil samples has a significant impact on monitoring results, and the errors caused by sampling may be greater than those caused by analysis and testing. The layout and sampling of soil environmental monitoring should strictly follow the relevant provisions in HJ/T 166-2004 Technical Specifications for Soil Environmental Monitoring.
Point placement: In order to ensure that the collected samples have equal representativeness, the placement of points should follow the principles of "random" and "equal quantity". There are three types of point placement methods: simple random, block random, and system random; The number of basic samples can be calculated from mean square deviation, absolute deviation, coefficient of variation, and relative deviation; The number of distribution points should meet the basic requirements of sample size. It is generally required that each monitoring unit should have at least 3 points, and in actual work, it should be determined based on factors such as investigation purpose, investigation accuracy, and environmental conditions of the investigation area. Sample collection: Sample collection is usually carried out in three stages, namely preliminary sampling, formal sampling, and supplementary sampling. Soil pollution investigations and sudden pollution accident investigations with smaller areas can be directly sampled.
Regional environmental background soil sampling, farmland soil sampling, construction project soil environmental assessment monitoring sampling, urban soil sampling, pollution accident monitoring soil sampling, different types have different characteristics and methods, and operations must be carried out in accordance with corresponding regulations and requirements.
Transportation and circulation: At the sample collection site, it is necessary to carefully fill out the sampling record, sample label, and sample information registration form. After verifying each sample one by one, the samples should be classified and packed, and the loss, confusion, and contamination of the samples should be strictly prevented during transportation. After the sample is delivered to the laboratory by a dedicated person, the sender and receiver should simultaneously count and verify the sample information, sign and confirm on the sample handover form, and each party should keep a copy of the handover form for future reference.
1.2 Quality assurance for sample preparation and preservation
Sample preparation: The laboratory needs to have an air drying room and a sample grinding room. The air drying room should be well ventilated, clean, dust-free, free of volatile chemicals, and avoid direct sunlight. The sample preparation personnel and the sample administrator shall simultaneously count, verify, and hand over the samples, and sign and confirm on the sample handover form. Corresponding tools and containers for air drying, rough crushing, sample grinding, sieving, and sample loading should also be available. Sample preparation operations should comply with regulatory requirements and strictly prevent mixing of labels and samples to prevent cross contamination caused by sample preparation tools.
Sample preservation: Whether the sample preservation method is appropriate and whether the preservation time meets the requirements will affect the test results of the sample. Samples of unstable components such as easily decomposed or volatile components should be stored and transported at low temperatures, and sent to the laboratory for analysis and testing as soon as possible; Soil samples that require fresh samples should be stored in a sealable polyethylene or glass container in a dark place below 4 ℃. The container should be filled with soil samples and testing should be completed within the effective storage time of the samples.
The test samples of volatile, semi volatile, and difficult to volatile organic compounds are stored in brown glass bottles for 7 days, 10 days, and 14 days, respectively. Arsenic, metal (excluding mercury and hexavalent chromium), and cyanide test samples can be stored in polyethylene or glass bottles with an effective storage time of 180 days. Hexavalent chromium and cyanide samples have an effective storage time of only 1 day and 2 days, respectively. Mercury test samples need to be packaged in glass bottles and can be stored for 28 days.
Reserve samples for record keeping in the sample library, usually for 2 years; The remaining samples after analysis and collection are also transferred to the sample library for storage, usually retained for six months. Registration is required for the storage, receipt, and cleaning of soil samples.
2 Laboratory Analysis Quality Control
2.1 Internal Quality Control in the Laboratory
Internal quality control in the laboratory refers to the self-control measures taken by laboratory analysis and testing personnel to control the quality of analysis, usually including precision control, accuracy control, and interference handling during the testing process.
Precision control: Precision refers to the degree of consistency between the measured values obtained from repeated analysis of homogeneous samples using specific analytical procedures. In soil environmental monitoring, 20% parallel samples must be taken for each item in each batch of samples. If the number of samples is less than 5, at least one parallel sample should be taken. Parallel samples can be either laboratory code parallel or on-site code parallel. The allowable range of error for parallel double sample measurement results of different measurement items is different, and it is judged as qualified within the corresponding allowable error range. If the pass rate of parallel double sample determination is lower than 95%, the batch of samples should be retested and the number of parallel samples should be increased by 10% to 20% until the pass rate of parallel double sample determination is higher than 95%.
Accuracy control: Accuracy is a comprehensive indicator that reflects the systematic error and random error of a method. Accuracy control can be achieved by using standard substances or quality control samples, or by measuring the recovery rate of spiked samples. Each batch of quality control parallel samples must be tested. On the premise of qualified precision, the measured values of the quality control samples must be within the guaranteed range (95% confidence level), otherwise this batch of samples needs to be retested. When there are no standard substances or quality control samples in the measurement project, accuracy can be determined through spiking recovery experiments. Randomly select 10% to 20% of each batch of samples for spiking recovery determination. If the number of samples is less than 10, increase the spiking rate appropriately. The amount of spiking depends on the content of the tested component. After spiking, the total amount of the tested component cannot exceed the upper limit of the method, and the spiked volume should not exceed 1% of the original sample volume. Otherwise, volume correction should be performed. The recovery rate of spiking should be within the allowable range. When the qualified rate of spiking recovery is less than 70%, the unqualified samples should be retested for recovery rate, and 10% -20% of the samples should be spiked for recovery until the total qualified rate is greater than or equal to 70%. The technical specifications for soil environmental testing require the preparation of an accuracy quality control chart for the required test items, using the guaranteed value (X) and standard deviation (S) of the quality control sample at a 95% confidence level, with X as the centerline and X ± 2S as the upper and lower limits
Quality control in laboratory management
3.1 Quality control of monitoring personnel
The quality of data is ensured by the quality of work, and the core of work quality lies in personnel. Monitoring personnel are the main implementers of soil environmental monitoring, and their professional quality and work ability directly affect the quality of monitoring. If the quality control concept of monitoring personnel is not strong and their quality awareness is insufficient, it is easy for them to be scattered, not rigorous, and not responsible enough during the implementation of monitoring. However, if the professional knowledge and skills of monitoring personnel are insufficient, standardized operations cannot be guaranteed during the monitoring process, which may lead to errors in the sampling process at the source of monitoring, errors in the laboratory, and unscientific data processing methods, So the final monitoring results are likely to deviate significantly from the actual situation. Therefore, it is very important to strengthen the construction of the talent team for soil environmental monitoring. All monitoring technicians should be certified to work, and the laboratory should regularly participate in or carry out training for soil environmental monitoring technicians. It is also necessary to strengthen the supervision of personnel's professional competence and technical ability, and continuously strengthen the construction of monitoring teams, qualities, and capabilities.
3.2 Quality control of instruments and equipment
The instruments and equipment for soil environmental monitoring, including sampling instruments, laboratory analytical instruments, and related auxiliary equipment, whose performance directly affects the efficiency and quality of monitoring work. The instruments and equipment should be operated according to the correct methods and daily maintenance should be carried out. Instruments that need to be calibrated or calibrated should be regularly sent to legal metrological verification institutions or authorized metrological calibration institutions for calibration or calibration, Confirm that the technical performance of the instrument meets the requirements of monitoring work and is used within the validity period of verification or calibration. For critical instruments with high accuracy requirements, or instruments that are prone to drift, aging, unstable performance, or frequent use, periodic verification should generally be conducted between two verifications or calibrations to ensure the accuracy and continuous stable operation of the instruments. Any suspicious situations that arise during use, significant changes in environmental conditions, or instruments returned after maintenance or lending should also be promptly verified.
3.3 Report quality control
The monitoring report is the final presentation of monitoring results. To ensure the accuracy and correctness of monitoring data, a data quality management responsibility system should be established, and a three-level audit system should be strictly implemented. From analysis, audit to issuance, division of labor and responsibility should be achieved, and layer by layer inspection should be carried out. If suspicious data or difficult problems are found, the monitoring person in charge should organize relevant personnel to verify, analyze and solve them, and trace the relevant data to ensure that any questionable data is never reported. In addition, attention should be paid to the training and education of report preparation personnel to avoid errors in the report preparation process as much as possible.
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