Quality control of water quality analysis experiments

1. What is quality control

Laboratory quality control includes quality control within the laboratory and quality control between laboratories. Its purpose is to control monitoring and analysis errors within the allowable range, ensure that measurement results have a certain degree of precision and accuracy, and ensure that the analysis data meets the required quality with confidence within the given confidence level.

(1) Quality control within the laboratory. Also known as internal quality control, it refers to the process in which laboratory analysts self control the quality of analysis. For example, relying on self prepared quality control samples, controlling the quality of analysis by analyzing and applying a certain quality control chart or other methods. It mainly reflects the stability of quality analysis, in order to timely detect certain accidental abnormal phenomena and take corresponding correction measures at any time.

(2) Quality control between laboratories. Also known as external quality control, it refers to the process in which external third parties, such as higher-level monitoring institutions, conduct regular or irregular inspections of the analytical quality of the laboratory and its analysts. It generally uses password standard samples for testing to determine the laboratory's ability to report acceptable analytical results and assist in determining the presence of systematic errors and checking the comparability of data between laboratories.

2. Quality control within the laboratory

(1) Determination of blank test values. In trace analysis commonly used in environmental monitoring, due to the small sample measurement values, they are often in the same order of magnitude as the blank test values. The size and dispersion of the blank test values have a significant impact on the precision of the analysis results and the detection limit of the analysis method. Moreover, the size and repeatability of blank test values to a considerable extent and comprehensively reflect the level of an environmental monitoring laboratory and its analysts. For example, the purity of experimental water and chemical reagents, the cleanliness of glass containers, the accuracy and usage of analytical instruments, the environmental pollution situation in the laboratory, and the level and experience of analysts can all affect the blank test values.

1) Measurement method. Measure two parallel samples of blank experiments every day, conduct public testing for 5 days, and calculate the standard deviation or intra batch standard deviation based on the selected formula.

2) Qualified requirements. Based on the measurement results of blank test values, calculate the detection limit using commonly used prescribed methods. If the value is higher than the specified value in the standard analysis method, the cause should be identified and corrected, and then retested until it is qualified.

(2) Determination of detection limit. The detection limit for general capacity analysis should be determined based on the detection limit given by the standard testing method.

Spectral spectrophotometry: The detection limit is the concentration value corresponding to the absorbance of 0.01 after deducting the blank value.

Instrument analysis: The determination of detection limit in instrument analysis should be based on the requirements of different instruments. Even the same instrument is affected by different models, while instruments of the same model are still affected by the signal-to-noise ratio of the instrument itself. Therefore, for instrument analysis, it is generally believed that the minimum distinguishable response signal should be 2-3 times the noise.

(3) Draw calibration curves. Each analysis method using the calibration curve method can determine its detection upper limit by plotting the calibration curve during initial use, and determine its detection range, i.e. linear range, by combining the detection lower limit.

(4) Calibration