Chemical Analysis - Accuracy and Precision Check!


2022-03-31 11:43

Accuracy is defined as how close a test result is to the true value. Precision is defined as the closeness between multiple measurements. A precise test result does not imply an accurate result (as shown in Figure 3-1). The accuracy and precision of test results can be assessed by spiking or standard solution check experiments. 

1. Standard solution

Standard solutions can be ordered as off-the-shelf reagents or prepared in the laboratory. It is a solution of known composition and concentration. The accuracy of the analytical system can be checked by substituting a standard solution for the sample during operation.

2. Standard solution addition experiment

Standard solution addition experiments are a common technique for checking test results. This method is also known as standard addition experiment and known concentration addition method. Through this technology, it can be judged whether there is interference, whether the reagent is invalid, whether the instrument is working normally, and whether the operation is correct.

A spike experiment is where a small amount of a standard solution is added to the sample and the test is repeated. Using the same reagents, instrumentation, and technique, you should get about 100% recovery. If not, it's confirmed that there is a problem.

If you want to carry out a standard addition experiment, you can select the standard solution addition function under the accuracy check menu in the instrument, and operate according to the instrument operation manual.

If the spiking experiment gives about 100% recovery, the experiment is correct and the results are accurate.

If the spiking experiment does not get 100% recovery, it indicates that there is a problem with the analysis process, and further work can be used to determine whether there is interference. Repeat the spiking experiment with deionized water as the sample, and if the spiking experiment yields approximately 100% recovery, the presence of an interference is evidenced.

If you still do not get good yields with deionized water, use the following checks to identify the problem.

(1) Check whether the operation is in accordance with the correct operation process: ① whether the reagents used and the order of addition are correct; ② whether the necessary time for color development has been reached; ③ whether the correct glassware is used; ④ whether the glassware is clean; Whether there are special requirements for the sample temperature; ⑥ Whether the pH value of the sample is in the appropriate range.

(2) Check the performance of the instrument according to the method in the instrument operation manual.

(3) Check the reagents. Repeat the spiking experiment with new reagents, and if you get good results, then the original reagents are substandard.

(4) If there are no other errors, it can almost be judged that the standard sample is unqualified. Repeat the spiking experiment with a new standard.

3. Solutions when there are doubts about the experimental results

If in doubt about the results of an analysis experiment, follow the procedure below to resolve the issue.

(1) Carry out an accuracy check. Prepare a standard solution of known concentration and perform the same procedure as the sample, including sample collection, storage, digestion, and photometric determination if applicable. If the result of the standard solution check is accurate, skip to step (4). If there is a deviation from the expected result, go to step (2) to check.

(2) If the experimental results of the standard solution check are different from those expected, follow the steps below to check the instrument settings and method flow.

① Confirm that the correct program number is selected for the analysis method.

②Confirm that the concentration unit of the standard solution used is consistent with the concentration unit displayed by the instrument (a certain concentration unit of the analyte will be displayed on the instrument screen), for example, molybdenum (Mo) can be expressed in two concentration units of Mo or MoO4.

③ Confirm that the sample cell required in the analysis manual is used for operation.

④ Confirm that the correct analytical reagent is used.

⑤ Confirm that the reagent blank stored in the instrument is the reagent of the current batch. Reagent blanks are different for different batches of reagents.

⑥ Confirm that the standard curve used has been adjusted (standard adjustment). It is recommended to use the factory default standard curve for standard solution inspection.

⑦ Confirm that the dilution factor is correct.

If the instrument settings and method flow are correct, proceed to step (3).

(3) If the results of the standard solution check experiment are different than expected, follow the steps below to check the reagents and techniques used in the analysis.

①Check the elapsed time of the reagents used in the analysis. There are many factors that affect the shelf life of reagents (eg storage temperature, storage conditions, microbial contamination, etc.). Replace the possibly deteriorated reagent with a new reagent for the standard solution check.

② Use deionized water or distilled water for a complete analysis process to determine the blank, including sample collection, preservation, digestion and photometric determination. Some chemicals will introduce some color, this is normal. However, if this color change results in a blank exceeding 10% of the test span, there is a problem with a chemical or dilution water.

③ Step by step to eliminate the problems existing in the operation process. First, using a standard solution, no preservation and storage of the sample, only digestion and photometric determination are performed. If the analytical results are accurate, check that the sample was stored and stored correctly. Confirm that the storage and storage methods used are suitable for the parameters to be measured; if the sample needs to be stored in acid, confirm that the correct acid solution is used, and adjust the pH value to the analytical range before analysis.

If the standard solution check is still inaccurate, directly use the standard solution for photometric analysis. If the results are correct, check the digestion process. Make sure that the reagent dosage and the pH value after digestion are within the analytical range (for details, please refer to the analysis operation manual).

(4) If the standard solution check gives accurate results, but the analytical results of the sample are still questionable, possibly caused by interference. Follow the steps below to check for possible interference.

① Carry out the sample addition experiment. A spike experiment was used instead of the standard solution experiment so that all interference factors were included.

② Add fresh water samples to be tested in two sample pools, and add a certain amount of standard solution to one of the sample pools.

③ Analyze the two samples using the same reagents, instruments, and analytical techniques. The increase of the test result of the standard sample to the test result of the sample should be equal to the amount of the standard solution added.

④ Calculate the recovery rate using the following method. Ideally, 100% recovery should be obtained. In practice, recoveries of 90% to 110% are all within the acceptable range. If the recovery rate is not within this range, refer to the description and exclusion method of interfering substances in the analysis operation manual to eliminate possible interfering substances.

⑤ Dilute the sample to obtain a series of samples with different dilution gradients. Make sure the diluted sample concentration is within the analytical range. Samples at concentrations that are not within the analytical range can cause erroneous analytical results due to insufficient or excessive sample development, excessive turbidity, or the sample's ability to bleach. Check for this possibility by analyzing this series of diluted samples.

⑥Because it is unlikely that the interference in the sample is known, diluting the sample so that the concentration of the interfering substance in the sample is lower than the upper limit of the interference is an economical and effective method to accurately analyze the sample containing the interfering substance. If there is no way to dilute the concentration of interfering substances in the sample below the upper limit of interference under the premise of ensuring that the concentration of the diluted sample is within the analytical range, other testing methods, such as other chemical methods or ion-selective electrode methods, can only be used. to try to analyze these parameters.

The calculation method of the recovery rate is as follows.

(1) Analyze the concentration of the unknown sample.

(2) Calculate the theoretical concentration after the addition using the following formula.

In the formula, Cu is the concentration of the unknown sample; Vu is the volume of the unknown sample; Cs is the concentration of the standard sample added; Vs is the volume of the standard sample added.

(3) Analyze the concentration of the sample after adding the standard.

(4) The concentration of the sample after adding the standard is divided by the theoretical concentration and multiplied by 100.

[Example 3-2] A sample was used to test manganese, and the result was 4.5 mg/L. 97 mL of this sample was added with 3 mL of a standard solution of 100 mg/L manganese. The spiked sample was then tested for manganese using the same method and the result was 7.1 mg/L. Calculate its recovery rate.

The calculation method of USEPA is as follows. USEPA's formula for calculating recovery is more stringent. Only calculating the recovery rate of the standard solution added to the sample will result in a lower recovery rate than the calculation method above. A complete explanation of this method is in "SW-846" published by USEPA. Calculated as follows:

In the formula, Xs is the concentration of the sample after adding the standard; Xu is the concentration of the sample, and the concentration change caused by dilution is corrected by the standard addition volume; K is the concentration of the standard sample in the standard solution.

[Example 3-3] One sample measured 10 mg/L. Add 5 mL of 100 mg/L standard solution to 100 mL of sample. The spiked sample was tested using the same method as the original sample and the result was 13.7 mg/L. Find its recovery rate.

Using the USEPA calculation method, acceptable recovery values are 80% to 120%.

4. Adjust the calibration curve

In general, a number of pre-made programs are permanently stored in the memory of Hach's spectrophotometers. Each pre-programmed program contains a calibration curve. These calibration curves are made under ideal conditions and are suitable for most users' daily analysis. Deviations from the calibration curve can occur when degraded analytical reagents and defective sample cells are used, incorrect procedures, techniques, and other factors that require correction are used.

In some cases, the use of prefabricated procedures may not be appropriate because the reagents used in the analytical method vary greatly from batch to batch; some analytical methods require frequent calibration curve checks; the samples analyzed have fixed interference.

Before performing calibration curve adjustment, please consider the following questions: Will the analysis results be more accurate after calibration curve adjustment? Are there fixed interferences in all samples? The estimated detection limit, sensitivity, accuracy, range, etc. of the pre-programmed procedure may not necessarily apply to the adjusted calibration curve.

The method of adjusting the calibration curve can be found in the analysis operation manual, which is generally achieved by adding analytical reagents to the empty and standard solutions. Be careful during adjustment. The adjusted calibration curve needs to use the standard solution method to verify whether the calibrated curve is satisfactory, or it can be verified by the addition of the sample.

The adjustment of the analysis results is achieved in two steps. First, the instrument will give the analysis result according to the pre-programmed calibration curve, and then this analysis result will be multiplied by the correction factor. Correction factors were adjusted from the standard curve and applied to all concentrations. The reading displayed on the screen is adjusted and a calibration icon will appear next to the reading.