1、 Strictly follow the instrument calibration process to eliminate systematic errors

Calibration is a key step in eliminating instrument system errors, which directly determines the baseline of detection accuracy and follows the principle of "regular calibration+real-time calibration".

Select the appropriate standard solution

Priority should be given to using certified standard substances to prepare sodium ion standard solutions, avoiding the use of self prepared non-standard solutions that may cause calibration deviations. The concentration gradient of the standard solution should cover the concentration range of the sample to be tested, and at least three concentration points (low, medium, and high) should be set. For example, when detecting sodium ions in drinking water (usually at a concentration of 0.1-10mg/L), standard solutions of 0.1mg/L, 1mg/L, and 10mg/L can be prepared to ensure the linear correlation of the calibration curve (R ² ≥ 0.999).

For ion selective electrode monitoring instruments, it is necessary to prepare a total ion strength adjustment buffer (TISAB) and mix it with standard solutions and samples in a fixed ratio to maintain a constant ion strength of the solution, eliminate the influence of pH values and interfering ions.

Standardize calibration operation details

Before calibration, the standard solution and instrument electrode/detection module should be equilibrated in the same temperature environment for at least 30 minutes to avoid changes in ion activity caused by temperature differences; During the calibration process, it is necessary to measure the standard solution in the order of "low concentration to high concentration". After measuring each concentration, thoroughly clean the electrode/detection channel with deionized water and gently absorb the residual liquid with filter paper (do not wipe the electrode membrane to prevent damage to the sensitive membrane) to avoid cross contamination.

Regularly perform blank calibration using ultrapure water (conductivity ≤ 0.055 μ S/cm) as the blank sample, deducting background interference values; For instruments that are used for a long time, full range calibration should be performed once a week, and single point calibration should be performed before daily testing (selecting a standard solution close to the sample concentration) to ensure that the instrument is in optimal working condition.

2、 Optimize sample pretreatment to reduce matrix interference

The matrix factors such as impurities, coexisting ions, and pH value in the sample are important factors affecting the detection accuracy, and targeted pretreatment should be carried out according to the sample type.

Eliminate coexisting ion interference

Sodium ion monitoring devices are susceptible to cation interference such as potassium ions and ammonium ions, especially in ion selective electrode methods. These ions compete with sodium ions for sensitive electrode sites, resulting in high detection results.

For water quality samples, ion masking agents can be added, such as adding sodium tetraphenylborate to TISAB to form a precipitate with potassium ions and eliminate potassium ion interference;

For complex matrix samples (such as soil extracts and food extracts), dilution method (ensuring that the diluted sodium ion concentration falls within the calibration curve range) or solid-phase extraction method can be used to separate and remove interfering ions before detection.

Adjust the pH value of the sample

The ion selective electrode method is sensitive to the pH value of the sample. When the pH value is too low, hydrogen ions can interfere with the determination of sodium ions; When the pH value is too high, hydroxide precipitation may be generated to adsorb sodium ions. The pH value of the sample needs to be adjusted to a suitable range of 5.5~8.5, which can be fine tuned by adding dilute hydrochloric acid or sodium hydroxide solution dropwise. During the adjustment process, it is necessary to avoid introducing sodium ion contamination.

Handling turbid or colored samples

Suspended solids in turbid samples (such as wastewater and beverages) can adsorb sodium ions or block ion contact with electrodes, while colored samples can interfere with the optical signal detection of flame photometry. Suspended solids can be removed by filtration through a 0.45 μ m membrane, or the sample can be clarified by centrifugation (speed ≥ 3000r/min, centrifugation for 10 minutes); For high turbidity samples that cannot be filtered, the standard addition method can be used for determination to reduce matrix interference.

3、 Standardize operating procedures to avoid human errors

The standardization of human operation directly affects the repeatability of detection results, and it is necessary to establish standardized operating procedures and strictly implement them.

Guidelines for the Use of Samples and Reagents

When taking samples and reagents, calibrated pipettes or pipettes should be used to ensure accurate pipetting volume; Avoid using uncleaned containers to hold samples. All containers should be soaked in dilute nitric acid for 24 hours, then rinsed with ultrapure water and dried to prevent contamination caused by residual sodium ions on the inner walls of the containers.

During the detection process, it is necessary to ensure that the stirring speed of the sample is consistent (ion selective electrode method). Stirring too slowly can lead to uneven ion diffusion, while stirring too quickly can generate bubbles that adhere to the electrode surface, affecting the stability of the response signal.

Ensure stable detection signal before reading

The ion selective electrode method requires waiting for the electrode response to reach a stable state before recording data. The stability standard is usually a potential value change of ≤ 0.1mV/min to avoid fluctuation errors caused by premature readings; The flame photometry method requires waiting for the flame to stabilize (usually preheating for 10-15 minutes after ignition), and then sequentially measuring the sample to ensure stable light signal intensity.