Technical principle
Atomic fluorescence photometry fluoresces based on the excitation of the element to be measured at a specific wavelength, and its fluorescence intensity is linearly with the concentration of the element within a certain range. Arsenic and mercury are converted into gaseous hydrides or atomic vapors through pretreatment, which are introduced into the atomizer by the carrier gas, and emitted characteristic fluorescence after being excited by the light source, which is measured by the detection system. The determination of arsenic and mercury is usually done by hydride-atomic fluorescence spectroscopy, and its basic reactions can be expressed as:
As5+ + Reducing Agent → AsH3↑
Hg2+ + Reducing agent → Hg0↑
Fluorescence intensity IfThe relationship with the concentration of the DUT c can be approximately expressed as: If = K · c, where K is the constant related to instrument efficiency, quantum yield, etc.
Sample preparation
Water sample preparation ensures that arsenic and mercury are completely released in measurable forms and interference is eliminated. clean water can be directly determined after acidification; Samples containing organic matter or suspended solids need to be digested. Commonly used digestion methods include nitric acid-hydrochloric acid system heating digestion or microwave-assisted digestion. Pre-reduction is often required before arsenic determination to convert pentavalent arsenic into trivalent arsenic to improve hydride generation efficiency. Mercury determination should pay attention to preventing volatilization loss, and the digestion container should be airtight. After digestion, the solution should be clear and transparent, and the acidity should be controlled within an appropriate range.
The reasonable setting of instrument parameters has a key impact on the sensitivity and stability of the measurement. The light source lamp current, photomultiplier negative high voltage, atomizer height and temperature, carrier gas flow, etc. need to be optimized according to the instrument model and water sample matrix. Interference mainly comes from coexisting ions, such as copper, nickel, cobalt and other transition metals may inhibit hydride formation, which can be mitigated by adding masking agents such as thiourea-ascorbic acid. Mercury has a strong memory effect, so it is necessary to ensure sufficient cleaning time. When measuring continuously, baseline stability should be monitored and instrument drift correction should be performed if necessary.
| Key parameters: | Optimization considerations |
| Negative high pressure | Keep the signal-to-noise ratio as low as possible |
| lamp current | Taking into account the intensity and life of the light source |
| Carrier gas flow | Affects hydride transport efficiency |
| atomization temperature | Ensures atomization and reduces fluorescence quenching |
| Reducing agent concentration | Ensures quantitative reactions and reduces gas-phase interference |
The whole process of quality control should be implemented during the measurement process. This includes monitoring contamination using blank tests, controlling precision through parallel samples, and evaluating accuracy with reference materials or spike recovery. The calibration curve should be well linear within the expected concentration range, and the correlation coefficient is usually required to be no less than 0.995. Low-concentration samples can be enriched with concentration steps. The properties of arsenic and mercury vary greatly, and it is recommended to establish separate measurement procedures. The result report should indicate the detection limit, quantitative limit and source of uncertainty.
Operation and maintenance
The experiment involves strong acids, high-pressure gases and toxic hydrides, which need to be operated in a fume hood, and personnel should be equipped with protective equipment. Instrument maintenance should regularly check the airtightness, desiccant status and atomization window cleanliness. The light source should have sufficient warm-up time and should not be stored in accordance with regulations for a long time. Waste liquids should be collected and properly disposed of, and mercury-containing waste liquids should be specially disposed of.