Standard Overview and Scope of Application

GB/T 22105.2-2008 is Part 2 of "Soil Quality - Determination of Total Mercury, Total Arsenic, and Total Lead - Atomic Fluorescence Spectrometry," specifically specifying the method for the determination of total arsenic in soil. This standard is applicable to the laboratory analysis of arsenic content in various soil samples, especially suitable for environmental monitoring, agricultural geological surveys, and contaminated site assessments. Atomic fluorescence spectrometry has advantages such as high sensitivity, minimal interference, and a wide linear range, making it one of the mainstream methods for soil arsenic detection currently.

Method Principle

After acid digestion of soil samples, the arsenic therein is converted to trivalent arsenic (As³⁺). In an acidic medium, trivalent arsenic reacts with potassium borohydride (KBH₄) to generate arsine (AsH₃) gas. This gas is carried by a carrier gas (argon) into the atomizer, forming ground-state arsenic atoms in an argon-hydrogen flame. The ground-state arsenic atoms are excited by the characteristic light emitted by an arsenic hollow cathode lamp, producing atomic fluorescence. The fluorescence intensity is proportional to the arsenic content, and quantification can be achieved through a standard curve.

Reagents and Materials

Main Reagents

  • Hydrochloric acid (HCl): Guaranteed reagent grade, used for digestion and reaction medium.
  • Nitric acid (HNO₃): Guaranteed reagent grade, used for digestion.
  • Potassium borohydride (KBH₄): Analytical reagent grade, must be prepared fresh before use, dissolved in 0.5% potassium hydroxide solution.
  • Thiourea-ascorbic acid mixed solution: Used to reduce pentavalent arsenic to trivalent arsenic.
  • Arsenic standard solution: Certified reference material, used for drawing the standard curve.

Instruments and Equipment

  • Atomic fluorescence spectrometer: Equipped with an arsenic hollow cathode lamp and hydride generation system.
  • Electric hot plate or microwave digestion system: Used for soil sample digestion.
  • Analytical balance: Precision 0.0001 g.
  • Volumetric flasks, pipettes, and other conventional glassware.

Analysis Procedure

Sample Pretreatment

  1. Air-dry and grind the soil sample, pass through a 0.149 mm (100 mesh) nylon sieve.
  2. Weigh 0.2~0.5 g of sample (accurate to 0.0001 g) into a digestion tube.
  3. Add aqua regia (HCl:HNO₃=3:1) or reverse aqua regia, let stand overnight.
  4. Place on an electric hot plate for heating and digestion, controlling the temperature at 100~120°C, until the solution is clear.
  5. After cooling, transfer to a 50 mL volumetric flask, add thiourea-ascorbic acid mixed solution, dilute to volume, and let stand for 30 min.

Standard Curve Preparation

  1. Prepare a series of arsenic standard solutions, typically with a concentration range of 0~20 μg/L.
  2. Measure the fluorescence intensity of each standard point sequentially, draw the standard curve, and calculate the regression equation.

Sample Determination

  1. Set the parameters of the atomic fluorescence spectrometer: negative high voltage, lamp current, carrier gas flow rate, etc. (refer to the instrument manual).
  2. Introduce the blank solution, standard solutions, and sample solutions sequentially into the hydride generation system.
  3. Record the fluorescence intensity and calculate the arsenic concentration in the sample based on the standard curve.

Key Parameters and Comparison

Parameter Recommended Value/Range Description
Digestion temperature 100~120°C Avoid excessive temperature causing arsenic volatilization loss
Reductant concentration 1.0%~2.0% KBH₄ Too low concentration reduces sensitivity; too high causes bubble interference
Carrier gas flow rate 300~500 mL/min Needs optimization to obtain stable fluorescence signal
Acidity 10%~20% HCl Acidity affects the efficiency of arsine generation
Detection limit 0.01 mg/kg Depends on instrument performance and sample amount

Precautions and Quality Control

  • Prevent contamination: All vessels should be soaked in 10% nitric acid for 24 h and rinsed with ultrapure water.
  • Complete reduction: After adding thiourea-ascorbic acid, allow sufficient standing time to ensure complete reduction of pentavalent arsenic.
  • Blank control: At least 2 blanks should be prepared for each batch of samples; the blank value should be below the detection limit.
  • Spike recovery: Regularly conduct spike recovery experiments; the recovery rate should be between 90%~110%.
  • Reference materials: Use certified soil reference materials for verification to ensure accurate results.

Conclusion

GB/T 22105.2-2008 provides a standardized, operable atomic fluorescence spectrometry procedure for the determination of total arsenic in soil. By strictly controlling digestion, reduction, and instrument conditions, accurate and reliable detection data can be obtained. Engineering technical personnel and procurement personnel should focus on reagent purity, instrument performance, and quality control aspects to ensure the validity of soil environmental monitoring data.