Overview of HJ 780-2015 Standard

HJ 780-2015 is an industry standard issued by the Ministry of Environmental Protection of China, with the full title "Soil and Sediment - Determination of Inorganic Elements - Wavelength Dispersive X-ray Fluorescence Spectrometry". This standard specifies the method for the determination of multiple inorganic elements in soil and sediment using wavelength dispersive X-ray fluorescence spectrometry (WD-XRF).

This standard is applicable to the determination of elements such as sodium, magnesium, aluminum, silicon, phosphorus, potassium, calcium, titanium, manganese, iron, cobalt, nickel, copper, zinc, arsenic, strontium, yttrium, zirconium, niobium, barium, lead, thorium, uranium in soil and sediment. The method has advantages such as simple sample preparation, simultaneous multi-element analysis, and non-destructiveness, and is widely used in environmental monitoring, geological surveys, and agricultural fields.

Method Principle

Wavelength dispersive X-ray fluorescence spectrometry is based on the principle of X-ray fluorescence. When a sample is irradiated with high-energy X-rays, the inner-shell electrons of atoms in the sample are excited, and outer-shell electrons transition to fill the vacancies, releasing secondary X-rays (fluorescent X-rays) with specific wavelengths.

By measuring the wavelength and intensity of these fluorescent X-rays, the types and contents of elements in the sample can be qualitatively or quantitatively analyzed. Wavelength dispersive spectrometers use analyzing crystals to separate fluorescent X-rays of different wavelengths, and then measure the intensity through detectors, offering high resolution and accuracy.

Instruments and Reagents

Main Instruments

  • Wavelength dispersive X-ray fluorescence spectrometer: equipped with a rhodium target or chromium target X-ray tube, analyzing crystals (such as LiF, PET, TAP, etc.), and a flow proportional counter or scintillation counter.
  • Pellet press: used for preparing powder pressed pellets, with pressure typically 20-40 tons.
  • Agate mortar or ball mill: used for grinding samples.
  • Standard sieve: pore size 0.075mm (200 mesh).
  • Oven: controllable temperature up to 105℃.

Main Reagents

  • Boric acid: analytical grade, used as pellet backing or diluent.
  • Anhydrous ethanol: analytical grade, used as grinding aid.
  • Reference materials: used for calibration curve preparation, such as GBW series soil reference materials.

Sample Preparation Steps

Sample preparation is a key step to ensure analytical accuracy, and must be strictly operated according to the following steps:

  1. Sample drying: Place the collected soil or sediment sample in an oven and dry at 105℃ to constant weight.
  2. Grinding: Take about 10g of dried sample, grind with an agate mortar or ball mill until all passes through a 0.075mm standard sieve.
  3. Pressing: Weigh about 4g of ground sample, add an appropriate amount of boric acid as backing, and press on a pellet press at 20-40 tons pressure for 10-30 seconds to form a disc about 32mm in diameter.
  4. Labeling and storage: The pressed sample pellets should be labeled with numbers and stored in a desiccator to prevent moisture absorption.

Note: For samples with high water content, air-dry first then oven-dry; for samples with high organic matter content, appropriately increase drying temperature or extend drying time.

Analysis Steps

Calibration Curve Preparation

Use national reference materials or artificially prepared standard samples, and prepare a series of standard pellets according to the above sample preparation method. Measure the fluorescent X-ray intensity of each element under selected instrument conditions, and plot the calibration curve with intensity versus concentration.

Instrument Condition Settings

Select appropriate X-ray tube voltage and current, analyzing crystal, detector, and measurement time according to the elements to be determined. Typical conditions are shown in the following table:

Element Analytical Line Analyzing Crystal Detector Voltage (kV) Current (mA)
Na Kα TAP Flow proportional 30 40
Mg Kα TAP Flow proportional 30 40
Al Kα PET Flow proportional 30 40
Si Kα PET Flow proportional 30 40
Fe Kα LiF200 Scintillation 60 30
Cu Kα LiF200 Scintillation 60 30
Pb Lβ LiF200 Scintillation 60 30

Note: The above conditions are for reference only and should be optimized according to the instrument model and sample characteristics in practice.

Sample Measurement

Place the prepared sample pellet into the sample chamber, and measure the fluorescent X-ray intensity of each element under the same conditions as the calibration curve. Measure each sample at least twice and take the average.

Result Calculation

Calculate the concentration of the element to be determined in the sample according to the calibration curve, and express the result as mass fraction (mg/kg). Perform matrix effect correction and spectral overlap correction when necessary.

Method Advantages and Precautions

Method Advantages

  • Simultaneous multi-element analysis: Multiple elements can be determined simultaneously, with high analysis efficiency.
  • Non-destructive: Samples can be measured directly after preparation without destroying the sample structure.
  • Simple sample preparation: The powder pellet method is easy to operate and does not require complex digestion.
  • Wide linear range: Suitable for element analysis from trace to high content.
  • Environmentally friendly: Does not use dangerous reagents such as strong acids, reducing pollution.

Precautions

  • Matrix effect: The matrix of soil and sediment is complex, and correction by empirical coefficient method or fundamental parameter method is required.
  • Particle size effect: The sample grinding particle size must be sufficiently fine (<0.075mm), otherwise measurement precision is affected.
  • Mineral effect: Different mineral structures may cause analytical errors, and reference materials matching the sample matrix should be used.
  • Sample homogeneity: Ensure the sample is thoroughly mixed to avoid segregation.
  • Instrument calibration: Regularly perform instrument drift correction and energy calibration.

Application Fields

The HJ 780-2015 standard is widely used in the following fields:

  • Environmental monitoring: Soil pollution investigation, heavy metal analysis in sediment.
  • Geological survey: Regional geochemical mapping, mineral resource assessment.
  • Agriculture: Soil fertility evaluation, agricultural product safety monitoring.
  • Scientific research: Research in soil science, environmental science, geology, etc.

By following this standard, laboratories can obtain accurate and reliable analytical data for inorganic elements in soil and sediment, providing a scientific basis for environmental management and decision-making.

To view the full text of the standard, please download the PDF document: