Overview of X-ray fluorescence instrument (two)

(3) Composition and type of X-ray fluorescence spectrometer

1.

The basic composition of X-ray fluorescence spectrometer includes four parts: excitation system, sample system, detection system and instrument control and data processing system

Excitation system: X-rays are emitted once to excite the sample to emit radiation

Spectroscopic system: distinguish the characteristic X-rays from the sample elements

Detection system: Intensity detection of characteristic X-rays of sample elements

Instrument control and data processing system: process the detector signal and give the analysis result

2.

X-ray fluorescence spectrometers can be divided into two categories: energy dispersion and wavelength dispersion

The EDXRF detector mainly measures the energy of the characteristic spectrum from the sample.

The dispersion of the spectroscopic crystal used by WDXRF is different.

(4) Sample preparation and analysis

The key factors affecting the results are mainly sample preparation and accurate measurement

The qualitative analysis methods used by EDXRF and WDXRF are slightly different.

1.

X-ray fluorescence spectrometers generally analyze a small number of samples, and the sample requirements must be representative, reasonably processed, and the spectrometer must have high sensitivity

(1) Solid: The preparation of solid samples is simple, usually cleaning and polishing the surface

(2) Powder: The powdered sample can be directly measured on a supporting film, or compressed into a tablet under a tablet machine for measurement

(3) Melting sheet: An additive called flux can also be added to the powder sample to melt the sample into a glassy sample with beads at 1000-1200°C
.
This sample is uniform and can be measured directly
.
However, in the process of sample melting, part of the sample will be volatilized and lost in the form of H ₂ O and CO ₂ , and elements like S, Hg, and Cd will also volatilize when heated.
This loss is called loss on ignition
.

Weighing samples before and after melting can help determine the total loss on ignition.
The flux and loss on ignition should be considered when analyzing.
The flux usually uses materials composed of light elements such as Li ₂ B ₄ O ₇ because they are not measured.
To correct for this effect, consider the type of flux used and the amount added
.

(4) Liquid: Liquid samples can be placed in special sample cups with supporting membranes.
Sometimes the sample volume is not enough and appropriate diluents must be added
.
Liquids cannot be measured in a vacuum because they will volatilize and can be measured in air, but air will absorb a lot of radiation, making it difficult to measure light elements.
Therefore, liquid samples are measured in a helium-filled spectrometer chamber, which will not cause The liquid is volatilized and the rays are hardly absorbed
.

(5) Filter paper sample: Air dust or liquid can be analyzed by XRF after being filtered by filter paper.
The filter paper contains only a small amount of the substance to be analyzed, and the filter paper can be determined without special treatment
.

2.
Sample analysis

Sample measurement includes steps such as peak search and peak matching, peak height measurement and background subtraction, spectral line overlap correction, counting statistics, and detection limit
.

XRF determination requirements: ① XRF is a highly sensitive method, and qualified samples must be available
.
②The voltage of the light tube of the spectrometer, the detector and other parameters are adjusted to the best value of the analysis element
.
③The spectrum in EDXRF is measured at the same time.
The peak area is used to calculate the mass concentration of the element, and the peak height can also be selected for calculation, but a lot of information will be lost
.
Because relative to the peak height, the peak area is less affected by noise
.
④In EDXRF, the measurement is generally only performed at the top of the peak.
Because the position of the peak is known, measuring at the top of the peak can save time and get accurate results
.

Related Links: Overview of X-ray Fluorescence Analyzer (1)