EDS exposto: explorando a estrutura e os usos da espectroscopia de raios X

EDS Exposed: Exploring the Structure and Uses of X-ray Spectroscopy

What is EDS?

The full name of EDS is an energy-dispersive X-ray spectrometer that can record all X-ray spectra at the same time to measure the functional relationship between X-ray intensity and X-ray energy.

It is a quick method for analyzing the composition of microareas without damaging the sample.

Qualitative analysis of elements is carried out by measuring the characteristic energy of X-rays excited by the material, and quantitative analysis is carried out by measuring the characteristic intensity of X-rays.

EDS began to be commercialized in the early 1970s and today is basically the standard SEM equipment.

Characteristic x-ray

Definition: photons of electromagnetic radiation with characteristic energy generated by the transition from outer shell electrons to inner shell electrons after the inner shell electrons of the atom are ionized.

When the high-speed moving electron beam bombards the surface of the sample, the electrons collide with the atomic nucleus and the outer shell electrons of the element for one or more elastic and inelastic collisions.

About 1% of the incident electron energy excites various sample signals that reflect sample information: secondary electrons, characteristic X-rays, continuous X-rays, traverse electrons, backscattered electrons, etc.

Fig. 1: signal generated by the bombardment of high-energy electrons on the sample surface

Structure and working principle of EDS

The characteristic X-ray is special because the X-ray energy released by different elements is different, just like the fingerprint of the same person, with uniqueness.

Elemental analysis using different characteristic X-ray energies is called the energy dispersive method.

The structural schematic diagram is as follows:

Fig. 2: Structural schematic diagram of the energy spectrometer

The characteristic to know:
N = E / ε,

Where, ε is the energy (3.8 eV) generated to generate a pair of electron holes.

For example: FeKα- the energy is 6.403keV and 1685 electron hole pairs can be generated.

By biasing the Si detector (LI) (generally – 500 ~ – 1000 V), the electron and hole pairs can be separated and collected, converted into current pulses by the preamplifier, then converted into voltage pulses by the main amplifier and then sent to the multichannel pulse height analyzer.

The height of the output pulse is determined by N, forming the abscissa of the EDS spectrum: energy.

According to the number of characteristic X-rays recorded in different intensity ranges, the X-ray intensities of different elements can be determined to form the ordinate of the EDS spectrum: intensity.

Figure 3 EDS Diagram

Range of EDS analysis elements

The elements that can be analyzed by the energy spectrometer are affected by the type of window material.

The traditional beryllium window can only analyze the elements after sodium (Na) because it absorbs X-rays from ultralight elements.

The organic film's ultra-thin window can analyze all elements between (Be) – uranium (U).

Reliability

Some people always think that EDS is a semi-quantitative analysis and that the deviation of the result will be large.

In fact, real EDS is the most convenient, fast, accurate and reliable analysis method for microregion composition analysis, and the data stability and reproducibility are good.

Its accuracy is second only to WDS, which can reach 2-10%.

The quantitative error of the main element without overlapping peak of the median atomic number is 2-3%, and the detection limit is 0.1-0.5%.

Generally, reliability decreases as the atomic number decreases and the element content decreases.

The measuring depth is at the micron level.

The progress of silicon drift detector (SDD), large solid angle detector and various software processing also further reduces the measurement error of EDs.

Sample Requirements

The energy spectrometer has no special requirements on the sample surface.

The dry solid and stage can be placed without magnetism, radioactivity and corrosion.

If the conductivity of the sample is low, it can be sputtered with gold or carbon.

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