Common Elemental Analysis Methods

Elemental analysis in the analysis of foreign objects, materials testing commonly used analytical methods, in machinery, chemical industry, metallurgy, electronics, food and many other industries have a wide range of applications, this article briefly introduces some common methods of elemental analysis, mainly instrumental methods of analysis, does not involve the chemical titration aspects.

EDS, WDS

EDS and WDS belong to electron microprobe analysis, which is the most widely used micro-area elemental analysis method.

Principle

High-energy electron beam bombardment of the surface of the specimen with the determination of the region, so that the inner layer of the atom electron jump released characteristic X-rays, different elements issued by the characteristic X-rays have different frequencies (energy) and wavelengths, through the detection of the characteristic X-rays of the energy or wavelength of the qualitative analysis of the element, through the characteristic X-rays of the intensity of the element for quantitative analysis.

Range of elements analysed

Beryllium (Be)- Uranium (U), some EDS equipment Na-U

Analytical features

1. Combined with electron microscope for micro-area composition analysis;

2. It is the most convenient, fast, accurate and reliable method for micro-area composition analysis, with good data stability and reproducibility, in which the precision of WDS is higher than that of EDS;

3. It can carry out full elemental sweep, but the detection limit is high, usually 0.1-0.5%;

4. Data reliability decreases as atomic number decreases and elemental content decreases.

Sample requirements

EDS and WDS are usually only tested on solids as they need to be tested under vacuum conditions, and liquids need special treatment. There are no special requirements for the sample surface, which needs to be dry solid and the carrier table can be placed, no magnetic, radioactive and corrosive. If the conductivity of the sample is very poor, can be sprayed with gold or carbon.

XRF

XRF full name X-ray fluorescence spectrometer, is a widely used macro-substance elemental analysis method. Similar to EDS, it also uses characteristic X-rays for elemental analysis, only the excitation mode is different.

Principle

A beam of X-rays is used to irradiate a sample material, causing the sample to emit secondary characteristic X-rays, also called X-ray fluorescence. The energy or wavelength of these X-ray fluorescences is characteristic, and the concentration of the elements in the sample directly determines the intensity of the rays. Thus, according to the characteristic energy lines to identify the type of elements, according to the intensity of the spectral lines to quantitative analysis.XRF wavelength scattering type (WDXRF) and energy scattering type (EDXRF) two kinds of the former measurement of precision is good, high stability, but the structure of the complex and expensive, the latter structure is simple, the price is low, but the interference of more elements, and the accuracy of the low energy X-ray analysis effect is not good.

Range of analysed elements

EDXRF: No. 11 Na (Na) - No. 92 Uranium (U)

WDXRF: No. 4 Beryllium (Be) - No. 92 Uranium (U)

Analytical features

1. Fast analysis speed. Measurement time is related to the precision of the measurement. Usually, the analysis can be completed in 2 to 5 minutes.

2. X-ray fluorescence spectroscopy has nothing to do with the chemical bonding state of the sample, and is basically unrelated to the state of solids, powders, liquids, and crystalline and amorphous substances. (Gases can be analysed even if they are sealed in a container.) However, wavelength changes can be seen in high-resolution precision measurements. This effect is particularly noticeable in the ultra-soft X-ray range. Wavelength shifts are used for the determination of chemical potentials.

3. Non-destructive analysis. There is no change in the chemical state during the measurement, and there is no scattering of the specimen. The same specimen can be measured several times with good reproducibility.

4. High analytical precision. The analytical precision can reach 0.1% and the detection limit can reach 10ppm.

5. Solid, powder and liquid samples can be analysed.

Sample requirements

1. Solid samples

   For example, steel, copper alloy, aluminum alloy, precious metal, etc. The method is to use a lathe to turn the sample into a garden column sample, with one end of the surface to be smoothed and polished. Before use, do not touch the light surface, so as not to stain the surface with oil, affecting the measurement accuracy. If stained with oil, use a clean flannel to wipe clean.

2. Powder samples

   Generally, the powder sample can be measured by placing it directly in the sample cup. About 7 grams of sample is enough to cover the bottom of the sample cup about 1cm thick. Large instruments, in order to obtain a high degree of measurement accuracy, the sample is also often the following two kinds of processing:

   • 2.1. Pressing. A small amount of binder is placed in the sample and pressed into a sheet using a hydraulic press.

   • 2.2. Glass frit method. To the sample in the melt, in the crucible with high temperature made of glass tablets. With this approach to dilute the sample, the mineral effect can be eliminated or reduced.

3. Liquid samples

   For liquid samples, there are three main sampling methods:

   • 3.1. direct method. Liquid samples will be poured directly into the liquid sample tank to analyse.

   • 3.2. enrichment method. The elements to be measured in the liquid sample with a certain method (such as copper reagent, ion exchange resin, etc.) enrichment.

   • 3.3. Dot-drop method. The liquid sample will be analysed by dropping it on the filter paper.

   In the preparation of the sample, sometimes an element is intentionally added to the sample to make it an internal standard, this method is called internal standard method.

Plasma Emission Spectroscopy (ICP-OES)

Inductively Coupled Plasma Emission Spectroscopy (ICP-OES) is a method of simultaneous measurement of multiple elements. It is also known as ICP-AES, but the abbreviation ICP-OES is often used because the abbreviation for Auger Electron Spectroscopy is also AES.

Principle

A sample solution is evaporated and vaporised using a plasma-excited light source (ICP) and decomposed into an atomic state and possibly further ionised into an ionic state, where the atoms or ions are excited in the light source to emit the characteristic spectral lines of the elements they contain. Based on the presence or absence of the characteristic spectral lines, it is possible to identify whether a certain element is present in the sample (qualitative analysis), and based on the intensity of the characteristic spectral lines, it is possible to determine the content of the corresponding element in the sample (quantitative analysis).

Note: Solid samples must be liquefied, due to accuracy reasons, it is not suitable to determine the content of more than 30%, can be diluted.

Analytical features

mainly used for trace/micronutrient analysis of metallic elements, less suitable for halogen and carbon, hydrogen, oxygen, nitrogen and other elements; high accuracy, detection limit up to ppm or even ppb level; in addition to a small amount of aqueous liquids can be fed directly into the sample, other samples generally have to be pre-treated, i.e., the sample will be dissolved into a dilute acid solution of inorganic; can be used for simultaneous determination of multiple elements.

Sample requirements

Solid samples need to be dissolved, so that the elements to be measured completely into solution, and no loss of the elements to be measured during the dissolution process, no or as little as possible to introduce the elements to be measured components, the solvent has a high degree of purity.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS is the analytical technique of choice for trace elements.

Principle

ICP-MS is a plasma with a mass spectrometer as the detector, and its injection section and plasma are very similar to those of ICP-OES, which measures optical spectra, while ICP-MS measures ion-mass spectra. The sample is ionised by ICP, and then efficiently transported to the mass spectrometer through the ICP-MS interface, where ions with different mass-to-nucleus ratios (m/z) are selected for detection. By selecting ions with different mass-to-nucleus ratios (m/z), the intensity of a certain ion is detected, and then the intensity of a certain element is analysed and calculated.

Analytical features

ICP-MS can analyse most metallic elements and some non-metallic elements; high accuracy, detection limit up to ppb or even ppt level; each element has an isotope spectrum, not interfered by the spectrum of other elements, less interference in multi-element testing; simultaneous determination of multiple elements is possible; high cleanliness requirement, easy to be contaminated.

Sample requirements

a. Solid samples ≤ 0.01 per cent

b. Liquid samples ≤ 1 ppm (preferably ≤ 100 ppb)

The sample preparation process is demanding, with detailed requirements for each step.

Organic Elemental Analysis (EA)

Principle

Organic Elemental Analysers are used to determine the carbon, hydrogen, oxygen, nitrogen, and sulphur content of organic matter by combustion of the corresponding reagents in a pure oxygen environment or high temperature cracking in an inert gas. The test generally has CHN mode, CHNS mode and oxygen mode.

Analysing elements

Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulphur (S)

Analysing features

Fast testing speed, high accuracy; can test solid and liquid samples, mainly suitable for organic compounds testing.

Sample requirements

The instrument is suitable for measuring organic substances, explosive, corrosive, metal samples can not be measured Only solid and liquid samples The content of 100ppm or less can not be measured The samples need to be dry, solid samples through a 60-mesh sieve to ensure that the sample is uniform.