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Compositional Analysis

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Compositional Analysis

Compositional analysis always refers to quantitative chemical/elemental analysis of a given sample. In materials science research, compositional analysis is a valuable tool because it enables researchers to identify unknown material, and understand the structure–property relationships for each component and their synergistic behaviors to characterize significant changes in the material structure and elemental distribution. For example, it has been found that the char yield, char combustion time and char combustion rate of woody biomass pellets are mostly depended on their composition. Pellets made of bark had up to a 50% longer char combustion time compared to that of stem wood pellets, due to differences in char yield. Quantitative chemical analysis methods are also used occasionally for evaluation of foreign material contaminants in special cases for failure analysis or investigation of product manufacturing or handling problems. At Matexcel, through analysis on given samples such information could be obtained: Presence of an element or chemical group, quantity or concentration of an element, location of each element present with sub-nm resolution, relative concentration for each element present in a given 2D or 3D area. Traditional techniques such as wet chemistry include gravimetric and titrimetric techniques. Depends on the sample type, the analysis may be performed by one or more complimentary techniques:

Mass spectroscopy (MS)
Mass spectrometry is an analytical technique that ionizes chemical species and sorts the ions based on their mass-to-charge ratio. In a typical MS procedure, a sample, which may be solid, liquid, or gas, is ionized, for example by bombarding it with electrons. These ions are then accelerated by an electric or magnetic field. Ions of the same mass-to-charge ratio will undergo the same amount of deflection. The ions are detected and results are displayed as spectra of the relative abundance of detected ions as a function of the mass-to-charge ratio. The atoms or molecules in the sample can be identified by correlating known masses (e.g. an entire molecule) to the identified masses or through a characteristic fragmentation pattern. Mass spectrometry is always coupled with other instruments. For example, with different ionization tools such as electron ionization (EI), matrix-assisted laser desorption/ionization (MALDI), Inductively coupled plasma (ICP). Different separation techniques such as gas chromatography (GC) and liquid chromatography (LC) are also selected to enhance the mass resolving and determining capabilities of mass spectrometry.
Similar techniques: Gas chromatography-mass spectrometry (GC-MS), Liquid chromatography-mass spectrometry (LC-MS), Inductively coupled plasma-optical emission spectroscopy (ICP-OES), ICP-MS, Time-of-flight mass spectrometry (TOF-MS), Secondary ion mass spectrometry (SIMS).

High performance liquid chromatography (HPLC)
Under high pressure conditions, the liquid is used as the flow phase, and the sample solution enters the flow phase through the sampler and is loaded into the stationary phase. Since each component in the sample solution has a different partition coefficient in the two phases, the components have a different moving speed when the two phases are relatively moved. After repeated adsorption-desorption process, the sample solution was separated into individual components and sequentially flowed out of the column. The sample concentration is converted into an electrical signal via the detector and sent to the recorder. The data is printed as a map.
The system flow chart of the HPLC is shown as follow:

X-Ray fluorescence (XRF)
XRF is a fast and non-destructive technique. It can be used for direct analysis of solid samples, thin metal films, petroleum products and various other materials. An x-ray tube is used to irradiate the sample with a primary beam of x-rays. Some of the impinging primary x-rays are absorbed by an electron in an atom’s innermost electron shell. This causes excitation and ejection of the absorbing electron which is so called photoejection. The caused electron vacancies are then filled by electrons from higher energy states, and x-rays are emitted (fluorescence) to balance the energy difference between the electron states. The emitted x-ray energy is characteristic of the element from which it was emitted. The energy of each x-ray and number of x-rays at different energy are recorded. The x-ray intensities (counts) are compared to values for known standards for quantitatively analysis of the unknown specimen.

Atomic absorption spectroscopy (AAS)
When the element is excited and its atoms transit between the ground state and the first excited state to produce a spectral line with its own characteristics. AAS performs elemental analysis by this feature. When the characteristic line emitted by the element lamp to be tested passes through the atomic vapor generated by the atomization of the sample, it is absorbed by the ground state atom of the element to be tested in the vapor. The degree of weakening of the intensity of the radiation light is determined to test the content of the element in the sample.

Other similar techniques: Real Time Magnified X-ray imaging, Energy dispersive X-Rays (EDX), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), Micro X-ray absorption near edge spectroscopy (μXANES).
Vibrational spectroscopy

Vibrational spectroscopy consists of three main tools: Fourier Transform Infrared (FTIR), Near-Infrared (NIR), and Raman spectroscopy. FTIR is used to study functional groups of solid or liquid materials using the discrete energy levels for vibrations of atoms in these groups. When light with a specific energy is transmitted through a very thin sample it can be absorbed by groups of atoms in the material. This occurs when the frequency of the incoming light corresponds to the frequency of vibrations in bonds between atoms. The vibration energy depends on the masses and chemical environment of the atoms, and the type of vibration. The range of Infrared region is 12800 ~ 10 cm-1 and can be divided into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1). The common used region for infrared absorption spectroscopy is 4000 ~ 400 cm-1 because the absorption radiation of most organic compounds and inorganic ions is within this region. By scanning over a range of wavelengths (400–4000 cm−1) and recording the amount of transmitted light for each wavelength it is possible to determine which functional groups are present in the material. Each of those three techniques has strengths and limitations, making them complementary tools for the analysis of various materials.
Similar techniques: Attenuated total reflectance-FTIR (ATR-FTIR), Micro-FTIR/Raman spectroscopy, Nano-FTIR/Raman.

UV Spectrometer
Since various substances have different molecules, atoms and molecular space structures, the absorption of light energy will not be the same. Therefore, each substance has its own unique and fixed absorption spectrum curve. The content of the substance can be determined according to the absorbance at certain characteristic wavelengths on the absorption spectrum, which is the basis for qualitative and quantitative analysis of spectrophotometry. The basis of quantitative analysis is Lambert-Beer law. Advantages High sensitivity, applicable to a wide range of concentrations, low analysis cost, easy operation, and fast.

Nuclear magnetic resonance (NMR)
NMR spectroscopy is an analytical chemistry technique employed in quality control and research for determining the content and purity of a material as well as its molecular structure. For example, NMR has been widely used to quantitatively analyze mixtures containing known compounds. For unknown compounds, NMR can either be used to match against spectral libraries or to infer the basic structure directly. The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. Under an external magnetic field, an energy transfer is possible between the base energy to a higher energy level (generally a single energy gap). The energy transfer takes place at a wavelength that corresponds to radio frequencies and when the spin returns to its base level, energy is emitted at the same frequency. The signal of this transfer is measured i and processed in order to yield an NMR spectrum for the nucleus concerned.

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