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Everything you wnat to know about X-Ray Diffraction

Applications of XRD – Foods

Applications of XRD

The growth of our civilization has seen a parallel growth in the food industry. In pre-historic ages humanity was dependent mainly on vegetables, fruits and meat for its daily food intake. However, over the years there has been an increasing demand for processed synthetic foods to cater to the needs of a growing population and to meet scarcities in different regions having harsh climatic conditions.

Processed foods need to have some basic characteristic properties which make them acceptable to the consumer. Some of these features are:

Colour which can be introduced with a blend of natural and artificial colouring agents

Flavours need to be added to make foods appealing or suppress some inherent bad odours

Preservatives are needed to improve the shelf life for long time storage and use

Stabilisers are added to improve their texture. This means modification of the crystalline behaviour and characteristics of the phases of different constituents

Anti- oxidants are added to prevent decay of food by oxidation

Emulsifying agents help in facilitating uniform distribution of fats and oils in aqueous media

Buffering agents control the acidity or pH of the food over storage period

The number of additives added to foods are growing by the day to match consumer tastes and lead to a corresponding increase in  market potential.

Several analytical techniques are routinely used to monitor the d presence of such food additives in manufacturing processes. The role played by XRD cannot be overlooked. X-ray diffraction provides us information on polymorphism, degree of crystallinity and amorphism. Such parameters help control the texture and stability of foods under different storage conditions.

X-ray diffraction has helped conduct studies on following common food ingredients such as

Starches

Fats

Candies

Starches

X-ray diffraction complements the DSC studies in starch gelatinization in foods. Gelatinization results when starch crystals melt with increasing temperature. It has been observed that the melting temperature increases with the decrease in moisture content. Gelatinization decreases the degree of crystallinity of starch granules and increases the non-crystalline amorphous content. This results in apparent changes in optical and rheological flow properties of foods.

Fats

X-ray diffraction has been extensively used in studying the polymorphism (different molecular packing arrangements in crystals) of fats such as margarines. The three common polymorphs of fats are α, ß and ß’. Packing arrangement in α form is hexagonal, triclinic in ß and orthorhombic in ß’. The ß form is the most stable form whereas α form is the least stable of the three. Polymorphic differences are of significance in food industry as such differences bring about changes in physical properties such as texture. Such inter- conversions can result from transitions from ß’ to the more stable ß form under different processing conditions.

Candies

Candies and chocolates are very popular among children. X-ray diffraction plays a useful role in manufacturing products of consistent quality in terms of sweetness, texture and flavor. Such characteristics depend on the balance between crystalline and amorphous proportions of constituents which can be easily monitored with the help of the X-ray diffraction technique.

The scope of X-ray diffraction in the food industry is increasing as consumers demand consistency of quality and new ingredients are finding their way into new products which are developed at an amazing pace.

Material Characterization by X-ray Diffraction Studies

Symmetric Faced Crystal

X-ray diffraction is a valuable tool in the hands of the analytical chemist for material characterization which is based on arrangement of atoms in the crystal lattices. The biggest advantage is that it is a non-destructive technique and the sample can be reused for further investigations by other means. Samples can range from symmetrically ordered atomic arrangements to random arrangements as in amorphous substances. The common applications of x-ray diffraction studies include crystal lattice dimensional analysis, grain size, crystal defects and residual strain.

X-rays interact with solid materials to generate diffraction patterns. Data on diffraction patterns resulting from interaction of x-rays with inorganic and organic solids shows definite patterns which can be used as fingerprints in identification of such materials. Diffraction data base is maintained by the International Centre of Diffraction data (ICDD) which was formerly known as Joint Committee on Powder Diffraction Standards (JCPDS). Such reference data can be purchased direct from ICDD or through

x-ray diffraction instrument suppliers.

X-ray diffraction studies easily distinguish between single crystal orderly arrangements of atoms to polycrystalline arrangements. The atomic planes of crystal lattices responsible for scattering of x-rays are their reflective surfaces. Scattered beams when in phase interact constructively and intensities are maximum at particular angles. Such reflecting patterns on reaching the detector will generate a response which can be matched with the pattern from standard materials.

It is interesting to note that under the influence of heat small particles anneal to form larger aggregates. This becomes apparent as peak intensities get enlarged and help distinguish nano-particles from larger aggregates of particles.

Applications of XRD

X-ray diffraction is a powerful tool for characterization of

nano- materials, bulk materials and thin polymeric films.

Phase studies

Powder crystallographic studies help characterization on the basis of chemical composition of materials. Such studies provide valuable details on phase transformations resulting from subjecting materials to extremes of temperatures.

Degree of crystallization

Polymeric materials often exhibit mixed behaviour as they can be partially crystalline and partially amorphous. The degree of amorphous content can vary with the conditions used during their processing. The more the amorphous content the greater will be the peak broadening so the ratio between the peaks of pure crystalline standards and polymeric materials will give an idea on the degree of crystallinity in the polymeric sample.

Residual stress

Stress is defined as force acting on a material per unit area and any deformation per unit length is referred to as strain. Residual stress is the stress that remains in the material when the force responsible for it is removed. In synthetic materials residual stress results from material treatment processes such as machining, welding, heat quenching, etc. On the other hand in geological samples such stress could be the result from natural rock dynamics under the earth’s surface. X-ray diffraction is helpful in studies on residual stress introduced in materials through artificial or natural processes.

X-ray diffraction also helps study the dominant or preferred orientation of polycrystalline aggregates. Such information is beneficial in relating orientations of aggregates or texture to the desirable properties of materials.

In conclusion it can be said that x-ray diffraction studies provide valuable information which can help characterize both manufactured as well as naturally occurring materials.

Role of Bragg’s law in X-Ray Diffraction studies

X-ray Crystal diffraction
X-ray Crystal diffraction

Bragg’s law is the foundation stone on which the edifice of x-ray diffraction stands. It holds same significance in crystal structure determinations as the Beer- Lambert law(link) holds in light absorbance measurements. The law was established by Sir W.H Bragg and his son Sir W.L Bragg in 1913 to explain the diffraction of x-rays from atomic planes of crystals of sodium chloride, zinc sulphide and diamond.The Nobel prize for Physics was awarded to the Bragg duo in 1915 for their contribution in the field of crystallography.

A beam of x-rays incident to a crystal face gets partially scattered by the atoms of the crystal. The fraction that is not scattered reaches the next atomic layer where another part is scattered and remainder passes across to the next layer and so on. As a result the diffraction pattern is generated from the constructive and destructive interference of X-rays diffracted from each plane. The diffracted beams interact constructively if the beams are in phase or destructively if they are out of phase. A basic requirement for x-rays to diffract is that the sample exhibit crystallinity and the spacing between the atomic layers must lie in the wavelength range of x-ray radiation.

Diffraction pattern on film
Diffraction pattern on film

The Bragg’s law can be expressed mathematically as

nλ = 2d Sinθ

where,

 λ is the wavelength of x-ray beam

θ the angle of incidence

d  the spacing between the atomic planes

n is an integer

Diffracted beam from several randomly oriented crystals in the sample progresses in a conical shape from the crystal face and the diffraction pattern can be displayed on a photographic film as a series of concentric rings.

Based on XRD measurements you can record the distance between the atomic layers of the crystal lattice, estimate bond lengths and angles, and confirm the identity of the unknown materials by correlating their crystal lattice structure with standard reference materials.

The law though developed to  observe scattering of X-rays by  crystals is applicable to determine the structure using different beams such as electrons, ions, neutrons or protons with wavelengths in the range of distances between the atoms or molecules in the crystal.

Applications of XRD – Pharmaceuticals

Pharmaceutical formulations are available as tablets, capsules, ointments, pills, oral liquids, aerosol sprays, etc. Even a single drug can be found in several different formulations which impart   it different characteristics as per administration requirements .In case of solid formulations X-ray diffraction has been used as a non-destructive technique for testing of inconsistencies in manufactured batches, […]

Applications of XRD– Forensics

Forensic analysis is a scientific study which assists criminal investigations. Whenever any crime takes place there is exchange of material between the accused and the victim such as textile fibers, blood stains, gunshot residues, broken glass, saliva, semen, etc. Chemical and biological tests on such materials can provide undisputed evidence to nail down the suspect. […]

Applications of XRD – Geology and Mining

X-ray diffraction or XRD has made valuable contributions in the field of geology and mining. Geology is the scientific study on structure of planet earth, formation of rocks, sediments and minerals. Mining is an applied science which concerns with extraction of ores, fossil fuels for energy needs and minerals for commercial processing. Minerals are inorganic […]

Applications of XRD – Cosmetics

Cosmetics have been in use since time immemorial. In earlier ages several potentially toxic inorganic pigments such as malachite, mercuric sulphide or white lead were used in cosmetic preparations. In contemporary products several organic components such as parabens, alkylphenols, etc have also been included. It was a common concept that cosmetic products were only meant […]

Applications of XRD – Foods

The growth of our civilization has seen a parallel growth in the food industry. In pre-historic ages humanity was dependent mainly on vegetables, fruits and meat for its daily food intake. However, over the years there has been an increasing demand for processed synthetic foods to cater to the needs of a growing population and […]