Q.1. (b) Discuss theory and pharmaceutical applications of X-Ray diffraction spectroscopy
Ans.1. (b) X-ray Diffraction Spectroscopy
These methods are based on the scattering of X-rays by crystals. By these methods one can identify the crystal structures of various solid compounds. These methods are extremely important as compared with X-ray absorption and X-ray fluorescence methods. X-ray diffraction methods are generally used for investigating the internal structures. However, the following methods are used:
(1) Laue photographic method
(2) Bragg X-ray spectrometer method
(3) Rotating crystal method
(4) Powder method
Theory X-ray Diffraction Spectroscopy:
Origin of X-rays: The X-ray region of the electromagnetic spectrum consists of wavelength in the region of about 0.1 to 100 A°. For analytical purposes, the range of 0.7 to 2 A° is the most useful region. X-rays are generated when high velocity electrons impinge on a metal target
Interaction of X-ray with matter: The X-rays can interact with matter in three ways: absorption, scattering and diffraction. The scattering and diffraction method is discussed here. As the phenomenon of scattering forms the basis of diffraction, we will consider them together. When a beam of X-radiation is incident upon a substance, the electrons constituting the atoms of the substance become as small oscillators. These, on oscillating at the same frequency as that of incident X-radiation, emit electromagnetic radiations in all directions at the same frequency as the incident X-radiation. These scattered waves are coming from electrons which are arranged in a regular manner in a crystal lattice and then travelling in certain directions. If these waves undergo constructive interference, they are said to be diffracted by the crystal plane. Every crystalline substance scatters the X-rays in its own unique diffraction pattern producing a finger of its atomic and molecular structure. The conditions for diffraction are governed by Bragg’s law and the diffracted beams are often referred to as reflections. Constructive interference of the refracted beams emerging from two different planes will take place if the difference in the path lengths of two rays is equal to whole no. of wavelengths. This can be understood from fig 1.2. If one X-ray is striking the top crystal plane at A and the other X-ray is striking the second crystal plane at B, the path difference between two parallel X-rays is equal to CB+BD. But for constructive interference
nλ = CB+BD ---------------------- equ. 1
But CB = BD = 1.
Therefore nλ =1+1 = 21
But from ΔABC, 1 = d sinθ
Where d is the lattice spacing. Equation 1 is the familiar Bragg’s law. For Miller’s indices, equation 1 becomes as nλ = 2dhklsinθ Where, θ is the glancing angle of incidence, dhkl is the spacing between the set of crystal planes of Miller’s indices hkl and n is an integer. On putting the integer n equal to 1, 2, 3 etc. a series of angles is obtained at which reflections will occur for a given set of planes of spacing dhkl. These reflections are referred to as first order, second order, third order, etc. respectively. Through X-ray diffraction we can identify the crystal structures of various solid compounds and identify a compound from its structure. We can also determine the arrangement of molecules in a crystal. This has enabled us to obtain invaluable information on the structure of such diverse materials as chemical crystals, metals, and living tissue.
Pharmaceutical Applications of X-Ray Diffraction:
Phase analysis and polymorph screening:
XRPD is especially valuable for distinguishing different phases or polymorphs by their unique diffraction pattern. Since the choice of polymorph can be important in relation to properties such as solubility, bioavailability, and stability, the quest to produce and characterize all accessible polymorphs of a given drug substance has become an area that attracts intense activity. Since a new crystalline form of an API could be patented as a new pharmaceutical product, the search for every polymorph of a target compound is an invaluable step toward patent protection.
Crystallinity determination
XRPD is used successfully in the determination of percent crystallinity, where the volume concentration of, for example, amorphous filler to a crystalline active matrix is measured within a drug’s dosage form. Percent crystallinity can influence a drug’s processing behavior as well as its pharmacological performance.
R&D applications
The most fundamental, yet crucially important, application of XRPD is in the identification or fingerprinting of crystalline phases, with different crystal structures giving rise to distinct powder diffraction patterns. The qualitative characterization of materials in this manner finds applications in many areas, including quality control and polymorph screening.
Crystallography and crystal structure determination
Although the crystal structure of a material can be determined from a general powder diffraction pattern, in most cases it is sufficient simply to identify the lattice type and dimensions of the unit cell for this new material (indexing). This form of analysis is particularly useful for characterizing alternative forms or polymorphs of registered drugs where patents are about to expire. In addition to this technique, information such as displacement parameters coordinates of atoms within the unit cell, site occupancy, and preferred orientation can be obtained.
Compatibility studies
The non-destructive nature of XRPD makes it an ideal tool for systematic drug–excipient compatibility studies in Preformulation. Careful selection of the excipients along with the systematic evaluation of drug–excipient interaction is essential to achieve consistent release and bioavailability and to avoid unexpected formulation stability problems in later stages of formulation development.
Manufacturing and production
The ability of XRPD to detect and quantify the presence of any polymorphic contamination, the level of crystallographic changes, and the active ingredient in the final dosage form allows the technique to be used to monitor and improve production efficiency and cost. Once the active ingredient of a pharmaceutical product is in its final dosage form, the morphology parameters measured by X-ray diffraction can be related directly to the final drug performance.
Control of ingredients
XRPD is well suited for monitoring the crystal morphology of active ingredients or the excipients. This is important because any change in the morphology of fillers or in the crystalline state of active ingredients in the final product, as a result of the manufacturing process, can influence a drug’s bioavailability. With the X’ Celerator (PAN alytical), the lower limit of detection for minority phases has been reduced considerably; in some cases even down to 0.05%.
Stability studies
In situ powder diffraction studies carried out as a function of temperature and/or relative humidity can provide a direct means of characterizing the stability of a pharmaceutical compound and the occurrence of hydration/dehydration processes. Such non ambient diffraction experiments can be performed at any stage of the drug development process.
Process control
The ability of XRPD to determine structural parameters, together with its capacity for nondestructive analysis, makes it useful in diverse applications. One such example is the use of XRPD in determining the optimal range of tableting pressure. This allows manufacturers to track the crystallographic structure of an API to ensure that the finished tablet achieves its target dissolution rate.
Batch/dosage uniformity
Because XRPD allows materials to be investigated directly under the conditions in which they are used in specific applications, it is valuable for monitoring batch/dosage uniformity. It is possible to analyze the actual percentages of individual active ingredients in the final dosage form of a drug in situ, together with the percentage of any amorphous or crystalline packing ingredients used. XRPD can even be used to identify and quantify the small amounts of crystalline aerosol drug delivered by a pressurized metered dose inhaler.
.png)
