X-ray powder diffraction as a technique for detection of physical impurities
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The chemical purity of your Active Pharmaceutical Ingredient (API) is of importance in the development of your compound and much effort is often invested in obtaining the highest purity of the API.  In parallel, further attention is given to develop a chromatographic method to detect the slightest of impurities.  The real challenge, however, is not in the detection and quantification of chemical impurities but rather the detection and quantification of the physical impurities.

At Crystallics approximately one in five of the projects that we execute are of crystalline compounds that are phase contaminated. These contaminations may consist of a different polymorphic form or a solvate/hydrate of the API that have crystallized together with the main physical phase of the compound.  There are various techniques that have been developed during recent years that allow for determination of quantities below 5% of the minor phase.  Such techniques include raman (Farias and Carneiro, 2014; Kalantri et al., 2010) and solid state NMR spectroscopy (Barry et al., 2012; Paradowska and Wawer, 2014).  These methods are complicated in their execution and rely on the construction of a calibration curve to obtain accurate data.

X-ray powder diffraction (XRPD) provides an alternative for the quantitative phase analysis.  For the determination and quantification either Rietveld or Whole Powder Pattern Decomposition (WPPD) methods are efficient for up to five components (Toraya and Tsusaka, 1995).  For the Rietveld method the cell parameters and atom positions are required whereas for the WPPD method only the cell parameters and peak intensities are required.

To demonstrate the usefulness and robustness of the X-ray powder diffraction method for the identification and quantification of low level phase impurities we examined the detection level (LoD) and the practical quantification level (PQL) of 3 different compounds.

Figure 1 presents the final Rietveld calculation of the X-ray powder diffractogram obtained from a powder mixture that contained 99% of sodium citrate monohydrate, 0.5% of citric acid and 0.5% of sodium dihydrogen citrate.  The blue line represents the experimentally collected data points (Bruker D8 advance with a Lynx eye detector) and the red trace shows the calculated data values from the adopted and refined model. The black line in the figure shows the peaks that belong to the powder pattern of citric acid and the green line presents the peaks of the sodium dihydrogen citrate.

Figure 1.  Graphical representation of the Rietveld analysis of sodium citrate monohydrate spiked with nominal 0.5% of citric acid and 0.5% sodium dihydrogen citrate.

In Figure 1 at a 2θ value of about 17.8 a peak with shoulder is visible that represents both impurities.  The peaks (1 2 0 and 1 -1 -1 for citric acid and sodium dihydrogen citrate, respectively) do not overlap with the peak of the sodium citrate and, therefore, are ideal for the calculations.  Figure 1 and the calculations demonstrate that phase impurities at a 0.5% level are easily detected.  Quantification of very low amounts of phase impurities requires atom positions associated with cell parameters so that the Rietveld method can be applied.  To demonstrate this experiments were carried out on arpiprazole and the co-crystal between cinnamic acid and 3-nitrobenzene and the Rietveld analysis are outlined in Figure 2.

Figure 2.  Graphical representation of the Rietveld analysis of arpiprazole (left) and cinnamic acid:3-nitro-benzamide co-crystal.

The top picture in Figure 2 represents the Rietveld calculation of crystalline arpiprazole. The monoclinic polymorph of this compound was spiked with 1% of the triclinic form. The black line in the left picture of Figure 2 shows the calculated diffractogram of the minor phase (triclinic crystals). The presence of the triclinic form can be spotted by the peak at 2θ approximately 18.3°. The Rietveld calculation of its content revealed 1.1% with an error ± 0.1%.  This value is in good agreement with the spiked amount of the triclinic crystal form of aripiprazole.

The bottom picture in Figure 2 presents the Rietveld analysis performed on a co-crystal between cinnamic acid and 3-nitrobenzene.  The analysis indicates a small excess of one of the components.  In this case the analysis shows that there is about 3% free cinnamic acid present in the sample.  In the diffractogram there are peaks at 2θ 5.7° and 9.8° that do not belong to the co-crystal (major phase). Based on the structural data of cinnamic acid the peak at 2θ 5.7° was assigned to Form I of cinnamic acid (blue line below diffractogram) while the other peak at 2θ 9.8 could be assigned to Form II of cinnamic acid (black line). Final calculation revealed that the surplus of free cinnamic acid in the sample was quantified at 2.9% but divided over two different polymorphs, namely 1.2% of Form I and 1.7% of Form II, with an error ± 0.1% for both phases.

Summarizing, X-Ray powder diffraction is an excellent technique for detection of physical impurities at the level up to 0.5% and the quantification limit can be set at 1% with high confidence level. In addition, and contrary to the other methods in the literature, the X-ray powder diffraction method does not require the use of a calibration line.

· Barry, S.J., Pham, T.N., Borman, P.J., Edwards, A.J., Watson, S. a., 2012. A risk-based statistical investigation of the quantification of polymorphic purity of a pharmaceutical candidate by solid-state 19F NMR. Anal. Chim. Acta 712, 30–36. doi:10.1016/j.aca.2011.10.064

· Farias, M.A.D.S., Carneiro, R.L., 2014. Simultaneous Quantification of Three Polymorphic Forms of Carbamazepine using Raman Spectroscopy and Multivariate Calibration. Anal. Lett. 47, 1043–1051. doi:10.1080/00032719.2013.860537

· Kalantri, P.P., Somani, R.R., Makhija, D.T., 2010. Raman spectroscopy : A potential technique in analysis of pharmaceuticals. Der Chem. Sin. 1, 1–12.

· Paradowska, K., Wawer, I., 2014. Solid-state NMR in the analysis of drugs and naturally occurring materials. J. Pharm. Biomed. Anal. 93, 27–42. doi:10.1016/j.jpba.2013.09.032

· Toraya, H., Tsusaka, S., 1995. Quantitative Phase Analysis using the Whole-Powder-Pattern Decomposition Method. I. Solution from Knowledge of Chemical Compositions. J. Appl. Crystallogr. 28, 392–399. doi:10.1107/S0021889894014986