q-calibration with silver behenate

Transcription

1 q-calibration with silver behenate

2 Page 2 of 5 Theory Silver Behenate is one of the most frequently used standards for calibration of the medium to low angle q-range, both at synchrotrons as well as for lab instruments. Silver Behenate, commonly abbreviated as AgBeh, is the silver salt of behenic acid, a C22 carboxylic acid. It forms a lamellar phase with a d-spacing of Å (Fig 1A), using copper K α radiation (E = 8.0 kev, λ = 1.54 Å), this results in 13 evenly spaced peaks in the range between 0.1 and 1.4 Å -1 of which the principle peak is found at Å -1 (Fig 1B). 1,2 A B Fig 1. A) Chemical structure of silver behenate B) Radially averaged scattering pattern of silver behenate at four different sample-to-detector distances each with different resolutions.

3 Page 3 of 5 The sample-to-detector distance (D) can be calculated by comparing the actual position of the first order peak with the theoretical value using the equations below (eq. 1). While it is possible to fit the first peak and obtain the peak position, fitting the second or third order peak and dividing by the peak order usually gives more reliable results. tan( 2θ ) = d D with q = 4π λ sinθ and d = 2π q (eq.1) But in practice There is a large difference in the quality of silver behenate from supplier to supplier, from batch to batch, and even within one batch. This is clearly visible in the 2D images (Fig 2.) collected at a sample-to-detector distance of 1050 mm. These images show that the broadness of the first order ring varies significantly as illustrated in Fig 2. One of the issues that becomes clear from the radially averaged images (Fig 3) is splitting of the higher order peaks. Measuring at a sample-to-detector distance of 450 mm (configurations 2 and 22) gives a clear view on the 1 st to 6 th order peak. Zooming in on the region between 0.3 and 0.7 Å -1 reveals varying degrees of peak splitting and peak broadening for most silver behenate samples. Fig 2. Scattering patterns of two different silver behenate samples.

4 Page 4 of 5 Fig 3. Radially averaged scattering patterns of different batches of silver behenate, collected at two different instruments. The ratios of the two peaks vary between the different samples, which indicates this is not merely an instrument issue caused for example by a tilt of the detector, nor caused by the fact that the X-ray radiation is actually composed of two different bus closely spaced wavelengths: the Cu K α1 and K α2 line. 3 This is further supported by the fact that the peaks consistently appear 1.74% apart while a difference of only 0.25% would be expected as a result of considering both K α lines. Interestingly, in two cases the ratios of peaks are also different between two samples of the same batch, indicating that the samples are not homogeneous. Presumably the observed peak splitting is caused by impuriy of the sample either already present, or caused by degradation under the influence of air, light or temperature. 4 Concluding remarks While none of the samples so far tested seem to be perfect, SAXSLAB recommends the use of silver behenate form: Rose Chemicals Ltd. 73 Englefield Road London N1 4HD +44 (0) To keep the standard samples in good condition, care should be taken to prevent exposure to air, light and elevated temperatures.

5 Page 5 of 5 References and further reading 1. Huang, T. C., Toraya, H., Blanton, T. N. & Wu, Y. X-ray powder diffraction analysis of silver behenate, a possible low-angle diffraction standard. J. Appl. Crystallogr. 26, (1993). 2. Blanton, T. N. et al. JCPDS International Centre for Diffraction Data round robin study of silver behenate. A possible low-angle X-ray diffraction calibration standard. Powder Diffr. 10, (1995)