Method of XRD Analysis
Method of XRD Analysis |
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The aims of this work were threefold:
(Note: all work in this paper assumes the use of CuKa Radiation throughout)
Standard Materials intended for X-Ray powder diffraction intensity referencing are available from NBS. These may be used to establish a scale on which to compare one supposed 100% crystallinity standard HA material against another.The method is to thoroughly mix the reference material and the standard HA material 50%:50% w/w, followed by measurement of the XRD powder pattern under experimental conditions given in appendix A.
(1) The reference material should have a similar absorption coefficient to HA; hence both Titanium Di-Oxide (TiO2) and Chromium Oxide (Cr2O3) were tried. The TiO2 is preferred as it has a single strong reflection which is well clear of the HA reflections and there are no TiO2 reflections which interfere with those of the phosphate.
The integrated area under the TiO2 (110) reflection, at approximately 27.5 degrees (2 Theta) using Cu Ka radiation is then compared to the integrated area under the envelope formed by four HA reflections - (211),(112),(300),(202). These occur between 26.8 degrees and 27.9 degees using these experimental conditions. The ratio of :HA envelope Area / TiO2 (110) area provides the standardisation.
For the HA provided for the work at Staffordshire University this ratio was measured as: 0.97 +/- 0.01
An HA material giving a ratio higher than this would have a crystallinity proportionally higher than our standard and viceversa.
Note: that this standardisation must be carried out against the internationally available NBS Standard reference material 674a:TiO2.
A selection of samples containing the following phases in various w/w ratios were available:
Some samples were also prepared by mixing known weights of"pure" phases together. The XRD patterns for these samples andmixtures were obtained and computer-fitted to a linear combination of synthesised patterns - one for each component phase, using a least squares profile fitting program. This eliminates problems due to peak overlap. This process determines the scaling factor of each individual phase pattern within the experimental pattern from the mixture. If we assume that there is little or no amorphous material present and that all these calibrations samples were run under identical conditions, then
KHIH + KaIa + KbIb = 1
will hold for all samples containing some or all of these phases, and no others.
IH is the scaling factor for the HA pattern in a particular sample, from the fitting procedure; similarly for the other I values.
KH is a factor which allows for the relative scattering power of the HA phase compared to the other phases; it is sample independent.
KHIH is the fractional w/w of HA in the sample.
Since more than three samples were available, the quantity of data available was sufficient to allow calculation of the K factors. Using these factors and measured I values, the w/w fractions for phases in two other samples whose composition was known were determined.The results of this are given in table (1).
The values of the K factors depend on the experimental conditions e.g. incident X-Ray flux, but ratios of K factors donot as they measure the relative scattering power of two phases.
Thus we calculate:
Ka/KH = 1.43 and Kb/KH = 1.25
These may be used to determine the w/w ratios of alpha and beta tri-calcium orthophosphates in HA materials. These ratios are inversely proportional to the scattering power of the "strongest line" in the pattern of each impurity phase.
Given similarity in these two ratios it seems likely that the other phosphate impurity phases e.g. Calcium Oxide Phosphate will scatter similarly.
Having established the relative scattering power of each phaseof interest, it only remains to determine the most convenient peaks or peaks for the routine quantitative analysis. For lowerlevels of impurity, we inevitably must select the strongest peakfrom each impurity phase; however it is convenient to use the(210) reflection from HA (18% relative intensity) as it isadjacent to the impurity reflections and clear of overlap. Usingthe general equation given above and applying it to the system, gives;
% Beta TCP = 1.25 (Ib100 /IH18) x 0.18 x % HA
and
% Alpha TCP = 1.43 (Ia100 /IH18) x 0.18 x % HA
Where % HA is the % crystallinity of the HA in the sample(relative crystallinity), derived by the standard XRD experimental method.
Ia100 is the integrated intensity of the strongest (100) peak from the alpha phase;
Ib100 is the integrated intensity of the strongest (100) peak from the beta phase;
I H18 is the integrated intensity of the HA (210) reflection.
The integrated intensities are measured as follows:
IH 18 from 28.5 degrees to 29.5 degrees allowing flexibility to include thetails of the peak if needed.
Ia100 is at around 30.7 degrees and I b100 is at around 31.0 degrees; the area needs to be determined, taking into account the backgroundfrom the tails of strong HA reflections.
The experimental data should be obtained using the step-scan program given in appendix A(2)
This method indicates that :
a and b tri-calcium Orthophosphates may be quantitativelyanalysed in HA materials down to a level of around 2% w/w.
Between 1% and 2% the quantitative outcome is that we can state that the impurity level lies within this range, but the difficulty of accurately excluding the background from the integrated intensity measurement precluded an accurate % being calculated. The phrase "trace of a (or b) tri-calcium orthophosphate" may be used . a similar phase may be used for Calcium Oxide phosphate by implication.
Below 1%, identification of the phase itself becomes unreliable and the phrase "minor traces of other phosphates at the 1% orless level" can be used.
All scans for Cu Ka radiation.
| Scan count time per step | Step size | Start Angle: | Finish Angle; | |
| Scan 1. HA : HA/TiO2 | 5 Secs | 0.02 deg | 20 Deg | 40 Deg |
| Scan 2. Relative X-stal & other phases | 4 Secs | 0.02 deg | 20 Deg | 38 Deg |
| Sample | HA | Beta TCP | Alpha TCP |
| 1. White label whitlockite | 0% | 182 x 0.069 =12.56 =82.9% | 15x0.15=2.25 =17.1% |
| 2. White label whitlockite + HA [50:50] | 110x0.089=9.79 =51.9% | 101x0.066=6.67 =44.0% | 10x0.095=0.95 =7.2% |
| 3. White label whitlockite + HA [10:90] | 177x0.093=16.46 =87.2% | 25x0.060=1.5 =9.9% | 2.4x0.12=0.29 =2.2% |
| 4. Blue label whitlockite | 0% | 121x0.066=7.99= 52.7% | 98.8x0.063=6.22= 47.3% |
| 5. Biotal whitlockite + HA [50:50] | 114x0.083=9.46 =50.1% | 101x0.072=7.27 =51.0% | 0% |
| 6. White label whitlockite + HA [5:95] | 149x0.095=14.14 =95% | 9.7x0.056=0.54 =4.6% | 0% |
Samples 1-4 were used to deduce the calibration factors for quantification; these factors were then used to quantify the phases in samples 5 & 6.
PLASMA BIOTAL LIMITED
STANDARD OPERATING PROCEDURE SOP 046/00 Standards & Quantification in HA Materials
Work by Staffordshire University
Issue A, Date 01/01/97 Author:- Craig Adam. Transfered to Web Site by PD.Steverson.
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