Tin Nguyen Tran The chemical formula for hydroxyapatite is Ca10(PO4)6(OH)2. Why do you think that you'd get a peak for CaO in the XRD analysis (unless it was an impurity)? How are you expressing your ratio (Ca + Mg)/P? Is this by molar or weight?

I systhesis hydroxylapatite by using (Ca+Mg)/P ratio by molar but EDS result show that (Ca+Mg)/P ratio is higher than 1,8.

i also calculate (Ca + Mg)/P ratio in EDS result by molar ratio but ratio is always higher than 1,8 with 5% , 10%, 15%, 20% Mg/(Mg + Ca) (%mol). Some authors said that they detected CaO peak in XRD if their ratio is higher than 1,67 but i can detect CaO peak in mine

What is the ratio with no Mg present?

Alan F Rawle Ca/P of 5% Mg, 10% Mg, 15% Mg and 20% Mg are 2,23; 1,95; 1,86 and 1,68 but (Ca + Mg)/p are 2,33; 1,70; 2,14 and 2,00

I’m not sure you understood my question. If no Mg is present then what is the Ca/P molar ratio? Should be 10/6 or 1.67 approximately.

My first question is how reliable are the EDS results? Why do you suppose that they are correct? Who did the analyses? How was the sample prepared? If it was just a loose powder, I would expect that sort of trend in the results.

EDS can provide good results when done properly. It assumes flat, polished samples. If the surface is not perpendicular to the beam but leaning away from the EDS detector you will lose both Ca and P signal, but you will lose P faster. You would lose Mg even faster than P, but that may be why your ratios are better when the Mg content is higher.

If you are scanning multiple particles, that does not average things out. The net effect is still as if tilted away. The faces tilted toward the detector do not make up for the ones tilted away.

Does your EDS system model the spectrum as it has calculated it. Our Oxford Aztec does. If the model deviates in background, the numbers will be off.

Can you forward a spectrum with the conditions for analysis?

First, I sincerely thank for your answer Mr Alan F Rawle.

My HA and Mg-HA powder are dispersed in ethanol before scanning by SEM/EDS so i think the surface is perpendicular to the beam

10 mg HA or Mg-HA in 2 ml ethanol

I have not performed EDS measurements with Mg-free samples.

Tin, I have been doing EDS from 1977 to the present. I assure you that the points made in my previous answer are a very real concern.

It sounds like you are using a powdered sample and I assure you the take-off angles are not uniform. The interaction volume is only about 1 um at 10 kV. If your particles are larger than that, you will have issues as I described.

Please try analyses from just the top of single particles - not from a field of particles. Compare the spectra and see how consistent the backgrounds and elemental intensities are. I expect you will find them varying, but because of geometry, not because of real compositional variation.

It would help to know more about your technique: voltage, particle size, analyzer, count rate, etc.

I find many users stock values that are far from optimum for your samples. For example, 10 kV is plenty for your HA. 20 kV leads to more absorption issues.

Perhaps I can post a real-life example, but for now, I need to examine some other samples.

Tin Nguyen Tran The first thing to do is prepare what should be pure hydroxyapatite - that is Mg free.

Mr Alan F Rawle, i appreciate your feedback , i will do with Mg free HA and respond soon

Mr Warren Straszheim, particle size is about 17 - 55 nanometer, voltage 15kV, analysis by SEM conected with EDS Bruker Quantax 75 from Germany

Mr Warren Straszheim, i aprreciate your feedback, maybe my SEM system have low resolution to observe particles below nanosize so EDS can not analyze single particle

FWIW, I got my Ph.D in 1986 and my career has largely been SEM-EDS and it's application to various materials.

It appears that I should ask for your SEM specs. If it is a field emission SEM, you should be able to resolve 15-nm particles, unless there is something binding the particles together.

What kind of substrate is below the particles? How thick is the deposit of particles? Do you see any sign of the substrate in the EDS spectrum?

I presume the sample is coated. With what? With how much? Otherwise, you would be building up quite a charge and that would affect the results?

I would think that 8kV would be enough for the analysis. 2-4 times over-voltage is usually prescribed so 15 kV is more than necessary.

Have you checked an apatite reference under similar conditions?

Again, an actual spectrum or spectra with answers to these questions would do much to help.

Mr Warren Straszheim, my SEM system is not FE-SEM. The substrate bellow particle is carbon tape and i detect spectrum of carbon in my result but i substract in final result. I dispersed my powder in ethanol with ratio 10 mg/2mL then droped it in carbon tape before scanning. The sample below is Mg - HA sample with initial (Ca+Mg)/P ratio for the synthesis is 1.67 in molar ratio but result show the ratio is 2,33

Dr. Straszheim says,

I think I can make out your individual particles of HA. I would not care to estimate their size at this magnification. I think you could push that SEM to 20kx or even 50kx and get a sense of the size, if you wanted to.

These particles are rough by EDS standards. EDS quant methods assume a flat, homogeneous volume for analysis.

* If your beam penetrates through and excites the substrate, you violate one assumption. (8 or 10kV would help reduce that.)

* If the sample is rough so that all x-rays do not undergo the same path length on escape, you violate another assumption. (I think this is the main point.

Probing just the top of your biggest and brightest particles MIGHT give you a more correct answer. Try measuring several such spots and see if the results are consistent. Compare those results with those from thinner areas or the overview and see how the results change. (They will!)

I advise users to make the sample correctly and do the analysis right in the first place. There is a lot of physics that goes on that is accounted for in the quant routines. It is dangerous (i.e., wrong) to try to make up for errors later by ignoring elemental peaks that are plainly present. The answers will be wrong, but maybe they will be good enough. Since you are asking the question, the EDS results do not appear to be good enough.

I repeat my question - have you probed a reference, bulk sample of HA to know that EDS gives the right answer? Do not assume that it will. Some companies do better on some systems than others. Some have better standardization. If you probe a sample of KNOWN composition under IDENTICAL conditions (thickness, angle, voltage, etc) you may be able to use the known sample as a reference to correct the results from your samples.

EDS is often dismissed as an unreliable technique compared to WDS. I do not agree with that statement completely, nor would Dale Newbury. _IF_ the same care was used in EDS as is used for WDS and EPMA, EDS could give good results. The fact of the matter is that EDS is often done "quick and dirty". You cannot expect much of any technique in that case.

i sincerely thank your answer Dr. Straszheim