Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-03-23T09:37:54+00:00
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Rhodium Rh Analysis by XRF (11 replies)

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

Is it possible to analyze/assay rhodium in samples if your WD-XRF has a Rh target in the tube? I am presuming that it is not possible unless one has a tube with a different tartget element. If anyone has insight on this, it would be appreciated.

Helena Russell
1 year ago
Helena Russell 1 year ago

We us a W filter will eliminate the Rh signal from the tube anode in our ED-XRF, and analyze for Rh at about 55kV tube voltage.

John Koenig
1 year ago
John Koenig 1 year ago

We used the Rh tube with a 300 micron brass filter for Rh analysis down to the low ppm levels. Let me know if you have some additional questions and perhaps I can help out.  But really it should be OK as Rh will be a background and can be subtracted as a background. So you will need a blank sample.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

The ideal filter for a Rh tube is Mo. The Rh K-alpha lines are at 20.0216 and 20.073 keV, while the Mo K-edge is at 20.002keV. By being just slightly below the emission energies, the Mo K-edge is situated to provide the maximum absorption of the Rh lines coming out of the tube. Any other filter will be less efficient at selectively reducing the Rh lines without knocking out any more of the Brehmstrahlung above the Rh K-edge than is absolutely necessary. For catalyst that contain Rh, the problem is fairly extreme, where very low loadings are common (current market price of Rh is ~$1100/oz, but in the recent past, it's been dramatically higher). Using a different tube anode is a good solution too.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

The brass filter using the procedure described by Lars works well. I have done it myself many times. The brass does a good job at scrubbing the tube lines if you have enough signal from the Rh in the sample. You should be able to do this on your system. Let me know if you want any help.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

Thanks for the comments from everyone. I tried scans of a material spiked with Rh at ~2 ppm, using the brass filter and could see no peak. I will try with a higher concentration tomorrow. Any guidance on the detection limit for Rh would be appreciated.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

What's the matrix you are using?

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Can you also share the excitation and measurement conditions you are using and a little about the spectrometer set up you are using ?

My recollection is that in an Axios system typical settings for Rh K alpha using a Rh target tube (Super Sharp) would be 60 kV & 66 mA (~ 4 KW tube power) for the tube operating/excitation conditions. 300 micron brass filter. PX10 crystal. 300 micron collimator and Hi-Per Scint detector.

At lower excitation power the 300 micron brass filter could be too thick.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

The matrix is mainly glass. I am making a pressed powder pellet with cellulose: 4g sample: 0.5 g cellulose.

You got some of the conditions correct. Our XRF does not have the PX10, so uses the LiF220 and a has 100 or 400 micron brass filter. The 150 micron collimator is being used.

With the 100 micron brass filter, the 2 ppm sample and the blank display the same peak intensity, which is significant. With the 400 micron brass filter, no peak is seen for either the blank or 2 ppm sample. I have not attempted to run a sample with higher Rh, but will still do this. If anyone can share an estimate for how low the Rh concentration can be using my conditions, I'll start there. I have ICP standards I can use to make a test sample.

I recently ordered several standards which are automotive catalysts. These range from 135 to 512 ppm, so if samples in this range yield a signal that can be quantified, that will suit my needs. I know a ED-XRF Model XDV-SDD can do very precise Analysis of Rh, Pd and Pt in Automotive Catalytic Converters.

JohnnyD
1 year ago
JohnnyD 1 year ago

You should see a quantifiable Rh peak in the concentration range 20-50ppm on the WD-XRF.
Surprisingly our small 50W ED-XRF, also using a Rh tube and a filter is much more sensitive despite also having the tube lines filtered out, you can detect 2-5ppm, compared to the 20-50ppm on the 4kW WD-XRF. The other big problem is the resolution on the WD-XRF is not great in that energy range, you will have overlaps between Rh and Pd which is something you need to look out for with autocats. On the ED-XRF resolution in that energy range is fantastic. You should be able to quantify your autocats using the WD-XRF though.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

It is difficult to see a peak of any element at 2 ppm level on a scan. But it is possible to make a quantitative analysis at ppm levels using Axios WD systems. The key point here is in a counting statistics. During a scan, usually, the spectrometer records the intensity at any point in less than a half of a second while in conventional/classic analysis you perform measurements for tenths or hundreds of seconds in the peak and background positions. As a starting point about the statistics in XRF you can read help files of SuperQ, in particular chapters describing CSE and LLD. Otherwise, you can seek for a qualified assistance and training. It may dramatically boost you knowledge in a short time.

Besides, I agree on the proposed conditions. In your case it may be:

  • Brass400 - LiF200 - 150 micron collimator - (Hyper-) Scint detector and of course the largest possible sample mask.
Bill Fraser
1 year ago
Bill Fraser 1 year ago

The comments by the contributors to this discussion do a good job of highlighting the advantages of both the WDXRF & EDXRF systems. Among these are the flexibility to optimize the instrument parameters to the overall analytical task at hand and the ability to achieve good results with different instruments and configurations.

For example, in catalyst correlation studies performed with other labs including some of the automotive OEM’s we experienced good results on WDXRF instruments with Cr, Mo & Rh target tubes. We were also successful in getting good correlation between WDXRF and EDXRF.

The explanation about the use of a Mo filter is “on point”. A Mo filter would be very good for Rh analysis. I’m not sure if Mo is a standard offering in the filter line up from any of the major WDXRF suppliers so that might be a specialty or custom item and worthwhile investigating. Perhaps someone in the group can comment on the availability of Mo filters for WDXRF units.

Mo target tubes are also available. Our experience with the Mo target tubes was that these tubes were a good bit more expensive and at times availability was a bit of a problem.

With the improvements in spectrometers over the years we observed that in our applications the technical advantage of the Mo target tube over the Rh target tube for Rh determinations diminished to a point of being negligible. In some applications we observed better Rh detection limits using the Rh target tube.

With both the Mo and Rh target tubes we experienced very good sensitivity and reproducibility that provided calibrations that extended down into the single ppm levels.

In the case where we needed better resolution we used a finer collimator (150 micron) which was mentioned in posting. In general, when we did this we did not have to sacrifice much at all in terms of detection limits.

Our experience with EDXRF demonstrated that good sensitivity and calibrations were achievable. On the instruments which we used and tested the counting times to achieve good reproducibility for all the elements in our samples (and in particular in the Rh concentration ranges where we were working) were a limiting factor for a lab with very high sample throughput.

For labs that have both WD and ED XRF they do a nice job of complementing each other in terms of the range of analytical capability and supporting method development and investigations.

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