Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-04-04T06:57:57+00:00
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pXRF Portable XRF for Mining and Soil mode (6 replies)

David Kano
2 years ago
David Kano 2 years ago

For those who are not aware of this choice, the Mining mode of pXRFs (Handheld Portable XRF) is not supposed to fit the needs of the Mining activities, nor the Soil mode the needs of Environmental activities. The Mining mode is expected to offer a better calibration when some elements (Fe or ore) reach a high level (5% for Fe, for instance).

The Soil mode is expected to offer a better scan of trace elements when diluted in a Si-Al matrix. I would be happy to have your returns of experience about using both modes, either for mining, exploration or environmental surveys. What are your own findings about this?

John Koenig
2 years ago
John Koenig 2 years ago

It is a really good question. And you have the answer there already. I have found that in most cases it is not common to satisfy requirements of the Soil Mode, so I have used it rarely. I prefer to use Mining Mode unless the sample has really light matrix. The other important thing is that the user should not jump from one mode to another in analyzing batch of samples as this can be cause of error.

Some portable XRF manufacturers have combined Mining and Soil modes into one mode (such as TestAll Geo). I prefer to use this mode when I analyze unknown samples for the first time so I get an idea about overall composition of the sample; then I decide to use either Soil Mode or Mining Mode. And, as you know, still in Mining Mode you can report data in ppm or %.

Helena Russell
2 years ago
Helena Russell 2 years ago

A classic question! I would add that soil mode (compton normalization) does not measure light elements and mining mode (fundamental parameters) will analyze the lights- Mg, Al, Si, P.  Also, if your looking for high accuraccy opposed to screening or trending, analyzer bias corrections or calibration factors can be built if you have a set of known standards which match your unknown samples matrix.

David Kano
2 years ago
David Kano 2 years ago

It seems our XL3t does not provide any provision for Calibration Factors or bias correction. Only on some models ?

Bob Mathias
2 years ago
Bob Mathias 2 years ago

To over simplify, use soil mode where you want ppm detection and geochem/mining modes (fundamental parameters where you want percent detection >1% and sometimes >5% - depends on the matrix). To put it simply (over simplification). The two modes can sometimes cross over in effectiveness between 1 and 5% but this very much depends on what is in the sample.

Soil Mode calibrations are based on the expectation that there is a lot of lighter elements like SiO2 in the sample. What this mode does not do is take into account inter-element effects that tend to happen in percent-level concentrations. Soil mode uses 'compton normalization' and geochem/mining modes use fundamental parameters (you can look this up on the internet if you like) - one has the expectation of a lot of light element contact as highlighted in a previous comment and geochem takes in the total content. Yes, in some cases you have to use soil mode and Geochem but you need to do some orientation on your samples to determine this you can commence to get a lot of good, valuable data very quickly from that point on - it depends on your needs for detection. You really do need to put in the time and investigate detection on your sample matrix. As for site specific calibration, you can post-process your site-specific calibration(s) on your raw data - then you will always have your raw data to go back on.

Geochem or mining plus/mining modes compensates for expected inter-elemental effects seen in higher concentration matrices and, where Mg, Al, Si are important this becomes an issue. While ppm detection is possible here, if you're mainly after great ppm-level detection, use soil mode to get there.

There is literature discussing this in your XRF manuals, the applications departments of your XRF manufacturer are a great resource (not just the support department but the 'applications' scientists) - you paid for the instrument so don't be afraid to use these people, EPA Method 6200 and other papers discuss this in some detail though some of the detection information in that document is a bit outdated; the concepts remain very useful. Also please do not confuse the old "testallgeo" with geochem mode. These are very different though I know the manufacturers intent was to approach the same goal. My opinions would only seem biased so please contact your instrument applications department for more detail. If you have problems, send them your raw data file and spectra files as per their instructions - let them assist you.

If you put in a bit of work up front and work with your manufacturer, you will receive invaluable data (plus an education). Geochemistry is not a point and shoot application as some of you point out - most of the issues with detection in pXRF have to do with the education of the user and the sample. The reason this group was formed was to help in this via the general community. Thank you so much for posting your questions and your best replies - whether accurate of misinformed - it puts us all to the same end goal and contributes to our solutions.

David Kano
2 years ago
David Kano 2 years ago

Thank you all for these first contributions, more contributions are welcome of course. Thank you Cindy for your post, which confirms how far the manufacturer's development staff can be invaluable partners in building our own solutions. I feel myself halfway between them and users, because I provide custom applications and expert support to end users, but with lots of help from manufacturers.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

Bob makes a very good point to check with the vendor since some like Bruker have different names for these as well as different functionality. Having used a NITON XL3T as well as a Delta in addition to the TITAN I am aware of the other vendor modes and their differences. Especially with the CAMIRO test it was clear that neither pure compton nor pure FP was/is the answer....

E.g. BRUKER soil mode is designed as a hybrid between "classic" Compton Based correction and FP as well as empirical corrections. This is why it requires powdered samples with Prolene foil to measure the light elements as well. The EPA 6200 elements are all Compton corrected and used in Phase/Beam1 so a classic approach using bags and type standardization can be done.

The best results for all GEO methods (GENERAL and TRACE) are obtained when using the dual phase mode and measuring the light elements. As one can imagine the homogeneity of the sample is important.

It is important to repeat that measuring soil and mining samples NEVER is just a point a shoot method but requires a little more expertise than the alloy measurements. Sample preparation is required for best results in addition having correctly sampled ....

If you want quality data you will need to calibrate the unit to the task, the same way you would for a benchtop or big XRF.

A very good resource is the Australian company which has some good papers on their website detailing this issue as well as the sampling. They are currently doing a lot of work with Pb in soil sampling.

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