pXRF has many applications both within and outside of mining and mineral exploration. In no industry has a vendor ever told a client to take the pXRF, point, shoot, and voilà – you now have a robust dataset that is ready for analysis.
This post both builds on our prior pXRF post (where we talked about QA/QC) and finishes our glorious month of posts on orientation surveys (what is an orientation survey, residual soils, and rocks). In our final installment, we honor you all with some objectives and steps to design your pXRF orientation survey.
Some quick notes first:
- pXRF studies (like all studies) need to be thought out ahead of time. Is the goal to map stratigraphy, vector to mineralization, or select samples to send to the lab? This will guide how you set up each part of this survey.
- Understand beforehand the elements that are of interest to your program. Like every instrument, the pXRF has limitations, for example, due to spectrum overlaps there can be false positives and false negatives in your data (Au is affected by Zn and As/Co are affected by Fe).
- XRF results are influenced by the sample itself. The sample container/presentation, films used, particle size, sample moisture, heterogeneity, and sample matrix can all influence results.
- Be mindful of where you shoot! Are you doing a vein study? No? Well then don’t collect data on the veins, make sure you point at the matrix.
The pXRF is delivered with a default calibration. However, depending on your objectives, sample media, rock type, etc., it may be important to change these settings for your program. Indeed, this is why you should always conduct a pXRF orientation survey!
- Dwell time (no need to dwell for 180secs if 60secs will do!)
- Is calibration required for specific elements of interest? You can use standards or wet chemistry data from your project to help with this. There’s a great video showing one method of calibrating your pXRF that you can watch here.
- Which ‘mode’ are you working in? Soils, mining…? This differs for each manufacturer but generally speaking we are talking about targeting trace elements or higher grade elements (i.e., ppm v. percent levels), as well as whether you are interested in targeting lighter elements. It pays to put in the time to investigate detection of your elements of interest in your sample matrix.
- Where are you going to collect your point data from? It is important to understand the effect of heterogeneity (do you have a lot of different rock types or veins like in the beautiful core seen at the top of the page?), moisture, particle size, different sample containers, etc.
- Once you have calibrated your pXRF properly, write a standard operating procedure (SOP) and give a training to ensure that the program continues to run smoothly when handed over to junior geologists or techs.
We again refer you to our blog post on pXRF QA/QC to supplement this post, but as a quick reminder, use matrix-matched certified reference materials (MM-CRM) as your standards, in addition to using blanks and duplicates the same as you would for every other geochemistry program!
Need we say it?… garbage in, garbage out… don’t fall victim to collecting poor pXRF data, especially considering that its collection is labor intensive and therefore not an insignificant amount of money is spent to collect it.
Thanks for enjoying our Month of Orientation Surveys Series! We need a break, but let us know what more in the world of spectral and geochemistry you would like to hear about next!