Can XRF Analyzers Provide Accurate Results Through Plastic Bags?

Field Portable X-Ray Fluorescence (FPXRF) instruments are used extensively in various stages of mining activity from grass roots exploration to exploitation, ore grade control, and even environmental investigations. FPXRF is a surface analysis technique that is capable of analyzing more than 30 elements in handheld XRF and 42 elements in a portable field x-ray lab in seconds in any type of sample, from soil to rock chips and drill cores. In some cases, depending on the sample type and required accuracy, clients may want to prepare their samples by grinding and pulverizing them, and then assaying them in a plastic bag.

Depending on the thickness of bag and its composition, the number of x-rays reaching the detector will be lower than with direct assay. However, there are good correlations between lab assay data and portable XRF results. In such cases, calibration of the instrument using “cal-factors” is necessary.

In one particular test, pulverized samples from a sedimentary-hosted lead-zinc deposit were analyzed in plastic (0.06 mm thick or 2-2.5 mils, 38 samples) and the results were compared to their lab assays. Samples were crushed and pulverized in mill steel to 95% passing 105 micron mesh. The laboratory assay methods used were a combination of instrumental neutron activation (INAA) with a 30g aliquot and inductively coupled plasma optical emission spectrometry (ICP-OES), with a four acid, near-total sample digestion of 0.25 g. The portable analyzer used a PIN detector using a filter time of 30 seconds per filter with analysis through the sample bags returned from the lab. The analyzer was operated in different modes, including Mining and Soil.

Mining Mode is based on a Fundamental Parameters Calibration and relies on the detector’s response to pure element spectra, whereas Soil Mode uses Compton Normalization where scatter (Compton Peak) in spectrum is related to sample matrix. Mining Mode provides suitable assay data on samples tested in a plastic bag. Preliminary investigation of the samples and their assay data indicate that Soil Mode is not suitable due to high concentration of elements such as Fe, Pb and Zn.

Here are the concentrations of elements and Limits of Detection (LOD) for plastic bags using Mining Mode.

Element Symbol	Element	Concentration	LOD
Ag	Silver	<60 ppm	16
As	Arsenic	<70 ppm	88
Ca	Calcium	<11.1%	560
Cd	Cadmium	<1700 ppm	16
Cu	Copper	<1200 ppm	94
Fe	Iron	<40%	13245
K	Potassium	<1.7%	1032
Mn	Manganese	<2%	716
Ni	Nickel	<650 ppm	198
Pb	Lead	<9%	546
S	Sulfur	<35%	20954
Sr	Strontium	<300 ppm	44
Ti	Titanium	<4500 ppm	213
Zn	Zinc	<50%	252

Can XRF be useful when utilizing plastic bags? If an application does not need very low levels of detection, the analyzer can be calibrated to the sample through various media. This case study shows that for many applications, portable XRF analysis of samples through a plastic bag yields valuable and reliable data that can be used to make timely decisions in the field.

See the Effects of Paper and Plastic Bags on the Performance of Portable XRF Analyzers Application Note for more details, including methods, results, and spectra.