Highly Sensitive Nondestructive Rare Earth Element Detection by Means of Wavelength-Dispersive X-ray Fluorescence Spectroscopy Enabled by an Energy Dispersive pn-Charge-Coupled-Device Detector

Abstract

Detection of rare earth elements (REE) is commonly performed with destructive techniques such as (LA)-ICP-MS or coupled to a destructive sample preparation. When investigating unique geological samples, such as cometary, asteroidal or interstellar material from sample return missions or inclusions in deep-Earth diamonds, a non-destructive method is preferred. The presented non-destructive highly sensitive wavelength-dispersive X-ray fluorescence spectroscopy (WD-XRF) technique is designed to measure the L-lines of REE between 4.5 and 7 keV with a sensitivity down to the ppm level. REE fluorescence L-lines are often only separated by a few eV from neighboring XRF lines and cannot be resolved by an energy dispersive approach especially in the presence of transition metal K lines. In our spectrometer the characteristic X-rays emitted by the sample are dispersed by a fixed Ge(111) analyzer crystal over the active area of an energy dispersive pnCCD detector, enabling high energy resolution detection of X-rays differentiated by their corresponding Bragg angles. The use of an energy-dispersive 2D detector enables to simultaneously acquire XRF lines while eliminating any ambiguities due to potential contribution from higher order diffraction effects or other diffraction planes, and thereby increases the sensitivity by reducing the (scatter) background. This detection method shows an energy resolution of 12 eV for the Ti-Kα fluorescence line and has a sensitivity down to 0.50 ppm for REE L-lines. The method was optimized specifically for the non-destructive analysis of inclusions in deep Earth diamonds, yielding in situ quantitative information about up-to-now inaccessible elemental (REE) composition patterns together with the more abundant transition metals like Ti, Cr, Mn and Fe. This information is of great importance to decipher the role that deep Earth plays in the global carbon and fluid cycle.

Raman Identification of Inclusions in Diamond

“…Basic identification of mineral inclusions is often straightforward, especially by referring to the spectra of those minerals that have been found previously in diamond, or even those that are anticipated to occur in the deep mantle based on high-pressure experiments (Smith 2021). When used in conjunction with techniques to measure inclusion composition, such as EPMA analysis, or in situ with synchrotron-XRF (Laforce et al 2014;De Pauw et al 2020), Raman spectroscopy provides a simple way to confirm the crystal structure of mineral phases. The technique can also reveal hidden phases that might otherwise go unnoticed, such as molybdenite in sulfide inclusions (Kemppinen et al 2018), or invisible layers of fluid trapped at the interface between solid inclusions and the host diamond (e.g., Nimis et al 2016;Smith et al 2018).…”

2020 atomic spectrometry update – a review of advances in X-ray fluorescence spectrometry and its special applications

“…De Pauw et al 38 demonstrated a high-resolution method for the non-destructive detection of REEs . The characteristic X-rays emitted by the sample were dispersed by a fixed Ge(111) analyser crystal over the active area of a pnCCD detector thereby enabling the high-energy-resolution detection of X-rays differentiated by their corresponding Bragg angles.…”

Rapid quantitative analysis of rare earth elements Lu and Y in rare earth ores by laser induced breakdown spectroscopy combined with iPLS-VIP and partial least squares

“…The common used analytical methods for rare earth elements are neutron activation analysis (NAA), 9 inductively coupled plasma emission spectrometry (ICP-OES), 10 inductively coupled plasma mass spectrometry (ICP-MS) 11,12 and X-ray uorescence spectrometry (XRF), 13 etc. However, the above methods have more or less disadvantages.…”