Methods & Instrumentation
X-Ray Fluorescence (XRF)
XRF is a measurement technique that uses x-rays, an energetic portion of the electromagnetic spectrum that includes microwaves and visible light, to determine the elemental makeup of materials ranging from rocks to minerals to soil.
In an XRF spectrometer, high-energy x-rays are used to excite the atoms within a sample causing those atoms to emit x-rays of their own. These fluorescent x-rays are characteristic of each element, such as iron, sodium or copper, present in the sample. The x-rays that are counted by the instrument’s detectors per unit of time (intensity) are then converted to element concentrations via a computer algorithm.
Learn more about the underlying principles and operation of XRF spectrometers, courtesy of Bruker AXS.
The Thermo ARL Perform’X is a latest generation sequential x-ray fluorescence spectrometer. Routine operation uses 45 kV accelerating voltage at 45 mA. A normal, 29 mm dia, analysis requires approximately 2 hours to measure a suite of 44 elements (majors + traces). A programmable aperture allows analysis of small 15 or 5 mm dia. glass disks. Each sample size requires a unique calibration. The instrument is recalibrated on about 10 month intervals utilizing ~70 certified reference standards to build a normal calibration. Drift is monitored with in-house standards that are run several times per week. At least one international standard is run with each sample batch. In addition, at least one duplicate is prepared and analyzed for every batch of ≥ 10 samples to check for sample homogeneity and reproducibility of data.
The Perform’X goniometer is gearless and has a very short slew time (time between successive measurement settings). The power supply is cooled with the tube cooling water to maintain stability. Analyzing crystal temperatures are maintained with heaters at 43.00 +/- 0.02 °C. The combination of mid-range power and thermal stability makes for an exceptionally stable platform: to date the instrument has not required drift correction. A dual sample turret design allows the spectrometer vacuum to be maintained at a near constant 2.0 Pa even during sample changes. Thermo-ARL XRF spectrometers have very low backgrounds due to their unique optical alignment design wherein the tube axis is not co-planar with the sample - collimator - analyzing crystal axis. The low background improves signal to noise ratio for all elements and allows determination of several elements not normally measured with XRF in geologic samples, e.g. Sm and Dy. Our instrument is equipped with a He flush for the analysis of liquid and powdered samples. The spectrometer is equipped with LiF200, LiF220, Ge111, PET, and two synthetic long wavelength analyzing crystals for elements with atomic number down to Z = 5 (B). Currently under development are analytical protocols for powdered sediment and soil samples.
Most XRF data at HAL are acquired with the low-dilution fused bead method using graphite crucibles. Samples are chipped to pea gravel size in a hardened steel chipmunk, sub-sampled with a line pour if necessary (most metamorphic and plutonic rocks), and ground to a very fine powder in a Rocklabs tungsten carbide (WC) or alumina ring mill. The powder, normally 3.5 gm, is weighed with a Li-tetraborate only flux (Merck Spectromelt A-10) in the proportion 1 part rock to 2 parts flux. Raw powders are preferred for fusion beads due to the potential for damage when pre-ignition is employed. For samples with very high concentrations of Fe, Mg, Ca, Mn, or P, dilution with pure fused silica is employed to prevent crystallization of the glass upon cooling. Rock powder and flux are blended using a vortex mixer and fused in custom-machined graphite (Mersen grade UF-4S) crucibles at 1000 °C in an electronically controlled muffle furnace. The fused pellets are cleaned of residual carbon and then reground to fine powder in a WC ring-mill, and fused again at 1000 °C. The doubly fused pellets are lapped flat in four stages with progressively finer diamond laps to a surface finish of 15 microns. After an ultrasonic cleaning in ethanol, the pellets are ready for the spectrometer. We are calibrated for both full-size (29 mm dia.) and half-size (15 mm dia.) pellets, the latter requiring 1.0 gram of powder. Analytical regimens for 5 mm (50-200 mg samples) and 0.5 mm glass pellets (10-50 mg samples) are on the drawing board.
The advantages of the low-dilution, graphite fusion method are several:
- A single sample preparation suffices for major, minor and trace element analysis.
- The large sample mass, full-size = 3.5 gm, reduces the potential for nugget effects.
- Combining the determinations of trace, minor, and major elements allows for complete spectral interference correction. This is especially important when the concentration of "traces" or “minors” increases to minor or major, or when Compton tailing from major or minor element peaks interferes with trace element peaks or backgrounds.
- Complete inter-element matrix corrections including both absorption and enhancement terms can be calculated with no approximations.
- Low dilution and the high heat capacity of graphite produce robust pellets that are stable for decades.
- graphite is chemically inert and does not react or alloy with molten rock/flux mixtures.
Ignition loss is measured for all submitted samples by heating overnight in silica crucibles at 900 °C.