XRF Lab Procedures

The Earth Science X-ray Laboratory at Southern Connecticut State University produces chemical analyses of geological materials using wavelength-dispersive X-ray Fluorescence (WDXRF) spectrometry using both fused glass disks and pressed powder pellets.

Loss-on-Ignition (LOI):

Approximately 8-10 grams of powdered rock sample is placed in a pre-weighed 10ml porcelain crucible (Coors 60104) . Crucibles with sample powders are weighed at room temperature, and after heating to to 110 C (for two hours) and 1000 C (for 1 hour). For samples containing significant organic material (shales, sediments, soils) or carbonate (lime-rich rocks and sediments) additional heating steps can be included to estimate the proportion of organic carbon (550 C) and or carbonate (950 C).

Fused glass disks:

Fused glass disks are produced using 0.8000 grams of pre-ignited sample powder (from LOI procedure) is mixed with 5.5000 grams of a premixed anhydrous flux consisting of 35.3% lithium tetraborate and 64.7% lithium metaborate (eutectic composition). One drop of LiBr is added to the sampe-flux mixture in order to help release the glass disks from the Pt molds. The flux-sample mixture is fused in Pt- 5%Au crucibles using a Katanax K1 fusion system which melts samples at approximately 1000 C. Glass disks are cast in a 31mm Pt-5%Au mold. Calibration parameters have been determined by two different methods using either geologic standards (GEOM) of known composition or by using synthetic standards derived from pure chemical reagents (WROXI). Comparison of results for the USGS Standard Basalt BHVO-2 are given in the table below. Because of the large dilution of the original sample that occurs during production of the glass disks, trace element concentrations are better determined by the pressed powder pellet technique described below.

Pressed powder pellets:

Pressed powder pellets are produced using 0.900 grams of a binder material (N, N –Ethylenebisstearamide, Primier Lab Supply Pelletblend powder) and 4.000 grams of very finely ground sample powder. The mixture is blended in a glass vial and pressed at 15 tons in a 32mm stainless steel pellet-die. The pellet-die surface is covered with a 6 mm mylar film to avoid metal contamination (Fe, Cr, Ni) from the die surface. Elements heavier than Cr are calibrated using an extended Compton model based on the intensity of the Compton radiation scattered from the sample by the Mo secondary target. Elements lighter than Cr are calibrated using a model based on a combination of fundamental parameter and extended Compton methods. Pressed powder pellet results for the lighter elements, in particular those < Si, are only considered qualitative because their accuracy suffers significantly from particle size and mineralogical affects associated with the pelletized powders. The lighter elements (most major elements) are better analyzed quantitatively by fusion techniques described above.