Sample Preparation and Analysis

A rock sample collected from the field, in this case a basalt (A), is broken into small chips (B) less than 1 cm diameter with a hammer or rock chipper called a jaw crusher. These chips are then pulverized into a fine powder (C) using a shatterbox. We use this powder for the analytical technique.

A small amount of rock powder, typically less than 0.1g, is carefully weighed in a teflon digestion vessel. In this photo an undergraduate is weighing a sample of powdered andesite collected from a volcano in New Zealand.

A mixture of hydrofluoric, nitric, and hydrochloric acid is added to the powdered sample and sealed in the teflon container.

The sealed teflon containers are placed on a hot plate for times in excess of 12 hours to aid in the sample digestion process. As a result, the powdered rock sample is completely dissolved into its elemental components.

The dissolved rock sample is loaded onto a quartz column filled with chromatography resin. The elements which compose the rock variably adhere to the resin depending on their respective absorption coefficients. As a result, the elements are separated from each other and the element (or elements) of interest is isolated and collected in an aqueous form. The host liquid is then evaporated on the hot plate and the element of interest is concentrated as a solid in the base of the collection vessel.

The pure, solid concentrate is then re-dissolved in 1-2 microliters of acid and loaded onto a filament.

Ten loaded filaments are put into a barrel assembly and the barrel assembly is then put into the source of the mass spectrometer.

The source of the mass spectrometer is evacuated of atmospheric gases and the filaments are heated. The heating of the filaments causes the loaded element to ionize. The ions form a beam and are accelerated down the fight tube of the mass spectrometer.

The ion beam, which is composed of all the respective isotopes of the element of interest, is subjected to a intense magnetic field. The magnetic field has the result of defecting the path of the ions depending on the mass of the isotopes that comprise the ion beam. Lighter masses are deflected more, heavier masses less. This results in the generation of multiple ion beams formed from the single originating ion beam. Each individual resulting beam is composed of ions which only have one specific mass. These equal-mass beams are then allowed to continue traveling to the rear of the machine.

The mass-separated ion beams are then directed into individual faraday collectors and the charge resulting from the impingement of the ions on the individual collectors is measured. The NMSU machine is equiped with seven faraday collectors and a single Daly photomultiplier.

The relative charges (or intensities) are then compared and the data is compiled by a computer and operator. The results can be used to date rocks, geologic events, or trace the processes or sources giving rise to the rock samples analyzed. Analyses of Sr and Nd isotopic composition, as well as U isotope dilution are performed on the TIMS at NMSU.

Attaching digital volt meters to source feed lines on the TIMS helps isolate electronic problems. For example, if a voltage generation card in the programmable focus unit is unstable, it prevents optimization of focusing parameters which can result in bad peak shapes. An unstable voltage generation card can also cause the accelerating voltage to destabilize, resulting in unstable beam intensities. Pictured below are examples of a bad and good peak shape. Provided that the ion beam is well focused and the accelerating voltage is stable, the magnet position generally controls the peak shapes at the back end of the machine.

Example of a bad peak shape. Notice the rounded corners and assymmetry of the peak

Example of a good peak shape. Here the corners are sharp and the peak is symmetrical

Graduate student Corey Dimond conducting analyses on the thermal ionization mass spectrometer in the new analytical lab at NMSU.