Digestion procedures

Instrument operation and analysis

Other information

Analytical capabilities

Our instrument is a PerkinElmer/Sciex Elan 6100 DRC. It is capable of analyzing about 70 chemical elements in any sample that can be put into aqueous solution. Detection limits and analytical precision vary widely with analytical protocol, instrumental response, blank contamination, interferences, matrix composition, and isotope abundance, but ranges from <100 ppb to <1 ppt (parts per trillion) in aqueous solution. Analytical relative deviation is typically <3%, one standard deviation.

Brief notes on sample preparation

Choosing internal standards

Internal standards are valuable for routine analyses because they allow correction for some aspects of instrumental drift. You can choose any internal standard elements you want, but consider the following:

Internal standards typically used are:

Containers

If possible, all bottles, volumetric flasks, pipettes, and other things that hold the liquid solutions should be made of plastic, not glass. Plastic ware should be thoroughly washed and pre-leached in 20% reagent grade HNO3. Solutions should never be stored in glass.

Digestion containers

We use a wide variety of digestion containers, including graphite crucibles for LiBO2 fusions (note: not permitted with our present instrument), 17 ml Teflon Savillex® containers for low-pressure acid digestions, cheap polypropylene or polystyrene test tubes for easily dissolved samples like calcite or phosphate shells, and 30 ml Teflon Picotrace high-pressure digestion vessels. The graphite crucibles, with 1/4" walls, last 6 to 12, 10 minute fusions at 1000°C. The cheap test tubes are generally used only once, but the Teflon containers seem to last indefinitely.

Reagents

Reagents should preferably be of "high-purity" or "ultra high-purity" grade, not the commonly used "reagent grade". 18 MOhm deionized or double distilled water should be used.

Acids

HNO3  is preferred because it generally causes the least problems with polyatomic ion interferences (a major exception is 28Si). HCl, H3PO4, HClO4, and H2SO4 cause no problems with the instrument, but do result in in a variety of polyatomic ion interferences (e.g. ClO+, ArCl+, SO+, PO+). If these acids are used in your sample prep procedure, you need to make sure these new interferences will not compromise isotopes of analytical interest. The dynamic reaction cell can potentially reduce some of these interferences, but this benefit is limited to only certain polyatomic ions (e.g., ArCl+) and generally requires gasses other than ammonia in the dynamic reaction cell (which is what we have). HF can be used so long as it is largely removed prior to analysis so it does not damage the glassware.

Cleanliness

Although clean room facilities are not strictly necessary for preparation or routine samples, it is extremely important to keep the work area clean, espcially free of dust, including dust falling from above. We have found that pre-soaking all containers in 20% reagent grade HNO3 followed by rinsing with high-purity water eliminates many contamination problems. Don't assume new plasticware is free of contamination (e.g., polypropylene test tubes with Pb or Cu and Zn contamination). Teflon digestion vessels should be cleaned under the same conditions dissolutions are done.

Standards

Use standards as similar as possible to the unknowns. Because of the linear response of the instrument over 6 to 8 orders of magnitude (ppt-ppm), use of a blank and one well-characterized standard is possible, though perhaps not wise. It is wise not to use standards having element concentrations vastly different than the unknowns.

Timeliness of sample preparation

Most elements, including the rare earths, seem to be stable in dilute acid solutions forever. Other elements are not stable, depending on the composition of the host solution. For example, elements such as Ti, Nb, and Ta tend not to be stable in solution, though a little fluoride stabilizes these. Time also allows contaminants in the storage containers to leach into solution. In general, it is better to analyze samples quickly rather than waiting a long time

Sticky elements

Certain elements are "sticky" in the ICP-MS sample introduction system. The offenders I have come across can include Li, B, platinum group elements, Ag, Au, Hg, Nb, and Ta. Some can be rinsed from the sample introduction system by extended (>1 minute) wash times between samples with 10% HNO3. Others can be washed out using different acids or higher acid concentrations. Some require more exotic wash solutions.

 

The ICP-MS described here was funded in part by the National Science Foundation, grant DUE-9952410.