Elemental Concentration

This Lab is capable to analyze all types of geological materials including rocks, minerals, natural fluids and various type of organic materials including crude oil, biological tissue, and blood with high precision. The analysis can be done in solution mode after digestion or in-situ with LASER-Ablation.

1) Solution Mode

For solution mode analysis, the samples are introduced into the ICP-MS, ICP-OES or MP-AES in aqueous or organic solution form. The solution is aspired and conveyed into a nebulizer by a peristaltic pump. There, it is mixed with argon and then introduced inside the torch for ionization. Automation of the sample introduction process is made possible by the use of an autosampler.There are several advantages of solution mode including but not limited to: easy matrix match between sample and standard; flexibilty on dillution factor or enrichment factor; capability to use spikes etc. Thus solution mode tend to have better precisions and can be used for all kinds of material.

However, in most case, sepecialized sample preparation is required to convert the samples into solutions that are suitable to be introduced into the instruments. Here are a few general considerations for sample prepation.

Clean Reagents

To get the best detection limit and precise analysis, clean reagent should be used for sample preparation (Milli-Q H2O, Distilled Acid or trace metal grade acid). The users are responsible to provide a procedure blank as a quality control for the sample run.

No Particles (100% Clear Solution)

Samples must be free of particulates that could clog sample line tubing and the nebulizer. It is sometimes necessary to conduct filtration or centrifugation in advance so that samples contain only soluble metals.

Sample matrix in 2% HNO3 if possible

It is highly recommended that sample solutions match conventional ICP lab standards (typically 1-2% nitric acid). The laboratory blank acid is 2% HNO3 (v/v), similar to the matrix of most calibration standards, and this acid may be used to make appropriate dilutions in our lab. Increasing acidity and/or salt concentration beyond calibration standards will result in lower intensities.

Proper Dilution to Meet the Best Instrument Working Range

The optimum working range for ICP-MS and ICP-OES/MP-AES are 10-100 ppb and 1-5 ppm, respectively. Though we can work with a much broader range from ppt level up to few hundred ppm level. If you do not know the approximate concentration of your samples, it may be worth running a few representative samples over a range of dilutions (10,000 – 1000 – 100 – 10X) to see what works best. In cases where high and low concentration analytes are both of interest, running samples at two separate dilutions may provide the best results.

Minimum Sample Volume

A minimum of 5mL and 10 mL of appropriately diluted sample is requested for ICP-MS and ICP-OES analysis, respectiviely. This is sufficient for two or more analyses using conventional sample introduction with ~400µL/minute uptake rate, and also provides sufficient volume for making a matrix spike sample for quality control. We can also work with smaller volume samples (1mL) using a microflow nebulizer with ~100µL/minute uptake rate.

2) In-Situ LASER Ablation Mode

LA-ICP-MS is a useful analytical technique to derive spatial compositional information from solid sample surfaces. Spots or line scans are typically analyzed. The laser essentially acts as a high precision photon “jackhammer”, which generates a particulate aerosol that is carried to the torch and sample introduction system of the ICP-MS. Laser ablation energy is concentrated at a set focal distance, thus samples appropriate for LA-ICP-MS should present a flat uniform surface. Thin sections and polished slabs work very well and the laser cells are designed to accommodate such samples.

LASER ABLATION ICP-MS CAPABILITIES

  1. Laser Ablation ICP-MS (LA-ICP-MS) can be considered the trace element microprobe
  2. In-situ trace elements directly from solid
  3. Rapid, multi-element with sub-ppm detection
  4. Minimal or no sample preparation required
  5. Spatial relationship or phase association characterized at high spatial resolution
  6. Qualitative to fully quantitative
  7. Capable of < 5% RSD precision

However, the major challenges for LA-ICP-MS is calibration. Given a constant laser source, the ablation efficiency (laser coupling) can vary markedly among different materials. To compensate for this variation, the intensity ratio of an analyte to an internal standard in a sample can be normalized to that for a standard of known composition. Close match of the matrix between sample and standard is alos recommended to yield a good analysis. Thus the use of LA-ICP-MS typically require some method development from representative sample material from this point.

Zircon geochronology

Zircon can be analyzed in either grain mount or in-situ in standard polished thin sections. For defining magmatic crystallization ages, target precision of 0.5 to 1% (2σ) can typically be achieved for populations of at least 10 concordant grains requiring no common-Pb correction. For in-situ dating using thin sections, a minimum of 30 ablations is typically required to obtain coherent magmatic or metamorphic zircon populations. For detrital zircon studies, a minimum of 120 spots is recommended. For details of Zirocn dating please contactDr. Tom Lapen(tjlapen@uh.edu).