DEVELOPMENTS IN MICROWAVE
DIGESTION FOR GEOCHEMICAL
ANALYSIS
Brad Whisson
AIG Symposium, Bali
May, 2013
Mother, should I trust the government...
The “Master and Commander” series – Patrick O’Brian
“Wit is the unexpected copulation of ideas”
- Dr Stephen Maturin in Patrick O’Brian’s The Hundred Days
Background
• Microwaves:
ν= 300MHz – 300GHz
(λ = 1mm – 1m)
• Rapid energising of molecules
• Laboratory instrument features
• ~1400W output
• Corrosion Resistant
• Sealed pressure vessels (~20 bar)
• Pressure and Temperature control
• Recording of parameters
• Multiple safety measures
Technical Advantages
• Digestion time: 0.5 – 1 hour
• High P and T (~180°C, 18 bar):
• Not limited to b.p. of acids (HF~104°C)
• Acids more oxidising at high temp,
therefore dissociate matrices very effectively.
• Sealed vessel:
• More efficient use of acids
• Environmental advantages
• Sealed vessel retains volatiles
• Reproducible digestion conditions
Limitations
• Sample mass practically limited to <1g,
• a sampling issue for gold analysis
• Capital costs ~$50K - $60K per system
• Typical investment $120K to commence
• Limited capacity
• 24-48 samples
• Operating costs ~$350 per vessel
• Important to extend life of vessels
LabWest Experience
• Microwave technology adopted early
• > 150,000 mineral analyses using
microwave systems.
• Wide range of sample types handled.
• Methods developed and validated.
• Client feedback has been very positive.
• Very good for high-grade base metals as
well (a separate topic).
Applications –
Geochemical Exploration
• Excellent recoveries from aqua-regia
digests
• Highly reproducible
• Negligible losses of volatile elements
• Excellent control of cross-contamination
Applications –
Lithogeochemistry
• High recovery of refractory elements
(Hf, Nb, W, Ti, Ta, Zr)
• Detection Limits much better than
XRF, alkaline fusion.
• eg. 0.05ppm for Rare-Earths.
• Cost-effective method for gaining
a wide-picture view of the
rock (60+ elements).
Applications – Whole Rock
• Good recoveries of major oxides:
Fe, Mn, Na, K, Al, Ca, Mg etc.
• Low-cost alternative to XRF
or alkaline fusion/ICP
• Ability to determine traces off the
same digest by ICP-MS
• Useful iron exploration tool
Salt dome – southern Iran
Applications –
Biogeochemistry
• Efficient digestion of vegetation
• Decomposition of oils, etc.
• Low detection limits by ICP-MS
without pre-ashing (0.1ppb Au)
• Retains volatiles (Ge, Hg)
• Avoids cross-contamination
Future Directions
• Overcome limitations:
• Higher volumes for gold analysis
• Enhance strengths:
• Higher P/T vessels (40Bar/240°C)
• Complete dissolution of refractories
• Zircons, Carbon, Chromites, etc.
• Investigate for new applications:
• Alternative extractants,
• Client suggestions?
etc.
Thank you