TECHNICAL ARTICLE: MICROWAVE-ASSISTED DIGESTION OF MINERAL SAMPLES
- Cost effective, low DL, high recovery multielement analysis.
- High pressure and temperature digestion: more aggressive than hotplates.
- Excellent recovery of resistate minerals: good for lithogeochem work.
- Retention of volatiles.
- Comparable to fusion techniques, but with better DLs.
- Whole-rock analysis readily achievable.
- LabWest is a leader in this field – 9 years’ development, >350,000 samples analysed.
Microwave radiation generally refers to that part of the electromagnetic spectrum with frequencies between 300MHz and 300GHz (corresponding to wavelengths between 1mm and 1 metre). Heating of various materials by exposure to microwave radiation has been accepted practice for more than 60 years – the first patents for microwave ovens were taken out by the Raytheon Company (resulting from their work with Radar) in 1946. A range of industrial applications has been developed since then, though the most commonly known technology is the microwave oven.
The rapid energising of molecules and consequent acceleration of chemical reactions by absorption of microwave radiation has logically led to the use of microwave heating in chemical applications such as synthesis and digestion. Microwave-assisted digestion (MAD) of mineral samples has been developed to a usable technique over the past ten years or so. Modern laboratory microwave digestors are essentially the same as kitchen microwave ovens, albeit with chemically resistant surfaces, very advanced microprocessor controlled power output and a range of safety features.
LabWest’s digestors have dual magnetrons (for more accurate control), and produce up to 1400W of 2455 MHz microwave power. Samples are digested in sealed, PTFE-lined, microwave-transparent pressure vessels, and digest conditions are monitored via infra-red sensors and internal sensors. Digests can be controlled with respect to microwave power, vessel temperature and vessel pressure. This gives the ability to achieve reproducible digestion conditions across a wide range of sample materials. The process control software records the various parameters for each digest run, so the chemist can store and scrutinise the data if desired. Vessels are equipped with safety disks that will rupture if the internal pressure exceeds safe limits, and the digestor automatically shuts power and evacuates its interior in the event of a vessel rupture.
There are several significant advantages of the MAD approach over traditional hotplate/hotbox techniques, the most obvious being digest time. A MAD will typically reach completion in one tenth of the time taken by an equivalent hotplate method, which largely mitigates the capacity disadvantage mentioned below.
Technically, the MAD approach offers excellent recoveries for most mineral matrices when compared to other acid digest techniques. Traditionally, digests are performed in open vessels at atmospheric pressure on hotplates or in hotboxes, which means that the digest temperature is limited to the boiling points of the acids being used – in the case of HF around 104°C – and the acids rapidly evaporate as the temperature is increased above boiling point. On the other hand MADs are performed in sealed vessels at temperatures up to 200°C and pressures up to 20 Bar, which has environmental and operational benefits, in that acids are retained in the vessel rather than being boiled off in great quantities, and also means that all acids are present for the full duration of the digest.
As the oxidising power of acids generally increases with temperature, the oxidising effect of the acids is far greater in the MAD than in atmospheric pressure digests. The MAD techniques are therefore very effective at decomposing sulphides and other resistant matrices. The use of sealed vessels in MAD has the additional benefits of retaining volatile elements such as mercury and germanium in the digest vessel, and preventing cross-contamination from spitting.
LabWest has been continually developing MAD techniques for routine multielement analysis for geochemical exploration over the past five years, making the specific advantages of this technology available to minerals exploration geologists and geochemists. The full value of MAD’s multi-element extraction capability is attained by combining the analytical strengths of ICP-Optical Emission Spectrometry (ICP-OES) and ICP-Mass Spectrometry (ICP-MS) to determine levels of up to 63 metallic elements down to low detection limits.
Aqua-regia-based digests give consistently high recoveries for base metals, readily decomposing sulphides and most organic matter. Being a single-stage procedure, MAD aqua-regia digestion provides a cost-effective means of obtaining a wide range of information for the explorer, for example in LabWest’s 20-element gold-and base-metals exploration package “EXPRESS GOLD+20”.
Hydrofluoric acid (HF) based digests are used where high recoveries are required for more refractory minerals, or total decomposition of the host rock matrix is necessary. Good whole-rock approximations can be achieved. The technique is more involved than aqua-regia, as it requires a second digest stage to remove residual HF. This method enables LabWest to offer packages designed for Uranium explorers, such as the “MMA-01U” suite.
The ability of MAD techniques to achieve high recoveries of the metals that are traditionally difficult to extract by acid digestion (eg. Hf, Nb, W, Zr, Rare Earths) lends itself nicely to litho-geochemical studies. These elements typically show better than 90% recovery in all but the most refractory mineral matrices, for example in the “MMA-03” rare-earth element package.
A single MAD digest solution can therefore be determined by combined ICP-OES and ICP-MS for suites of up to 63 elements, including base and trace metals, major rock-forming oxides and rare-earths. This has significant cost-saving implications, and enables exploration geologists and geochemists to gather a very detailed picture of the mineral environment in one step.
The application of MAD technology to geochemistry is still relatively young, and new applications and improved methodology are being developed, to take advantage of improvements in analytical instrumentation. LabWest, as part of its commitment to the use of MAD as a routine digestion technology, pursues an active development program to keep it at the forefront of microwave applications.