The partners in the project have developed a microwave-based
system for rapidly dissolving, or digesting, samples in solution in
The digester is faster and more energy-efficient than the usual method
of heating the solution on a hotplate or in a water bath. The main
novelty is the ability to simultaneously heat four vessels containing
solutions. Multiple heating should speed up laboratory work and be
One of the routine tasks in chemical laboratories
is the preparation of samples for analysis by dissolving material
in solution by heating, a process often known as digestion. The
process is also known as mineralisation, particularly when organic
samples of heavy metals are prepared. Traditionally samples are
prepared by placing a beaker containing solution and the material
to be dissolved on a hotplate or in a water bath. The vessel has
to be heated, typically for five minutes.
The partners in the Multidigester 4000 project decided to investigate
the use of microwaves for more efficient and controllable heating
of vessels to improve the efficiency of sample preparation. An important
aim of the project was to produce a unit that could simultaneously
heat four open vessels of solutions and substances. The design directs
all the microwave energy into the vessels, increasing the energy
efficiency of the equipment. In addition, the equipment is automatic
in order to simplify use.
Better focus on heating
The use of microwaves for heating in the laboratory is not new.
There is already a piece of equipment which can heat a single open
vessel, and there is also an oven used with closed vessels in which
the pressure rises to improve the dissolving of materials. However,
equipment based on domestic microwave ovens produces a poorly-controlled
distribution of microwaves, leading to uneven heating and poor control
over the quality of the samples being prepared.
The Multidigester partners decided to develop a specialised unit
for laboratories. The partners included Soled Industries, which
manufactures a range of laboratory equipment, the instrumental analytical
equipment department of Gerhard-Mercator-University of Duisburg
in Germany, and Puls Plasma Technik of Germany, which produces pulsed
energy equipment such as ultrasonic machines to treat kidney stones.
The unit developed by the partners is based on a single 2.45 GHz
microwave generator, or magnetron. The microwaves are directed into
a square-section beam splitter to split the single flow of energy
into four equal and symmetrical beams, which are then directed to
four heating chambers.
Test tubes or other vessels containing sample solutions are placed
in the four chambers for heating by the microwave beams, with nearly
all of the microwave energy passing into the sample and solution.
The efficiency of heating means that simple samples can usually
be prepared within one minute, rather than the five minutes needed
when using a hotplate or water bath. The digester can produce between
10 W and about 200 W, depending on the samples being prepared.
The design could be developed for laboratories that prepare large
numbers of samples by adding further magnetrons for further multiples
of four heating chambers.
The heating chambers are in the form of openings in the top of the
unit, into which the sample vessels are placed. This allows a laboratory
to prepare samples in a range of vessels. The design also carries
out a heating cycle automatically, under the control of a microprocessor
that can be programmed by the user.
Trials show faster results
Puls Plasma Technik produced an initial version of the machine
for testing at Gerhard-Mercator-University and five further units
for wider trials. Faults that appeared during trials included small
cracks in welds which produced sparking, and a movement of the Teflon
base of some heating chambers, leading to irregular heating. The
faults were corrected, in the latter case through the use of electrically
insulating material to fix the bases.
The partners enlisted the help of a number of chemical laboratories,
including Shell Research in Amsterdam, Solvay in Brussels, Merck
in Darmstadt, British Steel in Teesside and INA in Paris. Trials
were also carried out by the project partner, the Gerhard-Mercator-University.
The trials have shown that the time needed to dissolve heavy metals
can be cut from a normal 120 minutes to 10 minutes, and gold is
dissolved in 7 minutes rather than the 60 minutes needed using conventional
Unequal heating of the four chambers on a machine was a problem
with the prototypes. The reasons were because of variations of the
flatness of the bases of the vessels placed in the chambers, and
because of poor positioning of the microwave guides. The first problem
has been solved by the production of sample vessels with flat, polished
glass disc bases. Better manufacturing control during commercial
production will solve the positioning problem.
There was also a need to eliminate any leakage of microwaves, particularly
when not all four chambers are in use. Since 1996, European legislation
specifies a leakage of 0.5 milliwatts/cm2. It was found that liquid
in the sample vessels emitted microwaves, and that the microwaves
escaped if the liquid level was too high or boiled.
This problem has now been solved, say the partners, however, work
is still under way on solving an additional problem of optimising
the splitting of the energy from the magnetron into four microwave
beams of equal power. This is particularly important to ensure good
control of sample preparation. The use of a machine for several
hours also raised questions about the risk of overheating, although
the partners do not believe this will be a concern with future commercial
versions of the design.
Wide market interest
The prototype unit has been shown at two exhibitions and generated
considerable interest. After evaluating a unit in trials, Merck
expressed an interest in participating in the commercial development
of the design. The system of directing the microwaves to the vessels
is the subject of a patent application.
The partners see a large potential market in chemical laboratories
in many industrial sectors. The design could prepare samples of
a wide range of materials, including soil, detergent, ore, industrial
waste, sand, water, dye, and geological samples. The steel sector
would be just one of many potential markets. The partners believe
a commercial stage could be reached by the middle of 1997.
In future, an automation system could be developed for use with
the digester, for even greater laboratory efficiency.