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A Platform System for Microwave Sample Preparation
Multiwave 3000

Dr. Elisabeth Kahr, Peter Kettisch
Anton Paar GmbH, Anton-Paar-Str. 20, A-8054 Graz, Austria

Today's technology enables the near-perfect determination of trace elements in all concentrations down to the ultra trace range. Modern AAS, ICP-OES, ICP-MS or electrochemical procedures such as voltammetry are reliable and accurate measuring methods for almost every type of task. However, in most cases the sample material has to be supplied in a suitable, measurable form, i.e. as a largely matrix-free solution without contamination or loss of analytes.

For more than 25 years, Anton Paar has been a leading supplier of sample preparation instruments. Our vast experience and thorough understanding of the needs of analytical chemists enable us to offer advanced solutions in sample preparation. The Multiwave 3000 is designed to bridge the gap between sample and analysis. It is ideal for wet acid digestion, solvent extraction and other special applications, e.g. oxygen combustion. The compatibility of system accessories makes it easy to customize and enhance your Multiwave 3000.

A Powerful Microwave Oven
A powerful microwave oven serves as a solid base for the modular construction. The 66 liter chamber is made of stainless steel with a multilayer fluoroplastic coating. This guarantees robustness, protection against corrosion and easy cleaning. The oven is supplied with up to 1400 W of microwave power via two magnetrons. This power can be adjusted unpulsed in 1W steps across the whole 1400 W range via state-of-the-art electronics. This is extremely useful when handling samples which react spontaneously and for the operation of certain accessories, such as UV-reactors.

The control of the microwave power, the processing of the sensor signals, documentation of the reaction parameters and management of the sample data is carried out by built-in microprocessors. The course of the reaction, pressure and temperature data are displayed on a built-in, easy-to-read display either graphically or in large figures.

To shorten the total required process time, both rapid microwave heating and a short cooling time are required. The proven air cooling unit is used to cool the vessels within around 15 minutes without external tools such as water baths or freezers. This results in quick and safe work procedures because there is no handling of hot vessels under pressure.

The stable safety door has an innovative, user-friendly magnetic seal. It functions as a safety relief shutter, which opens a little when there is overpressure in the cavity and is caught by strong locking hooks. In this case, the microwave power is turned off immediately via interlock switches. After the pressure has been released, the magnets reseal the door automatically.

Reaction Vessels and Digestion Rotors: Everything You Need
The reaction vessel is the heart of any digestion system and the key to successful sample preparation. The Multiwave 3000 system offers a wide range of vessels and rotors with exceptional modularity and interchangeability. Its flexibility allows you to select optimized systems for a wide variety of organic and inorganic matrices. Most of the applications can be resolved using compound designs of microwave-transparent ceramics together with precisely tooled PTFE-TFM liners. While ceramics give excellent temperature and pressure resistance, the fluoroplastic provides the required chemical inertness. High-purity quartz glass vessels are best suited for challenging digestion tasks which require both maximum temperature and pressure simultaneously (see Table 1).

The digestion rotors are designed for maximum productivity. The 16-position rotor is the perfect choice for a wide selection of samples which decompose at pressures and temperatures up to 40 bar and 240 °C. This rotor provides maximum sample throughput without compromising the safety aspects. The highest decomposition quality is achieved with the 8-position rotor, meeting demanding pressure and temperature requirements and converting samples that are difficult to decompose into clear solutions. Continuous pressure monitoring of all eight sample vessels means the maximum reaction parameters are reached quickly and safely.

Table 1: Vessel specifications

The Multiwave 3000 is characterized by its well thought-out, simple operation and multifunctional parts. The screw cap of the vessel, lip seal, rupture disk and release valve form one unit, although each part can be exchanged individually. The reaction vessel can be closed by hand; no further tools are needed.

Sensors: Sophisticated and Unique
For safety reasons and to confirm the quality of the decomposition, the important reaction parameters temperature and pressure have to be measured and controlled with sensors. The system offers several alternatives for measuring these important reaction criteria and provides precise data for reaction control along with detailed documentation.

In the 16-position rotor, one vessel can be equipped with a combined pressure and temperature module instead of the normal screw cap. This module enables accurate reaction control and the vessel therefore functions as a reference vessel. The microwave heating is adjusted according to the course of the reaction of this vessel. The pressure sensor, which is not in direct contact with the sample solution, uses a hydraulic system to measure the force with which the reaction pressure pushes the seal upwards.

Along with the pressure, the reaction temperature is the most important parameter in chemical decomposition. Therefore, a gas pressure thermometer, protected by a sapphire tube, is immersed into the reaction solution. The temperature is therefore determined accurately and without time delay directly from within the sample. The gas pressure thermometer works as follows: A small gas volume is enclosed in a glass bulb, which is connected to an electronic pressure sensor via a capillary. Temperature changes in the sample solution are measured as proportional pressure changes in this gas volume. This very robust, reliable and easy to use thermometer works without electrical elements in the reaction vessel so there is no distortion of the microwave field.

Another principle of temperature measurement in microwave reaction systems is based on the infrared emission of hot surfaces. IR-sensors measure the temperature at the base of each reaction vessel once per rotor turn, through holes in the base of the rotor.

Especially impressive is the patented pressure measurement for all vessels of the 8-position rotor. This rotor type is used for very demanding decomposition at operating pressures up to 80 bar and provides the highest level of safety even when samples which react differently are handled at the same time. An immersion thermometer for exact temperature measurement is also available as an accessory for the 8-position rotor. The sensor signals are sent to the control unit via optical transmission, no electrical or mechanical connections are needed. The sensors are easy to handle, no tools are required.

Innovative Accessories
Modern microwave systems are not limited to decomposition tasks. It is often beneficial to dry samples with microwave energy prior to analysis. Sample volumes from a few grams to 1 kg can be gently microwave-dried without carbonization or contamination of the sample, in a quarter of the time required by conventional techniques. The moisture from the sample and any odors (such as those which arise when sewage sludge is dried) are removed via the built-in exhaust unit.

Another application of microwave heating includes the concentration of aqueous samples and the fuming off of acids out of decomposition solutions. This is carried out by an evaporation rotor and an external scrubber, which sucks the resulting vapors out of the sample vessels. The evaporation can be done practically under clean room conditions because the incoming air for the vessels is supplied via sterile filters.

The Multiwave 3000 supports more than just digestion, drying and evaporation. The solvent rotor expands the possibilities of the Multiwave 3000 from inorganic to organic sample preparation. Extremely fast, safe and easy MAE (microwave-assisted extractions) improve the performance and throughput of HPLC- or GC-based measurements. This rotor is the microwave-heated alternative for various kinds of applications using organic solvents. It replaces time-consuming classical methods like Soxhlet extraction, or speeds up the dissolution of polymers or derivatization reactions prior to analysis.
An optional, programmable magnetic stirrer can be used to achieve a better mixing of sample and reagents. This is very useful for the decomposition of heavy, inert samples and for organic solvent extraction. By appropriate selection of components, the Multiwave 3000 can easily be configured for dual use: digestion and extraction.

Recently, innovative solutions have been released for special sample preparation tasks which are now opened up for the application of microwave technology. Microwave-induced oxygen combustion is an innovative method for sample preparation for all kinds of combustible solids such as wood, paper, coal, food or polymers. Volatile compounds such as halogens or several metals released during this combustion process are trapped in an absorption solution which is subsequently analyzed via ion chromatography or atomic spectrometry. Microwave-assisted UV digestion is a powerful technique for ultra-trace analysis of liquid samples with a high content of organic compounds, e.g. seawater, effluents, sewage, body fluids or beverages.