Sample Preparation

Sample Preparation

 

• What is Sample Preparation?
• Open vs. Closed Vessel
• HPA-S
• Microwave-heated Systems

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What is Sample Preparation?

 

In analytical chemistry, sample preparation describes the procedures prior to analysis which are necessary to make analyte contents 'measurable'.

 

 

 

 

 

 

 

 

 

The analytes of interest are only in rare cases directly accessible to measurement. Very often the analytical results are distorted by matrix components or other interfering species, therefore impeding appropriate analysis. Moreover, common analytical techniques require liquid samples which contain the analytes in a dissolved form. For this reason, sample preparation is required in order to transfer a sample into a measurable form.

 

Depending on the analytes of interest and the requirements of the subsequent analysis one of the following sample preparation techniques can be applied:

 

• Acid digestion/decomposition
• Acid leaching
• Oxygen combustion
• Fusion
• UV digestion
• Solvent extraction
• Preconcentration

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Open vs. Closed Vessels

 

Conventional open digestion methods (e.g. hot plate, hot block) have several advantages as they are easy to handle and have low investment costs - basically due to the absence of sophisticated technologies.
Nevertheless, they also suffer from severe drawbacks, such as high reagent consumption, risk of contamination, risk of analyte losses and long digestion time due to the limited temperature.
In a closed system the reaction temperature can exceed the boiling point of the reaction mixture. Therefore, the temperature will be limited by the pressure and/or temperature limit of the used vessel type only. As a consequence, even lower boiling reagents can be used at higher temperature in a closed vessel system.

 

Closed vessel digestion has following advantages:

 

• High reaction temperatures can be achieved, therefore shortening the digestion time and enhancing the digestion quality.
• Only low amounts of reagents are needed, without high-boiling reagents (like sulfuric acid).
• No loss of analytes and the minimized risk of contamination ensure the reproducibility of the methods.

Anton Paar offers two sorts of instruments for closed-vessel sample preparation:

1. Conventionally heated system:
   a. HPA-S High Pressure Asher
   
2. Microwave-heated systems:
   a. Multiwave ECO microwave digestion system
   b. Multiwave PRO microwave reaction system

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HPA-S

 

HPA-S is the reference instrument for chemical sample preparation!
The name "High Pressure Asher" refers to "wet chemical ashing", which is another term for "acid digestion".

 

The working principle of HPA S is simple and efficient.

Reaction vessels (quartz, or glassy carbon) filled with sample and digestion reagents are sealed with a PTFE strip and a quartz lid and placed inside a conventionally heated heating block.

The heating block is positioned inside a stainless steel autoclave, which is then filled with 100 bar of gaseous nitrogen. The surrounding nitrogen pressure hermetically closes the reaction vessel.

 

The autoclave is heated up according to a pre-programmed, sample-specific temperature program.

During heating, pressure inside the reaction vessel is built up, but the reaction vessel stays tight as long as the internal pressure is lower than the surrounding pressure of approx. 130 bar..

After the run the instrument cools down automatically and nitrogen is released from the autoclave. Once the autoclave is at normal pressure, the lid can be opened and the reaction vessels can be removed.

Specifications

Continuous operating temperatures up to 320 °C and unlimited decomposition time ensure the highest possible digestion quality.

 

 

• Uniform, sample- and acid-independent heating of all vessels ensures highest digestion reproducibility.
• No loss of elements and no cross-contamination occur in the vessels which are sealed hermetically up to a pressure of 130 bar. Quartz and glassy carbon vessels allow the use of all digestion reagents.
• Depending on the application various conversion kits are available, consisting of a heating block, a set of reaction vessels (5x90 mL, 7x50 mL, 14x15 mL, 21x15 mL) and the corresponding lids for vessel closure.
  

 

 

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Microwave-heated Systems

 

How does Microwave-assisted Heating Work?

Heating with microwaves (electromagnetic waves in the range of 300 MHz (0.3 GHz) to 3 000 GHz) is based on two mechanisms:

 

• Dipolar re-orientation: molecules with a dipolar structure (like water) try to align in the fluctuating microwave field at higher frequencies. This results in absorption of microwave energy and molecular motion, and therefore heat.
• Ionic conductance: at lower frequencies oscillation of ionic constituents (like salts) results in absorption of microwave energy, and also in heat.

 

 

 

 

 

 

 

 

 

 

In contrast to conventional heating, microwave heating directly heats the liquid bulk, while the surroundings (like vessels) remain comparably cool. As a consequence, microwave heating is fast and energy-efficient. As a further benefit, cooling can be more efficient, as primarily only the liquid bulk has to be cooled down.

 

 

Anton Paar offers two microwave-heated systems for sample preparation: Multiwave ECO, Multiwave PRO.

 

Multiwave ECO

The easy-to-use and budget-friendly Multiwave ECO microwave digestion system focuses on acid digestion and leaching of routine samples working with only three rotors:

 

• Rotor 64MG5,
• Rotor 1DRY and
  
• Rotor 16HVT50.

 

Rotor 16HVT50 is a brand-new design with vessel support tubes made of fluoropolymer-coated aluminum and 16 pressure-activated-venting vessels.

 

Multiwave PRO

Multiwave PRO is a high-end microwave reaction system.
Combining the benefits of a closed-vessel system with rapid microwave heating Multiwave PRO is a highly modular and therefore very versatile system which can cope with almost any sample preparation task:

 

• Acid leaching
• Acid digestion
• Microwave-assisted solvent extraction
• Evaporation (of aqueous solutions)
• Drying
• Microwave-induced oxygen combustion (MIC)
• Microwave-assisted UV digestion (MUV)
• Protein hydrolysis

 

 

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