OPEN VESSEL DIGESTION SYSTEM

Samples and reagents are heated on hot plates, sand baths or in microwave units in open beakers or flasks, made of glass, quartz, fluoroplastics (PTFE, PFA) or glassy carbon. Acid fumes are evaporated or (partially) condensed in reflux coolers. Max. Reaction temperature is the boiling point of the acid mixture at ambient pressure. •

·         Typical sample weight: 0.5 to 10 g

·         Typical reagent consumption: 10 to 100 mL

·         Typical process time: 2 to 10 hours

Typical applications :Agricultural and environmental analysis, analysis of solid waste and other inhomogeneous material for elements in higher concentrations, often in combination with flame AA measurement. Suitable for very large batches of similar, easy to decompose samples. Beneficial in routine analysis, for lower skilled lab personnel.
Users who worked with open systems for a long time, sometimes are difficult to convince about the benefits of closed systems, but for difficult samples they admit limitations of their equipment. A closed system may be a necessary complement for their lab.

Types of instruments

• Hot plates
 • Heating blocks with reflux vessels
 • Sand or salt bath systems
• Microwave waveguide systems
• Microwave oven systems w. open vessels

Standardization

Especially in the environmental analysis sector there are standardized procedures for conventional hot plate methods (US-EPA 3030, 3050, DIN 38414 S7, etc.). They are practiced in many labs, but are very time consuming in handling and vessel cleaning. Better alternatives are required.

Advantages

• Simple, inexpensive equipment, in its basic form as a beaker on a hot plate.
 • Handling is simple.
• High sample weight is possible, therefore ideal for inhomogeneous samples.
 • For conventional systems (heating blocks): high sample throughput.
• Reaction gases can escape Æ no pressure built up, no pressure sensors or overpressure safety systems required.

Disadvantages

• Temperature is limited with the boiling point of the acid at ambient pressure Æ poor decomposition quality, incomplete mineralization of organic matter (high DOC).
• Long decomposition times (2 to 10 hours).
 • Chemistry is more difficult than for closed systems, in many cases mixtures of up to four reagents are required.
• Methods must be varied and optimized for different sample materials.
• For increase of reaction temperature often H2SO4-mixtures are required. H2SO4 is a disliked reagent because it is difficult to purify, it may trouble the following analysis step (high viscosity) and it forms insoluble sulfates with elements like Pb or Cd, leading to minor recoveries for those.
• A lot of supervision and repeated addition of reagents is required to achieve acceptable decomposition results and avoid charring or dryness at an early stage of the process.
• If a sample gets charred or led to dryness, it may be difficult for further decomposition.
• Automatic reagent addition modules make operation more convenient but increase costs of the equipment considerably.
• High amount of reagents required, even for systems with reflux coolers (up to 10 times more than in closed systems!) Æ high costs for high purity reagents.
• High water consumption for reflux coolers.
• Contamination by reagent blanks and airborne dust.
 • Corrosive lab atmosphere, even when performed under a fume hood.
• Losses of elements by evaporation: Hg (elemental), As, B, Cr, Ge, Pb, Sn, Te, Ti, Zn, Zr (as halogen compounds), Os, Rh, Ru (under oxidizing conditions), Se, Te (under reducing conditions). • Big glassware (typ. 250 mL flasks) requires a lot of cleaning work.

CLOSED VESSEL DIGESTION

Technology Samples and reagents are treated in closed pressure vessels, made of fluoroplastics (PTFE, PFA), quartz glass or glassy carbon. Conventional heating (autoclaves or blocks) or microwave heating, reaction control via temperature- or pressure sensors. Max. Reaction temperature is depending on thermal stability and pressure rating of the reaction vessels.
• Typical sample weight: 0.1 to 1 g
• Typical reagent consumption: 2 to 10 mL
• Typical process time: 15 to 60 minutes
Typical applications
Trace and ultratrace analysis of almost all materials with all measuring techniques. Preferred for difficult decomposing matrices. Suitable for all spectroscopic measurements and voltammetry. General trend to replace open vessels by downscaled, more economic closed vessel methods. For quick processing of single samples up to large batches. For routine digestion of hundreds of samples (soil, sediments, plants) per day, closed vessel digestion requires too much handling. However, it depends on details of the analytical task to decide which system is most applicable.
Types of instruments
 • Traditional PTFE-Bombs
• “High Pressure Asher”
• Microwave oven system (low-, medium-, high pressure types)
 • Microwave waveguide systems
Standardization
Today most standardization of sample decomposition procedures is done for microwave heated closed vessel procedures (US-EPA 3015, 3051, 3052, ASTM, NPDES and other national and international standards). Many new standard procedures are in consideration and can be expected soon.

Advantages

•        High temperature operation, up to 320 °C (in the HPA-S) due to high pressure, provides for complete mineralization of sample material.
•        Chemistry can be kept simple: for organic matter HNO3 at elevated temperature can provide for complete mineralization without the need for H2SO4 or HClO4, as known from open vessel procedures.
•        Fast reactions, esp. in microwave heated systems.
·         Small amounts of reagents (2 to 10 mL) reduce reagent blanks, reduce consumption and disposal costs.
·         With few standard methods many different kinds of samples can be decomposed.
·         With small vessels a good surface to sample ratio can be achieved, which is important to reduce adsorption effects.
·         No volatilization losses of analyte.
·         No escape of acid fumes during operation Æ environmentally friendly

Disadvantages

·         Higher equipment costs, as sensors and overpressure safety systems become necessary and pressure vessels must be designed more sophisticated than simple open flasks. BUT: Reduced reagent consumption and time savings counter-balance the higher instruments investment. Always compare total costs of an operation period of 5 years!
•        Limitation of sample weight, depending on the stability and reaction control of a system. High sample loads produce a lot of reaction gas, the resulting pressure must not exceed the maximum vessel ratings.