Digestions

Mars6

Image: Ulrike Buhler

Microwave digestion

Microwave-assisted pressure digestion can be used to digest biological material (plants, microorganisms) and aqua regia digestion of geoscientific materials. The digestion solutions can be analysed for their element content using ICP-MS and ICP-OES.

Equipment

Mars 6 laboratory microwave (CEM, Kamp-Lintfort, Germany) for parallel microwave-assisted pressure digestion of up to 40 samples. PFA or TFM vessels are available in 10 ml, 55 ml and 110 ml sizes.

Sample preparation

For microwave digestion, the samples must be supplied dried and ground as a powder (minimum sample mass 500 mg; if only smaller sample masses are available, prior agreement is required). It is recommended to grind larger masses to ensure representativeness of the sample material.

Biological samples must be clearly labelled if they contain pathogenic/toxic micro-organisms.

Equipment and materials used

(Status: May 2024)

    • Microwave: ‘MARS 6’; CEM GmbH, Kamp-Lintfort
    • Digestion vessels: ‘Xpress’ volume 10 ml and 55 ml (material: PFA), ‘Xpress Plus’ volume 110 ml (material: PFA/TFM), quartz glass inserts for ‘Xpress Plus’; CEM GmbH
    • Analytical balance: Sartorius ENTRIS224i-1S
    • Acid dosing device: ‘Dispensette ’; Brand, Wertheim
    • Centrifuge: ‘Multifuge 3L’; Heraeus company
    • Ultrapure water system: ThermoScientific ‘GenPure UV-TOC’
    • Disposable weighing dishes: Material: PS; VWR, Darmstadt
    • Volumetric flasks: 10 ml, 25 ml, 50 ml, material: PMP, PP stopper, class A; Vitlab
    • Centrifuge tubes: 15 ml, 50 ml, material: PP; Greiner
    • Funnel: Material: PP
    • Sample bottles: acid-rinsed with diluted HNO3, HDPE 30 ml; Nalgene

Only ‘ultrapure water’ and chemicals/acids of ‘suprapure’ or ‘subboiled’ purity are used!

Digestion programmes (selection)

(Status: May 2024)

The microwave power works from 0-1800 watts depending on the programme and the number of containers used.

‘Plants 55 ml’ programme for organic material (plants, microbes)

Container: Xpress, 55 ml

Typical sample weight: ~ 0.2 g

Addition of acid: 5 ml HNO3 65%

Pre-reaction in open container: approx. 20 min.

Time ramp temperature

Heat up to 180°C: 15 min.

Hold at 180°C: 15 min.

Cool down: 30 min.

Transfer the finished digestion into a 25 ml volumetric flask using a funnel and fill up with ultrapure water. Transfer to 50 ml centrifuge tubes and centrifuge at 3000 rpm for 15 min. Centrifuge for 15 minutes to separate any undissolved components, e.g. silicates. Transfer the clear supernatant into acid-rinsed bottles.

KOEWA DIN Quartz’ programme (aqua regia digestion for e.g. soil and dust samples)

largely in accordance with DIN 13346: ‘Characterisation of sludges, determination of trace elements and phosphorus, extraction method with aqua regia, German version EN 13346:2000’

Container: XPRESS Plus, 110 ml incl. quartz glass inserts

Typical sample weight: ~ 0.5 g

Addition of acid: 2 ml HNO3 65% + 6 ml HCl 35% (‘aqua regia’)

Pre-reaction in open container: approx. 20 min.

Time ramp temperature

Heat up to 160°C: 15 min.

Hold at 160°C: 20 min.

Cooling: 30 min.

Using a funnel, transfer the completed digestion to a 50 ml volumetric flask and make up to volume with ultrapure water. Transfer to a 50 ml centrifuge tube and centrifuge at 3000 rpm for 15 minutes. Centrifuge for 15 minutes to remove any undissolved components such as silicates. Transfer the clear supernatant to acid rinsed bottles.

More info

Slightly modified extract from:

www.cem.deExternal link

In contrast to digestion systems with convective heating, ‘microwave pressure digestion systems’ are able to dissolve the solid sample in a much shorter time.

This time advantage is achieved by direct microwave heating of the digestion solution, rapid cooling after digestion and the achievement of digestion temperatures well above the 'normal' boiling point of the digestion acids.

Microwave heating is a special case of heating. In contrast to convective heating, where heat is applied to the product from the outside and distributed within the product by its thermal conductivity (surface heating), microwave heating generates heat in the product itself (volume heating).

Despite the term 'pressure digestion', pressure is not an essential criterion in the digestion process. In fact, the pressure that builds up during the digestion reaction is more of an annoying side-effect that has to be compensated for by complex vessel technology as well as measurement and control technology. This digestion technique is better characterised as a 'wet digestion method using dissolving and/or oxidising digestion reagents in a closed system'. The decisive criterion for digestion is the temperature at which the acids digest the sample. The pressure is mainly generated by the decomposition, i.e. the oxidation of organic matter to CO2 and H2O, and therefore limits the sample volume.

The resulting pressure plays a minor role in the digestion of inorganic materials.

Further information: www.cem.deExternal link

Contact

Ulrike Buhler