Principles of Digestion TechnologyContents1. Purpose and Objective3. Digestion using Liquid Reagents3.1 Introduction 3.2 Theory of Digestion Process4. Digestion in Practice 4.1 Open Digestion at the Reflux 4.2 Pressure Digestion in Steel Tanks 4.3 Pressure Digestion by Microwave5. Synopsis

1. Purpose and ObjectiveQuantitative detection of elements in solids usually by subsequent spectroscopic analysis Purpose

1. Purpose and ObjectiveClear dissolution of solidComplete destruction of matrix avoiding disturbances in subsequent analysisAvoidance of lossObjective

3. Digestion with Liquid Reagents - PrincipleChemical digestion of sample matrixHeating accelerates rate of reactionMax. temperature in open digestion limited by boiling point of solutionPressure build-up in closed vessels permits higher temperatures

3. Digestion with Liquid Reagents - General Aspects1. Homogenisation of sample2. Weigh-in of a representative aliquot3. Addition of digestion reagent4. Supply of energy (usually heat)General procedure

3. Digestion with Liquid Reagents - General AspectsReagentsAcid (e.g. with HCl, H2SO4...)Base (e.g. with NaOH, NH3...)Oxidising (e.g. with HNO3, H2O2, K2S2O8...)Reductive (e.g. with HJ, HBr...)Complexing agent (e.g. H3BO3...)

3. Digestion with Liquid Reagents - General AspectsChoice of reagentsOrganic matrices usually oxidising substances or mixtures (HNO3, H2O2, K2S2O8 and possibly H2SO4) Inorganic matrices usually mixtures with HNO3, HCl, (also aqua regia), HF and possibly H2SO4 pure metals: HCl, aqua regia, HCl/HF Oxides: H2SO4/HCl, H3PO4/HCl, mixtures containing HF

3. Digestion with Liquid Reagents - General AspectsOrganic matrices - choice of reagentsHNO3 (65%) Universally used For readily oxidisable samples (food, wood, fat, oil) Nitrate or nitrogen must not interfere with analysis Mixture of HNO3 (65%) / H2O2 (30%) approx. 4:1 Improved quality of digestion No improvement for samples that are difficult to digest (e.g. plastics)

3. Digestion with Liquid Reagents - General AspectsOrganic matrices - choice of reagentsAqueous samples (waste water) Digestion with H2O2 (30%) / H2SO4 (1:1) mixture Difficult-to-digest samples (e.g. plastics) Digestion with HNO3 / H2SO4 (1:1) mixture Carbon in matrix made more readily corridible by dehydration Higher digestion temperatures due to lower vapour pressure of mixture

3. Digestion with Liquid Reagents - General AspectsInorganic matrices - choice of reagentsPure metals Digestion with HCl, aqua regia or HCl / HF mixtures Oxides, including Al2O3 in particular Digestion in H2SO4 / HCl or H3PO4 / HCl or HF mixtures High proportion of high-boiling acid (approx. 80%) needed in order to achieve highest digestion temperatures at moderate pressures

3. Digestion with Liquid Reagents - TheoryDigestion process - critical parametersDigestion temperatureDigestion timeChemical potential of digestion reagents

3. Digestion with Liquid Reagents - TheoryDigestion temperatureHigh digestion temperatures shorter reaction timeDigestion temperatures are limited by:vapour pressure of digestion acidstemperature resistance of container/vessel materialspressure resistance of containers/vessels

3. Digestion with Liquid Reagents - TheoryPressure buildup in closed digestionTotal pressure pp = p(CO2) + p(acid) p(CO2) = partial pressure of CO2 produced p(acid) = partial pressure of acid mixtureCO2 pressure:dependent on carbon content of sample and weigh-in p(CO2) = 6.9 * mc [g] * T/V [K/ml]

Example: V = 30 ml, 0.2 g carbon, 200C p(CO2) = 22 bar V = 80 ml, 0.2 g carbon, 200C p(CO2) = 8 bar

3. Digestion with Liquid Reagents - TheoryExample (60 ml vessel at 200C):500 mg carbon develops 930 ml CO2 partial CO2 pressure of 26 barAcid pressure for HNO3 at 200C of approx. 10 bar Total pressure approx. 36 bar (60 ml vessel at 200C)

3. Digestion with Liquid Reagents - TheoryVapour-pressure curves/graphs of pure acidsa. Aqua regiab. HCl 36%c. HNO3 91%d. HCl 22.9%e. Waterf. Boling point HNO3 100%g. Boiling point H2SO4 100%h. Boiling point H3PO4 96%

(Panholzer, LaborPraxis, Oct. 1994, 32)

3. Digestion with Liquid Reagents - TheoryDigestion timeShort digestion times recommended greater throughput of samplesBut good control of the process takes priority, e.g.:Slow heating in warm-up phase avoids spontaneous reactionsOver-vigorous process control unnecessary wear and tear on material

3. Digestion with Liquid Reagents - TheoryChemical potential depends onConcentration of reagents employed in the solutionInteraction of reagentsInteraction of reagents with sample waterGoal:The concentration of acids should not reduce greatly during digestion.

4. Practical Conduct of DigestionOpen method at refluxMax. temperature limited by boiling point of acid mixture ( conc. H2SO4)Allows high weigh-insQuality of digestion not always sufficientLoss of volatile elements (e.g. Hg, lead salts)

4. Practical Conduct of DigestionPressure digestion in steel vesselsPressure resistance 200 barTemperature max. 230C (briefly 260C)Digestion times from about 2 hours to several daysFree from contamination due to PTFE-TFM liningDifferent internal volumes (25-250 ml) and therefore weigh-in quantitiesOutstanding quality of digestionNo loss of volatile elements (e.g. Hg, lead salts)High degree of safety, easy operation

4. Practical Conduct of DigestionPressure digestion in steel vessels - specimen application

Digestion Vessel DAB-3 (250 ml)

Matrix

Weigh-in

Acid

Temperature

Time

Cellulose/starch

1000 mg

HNO3

160C

2h

Flour/grain/leaves

1000 mg

HNO3/HF

180C

2h

Tissue/liver

1000 mg

HNO3

170C-190C

2h

Fat/oil

1000 mg

HNO3 (poss. H2O2)

200C

4h

Plastics

500 mg

HNO3 /H2SO4

200C

3-4h

Carbon/resin

500 mg

HNO3

220C

6h

Ceramics/oxides

500 mg

HF or HCl

230C

2-8h

Steel

500 mg

HNO3/HCl

200C

4h

4. Practical Conduct of DigestionPressure digestion with microwave heatingPressure resistance dependent on type of container/vessel (40-100 bar)Free from contamination through use of PTFE-TFM containersDifferent interior volumes (10-100 ml) and therefore weigh-insQuality of digestion mostly sufficientNo loss of volatile elements (e.g. Hg, lead salts)High throughput of samples due to short digestion times (10-60 mins.)

4. Practical Conduct of DigestionPressure digestion with microwave heatingIt is primarily the sample that is heatedContainer material (plastic) is only indirectly heated Relatively high digestion temperatures can be reached for short periods (30-40 minutes)

4. Practical Conduct of DigestionContainer materialsPTFE maximum 260 CPTFE-TFM maximum 260 CPFA maximum 200 CQuartz (silica) glass maximum 1,000 C (theoretically)

4. Practical Conduct of DigestionTemperature controlThe most important aspect of controlling microwave digestionRate of reaction depends on temperatureThe temperature in the various vessels may vary as a function of the type of sample and the weigh-in quantity Temperature control is required in all vessels

All samples can be subjected to non-contact IR temperature measurement

speedwave MWS-3+Mikrowave radiationIR-SensorFilter IR-radiationTFM

Temperature Control - Principle

IR-measurement at wavelength, where TFM does not absorb IR-radiationThermal radiation of the vessel is filtered outHeat radiation of vessel surfaceHeat radiation of sample

5. SynopsisWork in closed vessel wherever possible higher reaction temperature better quality of digestionParameters determining digestion temperature, limited by: pressure resistance of container/vessel material of vesselDigestion in steel vessel for most difficult samples or lower sample throughputDigestion under microwave especially for high sample throughput

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