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REALIZING YOUR IDEAS

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G r u n d l a g e n - A u f s c h l u s s . p

p t

ContentsPrinciples of Digestion Technology

1Purpose and Objective

Different Digestion Methods

Digestion using Liquid Reagents

3.1 Introduction

3.2 Theory of Digestion Process

Digestion in Practice

4.1 Open Digestion at the Reflux4.2 Pressure Digestion in Steel Tanks

4.3 Pressure Digestion by Microwave

Synopsis

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Purpose and Objective

Quantitative detection of elements in solids

usually by subsequent spectroscopic analysis

Purpose

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Clear dissolution of solid

Complete destruction of matrix

→ avoiding disturbances in subsequent analysis

Avoidance of loss

Objective

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Sample Collection

Sample

Pretreatment

Data Management

Analysis

Handling Time of Analytical ChemistsPurpose and Objective

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Sample Throughput

0

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0

0

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Food Plastics Ceramics/Oxides Tissue/Blood Water

T i m e ( h o u r s

Hot Plate

DAB-pressure vessels

Microwave


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Different Digestion MethodsDigestion by melting

Acid (e.g. with KHSO4, B2O3...)

Base (e.g. with NaOH, Na2CO

3...)

Oxidising (e.g. with KNO3, Na2CO3, Na2O2...)

Reductive (e.g. with Na, KCN...)

Disadvantages:

→ High use of reagents (blank values!)

→ Loss due to high digestion temperature

→ High salt content (matrix effects!)

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Different Digestion MethodsDigestion with gases

Oxidising (e.g. with O2, Cl2...; Wickbold combustion)

Reductive (e.g. with H2

...)

Disadvantages:

→ Specialised apparatus (cooling traps etc.)

→ Only suitable for organic substances in practice

→ Difficult to measure quantities of gas (blank

values!)

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Different Digestion MethodsTo sum up

Digestion with liquid reagents should always be attempted as a first option

Possible procedures:

Open digestion under atmospheric pressure at reflux

Closed pressure-decompostion at high temperatures

→ Heated by heating block → Microwave heating

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Digestion with Liquid Reagents - Principle

Chemical digestion of sample matrix

Heating accelerates rate of reaction

Max. temperature in open digestion limited by boiling point of solution

Pressure build-up in closed vessels permits higher temperatures

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Digestion with Liquid Reagents - General Aspects

1. Homogenisation of sample

2. Weigh-in of a representativealiquot

3. Addition of digestion reagent

4. Supply of energy (usually heat)

General procedure

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Nitric Acid

Oxidizing acid (CH2)x + 2 HNO3 —> CO2 + 2 NO + 2 H2O

Often in mixtures with H2O2 or HCl,HF, H2SO4

Boiling point: 122°C (HNO3 65%)

Vapour pressure ~25 bar (at ~225°C)

Forms soluble nitrates with all elements except:

Au, Pt, Al, B,Cr, Ti, Zr

Nitric Acid + Hydrogen Peroxide

Increases oxidation potential 2 H2O2 —> 2 H2O + O2

Reoxidizes NO to NO3-

Typical mixtures HNO3 : H2O2 = 4:1


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Digestion of Milk Powder – BCR-151Procedure

Sample: 300 mg Milk Powder

Acid Mixture: 7 ml HNO3 (65%)

0 - 2 ml H2O2 (30%)

Temperature Program

Step 1 2 3 4

Temp. [°C] 145 170 190 100

Rise time [min] 2 5 2 1

Time [min] 5 10 15 10

Result

Clear Solution

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Digestion of Food Samples in DAP-40+Procedure

Sample: 500 mg


Temperature Program

Step 1 2 3 4

Temp. [°C] 145 170 190

Rise time [min] 2 2 2

Time [min] 10 10 10

Result Clear Solution

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Hydrochloric Acid

Non oxidizing acid

Boiling point 84°C (HCl 32%)


forms soluble chlorides with all elements except:

AgCl, HgCl, TiCl

Dissolves salts of weak acids (carbonates, phosphates, borates)

Digestion of Iron-alloys

Following oxides are insoluble: Al, Be, Cr, Sb, Sn, Si, Ti, Zr


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Aqua Regia

HCl : HNO3 = 3:1

Forms NOCl 2 NOCl —> 2 NO + Cl2


Digestion of pricous metals, sulfides

Use always freshly prepared


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Partial Digestion of Sediment Samples - EPA 3051AProcedure

Sample: 0.5 - 1.0 g Sediment


3 ml HCl (37%)

Temperature Program

Step 1 2 3 4

Temp. [°C] 175 100

Rise time [min] 1 1

Time [min] 10 10

Result

Clear solution with white precipitate of SiO2.

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Hydrofluoric Acid

Non-oxidizing acid

Decomposes silicates SiO2 + 6 HF —> H2SiF6 + 2 H2O

Excess is necessary to prevent loss of BF3, SiF4, GeF4, SeF4

boiling point: 108°C (HF 40%)

vapour pressure ~25 bar (at ~240°C)

Mostly used in mixtures with other acids

Digestion of minerals, ores, soil, rock and pflants

Complexation required H3BO3 + 4 HF —> HBF4 + 3 H2O


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Complete Digestion of Glass and QuartzProcedure

Sample: 500mg

Acid Mixture: 4.0 ml HNO3 (65%)

4.0 ml HF (40%)

Temperature Program

Step 1 2 3 4

Temp. [°C] 200

Rise time [min] 5

Time [min] 15

Result Clear solution.

Complexation with 1 g H3BO3 at 170°C, 5min

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Sulfuric Acid

Non-oxidizing acid

Dehydration of organic materials

Boiling point 340°C (H2SO4 98%%)

Vapour pressure neglectabel

Mostly used in mixtures with other acids

Digestion of plastics, ores, minerals

Insoluble sulfates for Ba, Pb, Sr


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Complete Digestion of PET and PlasticsProcedure

Sample: 250 mg

Acid Mixture: 1.5 ml HNO3 (65%)

1.5 ml H2SO4 (98%)

Temperature Program

Step 1 2 3 4

Temp. [°C] 220 220 100

Rise time [min] 5 1 1

Time [min] 20 15 10

Result

Clear solution.

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Perchloric Acid

Strongest oxidizing acid

Boiling point 203°C (HClO4 72%%)

Expolsive decomposition at 245°C!


Mostly used in mixtures with other acids (< 20% in HNO3)

Always use digestion temperature < 200°C

Insoluble KClO4

Explosion Hazard ! Use only in exceptional cases!


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Digestion with Liquid Reagents - General AspectsChoice of reagents

Organic 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

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Digestion with Liquid Reagents - General AspectsOrganic matrices - choice of reagents

HNO3 (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)

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Digestion with Liquid Reagents - General AspectsOrganic matrices - choice of reagents

Aqueous 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

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Digestion with Liquid Reagents - General AspectsInorganic matrices - choice of reagents

Pure 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

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Digestion with Liquid Reagents - TheoryDigestion process - critical parameters

Digestion temperature

Digestion time

Chemical potential of digestion reagents

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Digestion with Liquid Reagents - TheoryDigestion temperature

High digestion temperatures → shorter reaction time

Digestion temperatures are limited by:

vapour pressure of digestion acids

temperature resistance of container/vessel materials

pressure resistance of containers/vessels

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Digestion with Liquid Reagents - TheoryPressure buildup in closed digestion

Total pressure p

p = p(CO2

) + p(acid) p(CO2

) = partial pressure of CO2

produced

p(acid) = partial pressure of acid mixture

CO2 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, 200°C → p(CO2) = 22 bar

V = 80 ml, 0.2 g carbon, 200°C → p(CO2) = 8 bar

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Digestion with Liquid Reagents - TheoryExample (60 ml vessel at 200°C):

500 mg carbon develops 930 ml CO2

→ partial CO2 pressure of 26 bar

Acid pressure for HNO3 at 200°C of approx. 10 bar

→ Total pressure approx. 36 bar (60 ml vessel at 200°C)

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Digestion with Liquid Reagents - Theory

Pressure build-up in closed digestion

Total pressure p

p = p(CO2) + p(acid) p(CO2) = partial pressure of CO2

produced

p(acid) = partial pressure of axid mixture

Acid pressure:

Sum of partial pressures of pure acid and water

p(acid) = PH20 + PHCI

Characteristic vapour-pressure curves of acid mixtures

Independent of container volume on condition that acid concentration of solution

remains constant

→ Acid volume must be matched to container/vessel volume

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Digestion with Liquid Reagents - Theory Acid pressure - adjustment of acid volume to container volume

Example:

20 ml HCl (36%, 10 mol/l) in 60 ml container at 200°C

→ vapour-pressure curves: PHCl 36% approx. 70 bar PH2O approx. 14 bar

produce: PHCl (gas) approx. 56 bar

→ nHCl = pHCl*VGas / (R*T) = 0.012 (mol*K/bar*ml) *

pHCl*VGas/T

nHCl = 0.057 mol in gaseous phase and

nHCl = 0.143 mol in solutionTo sum up:

• Acid concentration still comprises 72% of initial concentration

•In practice, less gas may be used, because gaseous reaction

products (CO2, H2, nitrous gas) further increase pressure

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Digestion with Liquid Reagents - TheoryVapour-pressure curves/graphs of pure acids

a. Aqua regia

b. HCl 36%

c. HNO3 91%

d. HCl 22.9%

e. Water

f. Boling point HNO3 100%

g. Boiling point H2SO4 100%

h. Boiling point H3PO4 96%

(Panholzer, LaborPraxis, Oct. 1994, 32)

T [°C]

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Digestion with Liquid Reagents - TheoryDigestion time

Short digestion times recommended → greater throughput of samples

But good control of the process takes priority, e.g.:

Slow heating in warm-up phase → avoids spontaneous reactions

Over-vigorous process control → unnecessary wear and tear on material

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Digestion with Liquid Reagents - TheoryChemical potential depends on

Concentration of reagents employed in the solution

Interaction of reagentsInteraction of reagents with sample water-

Goal:

The concentration of acids should not reduce greatlyduring digestion.

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Practical Conduct of Digestion

Open method at reflux

Max. temperature limited by boiling point of acid mixture

(→ conc. H2SO4)

Allows high weigh-ins

Quality of digestion not always sufficient

Loss of volatile elements (e.g. Hg, lead salts)

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Practical Conduct of DigestionOpen method at reflux

Max. temperature limited by

boiling point of acid mixture

Allows high weigh-ins

Quality of digestion not

always sufficient

Loss of volatile elements

(e.g. Hg, lead salts)

Closed vessel digestion

Max. temperature 260-300°C

Reduced acid consumption

High quality of digestion

No Loss of volatile elements

(e.g. Hg, lead salts)

→ Speed-up of digestion process in closed

vessel

by reaching higher temperatures

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Pressure digestion in steel vessels

Pressure resistance 200 bar

Temperature max. 230°C (briefly 260°C)

Digestion times from about 2 hours to several days

Free from contamination due to PTFE-TFM lining

Different internal volumes (25-250 ml) and therefore weigh-in quantities

Outstanding quality of digestion

No loss of volatile elements (e.g. Hg, lead salts)

High degree of safety, easy operation

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Pressure digestion in steel vessels

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Practical Conduct of DigestionPressure digestion in steel vessels - specimen application

Matrix Weigh-in Acid Temperature Time

Cellulose/starch mg0000 HNO0 °C111 h0

Flour/grain/leaves mg0000 HNO0/HF °C111 h0

Tissue/liver mg0000 HNO0 °C111 - °C111 h0

Fat/oil mg0000 HNO0(poss. H0O0) °C000 h0

Plastics mg000 HNO0/H0SO0 00°C0 0- h0

Carbon/resin mg000 HNO0 °C000 h0

Ceramics/oxides mg000 HF or HCl °C000 0- h0

Steel mg000 HNO0/HCl °C000 h0

Digestion Vessel DAB-3 (250 ml)

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Heat-up behaviour - DAB

0

00

000

000

000

000

000

0 00 00 00 00 00 00 00 00 00 000

Time [min.

T

e m p e r a t u r e [ ° C ]

Hot plate temp. °C000Hot plate temp. °C111

Hot plate temp. °C000

Hot plate temp. °C000

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Rapid, direct sample heating

Rapid, multiple samples thus cost effective

Closed vessel; reduced blank value

higher temperatures are possible

improved reproducibility

reduced amount of reagent

retaining of volatile elements

Why Microwave Digestion?

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Uniform microwave distribution to multiple samples

Pressure resistance of plastic, microwave transparent vessels

Monitoring and control of temperature and pressure

in microwave field

Pressure relief mechanism

Microwave Digestion - Problems and Risks

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Pressure digestion under microwave

Pressure resistance dependent on type of container/vessel (40-100 bar)

Free from contamination through use of PTFE-TFM containers

Different interior volumes (10-100 ml) and therefore weigh-ins

Quality of digestion mostly sufficient

No loss of volatile elements (e.g. Hg, lead salts)

High throughput of samples due to short digestion times (10-60 mins.)

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Pressure digestion under microwave

It is primarily the sample that is heated

Container material (plastic) is only indirectly heated

→ Relatively high digestion temperatures can be reached for short

periods

(30-40 minutes)

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Microwave Heating

Dipole rotation Ionic conduction

Microwave Microwave

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Microwave Heating

Dipole Substances and Ions of Acid and Sample contribute to

Microwave Heating

Blank solutions absorb less microwave energy than sample

solutions and reach lower temperatures therefore

Sample weight and acid volume has similar influence on

termperature

Homogeneous microwave distribution necessary for unifom heating

of multiple samples

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Digestion of 6 gras samples

5 ml HNO3 / 2 ml H2O2

→ 1, 3 ,5: 500 mg gras

→ 2, 4, 6: Blank

Blanks absorb less microwaveenergy and are heated to lower

temperatures, therefore

Detection of Temperature Deviations

Advantages mid-IR-Temperature Measurement – Fast Reaction

Control

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Organic sample

1.Solution is heated by microwave energy

2. At a certain temperature Activation Energy is provided

for spontaneous reaction following:

Eakt + (CH2)x + 2 HNO3 —> CO2 + 2 NO + 2 H2O + Energy

Additional Energy is provided, which heats-up solution

3.Microwave power must be reduced to

- prevent overheating

- allow cool-down of solution

Temperature Control of Exothermic Reactions Control

Microwave

mid-IR temperature control for all vessels required for safe digestion

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Detection of fast, exothermic reactions

Digestion of PET

→ approx. 200 mg

→ 1,5 ml HNO3 / 1,5 ml

H2SO4

Advantages mid-IR Temperature Measurement – Fast Reaction

Control

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Container materials

PTFE → maximum 260º C

PTFE-TFM → maximum 260º C

PFA → maximum 200º C

Quartz (silica) glass → maximum 1,000º C (theoretically)

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Solid PTFE-TFM vessels

DAP-40+ → 40 ml / 40 bar at 230°C

DAP-30+ → 30 ml / 80 bar at 230°C

DAP-60+ → 60 ml / 40 bar at 230°C

DAP-100 → 100 ml / 40 bar at 230°C

DAK-100 → 100 ml / 100 bar at

230°C

Quartz-glass inserts

DAQ-20: → 20 ml / 100 bar at 260°C

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Temperature control

The most important aspect of controlling microwave digestion

Rate of reaction depends on temperature

The 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

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Temperature Control - Principle

IR-measurement at wavelength, where TFM does not absorb IR-radiation

Thermal radiation of the vessel is filtered out

IR-Sensor

Filter

IR-radiation

TFM

Heat radiation of

vessel surface

Heat radiation

of sample

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Temperature profile

Digestion of 6 reference materials

in 5 ml HNO3 / 2 ml H2O2

→ 1, 2: 500 mg leaves

→ 3, 4: 500 mg grass

→ 5, 6: 500 mg tomato

leaves

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Digestion of 6 gras samples

5 ml HNO3 / 2 ml H2O2

→ 1, 3 ,5: 500 mg gras

→ 2, 4, 6: Blank

Blanks absorb less microwaveenergy and are heated to lower

temperatures, therefore

Detection of Temperature Deviations

Advantages mid-IR-Temperature Measurement – Fast Reaction

Control

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Requirements of a modern microwave digestion apparatus

Digestion temperatures of up to 260°C or higher

Sample weigh-ins of up to 1,000 mg

Temperature control in each digestion vessel

Temperature-controlled, programmed heat-up

Recording of all temperature profiles

Optionally: Pressure control in each digestion vessel

Recording of all pressure profiles

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The Only Microwave with Non-Contact Pressure

and Temperature Control

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speedwave MWS-2

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REALIZING YOUR IDEAS

Many thanks for your attention!

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