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ic_theory_20041011_e.ppt 1 IC IC IC IC IC An introduction to Ion Chromatography An introduction to Ion Chromatography Theory of IC K. H. Viehweger Metrohm AG, Herisau, Switzerland

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Page 1: Theory of ion chromatography - · PDF fileic_theory_20041011_e.ppt 1 IC IC IC IC IC An introduction to Ion Chromatography Theory of IC K. H. Viehweger Metrohm AG, Herisau, ... HPLC

ic_theory_20041011_e.ppt 1

IC ICIC IC

IC

An introduction to Ion ChromatographyAn introduction to Ion Chromatography

Theory of IC

K. H. ViehwegerMetrohm AG, Herisau, Switzerland

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ic_theory_20041011_e.ppt 2

IC ICIC IC

IC

What is chromatography? What is chromatography? about history and physical chemistry

Where does the chromatography actually happen?Where does the chromatography actually happen?from ion exchange to ion pairs

Which detection can be used?Which detection can be used?from voltammetry to conductivity

What is suppressed ion chromatography?What is suppressed ion chromatography?from high to low backgrounds

ChromatographyChromatographyfrom beginners to experts

ContentContent

Theory of IC

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IC ICIC IC

IC

DefinitionDefinition«An analytical method where a substance mixture appearing in just one colour is separated in a way that different colours become visible. The method is used to separate chemical substances which are chemically quite similar and though difficult to separate»

Greekchroma = colourgraphein = to write

HistoryHistory

Theory of IC

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IC ICIC IC

IC

185019351942

1947195319571959

1967-701975

1979

1976-80

topsoilssulfonated and aminated polymerssulfonated polystyrene/divenylbenzene resins(Manhattan project)aminatedion exclusion chromatographymacro pore iotheoretical backgroundpelicular ion exchange materialsion exchange chromatography wdetectioconducsuppressionion pair chromatography

as ion exchanger for Mg2+, Ca2+ and NH4+

PS/DVB resins as ion exchanger

n exchanger

ith conductivity n and stripper (suppressor)

tivity detection with electronic

Thomson, WayAdams, Holmsd’Alelio

McBurneyWheaton, BaumannCorte, Meyer et al.HelfferichHorvath, KirklandSmall, Stevens

Gjerde, Fritz

Waters, Bidlingmeier, et. al

, Baumann

, Schmuckler

LC

HPLC

HistoryHistory

Theory of IC

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IC ICIC IC

IC

take a piece of papermake a little drop with «black» ink

just add water to it’s centersee the colors that make up

the «black»ink

Going back to schoolGoing back to school

Chromatography

A simple experiment from schoolA simple experiment from school

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IC

DefinitionDefinition«The term «chromatography» is the general name for a wide range of physico-chemical separation processes in which the components to be separated are distributed between a stationary and a mobile phase.»

gaseousmobile phasemobile phase liquidstationary phasestationary phase liquid → GLC, LLC

solid → GSC, LSC(HPLC - IC)

Physico Physico chemicalchemical

Chromatography

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IC ICIC IC

IC

Types of chromatographyTypes of chromatography

Mobile phase dissolves and transports the analyteStationary phase retains the analyte

adsorbing ⇒ adsorption chromatography

dissolving ⇒ distribution chromatography

reacting ⇒ ion exchangechromatography

Column interactionsColumn interactions

Chromatography

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Methods of chromatographyMethods of chromatographyBased on the polarities of the stationary and mobile phases a distinction is made between the following methods:

Group 1 – traditional TLC + HPLC1. normal phase chromatography2. reversed phase chromatographyGroup 2 – Ion Chromatography4. ion exchange chromatography3. ion pair chromatography5. ion exclusion chromatography

PolaritiesPolarities

Chromatography

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Group 1 Group 1 –– traditional TLC + HPLCtraditional TLC + HPLC1. Normal phase chromatographyStationary p. = polar (e.g. SiO2) –mobile p. = non polar (e.g. n-hexane)

2. Reversed phasechromatography

Stationary p. = non-polar (e.g. C18) –mobile p. = polar (e.g. acetonitrile or methanol/water)

Traditional methodsTraditional methods

Chromatography

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IC

Group 2 Group 2 –– ICIC4. Ion exchange chromatographyCations and anions form a weak ionic binding with the stationary phase.C: Stationary p. = polar (e.g. R-SO3

–) – mobile p. = polar (e.g. HNO3 aq.) orA: Stationary p. = polar (e.g. R-NR3

+) – mobile p. = polar (e.g. Na2CO3 aq.)

IC methodsIC methods

Chromatography

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IC

Group 2 Group 2 –– ICIC3. Ion pair chromatographyCations and anions react to a non-ionic molecule by adding a lipophilic counter ion. The resulting non-polar molecules are then separated in the RP-mode.Stationary p. = non-polar (e.g. C18) – mobile p. = polar (e.g. acetonitrile or methanol/water)

IC methodsIC methods

Chromatography

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IC

Group 2 Group 2 –– ICIC5. Ion exclusion chromatographyBy adding H+-ions the stationary phase is transformed into a non ionic but polar Donan membrane. Only non dissociated molecules can enter this membrane. If they dissociate they are excluded from the stationary phase. The separation is though depending on the dissociation constant of the respective molecules.– mobile p. = polar (e.g. H2SO4 aq.)

IC methodsIC methods

Chromatography

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DefinitionDefinition«Ion chromatography includes all chromatographic methods that separate ionic substances and substances that dissociate easily. These methods are ion pair chromatography (3), ion exchange chromatography (4) and ion exclusion chromatography (5). Analyte and mobile phase are initially always polar and/or ionic. Ion exchange chromatography is the most important separation mechanism in ion chromatography.»

ResumeResume

Chromatography

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IC

LiquidLiquid--solidsolid--chromatographychromatographySince the introduction of high pressure or high performance chromatography (HPLC) at the end of the sixties, liquid chromatography has developed into one of the most comprehensive and important methods of modern instrumental analysis. Ion chromatography is an «aliquot» of the HPLC-family.

[mobile phase = liquid and stationary phase = solid]

HPLC and ICHPLC and IC

Chromatography

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IC ICIC IC

IC

Eluent Pump Injector ColumnSuppressor

Detector

Most popular set upMost popular set up

Chromatography

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IC

Eluent Pump Injector Column Detector

Where does it actually happen?Where does it actually happen?

Chromatography

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IC

The stationary phaseThe stationary phaseCations need a cation exchanger and anions need an anion exchangerHow a stationary phase is built

Styrene

Divinylbenzene

Cationexchanger

Ion exchangeIon exchange

Chromatography

⇒ ⇒

Styrene-divinylbenzene resinAnionexchanger

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IC

Composition of the Composition of the stationary phasestationary phase

SubstratesPolystyrene/divinylbenzenePolymethacrylatePolyalcoholHydroxyethylmethacrylate (HEMA)Silicate

Anion exchangerquaternary ammonium groupsalkyl amineshydroxy-alkylaminesalkyl amines with acrylate type crosslinking

Ion exchangeIon exchange

Chromatography

substrate / resin carriera spacer groupa group that carries the separating capacity

Cation exchangersulfonatescarboxylates

Spaceralkyl chain

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IC

Mobile PhaseMobile PhaseThe mobile phase dissolves and carries the sampleThe mobile phase is usually aqueous

anions (I)Phthalic acidSalicylic acid p-Hydroxybenzoic acidBenzoic acidBorateBorate/GluconatePotassium hydroxide

...

anions (II)Carbonat/bicarbonatePotassium hydroxideBoratecations (I)

Nitric acidTartaric acidTartaric acid/dipicolinic acid Tartaric acid/citric acidSodium dihydrogene phosphateoxalic acid/ethylene diamine/acetone

...

Ion exchangeIon exchange

Chromatography

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IC

Ion exchangeIon exchange

Chromatography

CationCation separation mechanismseparation mechanismStationary phase and mobile phase compete for the analyte

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IC ICIC IC

IC

C2 – 250Tartaric-/dipicolinic acid/crown

ether; 4/0.45/0.05 mmol/L1 Lithium

SodiumNH4

+

4 MEA 1.05 DEA 1.06 Potassium 4.87 TEA 5.08 MDEA 4.89 Calcium 5.010 Magnesium

0.52 1.03

4.0

1.0

Ion exchangeIon exchange

Chromatography

ppm

CationsCations

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IC

Ion exchangeIon exchange

Chromatography

I II IIIIII IVIV VV VIVI VIIVII VIIIVIII II IIII III IV V VI VII VIIIH He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac Ku

CationsCations (selected)(selected)

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IC

Anion separation mechanismAnion separation mechanismStationary phase and mobile phase compete for the analyte

Ion exchangeIon exchange

Chromatography

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IC

A Supp 5 – 250NaHCO3/Na2CO3; 1/3.2 mmol/L1 Fluoride

ChloriteBromate

4 Chloride 5.05 Nitrite 5.07 Chlorate 5.08 Bromide 5.09 Nitrate 5.010 Phosphate 5.011 Sulfate

5.02 5.03

5.0

5.0

Ion exchangeIon exchange

Chromatography

ppm

AnionsAnions

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IC

Ion exchangeIon exchange

Chromatography

I II IIIIII IVIV VV VIVI VIIVII VIIIVIII II IIII III IV V VI VII VIIIH He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac Ku

Anions (selected)Anions (selected)

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CationsCationsCations interact with the basic ion-exchange sites of the stationary phase. According to the strength of binding (ion exchange equilibrium constant) these will be eluted earlier or later by the following protons from the eluent.

Anions Anions Anions interact with the acidic ion-exchange sites of the stationary phase. According to the strength of binding (ion exchange equilibrium constant) these anions will be eluted earlier or later by the following carbonate ions from the eluent

SeparationDifferent ion exchange equlibrium constants lead to different retention times of the respective cations or anions and that means separation of the substances which are «chemically quite similar».

Ion exchangeIon exchange

Chromatography

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IC

Ion exclusionIon exclusion

Chromatography

Anion separation mechanismAnion separation mechanismLipophilic interaction between analyte and stationary phase

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IC

Metrosep Organic AcidsPerchloric acid; 0.5 mmol/L

1 LactateFormateAcetate

4 Propionate 1.05 Butyrate 1.07 iso-Butyrate 1.08 Valerate 1.09 iso-Valerate

1.02 1.03

1.0

1.0

Ion exclusionIon exclusion

Chromatography

ppm

Organic acidsOrganic acids

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Mobile PhaseMobile PhaseAnions are excluded and elute with the void volume in the solvent front. Anions of organic acids form with protons weak organic acids. These acids are only to a small degree dissociated and have a low polarity.

Stationary phaseStationary phaseOn the ion exchanger a Donan membrane is formed. This membrane has a low polarity.

InteractionInteractionAs long as the organic acids are not dissociated they can stay in the Donan membrane. If they are dissociated they become ionic. In this case they are excluded from membrane.

Attention! Attention! Cations will be separated by ion exchange and therefore may disturb the chromatogram.

Ion exclusionIon exclusion

Chromatography

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IC

Ion pairIon pair

Chromatography

CationCation and anion separation mechanismand anion separation mechanism«RP» reversed phase mechanism

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IC

Mobile PhaseMobile PhaseThe mobile phase is of high polarity (e.g. acetonitrile or methanol/water)

Stationary phaseStationary phaseThe stationary phase (e.g. C18) is of low polarity a type of «RP»-Phase.

CationsCationsAnalyte cation (e.g. Na+) plus lipophilic anion (e.g. alkyl sulphonic acid) form a non polar molecule

AnionsAnionsAnalyte anion (e.g. Cl–) plus lipophilic cation (e.g. tetra butyl ammonium) form a non polar moleculeThe lipophilic end of the modified anion or cation molecule interact with the stationary phase in the «RP»-mode. This interaction leads to different retention times and that means separation of substances which are «chemically quite similar».

Ion pairIon pair

Chromatography

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IC ICIC IC

IC

Eluent Pump Injector Column Detector

DetectionDetection

Chromatography

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IC

Conductivity detectorAmperometric detectorUV/VIS detector (optical)RI detector (optical)IC/MS

Types of detectionTypes of detection

DetectionDetection

Chromatography

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IC

Measuring principleMeasuring principleA defined decent potential is applied to a three electrode system consisting of reference-, working- and auxiliary-electrode. If any oxidizable substance pass these electrodes an oxidation process takes place. A current corresponding to the concentration of the respective substance is measured.

ElectrochemicalElectrochemical

Detection

Simple connection to any IC- or HPLC-systemLarge variety of electrode materials (glassy carbon, carbon paste, silver, gold, etc.)Highly sensitiveVery selectiveExcellent price/performance ratio

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CharacteristicsCharacteristicsSensitive to UV-active ions e.g. nitrite, nitrate, thiosulfateAlmost no interference by non-chromophoric ions e.g. chloride, sulphateEluent must be UV-transparentLimited possibilities of organic modifiers

UV/VISUV/VIS

Detection

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CharacteristicsCharacteristicsUV-adsorbing eluent e.g. phthalic acidsAll ions with no adsorbance at the UV-wavelength applied aredetectedDetector must be extremely sensitive because measurements are performed with very low light intensitySensitivity is lower than the sensitivity achieved by conductivity detection

UV/VIS indirectUV/VIS indirect

Detection

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IC

CharacteristicsCharacteristicsSelective determinationsTime consuming (wet chemistry is needed)More complex systemAdditional peak broadeningLast possibility if all other approaches fail

UV/VIS with UV/VIS with pcrpcr

Detection

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IC

CharacteristicsCharacteristicsQualitative and quantitative informationDetermination of single atomsDetermination of molecule fragmentsExpensive set upHigh operating costs

IC/MSIC/MS

Detection

++ ++++++ + ++ ++

Quadrupole

LensesFragmentation zone (CID)

IC outlet

SkimmersCapilla

ryOctopo

le Detector

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IC

What is What is conductometryconductometry??Conductometry is the measurement of conductivity – a conductometricdetector measures the electrical conductivity of ions in a solution. This is done by applying an electrical field between two electrodes. The ions migrate in this field. The anions migrate to the anode and the cations to the cathode. The electrical resistance of the solution is measured. The conductivity is the reciprocal of the resistance. Alternating current is used in order to avoid substance changes and the formation of diffusion boundary layers at the electrodes.

κ =1R cK*

R = resistance [Ω]

Kc = cell constant [1/cm]

κ = specific conductivity [1/ Ω or S]

ConductivityConductivity

Detection

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What is specific conductivity?What is specific conductivity?The specific conductivity κ of an eluting ion depends on the concentration c of the individual ions i, the charge Zi, and the equivalent conductivity Λi. The equivalent conductivity Λi is a concentration-dependent quantity. The values listed can be applied for concentrations below 0.001 mol/L. Zi is the charge of the respective ion.

Λi = equivalent conductivity [S∗cm2/mol]zi = charge [ ]ci = concentration [mol/L] κ = specific conductivity [1/ Ω or S]Σ = sum of all ions – anions and cations

κ = ∑ ( * * )Λ i i iz c

ConductivityConductivity

Detection

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IC

Values for equivalent conductivityValues for equivalent conductivityAnions

OH– 198F– 54Cl– 76Br– 78I– 77NO2

– 72NO3

– 71HCO3

– 45½CO3

2– 72½SO4

2– 80½Phthalate 38

CationsH+ 350Li+ 39Na+ 50K+ 74NH4

+ 73½Mg2+ 53½Ca2+ 60Sr2+ 59½Ba2+ 64½Zn2+ 53½Cu2+ 55

ConductivityConductivity

Detection

[c] < 0.001 mol/L; Λ∝ = equivalent conductivity [S∗cm2/mol]

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IC

What influences the measured value?What influences the measured value?

κ = ∑ ( * * )Λ i i iz cκ =

1R cK*

the equivalent conductivity of the respective ions

the charge of the respective ions

the cell constant ⇒ ⇒⇒ which are constant

the temperature ⇒ ⇒ ⇒ which should be constant

the concentration ⇒ ⇒ ⇒ which is the actual result

Λ ≈ T ≈ 2 %/°CκEl El El El Elc c. . . . .* *= ++ + − −Λ Λ

ConductivityConductivity

Detection

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IC ICIC IC

IC

Ion chromatography without suppression

Ion chromatography with suppression

With or without?With or without?

Chemical suppression

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IC

CharacteristicsCharacteristics• Background conductivity = eluent• Conductivity in a peak = eluent plus sample• Measured value = difference between eluent plus sample and

background

κ κ κDet Peak El. .= − κDet Smpl Smpl El Smpl Smpl Elc c. . . . . . .*( ) *( )= − + −+ + + − − −Λ Λ Λ Λ

WithoutWithout

Chemical suppression

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Determination of Determination of cationscations and anionsand anions

Keywords:one column technique – non suppressed ion chromatography –measurement of the difference between the conductivity of thesample ion and the eluent ion

κDet Smpl Smpl El Smpl Smpl Elc c. . . . . . .*( ) *( )= − + −+ + + − − −Λ Λ Λ Λ

κDet Smpl Smpl Elc. . . .* ( )= −− − −Λ ΛκDet Smpl Smpl Elc. . . .* ( )= −+ + +Λ Λ

The counter ion of the sample is not retained and eluted in the void volume so the counter ion is always that of the eluent

WithoutWithout

Chemical suppression

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Ion chromatography with chemical suppressionIon chromatography with chemical suppression

In cation chromatography an anion exchanger is used– all anions are replaced by OH–

in anion chromatography a cation exchanger is used– all cations are replaced by H+

By this reaction an eluent with high conductivity is trans-ferred to water (+ CO2↑) which is of low conductivity.

Suppression reduces the background conductivity.

Suppression changes the counter ions in the sample.

WithWith

Chemical suppression

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CharacteristicsCharacteristicsBackground conductivity = water = zeroConductivity in a peak = water plus sampleMeasured value = difference between water + sample and background

κ κ κDet Peak El. .= −

κDet Smpl Smpl El Smpl Smpl Elc c. . . . . . .* ( ) *( )= − + −+ + + − − −Λ Λ Λ Λ

WithWith

Chemical suppression

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Determination of Determination of cationscations and anionsand anions

Keywords:two column technique – suppressed ion chromatography – pcrmeasurement of the sum of the sample ion and H+ or OH–

κDet Smpl Smpl El Smpl Smpl Elc c. . . . . . .* ( ) *( )= − + −+ + + − − −Λ Λ Λ Λ

κDet Smpl Smpl Hc. . . .* ( )= +− +Λ ΛκDet Smpl Smpl OHc. . . .*( )= ++ −Λ Λ

Eluent conductivity is zero andthe counter ion is always OH– or H+

WithWith

Chemical suppression

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(Sample = NaCl – Eluent = HNO3)

κDet Smpl Smpl OHc. . .* ( )= ++ −−Λ Λκ Det Smpl Smpl Elc. . . .* ( )= −+ +Λ Λ

κ Det Smpl Na Hc. . * ( )= −+ +

+ +Λ Λκ D e t S m p lc. . * (5 0 )= − 3 5 0

300*.. −= SmplDet cκ

IC without suppression IC without suppression –– Cations Cations –– IC with suppressionIC with suppression

(Sample = NaCl – Eluent = HNO3)

κDet Smpl Na OHc. . * ( )= ++ −

+ −Λ Λκ D et S m p lc. . * (5 0 )= + 1 9 8

κ D e t S m p lc. . *= 2 4 8

ComparisonComparison

Chemical suppression

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(Sample = NaCl – Eluent = NaOH) (Sample = NaCl – Eluent = NaOH)

κ Det Smpl Smpl Elc. . . .* ( )= −− −Λ Λκ Det Smpl H Clc. . * ( )= ++ −

+ −Λ Λ)(* ... ΛΛ −−−− −= OHClSmplDet cκ

)19876(*.. −= SmplDet cκ

112*.. −= −SmplDet cκ

κ D et Sm plc. . * ( )= +350 76

κDet Smpl H Smplc. . .* ( )= ++

+ −Λ Λ

κ D e t S m p lc. . *= 4 2 6

IC without suppression IC without suppression –– Anions Anions –– IC with suppressionIC with suppression

ComparisonComparison

Chemical suppression

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ComparisonComparison

Chemical suppression

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high background conductivityanions with low sensitivitycations with high sensitivityhigh linearity in calibrationlow cost set upfreedom of choice for eluents

low background conductivityanions with high sensitivity(detection limits 2-4 x lower)cations with high sensitivitynon linear calibrationmore expensive set upeluents must react to H2O

ComparisonComparisonIC without suppression IC with suppressioIC without suppression IC with suppressionn

Chemical suppression

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Housing

Anode

Cathode[H2SO4]

Ion exchangemembranes

H+ ↓

OH–↑

OH–↑OH–↑

OH–↑

OH–↑

H+ ↓+ Na+ ↓

Na+ ↓Na+ ↓

OH–↑OH–↑

H2O ↓ Eluent/Sample ↓ ↑ H2O

H2 + O2

↑ Eluent/Sample

2 OH– – 4 e– → O2 + 2 H+

H2O → H+ + OH–

2 H+ + 2 e– → H2

Na+ → H+

Cl– → OH–

ASRSASRS

Suppressors

«Auto self regenerating suppressor» for anions or cations

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Membrane SuppressorMembrane SuppressorContinuous regenerationNo additional solventsNot solvent stableExpensiveLimited lifetimeNoise 3-5 nS

EvaluationEvaluation

Suppressors

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Metrohm Suppressor Module «MSM»Metrohm Suppressor Module «MSM»

Suppressors

Na+ → H+ / Na2CO3 → H2CO3

Me+ → H+ / MeCl → H+ + Cl–

← H2SO4

Regeneration

Rinsing← H2O

Suppression

Packed bed suppressorPacked bed suppressor

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Metrohm Suppressor Module «MSM»Metrohm Suppressor Module «MSM»Regeneration outside the analytical lineRegeneration with organic solvents (e.g. 25 % acetone) possibleRegeneration with strong acids possibleBack pressure stability 2 MPa without effecting the analytical runUnlimited pressure stability without being destroyedLong lifetime and though inexpensiveLow noise 0.2-0.5 nS

EvaluationEvaluation

Suppressors

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Suppressors

H2CO3 → H2O + CO2

From «MSM»

CO2 →

Removal

Outlet↓ H2O

Sequential suppressionSequential suppression

Metrohm COMetrohm CO22 Suppressor «MCS»Suppressor «MCS»

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EvaluationEvaluation

Suppressors

Metrohm COMetrohm CO22 Suppresser «MCM»Suppresser «MCM»separation advantages of the carbonate / bicarbonate eluent system in combination with the low background of a hydroxide eluentlower detection limits because of greater peak areasLOD 0.7 – 1.9 ppb with 20 µL injection volume using the Metrohm 761 Compact ICno increase of chemical and/or electronic noisegradient capabilitiessignificantly reduced carbonate influenceeasy upgrade of existing systems

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Ion ChromatographyIon ChromatographyPrinciples of chromatography Chromatographic separation techniquesIon chromatography separation techniquesDetection in ion chromatographyConductivity measurementsSuppression reactionConductivity after suppression reaction

ResumeResume

What is IC?