Ion Chromatography: Separation of Polar and Ionic Compounds

Tracy Benson, Ph.D.

Typical LC System Operation

!   Analytes adsorb onto solid surface due to analyte/surface intermolecular forces !  “Good” chromatography occurs from proper selection of adsorbent material and mobile phase !  Changes in conc gradient of mobile phase disturbs the analyte/surface forces and analyte migrates through bed and onto the dectector  

Mobile Phase Conc Gradient !

Packed Tubular Column

ADSORPTION  PHENOMENON  

Adsorption is " The attraction of molecules onto a

particle surface " Driven by a concentration gradient " Interacted by molecular forces

(short range and long range) between the adsorbate and adsorbent

" An exothermic process

CA,b

Porous Pellet

Internal

Diffusion

CA,s CA

IC Solid Surface Chemistry

Retention is based on the affinity of different ions for the site and on a number of other solution parameters (pH, ionic strength, counterion type, etc.).

Ion - Pair Chemistry

Ion: particle formed when a neutral atom or group of atoms gains or loses one or more electrons. An atom that loses an electron forms a positively charged ion (cation); an atom that gains an electron forms a negatively charged ion (anion).

In Solution

Aaqm+ + Baq

n- "! (nAm+ · mBn-)org

•  Ion pairs are oppositely charged ions held together by coulombic attractions, non-covalently, and behave as a single unit

On a Solid Surface

Am+ + Bm

- "! Abm

Am+ + Ym

- + Ls "! SYLs

Bm- + Zm

+ + Ls " ! BZLs

Am+ + Bm

- + Ls " ! ABLs

•  Ion pair formation in mobile phase

• Adsorption of solute ion onto ligand

• Adsorption of counter-ion onto ligand

• Adsorption of ion pair onto ligand

EQUILIBRIUM DRIVEN BUT KINETICALLY CONTROLLED

Eluent  Bo*le  (CO3/HCO3)  Pump  

Guard  Column  

Analy5cal  Column  

Suppressor  

Regen  In  (H2SO4)  

Conduc5vity  Cell  

Chromatograph  So?ware  Ion  

Exchange  Separa5on  

Post-­‐Suppression  Conduc5vity  

Data  Handling  and  Instrument  Control  

Sample  Injec5on  

Regen  Out  (H2SO4)  

Conventional Ion Chromatographic System Anion Analysis

Pump Module

Pump Module

Specification Highlights •  Flow

–  Range = 0.001 to 10 mL/min –  Accuracy = 0.1% at 1 mL/min –  Precision = 0.1% at 1 mL/min

•  Gradient reproducibility = +/- 0.5% •  Gradient accuracy = 0.5% •  Delay volume < 400 µL •  Pressure range = 50 to 5000 psi •  Pressure ripple typically < 1.0%

DP  Dual  Pump  

Eluent Generator Module

Eluent Generator Module

•  Same device supports single or dual chemistry

–  OH, MSA, CO3,

•  CO3:HCO3 eluents •  Supports CR-TC technology

•  Supports CRD technology •  Supports EPM

(Electrolytic pH Modifier)

•  Slide-out tray

•  Optical leak sensor •  Status bar for basic operation

•  Same footprint for single or dual

EG  Eluent  Generator  

RFIC  System  Advantages  •  Provides  reliable  and  convenient  eluent  sources  and  eliminate  the  conven5onal  eluent  prepara5on  errors  

•  Simplifies  and  performs  both  isocra5c  and  gradient  ion  chromatographic  separa5ons      

•  “Just  add  water”—prolongs  pump  seals,  reduces  maintenance  

•  Be*er  gradient  performance    •  Lower  cost  of  ownership    

Eluent Generator Module

Advantages of Using Eluent Generators

•  Use only deionized water •  On-line high purity eluent

generation •  Insure reproducible eluent

concentration— minimal gradient delay

•  Reduce pump maintenance •  Gradient eluent

programming now practical

MSA Eluent Generation for Cation Analysis

MSA–  Electrolyte  Reservoir  

EluGen®  MSA  Cartridge  

Anion-­‐Exchange  Connector  MSA  

Genera5on    Chamber  

Vent  

Pump  

H2O  

                   Pt  Anode  

Pt  Cathode  (2  H2O  +  2e–                    2OH–  +  H2)  

[MSA]  α Current  

Flow  Rate  [  –  ]  

[  +  ]  

MSA  +  O2   MSA  

O2  

Degas  

Unit  

(H2O                  2H+  +  1/2O2  +  2e–)  MSA  =  Methanesulfonic  Acid  

CR-­‐CTC  Ca5on  Trap  

MSA–  

Improved  Performance  for  Trace  Anion    Analysis  Using  a  KOH  Eluent  Generator  

Column: IonPac® AG11, AS11, 2 mm Eluent: (A) NaOH

(B) KOH (EGC-KOH cartridge) 0.5 mM to 2.5 min, to 5.0 mM at 6 min, to 26 mM at 20 min

Flow Rate: 0.5 mL/min Inj. Volume: 1.0 mL Suppressor: ASRS®, external water mode Detection: Suppressed conductivity

Peaks: 1. Fluoride 0.37 µg/L (ppb) 2. Acetate 1.0 3. Formate 0.93 4. Chloride 0.44 5. Nitrite 0.27 6. Bromide 1.0 7. Nitrate 0.33 8. Carbonate – 9. Sulfate 0.64 10. Oxalate 0.39 11. Phosphate 1.1

µS  

1.20  

0  

(A)  ConvenRonal                Gradient  

11  10  

9  

8  

7  6  

5  4  

3  2  1  

Minutes  0   5   10   15   20  

0  

µS  

1.20  

(B)  EG  Gradient  

11  10  

9  

8  

1  2  3   4

567

Column:    IonPac®  AG11,  AS11,  4  mm    

Eluent:  0.5  to  25  mmol/L  KOH  

From:  EG40  or  0.1  mol/L  KOH  

Flow  Rate:  2  mL/min  

Injec5on:  25  µL  

Detec5on:  Conduc5vity  a?er  ASRS®  

 suppression,  recycle  mode  

Peaks:  1.  Fluoride  0.2    mg/L    2.  Chloride  0.3    3.  Nitrate    1.0    4.  Sulfate    1.5    5.  Phosphate  1.5  

µS  

0   2   4   6   8   10  0  1  2  3  4  

(A)  Gradient  EG40  

5  4  3  

2  1  

Minutes  

0   2   4   6   8   10  0  

1  

2  

3  

4  

µS  

(B)  ConvenRonal  Gradient  

1   2  3  

4  

5  

Detector Module

•  Three  dis5nct  sec5ons  for  separa5on,  detec5on  and  automa5on  

•   Separate  temperature  control  for  columns  and  detectors  

Automa5on  

Detec5on  

Separa5on  10  to  70  °C  

15  to  40  °C  

Dual  Thermal    Compartments  

Sec5on   DC  Module  

Detector Module

Role of Suppression in Conductivity Detection

•  Neutralize eluent and reduce background conductance to a low or negligible level

•  Increase sensitivity by converting the analyte ions to highly conductive form

•  Removes sample counter ions (cations or anions)

λ = λΝa+ + λCl-

⇓

λ = λΗ+ + λCl-

Equivalent  Conduc5vi5es  

Anions λº OH– 198

F– 54

Cl– 76

NO3– 71

Acetate– 41

Benzoate– 32

20585  

Cations λº H+ 350

Li + 39

Na+ 50

K+ 74

CH3 NH3+ 58

N(CH3 CH2)4+ 33

Range of IC Autosamplers

5 mL

AS40 AS AS-HV

10 mL

250 mL