6.0 - Ion Chromatography Notes
Definitions
Ion Chromatography is based on the equilibration of solute ions between the solvent, acting
as the Mobile Phase and charged ion-exchange sites on a solid surface acting as the Stationary Phase.
Structural Definitions
The Mobile Phase is usually an aqueous solution of salts of weak or strong Bronstead acids
and bases.
The Stationary Phase is usually a “bed” of either organic plastic beads or inorganic particles.
In both cases, the surfaces of the Solid Phase carry fixed charged functional groups of weak or strong
Bronstead acids or bases.
The organic plastic Stationary Phase, often called a “resin bed”, consists of small, amorphous
non-crystalline particles of styrene and divinylbenzene (1%-16%) copolymers. This material can be
used over a pH range 2-12, for small atomic or molecular ions (M.W.<500) which can penetrate the
small pores of the resin. The resin can be made with “dissociable functional groups” fixed to the
surface. These groups can be designed to act as either Bronstead acids or bases and react with water to
give either negatively or positively charged groups respectively.
The inorganic particulate Stationary Phase usually consists of small, amorphous noncrystalline particles of “silica gel” or “alumina”. Silica gel is chemically acidic and acts as a cation
exchange medium over a pH range pH 2-8. Alumina is chemically basic and acts primarily as an anion
exchange medium over the pH range 6 to 11. Alumino-silicate materials called “zeolites” can be used
over almost the full pH range and exchange cations or anions as needed.
Functional Definitions
Retention is based on the attraction between solute ions and charged sites on the stationary
phase. The Retention Time is a function of Stationary Phase retention. The stronger the retention, of
an anion/cation exchanger Stationary Phase, the longer the Retention Time on the IC column
For ions of the same radius. Retention depends directly on ion charge. In general, the greater
the ion charge, the greater the attraction for ion exchange sites. Typically retention increases from
monovalent, to divalent to trivalent ion charge..
For ions of equal charge, retention also depends directly on ion size. The larger the ion radius,
the more polarizable the ion, and the more strongly it is attracted to ion exchange sites
The Elution Order depends on the Selectivity of a Stationary Phase. Strong versus weak
exchanger resins can produce quite different elution orders. The elution order of halides is F-, Cl-, Brand I-, which corresponds to increasing size of these ions.
The Eluent Ionic Strength is defined by the eluent buffer concentration at a fixed pH. Since
buffer ions replace analyte ions on the Stationary Phase, the higher the Ionic Strength, the shorter their
retention times. Since buffers are prepared with polyvalent weak acids, the ionic strength varies
with pH according to the buffer pKA ;
HPO42-(aq) + H+(aq) ⇆ H2PO4-(aq)
divalent
monovalent
thus the retention time of analyte ions can be changed with pH.
Applications
In anion-exchange chromatography, the positively charged sites on the stationary phase
attract solute anions
In cation-exchange chromatography, the negatively charged sites on the stationary phase
attract solute cations
Because the method meets the criteia of the Figures of Merit, Ion Chromatography is often the
method of choice for routine anion analysis and has many applications in cation analysis.
Anion Chromatography
Equilibrium
Solute anions are attracted to the positively charged sites covalently bound to the surface of the
Stationary Phase;
Strong
Quaternary ammonium groups (RNR'3+) remain cationic at all pH
Weak
Tertiary ammonium groups (RNR'2H+) are deprotonated in moderately basic solution
and lose anion-exchange ability at high pH
Elution of Anions
In general, trivalent, divalent and monovalent eluents are need to elute tri-, di- and mono-valent
analyte ions, respectively
Relative Elution Strengths
Eluent
NaOH
NaHCO3
Na2CO3
NaI
Strength
weak
weak
moderate
strong
Using aqueous OH- solution as eluent (e.g. 10 mM NaOH), anaytes such as F-, Cl- and SO42- are
eluted from the separator column. The anions then cannot be easily detected because the eluent
contains a high concentration of Na+ and OH- whose high conductivity obscures that of the analyte
ions
The advantage of using a mixture of weeak and moderate eluents, NaHCO3 / Na2CO3 is
that ;
1
they can separate both mono- and di-valent anions in a single injection
2
their eluent strength is easily adjusted by changing the ratio of NaHCO3 / Na2CO3
3
the constituent carbonate salts of high purity are inexpensive
Anion chromatography is widely used in the beverage industry to check the organic and
inorganic acid content of various soft drink (cola) formulations
Certain acids (e.g. citrate) contribute significantly to the flavor of a carbonated soft drink, so it
is important to monitor their types and concentrations
To analyze a soft drink (and resolve for citrate), the sample is first degassed, then diluted,
filtered, and injected directly onto the column for anion separation
Suppressed-ion Anion Chromatography
Using an aqueous CO32- solution as eluent (e.g. 2 mM Na2CO3),. analytes such as NO3- and SO4
2are eluted from the separator column. However, these also cannot be easily detected because the
eluent contains a high concentration of Na+ and CO3 2- whose high conductivity obscures that of the
analyte ions.
Sulfonate groups (-SO3-) in the ion suppressor membrane repel anions, but allow cations (Na+
and H ) to pass freely. Therefore, Na+ can be replaced by H+, making H2CO3, HNO3 and H2SO4 This
results in removal of electrolyte (Na2CO3) before the conductivity measurement, allowing the other
anions to be detected.
+
Dependence of Analyte Detection on Eluents
Eluent
Conductivity Detector Sensitivity Background Noise
Signal / Noise
H2CO3
very low
low
low
weak
HNO3 /H2SO4
high
high
low
strong
Cation Chromatography
Equilibrium
Solute cations are attracted to the negatively charged sites covalently bound to the Stationary Phase;
Strong
Sulfonate groups (RSO3-) remain anionic even in strongly acidic solutions
Weak
Carboxylate groups (RCO2-) are protonated at pH 4 and lose cation-exchange capacity
at lower pH
Elution of Cations
In general, monovalent cations are eluted using dilute HCl (5 mM). Divalent analyte ions
are eluted with a solution of m-phenylenediamine / HCl (1.5 mM)
Dependence of Eluent Ionic Strength on Ion Type
Ion Charge
Ionic Strength
trivalent
high
divalent
moderate
monovalent
low
Suppressed-ion Cation Chromatography
Using HCl as eluent,. KCl and CaCl2 emerge from the separator column. Using quaternary
amine groups (-NR 3+) in the ion suppressor membrane, Cl- is replaced by OH-, making HOH and a
solution of KOH and Ca(OH)2. This removes unwanted electrolyte (HCl) prior to conductivity
measurement. Then the resulting HOH has very low conductivity and does not interfere with detection
of KOH and Ca(OH)2 which have high conductivity detector response.
Implementation
Detectors for Ion Chromatography
Several types of detectors are theoretically possible but UV-visible absorbance,
electrochemical, and fluorescence detectors are not compatible with the ion chromatograph.
Conductivity Detector
This is based upon the electrical conductivity of an ionic solution when placed between two
oppositely charged electrodes. Any ions present in solution are attracted to electrodes of opposite
charge and as they move towards these respective electrodes, they carry an electrical current between
the electrodes.
Ohm's law is obeyed by these ionic solutions:
I = V/R = V κ
where κ is the conductance of the solution.
Conductivity detection is universal and has a high selectivity for ions (with pKA or pKB < 7).
Applying the Figures of Merit, it has limits of detection in the ppm range when the eluent conductivity
(background) is reduced to near zero by the ion-exchange suppression step prior to analyte detection.
Then, it has good sensitivity, a wide linear dynamic range.
26-1 Ion Chromatography
p. 588 Harris (7th)
A high-performance version of ion-exchange chromatography is the method of choice for routine
anion analysis and many applications in cation analysis.
It is used in the semiconductor industry to monitor anions and cations at 0.1 ppb levels in deionized
water.
Retention is based on the attraction between solute ions and charged sites on the stationary phase:
 In anion-exchange chromatography, positively charged sites on the stationary phase attract solute
anions
 In cation-exchange chromatography, negatively charged sites on the stationary phase attract solute
cations
Dionex DX-120 Ion Chromatograph




performs all types of isocratic IC separations
is compatible with all anion- and cation-exchange columns
uses digital conductivity detection and self-regenerating suppressor
provides highly selective and sensitive analysis, superior detection limits, and a broad dynamic
range with unparalleled ease of use
26-2
Ion
Chromatography p. 647 Harris (6th)

p. 594 Harris (7th)
Based on equilibration of solute ions between the solvent and charged ion-exchange sites fixed on
the stationary phase.
Anion chromatography
 solute anions are attracted to the positively charged sites covalently bound to the stationary resin
particles
strong quaternary ammonium groups (RNR'3+) remain cationic at all pH
weak tertiary ammonium groups (RNR'2H+) are deprotonated in moderately basic solution and lose
anion-exchange ability at high pH
lon-exchange resin particles are made of
(a) polystyrene - divinylbenzene which has good stability in the alkaline pH range.
(b) polymethacrylates which has good stability in the alkaline pH range.
(c) polystyrene - divinylbenzene which demonstrates poor stability in the alkaline pH range.
(d) polymethacrylates which demonstrate poor stability in the alkaline pH range.
(e) any other polymer.
Cation exchanger
 solute cations are attracted to the negatively charged sites covalently bound to the stationary resin
particles
strong sulfonate groups (RS03-) remain anionic even in strongly acidic solutions
weak carboxylate groups (RC02-) are protonated at pH 4 and lose cation-exchange capacity at lower
pH
Potassium (http://alwaysyourchoice.com/ayc/nutrition/micronutrients/potassium.php)
Potassium is a major mineral essential for the inner working of the cell. It is most abundant in fruits
and vegetables, but also ever present in dairy foods and other animal products. The biochemical role of
potassium is the body is maintenance of normal water balance, regulation of neuromuscular activity.
We need it for normal cell growth. Potassium is related to muscle mass and glucose storage, if muscle
is being formed, an adequate supply of potassium is essential.
Potassium and blood pressure-boosting your intake of potassium seems to be protective against
hypertension and may control blood pressure more effectively than a low sodium diet does. Potassium
helps to make arteries stronger and better able to withstand the blood vessel damage that occurs with
aging.
Retention time in ion chromatography is a function of resin type
Strong anion/cation exchanger resins retain anion/cation analytes strongly increasing their retention
times on the IC column
Elution order (selectivity) in ion chromatography is a function of resin type
Strong versus weak exchanger resins can produce quite different elution orders
The exchange capacity of an ion-exchange resin can be defined as the number of moles of charged
sites per gram of dry resin. Describe how you would measure the exchange capacity of an anionexchange resin using standard NaOH, standard HCI, or any other reagent you wish.
One way is to wash extensively with NaOH a column containing a weighed amount of resin to load all
of the sites with OH-. After a thorough washing with water to remove excess NaOH, the column can
be eluted with a large quantity of aqueous NaCI to displace the OH-. The eluate is then titrated with
standard HCI to determine the moles of displaced OH-.
Bonus question
The exchange capacity of an anion-exchange column was measured by washing extensively
with 1.0 M NaOH and next H2O. 200 mL of 1.0 M NaCI was passed through the column, and the
eluate was collected. Titration of the eluate with 0.010 M HCl required
10.7 mL to reach the end point. If the column contained 25.0 g of resin, calculate the exchange
capacity of the resin (as the number of micromoles of charged sites per gram of resin).
Exchange capacity
= (10.7 mL)(l L/1000 mL)(O.Ol mol/L) / (25.0 g)
= 4.3 x1O-3 umol/g
Retention time in ion chromatography
is a function of ion charge, ion size, ionic strength of eluent, and resin type
Ion charge
 in general, the greater the ion charge, the greater the attraction for ion exchange sites
 divalent ions (e.g. C032-) have a greater affinity than monovalent ions (e.g. HC03-)
 the typical elution order is first monovalent, then divalent, and last trivalent.
Ion size
 for different Ions of the same charge, the larger the ion radius, the more polarizable the ion, and
the more strongly It Is attracted to Ion exchange sites
OH
HC03
l1.32 A
2.20 A
the elution order of halides is F-, Cl- Br- and r, which corresponds to Increasing size of these ions.
Eluents for ion chromatography
Elution of anions in general, trivalent, divalent and monovalent eluents are need to elute tri-, di- and
mono valent analyte ions, respectively
Relative strength: NaOH weak, NaHC03 weak, Na2C03 moderate, Nal strong
Advantage of NaHC03/ Na2C03 eluents:
 can separate both mono- and di-valent anions in a single injection
 eluent strength is easily adjusted by changing the ratio of NaHCOa I Na2COa
 carbonate salts of high purity are inexpensive.
Ion chromatography is widely used in the beverage industry to check the organic and inorganic acid
content of various soft drink (cola) formulations
Certain acids (e.g. citrate) contribute significantly to the flavor of a carbonated soft drink
So it is important to monitor their types and concentrations
To analyze a soft drink (and resolve for citrate) the sample is first degassed, then diluted, filtered, and
injected directly onto the column for anion separation
Citrate analysis in a cola beverage
IC operating conditions
Column:
Waters IC.PAK anion column
Eluent:
potassium phthalate
Flow rate:
1.2 mL/min
Detector:
conductivity
Eluent ionic strength
 high ionic strength eluents are needed to elute trivalent
ions
 less concentrated eluents for divalent ions
 low ionic strength eluents for mono-valent ions
Eluent ionic strength
 the higher the eluent concentration, the faster ions are
eluted, and the shorter their retention times.
Detectors for Ion Chromatography
UV-visible absorption detector (for HPLC)
Fluorescence detector (for HPLC)
Electrochemical detector (for HPLC)
Flame ionization detector (for GC)
Thermal conductivity detector (for GC)
???
Conductivity Detector
 based upon the electrical conductivity of an ionic solution when placed between two oppositely
charged electrodes
 ions present in solution allows electrical current to flow between the electrodes
Ohm's law is obeyed by ionic solutions: I = VIR = VG
where G is the conductance of the solution G is proportional to ionic species concentration
Advantages
 Universal and high selectivity for ions (with pKa or pkb < 7)
 Good sensitivity - detection limits in the sub ppm range
 Wide linear dynamic range
Requirement
 Eluent conductivity (background signal) is reduced to near zero exchange suppression (an extra
step) prior to analyte detection
Anion chromatography
 Aqueous OH- solution as eluent (e.g. 10 mM NaOH).
 Anaytes such as F-, Cl- and S042- are eluted from the separator column
 They cannot be easily detected because the eluent contains a high concentration of Na+ and OHwhose high conductivity obscures that of the analyte ions