AN INTRODUCTION
TO
ION CHROMATOGRAPHY
Prepared by
Kenneth Piers
Professor of Chemistry
Calvin College
Ion Chromatography: Analysis of Natural Water for Some Common Anions
Introduction
The concentration of individual ions in natural water systems is of
considerable environmental interest. For example high concentrations of
chloride may indicate that a body of water is receiving run-off from roads
and parking lots where salt is commonly used for de-icing in the winter.
High chloride concentrations contribute to salinization of the water. High
concentrations of sulfate ion may indicate that the body of water is
receiving significant quantities of acid precipitation in which sulfuric acid
plays a large role. Or high concentrations of nitrate and phosphate ions
may indicate that the body of water is receiving run-off from lawns or
agricultural land which often are treated with high levels of synthetic
fertilizer which contain high concentrations of these nutrient ions to
encourage plant growth. High nitrate and phosphate concentrations in
water stimulate the growth of algae which, when it dies in late summer,
contributes to eutrophication (oxygen depletion) of the body of water and
subsequent death of aquatic animals.
Until recently, determination of the concentration of individual ions
such as chloride, sulfate, nitrate and phosphate in a water sample was both
a time demanding and quite tedious process. Each ion had to be
determined one at a time separately from all other ions present and sample
preparation time was long, often involving lengthy protocols, which
required the use of potentially hazardous analytical reagents. However,
within the past ten years or so, commercial instruments have become
available which greatly reduce the sample preparation time and which
allow simultaneous determination of all anions of interest in a sample
during a single pass. This instrumental method is called Ion
Chromatography.
Basic Principles of Chromatography
Chromatography is an experimental technique used for analyzing
and/or separating mixtures of chemical substances. There are many
different kinds of chromatography, among them paper chromatography,
gas chromatography, liquid chromatography, and ion-exchange
chromatography. An ion chromatograph is an example of the latter kind of
chromatography.
All chromatographic methods share the same basic principles and
mode of operation. In every case, a sample of the mixture to be analyzed
(the analyte) is applied to some stationary fixed material (the adsorbent)
and then a second material (the eluent) is passed through or over the
stationary phase. The compounds contained in the analyte are then
partitioned between the stationary adsorbent and the moving eluent. The
success of the method depends on the fact that different materials adhere
to the adsorbent with different forces. Some adhere to the adsorbent more
strongly than others and are therefore moved through the adsorbent more
slowly as the eluent flows over them. Other components of the analyte are
less strongly adsorbed on the stationary phase and are moved along more
quickly by the moving eluent. So, as the eluent flows through the column,
the components of the analyte will move down the column at different
speeds and therefore separate from one another, as shown in the diagram
below. Now if we monitor the end of the column, at some point we will
observe molecules or ions of the fastest moving substance (least tightly
bound to the adsorbent) emerging from the column - usually in a narrow
band if things are working right. Shortly thereafter we see molecules of the
second fastest moving substance emerge from the column and so forth.
If we now attach a detector to analyze the output at the end of the
column we should be able to determine whenever a group of analyte
molecules emerge from the column. This is, in practice, what is done in
most commercial chromatography instruments. Each time analyte
molecules/ions emerge from the chromatography column the detector
generates a measurable signal which is usually printed out as a peak on
the chromatogram. The chromatogram is a record of detector output Vs
time as the analyte passes through the chromatography system. It usually
consists of a series of several peaks corresponding to the different times in
which components of the analyte mixture emerge from the column. The
number of peaks corresponds to the minimum number of different
substances (compounds or ions) contained in the analyte. If the analyte is
found to display only a single peak, it is an indication that it is composed of
only a single component, i.e., it is pure, although rigorous confirmation of
purity may require additional testing.
Ion Chromatography
A. The Injector and the Eluent
Commercial ion chromatography systems have become available
within the past ten to fifteen years. The basic components of such a system
are shown in the cartoon below:
For anion chromatography the eluent is a dilute aqueous solution of
sodium bicarbonate and sodium carbonate, each about 1-4 mM in
concentration. The water used for the preparation of the eluent is of the
highest purity - Type I water which is virtually free from any contaminating
ions. The sample is injected into the instrument through the injection
system. The eluent, which the pump draws from the eluent jar and forces
through the system, then carries the analyte onto the ion exchange column
under a pressure of about 1000 psi (ca 65 atm).
B. The Ion Exchange Column
The ion exchange column (sometimes called the analytical column)
is tightly packed with the stationary adsorbent. This adsorbent is usually
composed of tiny polymer beads which have positive charge centers
(actually ammonium type cations) attached to their surfaces with covalent
chemical bonds. If we could magnify the column packing, we imagine that
it would look something like the cartoon which follows:
When just the pure eluent is flowing through the column these
positive centers on the polymer surface attract the negatively charged
bicarbonate and carbonate ions in the eluent. But as the anions contained
in the analyte sample begin to enter the column, these anions, which are
also attracted to the positive centers on the polymer surface, may replace
(i.e., they exchange with) the bicarbonate and carbonate ions stuck on the
polymer surface. The analyte anions and the eluent anions compete with
each other for the positive centers on the polymer surface. As a result of
these interactions a rather complex equilibrium is set up as the analyte is
carried through the column by the eluent. This equilibrium can be
described by the cartoon below:
How tightly the competing anions stick (i.e., how strongly they are
attracted) to the surface of the polymer beads depends on the size of the
anion and its charge. Usually, the greater the charge on the anion the more
strongly it is attracted to the surface of the polymer beads (so, for example,
sulfate ions are more tightly held to the polymer beads than chloride ions).
Also, larger anions generally move more slowly through the column than
smaller anions (e.g., bromide ions usually are more tightly bound to the
polymer beads than chloride ions).
So now we have a situation in which the eluent is trying to move the
anions of the analyte forward through the column, but the polymer beads
are trying to hold the anions in place by attracting them to the positive
charge centers. Obviously, anions can only move forward through the
column when they are in solution. So the speed at which a given type of
anion moves through the column is dependent on the relative amount of
time the ion is in solution. If an ion is only weakly attracted to the positive
centers on the surface of the polymer beads, it will spend relatively more of
its time in the solution phase than an anion that is strongly attracted to the
polymer surface. So these anions move through the column more quickly.
The more strongly the anion is attracted to the surface of the polymer
beads, the more slowly it moves through the column.
The result is that what was injected as a homogeneous solution of
analyte anions at the top of the column, separates into bands of different
kinds of ions as they travel through the column. For example, if we could
see what happened to a solution of chloride, bromide and sulfate ions as
they traveled through the ion exchange column, after sometime the column
might look some thing like this:
While the details of what goes on inside the column is more
complicated than described above, perhaps this description will provide
you with a qualitative understanding of how different anions can be
separated on such a column.
Detection and the Suppresser Column
The detector that is most commonly used in ion chromatography is
a conductivity cell which measures the conductance of the solution
passing through it. The conductance of a solution is proportional to the
concentration of the ions dissolved in the solution. Because we use
conductivity measurements to detect the different anions as they emerge
from the column, it is essential to pass the eluent through a suppresser
column before entering the detector. Here’s why. The eluent contains
relatively high concentrations (1-5 mM) of sodium bicarbonate and sodium
carbonate - both strong electrolytes. If such a solution were passed directly
into a conductivity detector (which measures the electrical conductance of
the eluent solution), a large signal would be obtained just from the
presence of these eluent ions. The actual concentration of the analyte
anions of interest is usually only a tiny fraction of the concentration of the
bicarbonate and carbonate ions contributed by the eluent itself. Usually the
analyte anions of interest are present in the parts per million level or less;
however the bicarbonate and carbonate ions on the eluent are present in
much greater concentration – approximately 100 times as large as the
anions of interest. Thus the bicarbonate and carbonate ions impart a large
conductivity to the solution entering the detector and the additional
conductivity imparted to the solution of the eluent by the presence of a tiny
amount of an anion of interest coming from the sample is extremely small.
That is, we would have a very hard time measuring the difference in the
conductivity of the pure eluent itself and the conductivity of the eluent
containing a tiny amount of an anion of interest. In other words, the
sensitivity of the measurement would be too low to be of value.
To get around this problem, after emerging from the column and
before entering the conductivity detector, we pass the solution through a
suppresser column which effectively removes all the bicarbonate and
carbonate ions from the solution, leaving only the analyte ions of interest
dissolved in the solution. When the resulting solution is now passed
through the conductivity cell, we can readily detect the signal due to the
presence of a small amount of the analyte ion, because in this case the
reference signal we are comparing it to is that of pure water which has an
exceedingly low conductivity. So by removing the large quantities of the
eluent anions, bicarbonate and carbonate, we greatly increase the
sensitivity of the measurement. Indeed, using suppression, it is possible to
detect ion concentrations as low as parts per billion in the analyte sample.
The suppresser column essentially is another ion exchange column.
Although there are different designs for such columns, we may think of
these columns also as being packed with small polymer beads, but these
beads carry acidic protons (H+1) on their surface:
As the solution from the analytical column flows into the suppresser
column, the carbonate and bicarbonate ions, both of which are basic,
combine chemically with the protons on the polymer surface forming
carbonic acid which, being unstable in aqueous solution, decomposes to
carbon dioxide gas and water. In this way the carbonate and bicarbonate
ions are removed from the solution:
HCO3-1 + H+1 ----> [H2CO3] ---> H2O + CO2
CO3-2 + 2 H+1 ----> [H2CO3] ---> H2O + CO2
The spaces that are left on the polymer surface by the departure of the
protons are filled by sodium ions provided by the eluent solution (i.e.,
sodium ions undergo exchange with protons on the surface of the polymer
beads). This process essentially removes all of the anions (bicarbonate and
carbonate) that were present in the eluent solution but leaves the anions
that were contained in the analyte dissolved in the solution. The reason
that the analyte anions are not removed from the solution by the
suppresser column is that anions like chloride, bromide, nitrate, sulfate,
etc., are non-basic and do not combine with protons as they pass through
the suppresser column.
When only pure eluent is entering the suppresser column, the
bicarbonate and carbonate anions are removed, the sodium ions are
retained on the column, and what emerges from the end of the suppresser
column is essentially pure water which has exceedingly low conductivity.
In this condition the conductivity detector is zeroed. Then when eluent
carrying a band of analyte anions enters the suppresser column, the
bicarbonate and carbonate anions are again removed as before, but the
analyte anions emerge from the suppresser column unscathed. When they
enter the conductivity cell of the detector, the conductance of the solution
increases and this increase is measured. The greater the concentration of
the analyte anions the greater the conductivity and the larger is the peak
that is recorded in the printout.
Qualitative Identification of Anions
The identification of the anions present in an analyte mixture is
achieved by comparing the results of the analysis with results obtained by
analyzing samples of known substances. The key to such identification
rests on the fact that under a fixed set of analytical conditions (type of
analytical column packing, concentration of eluent electrolytes, etc.) a
given anion will always take the same amount of time to travel through the
analytical column. This time is called the retention time (RT) see diagram
following:
.
Each anion has its own characteristic retention time under a given
set of conditions. Thus, for example, if we find that a known solution
containing only sodium chloride, takes 2.76 min to emerge from the
column, then finding a peak in the chromatogram of an unknown analyte
which contains a peak at 2.76 minutes allows us to conclude that the
sample contains chloride ion. In all cases identification of an anion needs
to be done by comparison of the sample results with the results obtained
by analysis of known solutions.
Quantitative Identification of Anions
One can also quite readily obtain a measurement of the
concentration of a given anion in an unknown sample. This measurement
rests on the fact that the conductivity that a solution displays is directly
proportional to the concentration of ions it contains. So the larger the peak
for a given ion in a chromatogram the greater is its concentration, that is,
the area of the chromatogram peak for a given ion is proportional to the
concentration of the ion in solution. To determine the concentration of an
ion in an unknown solution one needs to compare the peak area of the ion
in the unknown solution with the peak area of that ion is some solution(s)
in which the concentration of the ion is known, i.e., a standard solution.
Typically quantitative results are achieved by using external or internal
standards. The use of external standards is more common.
Suppose one wants to determine the precise concentration of
chloride in an unknown solution. First, a set of standard solutions
(prepared by dissolving pure sodium chloride in pure water) is made in
which the precise concentration of the chloride ion is known. Typically a
minimum of three standards are used - say a 100 ppm, 50 ppm and 10 ppm
solution (ppm = parts per million). Then each of these solutions is analyzed
on the ion chromatograph and from the results a standard curve is
obtained. Such a curve typically is obtained by plotting peak height (or
peak area) in the chromatogram Vs concentration of chloride ion which
usually produces close to a straight-line graph. A typical result would look
something like that shown below:
Finally, the unknown solution is analyzed and, by measuring
the peak area for the chloride ion (identified by retention time), and
comparing it with the standard curve, the concentration of chloride ion in
the unknown solution can be calculated.
The Ion Chromatography Experiment
A. Sample Preparation and Analysis
The actual experiment is very simple. All that needs to be done is to
prepare the sample by gravity filtering a small portion (10 mL) of the water
sample into an ion chromatography analysis tube, label the tube and place
it into the ion chromatography auto-sampler. An identifying name for the
sample needs to be entered on the computer. After all samples are loaded
into the auto-sampler and entered into the computer, the analysis begins
and needs no further attention. The results will be distributed next week
B. Analysis of Results
To analyze the results of the ion chromatography experiment, we
will pool the results that were obtained for water samples that were
collected from the same source. So there will be different sets of results one each for samples collected from each of the different rain samples (or
Lisa's well water). For each of these locations we will prepare a
spreadsheet using Excel or some other spreadsheet program. After all the
individual results are entered, we will calculate an average concentration
for each ion and a standard deviation for the measurement of each ion
concentration.