Environmental Data Analysis EESC
BC
3017
Ion Chromatograph to detect major anions in precipitation (snow),
groundwaters and drinking waters from New York
Ions and anions
- Ion: an atom or a group
of atoms that has either an excess or a
deficiency of electrons
- major ions in water, positive charge (cations), negative charge
(anions):
- H+, Na+, K+, Ca2+,
Mg2+,
OH-, Cl-, SO42-, NO3-,
HCO3-, PO43-
- Mole: unit of amount of
substance.
- By definition, the mole is the amount of substance of a system
that
contains as many elementary entities as there are atoms in 0.012
kilogram (12g) of carbon-12. The elementary entities, which must
be specified, may be atoms, molecules, ions, electrons, other
particles, or even specified groups of such particles.
- 1 mole = 6.022*1023 molecules or atoms (or other
units)
- conversion from mg/L to mmol/L
- On-line periodic table
How does an Ion Chromatograph (IC) work? Basic
principle (Fig),
(Fig):
- A sample of the mixture to be analyzed (analyte) is injected into a carrier
fluid (the eluent)
- the combination is passed through a column containing a
stationary fixed material (adsorbent)
- Compounds contained in the analyte are then partitioned between
the
stationary adsorbent and the moving eluent/analyte mixture
- Different dissolved materials adhere to the adsorbent with
different
forces
- The ones that adhere strongly are moved through the adsorbent
more
slowly as the eluent flows over them
- 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 (Fig),
(Fig)
- A detector is used to
analyze the output at the end of the
column.
There are various detectors, our IC has an electrical conductivity
detector.
- 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.
- A suppressor is being
used to reduce the background conductance
of
the
Eluent and at the same time enhance the conductance of the sample ions (Fig)
- implementation of the chromatographic method (Fig)
Detection of ions
- The chromatogram is
a
record of detector output (electrical conductivity) versus 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 (Fig).
- Siemens: unit of the conductance,
the
reciprocal of the resistance in ohms (S = amperes/volts), or in SI
units: one second cubed- ampere squared- per kilogram- per
meter squared
(1s3 A2 · kg-1 · m-2),
1
uS is 1 micro Siemens or
10-6S
- Chromatogram
and
determination of peak areas (Fig)
Application of ICs
Procedures
- Eluent
- Eluent is the mobile phase that
transports
the sample ions through the ion chromatograph plumbing, including the
columns. For anion analysis we are using a mixture of Sodium Carbonate
(Na2CO3) Sodium Bicarbonate (NaHCO3)
dissolved in water
- The pH, ion concentration, and ion types in the eluent
determine
the
partitioning of the analyzed ions between the stationary phase (ion
exchange
resin in the columns) and the eluent.
- Calibration
- Standards are needed to calibrate the IC (to identify peaks and
to determine the concentrations of our samples)
- We are making standards by diluting a commercial solution, named Dionex Seven
Anion Standard. This solution
is designed for routine
anion determinations.It contains fluoride (20 mg/L); chloride (30
mg/L);
nitrite (100 mg/L); bromide (100 mg/L); nitrate (100 mg/L); phosphate
(150 mg/L); sulfate (150 mg/L)
- peak area is plotted versus anion concentration and a
trendline is fitted through the data. The equation of the trendline is
then used to convert peakareas from measured samples into
concentrations.
- Construction of a Calibration
Curve: of peak area vs
known concentration (data)
- Blank measurments
- distilled water is used as a sample to check
if
the dilutions will not affect measured concentrations and to
make sure that there are no significant sources of ions in the
instument
- Sample Collection and Filtration
- Sample and standard measurements
- Chromatogram
and
determination of peak areas (Fig)
- Diagram
of our Departmental IC -1 (Fig), Diagram of
Our
Departmental IC -2 (Fig).
- The detailed procedures are summarized in this file (Protocol
/ PDF
file)
- Sequence
of a typical run (data)
Resources
- Ion Chromatography., James Fritz and Douglas Gjerde; CHEM QD
79.C453 G52 2000
- Ion Chromatography in Water Analysis., Shpigun and Zolotov;
CHEM
QD 79.C453 S47 1988
- Dionex
Corporation