ION CHROMATOGRAPHY WITH CONDUCTOMETRIC DETECTION
Useful information about the specific chromatographic instrumentation:
Points to enquire about the specific chromatographic instrumentation:
composition and flow rate of the eluent;
type and handling of the pump;
injection loop volume;
type of the column;
type of the detector.
Chemicals:
1. 3 ppm NaCl
2. Standard solution #5: 2 ppm F-, 300 ppm Cl-, 2 ppm Br-, 60 ppm NO3-, 60 ppm
SO423. Standard solution #4: 1 ppm F-, 150 ppm Cl-, 1 ppm Br-, 30 ppm NO3-, 30 ppm
SO424. Effluent solution: 1.8 mM Na2CO3 / 1.7 mM NaHCO3
5. Deionized water: Type I Reagent Grade Water with a specific resistance of 18
M·cm or greater. Free of ionized impurities, organics, microorganisms and
particulate matter larger than 0.2m.
Column:
Designed for analysis of the above mentioned anions
Detector:
Conductivity detector
Procedure
Part 1. Start-up
Turn on the ion chromatograph according to the requirements of the
instrument in use. Set the flow rate of the pump to 2.0 ml/min. Wait at least 10
minutes after starting the pump or changing the flow rate before beginning the
analysis. This allows the pump's real-time electronic pulse dampering circuitry to
stabilize the flow rate.
Monitor the baseline. If the stability of the baseline is satisfactory, the system
is ready for analysis.
Part 2. Characteristic parameters of the analytical method for determination of
chlorides
2.1. Precision
The precision of an analytical method is the degree of mutual agreement
among results obtained under identical conditions. Statistically, the precision is
expressed as the relative standard deviation of a set of measurements.
The precision of the system is tested with 3 ppm Cl- solution. Run five
subsequent chromatograms with the chloride solution at a sensitivity of 10 mS/cm full
scale.
Report the peak heights and areas. Calculate the average and the relative
standard deviation of each parameter. Discuss with the instructor the acceptability of
the results.
2.2. Calibration curves
The composition and the concentrations of the standard solutions must suit the
analyte. In the analysis of drinking water the calibration curves for the five anions of
interest (F-, Cl-, Br-, NO3- and SO42-) are constructed in a concentration range
reflecting the composition of various sources of drinking water:
Anion
Concentration range, ppm
F-
0.1 - 2
Cl-
15 - 300
Br-
0.1 - 2
NO3-
3 - 60
SO42-
3 - 60
You are provided with the two most concentrated standard solutions
(Standards #5 and #4). Prepare in 25 ml volumetric flasks three additional standard
solutions (concentrations of the anions are in ppm):
F-
Cl-
Br-
NO3-
SO42-
Standard solution #3
0.4
60
0.4
12
12
Standard solution #2
0.2
30
0.2
6
6
Standard solution #1
0.1
15
0.1
3
3
Record chromatograms of the standard solutions, in increasing order of
concentrations (from #1 to #5).
Observe the shapes of the peaks. Comment on the peculiar shape of the
chloride peaks for the most concentrated solutions.
Obtain calibration plots (in respect to both height and area) for each
component. Comment on the quality of the calibration curves and on their suitability
for quantitative determination of the analytes. Is there a connection between the
asymmetric shape of the chloride peaks and the poor quality of the calibration curve
of chlorides in respect to height? How do you explain the fact that the calibration
curve of chlorides in respect to area is satisfactory?
2.3. Calibration sensitivity
Sensitivity of an analytical method, m, called calibration sensitivity, is defined
as the slope of a calibration curve
m = d(signal)/dC
In order to determine the calibration sensitivity, use the calibration curve for
the five components. For each species to be quantitatively determined calculate the
calibration sensitivity m (for height and area data).
2.4. Detection limit
Detection limit of a specific analyte, Cdl, is determined as
C dl  3sbl / m
where sbl is the standard deviation of the blank.
1. Estimation of the standard deviation of the baseline. Three factors can
characterize the baseline and should be accounted in estimation of the standard
deviation of the baseline: a) average amplitude of the noise, b) noise frequency and c)
drift of the baseline.
Inject deionized water and record the baseline at high sensitivity. (Consult the
instructor about the value of conductivity full scale).
Observe the baseline in the region from which the peak of interest is detected.
Note the quality of the baseline in respect of the amplitude, frequency and drift of the
noise. Make a rough visual estimation of the average amplitude of the noise. This
value can be used as an approximate value of sbl. Consult the instructor about the
acceptability of this parameter.
Calculate the detection limit Cdl for chloride and for other anions, as
recommended by the instructor. Use the respective value of m, calculated from the
calibration curve in respect to height.
2. Verification of the value of the detection limit. Prepare a solution of the
anion of interest at a concentration corresponding to the calculated value of Cdl.
Record the chromatogram for this solution. Comment on the quality of the peaks.
Does this concentration correspond indeed to the detection limit? If not, can you make
a better estimation? Consult the instructor.
Part 3. Quantitative determination of anions in drinking water
Record chromatograms of the following solutions (each one in the proper
sensitivity):
a) tap water from different sources;
b) mineral water;
c) the general purpose deionized water used in the laboratory.
The determination of high concentration constituents in tap water samples may
require recording of an additional chromatogram in diluted solution. (Consult the
instructor.)
Determine the concentration of chlorides using the calibration curve.
Estimate the concentrations of the rest of the anions.
Fig.8-1 Example of ion chromatogram of anions:
1. Fluoride (1.0 ppm)
5. Nitrate (15.0ppm)
2. Chloride (1.5ppm)
6. Phosphate (20.0ppm)
3. Nitrite (7.5 ppm)
7. Sulfate (25.0ppm)
4. Bromide (10.0ppm)
8. Oxalate (25.0ppm)
Dionex DX 500 Chromatography System;
Column: IonPac AS4A-SC;
Detector: ED 40 conductivity detector;
Sample loop volume: 20 l;
Eluent: 1.8 mM Na2CO3 / 1.7 mM NaHCO3, flow rate 2.0 ml/min.