8.  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 SO42-

            3.  Standard solution #4: 1 ppm F-, 150 ppm Cl-, 1 ppm Br-, 30 ppm NO3-, 30 ppm SO42-

            4.  Effluent solution: 1.8 mM Na2CO3 / 1.7 mM NaHCO3

            5.  Deionized water: Type I Reagent Grade Water with a specific resistance of 18 MWcm

                 or greater. Free of ionized impurities, organics, microorganisms and particulate matter

                 larger than 0.2mm.

 

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

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 ml;

            Eluent: 1.8 mM Na2CO3 / 1.7 mM NaHCO3, flow rate 2.0 ml/min.

 

 

Recommended Literature

1.         D. A. Skoog and J. J. Leary, Instrumental Analysis.

 

 

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