Ion Chromatograph Data Analysis
Soon, you all will have completed your 3 workshops on how to operate the Dionex Model 2020
Ion Chromatograph (IC). Now it is time to analyze the IC data to produce anion concentrations that
you can report with confidence. We need to get through three steps – measure a peak area for each
anion in each sample, prepare a standard curve relating conductivity response of IC to anion
concentration, and calculate anion concentrations in samples from IC peak height or area.
Logger Pro is the software used to collect the IC data on the computer attached to the
instrument. A 10 min acquisition time should be adequate for recording IC data. When you begin to
do sample runs make the best use of your time on the instrument. Be prepared to run samples when
your time slot begins. When you complete a run, save the data directly to your 3.5-inch disk. Analyze
your data on one of the fast computers in the Cal lab or the computer in the EA homeroom. All of
these computers have Logger Pro software loaded.
There are 3 files in this directory from a calibration. Standards contained chloride, nitrate,
phosphate, and sulfate. The three files are named high.mdl, med.mdl, and low.mdl for anion
concentrations of high, medium, and low standards described in your IC handout.
Measure the peak area. Start Logger Pro. Open a *.mbl file. Let’s do “high.mbl”. The
familiar chromatogram appears on the screen with time as the x-axis and potential as the y-axis. The
peaks are assigned to chloride, nitrite, nitrate, phosphate, and sulfate in order of increasing retention
time. Click the Examine button on the tool bar. A vertical line appears on the screen together with a
window that shows the retention time in min and the intensity. To find peak locations you can use the
table on the right hand side where the outline box corresponds with the line position. Try it out. The
time to each peak is referred to as the retention time for that compound. Record the retention time for
each peak to confirm the identity of peaks in your water sample.
Now a constant baseline will be subtracted from the chromatogram. View the baseline at the
beginning of the trace. Select a representative value for the baseline. I picked 0.465. Click on Data,
New Column, Formula. Fill in a name – I used 465 for the name. Select the Definition tab. In the
Formula box enter 0.465, or whatever value you selected for the baseline. Click OK. You have now
filled a new data column named 465 with the value 0.465. Now we need to subtract this baseline from
the chromatogram.
Click on Data, New Column, Formula. Fill in a name – I used “Correct” for the name. Select
the Definition tab. Click the arrow on the right of the Variables box. Select Potential. “Potential”
should appear in the Formula box. After “Potential” enter a -. Using the Variables box select 465.
The Formula box should now read “Potential”-“465”. Click OK. You have now filled a new data
column named Correct with the baseline corrected data.
Measure area under peaks. Start with the rightmost peak – sulfate. Click the mouse on the
right side of the peak, hold the mouse button down and drag to the left side. Release the button. Two
lines should be bracketing the sulfate peak. Select Analyze, Integral. The peak will be filled with
color and a small box will appear tint he upper left hand corner containing the area of the peak. I got
0.251. With the mouse you can drag the box over or next to the peak you measured. Repeat this for
the other peaks and the medium and low curves.
Prepare a standard curve. Now we must figure out what is the relationship between the peak
area and anion concentration. We will use 3 standards of different concentration.
We will use Excel to analyze the standard curve. Start Excel. In column A enter the anion
concentrations in mg/L. In column B enter the corresponding peak areas. Find the best-fit straight line
through the data using a regression analysis. Click on Tools, Data Analysis, Regression, OK. Click on
“Input Y Range” and highlight the peak area data. Click on “Input X Range” and highlight the mg/L
concentration data. Under “output options”, click on “new worksheet ply”. Under “residuals”, click
on “residual plots” and “line fit plots”. Click OK and Excel will analyze the data.
Regression statistics provide some useful information about the linear regression.
R2 is the coefficient of determination. It is the square of the correlation coefficient. R 2
represents the fraction of the variance in the data that is described by the linear regression line. The
closer the value is to 1.0, the better the fit.
Standard error gives you an estimate of the error in concentration as estimated from the IC peak
intensity.
The regression line is defined by the linear equation y=mx + b.
X-variable 1 is the slope of the line, m.
Intercept is the y-axis intercept, b.
You should be able to calculate the concentration of an unknown sample from the peak
intensity by writing the linear equation as x=(y-b)/m. Try this by identifying the two peaks in the Soil
Extract.mbl file. Measure the areas under the peaks and calculate the anion concentrations using your
linear equation.