Retention
In IC, analyte retention is dependent upon eluent strength and, for weak anion and cation exchangers, pH.
Variation in retention is generally the result of eluent preparation errors or pump issues.
Symptom Solution
Decreasing
retention times
Column usage can affect capacity. Reproduce the production test chromatogram using conditions
noted on the Quality Assurance Report that was shipped with the column to assess column
performance. If capacity has been lost, attempt column regeneration per the manufacturer’s
recommendations or replace the column. Check the eluent concentration. Irreversible adsorption of
highly charged species from samples will reduce available capacity. Check the sample for potential
sources of column fouling. Use IC sample preparation to minimize matrix effects and
prevent column fouling.
Increasing
retention times
Verify that the actual eluent concentration and flow rate are what have been specified in the method.
For organic ions, check secondary hydrophobic interactions (switch to more hydrophilic ion
exchange column if available).
Variable
retention times
Check the eluent concentration and pump stability. Ensure that there is sufficient equilibration time
between runs. Check injection volumes and concentrations of sample and standard; if the column is
being overloaded, try a higher capacity column, or inject a smaller volume. To eliminate fluctuations
due to changes in room temperature, place column in thermostatted oven.
Detection and detectors
Suppressed conductivity detection is by far the most common detection mode for modern IC. Other commonly used
detectors include UV absorbance and electrochemical. For all detector types,
eluent purity and correct preparation are
key to successful IC separations.
Symptom Solution
Excessive noise Check all predetection modules (including suppressor) for sources of noise and eliminate.
Check for trapped air bubbles in detection cell. Check detection cell for any visible contamination and
clean if necessary. Check lamp usage time (UV absorbance detection), replace if necessary.
Baseline drift Check for changes or drift in flow rate. Check for fluctuations in room temperature. Use column
and detector cell temperature control (conductivity detection). Check for column bleed and
contamination (remove column and remeasure baseline drift). For absorbance detection, ensure
sufficient warm-up and check lamp usage time.
Periodic baseline
fluctuation
If fluctuations appear to be synchronous with pump cycle, purge the pump and check for general
functionality. To eliminate the potential for air in the fluidics, degas eluents or source water
(for eluent generation) and purge pump.
No peaks Check the injector operation. Check standard integrity. As above, ensure that all predetector
modules are operating correctly. Check the baseline noise. To verify detector operation, remove
the column and inject standard to observe response. For UV absorbance detection, ensure correct
wavelength setting.
Lack of analyte
sensitivity
Carry out a signal-to-noise analysis and compare to previous data. Verify the eluent conditions
and suppressor function (conductivity detection). Check the eluent composition and background
conductivity or absorbance (high background signal can obscure analyte peaks.) Check for proper
injector functioning.
Nonlinearity Check the system precision, both with and without column. Check the baseline and standard
stability over time. Ensure that the signal-to-noise ratio is constant. Work above 10:1 signal-to-noise
ratio for quantitation. Make sure you are not overloading the column. Establish and work within linear
range. Note: For weak acid and weak base species, conductivity is fundamentally nonlinear.
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