Charge Detection in Ion Chromatography

• What is charge detection?
• How does charge detection work?
• What are the advantages of charge detection?
• What applications benefit most from charged detection?
• Which IC systems can be used with the Dionex QD Charge Detector?

For ion chromatography, charge detection can be used in combination with suppressed conductivity detection as a confirmatory, orthogonal detector to obtain additional analytical information.  The Dionex QD charge detector uses a membrane-based technology to detect ions in proportion to their charge and concentration. It produces up to three- times greater signal for triply charged ions, such as phosphate, than for singly charged ions such as chloride. Ions with the same charge and concentration yield similar response, permitting reliable quantification of known and unknown compounds with a single standard.

Charge detection works by dissociating ionizable species and measuring the resulting current, thereby driving the dissociation of weakly ionized molecules. This generates a stronger signal for weak electrolytes than would be possible using conductivity detection, which measures current from only the already ionized portion. In combination with conductivity detection, charge detection provides improved peak identification and peak purity analysis for your ion chromatography (IC) applications.

• Detects all ionizable species
• Provides stronger response for very weakly dissociated and multivalent ions, compared to conductivity detection
• Gives linear response for weakly dissociated ions
• Complements conductivity detection
• Enables quantification of unknowns with universal calibration
• Provides peak purity information
• Improves peak identification and peak purity analysis
• Fills the detection gap between suppressed conductivity and mass spectrometry

The Dionex QD is well suited to the analysis of phosphates in environmental samples, organic acids in food and beverages, and amines in chemicals.

For Weakly Dissociated Ions:

The Dionex QD detector provides a unified response for strongly and weakly dissociated ions. However, weakly dissociated ions (e.g. ammonia, organic amines, organic acids and silicate) and multivalent ions will show a higher QD response than with a conductivity detector (CD). As a result, there is a linear correlation between response and concentration for these types of weakly dissociated species when using a QD.

For Quantification of Unknowns:

The combination of QD and CD allows for the quantification of unknown analytes. Chemists can compare the predicted concentrations for unknown analytes from the calibration of standards from each. In addition to the information found using retention time-based identification, if the analyte concentrations predicted by both QD and CD match, there is increased confidence in narrowing down the identity of the unknown analyte. It is important to note that for co-eluting species, predictions may favor the largest response contributor. 

The Dionex QD charge detector is integrated into the Dionex ICS-4000 HPIC System.