The Dionex Carbohydrate Membrane Desalter (CMD^TM I) is an on-line device for users of HPAE-PAD who wish to collect and further analyze carbohydrate samples.

• Collected fractions can be lyophilized without dialysis, leaving the sample ready for further manipulation, such as enzymatic or chemical digestion, NMR, mass spectrometric, or further chromatographic analysis.
• Reduces the pH of the effluent from pH ~13 to between pH 2–6, depending upon the concentration of sodium acetate in the eluent, for subsequent saccharide characterization.
• Provides minimal dispersion of the sample exiting the detector cell, allowing accurate fraction collection.
• >99% sodium ions in eluents that contain up to 0.35 M sodium ions flowing at a rate of 1 mL/min will be removed from the CMD.

High-pH HPAE-PAD has become the method of choice for oligosaccharide purification because it permits very high resolution of neutral and charged oligosaccharide isomers. Subsequent characterization of purified oligosaccharides often requires removing the eluent salts. Although manual desalting methods are available, it is highly desirable to have an automated method for the process.

The CMD I exchanges sodium ions in the eluent for hydronium ions. This process converts sodium hydroxide and sodium acetate eluents to water and acetic acid, immediately lowering the sample pH after leaving the detector cell. Because acetic acid is volatile, collected fractions can then be lyophilized without dialysis, leaving the purified carbohydrate sample ready for further manipulation. The CMD I has been used for applications ranging from on-line desalting prior to mass spectrometry, to oligosaccharide purification prior to NMR analysis, to general oligosaccharide characterization.

The CMD I reliably desalts up to 0.35 M Na⁺ at an eluent flow of 1.0 mL/min. Oligosaccharides eluting in up to 0.35 M Na⁺, collected after on-line desalting and evaporated in a centrifugal vacuum evaporator, exhibited residual [Na⁺] below 200 µM after resuspension to original volume in deionized water. This represents a desalting efficiency of >99.9%. The recovery of neutral and sialylated oligosaccharides under these conditions ranged from 75 to 100%. For people wishing to collect fractions following desalting, loss of resolution as a result of the added volume between detection and collection points is measurable (~6%) (and illustrated in the figure above), but sufficient to allow acceptable purification.