Concurrent High-Sensitivity Conductometric Detection of Volatile Weak Acids in a Suppressed Anion Chromatography System

A suppressed hydroxide eluent anion chromatograph effluent flows through the outside of a gas-permeable membrane tube while electrogenerated 100–200 μM LiOH flows through the lumen into a second conductivity detector. Undissociated volatile acid eluites (e.g., H₂S, HCN, H₂CO₃, etc., represented as HA) transfer through the membrane and react as OH^– + HA → A^– + H₂O; the conversion of high-mobility OH^– to lower mobility A^– results in a significant negative response for these analytes. With the chromatograph operated at a macroscale (0.3 mL/min) the LiOH flow can be 3–30-fold lower, resulting in corresponding enrichment of the transferred analyte prior to detection. Because there is no mixing of liquids, the detector noise is very low (<0.1 nS/cm), comparable to the principal chromatographic detector. Thus, despite a background of 25–45 μS/cm, limits of detection for sulfide and cyanide are in the submicromolar level, with a linear dynamic range up to 100 μM. Carbonate/bicarbonate can also be sensitively detected. We demonstrate adaptation in a standard commercial system. We also show that Microsoft Excel-based numerical simulations of transport quantitatively predict the observed behavior well

Enigmatic Ion-Exchange Behavior of <i>myo</i>-Inositol Phosphates

The separation of <i>myo</i>-inositol mono-, di-, tri-, tetra-, pentakis-, and hexakisphosphate (InsP₁, InsP₂, InsP₃, InsP₄, InsP₅, InsP₆) was carried out using hydroxide eluent ion chromatography. Acid hydrolysis of InsP₆ (phytate) was used to prepare a distribution of InsPs, ranging from InsP₁ to InsP₅’s and including unhydrolyzed InsP₆. Counting all possible positional isomers (many of which have stereoisomers that will not be separable by conventional ion exchange), 40 chromatographically separable peaks are possible; up to 22 were separated and identified by mass spectrometry. InsPs show unusual ion-exchange behavior in two respects: (a) the retention order is not monotonically related with the charge on the ion and (b) at the same hydroxide eluent concentration, retention is greatly dependent on the eluent metal cation. The retention of InsP₃–InsP₆ was determined to be controlled by steric factors while elution was influenced by eluent cation complexation. These highly phosphorylated InsPs have a much greater affinity for alkali metals (Li⁺ > Na⁺ > K⁺) than quaternary ammonium ions. This difference in cation affinity was exploited to improve separation through the use of a tetramethylammonium hydroxide–sodium hydroxide gradient

Flow-Cell-Induced Dispersion in Flow-through Absorbance Detection Systems: True Column Effluent Peak Variance

Following a brief overview of the emergence of absorbance detection in liquid chromatography, we focus on the dispersion caused by the absorbance measurement cell and its inlet. A simple experiment is proposed wherein chromatographic flow and conditions are held constant but a variable portion of the column effluent is directed into the detector. The temporal peak variance (σ_t,obs²), which increases as the flow rate (<i>F</i>) through the detector decreases, is found to be well-described as a quadratic function of ¹/_<i>F</i>. This allows the extrapolation of the results to zero residence time in the detector and thence the determination of the true variance of the peak prior to the detector (this includes contribution of all preceding components). This general approach should be equally applicable to detection systems other than absorbance. We also experiment where the inlet/outlet system remains the same but the path length is varied. This allows one to assess the individual contributions of the cell itself and the inlet/outlet system.to the total observed peak. The dispersion in the cell itself has often been modeled as a flow-independent parameter, dependent only on the cell volume. Except for very long path/large volume cells, this paradigm is simply incorrect