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

Indirect Potentiometric pH Detection of Weak Acids with Absolute Quantitation by a Theoretical Approach

A fast response potentiometric flow-through pH sensor was applied for organic acid determination. The analyte response with the pH sensor was obtained by eluent pH modification following ion exclusion chromatography with HClO₄ as an eluent. The response characteristics depend on the adjusted baseline pH. The baseline pH adjustment was successfully done with an ammonia permeation device without solution mixing, which may cause analyte dilution, dispersion, and mixing noise. After pH adjustment, the pH response was universal to the equivalent of introduced analyte acids because the pH response was obtained by the titration of the permeant ammonia by the analytes. The average of limit of detections (S/N = 3) was 0.06 mM for seven target organic acids. Furthermore, the pH response follows the theoretical pH calculation with the concentrations of the eluent, pH modifier, and analyte. Thus, the analyte concentration in the sample can be theoretically calculated from the pH response without calibration by the analyte standard. Predicted concentrations of injected standards were within 5% of the actual standard concentration. Additionally, analysis of real samples was performed and compared with the conventional post-column reaction with a bromothymol blue (BTB) method. The results obtained with the present system (absolute quantification with theoretical calculation) and conventional BTB method agreed within 10% of errors.

Reversed-phase chromatomembrane extraction as a novel approach for automated sample pretreatment: Anions determination in biodiesel by ion chromatography with conductivity detection

In this study, a reversed-phase chromatomembrane extraction (RP-CME) method as a novel approach for automated sample pretreatment was suggested for the first time. The RP-CME was applied to automated separation of anions (formate, chloride, nitrate, phosphate and sulfate) from biodiesel samples as a proof-of-concept example. The novel design of chromatomembrane cell was developed for on-line RP-CME. The RP-CME procedure assumed mass-transfer of water-soluble analytes from organic sample phase (biodiesel sample) to aqueous phase supported in a porous composite mass-transfer block. The composite mass-transfer block based on microporous hydrophobic poly (tetrafluoroethylene) and hydrophilic glass fiber was developed for the RP-CME implementation. The block provided the effective retention of aqueous phase into the cell and simultaneous penetration of organic phase. The hydrophilic membrane-based sheet was used for the on-line separation of hydrophilic emulsion (biodiesel in water) containing target analytes obtained during analytes elution by aqueous phase from the mass-transfer block. The RP-CME was successfully coupled with an ion chromatography with conductivity detection. The limits of detection, calculated from a blank test based on 3σ, were 5 μg kg^-1 for sulfate, 6 μg kg^-1 for nitrate, 3 μg kg^-1 for chloride, 5 μg kg^-1 for phosphate and 1 μg kg^-1 for formate.

Conductometric Gradient Ion Exclusion Chromatography for Volatile Fatty Acids

We describe a fatty acid vapor extractor (FAVE) as a postcolumn device for sensitive detection following ion exclusion chromatographic (ICE) separation of weak acids. The device consists of a single length of a permselective membrane tube surrounded by a jacket that consists of two isolated sections. The separation column effluent flows through the lumen. A suitable strong acid is put in the upstream, short section of the jacket and permeates in, rendering the lumenal flow strongly acidic (pH ≤ 2) that suppresses eluite weak acid dissociation. A lipophilic polysiloxane membrane is selectively permeable to volatile fatty acids (VFAs). A small fraction of the VFAs transfer to a cocurrent receptor stream of water (or a weak base, e.g., dilute hydroxylamine), flowing through the second, longer section of the jacket. Even though the transferred amount of VFAs may be very small (0.5-5%), significantly better detection limits than conventional suppressed conductometric ICE (SCICE) is possible because of the low and stable background (noise <1 nS/cm). It also permits gradient elution, not possible in SCICE. The polysiloxane based FAVE device is highly selective for VFAs, it shows no response to dicarboxylic acids, hydroxycarboxylic acids, or aromatic acids. As such, trace detection of VFAs in the FAVE extractant is possible while other components can still be monitored conventionally in the FAVE lumenal effluent. Various parameters, related both to device design and operation were studied. The FAVE provides isolation from the eluent matrix and can be used for other detectors where the eluent matrix is incompatible with the detector.

Magnetic Solid-Phase Extraction Based on β-Cyclodextrins/Acrylic Acid Modified Magnetic Gelatin for Determination of Moxidectin in Milk Samples

β-Cyclodextrins/acrylic acid modified magnetic gelatin was prepared and then employed as the magnetic solid-phase extraction (MSPE) sorbent for extraction of moxidectin in milk samples. Due to the rigidity of hydrophobic cavity of β-cyclodextrins and carboxyl groups of acrylic acid, magnetic composites are prepared to form a complex with target molecules through various kinds of chemical reactions and then showed excellent extraction performance. This method exhibits the advantages of simplicity of implementation, short extraction time (5 min), low solvent consumption, and high extraction efficiency. A rapid, simple, and effective method for the analysis of moxidectin in milk samples was established by MSPE coupled with liquid chromatography-fluorescence detection. The limit of detection was 0.1 ng·mL(-1) and the recoveries from milk samples were in the range of 93.8%-112.5%. The relative standard deviation was not higher than 6.4%. In conclusion, magnetic solid-phase extraction is a simple and robust preconcentration technique that can be coupled to other analytical methods for the quantitative determination of target molecules in complex samples.

Permeative Amine Introduction for Very Weak Acid Detection in Ion Chromatography

A permeative amine introduction device (PAID) is placed after a conventional KOH eluent-suppressed conductometric anion chromatography (SCAC) system. The PAID converts the suppressed eluites from the acid form to the corresponding ammonium salt (NR2H + HX → NR2H2(+) + X(-)) and allows very weak acids HX (pKa ≥ 7.0) that cannot normally be detected by SCAC to be measured by a second conductivity detector following the PAID. Permeative reagent introduction is dilutionless, can be operated without pumps, and provides good mixing (baseline noise 0.8 nS/cm for 27 μM diethylamine) with low band dispersion (as small as 30 μL). Diethylamine (DEA) was chosen as the amine source due to its low pKb value (3.0), high vapor pressure, low toxicity, and low odor. The eluites are thus detected against a low diethylammonium hydroxide (DEAOH) background (5-31 μS/cm) as negative peaks because the equivalent conductance of OH(-) is greater than that of X(-). Reducing the background DEA concentration enhances the detectability of traces of weak acids. Lower background [DEA] will limit the maximum concentration of analyte acids that can be determined; a general concept of peak width measurement at a fixed height is proposed as a solution. Trace impurities formed during electrodialytic suppression play a role in background noise; for the first time, we look at the nature of such impurities. The appearance of silicate in a sample put in a glass container as a function of pH can be readily followed. The maximum silica level in high purity type 1 water is 50 nM (1.40 μg/L Si), which is a measurement challenge in particular. A large injection volume (1 mL) permits detection limits of 21 nM silicate, 3 nM taurine, 3 nM sulfide, and 13 nM cyanide.