Electrodialytic Capillary Suppressor for Open Tubular Ion Chromatography

We describe an electrodialytic capillary suppressor for suppressed conductometric open tubular ion chromatography (SC-OTIC). For practical eluent concentrations, suppressor active lengths less than ∼1 mm are adequate. In its preferred embodiment, the suppressor consists of a solid polymer ion exchanger block. Two 0.45-mm-diameter parallel cylindrical passages are provided: these provide for passage of regenerant water and placement of platinum wire electrodes. The suppression channel is made by making a crack in the soft polymer block, using a needle 0.3 mm in diameter. The suppression channel runs parallel to and is flanked by the two aforementioned electrode channels through which water is pumped. The suppressor ends of the separation and the detection capillaries are tapered. The tapered ends of the capillaries are inserted into the suppression channel with the tips 0.4–1.1 mm apart. Using a suppression length of 1 mm, we were able to suppress 100 mM hydroxide @ 100 nL/min (10 neq/min). With such a suppressor coupled to an AS18 latex-coated surface-sulfonated cyclo-olefin polymer (COP) capillary column with an inner diameter (i.d.) of 28 μm and using an on-capillary admittance detector (AD), the feasibility of both isocratic and gradient SC-OTIC was demonstrated. At 170 nL/min (substantially above the Van Deemter optimum), the plate count for fluoride exceeded 70 000 plates/m under isocratic conditions. The dispersion induced by the suppressor could not be determined, because the peak half-widths after suppression were paradoxically less than those without suppression; this apparent negative dispersion is likely an artifact of the detector

Electrochemical Arsine Generators for Arsenic Determination

Arsine generation is the gateway for several sensitive and selective methods of As determination. An electrochemical arsine generator (EAG) is especially green: we report here the use of two electrode materials, aluminum and highly oriented (ordered) pyrolytic graphite (HOPG) never before used for this purpose. The first is operated on a novel constant voltage mode: current flows only when the sample, deliberately made highly conductive with acid, is injected. As a result, the cathode, despite being a highly active metal that will self-corrode in acid, lasts a long time. This EAG can be made to respond to As­(III) and As­(V) in an equivalent fashion and is fabricated with two readily available chromatographic T-fittings. It permits the use of a wire roll as the cathode, permitting rapid renewal of the electrode. The HOPG-based EAG is easily constructed from ion chromatography suppressor shells and can convert As­(III) to AsH₃ quantitatively but has significantly lower response to As­(V); this difference can be exploited for speciation. The success of Al, an active metal, also dispels the maxim that metals with high hydrogen overpotential are best for electrochemical hydride generation. We report construction, operation, and performance details of these EAGs. Using gas phase chemiluminescence (GPCL) with ozone as a complementary green analytical technique, we demonstrate attractive limits of detection (LODs) (S/N = 3) of 1.9 and 1.0 μg/L As­(V) and As­(III) for the HOPG-based EAG and 1.4 μg/L As­(V) or As­(III) for the Al-based EAG, respectively. Precision at the ∼20 μg/L As­(V) level was 2.4% and 2.1% relative standard deviation (RSD) for HOPG- and Al-based EAGs, respectively. Both HOPG- and Al-based EAGs permitted a sample throughput of 12/h. For groundwater samples from West Texas and West Bengal, India, very comparable results were obtained with parallel measurements by induction coupled plasma-mass spectrometry

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 (p<i>K</i>_a ≥ 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 p<i>K</i>_b 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

Polymethylmethacrylate Open Tubular Ion Exchange Columns: Nondestructive Measurement of Very Small Ion Exchange Capacities

We describe an approach to prepare an open tubular ion exchange (OTIE) column by coating a monolayer of anion exchange nanoparticle to a 16–20 μm bore polymethylmethacrylate (PMMA) capillary. The latex nanoparticle was electrostatically attached to carboxylate groups on the inner wall of capillary, pretreated with strong base for hydrolyzing the ester. Several approaches to nondestructively measure ion exchange capacities (IEC) of the columns were examined: (a) adsorption–desorption of an intensely fluorescent ion, e.g. fluorescein, and off-line fluorometry, (b) loading a weakly retained ion (e.g., IO₃^–), frontal displacement by a strongly bound ion (e.g., Cl^–), and online optical or conductometric boundary detection, and (c) similar to the above except displacement being accompanied by reaction (e.g., acid–base titration). To our knowledge, this is the first time on-column titration has been used to measure capacities. By using different pH displacer solutions, we demonstrate for the first time the possibility of p<i>K</i>_a-differentiated ion exchange capacity measurements. The cation exchange capacity of bare PMMA capillaries was on the order of 1 pequiv/mm² with little dependence on time and temperature of hydrolysis conditions. After AS18 latex coating, the strong base anion exchange capacity was on the order of 10 pequiv/mm², very close to what would be estimated on the basis of monolayer coverage of the surface by individual latex particles. The latex used contained a significant, additional amount of weak base character, about the same as the strong base ion exchange capacity

Inline Shunt Flow Monitor for Hydrocephalus

In hydrocephalus, cerebrospinal fluid (CSF) builds up in the cranial cavity causing swelling of the head and potentially brain damage. A shunt to drain the fluid into a body cavity is now universally used, but failure is all too common. Techniques for ascertaining shunt failure are time-consuming, expertise-dependent, and often inconclusive. We report here an inline system that reliably and quantitatively measures the CSF flow rate. The system uses a single thermistor to both heat the surrounding and to sense the temperature. In the heating mode, the thermistor is subjected to a 5 s voltage pulse. In the sensing mode, it is part of a Wheatstone’s bridge, the output being proportional to temperature. The signal, <i>V</i>_i – <i>V</i>_f, which is the net change Δ<i>V</i> in the bridge output immediately before and after the heat pulse, depends both on the flow rate and the surrounding temperature. In vitro, a single equation, flow rate = 3.75 × 10^–6 × Δ<i>V</i>^{(−9.568+1.088 <i>V</i>_i)} provided good prediction for the flow rate, with 6.3% RMS relative error. The sensor behavior is reported for flow rates between 0–52.5 mL/h at 32–39 °C, adequately covering the range of interest

Electrodialytic Membrane Suppressors for Ion Chromatography Make Programmable Buffer Generators

The use of buffer solutions is immensely important in a great variety of disciplines. The generation of continuous pH gradients in flow systems plays an important role in the chromatographic separation of proteins, high-throughput p<i>K</i>_a determinations, etc. We demonstrate here that electrodialytic membrane suppressors used in ion chromatography can be used to generate buffers. The generated pH, computed from first principles, agrees well with measured values. We demonstrate the generation of phosphate and citrate buffers using a cation-exchange membrane (CEM) -based anion suppressor and Tris and ethylenediamine buffers using an anion-exchange membrane (AEM) -based cation suppressor. Using a mixture of phosphate, citrate, and borate as the buffering ions and using a CEM suppressor, we demonstrate the generation of a highly reproducible (avg RSD 0.20%, <i>n</i> = 3), temporally linear (pH 3.0–11.9, <i>r</i>² > 0.9996), electrically controlled pH gradient. With butylamine and a large concentration (0.5 M) of added NaCl, we demonstrate a similar linear pH gradient of large range with a near-constant ionic strength. We believe that this approach will be of value for the generation of eluents in the separation of proteins and other biomolecules and in online process titrations

Electrodialytic Membrane Suppressors for Ion Chromatography Make Programmable Buffer Generators

The use of buffer solutions is immensely important in a great variety of disciplines. The generation of continuous pH gradients in flow systems plays an important role in the chromatographic separation of proteins, high-throughput p<i>K</i>_a determinations, etc. We demonstrate here that electrodialytic membrane suppressors used in ion chromatography can be used to generate buffers. The generated pH, computed from first principles, agrees well with measured values. We demonstrate the generation of phosphate and citrate buffers using a cation-exchange membrane (CEM) -based anion suppressor and Tris and ethylenediamine buffers using an anion-exchange membrane (AEM) -based cation suppressor. Using a mixture of phosphate, citrate, and borate as the buffering ions and using a CEM suppressor, we demonstrate the generation of a highly reproducible (avg RSD 0.20%, <i>n</i> = 3), temporally linear (pH 3.0–11.9, <i>r</i>² > 0.9996), electrically controlled pH gradient. With butylamine and a large concentration (0.5 M) of added NaCl, we demonstrate a similar linear pH gradient of large range with a near-constant ionic strength. We believe that this approach will be of value for the generation of eluents in the separation of proteins and other biomolecules and in online process titrations

Admittance Detector for High Impedance Systems: Design and Applications

We describe an admittance detector for high impedance systems (small capillary bore and/or low solution specific conductance). Operation in the low frequency range (≤1 kHz, much lower than most relevant publications) provides optimum response to conductance changes in capillaries ≤20 μm in bore. The detector design was based on studies described in a preceding companion paper (Zhang, M.; Stamos, B. N.; Amornthammarong, N.; Dasgupta, P. K. Anal. Chem. 2014, 86, DOI 10.1021/ac503245a.). The highest <i>S</i>/<i>N</i> for detecting 100 μM KCl (5.5 μM peak concentration, ∼0.8 μS/cm) injected into water flowing through a capillary of 7.5 μm inner radius (<i>r</i>) was observed at 500–750 Hz. A low bias current operational amplifier in the transimpedance configuration permitted high gain (1 V/nA) to measure pA–nA level currents in the detection cell. Aside from an oscillator, an offset-capable RMS-DC converter formed the complete detection circuitry. Limits of detection (LODs) of KCl scaled inversely with the capillary cross section and were 2.1 and 0.32 μM injected KCl for <i>r</i> = 1 and 2.5 μm capillaries, respectively. When used as a detector on an <i>r</i> = 8 μm bore poly­(methyl methacrylate) capillary in a split effluent stream from a suppressed ion chromatograph, the LOD was 27 nM bromide (<i>V</i>_ex 22 V p-p), compared to 14 nM observed with a commercial bipolar pulse macroscale conductivity detector with an actively thermostated cell. We also show applications of the detector in electrophoresis in capillaries with <i>r</i> = 1 and 2.5 μm. Efficient heat dissipation permits high concentrations of the background electrolyte and sensitive detection because of efficient electrostacking

Water ICE: Ion Exclusion Chromatography of Very Weak Acids with a Pure Water Eluent

Separation of ions or ionizable compounds with pure water as eluent and detecting them in a simple fashion has been an elusive goal. It has been known for some time that carbonic acid can be separated from strong acids by ion chromatography in the exclusion mode (ICE) using only water as the eluent. The practice of water ICE was shown feasible for very weak acids like silicate and borate with a dedicated element specific detector like an inductively coupled plasma mass spectrometer (ICPMS), but this is rarely practical in most laboratories. Direct conductometric detection is possible for H₂CO₃ but because of its weak nature, not especially sensitive; complex multistep ion exchange methods do not markedly improve this LOD. It will clearly be impractical in acids that are weaker still. By using a permeative amine introduction device (PAID, Anal. Chem. 2016, 88, 2198–2204) as a conductometric developing agent, we demonstrate that a variety of weak acids (silicate, borate, arsenite, cyanide, carbonate, and sulfide) cannot only be separated on an ion exclusion column, they can be sensitively detected (LODs 0.2–0.4 μM). We observe that the elution order is essentially the same as that on a nonfunctionalized poly­(styrene-divinylbenzene) column using 1–10% acetonitrile as eluent and follows the reverse order of the polar surface area (PSA) of the analyte molecules. PSA values have been widely used to predict biological transport of pharmaceuticals across a membrane but never to predict chromatographic behavior. We demonstrate the application of the technique by measuring the silicate and borate depth profiles in the Pacific Ocean; the silicate results show an excellent match with results from a reference laboratory

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