29 research outputs found
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 AsÂ(III) and
AsÂ(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 AsÂ(III) to AsH<sub>3</sub> quantitatively but
has significantly lower response to AsÂ(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 AsÂ(V) and AsÂ(III) for the HOPG-based
EAG and 1.4 μg/L AsÂ(V) or AsÂ(III) for the Al-based EAG, respectively.
Precision at the ∼20 μg/L AsÂ(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 (NR<sub>2</sub>H + HX →
NR<sub>2</sub>H<sub>2</sub><sup>+</sup> + X<sup>–</sup>) and
allows very weak acids HX (p<i>K</i><sub>a</sub> ≥
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><sub>b</sub> 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<sup>–</sup> is greater than
that of X<sup>–</sup>. 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., IO<sub>3</sub><sup>–</sup>), frontal displacement
by a strongly bound ion (e.g., Cl<sup>–</sup>), 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><sub>a</sub>-differentiated ion exchange capacity measurements.
The cation exchange capacity of bare PMMA capillaries was on the order
of 1 pequiv/mm<sup>2</sup> 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/mm<sup>2</sup>, 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><sub>i</sub> – <i>V</i><sub>f</sub>, 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<sup>–6</sup> × Δ<i>V</i><sup>(−9.568+1.088 <i>V</i><sub>i</sub>)</sup> 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><sub>a</sub> 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><sup>2</sup> > 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><sub>a</sub> 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><sup>2</sup> > 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 polyÂ(methyl methacrylate)
capillary in a split effluent stream from a suppressed ion chromatograph,
the LOD was 27 nM bromide (<i>V</i><sub>ex</sub> 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 H<sub>2</sub>CO<sub>3</sub> 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 polyÂ(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
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.,
H<sub>2</sub>S, HCN, H<sub>2</sub>CO<sub>3</sub>, etc., represented
as HA) transfer through the membrane and react as OH<sup>–</sup> + HA → A<sup>–</sup> + H<sub>2</sub>O; the conversion
of high-mobility OH<sup>–</sup> to lower mobility A<sup>–</sup> 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