Source apportionment of gaseous Nitrophenols and their contribution to HONO formation in an urban area.

Nitrophenols (NPs) have significant impacts on human health, climate, and atmospheric chemistry. Despite numerous measurements of particulate NPs, still little is known about their gaseous atmospheric abundances, sources, and fate. Here, four gaseous NPs [2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), and 2-Methyl-4-nitrophenol (2-Me-4-NP)] were continuously monitored during late Spring at an urban site in Houston, Texas. Among the four NPs, 4-NP showed the highest abundance, followed by 2-Me-4-NP, 2-NP, and 2,4-DNP with average concentrations of 1.07 ± 0.19 ppt, 0.47 ± 0.12 ppt, 0.41 ± 0.16 ppt, and 0.27 ± 0.09 ppt, respectively. The positive matrix factorization (PMF) model identified seven sources: industrial NPs, secondary formation, phenol sources, acetonitrile source, natural gas/crude oil, traffic, and petrochemical industries/oil refineries. A zero-dimensional photochemical box model was used to simulate the observed 2-NP and 2,4-DNP. A 50.0% and 70.0% jNO2 was found to be consistent with the measured 2-NP and 2,4-DNP. This yields a nitrous acid (HONO) production of 7.5 ± 2.5 ppt/h from 06:00 to 18:00 Central Standard Time (CST) from both NPs. An extrapolation including other known NPs suggests a maximum HONO formation of 13.8 ppt/h. The results of this study suggest that using PMF analysis supplemented by photochemical box model provides identification of the NPs sources and their atmospheric implication to HONO formation.

Geometric characterization of polymeric capillaries.

There are few methods in the literature to measure the inner diameter of very small capillaries. Although silica capillaries are more commonly used, synthetic polymer capillaries are preferred in some applications. The technology for producing them is not as mature. Aside from the absolute value of the inner diameter, the circularity, concentricity (a quantitative index is defined here for the first time) and the bore uniformity of such capillaries are of interest. Beyond microscopy, we describe multiple methods that determine the capillary inner diameter, averaged over a given length. The measurements variously depended on the capillary internal volume, length and cross section, and the resistance to fluid flow. The different approaches produced mutually consistent results. We show that when the internal diameter is not uniform, the different dependence on diameter that two such methods may exhibit, can be exploited to determine the true mean diameter as well as its variance. Finally, for open tubular liquid chromatography, where performance acutely depends on the inner diameter, we surprisingly find that while the mean i.d. may be the dominant determinant of efficiency, bore variance has little to no effect on the performance.

Automated Programmable Generation of Broad pH Range Volatile Ionic Eluents for Liquid Chromatography.

Many of the universal detectors in liquid chromatography, including mass spectrometry, must completely volatilize the chromatographic eluent first before further processing and detection of the analytes. A basic requirement is that the eluent does not contain a nonvolatile dissolved component. However, separation of biomolecules must be conducted in mostly aqueous media of compatible pH and ionic strength if their biological activity must survive the separation process. Combinations of ammonia with acetic and formic acids are commonly used as eluent for this purpose but generally maximum concentrations that can be tolerated are relatively low. Further, buffering is good only over a limited pH range. We describe a system where the eluent is generated in an automated pressure-programmed manner from high-purity gaseous NH3 and CO2 through gas-permeable membrane devices. This can be aided by the prior presence of formic/acetic acids in the mobile phase to extend the attainable low pH limit. We outline the fundamental pH, ionic strength, and buffer intensity considerations and demonstrate the application of such eluents in the separation of amino acids, proteins, and monoclonal antibodies. We also demonstrate the use of dissolved CO2 as an ion-pairing agent in the separation of chiral amines.

Shape-Based Peak Identity Confirmation in Liquid Chromatography.

Chromatography is used to separate analytes in a mixture. In the absence of a detector providing analyte-specific information, the retention time is used to identify the analyte, typically by comparing the retention time of the unknown with that of a standard. This is the simplest of identification routines, but it remains the most widely used. The challenge is that analyte retention times can and do shift; sole reliance on retention time for identification is not prudent. Retention time is influenced by the total sample concentration, elution of components near the peak of interest, aging of a column, and so on, complicating time window-based identification. We propose a sameness threshold (ST) that is based on numerical comparison of shapes of chromatographic peaks for identification confirmation. We demonstrate shape-based identification confirmation or lack thereof of an unconfirmed peak by comparison with a shape library of standards. In numerous cases we look at, only one analyte in the shape library fits the putative identity. ST can be based on r2, the commonly used coefficient of determination used in regression analysis, or on an "index of width mismatch" that compares the widths of the peaks throughout the available height range.

Exploiting adduct formation through an auxiliary spray in liquid chromatography-electrospray ionization mass spectrometry to improve charge-carrier identification.

We describe a simple and effective approach to probe adduct formation in liquid chromatography - electrospray ionization mass spectrometry (LC-ESI-MS) and help designate and/or confirm which particular analyte is leading to which particular charged species that is detected. A compound tends to form similar adducts with adduct-forming analogs, at various abundance levels, of course. It is based on this understanding that in this work we probed adduct formation by modulating the adduct-forming analogs and observing the adducts formed with these analogs to lend experimental evidence to adduct annotation. The approach was implemented through an auxiliary spray and made possible thanks to the interaction between the plumes of the sample spray or main spray and the auxiliary spray. Changing adduct-forming analogs by switching the auxiliary spray solution, or simply turning on and off the auxiliary spray facilitated the observation of the adducts corresponding to these analogs or lack thereof, giving rise to a simple and effective approach to probe adduct formation and thus help annotate the analyte ions.

Moldable Strong Cation Exchange Polymer and Microchannel Fabrication.

We characterize a high-capacity cation exchange membrane (CEM) synthesized from an aqueous poly(vinyl alcohol) (PVA) solution and varying amounts of a water-soluble ionic monomer, sodium styrenesulfonate. Highly hydrophilic but water-insoluble transparent polymers with a range of ion exchange capacities (IECs) can be made; at the high end, the ion exchange capacities (IEC) is >2× that of the benchmark CEM, Nafion. The water uptake of the polymer (as moles of water/mole H+) is 5-10× greater than that of Nafion. Except at the highest IECs (where steeply increasing water sorption and resultant swelling outpaces IEC increase), the specific conductance increases with increasing IEC while the conductance anisotropy decreases. The material withstands repeated regeneration cycles and hour-long boiling in water or alcohols. The aqueous prepolymer mixture can be cast around a fine wire acting as a mandrel. Fabrication of an ion exchanger microchannel, capable of withstanding at least 300 psi, is demonstrated. We also discuss entirely novel considerations on ab initio limits of conductance of ion exchange membranes.

Optimum Cell Pathlength or Volume for Absorbance Detection in Liquid Chromatography: Transforming Longer Cell Results to Virtual Shorter Cells.

The cell volume permissible for a specified degree of loss of efficiency can be computed from response volume considerations. For an open tubular column, the permissible illuminated length can be computed for the unretained peak. Similar estimations can be made for the maximum permissible cell volume with a packed column for a given column efficiency and flow rate. The packed column case does require an assumption on the degree to which the cell behaves as a mixer. An altogether different question is if the data from a long cell, with its considerable advantage in S/N, can be used and the associated dispersion mathematically removed. Experimental data for a variable path length (0-60 mm) HPLC detection cell indicate that an exponential model fits the observed dispersion. Once fit parameters are determined, the same can be applied to a peak, not part of the original training set, obtained in a longer path cell and the effects of dispersion mathematically reversed without major loss of S/N. Results with shorter path cell dispersion characteristics are then obtained with much higher S/N. A comparison is made with Fourier transform-inverse Fourier transform based deconvolution that can be used to achieve the same ends.

Nanovolume Gas-Free Hydroxide Eluent Generator for Open Tubular Ion Chromatography.

A gas-free KOH eluent generator (EG) with 210 nL of internal volume is described. It utilizes a two-membrane configuration where there is a single CEM layer on one side and a single BPM layer on the other side for use in open tubular ion chromatography systems with typical back pressures < 50 psi. At a flow rate of ∼190 nL/min, the 10-90% gradient rise time is 3.5 min. The device shows good linearity between applied current and concentration of KOH generated, which is stable over extended periods. The overall system reproducibility (that includes contributions from any changes in flow rate), as judged by the relative standard deviation (RSD) of the retention times of individual separated ions in repeat measurements (n = 6), ranged from <0.5% for isocratic to <1.2% for gradient elution schemes. Perceptible current flow and KOH production in the BPM-based EG begins at subelectrolytic applied voltages, prompting us to look more closely at exact field strength necessary for field-enhanced dissociation of water. An increase in the specific conductance of pure water is noticeable by a field strength of 105 V/m.

Time-of-Sight Liquid Flow Measurements in the Low Nanoliters per Minute Scale.

We describe an affordable and robust measurement technique applicable to nanoscale liquid flow. The approach can provide good precision (<1% RSD) in the 1.5-15 nL/min flow range. The motion of a conductive/nonconductive immiscible segmental interface in a capillary is followed by an admittance detector. The conductive marker segment, e.g., a salt solution, is protected on both sides from the principal flow stream by immiscible guard segments, typically a fluorocarbon (FC) liquid, of significantly greater impedance. Fluorosilylation of the capillary ensures no other liquid film between the FC segments and the wall (perfect piston). A given interface/marker can typically be used only once in interface/front tracking systems. We overcome this by putting the sensor capillary in a valved configuration where the flow direction in the sensor is reversed before the guard/marker segments escape. Several strategies are possible to interpret flow rate from the sensor output, including the rate of the interface movement. During the measurement process, a change in this rate of movement can be detected in <1 s. Small temperature variations in the 25-35 °C range did not affect sensor behavior.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length.

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/αa{ln[1 + 2(αa/αb)]}, approximately 2/αb under most conditions, αa and αb being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable αb value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed αb values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

Ion exchange membranes in ion chromatography and related applications.

This review is a broad survey of ion exchange membranes (IEMs) in chosen areas of analytical chemistry. The coverage is comprehensive in ion chromatography (IC) applications, this includes in-line water purification for IC use, chemical and electrodialytic eluent suppression, eluent generation, eluent purification, pH modification, buffer generation, use in ion exclusion chromatography, charge-based detection, as well as suppressor-detector combinations. The use of IEMs in sample treatment using ion transfer devices and Donnan dialysis preconcentration are discussed. Introduction of reagents through an IEM, whether through a concentration gradient large enough to overcome the Donnan barrier or active electrodialysis, does not involve volumetric dilution. Reagent introduction in this fashion, enabling conductometric detection of otherwise non-ionized weak electrolytes, is discussed. Several important applications of IEMs, in which ion exchange plays little or no role, are discussed; these include (a) diffusion-based gas collection and analysis, (b) evaporative concentration in a flow system, (c) drying of gas streams and (d) sensing of moisture in gases and liquids. An important area that is not covered is ion-sensing with IEMs and/or ionophores.

Inline flow sensor for ventriculoperitoneal shunts: Experimental evaluation in swine.

Shunts are commonly employed to treat hydrocephalus, a severe central nervous disease caused by the buildup of cerebrospinal fluid in the brain. These shunts divert excessive cerebrospinal fluid from brain ventricles to other body cavities, thereby relieving the symptoms. However, these shunts are highly prone to failure due to obstruction from cellular debris, leading to cerebrospinal fluid accumulation in the brain and exacerbation of neurological symptoms. Therefore, there is a clinical need for a reliable, non-invasive method of monitoring shunt performance. Recently, a simple inline flow sensor was reported for monitoring ventriculoperitoneal shunting of cerebrospinal fluid in hydrocephalus treatment. The present work aimed to evaluate performance of the device in an animal model of hydrocephalus. Sensor-equipped shunt tubes were placed in anesthetized, juvenile swine. The flows reported by the sensor were compared with gravimetric flow measurements. Robust correlations (r ≈ 0.87-0.96) between the gravimetric and sensor-reported flows were obtained in 4 of the 6 experiments. The mean slope of the linear relationship of the gravimetrically determined vs. sensor flow rates was 0.98 ± 0.09 in the 6 experiments, indicating the sensor accurately reported shunt flows up to 35 ml/h. The sensor responded immediately to abrupt flow changes following cerebroventricular fluid injections. Minor hardware problems were identified and corrected. These experiments provide practical guidance for future preclinical testing of the device.

Carbonic Acid Eluent Ion Chromatography.

Alkali metals, amines and alkanolamines are separated on a poly(butadiene)-maleic acid on silica stationary phase using a carbonic acid (H2CO3*) eluent with and without a mineral acid. The H2CO3* eluent is prepared in situ by high pressure permeative introduction of gaseous CO2 through thin membranes supported upon a porous steel disk. The permeation flux and thus the eluent concentration is controlled by varying the applied CO2 pressure. This novel frit-supported membrane device tolerates much higher liquid and gas pressures than Teflon AF capillaries, permitting [H2CO3*] exceeding 0.53 M and attaining a pH of 3.3. Silicone was presently preferred over Teflon AF, both as planar membranes, as mechanical properties of the latter change as large amounts of CO2 dissolve in it. After separation, the CO2 can be efficiently removed via another gas permeable membrane device permitting detection of the eluting bicarbonate salt conductometrically in a background of nearly pure water. Most analytes are more sensitively detected after anion conversion to hydroxide using a standard suppressor, permitting 3-17 pmol LODs on 2 mm bore columns. The data, particularly comparisons with an HNO3 eluent, with or without H2CO3*, indicate that proton exchange alone does not account for the retention behavior; some reactive addition of HCO3- is involved.

Direct Photothermal Measurement of Optical Absorption in a Flow System.

We describe here a simple photothermal detection scheme in a flow stream based on the temperature difference upstream and downstream of the point of illumination. We use a single, two-junction 25 µm diameter thermocouple to measure the temperature change. The baseline standard deviation in the dark is ∼0.001 °C that increases up to 0.0016 °C depending on the illumination source. We demonstrate the detection of several chromatographically separated dyes both with a 1.5 mm and a 0.1 mm i.d. detection cell, respectively, with a white LED and a solid-state laser source. With an inexpensive 660 nm, 19 mW laser as the light source, the estimated detection limit for methylene blue (MB) was 30 nM, corresponding to 120 amol in the illuminated volume. The dimerization constant of MB and the quantum efficiency of the monomer was determined.

Capillary Scale Admittance and Conductance Detection.

Admittance detection, more commonly called capacitively coupled contactless conductivity detection, is widely used. While the true conductance of a solution is linear with concentration up to ∼1 mequiv/L, the admittance signal is nonlinear. In small-bore capillaries and highly resistive solutions, such as in suppressed open tubular ion chromatography (SOTIC), the admittance signal is exponentially related to concentration. Solution contact conductivity detection is common in microfluidics, but no true conductivity measurement efforts have been made with conventional capillaries in the last two decades. We examine five solution contact cell geometries: (A) wire/disk electrodes perpendicular to the flow direction, (B) annular tubular electrodes facing the flow (that exits through the center tube), (C) ring-disk electrodes facing the flow that exits through the annulus, (D) coplanar parallel wire/disk electrodes facing the flow that exits around the electrodes, and (E) planar electrodes separated by a thin insulating layer with a through-hole in which the liquid flows. Present limitations of the available components as well as the designs themselves do not allow type A and B cells to reach low enough dispersion levels. Types C-E all produce the same efficiency for chloride in a SOTIC setup (12000 ± 200 plates/m), limited by dispersion in the suppressor. Further choice and refinement will be dictated by the availability of a lower dispersion suppressor. All such contact conductance results were linear with concentration, and initial results suggest that attainable LODs will be competitive with those from benchtop ICs.

Characterization of ion exchange functionalized cyclic olefin polymer open tubular columns.

We describe both the cation-exchange (CEX) and anion-exchange (AEX) capacity measurements of functionalized cyclic olefin polymer (COP) open tubular (OT) columns. COP capillaries were sulfonated to provide CEX functionality and then coated with AEX latex to provide AEX functionality. We measure functionalization uniformity along the column by (a) separately measuring capacities of two halves of a column, (b) measuring retention factors for several ions in successive 10 cm segments of a column in multiple injections and (c) measuring retention variance of a single ion in different segments of a column from a single injection. Data were collected in both flow directions. Errors in CEX capacity determination arises primarily from uncertainties in titration end point location. AEX capacities are typically 10x higher, reducing the relative measurement error. In the AEX case, both titration end point location error and variations in column internal diameter contribute to the overall uncertainty. Although sulfonation of COP results in nonuniform CEX functionalization, AEX latex coating of this surface results in uniform AEX coverage. Frontal displacement chromatography (using both admittance and optical detection) and acid-base titrimetry were compared for capacity measurement. In a pooled standard deviation based t-test for the data on 8 columns, at the 95% confidence level, CEX capacity differed significantly between two halves of a column. But AEX capacity of AEX columns prepared by coating the same CEX columns with AEX latex did not.

Low-Bleed Silica-Based Stationary Phase for Hydrophilic Interaction Liquid Chromatography.

Bleeding of hydrophilic interaction liquid chromatographic (HILIC) phases is a major problem. A hydrophobic silica surface exhibits low bleeding but does not behave as a HILIC phase. A hydrophilic coating, such as that of poly(vinyl alcohol) (PVA) on a hydrophobic particle should result in a low-bleed hydrophilic phase. However, a silica particle functionalized in a hydrophobic manner is not wetted by PVA and cannot be coated with it. Coating with PVA becomes possible if one region of the hydrophobic functionality is polar. We describe a low-bleed HILIC stationary phase, PVA-coated benzylthioethyl-silica. The benzyl groups shield the silica from water erosion, while the thiol group provides sufficient local polarity for PVA to wet the material. The new stationary phase demonstrated good chromatographic performance and typical HILIC retention behavior. The measured silica concentration in the effluent was ∼80-fold lower than that from a bare silica column. The new stationary phase exhibited a lower background level, lower background noise, and lower background drift under gradient conditions than benchmark commercial columns.

Ion exchange column capacities. Predicting retention behavior of open tubular columns coated with the same phase.

We discuss the reported capacities of available packed ion exchange columns and the different methods used for their measurement. We outline basic considerations related to both packed and open tubular columns based on ion exchange latex particles. There is a large body of information covering the retention behavior of packed ion exchange columns based on ion exchange latex particles. We propose a parameter γiex, which is the ion exchange capacity of a column (packed or open tubular) per unit liquid volume present in the column (including accessible volume within pores) and show that the retention factor for any given ion is directly related to γiex. On this basis, if based on the same type of latex, the behavior of one type of column can be reasonably predicted from the known behavior of the other, even when the absolute capacities differ by more than 5 orders of magnitude.

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 (σt,obs2), which increases as the flow rate (F) through the detector decreases, is found to be well-described as a quadratic function of 1/F. 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.

Continuous measurement of elemental composition of ambient aerosol by induction-coupled plasma mass spectrometry.

There is presently no instrumentation that can provide (near) real time information on elemental composition of atmospheric aerosols. We describe an arrangement where air is sampled through a cyclone @30L/min with a 50% cutoff @ ~250nm. The particles deposit into a cup through which deionized water is continuously flowing. High purity HNO3 is added downstream and the mixed stream optionally flows through a quartz photo reactor (185nm, ~90°C, tR ~1.2min) and is aspirated by an induction coupled plasma mass spectrometer (ICP-MS). Comparative batch experiments in which samples were not photodigested at all or thermally digested off-line for an extended period indicated no statistically significant difference in the results. This observation agrees with early theoretical and experimental work. Some 22 elements were quantifiable (S/N > 10) at all times in the aerosol samples collected in our highly urban sampling location; an additional 4 elements were quantifiable at times of construction activity in the general area. Presently attained system limits of detection (LODs) are orders of magnitude higher than the instrumental LOD, both because of the purity of the acid and pump-induced contamination. These aspects can be vastly improved and will need to be improved to determine background concentrations.