Akinde F. Kadjo

A Comprehensive Analysis of Groundwater Quality in The Barnett Shale Region

The exploration of unconventional shale energy reserves and the extensive use of hydraulic fracturing during well stimulation have raised concerns about the potential effects of unconventional oil and gas extraction (UOG) on the environment. Most accounts of groundwater contamination have focused primarily on the compositional analysis of dissolved gases to address whether UOG activities have had deleterious effects on overlying aquifers. Here, we present an analysis of 550 groundwater samples collected from private and public supply water wells drawing from aquifers overlying the Barnett shale formation of Texas. We detected multiple volatile organic carbon compounds throughout the region, including various alcohols, the BTEX family of compounds, and several chlorinated compounds. These data do not necessarily identify UOG activities as the source of contamination; however, they do provide a strong impetus for further monitoring and analysis of groundwater quality in this region as many of the compounds we detected are known to be associated with UOG techniques.

Temporal variation in groundwater quality in the Permian Basin of Texas, a region of increasing unconventional oil and gas development.

The recent expansion of natural gas and oil extraction using unconventional oil and gas development (UD) practices such as horizontal drilling and hydraulic fracturing has raised questions about the potential for environmental impacts. Prior research has focused on evaluations of air and water quality in particular regions without explicitly considering temporal variation; thus, little is known about the potential effects of UD activity on the environment over longer periods of time. Here, we present an assessment of private well water quality in an area of increasing UD activity over a period of 13months. We analyzed samples from 42 private water wells located in three contiguous counties on the Eastern Shelf of the Permian Basin in Texas. This area has experienced a rise in UD activity in the last few years, and we analyzed samples in four separate time points to assess variation in groundwater quality over time as UD activities increased. We monitored general water quality parameters as well as several compounds used in UD activities. We found that some constituents remained stable over time, but others experienced significant variation over the period of study. Notable findings include significant changes in total organic carbon and pH along with ephemeral detections of ethanol, bromide, and dichloromethane after the initial sampling phase. These data provide insight into the potentially transient nature of compounds associated with groundwater contamination in areas experiencing UD activity.

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 myo-Inositol Phosphates

The separation of myo-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 InsP5s 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.

Evaluation of Amount of Blood in Dry Blood Spots: Ring-Disk Electrode Conductometry

A fixed area punch in dried blood spot (DBS) analysis is assumed to contain a fixed amount of blood, but the amount actually depends on a number of factors. The presently preferred approach is to normalize the measurement with respect to the sodium level, measured by atomic spectrometry. Instead of sodium levels, we propose electrical conductivity of the extract as an equivalent nondestructive measure. A dip-type small diameter ring-disk electrode (RDE) is ideal for very small volumes. However, the conductance (G) measured by an RDE depends on the depth (D) of the liquid below the probe. There is no established way of computing the specific conductance (σ) of the solution from G. Using a COMSOL Multiphysics model, we were able to obtain excellent agreement between the measured and the model predicted conductance as a function of D. Using simulations over a large range of dimensions, we provide a spreadsheet-based calculator where the RDE dimensions are the input parameters and the procedure determines the 99% of the infinite depth conductance (G99) and the depth D99 at which this is reached. For typical small diameter probes (outer electrode diameter ∼ <2 mm), D99 is small enough for dip-type measurements in extract volumes of ∼100 μL. We demonstrate the use of such probes with DBS extracts. In a small group of 12 volunteers (age 20-66), the specific conductance of 100 μL aqueous extracts of 2 μL of spotted blood showed a variance of 17.9%. For a given subject, methanol extracts of DBS spots nominally containing 8 and 4 μL of blood differed by a factor of 1.8-1.9 in the chromatographically determined values of sulfate and chloride (a minor and major constituent, respectively). The values normalized with respect to the conductance of the extracts differed by ∼1%. For serum associated analytes, normalization of the analyte value by the extract conductance can thus greatly reduce errors from variations in the spotted blood volume/unit area.

Width Based Characterization of Chromatographic Peaks: Beyond Height and Area

The preceding paper ( Anal. Chem. 2017 , 10.1021/acs.analchem.6b04857 ) introduced width-based quantitation (WBQ). The present paper focuses on (1) situations where WBQ is effective while height/area-based linear calibrations fail, e.g., when (a) the detector is in a nonlinear response region, (b) the detector/data system is saturated, causing clipping/truncation of the signal, or (c) the detector signal is not a single-valued function of concentration, as when a fluorescence signal goes into the self-quenched domain. (2) Utilization of WBQ in postcolumn reagent addition methods where the reagent produces a significant detector background. WBQ can minimize added reagent without sacrificing the upper determination limit; a limited reagent amount truncates peaks from high analyte concentrations but does not hamper WBQ at a low height. (3) A description of peak asymmetry via leading/trailing half-widths vs relative height (fraction of maximum height) plots. (4) A holistic description of chromatographic peaks through six parameters describing the two independent generalized Gaussian distributions that underlie the WBQ chromatographic peak model. (5) Characterization of shape by widths at multiple heights and shape-based impurity detection.

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.

Width Based Quantitation of Chromatographic Peaks: Principles and Principal Characteristics

Height- and area-based quantitation reduce two-dimensional data to a single value. For a calibration set, there is a single height- or area-based quantitation equation. High-speed high-resolution data acquisition now permits rapid measurement of the width of a peak (Wh), at any height h (a fixed height, not a fixed fraction of the peak maximum) leading to any number of calibration curves. We propose a width-based quantitation (WBQ) paradigm complementing height or area based approaches. When the analyte response across the measurement range is not strictly linear, WBQ can offer superior overall performance (lower root-mean-square relative error over the entire range) compared to area- or height-based linear regression methods, rivaling weighted linear regression, provided that response is uniform near the height used for width measurement. To express concentration as an explicit function of width, chromatographic peaks are modeled as two different independent generalized Gaussian distribution functions, representing, respectively, the leading/trailing halves of the peak. The simple generalized equation can be expressed as Wh = p(ln h̅)q, where h̅ is hmax/h, hmax being the peak amplitude, and p and q being constants. This fits actual chromatographic peaks well, allowing explicit expressions for Wh. We consider the optimum height for quantitation. The width-concentration relationship is given as ln C = aWhn + b, where a, b, and n are constants. WBQ ultimately performs quantitation by projecting hmax from the width, provided that width is measured at a fixed height in the linear response domain. A companion paper discusses several other utilitarian attributes of width measurement.

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.

Transient Ion-Pair Separations for Electrospray Mass Spectrometry.

We report a novel ion-pair chromatography (IPC) approach for liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS), where the eluent does not contain any ion-pairing reagent (IPR). The IPR is injected on the column, much like the sample, and moves down the column. Significant amounts of a high retention factor IPR is injected, resulting in a transient but reproducible regional coating that progresses along the column. The sample is injected after a brief interval. The sample components interact with the IPR coated region during their passage; the chosen eluent gradient elutes the analytes of interest into the mass spectrometer before the IPR. Following analyte elution, the gradient is steeply raised, the IPR is washed out, and the effluent is sent to waste via a diverter valve until it is fully removed. As the nature of the analyte retention continuously changes along the column and with time, we call this transient ion-pair separation (TIPS). As the IPR never enters the MS, TIPS addresses two major drawbacks of IPC for ESI-MS: it avoids both ion suppression and ion source contamination. The potential of the generic approach for other modes of separation is discussed. An illustrative separation of two small inorganic ions, iodate and nitrate, is demonstrated on a reverse phase column by a transient prior injection of hexadecyltrimethylammonium chloride as IPR.