Admittance Scanning for Whole Column Detection.

Whole column detection (WCD) is as old as chromatography itself. WCD requires an ability to interrogate column contents from the outside. Other than the obvious case of optical detection through a transparent column, admittance (often termed contactless conductance) measurements can also sense changes in the column contents (especially ionic content) from the outside without galvanic contact with the solution. We propose here electromechanically scanned admittance imaging and apply this to open tubular (OT) chromatography. The detector scans across the column; the length resolution depends on the scanning velocity and the data acquisition frequency, ultimately limited by the physical step resolution (40 µm in the present setup). Precision equal to this step resolution was observed for locating an interface between two immiscible liquids inside a 21 µm capillary. Mechanically, the maximum scanning speed was 100 mm/s, but at 1 kHz sampling rate and a time constant of 25 ms, the highest practical scan speed (no peak distortion) was 28 mm/s. At scanning speeds of 0, 4, and 28 mm/s, the S/N for 180 pL (zone length of 1.9 mm in a 11 µm i.d. column) of 500 µM KCl injected into water was 6450, 3850, and 1500, respectively. To facilitate constant and reproducible contact with the column regardless of minor variations in outer diameter, a double quadrupole electrode system was developed. Columns of significant length (>1 m) can be readily scanned. We demonstrate its applicability with both OT and commercial packed columns and explore uniformity of retention along a column, increasing S/N by stopped-flow repeat scans, etc. as unique applications.

Xeropreservation of functionalized lipid biomarkers in hyperarid soils in the Atacama Desert.

Our understanding of long-term organic matter preservation comes mostly from studies in aquatic systems. In contrast, taphonomic processes in extremely dry environments are relatively understudied and are poorly understood. We investigated the accumulation and preservation of lipid biomarkers in hyperarid soils in the Yungay region of the Atacama Desert. Lipids from seven soil horizons in a 2.5 m vertical profile were extracted and analyzed using GC-MS and LC-MS. Diagnostic functionalized lipids and geolipids were detected and increased in abundance and diversity with depth. Deeper clay units contain fossil organic matter (radiocarbon dead) that has been protected from rainwater since the onset of hyperaridity. We show that these clay units contain lipids in an excellent state of structural preservation with functional groups and unsaturated bonds in carbon chains. This indicates that minimal degradation of lipids has occurred in these soils since the time of their deposition between >40,000 and 2 million years ago. The exceptional structural preservation of biomarkers is likely due to the long-term hyperaridity that has minimized microbial and enzymatic activity, a taphonomic process we term xeropreservation (i.e. preservation by drying). The degree of biomarker preservation allowed us to reconstruct major changes in ecology in the Yungay region that reflect a shift in hydrological regime from wet to dry since the early Quaternary. Our results suggest that hyperarid environments, which comprise 7.5% of the continental landmass, could represent a rich and relatively unexplored source of paleobiological information on Earth.

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.

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, 8 , DOI 10.1021/ac503245a.). The highest S/N 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 (r) 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 r = 1 and 2.5 µm capillaries, respectively. When used as a detector on an r = 8 µm bore poly(methyl methacrylate) capillary in a split effluent stream from a suppressed ion chromatograph, the LOD was 27 nM bromide (Vex 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 r = 1 and 2.5 µm. Efficient heat dissipation permits high concentrations of the background electrolyte and sensitive detection because of efficient electrostacking.

Capillary scale admittance detection.

Techniques that have been variously termed oscillometric detection or (capacitively coupled) contactless conductivity detection (C(4)D) are known actually to respond to the admittance. It is not often appreciated that the frequency range (f) over which such systems respond (quasi)linearly with the cell conductance decreases acutely with increasing cell resistance. Guidance on optimum operating conditions for high cell resistance, such as for very small capillaries/channels and/or solutions of low specific conductance (σ), is scant. It is specially necessary in this case to take the capacitance of the solution into account. At high frequencies and low σ values, much of the current passes through the solution behaving as a capacitor and the capacitance is not very dependent on the exact solution specific conductance, resulting in poor, zero, or even negative response. We investigated, both theoretically and experimentally, capillaries with inner radii of 5-160 µm and σ ≈ 1-1400 µS/cm, resulting in cell resistances of 51 GΩ to 176 kΩ. A 400-element discrete model was used to simulate the behavior. As model inputs, both the wall capacitance and the stray capacitance were measured. The solution and leakage capacitances were estimated from extant models. The model output was compared to the measured response of the detection system over broad ranges of f and σ. Other parameters studied include capillary material and wall thickness, electrode spacing and length, Faraday shield thickness, excitation wave forms, and amplitude. The simulations show good qualitative agreement with experimental results and correctly predict the negative response behavior observed under certain conditions. We provide optimum frequencies for different operating conditions.

An open tubular ion chromatograph.

We describe an open tubular ion chromatograph (OTIC) that uses anion exchange latex coated 5 µm radius silica and 9.8 µm radius poly(methyl methacrylate) tubes and automated time/pressure based hydrodynamic injection for pL-nL scale injections. It is routinely possible to generate 50,000 plates or more (up to 150,000 plates/m, columns between 0.3 and 0.8 m have been used), and as such, fast separations are possible, comparable to or in some cases better than the current practice of IC. With an optimized admittance detector, nonsuppressed detection permits LODs of submicromolar to double digit micromolar for a variety of analytes. However, large volume injections are possible and can significantly improve on this. A variety of eluents, the use of organic modifiers, and variations of eluent pH can be used to tailor a given separation. The approach is discussed in the context of extraterrestrial exploration, especially Mars, where the existence of large amounts of perchlorate in the soil needs to be confirmed. These columns can survive drying and freezing, and small footprint, low power consumption, and simplicity make OTIC a good candidate for such a mission.

Portable light weight open tubular ion chromatograph for field determination of environmental anions.

We report a battery powered non-suppressed open tubular ion chromatograph (NSOTIC) that weighs less than 3 kg with on-board rechargeable lithium-ion batteries that provide power for 18 h of operation. It is contained in an aluminum case measuring 30 × 25 × 16 cm. Separation relies on open tubular (OT) chromatographic columns which eliminate the need for high pressure pumps, drastically reducing weight and complexity. Eluent consumption is less than 100 µL per separation. Eluent is supplied from a pressurized vessel via a voltage-controlled electronic pressure controller. Flow rates are typically <200 nL/min which allows a single 16-20 g gas cartridge to perform hundreds of separations. Two anions, chloride and nitrate, in Atacama soil samples were field determined by running the portable NSOTIC. More samples were lab analyzed by commercial IC and IC/MS-MS (only perchlorate due to its low concentration level). To demonstrate the feasibility of running NSOTIC on sample analysis, samples were tested by both non-portable and portable NSOTIC.