University of North Florida
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Stuart Chalk, Ph.D.
Department of Chemistry
University of North Florida
Phone: 1-904-620-1938
Fax: 1-904-620-3535
Email: schalk@unf.edu
Website: @unf

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Robert E. Synovec

Abbrev:
Synovec, R.E.
Other Names:
Address:
Department of Chemistry, Center for Process Analytical Chemistry, University of Washington, P.O. Box 351700, Seattle, WA 98195-1700, USA
Phone:
+1-206-685-2328
Fax:
+1-206-685-8665

Citations 19

"A Dynamic Liquid-liquid Interfacial Pressure Detector For The Rapid Analysis Of Surfactants In A Flowing Organic Liquid"
Talanta 2005 Volume 65, Issue 3 Pages 722-729
Narong Lenghor, Bethany A. Staggemeier, Mazen L. Hamad, Yuthapong Udnan, Sumalee Tanikkul, Jaroon Jakmunee, Kate Grudpan, Bryan J. Prazen and Robert E. Synovec

Abstract: Design and development of a dynamic interfacial pressure detector (DIPD) is reported. The DIPD measures the differential pressure as a function of time across the liquid-liquid interface of organic liquid drops (i.e., n-hexane) that repeatedly grow in water at the end of a capillary tip. Using a calibration technique based on the Young-Laplace equation, the differential pressure signal is converted, in real-time, to a relative interfacial pressure. This allows the DIPD to monitor the interfacial tension of surface active species at liquid-liquid interfaces in flow-based analytical techniques, such as flow injection analysis (FIA), sequential injection analysis (SIA) and high performance liquid chromatography (HPLC). The DIPD is similar in principle to the dynamic surface tension detector (DSTD), which monitors the surface tension at the air-liquid interface. In this report, the interfacial pressure at the hexane-water interface was monitored as analytes in the hexane phase diffused to and arranged at the hexane-water interface. The DIPD was combined with FIA to analytically measure the interfacial properties of cholesterol and Brij®30 at the hexane-water interface. Results show that both cholesterol and Brij®30 exhibit a dynamic interfacial pressure signal during hexane drop growth. A calibration curve demonstrates that the relative interfacial pressure of cholesterol in hexane increases as the cholesterol concentration increases from 100 to 10,000 µg mL-1. An example of the utility of the DIPD as a selective detector for a chromatographic separation of interface-active species is also presented in the analysis of cholesterol in egg yolk by normal-phase HPLC-DIPD. © 2004 Elsevier B.V. All rights reserved.

"Sequential Injection Analysis With Dynamic Surface Tension Detection: High Throughput Analysis Of The Interfacial Properties Of Surface-active Samples"
Talanta 2003 Volume 59, Issue 6 Pages 1153-1163
Narong Lenghor, Kate Grudpan, Jaroon Jakmunee, Bethany A. Staggemeier, Wes W. C. Quigley, Bryan J. Prazen, Gary D. Christian, Jaromir Ruzicka and Robert E. Synovec

Abstract: A sequential injection analysis (SIA) system is coupled with dynamic surface tension detection (DSTD) for the purpose of studying the interfacial properties of surface-active samples. DSTD is a novel analyzer based upon a growing drop method, utilizing a pressure sensor measurement of drop pressure. The pressure signal depends on the surface tension properties of sample solution drops that grow and detach at the end of a capillary tip. In this work, SIA was used for creating a reagent concentration gradient, and for blending the reagent gradient with a steady-state sample. The sample, consisting of either sodium dodecyl sulfate (SDS) or poly(ethylene glycol) at 1470 g mol-1 (PEG 1470), elutes with a steady-state concentration at the center of the sample plug. Reagents such as Brij(R)35, tetrabutylammonium (TBA) hydroxide and β-cyclodextrin were introduced as a concentration gradient that begins after the sample plug has reached the steady-state concentration. By blending the reagent concentration gradient with the sample plug using SIA/DSTD, the kinetic surface pressure signal of samples mixed with various reagent concentrations is observed and evaluated in a high throughput fashion. It was found that the SIA/DSTD method consumes lesser reagent and required significantly less analysis time than traditional FIA/DSTD. Four unique chemical systems were studied with regard to how surface activity is influenced, as observed through the surface tension signal: surface activity addition, surface activity reduction due to competition, surface activity enhancement due to ion-pair formation, and surface activity reduction due to bulk phase binding chemistry.

"Review Of Analytical Measurements Facilitated By Drop Formation Technology"
Talanta 2000 Volume 51, Issue 5 Pages 921-933
Keith E. Miller and Robert E. Synovec

Abstract: The use of drops in chemical analysis methodology and instrumentation has a deeply rooted past in the area of electrochemistry through the evolution of the dropping mercury electrode (DME). This history has also been deeply rooted in the field of surface science due to the inextricable connection between surface tension forces and drop formation. While the use of the DME is well established, the evolution of drop-based analytical measurements using aqueous and/or organic drops is a rapidly emerging and diverse field, encompassing several interdisciplinary areas of science: surface science and interfacial surface tension phenomena, spectroscopic detection, analytical instrumentation hyphenation, liquid membrane separation, reagent chemistry, electrochemistry, and so on. This review of 112 references covers various aspects of drop-based analytical measurements involving aqueous and/or organic drops. The review is divided into four sections, although the classification of a particular reference into a given section can sometimes be argued. The first section considers the use of drops as a detector component. The second section deals with fundamental studies that probe drop-related chemical and physical phenomena that are relevant to current and future developments in analytical chemistry. The next section covers recent advances in the area of microfluidic sample handling and instrumentation hyphenation. The final section reports upon emerging technologies aimed toward drop-based chemical analyzers that incorporate a number of steps in a chemical analysis: microextraction, pre-concentration, reagent chemistry, microfluidic handling, and detection.

"Novel Calibration Of A Dynamic Surface Tension Detector: Flow Injection Analysis Of Kinetically-hindered Surface Active Analytes"
Talanta 1999 Volume 50, Issue 5 Pages 1045-1056
Keith E. Miller, Kristen J. Skogerboe and Robert E. Synovec

Abstract: First, a novel technique for calibration of a dynamic surface tension detector (DSTD) is described. The DSTD measures the differential pressure as a function of time across the liquid-air interface of growing drops that repeatedly form and detach at the end of a capillary tip. The calibration technique utilizes the ratio of pressure signals acquired from the drop growth of two separate solutions, i.e. a standard solution and a corresponding mobile phase, such as water, both of which have a known surface tension. Once calibrated, the dynamic surface tension of an analyte is obtained from the ratio of the pressure signals from the analyte solution to that of the mobile phase solution. Thus, this calibration technique eliminates the need to optically image the radius of the expanding drop of liquid. Accurate dynamic surface tension determinations were achieved for aqueous sodium dodecyl sulfate (SDS) solutions over a concentration range of 0.5-5.4 mM. The measured surface tensions for these SDS solutions range from 70.3 to 46.8 dyne/cm and were in excellent agreement with the literature. A precision of 0.2 dyne/cm (1 S.D.) was routinely obtained. Second, the DSTD with this calibration technique was combined with flow injection analysis (FIA) for the study of model protein solutions and polymer solutions. The kinetic surface tension behavior of aqueous bovine serum albumin (BSA) solutions as a function of concentration and flow rate is presented. Evaluation of the dynamic surface tension data illustrates that a protein such as BSA initially exhibits kinetically-hindered surface tension lowering, i.e. a time dependence, as BSA interacts with the liquid-air interface of an expanding drop. FIA/DSTD is then shown to be an effective tool for the rapid study of kinetically-hindered surfactant mixtures. It was found that mixtures of SDS and the polymeric surfactant Brij(R)-35 (lauryl polyoxyethylene ether with an average molecular weight of 1200 g/mol) result in essentially an additive lowering of the surface tension. Mixtures of polyethylene glycol (PEG), with an average molecular weight of 1470 g/mol, and Brij(R)-35, however, result in a competitive (non-additive) surface tension with the Brij(R)-35 dominating the response.
Adsorption

"Enhanced Surfactant Determination By Ion-pair Formation Using Flow Injection Analysis And Dynamic Surface Tension Detection"
Talanta 1996 Volume 43, Issue 6 Pages 889-899
Toby E. Young and Robert E. Synovec*

Abstract: Ion-pair reagent was injected into a carrier stream of water, sample was then injected into the stream and passed through a mixing coil. The resulting stream passed to a stainless steel capillary (5 cm x 0.007 in. i.d.) with a tapered tip. A drop was produced at the end of the capillary which was detected by a diode laser with an attached focusing lens which passed through a 0.25 in. pin hole aperture to a photodiode. The larger the drop produced the more light is block out from the diode laser. When surfactant is present in the sample, the surface tension in the drop decreases causing a reduction in drop size and an increase in laser light intensity detected. The detection limit for dodecylsulfate using tetrabutylammonium hydroxide as the ion-pair reagent was 40 ppb.
Surfactants Sensor Ion pair formation Laser diode

"Comparison Of The Binding Constant Of Decanesulfonate With β-cyclodextrin As Determined By Liquid Chromatography With A Water Mobile Phase And Flow Injection Analysis Coupled With Dynamic Surface Tension Detection"
Microchem. J. 1999 Volume 62, Issue 1 Pages 70-82
Toby E. Young, Keith E. Miller and Robert E. Synovec

Abstract: This paper compares two independent methods for the determination of the inclusion complex binding constant between β-cyclodextrin (β-CD) and decanesulfonate. First, reverse-phase liquid chromatography (RP-LC) separations of four surfactants are examined under two conditions for the binding constant determination. The first separation relies on RP-LC using a 100% water mobile phase and a cyanopropyl stationary phase. Hexanesulfonate, octyl sulfate. and decanesulfonate were separated in under 10 min. With the addition of only 1 mM β-CD to the water mobile phase, the same three surfactants, in addition to dodecyl sulfate, were separated in 6 min. Limits of detection were in the parts-per-billion range using conductivity detection but did not require a mobile phase conductivity suppressor. The binding constant for decanesulfonate with β-CD using RP-LC with a water mobile phase was found to be (3.13±0.38) x 10(3) M-1. Second, flow injection analysis (FIA) is coupled with dynamic surface tension detection (DSTD) to provide an independent method to determine the binding constant of β-CD with decanesulfonate. Using FIA with DSTD, a decanesulfonate binding constant with β-CD of (4.75±1.25) x 10(3) M-1 was obtained. The two binding constant values were in close agreement, suggesting that secondary equilibria were minimal in RP-LC with a water mobile phase. The combination of FIA with DSTD is shown to be an excellent method to help study reverse-phase retention mechanisms involving complexation equilibria.

"Diffusion Coefficient Measurement In A Microfluidic Analyzer Using Dual-beam Microscale-refractive Index Gradient Detection: Application To On-chip Molecular Size Determination"
J. Chromatogr. A 2003 Volume 1013, Issue 1-2 Pages 77-91
Colin D. Costin, Roy K. Olund, Bethany A. Staggemeier, Ana Kristine Torgerson and Robert E. Synovec

Abstract: We report a microchip-based detection scheme to determine the diffusion coefficient and molecular mass (to the extent correlated to molecular size) of analytes of interest. The device works by simultaneously measuring the refractive index gradient (RIG) between adjacent laminar flows at two different positions along a microchannel. The device, referred to as a microscale molecular mass sensor (µ-MMS), takes advantage of laminar flow conditions where the mixing of two streams occurs essentially by diffusion across the boundary between the two streams. Two flows merge on the microchip, one containing solvent only, referred to as the mobile phase stream and one which contains the analyte(s) of interest in the solvent, i.e. the sample stream. As these two streams merge and flow parallel to each other down the microchannel a RIG is created by the concentration gradient. The RIG is further influenced by analyte diffusion from the sample stream into the mobile phase stream. Measuring the RIG at a position close to the merging point (upstream signal) and simultaneously a selected distance further down the microchannel (downstream signal) provides real-time data related to the extent a given analyte has diffused, which can be readily correlated to analyte molecular mass by taking the ratio of the downstream-to-upstream signals. For the dual-beam RIG measurements, a diode laser output is coupled to a single mode fiber optic splitter with two output fibers. Light from each fiber passes through a graded refractive index (GRIN) lens forming a collimated beam that then passes through the microchannel and then on to a position sensitive detector (PSD). The RIG at both detection positions deflects the two collimated probe beams. The deflection angle of each beam is then measured on two separate PSDs. The µ-MMS was evaluated using polyethylene glycols (PEGs), sugars, and as a detector for size-exclusion chromatography (SEC). Peak purity can be readily identified using the µ-MMS with SEC. The limit of detection was 0.9 ppm (PEG at 11840 g/mol) at the upstream detection position corresponding to a RI limit of detection (LOD) (3s) of 7.10-8 RI. The pathlength for the RIG measurement was 200 µm and the angular LOD was 0.23 µrad with a detection volume of 8 nl at both positions. The average molecular mass resolution was 9% (relative standard deviation) for a series of PEGs ranging in molecular mass from 106 to 22800 g/mol. With this excellent mass resolution, small molecules such as monosaccharides, disaccharides, and so on, are readily distinguished. The sensor is demonstrated to readily determine unknown diffusion coefficients.

"High-throughput Screening Of Protein Surface Activity Via Flow Injection Analysis-pH Gradient-dynamic Surface Tension Detection"
Anal. Chem. 2005 Volume 77, Issue 1 Pages 250-258
Bethany A. Staggemeier, Emilia Bramanti, Chiara Allegrini, Kristen J. Skogerboe, and Robert E. Synovec

Abstract: Using flow injection analysis (FIA), a pH gradient is blended in real time with a protein sample as the pH-dependent protein surface activity is measured by a dynamic surface tension detector (FIA-pH-DSTD). This instrumental system was developed as a high-throughput method for the screening of protein surface activity at the air/liquid interface as a function of pH. This method utilizes the continuous flow, drop-based dynamic surface tension detector in combination with flow injection sample introduction and blending of a steady-state concentration of protein sample with a pH gradient ranging from pH 2.0 to pH 11.5. Dynamic surface tension is measured through the differential pressure across the air/liquid interface of repeatedly growing and detaching drops. Continuous surface tension measurement is achieved for each eluting drop of 2-s length (2 µL), providing insight into both the kinetic and thermodynamic behaviors of molecular orientation processes at the liquid/air interface. Three-dimensional data are obtained, with surface tension first converted to surface pressure, which is collected as a function of elution time versus drop time. In FIA-pH-DSTD, a commercial pH probe is used to measure pH during elution time, enabling surface pressure throughout drop time to be subsequently plotted as a function of eluting pH. An automated DSTD calibration procedure and data analysis method is applied, which allows simultaneous use of two different solvents, permitting real-time dynamic surface tension data to be obtained. The method was applied to the analysis of 14 commercial purified proteins, yielding characteristic features of surface activity as a function of pH. The reproducibility of the measurement and selectivity advantage of the DSTD was shown for the analysis of serum albumins from various mammalian sources. Several applications were also suggested and discussed in order to show the potential of the method for protein and food chemistry studies and in the study of protein-polymer interactions.

"A Microscale-Molecular Weight Sensor: Probing Molecular Diffusion Between Adjacent Laminar Flows By Refractive Index Gradient Detection"
Anal. Chem. 2002 Volume 74, Issue 17 Pages 4558-4565
Colin D. Costin and Robert E. Synovec

Abstract: A detection scheme that measures the refractive index gradient (RIG) between adjacent laminar flows in a microfluidic device has been used to develop a microscale-molecular weight sensor. The behavior of low Reynolds number flows has been well documented and shows that molecular transport (mixing) between adjacent laminar flows occurs by molecular diffusion between flow boundaries. A diode laser beam, incident upon and illuminating the entire width of a microchannel, measured the transverse concentration gradient at two different positions along a microchannel. The concentration gradient is impacted by the transverse diffusion from a flow with analyte into a flow initially without analyte. The RIG that forms as analyte diffuses from one adjacent flow to the other causes the laser beam, impinging orthogonal to the RIG through the microchannel, to be deflected. The angle of deflection is then monitored on a position-sensitive detector (PSD) at two different positions along the axis of flow to provide a measurement of analyte diffusion. The two positions are just after the flow initially without analyte merges with the flow initially containing all of the analyte (upstream) and then after the two streams have had more time to diffuse together (downstream). The ratio of the PSD signals obtained at the two positions along the flow, downstream signal divided by the upstream signal, is readily correlated to the analyte diffusion coefficient and, thus, the analyte molecular weight for a given class of compounds. The device was evaluated as a molecular weight sensor for poly(ethylene glycol) (PEG) solutions over a molar mass range from 106 to 22 800 g/mol. The ratio signal was found to be both independent of PEG concentration and sensitive to molecular weight changes for samples ranging from 960 to 22 800 g/mol. Independence of concentration is important for obtaining a reliable molecular weight measurement. The limit of detection for 11 840 g/mol PEG measured at the upstream detection position was determined to be 56 ppm, equivalent to 4.5 x 10^-6 RI (3s). This technique provides a much needed universal detection method, without requiring analyte derivatization chemistry (e.g., fluorescence), for microfluidic analyzes that are becoming increasingly useful in monitoring chemical systems such as continuous-flow reactors or batch polymerization processes. Thus, the molecular weight determination capability is potentially applicable to other compound classes, such as DNA or proteins.

"Toward A Fully Integrated Positive-pressure Driven Microfabricated Liquid Analyzer"
Anal. Chem. 2002 Volume 74, Issue 1 Pages 177-184
Paul G. Vahey, Sean A. Smith, Colin D. Costin, Younan Xia, Anatol Brodsky, Lloyd W. Burgess, and Robert E. Synovec

Abstract: A versatile integrated analyzer with a flow-programmed injection strategy and multiwavelength detection is described with applications toward sampling, flow injection analysis, and capillary separations. Continuous near-real-time sampling is a major benefit of the flow-programmed injection technique. Injection volumes ranging from 250 pL to several microliters were made without electrophoretic flow. Multiwavelength grating light reflection spectroscopy (GLRS) and transmission absorbance spectroscopy were performed simultaneously in a detection volume of 150 pL The utility of these detection methods for refractive index (RI) and absorbance detection in capillary channels is demonstrated through analysis of salt, indicator, and dyes. GLRS is a unique, selective, and path-length-independent technique for probing RI, absorbance, and other optical properties. A limit of detection (LOD) of 170 muM was achieved for GLRS interferometric detection of FD&C Red #3, which corresponded to 2.6 fmol of analyte in the 150 pL detection volume. A LOD of 2 mM for phosphate buffer, or 3 fmol in the 150 pL detection volume will also be demonstrated. A siloxane coating on the GLRS grating was employed as a sensing layer to probe interactions between the sample and stationary phase. The combined GLRS interferometric response provided insight into both optical and chromatographic properties of samples. Open tubular capillary liquid chromatography with multidimensional multiwavelength detection is demonstrated for the analysis of three food dyes. Separation efficiency, N, of 16 000 was achieved for an unretained dye peak eluting at 12 min. Integration of novel sampling and detection schemes makes this a broadly applicable liquid analyzer.

"Multidimensional Analysis Of Poly(ethylene Glycols) By Size Exclusion Chromatography And Dynamic Surface Tension Detection"
Anal. Chem. 2000 Volume 72, Issue 18 Pages 4372-4380
Keith E. Miller, Emilia Bramanti, Bryan J. Prazen, Marina Prezhdo, Kristen J. Skogerboe, and Robert E. Synovec

Abstract: Substantial improvements in a multidimensional dynamic surface tension detector (DSTD) are presented. Rapid, online calibration and measurement of the dynamic surface tension for high-performance liquid chromatography separations is achieved. Dynamic surface tension is determined by measuring the differential. pressure across the liquid-air interface of repeatedly growing and detaching drops. Continuous surface tension measurement throughout the entire drop growth (50 ms to 2 s) is achieved, for each eluting drop, providing insight into the kinetic behavior of molecular orientation processes at the liquid-air interface. Three-dimensional data are obtained, with surface tension first concerted to surface pressure, which is plotted as a function of elution time axis versus drop time axis. Two key innovations will be reported. First, a novel calibration procedure is described and implemented. Differential pressure signals from three drops (mobile phase, standard in mobile phase, and analyte in mobile phase) are utilized to make the dynamic surface tension measurement, thereby eliminating the need for optical imaging, and viscosity and hydrostatic pressure corrections, as required by other methods. Only pressure signals from one mobile-phase drop and one standard drop pressure signal are required, while the analyte drop pressure signal is measured along the chromatographic time axis. Second, corrections for drop elongation are not required, because the drops are precisely detached bq an air burst actuation method in a regime were the surface tension forces significantly dominate gravitational forces. Drops that would fall with a volume of similar to 10 µL due to gravity are precisely and repeatedly detached earlier at a volume of 2 µL. The sensitivity and unique selectivity of the DSTD opens up new possibilities in the analysis of small molecular weight polymers of varying degrees of surface activity, as illustrated for the size-exclusion chromatography analyzes of complex poly(ethylene glycol) (PEG) samples. Using partial least squares for data analysis, polydispersity of complex PEG samples is determined at a relative precision of similar to 1%.
Interface

"Fibre-optic Based Mode-filtered Light Detection For Small-volume Chemical Analysis"
Anal. Chem. 1995 Volume 67, Issue 3 Pages 473-481
Robert E. Synovec, Andrew W. Sulya, Lloyd W. Burgess, Marc D. Foster, and Carsten A. Bruckner

Abstract: An unjacketed optical fiber was inserted into a transparent capillary, such that the i.d. of the tube was slightly larger than the o.d. of the fiber cladding; full details are given. A sample was introduced into this annular column at a low flow rate; propagated light from a He-Ne laser source was mode filtered due to partitioning of the chemical species into the polymeric cladding. Light emitted at right angles to the fiber axis was determined with a photomultiplier tube. The sensor exhibited chemical selectivity provided by differences in analyte RI, distribution coefficient and transient response time. The detection limit was 20 ppm for butylbenzene using a polysiloxane-clad fiber and an aqueous 60% methanol mobile phase. The annular column sensor is a potential detector for LC, FIA and GC for process analysis and environmental monitoring. A novel chemical analyzer is described in which an unjacketed optical fiber is inserted into a transparent capillary tube, such that the inner diameter of the tube is slightly larger than the outer diameter of the fiber cladding. This configuration is referred to as an annular column. When a sample volume is introduced to the annular column at a low flow rate, propagated light is mode-filtered due to a change in the critical angle at the core/clad interface, as a result of partitioning chemical species. Conventionally, chemical species partitioning into a fiber-optic cladding are sensed as a change in the transmitted light at the end of the fiber. An alternative approach, measuring this mode-filtered light directly along the side of the fiber, is reported. The new approach has a signal-to-noise advantage over the conventional approach, since the analyte signal is measured against a low background instead of a high background. The result is a low-volume chemical sensor that temporally separates, as well as detects, chemical species that partition into the fiber cladding. The temporal information enhances sensor selectivity. We have examined the modulation of the critical angle by chemical species of interest at steady-state concentrations and as transient concentration profiles that were shifted in time due to chromatographic retention within the sensor. A detection limit of 20 ppm was achieved for butylbenzene using a polysiloxane-clad fiber and a 60/40 methanol to water (by volume) mobile phase. In summary, the sensor has chemical selectivity provided by differences in analyte refractive index, distribution coefficient, and transient response time. The annular column sensor is shown to be a potential detector for liquid chromatography, flow injection analysis, and gas chromatography, in particular for process analysis and environmental monitoring applications. Likewise, solid phase extraction and sensing for online sampling applications are demonstrated. Copyright 1995, American Chemical Society.
Butylbenzene GC LC Sensor Sensor Optical fiber Refractive index

"Dynamic Surface Tension Detection By Optically Probing A Repeating Drop Rate"
Anal. Chem. 1994 Volume 66, Issue 8 Pages 1209-1216
Lawrence R. III Lima, Darren R. Dunphy, and Robert E. Synovec

Abstract: The output (5 mW at 633 nm) from a He:Ne laser incorporated in a dynamic surface tension detector (DSTD; construction described) was focused 2 cm below the end of a suspended glass capillary (0.33 mm o.d., 0.2 mm i.d.) and provided measurements of the repeating drop rate associated with linked FIA or HPLC separation systems. Surface-active analytes (SAA) caused a significant decrease in drop volume which was readily measured. A calibration graph for aqueous PEG 1470 (molecular mass 1470 g/mol, nominal), at a flow rate of 66 µL/min, was linear for 4 (detection limit) to 20 ppm of PEG 1470. ppm. The linear calibration range was extended to 8-40 ppm with a flow rate of 133 µL/min but the sensitivity was decreased 2-fold. The dependency of DSTD signal and flow rate (13-266 µL/min) was examined with a series of PEG and a good correlation between analyte transitional diffusion coefficient and detector signal was observed. The selectivity of DSTD for SAA was illustrated with 0.04% PEG 8650 separated (66 µL/min) from 0.04% ethylene glycol (EG) by size-exclusion chromatography. PEG 8650 was detected with a signal-to-noise ratio of 25 but no signal was observed with EG. The RI responses for the two analytes were similar.
Surfactants HPLC Diffusion coefficients

"Uncoupling The Effects Of Convection And Diffusion On Refractive Index Gradient Detection In High-temperature Liquid Chromatography"
Anal. Chem. 1993 Volume 65, Issue 2 Pages 128-134
Lawrence R. Lima and Robert E. Synovec

Abstract: A model based on Poiseuille flow was expanded to demonstrate the effects of analyte diffusion and convection on the sensitivity of the radial refractive index gradient (RIG) measurement using the Z-configuration flow cell for HPLC detection. Analytes were a series of poly(ethylene glycol)s (PEG) with a mol. wt. range up to 23,000. Separations were performed by LC on a column (25 cm x 2 mm) of Asahipak (9 m), operated at temperature from 25 to 125°C. Mobile phase flow rates were 10 to 130 l min-1. Diffusion affected RIG appreciably when analyte translational migration was >20 m. Sensitivity for analytes in the convection-dominated response region were constant with temp., while those in the diffusion-sensitive region showed a temperature dependence. Baseline noise levels increased by less than a factor of 3 over the temperature range thus the RIG detector is compatible with high temperature and thermal gradient microbore HPLC.
LC Diffusion Diffusion Refractive index

"Thermal-gradient Micro-bore Liquid Chromatography With Dual-wavelength Absorbance Detection"
Anal. Chem. 1991 Volume 63, Issue 6 Pages 568-574
Curtiss N. Renn and Robert E. Synovec

Abstract: A single fiber-optic two-wavelength detector is described for remote sensing in thermal-gradient microbore LC (TGMLC). Theoretical relationships are derived relating changes in the refractive index of the LC mobile phase to aperture-limited absorbance measurements. The use of the detector system was illustrated by the reversed-phase separation of unleaded gasoline on a C18 column, with a temperature gradient of 25°C to 150°C over 30 min, methanol - water as the mobile phase and detection at 230 and 295 nm. The detector reduced the baseline drift associated with thermally induced refractive index aberrations, not only for TGMLC but also for mobile-phase gradient LC and flow injection analysis. The advantages of the TGMLC technique are discussed.
LC Spectrophotometry C18 Column Gradient technique Heated reaction Optical fiber Refractive index

"Flow Dependence And Sensitivity Of The Refractive Index Gradient Measurement With The Z-configuration Flow Cell At Low Reynolds Number"
Anal. Chem. 1990 Volume 62, Issue 22 Pages 2441-2447
Darrell O. Hancock, Curtiss N. Renn, and Robert E. Synovec

Abstract: A simple model based upon Poiseuille flow is reported that allows the prediction of the sensitivity in the refractive index gradient (RIG) measurement with the Z-configuration flow cell. The RIG measurement is shown to depend upon carefully probing the radial-concentration gradient (orthogonal to the direction of flow) of an analyte. A fiber optic graded refractive index (GRIN) lens combination provided a narrow collimated beam that facilitated accurate probing of the RIG and far-field observation of the beam deflection, for subsequent comparison to the position-sensitive detector output. The flow rate range investigated was at a low Reynolds number, Re I 10 in the cylindrical flow cell bore. Both the predicted and experimentally measured RIG data are directly proportional to linear flow velocity and inversely proportional to both the axial length variance of the analyte concentration and the analyte traditional diffusion coefficient. The linear flow velocity and axial length variance dependence on RIG sensitivity were examined, and the model was found to closely predict the experimentally observed RIG sensitivity. Band-broadening contributions in the RIG detector are critically accounted for in the calculations. Measuring the radial concentration gradient was found to be over two orders of magnitude more sensitive than measuring the axial concentration gradient of a typical analyte peak for microbore ilquld chromatography conditions.
Sensitivity Flowcell Refractive index Gradient technique

"An Absorbance-based Micro-fluidic Sensor For Diffusion Coefficient And Molar Mass Determinations"
Anal. Chim. Acta 2006 Volume 575, Issue 2 Pages 151-158
Adam D. McBrady, Rattikan Chantiwas, Ana Kristine Torgerson, Kate Grudpan and Robert E. Synovec

Abstract: The H-Sensor reported herein is a micro-fluidic device compatible with flow injection analysis (FIA) and high performance liquid chromatography (HPLC). The device detects analytes at two separate off-chip absorbance flow cells, providing two simultaneous absorbance measurements. The ratio of these two absorbance signals contains analyte diffusion coefficient information. A theoretical model for the sensing mechanism is presented. The model relates the signal Ratio to analyte diffusion coefficient. The model is qualitatively evaluated by comparing theoretical and experimental signal Ratio values. Experimental signal Ratios were collected via FIA for a variety of analytes, including sodium azide, benzoic acid, amino acids, peptides, and proteins. Measuring absorbance at multiple wavelengths provides higher order data allowing the analyte signals from mixtures to be deconvolved via classical least squares (CLS). As a result of the H-Sensor providing two simultaneous signals as a function of time for each sample injection, two simulated second-order HPLC chromatograms were generated using experimental H-Sensor data. The chemometric deconvolution method referred to as the generalized rank annihilation method (GRAM) was used to demonstrate chromatographic and spectroscopic deconvolution. GRAM also provides the signal Ratio value, therefore simultaneously obtaining the analyte diffusion coefficient information during deconvolution. The two chromatograms successfully serve as the standard and unknown for the GRAM deconvolution. GRAM was evaluated on chromatograms at various chromatographic resolutions. GRAM was found to function to a chromatographic resolution at and above 0.25 with a percent quantitative error of less then 10%. © 2006 Elsevier B.V. All rights reserved.

"Rapid Polymeric Surfactant Characterization Using A Novel Flow Injection System With Dynamic Surface Tension Detection"
Anal. Chim. Acta 2000 Volume 412, Issue 1-2 Pages 149-160
Keith E. Miller and Robert E. Synovec

Abstract: Substantial improvements in a dynamic surface tension detector (DSTD) are presented, providing the rapid measurement of the dynamic surface tension of Rowing liquids. The dynamic surface tension is determined by measuring a differential pressure across the liquid-air interface of growing drops. A novel calibration procedure is described and implemented. Differential pressure signals from three drops (water, standard, and analyte) are utilized to make the dynamic surface tension measurement, thereby eliminating the need for optical imaging as required by current methods. Furthermore, corrections for drop elongation are not required, since the growing drops are precisely detached by an air burst actuation method in a regime where the surface tension forces significantly dominate gravitational forces. Drops that would fail with a volume of about 10 µl due to gravity, are precisely detached earlier at a volume as low as 1 µl. Two model surfactant systems, sodium dodecyl sulfate (SDS) and polyoxyethylene-20-cetyl ether (Brij(R) 58), are evaluated to demonstrate the significance of these improvements to the DSTD. Using 0.5-3.0 mM SDS solutions, the precision of the instrument is demonstrated, with a precision of 0.2 dyn/cm routinely obtained. A reproducible method to perform stress-relaxation experiments of surfactant systems, similar to those reported in the literature, is demonstrated using Brij(R) 58 solutions. Additional characterization of Brij(R) 58 solutions above the critical micelle concentration (CMC) is presented, providing insight into the impact that micelles have on dynamic surface properties. Finally, combination of a flow-injection system with the DSTD provides information-rich, multi-dimensional data. The smaller drop volume provided by automated drop detachment results in excellent data density for rapid chemical analysis. The flow injection system with the DSTD provides rapid characterization of polymeric surfactant solutions above the CMC based on diffusional properties of the micelles and on surface active properties of the micelle/monomer system.
Polymers Surfactants Tensitometry Micelle

"Molecular Weight Sensing Of Polyethylene Glycols By Flow Injection Analysis And Refractive Index Gradient Detection"
Anal. Chim. Acta 1991 Volume 246, Issue 1 Pages 241-249
Veeravagu Murugaiah and Robert E. Synovec

Abstract: Fundamental aspects of a novel molecular weight sensor (MWS) will be described with application to polyethylene glycol (PEG) analysis in the range of 1470 g mole-1 to 12600 g mole-1. The MWS incorporates controlled dispersion due to convection and diffusion contributions resulting from flow of a sample plug through a narrow diameter tube and flow cell. The controlled dispersion results in a concentration profile that is a strong function of the analyte diffusion coefficient and is readily correlated to molecular weight. The concentration profile is sensitively measured by a refractive index gradient (RIG) detector. This detector provides excellent sensitivity without requiring analyte absorbance. The data is readily converted to an asymmetry ratio (AR), a quality parameter that is independent of analyte concentration between the range of 100 ppm to 800 ppm injected PEG. The asymmetry ratio is the ratio of the relative maximum to the relative minimum on the RIG detected signal. The concentration gradient at the maximum and minimum were found to be dominated by different dependencies on the analyte diffusion coefficient. When the asymmetry ratio is calculated, the diffusion effects constructively add to the sensitivity of the molecular weight prediction. The precision of the molecular weight determination for 1470 g mole-1 PEG was 117 g mole-1 (n=5 trials).
Ethylene glycol SEC Refractive index Gradient technique