Biomedical subjects

Robert E Synovec

Publications and source records attributed to Robert E Synovec.

At least 19 recordsLinked to original sources

PubMed2006

Unsupervised parameter optimization for automated retention time alignment of severely shifted gas chromatographic data using the piecewise alignment algorithm.

Simulated chromatographic separations were used to study the performance of piecewise retention time alignment and to demonstrate automated unsupervised (without a training set) parameter optimization. The average correlation coefficient between the target chromatogram and all remaining chromatograms in the data set was used to optimize the alignment parameters. This approach frees the user from providing class information and makes the alignment algorithm applicable to classifying completely unknown data sets. The average peak in the raw simulated data set was shifted up to two peak-widths-at-base (average relative shift=2.0) and after alignment the average relative shift was improved to 0.3. Piecewise alignment was applied to severely shifted GC separations of gasolines and reformate distillation fraction samples. The average relative shifts in the raw gasolines and reformates data were 4.7 and 1.5, respectively, but after alignment improved to 0.5 and 0.4, respectively. The effect of piecewise alignment on peak heights and peak areas is also reported. The average relative difference in peak height was -0.20%. The average absolute relative difference in area was 0.15%.

Algorithms↗

PubMed2006

Ultrafast gas chromatography on single-wall carbon nanotube stationary phases in microfabricated channels.

The key to rapid temperature programmed separations with gas chromatography are a fast, low-volume injection and a short microbore separation column with fast resistive heating. One of the major problems with the reduction of column dimensions for micro gas chromatography is the availability of a stationary phase that provides good separation performance. In this report, we present the first integration of single-wall carbon nanotubes (SWNTs) as a stationary phase into 100 mum x 100 mum square and 50-cm-long microfabricated channels. The small size of this column with integrated resistive heater and the robustness of the SWNT phase allow for fast temperature programming of up to 60 degrees C/s. A combination of the fast temperature programming and the narrow peak width of small-volume injections that can be obtained from a high-speed, dual-valve injection system allows for rapid separations of gas mixtures. We demonstrate highly reproducible separations of four-compound test mixtures on these columns in less than 1 s using fast temperature programming.

Journal Article↗

PubMed2006

Classification of high-speed gas chromatography-mass spectrometry data by principal component analysis coupled with piecewise alignment and feature selection.

A useful methodology is introduced for the analysis of data obtained via gas chromatography with mass spectrometry (GC-MS) utilizing a complete mass spectrum at each retention time interval in which a mass spectrum was collected. Principal component analysis (PCA) with preprocessing by both piecewise retention time alignment and analysis of variance (ANOVA) feature selection is applied to all mass channels collected. The methodology involves concatenating all concurrently measured individual m/z chromatograms from m/z 20 to 120 for each GC-MS separation into a row vector. All of the sample row vectors are incorporated into a matrix where each row is a sample vector. This matrix is piecewise aligned and reduced by ANOVA feature selection. Application of the preprocessing steps (retention time alignment and feature selection) to all mass channels collected during the chromatographic separation allows considerably more selective chemical information to be incorporated in the PCA classification, and is the primary novelty of the report. This methodology is objective and requires no knowledge of the specific analytes of interest, as in selective ion monitoring (SIM), and does not restrict the mass spectral data used, as in both SIM and total ion current (TIC) methods. Significantly, the methodology allows for the classification of data with low resolution in the chromatographic dimension because of the added selectivity from the complete mass spectral dimension. This allows for the successful classification of data over significantly decreased chromatographic separation times, since high-speed separations can be employed. The methodology is demonstrated through the analysis of a set of four differing gasoline samples that serve as model complex samples. For comparison, the gasoline samples are analyzed by GC-MS over both 10-min and 10-s separation times. The successfully classified 10-min GC-MS TIC data served as the benchmark analysis to compare to the 10-s data. When only alignment and feature selection was applied to the 10-s gasoline separations using GC-MS TIC data, PCA failed. PCA was successful for 10-s gasoline separations when the methodology was applied with all the m/z information. With ANOVA feature selection, chromatographic regions with Fisher ratios greater than 1500 were retained in a new matrix and subjected to PCA yielding successful classification for the 10-s separations.

Analysis of Variance↗

PubMed2006

Fisher ratio method applied to third-order separation data to identify significant chemical components of metabolite extracts.

This report is about applying a Fisher ratio method to entire four dimensional (4D) data sets from third-order instrumentation data. The Fisher ratio method uses a novel indexing scheme to discover the unknown chemical differences among known classes of complex samples. This is the first report of a Fisher ratio analysis procedure applied to entire 4D data sets of third-order separation data, which, in this case, is comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry analyses of metabolite extracts using all of the collected mass channels. Current analysis methods for third-order separation data use only user-defined subsets of the 4D data set. First, in a validation study, the Fisher ratio method was demonstrated to objectively evaluate and determine the chemical differences between three controlled urine samples that differed by known spiked chemical components. It was determined that, out of more than 600 recognizable chemical components in a single sample, the six spiked components, along with only two other matrix components, differed most significantly in concentration among the control samples. In a second study, the Fisher ratio method was used in a novel application to discover the unknown chemical differences between urine metabolite samples from pregnant women and nonpregnant women. A brief list of the top 11 components that were most significantly different in concentration between the pregnant and nonpregnant samples was generated. Because the Fisher ratio calculation statistically differentiates regions of the chromatogram with large class-to-class variations from regions containing large within-class variations, the Fisher ratio method should generally be robust against biological diversity in a sample population. Indeed, application of principal component analysis in this second study failed due to biological diversity of the samples.

Algorithms↗

PubMed2006

Comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry analysis of metabolites in fermenting and respiring yeast cells.

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry coupled with rapid chemometric analysis were used to identify chemical differences in metabolite extracts isolated from yeast cells either metabolizing glucose (repressed (R) cells) via fermentation or metabolizing ethanol by respiration (derepressed (DR) cells). Principal component analysis (PCA) followed by parallel factor analysis (PARAFAC) in concert with the LECO ChromaTOF software located and identified the differences in composition between the two types of cell extracts and provided a reliable ratio of the metabolite concentrations. In this report, we demonstrate the analytical method developed to provide relatively rapid analysis of three selective mass channels (m/z 73, 205, 387), although in principle all collected mass channels could be analyzed. Twenty-six metabolites that differentiate repressed cells from derepressed cells were identified. The DR/R ratio of metabolite concentrations ranged from 0.02 for glucose to 67 for trehalose. The average biological variation of the sample extracts was 31%. This analysis demonstrates the utility and benefit of using PCA combined with PARAFAC and ChromaTOF software on extremely complex samples to derive useful information from complex three-dimensional chromatographic data objectively and relatively rapidly.

Chromatography, Gas↗

PubMed2006

Simple sequential injection analysis systems with a dynamic surface tension detector.

Simple sequential injection analysis systems with DSTD (SIA/DSTD) have been developed. One was employed for the study of the effects of the ion contents in solutions to the dynamic surface pressure of ionic surfactants. The results from the studies show the possibility for an alternative simple fast screening, but also a sensitive procedure for water quality determination. Another simple SIA/DSTD system has been demonstrated for the quantification of an anionic surfactant using a single standard calibration.

Calibration↗

PubMed2005

Flow injection analysis with diode array absorbance detection and dynamic surface tension detection for studying denaturation and surface activity of globular proteins.

In this article, a multidimensional dynamic surface tension detector (DSTD), in a parallel configuration with a UV-visible diode array absorbance detector, is presented in a novel flow injection analysis (FIA) application to study the effects of chemical denaturants urea, guanidinium hydrochloride (GdmHCl), and guanidinium thyocyanate (GdmSCN) on the surface activity of globular proteins at the liquid-air interface. The DSTD signal is obtained by measuring the changing pressure across the liquid-air interface of 4-mul drops repeatedly forming at the end of a capillary using FIA. The sensitivity and selectivity of the DSTD signal is related to the surface-active protein concentration in aqueous solution combined with the thermodynamics and kinetics of protein interaction at a liquid-air drop interface. Rapid on-line calibration and measurement of dynamic surface tension is applied, with the surface tension converted into surface pressure results. Continuous surface tension measurement throughout the entire drop growth is achieved, providing insight into kinetic behavior of protein interactive processes at the liquid-air drop interface. Specifically, chemical denaturation of 12 commercial globular proteins-chicken egg albumin, bovine serum albumin, human serum albumin, alpha-lactalbumin (alpha-Lac), myoglobin, cytochrome c, hemoglobin, carbonic anhydrase, alpha-chymotrypsinogen A, beta-lactoglobulin (beta-LG), lysozyme, and glyceraldehyde-3-phosphate-dehydrogenase-is studied in terms of surface pressure (i.e., surface activity) after treatment with increasing concentrations of urea, GdmHCl, and GdmSCN in the 0-8, 0-6, and 0-5 M ranges, respectively. For several of these proteins, the spectroscopic absorbance changes are monitored simultaneously to provide additional information prior to drop formation. Results show that surface pressure of proteins generally increases as the denaturant concentration increases and that effectiveness is GdmSCN > GdmHCl > urea. Protein unfolding curves obtained by plotting surface pressure as a function of denaturant concentration are presented and compared with respect to unfolding curves obtained by using UV absorbance and literature data. Kinetic information relative to the protein adsorption to the air-liquid interface of two proteins, alpha-Lac and beta-LG (chosen as representative proteins for comparison), denatured by the three denaturants is also studied and discussed.

Animals↗

PubMed2005

A comprehensive two-dimensional retention time alignment algorithm to enhance chemometric analysis of comprehensive two-dimensional separation data.

A comprehensive two-dimensional (2D) retention time alignment algorithm was developed using a novel indexing scheme. The algorithm is termed comprehensive because it functions to correct the entire chromatogram in both dimensions and it preserves the separation information in both dimensions. Although the algorithm is demonstrated by correcting comprehensive two-dimensional gas chromatography (GC x GC) data, the algorithm is designed to correct shifting in all forms of 2D separations, such as LC x LC, LC x CE, CE x CE, and LC x GC. This 2D alignment algorithm was applied to three different data sets composed of replicate GC x GC separations of (1) three 22-component control mixtures, (2) three gasoline samples, and (3) three diesel samples. The three data sets were collected using slightly different temperature or pressure programs to engender significant retention time shifting in the raw data and then demonstrate subsequent corrections of that shifting upon comprehensive 2D alignment of the data sets. Thirty 12-min GC x GC separations from three 22-component control mixtures were used to evaluate the 2D alignment performance (10 runs/mixture). The average standard deviation of first column retention time improved 5-fold from 0.020 min (before alignment) to 0.004 min (after alignment). Concurrently, the average standard deviation of second column retention time improved 4-fold from 3.5 ms (before alignment) to 0.8 ms (after alignment). Alignment of the 30 control mixture chromatograms took 20 min. The quantitative integrity of the GC x GC data following 2D alignment was also investigated. The mean integrated signal was determined for all components in the three 22-component mixtures for all 30 replicates. The average percent difference in the integrated signal for each component before and after alignment was 2.6%. Singular value decomposition (SVD) was applied to the 22-component control mixture data before and after alignment to show the restoration of trilinearity to the data, since trilinearity benefits chemometric analysis. By applying comprehensive 2D retention time alignment to all three data sets (control mixtures, gasoline samples, and diesel samples), classification by principal component analysis (PCA) substantially improved, resulting in 100% accurate scores clustering.

Journal Article↗

PubMed2005

Classification of gasoline data obtained by gas chromatography using a piecewise alignment algorithm combined with feature selection and principal component analysis.

A fast and objective chemometric classification method is developed and applied to the analysis of gas chromatography (GC) data from five commercial gasoline samples. The gasoline samples serve as model mixtures, whereas the focus is on the development and demonstration of the classification method. The method is based on objective retention time alignment (referred to as piecewise alignment) coupled with analysis of variance (ANOVA) feature selection prior to classification by principal component analysis (PCA) using optimal parameters. The degree-of-class-separation is used as a metric to objectively optimize the alignment and feature selection parameters using a suitable training set thereby reducing user subjectivity, as well as to indicate the success of the PCA clustering and classification. The degree-of-class-separation is calculated using Euclidean distances between the PCA scores of a subset of the replicate runs from two of the five fuel types, i.e., the training set. The unaligned training set that was directly submitted to PCA had a low degree-of-class-separation (0.4), and the PCA scores plot for the raw training set combined with the raw test set failed to correctly cluster the five sample types. After submitting the training set to piecewise alignment, the degree-of-class-separation increased (1.2), but when the same alignment parameters were applied to the training set combined with the test set, the scores plot clustering still did not yield five distinct groups. Applying feature selection to the unaligned training set increased the degree-of-class-separation (4.8), but chemical variations were still obscured by retention time variation and when the same feature selection conditions were used for the training set combined with the test set, only one of the five fuels was clustered correctly. However, piecewise alignment coupled with feature selection yielded a reasonably optimal degree-of-class-separation for the training set (9.2), and when the same alignment and ANOVA parameters were applied to the training set combined with the test set, the PCA scores plot correctly classified the gasoline fingerprints into five distinct clusters.

Algorithms↗

PubMed2005

Evaluation of the DotMap algorithm for locating analytes of interest based on mass spectral similarity in data collected using comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry.

Comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC x GC-TOF-MS) is a highly selective technique ideal for the analysis of complex mixtures. The instrument yields an abundance of data, with complete mass spectral scans at every time point in the GC x GC separation space. The development and application of appropriate tools for data mining is essential in making sense of the wealth of information available. An algorithm for locating analytes of interest based on mass spectral similarity in GC x GC-TOF-MS data, called DotMap, has been previously reported and is rigorously evaluated herein. A thorough investigation into the performance characteristics of DotMap, including the performance near the limit of detection and dynamic range of the algorithm as well as the capacity of the algorithm to deal with peak overlap, is investigated using jet fuel as a complex sample matrix. For instance, the algorithm can successfully identify a spiked compound at the single microg/ml level in a jet fuel sample with an overlapping interferent. The performance of the DotMap algorithm in situations with very limited mass spectral selectivity, specifically in the evaluation of spectra from isomer compounds, as well as the ability to tune DotMap results to provide the location of a specific analyte or of a class of compounds is demonstrated. The DotMap algorithm is demonstrated to be a sensitive tool that is useful in the analysis of complex mixtures and which possesses the capacity to be easily "tuned" to discern the location of analytes of interest.

Algorithms↗

PubMed2005

Microfabricated refractive index gradient based detector for reversed-phase liquid chromatography with mobile phase gradient elution.

Typical refractive index (RI) detectors for liquid chromatography (LC) are not well suited to application with mobile phase gradient elution, due to the difficulty in correcting for the detected baseline shift during the gradient. We report a sensitive, highly reproducible, microfabricated refractive index gradient (micro-RIG) detector that performs well with mobile phase gradient elution LC. Since the micro-RIG signal remains on-scale throughout the mobile phase gradient, one can apply a baseline correction procedure. We demonstrate that by collecting two mobile phase gradient blanks and subtracting one of them from the other, a reproducible, flat baseline is achieved. Therefore, subtracting a blank from a separation provides a baseline corrected chromatogram with reasonably high signal-to-noise ratio for eluting analytes. The micro-RIG detector uses a collimated diode laser beam to optically probe a RIG formed perpendicular to the laminar flow direction within a microfabricated borosilicate glass chip. The chip-based design of the detector is suitable for either traditional bench-top or LC-on-a-chip technologies. We report reversed phase high performance liquid chromatography (RP-HPLC) separations of proteins and polymers, over mobile phase gradient conditions of 67% A:33% B to 3% A:97% B by volume, where A is 96% methanol:3.9% water:0.1% trifluoroacetic acid (TFA), and B is 3.9% methanol:96% water:0.1% TFA. The separations were performed on a Jupiter 5 mu C4 300 A 150 mm x 1.0 mm Phenomenex column at a flow rate of 20 microl/min. Viscosity changes during the mobile phase gradient separation are found to shift the on-chip merge position of the detected concentration gradient (i.e., RIG), in a reproducible fashion. However, this viscosity effect makes detection sensitivity vary throughout the mobile phase gradient, due to moving the optimized position of the probe beam in relation to the analyte concentration gradient being probed. None-the-less, consistent limits of detection (LODs) were achieved. The 3-sigma deflection angle LOD was 16 microrad for micro-RIG detection, corresponding to an injected concentration LOD of 7 ppm (mass/mass) for cytochrome c.

Chromatography, High Pressure Liquid↗

PubMed2005

High-throughput screening of protein surface activity via flow injection analysis-pH gradient-dynamic surface tension detection.

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 muL), 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.

Journal Article↗

PubMed2004

Development and evaluation of gold-centered monolayer protected nanoparticle stationary phases for gas chromatography.

The current status for the development of novel open-tubular gas chromatography (GC) stationary phases consisting of thin films of gold-centered monolayer protected nanoparticles (MPNs) is reported. Dodecanethiol MPNs, in which the monolayer is dodecanethiol linked to the gold nanoparticle, have shown great promise as a GC stationary phase with efficient columns having been produced in a variety of capillary i.d.'s with stationary phase film depths ranging from 10 to 60 nm, +/-2 nm at a given film depth. Stationary phase operational parameters are discussed including maximum operating temperature, sample capacity, and stationary phase lifetime and robustness. An overview of the general method employed for column production is also included. The sample capacity was determined for a 2.5 m, 250 microm i.d. column with a stationary phase film thickness of 40 nm, at 50 degrees C using anisole (k' = 1.86) as the probe analyte. The sample capacity was experimentally found to be 2.3 ng under these conditions, similar to values reported for thicker, polymer stationary phases. The efficiency of the dodecanethiol MPN stationary phase was determined with a 100 microm i.d. capillary and found to have a reduced plate height hmin value of 0.95 for octane (k' = 0.68). Areas of application illustrated and discussed utilizing the dodecanethiol MPN stationary phase include complementary separations such as two-dimensional GC (GC x GC), potential utilization within a model system for a micro-fabricated GC (microGC), as well as efficient single dimension high-speed separations. Initial development of polar stationary phases utilizing 4-chlorobenzenethiol MPNs and 4-(trifluoromethyl)benzenethiol MPNs is discussed. Included is a selectivity comparison of the retention behavior of the 4-chlorobenzenethiol MPN stationary phase and the dodecanethiol MPN stationary phase.

Chromatography, Gas↗

PubMed2004

Algorithm for locating analytes of interest based on mass spectral similarity in GC x GC-TOF-MS data: analysis of metabolites in human infant urine.

The developed algorithm reported herein, referred to as "DotMap," addresses the need to rapidly identify analyte peak locations in gas chromatography x gas chromatography-time of flight mass spectrometry (GC x GC-TOF-MS) data. The third-order structure of GC x GC-TOF-MS data is such that at each point in the GC x GC chromatogram, a complete mass spectrum is measured. DotMap utilizes this third-order structure to search for the location of a given spectrum of interest in a complete data set, or in a user selected portion of the complete data set. The algorithm returns a contour plot indicating the location of signal(s) with the most similar mass spectra to the analyte of interest. A spectrum from the region indicated is then subjected to an automated mass spectral search to give immediate feedback on the accuracy of the analysis. This algorithm was investigated with a trimethylsilyl (TMS) derivatized human infant urine sample that contained organic acid metabolites. One hundred percent of 12 selected TMS derivatized organic acid metabolites in human infant urine were located with the DotMap algorithm. A typical automated DotMap analysis takes 30 s on a 1.6 GHz PC with 1024 MB of RAM. Vanillic acid (TMS) was located by DotMap, but also exhibited overlap with other organic acids. The presence of vanillic acid (TMS) was confirmed by subjecting the appropriate GC x GC region to chemometric signal deconvolution by PARAFAC to yield pure component information suitable for subsequent quantification.

Algorithms↗

PubMed2004

Multivariate selectivity as a metric for evaluating comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry subjected to chemometric peak deconvolution.

Two-dimensional gas chromatography (GC x GC) coupled to time-of-flight mass spectrometry (TOFMS) [GC x GC-TOFMS)] is a highly selective technique well suited to analyzing complex mixtures. The data generated is information-rich, making it applicable to multivariate quantitative analysis and pattern recognition. One separation on a GC x GC-TOFMS provides retention times on two chromatographic columns and a complete mass spectrum for each component within the mixture. In this report, we demonstrate how GC x GC-TOFMS combined with trilinear chemometric techniques, specifically parallel factor analysis (PARAFAC) initiated by trilinear decomposition (TLD), results in a powerful analytical methodology for multivariate deconvolution. Using PARAFAC, partially resolved components in complex mixtures can be deconvoluted and identified without requiring a standard data set, signal shape assumptions or any fully selective mass signals. A set of four isomers (iso-butyl, sec-butyl, tert-butyl, and n-butyl benzenes) is used to investigate the practical limitations of PARAFAC for the deconvolution of isomers at varying degrees of chromatographic resolution and mass spectral selectivity. In this report, multivariate selectivity was tested as a metric for evaluating GC x GC-TOFMS data that is subjected to PARAFAC peak deconvolution. It was found that deconvolution results were best with multivariate selectivities over 0.18. Furthermore, the application of GC x GC-TOFMS followed by TLD/PARAFAC is demonstrated for a plant metabolite sample. A region of GC x GC-TOFMS data from a complex natural sample of a derivatized metabolic plant extract from Huilmo (Sisyrinchium striatum) was analyzed using TLD/PARAFAC, demonstrating the utility of this analytical technique on a natural sample containing overlapped analytes without selective ions or peak shape assumptions.

Evaluation Studies as Topic↗

PubMed2004

High-speed gas chromatography using synchronized dual-valve injection.

A novel injection technique for high-speed gas chromatography is demonstrated. Synchronized dual-valve injection is shown to provide peak widths as low as 1.5 ms (width at half-height) for an unretained analyte. This was achieved using a 0.5-m DB-5 column with an internal diameter of 100 microm and a film thickness of 0.4 microm operated at a temperature of 150 degrees C with a column absolute head pressure of 85 psi, resulting in a dead time of only t(o) = 26 ms ( approximately 1900 cm/s, 26 mL/min). Using the DB-5 column in a 1-m length under the same instrumental parameters, with a resulting linear flow velocity of 935 cm/s (12.7 mL/min, t(o) = 117 ms), a minimum peak width of 3.3 ms was obtained. During an isothermal separation, 10 analytes were separated in a time window of 400 ms. A rigorous comparison of experimental and theoretical band-broadening data based on the Golay equation showed that band broadening is limited almost entirely by the chromatographic band broadening terms expressed by the Golay equation and not by extra column band broadening due to the injection process. Synchronized dual-valve injection offers a rugged and inexpensive design, providing extremely reproducible injections with peak height precision of 2.4% (RSD) and low run-to-run variation in retention times, with an average standard deviation less than 0.1 ms. Herein, synchronized dual-valve injection is demonstrated as a proof of principle using high-speed diaphragm valves. It is foreseen that the injection technique could be readily implemented using a combination of thermal modulation and high-speed valve hardware, thus optimizing the mass transfer and not significantly sacrificing the limit of detection performance for high-speed GC. Further implications are that, if properly implemented, high-speed temperature programming coupled with this new technology should lead to very large peak capacities for approximately 1-s separations.

Chromatography, Gas↗

PubMed2004

Theoretical modeling and experimental evaluation of a microscale molecular mass sensor.

A theoretical model for a recently developed microscale molecular mass sensor (micro-MMS) is presented. The micro-MMS employs a widely applicable technique of measuring the refractive index gradient (RIG) in a microchannel created after two adjacent streams merge: a "sample stream" containing analyte(s) of interest in a host solvent and a "mobile-phase" stream containing only the host solvent. Because the flow in the microchannel is laminar, the analytes in the sample stream mix with the mobile-phase stream primarily by diffusion. The diffusion-induced RIG in the microchannel is measured by monitoring the deflection angle of a diode laser probe beam, which is orthogonal to both the direction of flow and the direction of analyte diffusion. The micro-MMS samples the RIG with probe beams at two positions along the direction of flow, and the ratio of the downstream to the upstream signal monitors the diffusion coefficient. Following calibration for a given class of compounds, the molecular mass of an analyte of interest can be determined. Along with the analyte diffusion coefficient, the theoretical model indicated three other specific parameters are important to interpret the micro-MMS output: the radius of the interrogating light probe beams, the time intervals between each of the detection positions, and the merge point relative to the detection positions. A series of experiments were conducted at different beam radii and flow rates to investigate these parameters, and the results are consistent with the model. The model shows that by using smaller beam radii and altering flow rates the molecular mass range of the micro-MMS can be, in principle, tuned from less than 10(2) g/mol to greater than 10(8) g/mol. The ratio data from the micro-MMS is also demonstrated to readily provide a "universal calibration", from which the determination of unknown diffusion coefficients can be readily obtained.

Journal Article↗

PubMed2004

Monolayer-protected gold nanoparticles as an efficient stationary phase for open tubular gas chromatography using a square capillary model for chip-based gas chromatography in square cornered microfabricated channels.

The application of a dodecanethiol monolayer-protected gold nanoparticle (MPN) stationary phase within a microchannel environment was explored using a square capillary column as a model for high-speed, microfabricated gas chromatography (microGC). Successful deposition and evaluation of a dodecanethiol MPN phase within a 1.3 m long, 100 microm x 100 microm square capillary is reported. The thickness of the MPN phase was evaluated using SEM analysis. An average thickness of 15 nm along the capillary walls was determined. While the film depth along the walls was very uniform, the corner depths were greater with the largest observed depth being 430 nm. Overall, an efficient chromatographic system was obtained with a minimum reduced plate height, h(min), of 1.2 for octane (k = 0.22). Characterization of the MPN column was completed using four compound classes (alkanes, alcohols, ketones, and aromatics) that were used to form a seven-component mixture with a 2-s separation. A mixture consisting of a nerve agent simulant in a sample containing analytes that may commonly interfere with detection was also separated in only 2 s, much faster than a similar separation previously reported using a microGC system requiring 50 s. A comparison of the MPN stationary phase to phases employed in previously reported microGC systems is also made. Application of the square capillary MPN column for a high-speed separation as the second column of a comprehensive 2-D gas chromatography system (GC x GC) was also explored.

Chromatography, Gas↗