Microfluidic deans switch for comprehensive two-dimensional gas chromatography

A microfluidic Deans switch was used as a comprehensive two-dimensional gas chromatography (GCxGC) modulator. The simplicity and wide temperature range of the Deans switch make it a promising alternative to existing modulation techniques. However, the Deans switch is a low duty cycle modulator; that is, it samples only a small portion of the primary column effluent. Like all low duty cycle modulators, the Deans switch produces inconsistent transfer of components from the primary to the secondary column if the primary peaks are undersampled. Theoretical simulations and experimental studies show that the relative standard deviation (RSD) of the fraction of material transferred from the primary column to the secondary column is less than 1% if the modulation ratio is greater than 2.5. But the RSDs increase rapidly as the modulation ratio is decreased below 2.5. Deans switch GCxGC was validated by analyzing the aromatic content of gasoline. A fast analysis (<10 min) produced narrow primary peaks and a modulation ratio of 1.7. The quantitative results were in good agreement with results obtained with differential flow modulation GCxGC and GC/MS, but the RSDs of single-component levels were approximately three times greater. The Deans switch modulator was also used for a slower gasoline analysis (33 min run time) that produced modulation ratios near 5. In this case, the quantitative results and RSDs were in excellent agreement with the differential flow GCxGC and GC/MS results. These studies demonstrate that a Deans switch can be an effective modulator provided that modulation ratios greater than approximately 2.5 are employed.     

Using comprehensive two-dimensional gas chromatography retention indices to estimate environmental partitioning properties for a complete set of diesel fuel hydrocarbons

Comprehensive two-dimensional gas chromatography (GCxGC) provides nearly complete composition data for some complex mixtures such as petroleum hydrocarbons. However, the potential wealth of physical property information contained in the corresponding two-dimensional chromatograms is largely untapped. We developed a simple but robust method to estimate GCxGC retention indices for diesel-range hydrocarbons. By exploiting n-alkanes as reference solutes in both dimensions, calculated retention indices were insensitive to uncertainty in the enthalpy of gas-stationary-phase transfer for a suite of representative diesel components. We used the resulting two-dimensional retention indices to estimate the liquid vapor pressures, aqueous solubilities, air-water partition coefficients, octanol-water partition coefficients, and vaporization enthalpies of a nearly complete set of diesel fuel hydrocarbons. Partitioning properties were typically estimated within a factor of 2; this is not as accurate as some previous estimation or measurement methods. However, these relationships may allow powerful and incisive analysis of phase-transfer processes affecting petroleum hydrocarbon mixtures in the environment. For example, GCxGC retention data might be used to quantitatively deconvolve the effects of water washing and evaporation on environmentally released diesel fuels.     

Application of time-of-flight mass spectrometry with laser-based photoionization methods for time-resolved on-line analysis of mainstream cigarette smoke

The application of soft photoionization mass spectrometry methods (PIMS) for cigarette mainstream smoke analysis is demonstrated. Resonance-enhanced multiphoton ionization (REMPI) at 260 nm and vacuum ultraviolet light single-photon ionization (SPI) at 118 nm were used in combination with time-of-flight mass spectrometry (TOFMS). An optimized smoking machine with reduced memory effects of smoke components was constructed, which in combination with the REMPI/SPI-TOFMS instrument allows PIMS smoke analysis with a time resolution of up to 10 Hz. The complementary character of both PIMS methods is demonstrated. SPI allows the detection of various aliphatic and aromatic compounds in smoke up to approximately 120 m/z while REMPI is well suited for aromatic compounds. The capability of the instrument coupled to the novel sampling system for puff-by-puff resolved measurements is demonstrated. The feasibility of using the experimental system for intrapuff smoke measurements is also shown. Two main patterns of puff-by-puff behaviors are observed for different smoke constituents. The first group exhibits a constant increase in smoke constituent yield from the first to the last puff. The second group shows a high yield of the constituent in the first puff, with lower and constant or slowly increasing yields in the following puffs. A third group cannot be clearly classified and is a combination of both observed profiles.     

Hollow-fiber flow field-flow fractionation for whole bacteria analysis by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

This work proposes for the first time the use of hollow-fiber flow field-flow fractionation (HF FlFFF) for improved matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI/TOFMS) of whole bacteria. HF FlFFF has proved to be able to prepurify or fractionate different species of whole bacteria. Sample preparation by HF FlFFF gives improved spectra quality because noncellular components possibly present in the sample can be separated from the cells. When a mixture of two bacteria (Bacillus subtilis and Escherichia coli) is fractionated through HF FlFFF, MALDI/TOFMS analysis of each separated bacterial species preserves the most characteristic ion signals of the species without the presence of characteristic signals of the other species. The main advantages of HF FlFFF for MALDI/TOFMS analysis of whole bacteria are miniaturization, simplicity, and low cost of the fractionator components. This low cost makes disposable usage of the fractionator possible, thus eliminating the risk of run-to-run contamination of spectra due to sample carryover. The low fractionator volume yields bacterial fractionation on the order of a few minutes, which is comparable to MALDI/TOFMS analysis time. The small fractionation volume makes sample dilution low enough so that additional sample concentration steps are not strictly required to preserve MALDI/TOFMS detection.     

Analysis of street drugs in seized material without primary reference standards

A novel approach was used to analyze street drugs in seized material without primary reference standards. Identification was performed by liquid chromatography/time-of-flight mass spectrometry (LC/TOFMS), essentially based on accurate mass determination using a target library of 735 exact monoisotopic masses. Quantification was carried out by liquid chromatography/chemiluminescence nitrogen detection (LC/CLND) with a single secondary standard (caffeine), utilizing the detector's equimolar response to nitrogen. Sample preparation comprised dilution, first with methanol and further with the LC mobile phase. Altogether 21 seized drug samples were analyzed blind by the present method, and results were compared to accredited reference methods utilizing identification by gas chromatography/mass spectrometry and quantification by gas chromatography or liquid chromatography. The 31 drug findings by LC/TOFMS comprised 19 different drugs-of-abuse, byproducts, and adulterants, including amphetamine and tryptamine designer drugs, with one unresolved pair of compounds having an identical mass. By the reference methods, 27 findings could be confirmed, and among the four unconfirmed findings, only 1 apparent false positive was found. In the quantitative analysis of 11 amphetamine, heroin, and cocaine findings, mean relative difference between the results of LC/CLND and the reference methods was 11% (range 4.2-21%), without any observable bias. Mean relative standard deviation for three parallel LC/CLND results was 6%. Results suggest that the present combination of LC/TOFMS and LC/CLND offers a simple solution for the analysis of scheduled and designer drugs in seized material, independent of the availability of primary reference standards.     

Measurement of selected polybrominated diphenyl ethers, polybrominated and polychlorinated biphenyls, and organochlorine pesticides in human serum and milk using comprehensive two-dimensional gas chromatography isotope dilution time-of-flight mass spectrometry

A new method using comprehensive two-dimensional gas chromatography and isotope dilution time-of-flight mass spectrometry (GCxGC-IDTOFMS) for the simultaneous measurement of selected polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), and brominated flame retardants is presented. In contrast to the reference methods based on classical GC/MS, a single injection of the extract containing all compounds of interest results in accurate identification and quantification. Using GCxGC ensures the chromatographic separation of most compounds, and TOFMS allows mass spectral deconvolution of coeluting compounds as well as the use of (13)C-labeled internal standards for quantification. Isotope ratio measurements of the most intense ions for both native and labels ensure the required specificity. The use of this new method with an automated sample preparation procedure developed at the Centers for Disease Control and Prevention (CDC) for the analysis of human serum and milk compared favorably to conventional isotope-dilution one-dimensional gas chromatography-high-resolution mass spectrometry (GC-IDHRMS) for the different human serum and milk pools tested. The instrumental detection limits ranged between 0.5 pg/microL and 10 pg/microL and the method detection limits ranged between 1 and 15 pg/microL (N = 59 analytes). The reproducibility of the method was almost as good as with GC-IDHRMS, the relative standard deviations ranging between 1 and 11% for OCPs measured in human serum. OCP, PBDE, and PCB levels measured using the two methods were highly correlated, and the deviations between the two methods were below 20% for most analytes with concentrations above 1 ng/g milk lipids.     

Fast GCxGC with short primary columns

A novel approach to comprehensive two-dimensional gas chromatography (GCxGC) separations is presented, which operates in a new region of the "GCxGC optimization pyramid". The technique relies on the use of short primary columns to decrease elution temperatures (Te) of analytes from the primary column, with a Te reduction of up to 50 degrees C illustrated. This in turn has implications that will expand the areas where GCxGC can be used, as decreased elution temperatures will allow GCxGC to be applied to mixtures of less volatile compounds or permit the use of less thermally stable stationary phases in the column ensemble. As well, it will allow GCxGC to be applied to thermally labile compounds through a reduction in elution temperature. With short primary columns, resolution and efficiency in the first dimension is sacrificed, but speed is gained; however, the second column in GCxGC provides additional resolution and separation of compounds of differing chemical properties. Thus, it is possible to recover some of the analytical separation power of the system to provide resolution of target analytes from sample impurities. As an example, a case study using short primary columns for the separation of natural pyrethrins, which degrade above 200 degrees C, is described. Even with the sacrifices of overall separation power that are made, there is still sufficient resolution available to separate the six natural pyrethrins from each other and the complex chrysanthemum extract matrix. The use of cold-on-column injection, a short primary column, and a high carrier gas flow rate allow the pyrethrins to be eluted below 200 degrees C, with separation in 17 min and complete resolution from sample matrix.     

GCxMS of diesel: a two-dimensional separation approach

Comprehensive two-dimensional gas chromatography can be viewed as a traditional gas chromatography with a sophisticated "elution-resolved" flame ionization detector (GC/FID) or a detector with separation capability. The concept of multidimensional chromatography can be extended to other detectors that also have separation capability, such as ultraviolet, infrared, and mass spectrometer. Mass spectrometry, combined with gas chromatography, GC/MS, has been a powerful separation/identification device for many years. However, if one applies the multidimensional separation concept to this combination with a nonfragmentation ionization method, GC/MS not only can be used as a separation/identification tool, but also a two (multiple) dimensional separation device, GCxMS. In this study, a two-dimensional separation (GCxMS) study of diesel composition is demonstrated and compared with the GCxGC technique. The major advantage of GCxMS is the compound class separation. The compound groups within a compound class are also well-separated on the basis of their parent masses. Because of the exact mass operation, the specific element containing compound distribution can also be generated through the extraction of specific mass groups. For qualitative analysis, GCxMS is a technique where one experiment can generate a wide range of information. GCxMS may also perform quantitative analysis when appropriate response factors for various compound groups are available. From GCxGC to GCxMS, the power of two (multiple) dimensional separation has just started exposing its advantages for complex mixture analysis. To achieve multiple dimensional separation in different forms, many improvements remain to be made. The challenge now is to combine/accommodate two or more different techniques to solve a specific complex separation problem. The GCxMS experience has pushed this effort one step ahead toward complete application of this new concept in the analysis of complex mixtures.     

Single photon ionization time-of-flight mass spectrometry with a pulsed electron beam pumped excimer VUV lamp for on-line gas analysis: setup and first results on cigarette smoke and human breath

Single-photon ionization (SPI) using vacuum ultraviolet (VUV) light produced by an electron beam pumped rare gas excimer source has been coupled to a compact and mobile time-of-flight mass spectrometer (TOFMS). The novel device enables real-time on-line monitoring of organic trace substances in complex gaseous matrixes down to the ppb range. The pulsed VUV radiation of the light source is employed for SPI in the ion source of the TOFMS. Ion extraction is also carried out in a pulsed mode with a short time delay with respect to ionization. The experimental setup of the interface VUV light source/time-of-flight mass spectrometer is described, and the novel SPI-TOFMS system is characterized by means of standard calibration gases. Limits of detection down to 50 ppb for aliphatic and aromatic hydrocarbons were achieved. First on-line applications comprised real-time measurements of aromatic and aliphatic trace compounds in mainstream cigarette smoke, which represents a highly dynamic fluctuating gaseous matrix. Time resolution was sufficient to monitor the smoking process on a puff-by-puff resolved basis. Furthermore, human breath analysis has been carried out to detect differences in the breath of a smoker and a nonsmoker, respectively. Several well-known biomarkers for smoke could be identified in the smoker's breath. The possibility for even shorter measurement times while maintaining the achieved sensitivity makes this new device a promising tool for on-line analysis of organic trace compounds in process gases or biological systems.     

Toxicological screening with formula-based metabolite identification by liquid chromatography/time-of-flight mass spectrometry

An analytical procedure was evaluated for the comprehensive toxicological screening of drugs, metabolites, and pesticides in 1-mL urine samples by TurboIon spray liquid chromatography/time-of-flight mass spectrometry (LC/TOFMS) in the positive ionization mode and continuous mass measurement. The substance database consisted of exact monoisotopic masses for 637 compounds, of which an LC retention time was available for 392. A macroprogram was refined for extracting the data into a legible report, utilizing metabolic patterns and preset identification criteria. These criteria included +/-30 ppm mass tolerance, a +/-0.2-min window for absolute retention time, if available, and a minimum area count of 500. The limit of detection, determined for 90 compounds, was <0.1 mg/L for 73% of the compounds studied and >1.0 mg/L for 6% of the compounds. For method comparisons, 50 successive autopsy urine samples were analyzed by this method, and the results confirmed by gas chromatography/mass spectrometry (GC/MS). Findings for parent drugs were consistent with both methods; in addition, LC/TOFMS regularly revealed apparently correct findings for metabolites not shown by GC/MS. Mean and median mass accuracy by LC/TOFMS was 7.6 and 5.4 ppm, respectively. The procedure proved well-suited for tentative identification without reference substances. The few false positives emphasized the fact that all three parameters, exact mass, retention time, and metabolite pattern, are required for unequivocal identification.     

Dual-injection system with multiple injections for determining bidimensional retention indexes in comprehensive two-dimensional gas chromatography

A new instrumental approach for collection of retention index data in the first (1D) and second (2D) dimensions of a comprehensive two-dimensional (2D) gas chromatography (GCxGC) experiment has been developed. First-dimension indexes were determined under conventional linear programmed temperature conditions (Van den Dool indexes). To remove the effect that the short secondary column imposes on derived 1D indexes, as well as to avoid handling of pulsed GCxGC peaks, the proposed approach uses a flow splitter to divert part of the primary column flow to a supplementary detector to simultaneously generate a conventional 1D chromatogram, along with the GCxGC chromatogram. The critical 2D indexes (KovAts indexes) are based upon isovolatility curves of normal alkanes in 2D space, providing a reference scale against which to correlate each individual target peak throughout the entire GCxGC run. This requires the alkanes to bracket the analytes in order to allow retention interpolation. Exponential curves produced in the 2D separation space require a novel approach for delivery of alkane standards into the 2D column by using careful solvent-free solid-phase microextraction (SPME) sampling. Sequential introduction of alkane mixtures during GCxGC runs was performed by thermal desorption in a second injector which was directly coupled through a short transfer line to the entrance of the secondary column, just prior to the modulator so that they do not have to travel through the 1D column. Thus, each alkane mixture injection was quantitatively focused by the cryogenic trap, then launched at predetermined times onto the 2D column. The system permitted construction of an alkane retention map upon which bidimensional indexes of a 25-perfume ingredient mixture could be derived. Comparison of results with indexes determined in temperature-variable one-dimensional (1D) GC showed good correlation. Plotting of the separation power in the second dimension was possible by mapping Trennzahl values throughout the 2D space. The methodology was applied to the separation of a standard mixture composed of 25 analytes (very diverse in polarity and structure) suspected to be allergens in perfume samples. The method will allow straightforward determination of temperature-variable retention indexes of target analytes.     

Differential metabolomics using stable isotope labeling and two-dimensional gas chromatography with time-of-flight mass spectrometry

This work describes an approach to differential metabolomics that involves stable isotope labeling for relative quantification as part of sample analysis by two-dimensional gas chromatography/mass spectrometry (GCxGC/MS). The polar metabolome in control and experimental samples was extracted and differentially derivatized using isotopically light and heavy (D6) forms of the silylation reagent N-methyl-N-tert-butyldimethylsilyl)trifluoroacetamide (MTBSTFA). MTBSTFA derivatives are of much greater hydrolytic stability than the more common trimethylsilyl derivatives, thus diminishing the possibility of isotopomer scrambling during GC analysis. Subsequent to derivatization with MTBSTFA, differentially labeled samples were mixed and analyzed by GCxGC/MS. Metabolites were identified, and the isotope ratio of isotopomers was quantified. The method was tested using three classes of metabolites; amino acids, fatty acids, and organic acids. The relative concentration of isotopically labeled metabolites was determined by isotope ratio analysis. The accuracy and precision, respectively, in quantification of standard mixtures was 9.5 and 4.77% for the 16 amino acids, 9.7 and 2.83% for the mixture of 19 fatty acids, and 14 and 4.53% for the 20 organic acids. Suitability of the method for the examination of complex samples was demonstrated in analyses of the spiked blood serum samples. This differential isotope coding method proved to be an effective means to compare the concentration of metabolites between two samples simultaneously.     

High-speed characterization and analysis of orange oils with tandem-column stop-flow GC and time-of-flight MS

High-speed GC with time-of-flight (TOF) MS detection is used for the characterization and analysis of oils rendered from the peel of five diverse species of orange including bergemot orange, bitter orange, tangerine, mandarin orange, and sweet orange. With a user-defined signal-to-noise threshold of 100, 44 peaks were found and 36 compounds identified in the various oils. Some major constituent components show large concentration ranges over the five species. A 14-m-long, 0. 18-mm-i.d. column ensemble consisting of 7.0-m lengths of a trifluoropropylmethyl polysiloxane and a 5% phenyl dimethyl polysiloxane column was temperature-programmed at 50 degrees C/min starting at the time of injection to achieve analysis times under 140 s. The TOFMS was operated with a spectral acquisition rate of 25 spectra/s, and automated peak finding software successfully found all of the components, with the exception of one severely overlapping peak pair in bitter-orange oil. Of the 44 peaks, 25 were identified by use of a TOFMS library created for this study; another 11 were identified with a commercial terpene library, and 8 were not identified. A quantitative comparison (percent of total peak area) is presented for 16 components, which comprise 98.8-99.5% of the total peak area for the five orange species. Stop-flow operation of the column ensemble is used to enhance selectivity for targeted component pairs to facilitate single-channel detection for QA/ QC analysis of characterized samples and to enhance column selectivity for TOFMS characterization in cases in which peak overlap is so severe that only a single peak is observed.     

Assessment of purity and screening of peptide libraries by nested ion mobility-TOFMS: identification of RNase S-protein binders

Combinatorial peptide synthesis in combination with affinity selection and high-resolution ion mobility/time-of-flight mass spectrometry (IM/TOFMS) analysis has been used to investigate the binding of a series of 96 related eight-residue peptides (with the general sequence NH2-GX1X2FX3X4X5G-CO2H, where X1 = L, F, V, Y; X2 = N, F; X3 = E, V, T; X4 = V, L; X5 = V, L) to the ribonuclease S protein. A key advantage of this strategy is that the IM/ TOFMS approach allows the relative abundances of individual library components (including numerous sequence and structural isomers) to be characterized before and after screening. The relative binding interactions of different sequences are assessed by comparing IM/TOFMS data for those components that pass through the column (as well as those that bind) to data for the library prior to screening. The high-affinity sequences that are found in this study are compared with those selected from much larger combinatorial libraries. The results suggest that many expected sequences in the large libraries may be missing (e.g., due to issues such as failure of specific steps during the synthesis or differences in solubility). Comparison of the binding sequences obtained in these studies and those reported previously indicates that screening results from large libraries should be interpreted with caution.     

Data analysis tool for comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry

Data processing and identification of unknown compounds in comprehensive two-dimensional gas chromatography combined with time-of-flight mass spectrometry (GC×GC/TOFMS) analysis is a major challenge, particularly when large sample sets are analyzed. Herein, we present a method for efficient treatment of large data sets produced by GC×GC/TOFMS implemented as a freely available open source software package, Guineu. To handle large data sets and to efficiently utilize all the features available in the vendor software (baseline correction, mass spectral deconvolution, peak picking, integration, library search, and signal-to-noise filtering), data preprocessed by instrument software are used as a starting point for further processing. Our software affords alignment of the data, normalization, data filtering, and utilization of retention indexes in the verification of identification as well as a novel tool for automated group-type identification of the compounds. Herein, different features of the software are studied in detail and the performance of the system is verified by the analysis of a large set of standard samples as well as of a large set of authentic biological samples, including the control samples. The quantitative features of our GC×GC/TOFMS methodology are also studied to further demonstrate the method performance and the experimental results confirm the reliability of the developed procedure. The methodology has already been successfully used for the analysis of several thousand samples in the field of metabolomics.     

Analysis of coal tar pitch and smoke extract components and their cytotoxicity on human bronchial epithelial cells

Coal tar pitch and its smoke are considered hazardous by-products and common pollutant generated from coal industry processing. In this study, coal tar pitch and its smoke extracts were characterized by gas chromatography/mass spectrometry (GC/MS) with dimethylsulfoxide. We identified only 0.3025% of components in the total coal tar pitch using GC/MS. Among 18 identified compounds, polycyclic aromatic hydrocarbons (PAHs) has the highest relative abundance (0.19%). The remaining components were composed of monocyclic aromatic hydrocarbons, heterocyclic compounds and alkenes. In contrast, among 38 coal tar pitch smoke extract constituents that have been profiled, 87.91% were PAHs, and the remaining 12.09% were composed of monocyclic aromatic hydrocarbons, heterocyclic compounds and alkenes. The cytotoxic effect of coal tar pitch and its smoke extracts on BEAS-2B cells were also evaluated by MTT assay. BEAS-2B cells exposed to coal tar pitch showed a non dose-dependent U-shaped cytotoxicity with a dosage for maximal inhibitory of 3.75 mg/L. In contrast, BEAS-2B cells exposed to coal tar pitch smoke extracts showed a dose dependent cytotoxicity with a LC(50) of 8.64 mg/L. Our study demonstrated the significant different composition and cytotoxicity of coal tar pitch and its extracts, suggesting two different underlying mechanisms that are pending future investigation.     

Combined approach to the analysis of recombinant protein drugs using hollow-fiber flow field-flow fractionation, mass spectrometry, and chemiluminescence detection

The impurities present in recombinant protein drugs produced by large-scale refolding processes can not only affect the product safety but also interact with the expressed protein. To relate the impurity profile to conformation and functionality of the protein drug, analytical methods able not to degrade the sample components should be preferred. In this work, an urate oxidase (uricase) drug from Aspergillus flavus expressed in Saccharomyces cerevisiae, and a reagent-grade uricase from Candida sphaerica expressed in Escherichia coli, are analyzed by combining hollow-fiber flow field-flow fractionation with matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI/TOFMS) and with chemiluminescence enzyme activity assay. Preliminary detection and identification of sample impurities is performed by means of conventional methods such as RP HPLC with electrospray ionization quadrupole-TOF MS and MALDI/TOFMS with SDS PAGE and 2D SDS PAGE. Results show that the recombinant uricase samples obtained from different microorganisms have different impurities and different enzymatic activity and that different uricase oligomers are present in solution.     

Electrically heated, air-cooled thermal modulator and at-column heating for comprehensive two-dimensional gas chromatography

An instrument for comprehensive two-dimensional gas chromatography (GCxGC) is described using an electrically heated and air-cooled thermal modulator requiring no cryogenic materials or compressed gas for modulator operation. In addition, at-column heating is used to eliminate the need for a convection oven and to greatly reduce the power requirements for column heating. The single-stage modulator is heated by current pulses from a dc power supply and cooled by a conventional two-stage refrigeration unit. The refrigeration unit, together with a heat exchanger and a recirculating pump, cools the modulator to about -30 degrees C. The modulator tube is silica-lined stainless steel with an internal film of dimethylpolysiloxane. The modulator tube is 0.18 mm i.d. x 8 cm in length. The modulator produces an injection plug width as small as 15 ms.     

Kinetic Theory of a Spin-1/2 Bose-Condensed Gas

We derive a kinetic theory for a spin-1/2 Bose-condensed gas of two-level atoms at finite temperatures. The condensate dynamics is described by a generalized Gross–Pitaevskii equation for the two-component spinor order parameter, which includes the interaction with the uncondensed fraction. The noncondensate atoms are described by a quantum kinetic equation, which is a generalization of the spin kinetic equation for spin-polarized quantum gases to include couplings to the condensate degree of freedom. The kinetic equation is used to derive hydrodynamic equations for the noncondensate spin density. The condensate and noncondensate spins are coupled directly through the exchange mean field. Collisions between the condensate and noncondensate atoms give rise to an additional contribution to the spin diffusion relaxation rate. In addition, they give rise to mutual relaxation of the condensate and noncondensate due to lack of local equilibrium between the two components.     

Design and Optimization of a Corona Discharge Ion Source for Supercritical Fluid Chromatography Time-of-Flight Mass Spectrometry

The interfacing of capillary column supercritical fluid chromatography (SFC) to time-of-flight mass spectrometry (TOFMS) through atmospheric pressure chemical ionization (APCI) was investigated. An ion source chamber and a new, flexible, and efficient transfer line from the SFC to the TOFMS system were designed to accommodate the requirements of this study. Ionization of analytes was performed using a corona discharge needle. The interface was equipped with two multiple-axis translation stages for positioning of the transfer line tip and the discharge needle inside the ion chamber. The investigations were oriented toward the optimization of parameters which have a strong effect on the intensity and stability of the analyte signal, including background stability, corona discharge needle positioning in the ion source, transfer line tip and discharge needle relative positioning, curtain gas and makeup gas flow interactions, ion chamber temperature, and elution pressure of analytes from the SFC system.