Computer reconstructed gas and liquid chromatograms

This paper describes a Lab Basic II program which allows the reconstruction of GLC-chromatograms on a Hewlett-Packard 2648A graphics terminal, starting from raw data gathered by the HP-3354A Laboratory Data System. The introduction of a considerable number of automatic parameter selections restricts user's interventions. The program can be run in terminal mode or be recalled as a post-analysis program when working in autocall mode.     

Computer reconstruction of high resolution gas chromatograms

This paper describes two programs that are used to plot digital area slice data from glass capillary gas chromatograms. The programs are written in H/P LAB BASIC II, and can be executed on Hewlett-Packard 3354A Laboratory Data Systems using digital plotting devices. The programs are designed to plot high resolution chromatograms, allow horizontal and vertical scaling, and the plots can be numbered and labelled.     

The dynamic properties of plasticized epoxies over a wide frequency range

Summary The dynamic mechanical properties of a class of plasticized epoxies were investigated over a wide temperature range by means of a Rheovibron Viscoelastometer. Master curves for storage and loss moduli were produced by means of the frequency-temperature superposition principle, as well as for relaxation modulus by a conversion formula. The effect of plasticizer percentage on the said parameters, as well as the loss peaks, corresponding to and relaxations was investigated.     

Multivariate profile analysis of gas chromatograms

Summary Pyrolysis gas chromatography is commonly used for microstructural studies in a wide range of applications. A multivariate statistical computer program has been developed which allows the comparison of closely related complex pyrograms. If the patterns are shown to be statistically different within the reproducibility limits of the experiments, those peaks responsible for the differentiation are denoted. Utilization of such computer matching of chromatographic patterns, in addition to other spectral or microchemical analytical data, aids in the identification of organic unknown structures.     

Computer analysis of amino acid chromatograms.

A procedure is described for the automatic off-line analysis of amino acid chromatograms of protein hydrolysates, using a small computer. The data requirements are basic, and, unlike previous programs, the present system allows the separation and identification of bands, as well as the quantitative determination of composition. With minor modification, the program could be extended for use with most types of chromatographic data. The validity of the application of the program to experimental data is discussed.     

Quantitative evaluation of thin-layer chromatograms

A commercially available scanning photometer, designed to scan paper electrophoresis strips, has been used to evaluate thinlayer chromatograms. The chromatogram may be lifted off the glass plate with cellophane tape which is then cut into strips and scanned, or the thin-layer chromatogram may be photographed and the photograph scanned. Newly developed cellulose-backed thin layers of silica or alumina may also be cut into strips and scanned. The instrument has also been modified to scan glass plates. A correction is applied for non-uniformity of the thin layer. Zones containing colourless components can be located by means of an auxiliary chromatogram, and the components present determined by carbon analysis.     

Validating regulatory-compliant wide dynamic range bioanalytical assays using chip-based nanoelectrospray tandem mass spectrometry

Automated chip-based infusion nanoelectrospray ionization coupled to tandem mass spectrometry (nanoESI-MS/MS) was used to validate a bioanalytical assay conforming to United States Food and Drug Administration (FDA) regulatory guidelines and Good Laboratory Practices (GLP). Reboxetine was used as the analyte fortified in dog plasma along with an analog internal standard (IS). The best nanoESI response for reboxetine was observed with 90% acetonitrile (ACN)/water without any mobile phase modifiers. The analyte and IS were extracted from dog plasma samples by liquid-liquid extraction (LLE). The supernatant was concentrated to dryness and redissolved in 90% ACN/water for nanoESI. Selected reaction monitoring (SRM) data were collected for all samples to generate ion current profiles with a base width of approximately 20 s. Selectivity experiments showed no interferences in blank plasma samples. Interferences as a result of in-source collision-induced dissociation of metabolites were not an issue due to the previously documented metabolism of reboxetine. Matrix suppression was evaluated across multiple lots of dog plasma as well as over different animal species (rabbit, rat, mouse) and different anticoagulants (heparin, EDTA). Matrix suppression ranged from approximately 30-60% across the different lots, species etc.; however, in all instances, the analyte and the IS were suppressed by similar amounts, suggesting the similarity in ionization properties between the two. A three-batch validation was performed (each batch consisting of four different concentrations, six replicates of each concentration) and demonstrated inter-assay accuracy (% relative error; RE) of less than +/-8% and an inter-assay precision (% relative standard deviation; RSD) of less than 7%, thus meeting regulatory guidelines. A comparison of analyses by nanoESI-MS/MS and liquid chromatography coupled to tandem mass spectrometry (LC/MS/MS) showed that nanoESI-MS/MS had a greater slope for the calibration standard curve compared to LC/MS/MS, indicating greater sensitivity for the former technique. It is also noteworthy that the amount of sample infused during nanoESI-MS/MS was approximately 80-fold less compared to the amount of sample injected during LC/MS/MS. The absence of carryover (attributed to the lack of a common fluid path) in the nanoESI technique enabled the extension of the assay linear dynamic range to 500,000-fold, and the possibility of analyzing samples in a single batch without the need for re-analysis of samples with high concentrations. This technology offers the possibility for increased throughput for studies supporting drug development by providing fast data turnaround for assays conforming to regulatory guidelines and GLPs.     

Measurement of pH by flow injection over a wide dynamic range with PVC/neutral carrier electrodes

Two pH-sensitive neutral carrier/PVC electrodes are used simultaneously for the measurement of pH by flow injection. One of these is based on the neutral carrier tridodecylamine and the other on octadecyl isonicotinate, and together they allow the pH range 1–13 to be covered. These electrodes have been used in a very low dispersion miniature potentiometric flow cell designed specifically for use in flow injection in conjunction with a multi-channel data acquisition system. The effects of the solution ionic strength and buffer capacity on the pH measurement are discussed. A flow-injection manifold is proposed which can be used for high-accuracy pH measurements without ionic strength adjustment of the sample and for simultaneous pH and ion concentration measurements with ionic strength adjustment. This has been tested on some synthetic samples for the simultaneous determination of potassium and calcium and the measurement of pH.     

Recent trends in image evaluation of HPTLC chromatograms

Abstract

In the current article, an overview of recent applications and opportunities of image evaluation of high-performance thin-layer chromatograms (HPTLC) in food analysis and natural product research is presented. The article shortly covers the aspects of specialized software packages for image analysis, image post-correction, signal acquisition, and preprocessing. Contemporary aspects of image-based HPTLC fingerprinting, quantification of target compounds, and advanced chemometric modeling were reviewed. Special attention was dedicated to freely available software packages. Advantages and disadvantages of each one were discussed in terms of abilities to obtain chromatographic profiles and perform crucial steps in signal manipulation, such as background subtraction, denoising, and background detrending. The most frequently applied techniques for signal manipulation have been discussed and recommendation provided. In that sense, the article aims to provide a valuable guideline for readers dealing with the application of HPTLC image analysis in food and natural products research, especially in connection with the most frequently used chemometric techniques, in domains of pattern recognition, classification, and regression.     

Validation of theory of n-column separations with gas chromatograms predicted by commercial software

A probability theory for the average number of compounds resolved by the partial separation of complex mixtures on n columns was tested using commercial-software predictions of gas chromatograms. Such n-column separations are traditional means for addressing peak overlap, in which one chooses additional columns of different selectivity to separate compounds that cannot be separated by a single column. Gas chromatograms of five types of complex mixtures containing from 99 to 283 compounds were predicted for eight stationary phases using both optimized and other temperature programs. The number n of columns for different mixtures varied from 2 to 5. The numbers of compounds separated as singlet peaks at different resolution thresholds were compared to predictions, as evaluated with point-process statistical-overlap theory based on a Poisson distribution. A good agreement between theory and results was found in all cases corresponding to low saturation. Both good and poor agreements were found for cases corresponding to high saturation. A good agreement also was found for results based on resolving complex mixtures by a single column subject to two temperature programs. The moments and distribution of the number of resolved compounds were computed by Monte Carlo simulation, thus gauging the significance of departures between results and theory. The potential of such simulations to explore the limitations of theory was briefly investigated.     

Real-time target selection optimization to enhance alignment of gas chromatograms

An improved method for real-time selection of the target for the alignment of gas chromatographic data is described. Further outlined is a simple method to determine the accuracy of the alignment procedure. The target selection method proposed uses a moving window of aligned chromatograms to generate a target, herein referred to as the window target method (WTM). The WTM was initially tested using a series of 100 simulated chromatograms, and additionally evaluated using a series of 55 diesel fuel gas chromatograms obtained with four fuel samples. The WTM was evaluated via a comparison to a related method (the nearest neighbor method (NNM)). The results using the WTM with simulated chromatograms showed a significant improvement in the correlation coefficient and the accuracy of alignment when compared to the alignments performed using the NNM. A significant improvement in real-time alignment accuracy, as assessed by a correlation coefficient metric, was achieved with the WTM (starting at ∼1.0 and declining to only ∼0.985 for the 100th sample), relative to the NNM (starting at ∼1.0 and declining to ∼0.4 for the 100th sample) for the simulated chromatogram study. The results determined when using the WTM with the diesel fuels also showed an improvement in correlation coefficient and accuracy of the within-class alignments as compared to the results obtained from the NNM. In practice, the WTM could be applied to the real-time analysis of process and feedstock industrial streams to enable real-time decision making from the more precisely aligned chromatographic data.     

Computer analysis of amino acid chromatograms and other linear elution spectra

A computer-aided method of evaluation of amino acid column chromatograms is proposed and compared with the usual synchronous peak-integration method. Spectra and their backgrounds are digitized separately by a curve digitizer; this allows a better estimate of the background time courses. A computer program then fits a set of superimposed Gaussian distributions to each corrected spectrum, thus circumventing the problems arising from incompletely separated peaks. Samples of known composition, run intermittently through the analyzer between the unknown spectra, allow a determination of the time dependence of the ratios of “peak areas over amounts of amino acids”; hence, the amounts of amino acids can automatically be corrected for aging effects of the analyzing system. A modified version of the computer program allows resolution of any spectrum to a sum of Gaussians, with least squares fitting of their amplitudes, widths and locations.     

Combined electron gas SNMS and SIMS instrument for trace and depth profile analysis with high dynamic range

Summary Basic physics, technical implementation and applications of a combined SNMS and SIMS instrument improved in dynamic range and detection limits are discussed.

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Kombiniertes SNMS- und SIMS-Gerät zur spurenund Tiefenprofilanalyse mit großem dynamischem Bereich

     

Combination of dynamic time warping and multivariate analysis for the comparison of comprehensive two-dimensional gas chromatograms: application to plant extracts

Comprehensive two-dimensional gas chromatography (GC x GC) is now recognized as the preferred technique for the detailed analysis and characterization of complex mixtures of volatile compounds. However, for comparison purposes, taking into account all the information contained in the chromatogram is far from trivial. In this paper, it is shown that the combination of peak alignment by dynamic time warping and multivariate analysis facilitated the comparison of complex chromatograms of tobacco extracts. The comparison is shown to be efficient enough to provide a clear discrimination among three types of tobacco. A tentative interpretation of loadings is presented in order to give access to the compounds which differ from one sample to another. Once located, mass spectrometry was used to identify markers of tobacco type.     

Conditions for accurate split injection of samples with wide boiling point range

The effect of injection temperature, carrier gas flow rate, geometry of the glass insert, and column temperature program on the precision and accuracy of split injections was measured. Three types of injection techniques were compared: injection into a hot isothermal injector, isothermal injection with the injector at the solvent boiling point temperature, and programmed injection temperature. The last of these techniques produced the best accuracy and precision of analysis. Conditions for complete sample trapping at the beginning of programmed temperature analysis are described.     

An interferometric spectrum analyzer with high dynamic range

The dynamic range improvement and realization of the acousto-optic spectrum analyzer are reported. When two RF signals of equal strengths and frequencies f₁ and f₂ are applied simultaneously to the signal Bragg cell, the analyzer demonstrates a dynamic range of 50 dB and frequency resolution 10 kHz.