Bob W. Wright

High-speed peak matching algorithm for retention time alignment of gas chromatographic data for chemometric analysis

A rapid retention time alignment algorithm was developed as a preprocessing utility to be used prior to chemometric analysis of large datasets of diesel fuel profiles obtained using gas chromatography (GC). Retention time variation from chromatogram-to-chromatogram has been a significant impediment against the use of chemometric techniques in the analysis of chromatographic data due to the inability of current chemometric techniques to correctly model information that shifts from variable to variable within a dataset. The alignment algorithm developed is shown to increase the efficacy of pattern recognition methods applied to diesel fuel chromatograms by retaining chemical selectivity while reducing chromatogram-to-chromatogram retention time variations and to do so on a time scale that makes analysis of large sets of chromatographic data practical. Two sets of diesel fuel gas chromatograms were studied using the novel alignment algorithm followed by principal component analysis (PCA). In the first study, retention times for corresponding chromatographic peaks in 60 chromatograms varied by as much as 300 ms between chromatograms before alignment. In the second study of 42 chromatograms, the retention time shifting exhibited was on the order of 10 s between corresponding chromatographic peaks, and required a coarse retention time correction prior to alignment with the algorithm. In both cases, an increase in retention time precision afforded by the algorithm was clearly visible in plots of overlaid chromatograms before and then after applying the retention time alignment algorithm. Using the alignment algorithm, the standard deviation for corresponding peak retention times following alignment was 17 ms throughout a given chromatogram, corresponding to a relative standard deviation of 0.003% at an average retention time of 8 min. This level of retention time precision is a 5-fold improvement over the retention time precision initially provided by a state-of-the-art GC instrument equipped with electronic pressure control and was critical to the performance of the chemometric analysis. This increase in retention time precision does not come at the expense of chemical selectivity, since the PCA results suggest that essentially all of the chemical selectivity is preserved. Cluster resolution between dissimilar groups of diesel fuel chromatograms in a two-dimensional scores space generated with PCA is shown to substantially increase after alignment. The alignment method is robust against missing or extra peaks relative to a target chromatogram used in the alignment, and operates at high speed, requiring roughly 1 s of computation time per GC chromatogram.

Investigation of polar modifiers in carbon dioxide mobile phases for capillary supercritical fluid chromatography

Abstract The effect of low concentrations of polar modifiers on the selectivity of a non-polar carbon dioxide mobile phase in capillary supercritical fluid chromatography (SFC) was determined. The selectivity of mixtures of carbon dioxide-water (approximately 0.3—0.9 mole%) and carbon dioxide-methanol (approximately 0.8 mole%) were compared to the selectivity of pure carbon dioxide using polarity-test mixtures. Under the conditions of this study, water modifiers did not significantly affect selectivity or retention. Slight changes in selectivity and decreased retention, however, were observed for the methanol modifier. These data contrast with those obtained for packed-column SFC in which similar concentrations of modifiers produced significant changes in selectivity and greatly reduced retention, and are consistent with results indicating these previous results were due to modification of the stationary phase rather than changes in fluid phase solubilities.

Blind field test evaluation of Raman spectroscopy as a forensic tool

Analytical instrumentation for Raman spectroscopy has advanced rapidly in recent years to the point where commercial field-portable instruments are available. Raman analysis with portable instrumentation is a new capability that can provide emergency response teams with on-site evaluation of hazardous materials. Before Raman analysis is accepted and implemented in the field, realistic studies applied to unknown samples need to be performed to define the reliability of this technique. Studies described herein provide a rigorous blind field test that utilizes two instruments and two operators to analyze a matrix that consists of 58 unknown samples. Samples were searched against a custom hazardous materials reference library (Hazardous Material Response Unit (HMRU) Spectral Library Database). Experimental design included a number of intentionally difficult situations including binary solvent mixtures and a variety of compounds that yield medium-quality spectra that were not contained in the HMRU library. Results showed that over 97% of the samples were correctly identified with no occurrences of false positive identifications (compounds that were not in the library were never identified as library constituents). Statistical analysis indicated equivalent performance for both the operators and instruments. These results indicate a high level of performance that should extrapolate to actual field situations. Implementation of Raman techniques to emergency field situations should proceed with a corresponding level of confidence.

Supercritical fluid chromatography and supercritical fluid chromatography—mass spectrometry of marine diesel fuel

Abstract The application of capillary column suprciritcal fluid chromatography (SFC) and supercritical fluid chromatography—mass spectrometry (SFC—MS) to the analysis of the polycyclic aromatic hydrocarbon fraction of selected marine diesel fuel samples is described. The chromatographic separation methods utilized pressure or density programming techniques and small diameter (50 μm) capillary columns. This work demonstrates the feasibility of achieving hihg-resolution separations of medium polarity compounds using nonpolar carbon dioxide as the mobile phase in SFC and the successful coupling of SFC to MS to obtain mass chromatograms and reliable chemical ionization mass spectra. High resolution separations of the polycyclic aromatic hydrocarbon fractions of marine diesel fuel samples were combined with isobutane chemical ionization MS. Analysis of the more polar components will require a more polar supercritical fluid mobile phase.

Sample introduction and separation in capillary electrophoresis, and combination with mass spectrometric detection

Capillary-electrophoresis methods are attracting interest owing to the ability to yield rapid high-resolution separations, but many aspects, such as sample injection, separation conditions and detection, need further development. Effects related to sample injection and buffer composition have been investigated. Automated methods for electromigration injection of nl-size sample volumes are shown to give a precision of approximately +/-1%. Problems encountered with manual injection procedures have been examined by an electric field reversal technique. The effect of buffer pH on capillary zone-electrophoresis (CZE) separations can be attributed to changes in electro-osmotic flow velocities and to changes in the isoelectric points of analytes. The interfacing of capillary electrophoresis with mass spectrometry is described and demonstrated for a range of conditions, with a quaternary phosphonium salt mixture. Separations obtained by CZE and capillary isotachophoresis are compared and the relative advantages of the two techniques discussed.

Effect of temperature and modifier concentration on retention in supercritical fluid chromatography

Abstract Use of binary modifiers in supercritical fluid chromatography (SFC) enhances the solvating capabilities of the supercritical mobile phase, allowing the mobile phase to be chemically tailored to a specific separation. The effects of temperature, density and modifier mole fraction on the retention process in capillary SFC was studied. Selectivity, as well as the enthalpy of solute transfer, were seen to be dependent on modifier concentration. Specific molecular interactions between the binary fluid and the solutes as well as enhanced solvation of the bonded stationary phase are suggested by the experimental data.

Selective Stationary Phase for Solid-Phase Microextraction Analysis of Sarin (GB)

A number of critical field applications require monitoring air samples for trace levels of chemical warfare agents. Solid-phase microextraction (SPME) is a convenient format to conduct these analyses. Measurements could be significantly improved if a SPME phase selective for nerve agents were substituted for non-selective polymers typically used (e.g., polydimethylsiloxane). This paper evaluates a novel stationary phase, previously developed for methylphosphonate sensor applications, for use with SPME sampling. The phenol-based polymer, BSP3, was found to offer far higher selectivity toward sarin (GB) than polydimethylsiloxane due to a pronounced affinity toward the target analyte and a lower affinity toward hydrocarbons.

Analytical applications of capillary supercritical fluid chromatography-mass spectrometry

Abstract Capillary chromatographic analyses of thermally labile, less volatile and higher molecular weight compounds can be accomplished using supercritical mobile phases due to the combination of solvating powers and attractive mass transport properties. Recent advances in the use of small bore (25–50μm) capillary columns, rapid pressure programming methods, and polar mobile phases have yielded high-sensitivity and high-speed analyses with chromatographic resolution approaching that of conventional capillary gas chromatography. Mass spectrometry (MS) provides a nearly universal detector for supercritical fluid chromatography (SFC), and the ease with which capillary SFC can be directly coupled with the mass spectrometer avoids the complications inherent in liquid chromatography (LC)—MS interfaces. Capillary SFC can be interfaced with both electron impact and chemical ionization (CI) modes of operation, and high selectivity and sensitivity as well as structural data can be obtained through appropriate choice of CI reagents. Recent developments in capillary SFC—MS and applications to several compound classes are described.

Application of capillary supercritical fluid chromatography to the analysis of a middle distillate fuel

SummaryThe application of capillary supercritical fluid chromatography (SFC) to the analysis of a middle distillate fuel is described. Small diameter (50μm i.d.) fused-silica capillary columns coated with crosslinked 50% phenyl polymethylphenyl siloxane provided high separation efficiency and good compatibility with flame ionization detection. High resolution separations of the chemical class fractions obtained by adsorption chromatography on alumina were obtained using carbon dioxide as the supercritical mobile phase and simple pressure programming techniques. In addition to the less polar fuel components, supercritical carbon dioxide allowed chromatography of the nitrogen-containing polycyclic aromatic hydrocarbon fraction and the hydroxylated polycyclic aromatic materials.

Chemometric analysis of gas chromatography-mass spectrometry data using fast retention time alignment via a total ion current shift function.

A critical comparison of methods for correcting severely retention time shifted gas chromatography-mass spectrometry (GC-MS) data is presented. The method reported herein is an adaptation to the piecewise alignment algorithm to quickly align severely shifted one-dimensional (1D) total ion current (TIC) data, then applying these shifts to broadly align all mass channels throughout the separation, referred to as a TIC shift function (SF). The maximum shift varied from (-) 5s in the beginning of the chromatographic separation to (+) 20s toward the end of the separation, equivalent to a maximum shift of over 5 peak widths. Implementing the TIC shift function (TIC SF) prior to Fisher Ratio (F-Ratio) feature selection and then principal component analysis (PCA) was found to be a viable approach to classify complex chromatograms, that in this study were obtained from GC-MS separations of three gasoline samples serving as complex test mixtures, referred to as types C, M and S. The reported alignment algorithm via the TIC SF approach corrects for large dynamic shifting in the data as well as subtle peak-to-peak shifts. The benefits of the overall TIC SF alignment and feature selection approach were quantified using the degree-of-class separation (DCS) metric of the PCA scores plots using the type C and M samples, since they were the most similar, and thus the most challenging samples to properly classify. The DCS values showed an increase from an initial value of essentially zero for the unaligned GC-TIC data to a value of 7.9 following alignment; however, the DCS was unchanged by feature selection using F-Ratios for the GC-TIC data. The full mass spectral data provided an increase to a final DCS of 13.7 after alignment and two-dimensional (2D) F-Ratio feature selection.