High-speed gas chromatographic separations with diaphragm valve-based injection and chemometric analysis as a gas chromatographic “sensor”

Optimizing retention time and peak width reproducibility with high peak capacity in high-speed gas chromatography using dynamic pressure gradient injection

We are developing high speed gas chromatographic (HSGC) instrumentation with an optimizable injection system, referred to herein as dynamic pressure gradient injection (DPGI). In the present study, we examine the effects of the DPGI pulse width and linear flow velocity on the resultant chromatographic peak widths and separation peak capacity. DPGI readily yields reproducible peak widths and retention times in a sub-second separation runtime regime over long periods of repeated injections. These repeated measurements facilitate a statistically rigorous analysis of the relationships between peak widths obtained and injection pulse width and/or linear flow velocity. Chromatographic performance was studied using a 1 m × 100 µm × 0.1 µm Rtx-5 chromatographic column at various linear flow velocities with hydrogen as the carrier gas, an isothermal temperature of 100 °C, with a test mixture of acetone, nonane, decane and undecane. At this column temperature, acetone is nominally unretained. For conditions where plate height is minimized (Hmin) at the so-called optimum linear flow velocity, uopt, and with the off-column band broadening approaching zero by optimizing DPGI performance, an Hmin of 77 µm was obtained. The chromatographic data corresponding to this Hmin included a minimum peak width-at-half height (w1/2) of 8±0.2ms for acetone, and a peak capacity (nc)of ∼30 for a separation runtime of 1.2 s. When all that is needed is the separation of a few key analytes as fast as possible, and if some peak capacity can be sacrificed, the fastest separation studied yielded a minimum peak width at half-height w1/2=5.5±0.09ms for acetone, and a nc of 10 with a separation runtime of 325ms.

Preconcentration and partial separation of nitroaromatic vapors using a methyltrimethoxysilane-based sol-gel

Typical trace vapor analysis involves sorbent trapping, followed by desorption and chromatographic separation. This communication describes a method for streamlining this process by combining sorbent sampling/preconcentration with partial separation achieved through temperature-programmed thermal desorption. A novel sorbent trap was formulated in which tubular glass liners for a programmable-temperature gas chromatograph inlet were coated with a sol-gel based polymer stationary phase synthesized from methyltrimethoxysilane precursor and installed into the inlet, which was directly connected to a mass-selective detector by a fused silica capillary transfer line. This method is shown to achieve partial separation of two nitroaromatic vapors in a total 3-5min analysis time, which represents a tenfold improvement in speed in terms of the overall cycle time compared to an analogous conventional vapor analysis method. Both analytes proved to have a high dynamic range and loading capacity, with nitrobenzene achieving both high and low sampling extremes (0.32ng-4μg sampling concentration) with only a slight compromise in peak broadening. The multivariate curve resolution by alternating least squares algorithm (MCR-ALS) was shown to successfully resolve the overlapped elution profiles of the two nitroaromatic test vapors examined in this study.

Investigation of high-speed gas chromatography using synchronized dual-valve injection and resistively heated temperature programming

High-speed temperature programming is implemented via the direct resistive heating of the separation column (2.3m MXT-5 Silicosteel column with a 180 microm I.D. and a 0.4 microm 5% phenyl/95% dimethyl polysiloxane film). Resistive temperature programming was coupled with synchronized dual-valve injection (with an injection pulse width of 2 ms), producing a complete high-speed gas chromatography (GC) system. A comparison of isothermal and temperature programmed separations of seven n-alkanes (C(6) and C(8)-C(13)) shows a substantial improvement of peak width and peak capacity with temperature programming. The system was further implemented in separations of a mixture of analytes from various chemical classes. Separations of the n-alkane mixture using three different temperature programming rates are reported. A temperature programming rate as high as 240 degrees C/s is demonstrated. The method for determination of temperature programming rate, based on isothermal data, is discussed. The high-speed resistive column heating temperature programming resulted in highly reproducible separations. The highest rate of temperature programming (240 degrees C/s) resulted in retention time and peak width RSD, on average, of 0.5 and 1.4%, respectively, for the n-alkane mixture. This high level of precision was achieved with peak widths-at-half-height ranging from 13 to 36 ms, and retention times ranging from 147 to 444 ms (for n-hexane to n-tridecane).

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.

Determination of methyl tert-butyl ether in gasoline: a comparison of three fast methods based on mass spectrometry

A high-speed quantitative analysis of methyl tert-butyl ether (MTBE) using three different methods with mass spectrometry detection has been performed. The first method is based on fast chromatography and required an analysis time of 5.23 min per sample, although a certain period (6 min) was necessary for the initial measurement conditions to be regained prior to analysing the next sample. The other two are non-separative methods and are based on direct injection and headspace generation. The analysis times were 1.5 and 3.5 min, respectively, although in the latter case an additional period of time was required to extract volatiles from the sample. The analytical characteristics of all three methods are highly satisfactory in terms of linearity, lack of fit, precision and accuracy. The methods were applied to the determination of MTBE in different gasoline samples. The non-separative methods afforded slightly higher concentrations than those found when fast chromatography was used; this is due to the presence of other minor components that contribute to the abundance of the ion at m/z 73, characteristic of MTBE. We propose a correction that removes this error very satisfactorily and allows the same results to be obtained with all three methodologies proposed.

Monolayer-protected gold nanoparticles as an efficient stationary phase for open tubular gas chromatography using a square capillary

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.