Temperature Programming of the Second Dimension in Comprehensive Two-Dimensional Gas Chromatography

Second-Dimension Temperature Programming System for Comprehensive Two-Dimensional Gas Chromatography. Part 2: Technical Improvements and Compatibility with Flow Modulation and Time-of-Flight Mass Spectrometry

The second-dimension (²D) temperature programming system (²DTPS) for comprehensive two-dimensional gas chromatography (GC × GC) described in Part 1 was updated and tested with the time-of-flight mass spectrometer (TOFMS) and flow modulator. Addition of a real-time clock and remote port allowed the ²DTPS to be a truly standalone system to be used with any GC × GC instrument. GC × GC reproducibility with the ²DTPS was tested with thermal and flow modulation, coupled with the TOFMS and/or FID to demonstrate compatibility with all typical GC × GC setups. An improvement in the match factor, reverse match factor, and signal-to-noise ratio was found when performing ²D temperature programming. Within-day and day-to-day reproducibility of the ²DTPS for the ¹D retention time (≤0.04 and ≤0.05%), ²D retention time (≤0.36 and ≤0.52%), and peak area (≤2.47 and ≤3.37%) were acceptable, while providing flexibility in ²D optimization and improved peak capacity.

Second-Dimension Temperature Programming System for Comprehensive Two-Dimensional Gas Chromatography. Part 1: Precise Temperature Control Based on Column Electrical Resistance

A second-dimension temperature programming system (²DTPS) for comprehensive two-dimensional gas chromatography (GC × GC) is introduced, and its performance is characterized. In the system, a commercial stainless-steel capillary column was used for the separation, as a heating element, and as a temperature sensor. The second dimension (²D) column was resistively heated and controlled using an Arduino Uno R3 microcontroller. Temperature measurement was accomplished by measuring the overall ²D column's electrical resistance. A diesel sample was used to compare the ²D peak capacity (²n_c) and resolution (²R_s), while a perfume sample was used to compare the reproducibility of the system for within-day (n = 5) and day-to-day (n = 5) results. The ²n_c improved by 52% with the ²DTPS compared to the secondary oven. The GC × GC system utilizing the ²DTPS had an average within-day and day-to-day relative standard deviation (RSD) of 0.02 and 0.12% for the ¹D retention time (¹t_R), 0.56 and 0.58% for the ²D retention time (²t_R), and 1.18 and 1.53% for the peak area, respectively.

Biological studies with comprehensive 2D-GC-HRMS screening: Exploring the human sweat volatilome

A key issue in GCxGC-HRMS data analysis is how to approach large-sample studies in an efficient and comprehensive way. We have developed a semi-automated data-driven workflow from identification to suspect screening, which allows highly selective monitoring of each identified chemical in a large-sample dataset. The example dataset used to illustrate the potential of the approach consisted of human sweat samples from 40 participants, including field blanks (80 samples). These samples have been collected in a Horizon 2020 project to investigate the capacity of body odour to communicate emotion and influence social behaviour. We used dynamic headspace extraction, which allows comprehensive extraction with high preconcentration capability, and has to date only been used for a few biological applications. We were able to detect a set of 326 compounds from a diverse range of chemical classes (278 identified compounds, 39 class unknowns, and 9 true unknowns). Unlike partitioning-based extraction methods, the developed method detects semi-polar (log P < 2) nitrogen and oxygen-containing compounds. However, it is unable to detect certain acids due to the pH conditions of unmodified sweat samples. We believe that our framework will open up the possibility of efficiently using GCxGC-HRMS for large-sample studies in a wide range of applications such as biological and environmental studies.

A fast temperature-programmed second-dimension column for comprehensive two-dimensional gas chromatography

The short analysis time and constant temperature environment in the second dimension of two-dimensional comprehensive chromatography frequently causes wraparound problems, especially for complex high boilers. This problem can be solved by temperature programming on the second column, but since this requires heating and cooling the column in a matter of seconds, it is difficult to implement. In this study, we describe a method of accomplishing rapid heating and cooling with a resistively heated column cooled by compressed air. Critical to this method is minimizing the lag time between the actual temperature and the reported temperature by using the column heating element as the temperature sensor, virtually eliminating the danger of overshooting the temperature setpoint. This technique facilitates a ramp rate of up to 100 °C/s with minimal overshooting-well beyond the requirements of gas chromatography. A single-layer column bundle design allows a compressed-air cooling device to cool the column from 200 to 50 °C at an average rate of -21 °C/s. The secondary dimension temperature programming is facilitated by the longer secondary dimension time made possible by the direct flow modulation method. We evaluated the performance of the single-layer column bundle and demonstrated this method by applying it to a gasoline sample. We also compared this method with the traditional isothermal approach and found that use of the secondary temperature program reduced the naphthalene retention time from 12.1 to 6.3 s and its peak width at half height from 846 to 126 ms.

A high-repetition-rate, fast temperature-programmed gas chromatograph and its online coupling to a supercritical fluid chromatograph (SFC × GC)

We present a fast gas chromatographic system that can be used as a second dimension in comprehensive two-dimensional (supercritical fluid × gas) chromatography (SFC × GC). The temperature of the short (1 m long) capillary column is controlled by a resistively heated coaxial stainless-steel tube. The electrical resistance and, therefore, temperature of the stainless-steel tube are measured by continuous monitoring of the current/voltage ratio. Highly repeatable heating rates of up to 2100 °C min^-1 (35 °C s^-1) are obtained, which should be high enough for the most demanding fast chromatograms. To reduce the cooling time between temperature programs, the column is cooled by injecting evaporating carbon dioxide into the space between the coaxial heater and the column. This gives cooling rates of 5100 °C min^-1 (85 °C s^-1), which allows quick succession of temperature programs. More repeatable heating profiles with stable GC retention times together with faster cooling are significant improvements on previous SFC × GC systems. Cycle times of four gas chromatograms per minute could readily be achieved, which allows efficient coupling to high-resolution stop-flow SFC in the first dimension. We demonstrate the fast chromatograph by separating fatty acid methyl esters, yielding information that would be useful in the food and biodiesel industries.