Applications of resistive heating in gas chromatography: A review

Fuel property modeling by high-speed gas chromatography coupled with partial least squares data analysis

Partial least squares (PLS) regression is a valuable chemometric tool for property prediction when coupled with gas chromatography (GC). Since the separation run time and stationary phase selection are crucial for effective PLS modeling, we study these GC parameters on the prediction of viscosity, density and hydrogen content for 50 aerospace fuels. Due to the diversity of compounds in the fuels (primarily alkanes, cycloalkanes, and aromatics), we explore both polar and non-polar stationary phase columns. The robustness for the PLS models was evaluated by their normalized root mean square error of cross-validation (NRMSECV). PLS models built for viscosity across 1-min, 3-min, 7-min, and 10-min time window (TW) high-speed GC separations produced nearly the same NRMSECV with the polar column data with an average (standard deviation) of 4.41 % (0.34 %) versus the non-polar column data of 4.69 % (0.15 %). In contrast, while the NRMSECV of density modeling with the polar column data varied more than the viscosity models, averaging 7.54 % (0.67 %), the non-polar column data produced a significantly higher average NRMSECV of 10.06 % (0.35 %). Similarly, for hydrogen content, the NRMSECV with the polar column data averaged 9.50 % (0.87 %), which was significantly lower than the NRMSECV with the non-polar column data averaging 12.10 % (0.88 %). We also investigated the impact of smoothing the GC data on the corresponding PLS models. By applying varying degrees of smoothing, we can effectively obtain similar chromatographic peak patterns in a shorter TW. For example, a 10-min smoothed chromatogram appears like the 1-min separation with no smoothing but resulted in nearly the same NRMSECV. Overall, the fast separation with a 1-min TW produced robust PLS models for viscosity with either stationary phase column, whereas for density and hydrogen content the polar stationary phase column produced superior PLS models, thus with proper stationary phase selection, a fast separation run time could be readily applied with optimal PLS property modeling results.

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.

Optimizing column-to-column retention time alignment in high-speed gas chromatography by combining retention time locking and correlation optimized warping

We examine and then optimize alignment of chromatograms collected on nominally identical columns using retention time locking (RTL), an instrumental alignment tool, and software-based alignment using correlation optimized warping (COW). For this purpose, three samples are constructed by spiking two sets of analytes into a base test mixture. The three samples are analyzed by high-speed gas chromatography with four nominally identical columns and identical separation conditions. The data is first analyzed without alignment, then using COW alone, then RTL alone, and finally with RTL followed by COW to correct the severe column-to-column misalignment. Principal component analysis (PCA) is used to investigate how well each alignment method clustered the chromatograms into the three sample classes via a scores plot without being compromised by the specific column(s) used. The degree-of-class separation (DCS) is used as a classification metric, measured as the Euclidian distance between the centroids of two clusters in PC space in the scores plot, normalized by their pooled variance. With no alignment, the average DCS between sample classes (DCS_sam) was 3.0, while the average DCS between the four nominally identical columns, i.e., column classes (DCS_col) was 76.1 (ideally the DCS_col should be 0), indicating the chromatograms were initially classified by the columns used. Using either COW or RTL alone also produced unsatisfactory results, with COW alone incorrectly aligning many peaks, leading to a DCS_sam of only 1.9 and DCS_col of 1.7, while RTL alone provided a DCS_sam of 4.7 and DCS_col of 4.2. Finally, using RTL followed by COW alignment, DCS_sam increased to 32.5, indicating successful classification by chemical differences between sample classes, while the DCS_col decreased to 0.4, indicating virtually no classification due to column-to-column differences, as desired. Thus, RTL provided a "first-order" correction of the initial retention mismatch observed for the nominally identical columns, while additional alignment via COW was required to optimize sample classification by PCA.

Programmable smart fast gas chromatograph and open probe controller

Today, labs that carry out chemical analyses for regulation, food safety, health, forensics, or even security purposes are looking for ways to accelerate the analytical process. Slow procedures are costly because the necessary instruments are expensive and require maintenance and a highly trained staff to operate them. One of the more ubiquitous instruments in such labs is a Gas Chromatograph (GC), which accepts a solution and outputs each of the compounds within it in a gaseous form, one by one to be further analyzed and identified, usually by a Mass Spectrometer (MS). This separation process in a GC can be rather time-consuming, partly due to the slow heating and cooling of the GC column through which the compounds move, which happens inside a box-shaped oven. This paper describes a controller developed for a unique Open Probe Fast GC instrument that enables, among other things, high-speed and controlled heating and cooling of a gas-carrying capillary transfer line. Fast heating is achieved by precisely controlling the electrical current flowing through the small inner-diameter steel tube through which the GC column passes. The fast cooling occurs by exposing the low-mass heated tube to room temperature, along with the assistance of a simple fan that carries the heated air away. This technology also supports control of other system parts, including a unique quick sampling device called an Open Probe that allows for an even faster analysis cycle. Our design is based entirely on a digital signal processor (DSP) and digital control. The use of pulse width modulation (PWM) control enables a compact and efficient system.

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.

Application of chromatographic techniques in the analysis of total nitrosamines in water

The use of ozone, chloramine and chlorine dioxide for water treatment results in the formation N-nitrosamines in the treated water. These groups of chemicals and other nitrogen-containing compounds have been described as disinfection by-products (DBPs) which are known for their toxicity. Nitrosamines are a potential source of nitric oxide (NO) which can bind with metals present in the sample matrix leading to formation of metal - nitrosyl complexes and dissolved metals have the potential to increase the total nitrosamines in water. This phenomenon has not received the desired attention and determination of metal-nitrosyl complexes lack standard analytical technique. Chromatography linked to various detectors is the commonest of the techniques for nitrosamine analysis but it is beset with reduced sensitivity as a result of inappropriate choice of the column. Incidentally, chromatographic techniques have not been really adapted for the analysis of metal-nitrosyl complexes. Therefore, there is need for the survey of existing techniques vis-à-vis metal-nitrosamine analysis and to suggest possible areas for method optimization.