Retention index based approach for simulation of results and application for validation of compound identification in comprehensive two-dimensional gas chromatography.

In this work, first and second dimensional retention index (1I and 2I) based calculation approach is established to simulate peak retention times (1tR and 2tR) of samples for the given sets of volatile compounds in comprehensive two-dimensional gas chromatography-mass spectrometry (GC×GC-MS). For the result without 1tR and 2tR data of alkane references (1tR(n) and 2tR(n)), the following steps were applied: (1) curve fitting based on van den Dool and Kratz relationship in order to simulate 1tR(n) using a training set of volatile compounds in a sample with their experimental 1tR data, and (2) simulation of 2tR(n) at different 1tR(n) to construct their isovolatility curves based on a nonlinear equation with p1-p5 parameters and a constant (within the ranges of -0.0052 to 0.0049, -0.6181 to -0.0230, -26.4775 to -0.2698, 0.0050 to 9.6259, -7.2976 to -3.9524 and 0.9157 to 4.0779, respectively). These parameters were obtained by performing curve fitting according to the experimental 2tR data of the same training set with the least square values ranging from 4.58×10-15 to 32.55. Simulation of 1tR and 2tR of target analytes (1tR,sim and 2tR,sim) with known 1I and 2I were performed using 1tR(n) and the simulated isovolatility curves. All the calculations and curve fittings were carried out by using Solver in Microsoft Excel. The approach was applied to simulate results for 542 compounds in several samples including analysis of saffron (Crocus sativas L.), Boswellia papyrifera, acacia honey and incense powder/smoke, perfume and cannabis either reported from literature or from the experiments in this work using different experimental approaches. These were compared with the experimental data showing correlation with the R2 ranges of 0.98-1.00 and 0.80-0.97 for 1tR and 2tR, respectively. This approach was then applied to propose 6 compounds which may be incorrectly identified based on the differences of >2 times of the standard deviations between 2tR,sim and the experimental 2tR in both residue and leave-one-out analyses.

Analysis of low-volatility pesticides in cabbage by high temperature comprehensive two-dimensional gas chromatography.

High-temperature comprehensive two-dimensional gas chromatography (HTGC × GC) using a longitudinally modulated cryogenic system (LMCS) was developed for the analysis of low-volatility pesticides in cabbage. The method applied DB-17HT and DB-5HT as the first and second dimensional (1D and 2D) columns, respectively. Twelve pesticides, namely 6 organochlorines (4,4'-DDT, ß-endosulfan, endosulfan sulfate, endrin, heptachlor, and dicofol), 4 carbamates (metolcarb, isoprocarb, methiocarb, and carbofuran), 1 organophosphate (chlorpyrifos), and 1 pyrethroid (permethrin), were spiked into cabbage samples and prepared using QuEChERS. The applied oven temperature was up to 340 °C, enabling the elution of all the target pesticides and the matrix. The effects of initial oven temperature program, temperature ramp rate, LMCS trap temperature, and modulation period (PM) on the separation results were investigated, leading to the suitable conditions of 80 °C, 15 °C min-1, 10 °C, and 12 s, respectively. The method detection limits, signal-to-noise ratio, and recoveries of the compounds were within the ranges of 0.01-0.09 mg kg-1, 4.26-32.7, and 78-104%, respectively. Good linearity ranges within the concentration range of 0.1-1 ppm with R2 > 0.9134 were also obtained with the intra and interday precisions of the peak areas of 0.4-9.8% and 1.0-10.2%, respectively.

Strategies towards simpler configuration and higher peak capacity with comprehensive multidimensional gas chromatography.

Experimental and data analysis approaches in multidimensional gas chromatography (MDGC) comprising comprehensive multiple heart-cut (H/C) and comprehensive two dimensional GC (GC × GC) were developed with an example application illustrated for analysis of a technical glycol precursor sample. The GC × GC system employed a long 1D (30 m) and a short 2D (5 m) column with a flow modulator and a Deans switch (DS) as a splitter; meanwhile. The H/C system was applied solely as a DS located between long 1D (30 m) and 2D (60 m) columns without use of cryogenic trapping devices. The effects of injection time and 2D column flow in GC × GC and the impacts of H/C window and number of injections (total analysis time) in H/C analysis were investigated. The analysis performance for each condition was evaluated according to peak capacity and number of separated compounds. The continuum between the two techniques was then established via the relationship between analysis time and analysis performance. The separation performances were improved with longer analysis time so that the suitable condition was selected within this compromise. Under the selected conditions, volatile compounds in the technical glycol precursor sample were identified according to the match between the experimental MS spectra and first dimensional retention indices (1 I) with that from the NIST2014 database and literature. An hour analysis with GC × GC resulted in a total peak capacity of 798, number of separated peaks of 61 and average MS match score of 887 ± 35; meanwhile, the corresponding numbers were improved to be 9198, 107 and 898 ± 24, respectively, with the 25 h comprehensive H/C analysis.

A multi-location peak parking approach for calculation of second dimensional retention indices for improved volatile compound identification with cryogen-free comprehensive heart-cut two-dimensional gas chromatography.

Comprehensive heart-cut multidimensional gas chromatography (CH/C MDGC) without a cryogenic trapping device was developed with an established approach for calculation of first and second dimensional retention indices (1I and 2I) for improved compound identification. A first dimensional (1D) DB-1MS column (60 m) and a second dimensional (2D) DB-WAX column (60 m) were applied with a Deans switch (DS) using a constant H/C window of 0.2 min and a periodic multiple heartcut strategy comprising 225H/C throughout the CH/C. 1I was calculated based on comparison of the middle of the heartcut time with the alkane retention times on the 1D column. A multi-location peak parking approach using sixteen sets of automated injections of alkane references was also established with the least square curve fitting method for construction of the alkane isovolatility curves which were applied for 2I calculation. The untargeted compound analysis of a perfume sample was then performed according to comparison with the libraries of mass spectra, 1I and 2I. The CH/C MDGC system with a 25 h analysis time showed a peak capacity (nc) of 9198 and 128 separated peaks with 71 compounds successfully identified according to MS, 1I and 2I library match under the established error approximation criteria. Furthermore, relationship between the analysis time and number of separated peaks was proposed based on the set of 84 identifiable compounds. With the compensation of lower separation performance and greater I errors, the analysis time could be reduced by applying a 2.5 min H/C window with a total analysis time of 2 h and nc of 1134.

Cryogen-free comprehensive heartcut multidimensional gas chromatography using a Deans switch for improved analysis of petrochemical products derived from palmitic acid oxidation.

Comprehensive heartcut multidimensional gas chromatography was applied with example application for analysis of a sample obtained from palmitic acid oxidation in a Rancimat instrument. The system utilized a single Deans switch (DS) located between first dimensional semi-standard nonpolar (30 m) and second dimensional polar (60 m) columns. A cyclic multiple heartcut strategy consisting of 150 heartcuts with a 0.2 min window was applied offering comprehensive analysis and injection of a narrow band of compounds onto the second column without use of cryogenic trapping devices. Untargeted compound analysis of the sample prepared by solid phase micro-extraction was performed based on match between the experimental MS spectra and first dimensional retention indices with that from the NIST library. The sample contained the major compounds of 2-octanone, 1-methylcyclohexanol, 2,3,6-trimethylphenol, 3-phenylpropanol and 2-nonanone. This approach was then evaluated based on peak capacity and the number of identified compounds. Compared with one dimensional gas chromatography providing a total peak capacity of 172 and 43 identified compounds, the analysis performance was much more improved with a capacity of 5840 and 235 identified compounds by using comprehensive heartcut multidimensional gas chromatography with the total analysis time of 15.3 h. By comparison within the same set of identified compounds, the one dimensional and multidimensional approaches provided the MS match scores of 769 ± 81 and 836 ± 88, respectively. In addition, the nonlinear relationship between the analysis time and number of identifiable peaks was calculated according to the set of 235 compounds. This revealed that the analysis time could be shortened with the compensation of lower separation performance, where application of a 2.5 min heartcut window with the total analysis time of 1.2 h could result in the total peak capacity of 390 with 150 identifiable compounds.