A method of calculating the second dimension retention index in comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry

Abstract

A method was developed to calculate the second dimension retention index of comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC/TOF-MS) data using n-alkanes as reference compounds. The retention times of the C(7)-C(31) alkanes acquired during 24 isothermal experiments cover the 0-6s retention time area in the second dimension retention time space, which makes it possible to calculate the retention indices of target compounds from the corresponding retention time values without the extension of the retention space of the reference compounds. An empirical function was proposed to show the relationship among the second dimension retention time, the temperature of the second dimension column, and the carbon number of the n-alkanes. The proposed function is able to extend the second dimension retention time beyond the reference n-alkanes by increasing the carbon number. The extension of carbon numbers in reference n-alkanes up to two more carbon atoms introduces <10 retention index units (iu) of deviation. The effectiveness of using the proposed method was demonstrated by analyzing a mixture of compound standards in temperature programmed experiments using 6 different initial column temperatures. The standard deviation of the calculated retention index values of the compound standards fluctuated from 1 to 12 iu with a mean standard deviation of 5 iu.

Figure 1

Cumulative probability curves of standard deviation of the second dimension retention times of the n-alkanes (in blue) and compounds detected in MegaMix (in red). A mixture of n-alkanes and MegaMix was analyzed on a GC×GC/TOF-MS system operated in temperature programmed mode with 6 replicate injections every day for five continuous days.

Figure 2

Retention time map of the n-alkanes analyzed in temperature programmed mode with different initial first oven temperatures of 60 (□), 80 (■),100 (□), 120 (●), 140 (▲) and 160 (○) °C, respectively. There is an inflection point in each curve, at which the temperature of the second dimension column was 285 °C and was held for 20 min.

Figure 3

The second dimension retention map of the n-alkanes constructed from the isothermal experimental data. The proposed ²t_R-²T_e function was used to fit the retention time values of the n-alkanes. The fitted curves are displayed as solid lines.

Figure 4

The second dimension retention map constructed from the isothermal data of the n-alkanes from C₇ to C₂₆ using the proposed empirical function. The ²t_R-²T_e curves for C₂₇-, C₂₈-, C₂₉-, and C₃₀-alkane were predicted based on the retention time values of n-alkanes from C₇ to C₂₆ using the proposed ²t_R-²T_e fucntion.

Figure 5

The relationship of the mean absolute deviation between the predicted retention index values and the true retention index values defined the number of carbon atoms in n-alkanes. The retention index was converted from the retention time of n-alkanes analyzed in the isothermal mode. The red curve shows the mean absolute deviation obtained using all n-alkanes from C₇ to C₃₁ to fit the ²t_R-²T_e function. The black curve is the mean absolute deviation using the n-alkanes from C₇ to C₂₆ to fit the parameters in the ²t_R-²T_e function. The ²t_R-²T_e function with the fitted empirical parameters was then used to predict the column temperature – retention time curves for the n-alkanes from C₂₇ to C₃₁.