Alignment of retention time obtained from multicapillary column gas chromatography used for VOC analysis with ion mobility spectrometry

Abstract

Multicapillary column (MCC) ion mobility spectrometers (IMS) are increasingly in demand for medical diagnosis, biological applications and process control. In a MCC-IMS, volatile compounds are differentiated by specific retention time and ion mobility when rapid preseparation techniques are applied, e.g. for the analysis of complex and humid samples. Therefore, high accuracy in the determination of both parameters is required for reliable identification of the signals. The retention time in the MCC is the subject of the present investigation because, for such columns, small deviations in temperature and flow velocity may cause significant changes in retention time. Therefore, a universal correction procedure would be a helpful tool to increase the accuracy of the data obtained from a gas-chromatographic preseparation. Although the effect of the carrier gas flow velocity and temperature on retention time is not linear, it could be demonstrated that a linear alignment can compensate for the changes in retention time due to common minor deviations of both the carrier gas flow velocity and the column temperature around the MCC-IMS standard operation conditions. Therefore, an effective linear alignment procedure for the correction of those deviations has been developed from the analyses of defined gas mixtures under various experimental conditions. This procedure was then applied to data sets generated from real breath analyses obtained in clinical studies using different instruments at different measuring sites for validation. The variation in the retention time of known signals, especially for compounds with higher retention times, was significantly improved. The alignment of the retention time--an indispensable procedure to achieve a more precise identification of analytes--using the proposed method reduces the random error caused by small accidental deviations in column temperature and flow velocity significantly.

Fig. 1

An ion mobility spectrometer (IMS) with optional preseparation as designed at ISAS for breath analysis [16]

Fig. 2

Variation of the retention time of the selected analytes with different carrier gas flow rates. An example potential regression for 1-decanol is indicated by the dotted line (R² = 0.9931)

Fig. 3

Variation of the retention times of 2-undecanol and decanal as examples of the 15 compounds investigated with different multicapillary column (MCC) temperatures. Potential fittings are indicated by solid lines

Fig. 4

Variation of the retention time of 2-nonanone with different MCC temperatures around the normal operation temperature of 40 °C. The linear regression is indicated by the solid line. Error bars mark the time range of one spectrum in the MCC-IMS chromatogram for the normal set-up

Fig. 5

Example alignment of a standard gas mixture. The ovals indicate the expected position of compounds in the topographic plot (BB_IMSAnalysis 1.0, ISAS, Dortmund, Germany) before (a) and after (b) alignment. M monomer, D dimer

Fig. 6

Example alignment of real breath analysis. The topographic plot (BB_IMSAnalysis 1.0, ISAS, Dortmund, Germany) is presented before (a) and after (b) alignment