A fast thermal desorption unit for micro thermal desorption tubes, Part I: Development of the system and proof of concept measurements with hyper-fast gas chromatography.

A system consisting of a thermal desorption unit (TDU) and micro thermal desorption tubes (µTD-tubes, 1.4 mm I.D., 10mg Tenax TA) for fast desorption of analytes was developed for the efficient combination of hyper fast gas chromatography with thermal desorption. The fast desorption is achieved by a significantly reduced thermal mass compared to conventional thermal desorption tubes. Therefore, extremely fast heating and cooling cycles are possible. Proof of concept measurements combining the new setup with a flow-field thermal gradient gas chromatograph (FF-TG-GC) and FID detection show good precision and linearity with R2≥0.995 in the analysis of an n-alkane mix (C8-C20). Thermal desorption occurs within 12s. The impact of reduced µTD-tube dimensions on desorption time, full width at half maximum (FWHM), breakthrough volumes, tube flow rates ergo linear velocities, porosity and back pressure is discussed.

Residual solvent analysis with hyper-fast gas chromatography-mass spectrometry and a liquid carbon dioxide cryofocusing in less than 90 s.

A new hyper-fast gas chromatography method with less than 90 s runtime including the column cool down was developed for the analysis of four gases and 16 residual solvents, combining a CO2 cryofocusing with a flow-field thermal gradient gas chromatograph (FF-TG-GC) and ToF-MS. The extremely low analysis time can be achieved by combining the new FF-TG-GC and a very short Rxi-624 Sil MS separation column with a small inner diameter and small film thickness (2.05 m × 0.1 mm × 1.0 µm). The column is inserted into a low thermal mass, resistively heated stainless steel capillary. This enables fast temperature programs with heating rates up to 3000 °C/min and a column cool down within a few seconds. In addition to temporal temperature gradients, the FF-TG-GC can generate a spatial temperature gradient that leads to an improved peak shape. Further, an external liquid CO2 cryo-trap was designed in order to reduce the injection bandwidths of analytes and to take full advantage of the resolving power of the separation column. No modifications are required to the FF-TG-GC for the use of the cryogenic trap, as the cooled spot is heated by the resistively heated stainless steel capillary during the temperature program. With cryofocusing, analyzed residual solvents are baseline separated. R2 values over 0.99 for calibration curves and low relative standard deviations (mainly < 3%) for repeatability tests were obtained.

Simulation of spatial thermal gradient gas chromatography.

A model to simulate the gas chromatographic separation in the presence of a spatial thermal gradient is presented. This model is developed from existing models for the prediction of retention times in temperature programmed GC. It is based on basic fluid mechanics of gasses in capillaries to describe the properties of the mobile phase and a thermodynamic model to describe retention of solutes in a stationary phase. This model is expanded to incorporate a spatial thermal gradient. The development of the peak width during the chromatographic separation is modeled by a differential equation, which uses the solute residency, the inverse of the solute velocity, instead of the solute velocity. The presented model is compared to measurements of n-alkanes with conventional temperature programmed GC-FID and to measurements with a hyper-fast flow-field thermal gradient GC (FF-TG-GC) coupled with a MS. The FF-TG-GC enables the use of a spatial thermal gradients. For temperature programmed GC-FID, without spatial thermal gradients, calculated retention times are mostly within 1% of measured values. For the FF-TG-GC-MS with a thermal gradient the simulation showed a deviation of the spatial thermal gradient from a linear to a nonlinear gradient, which could be confirmed by measuring the shape of the spatial gradient. The calculated retention times for the nonlinear gradient are also mostly within 1% of measured values. Calculated peak widths are smaller than measured peak widths by 10 to 15% in the case of the conventional GC-FID and by 30 to 50% for the FF-TG-GC-MS. The relation between the measured and calculated variances shows a linear correlation which can be used to correct the calculated variance and peak width. With this correction the difference for the peak widths is reduced to 4-10% for the conventional GC and below 10% for the majority of solutes with exceptions for early and late eluted n-alkanes (up to 25% difference).