doi:10.1016/j.chroma.2006.11.016 
Copyright © 2006 Elsevier B.V. All rights reserved.
Rapid column heating method for subcritical water chromatography
Michael O. Fogwilla and Kevin B. Thurbide
, a, 
aDepartment of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada
Received 7 September 2006;
revised 1 November 2006;
accepted 6 November 2006.
Available online 28 November 2006.
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Abstract
A novel resistive heating method is presented for subcritical water chromatography (SWC) that provides higher column heating rates than those conventionally obtained from temperature-programmed gas chromatography (GC) convection ovens. Since the polarity of water reduces dramatically with increasing temperature, SWC employs column heating to achieve gradient elution. As such, the rate at which the mobile phase is heated directly impacts the magnitude of such gradients applied in SWC. Data from the current study demonstrate that the maximum column heating rate attainable in a typical SWC apparatus (i.e. using a GC convection oven) is around 10 °C/min, even at instrument oven settings of over three times this value. Conversely, by wrapping the separation column with ceramic insulation and a resistively heated wire, the column heating rates are increased five-fold. As a result, elution times can be greatly decreased in SWC employing gradients. Separations of standard alcohol test mixtures demonstrate that the retention time of the latest eluting component decreases by 35 to 50% using the prototype method. Additionally, solute retention times in this mode deviate by less than 1% RSD over several trials, which compares very well to those obtained using a conventional GC convection oven. Results suggest that the developed method can be a useful alternative heating technique in SWC.
Keywords: Gradient elution; Liquid chromatography; Resistive heating; Subcritical water; Temperature programming
Fig. 1. Schematic diagram of the SWC-FID system.
Fig. 2. SWC separation of alcohols using GC oven heating at programmed rates of 16 (upper) and 32 °C/min (lower). The temperature program for each was 70 °C initial until injection, then increasing at the posted rate to 130 °C final. Overlaid are the observed oven temperature (○) and the actual column temperature (□). The elution order is methanol, ethanol, 2-propanol, 1-propanol, 2-butanol, and 1-butanol.
Fig. 3. Temperature inside the GC oven (●) and the separation column (■) using conventional heating at programmed rate set values of 5 °C/min (upper) and 32 °C/min (lower). Overlaid on these plots is the temperature inside the separation column when using resistive heating (○) set to the same rates. The temperature program is 70 °C initial until injection, then ascending at the posted rate to 130 °C final.
Fig. 4. SWC separation of alcohols using resistive heating at a rate of 54 °C/min (solid line) and conventional oven heating at a set rate of 32 °C/min (dashed line). Other details are as in Fig. 2.
Fig. 5. SWC separation of an n-alcohol mixture using three different heating modes: a conventional GC oven set at 32 °C/min (top), a conventional GC oven in a ballistic heating mode (middle), and resistive heating at a rate of 57 °C/min (bottom). The temperature program is 70 °C initial for one minute, then ascending at the posted rate to 160 °C final. The order of elution is methanol, ethanol, propanol, butanol and pentanol.
Fig. 6. SWC separation of an n-alcohol mixture using aggressive heating to the largest final temperature attainable. Both the GC oven heating (upper) and resistive heating (lower) modes used their fastest possible heating rates in a program that started at 70 °C for 2 min and then ascended to 200 °C final. The order of elution is the same as in Fig. 5.
Table 1.
Retention times of n-alcohols using various conventional GC oven heating ratesa
a The temperature program used for each rate trial was 70 °C initial until injection and then to 130 °C final.
b Describes a program where the final temperature is input after injection and the oven is immediately allowed to heat to that value.
Table 2.
Set values and actual column heating rates obtained when using a GC oven
Table 3.
Column heating rates obtained by resistive heating
a Notation describes the transformer settings employed, where 13% → 25% (210 s) → 19% denotes a separation started at 13%, moved to 25% upon injection, and returned to 19% after 210 s elapsed.
Table 4.
Retention times of n-alcohols using various resistive heating ratesa
a The temperature program used for each rate trial was 70 °C initial until injection and then to 130 °C final.
Table 5.
Reproducibility of n-alcohol retention times using various heating methodsa
a The temperature program used for each trial was 70 °C for 1 min, then heated to 160 °C final by the specified method.
b n = 5.
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