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Methods of making columns electrically conductive
Resistive heating of flexible fused silica columns with metal cladding was first suggested by Lee in 198415. The first practical demonstration of this technology for GC analysis was by Hail and Yost16 who used a short section of a commercial aluminum clad fused silica column. A programmable DC power supply was used to regulate the voltage across the column. The power supply output was regulated through a 0-5V signal derived from a digital to analog board.
Philips and Jain painted fused silica columns with a thin layer of electrically conductive paint and regulated the output from a programmable DC power supply in a similar way17. Mechanical instability due to differences between the thermal expansion coefficients of the fused silica and the coatings caused rupture of the conductive layer. This was further accelerated by local hot and cold spots caused by uneven coating. Chromatographic efficiency was degraded and the analytical column was damaged. Mechanical stability was improved by Ehrmann et al18. They compared the use of a coaxial metal tube or collinear heater wire as an at-column heating element. Both approaches proved to be satisfactory, but the coaxial heater provided better retention time reproducibility. The tubular design allowed the use of an auxiliary sheath gas to even out heat distribution along the column. While this gas had a statistically significant advantageous effect, the benefit was too small to justify the additional instrumental complexity. A pulse width modulator operating at 100Hz was used to control the temperature.
An instrument using a coaxial heater is commercially available. The Flash-GC19 embodies a conventional 0.25mm id column, either 6 or 12 meters long, placed inside a precision-engineered metal tube. The tube can be heated up to 1200°C/min but was sold in 2001 for approx. £20 000 pounds.
Methods of sensing temperature
In the beginning, resistive heating was calibrated through an iterative process where the heating profile was changed until the desired normal paraffin separation was obtained17. This time consuming process was not very flexible and discouraged the changing of ramp rates. The actual column temperature and heating rates were also unknown. Thermocouples could be considered impractical for this application due to their relatively large thermal mass in comparison to the capillary column wall onto which they are to be connected. Even if small sensors were placed on the column, local cold spots would be caused and this may lead to inaccurate temperature measurements, possibly producing peak tailing due to local cooling.
Resistance measurement with superimposed AC signal
Philips measured the resistance of the column by superimposing a supposedly constant current 10kHz square wave on top of the DC heating current. The square wave was sampled at 20kHz and the amplitude of this voltage measurement was proportional to the resistance and hence the temperature of the column17. This circuit (Figure 4-2) was somewhat delicate and required empirical tuning to obtain good results. Very small signals of about 10 to 20mV had to be measured at high frequency. Electronic noise is definitely a problem in a laboratory environment with lots of electronic equipment. Despite the incorporated band pass filters, noise equal to 10°C can be discerned from the published graphic results.
Chapter 1 Introduction
1.1 Project history
1.2 Background
1.3 Comprehensive supercritical fluid and gas chromatography
1.4 Approach
1.5 Presentation and arrangement
Chapter 2 Fundamental principles of comprehensive multidimensional chromatography
2.1 Multidimensional techniques
2.2 Multidimensional chromatography
2.3 Comprehensive Multidimensional Chromatography
2.4 Orthogonality
2.5 Resolution in two-dimensional chromatograms
2.6 Peak capacity of comprehensive multidimensional systems
2.7 Sample dimensionality and Ordered Chromatograms:
2.8 Historical overview of multidimensional instrumentation
2.9 Chapter Conclusion
Chapter 3 Fast Gas Chromatography: Theoretical considerations
3.1 Introduction
3.2 Optimization of resolution for fast gas chromatography
3.3 Optimization of separation speed
3.4 Temperature programmed analysis
3.5 Achieving fast heating rates
3.6 Chapter conclusion
Chapter 4 Fast gas chromatography: Design, construction and evaluation
4.1 Introduction
4.2 Instrumentation
4.3 Results and discussion
4.4. Conclusions
Chapter 5 SFC: Theoretical considerations
5.1 Introduction
5.2 Supercritical fluids defined
5.3 Supercritical fluids as mobile phase in chromatography
5.4 Parameters affecting retention in SFC
5.5 Stationary phases used with SFC
5.6 Using phase ratio to reduce retention of oxygenates
5.7 Conclusions
Chapter 6 SFC: Demonstration of group separation
6.1 Introduction
6.2 Experimental
6.3 Results and discussion
6.4 Conclusions
Chapter 7 The Modulator: Background and literature survey
7.1 Introduction
7.2. The modulator
7.3 Stationary Phase Focusing
7.4 The sweeping arm thermal modulator
7.5 The Cryogenic Modulator
7.6 Diaphragm Valve Modulator
7.7 Non-mechanical modulators
7.8 Conclusion
Chapter 8 The modulator: Design, construction and characterization
8.1 Introduction
8.2 Suggested modulator designs
8.3 Experimental
8.4 Results and Discussion
8.5 Conclusions
Chapter 9 Demonstration of the comprehensive two-dimensional SFCxGC
9.1 Introduction
9.2 Experimental
9.3 Results and Discussion
9.4 Conclusions
Chapter 10 Conclusions
10.1 Group separation with SFC as a first separation dimension
10.2 Fast resistive heating for boiling point distribution in SFCxGC
10.3 The stopped flow pressure drop modulation interface
10.4 Advantages to SFCxGC
10.5 SFCxGC Applications
