Details for Sub-ppt Atmospheric Measurements Using PTV-GC-FID and realtime Digital Signal Processing

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Name:Sub-ppt Atmospheric Measurements Using PTV-GC-FID and realtime Digital Signal Processing
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J.B McQuaid, A.C Lewis and K.D Bartle. , Sub-ppt Atmospheric Measurements Using PTV-GC-FID and Real-Time Digital Signal Processing., Journal of High Resolution Chromatography:


Abstract


Automated on-line trace level measurements in the atmosphere have become possible over the past ten years due to developments in large volume injection technology and the increased availability of  non-cryogenic cooling devices using phenomena such as the Peltier effect. The analysis of hydrocarbon (HC) species in the atmosphere is a particularly challenging area where individual
ambient concentrations as low as 1 ppt may be encountered in clean tropical marine air. From measurements of hydrocarbon species taken in clean air it has been demonstrated that small but significant concentrations of some species remain constantly present. The presence of certain reactive alkene species even at levels below 5 ppt may have a significant impact on the oxidative capacity ofthe clean atmosphere. Whilst halogenated species have been routinely measured on-line at sub-ppt levels for several years, the analysis of HC species has been limited by the flame ionisation detector - the most popular detector for field measurements. The limiting parameter in using FID for trace determination is often the noise generated within the FID amplifier electronics rather than by the absolute detection limit of the FID itself.  The flame ionisation detector amplifier uses a high gain transimpedance stage which has three main sources of noise. The feedback resistor generates both Johnson and Shot noise. This noise passes through the output without amplification as the voltage gain of a transimpedance amplifier is essentially unity over the bandwidth of interest. The third source of noise is due to current noise generated within the amplifier input stages. This noise is subsequently amplified along with the signal. Determinations at very low concentrations using automatic peak height or area integration prove difficult to perform with reliability due to a combination of poor signal to noise and transient detector spikes. By increasing data acquisition sampling rates coupled to the application of a digital signal processing algorithm, the electrometer output can be bandwidth limited to a frequency which allows peak information to be accurately represented whilst substantially reducing noise of higher frequencies. The roll off rate of a digital filter is much greater than can be achieved using an analogue filter. A rejection of greater than 150 dB can be achieved with a pass band to stop band delta of a fraction of 1 Hz. The benefit of this is that chromatographic resolution and efficiency are not affected by the process, i.e. the peaks are not broadened. This communication reports a digital signal processing (DSP) unit tested in parallel to standard data capture system coupled to an automated trace analysis field GC instrument. The basic principle of DSP is to convert a dynamic analogue signal into discrete values by sampling at certain intervals, followed by the application of mathematical filter algorithms to remove any interference or noise that may be obscuring the desired signal. Once processed, the discrete signal may then be converted back to analogue if necessary. Comparison of data from both systems demonstrates a large enhancement in minimum detectable amount and improved integration reliability. Use of a DSP unit has resulted in a lowering of detection limits to allow automated sub-ppt measurements to be performed. Comparison of trace level analysis has shown that raw FID output logged straight to PC leads to a consistent overestimation of peak height of the order of 25% at low signal to noise levels. Post processing of chromatograms using enhancement in digital gain has also been performed. This has demonstrated that a significant amount of noise remaining following on-line processing is induced at the analogue input stage to PC data capture system.

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