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• W249197166 abstract "Analysis of volatile and semi-volatile organic compounds using field instruments is gaining popularity due to faster field screening, and sample preparation. A substantial reduction in the cost of sample analysis can be achieved if the field screening can be performed accurately. Although a number of field screening instruments such as gas chromatographs (GC) have emerged recently particularly for volatile organic chemicals measurement, there is still a lack of instruments versatile enough to analyse a wide range of organic compounds. This paper details the development and application of a novel field portable GC system equipped with a Surface Acoustic Wave (SAW) detector. Unlike other types of GC detectors, the SAW detector is a temperature controlled integrating mass detector with zero retention volume and the ability to operate with chromatography peak widths measured in milliseconds. An internal sample pump and Tenax trap collect analyte vapours entrained within ambient inlet air. Speciation is accomplished using a short, temperature ramped GC column selected for its ability to separate analytes of interest. Detecting the total amount of each analyte as it exits the column performs quantification. By focusing the effluent from the column onto a specific area within a surface acoustic wave resonance-field on the surface of a temperature controlled piezoelectric crystal, picogram sensitivity is achieved. Results from recent US EPA field validation studies using a GC/SAW are presented. Measurements of volatile compounds such as benzene, toluene and xylene (BTX) and semi-volatile Polychlorinated Biphenyl (PCB) in various matrices have been completed. The results of field tests have shown that it is possible to speciate and quantify a wide range of environmental pollutants in near real time (less than 10 seconds) with good precision. Utilising only 5 or 10 seconds to sample vapours containing these compounds, minimum detection levels are at parts per billion levels for volatiles and parts per trillion for semi-volatiles have been achieved. Early separations of field samples that are below the regulatory level from those that are above and therefore require laboratory validation with a GC/MS is the main goal of field screening methods. Application of field screening methods using a GC/SAW system can reduce the cost associated with environmental site characterisation and monitoring. Environmental Strategies for the 21 Century, Asia Pacific Conference, 8-10 April 1998, Singapore 2 SURFACE ACOUSTIC WAVE DETECTOR FOR GAS CHROMATOGRAPHY A large amount of research has been performed with chemical coatings applied to SAW crystals. A common approach is to expose an array of SAW crystals with different polymer coatings to the vapour to be characterised. In theory each polymer coating will adsorb the vapours differently and by comparing response patterns from the array of sensing crystals, identification can be accomplished. However, polymer coatings reduce the sensitivity of the SAW crystal and limit detection to nanogram levels. Further loss in sensitivity results because the vapour sample must be split between many sensing crystals. Polymer coatings are not highly specific and in general each coated crystal response overlaps the response of other crystals to some extent and in this case pattern recognition with non-orthogonal (over-lapping) responses is very difficult. A new type of SAW vapour detector with picogram sensitivity and which does not use polymer coatings has been developed [1]. The sensing crystal comprises a very high Q SAW resonator placed in contact with a small thermoelectric cooling element as depicted in Figure 1. The thermoelectric element provides the precise control of cooling needed for vapour adsorption and simultaneously the ability to clean the crystal using thermal desorption when needed. The focused SAW resonator sensing element provides part per billion sensitivity for volatile organics and part per trillion sensitivity for semi-volatile compounds The crystal operates by maintaining highly focused and resonant surface acoustic waves at 500 MHz on the face of a single crystal quartz chip. By focusing the vapour through a micro-nozzle as shown in Figure 2, femtogram sensitivity can be achieved. This result [2] is 1000 times lower than SAW crystals coated with polymers. Because the crystal is manufactured from single crystal quartz without polymer coatings, long term stability and precision is achieved over a wide temperature range. Hi Q Resonator Sensing Crystal Thermoelectric Heating and Cooling Element. Figure 1Thermoelectrically cooled SAW detector crystal. Environmental Strategies for the 21 Century, Asia Pacific Conference, 8-10 April 1998, Singapore 3 The SAW sensor only requires a low voltage power source and because it is non-ionic, does not require a radioactive ionisation source. The ability to detect compounds based upon their ability to absorb onto a cooled surface provides detection capabilities which can be extended to an indefinite analyte list without regard to analyte polarity or electronegativity. The uncoated SAW detector is only specific to vapour pressure. The specificity of the uncoated SAW detector is based upon the temperature of the crystal surface and the vapour pressure characteristics of the condensate itself. At a given crystal temperature only those analytes with dew points below the crystal temperature will condense and be detected. This provides a general method for separating volatile from non-volatile vapours based upon the selected operating temperature of the SAW crystal. GC/SAW FAST CHROMATOGRAPHY SYSTEM By combining SAW detectors with high speed temperature programmed chromatographic columns, specificity over a wide range of vapours at the part per billion level in near real time (10 seconds) has been achieved [3]. The GC/SAW offers the advantages of a low cost solid state detector and the specificity of a temperature programmed GC column. The major elements of a GC/SAW vapour detection system are shown in Figure 3. The analysis is performed in two steps corresponding to the two positions of the GC rotary valve. In the sample Figure 2GC/SAW nozzle interface showing interaction of column and acoustic cavity. Inlet filter, water trap, or membrane Loop Trap Capillary GC column SAW Detector" @default.
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• W249197166 date "1999-01-25" @default.
• W249197166 modified "2023-09-23" @default.
• W249197166 title "Real Time Environmental Screening of Air, Water and Soil Matrices Using a Field Portable GC / SAW System" @default.
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