Numerous researches have been performed with chemical coatings applied to SAW crystals. A typical method is to expose an array of SAW crystals with varied polymer coatings to the vapor to be characterized.
Theoretically, each polymer coating will adsorb the vapors in a different way and by comparing reaction patterns from the array of sensing crystals, identification can be done. However, polymer coatings decrease the sensitivity of the SAW crystal and restrict detection to nanogram levels. Additional loss in sensitivity happens because the vapor sample has to be divided between several sensing crystals. Polymer coatings are not very specific and generally each coated crystal response overlaps the response of other crystals to a certain extent and in this case pattern recognition with non-orthogonal (over-lapping) responses is very hard.
A new type of SAW vapor detector possessing picogram sensitivity and which does not use polymer coatings has been created . The sensing crystal contains a very high Q SAW resonator positioned in contact with a small thermoelectric cooling element as shown in Figure 1.
The thermoelectric element offers the precise control of cooling required for vapor adsorption and simultaneously the ability to clean the crystal using thermal desorption when required. The focused SAW resonator sensing element offers part-per-trillion sensitivity for semi-volatile compounds and part-per-billion sensitivity for volatile organics. The crystal functions by maintaining highly focused and resonant surface acoustic waves at 500 MHz on the face of a single crystal quartz chip. By aiming the vapor through a micro-nozzle as illustrated in Figure 2, femtogram sensitivity can be attained.
Figure 1. Thermoelectrically cooled SAW detector crystal
This result  is 1000 times lesser than SAW crystals coated with polymers. Since the crystal is produced from single crystal quartz without polymer coatings, both precision and long term stability are attained over a broad temperature range.
The SAW sensor just needs a low voltage power source and since it is non-ionic, does not need a radioactive ionization source. The ability to detect compounds according to their ability to absorb onto a cooled surface offers detection capabilities which can be stretched to an indefinite analyte list, irrespective of analyte polarity or electronegativity.
The uncoated SAW detector is only specific to vapor pressure. The specificity of the uncoated SAW detector is centered on the temperature of the crystal surface and the vapor pressure attributes of the condensate itself. At a specified crystal temperature, only those analytes with dew points lower than the crystal temperature will condense and then ultimately detected. This offers a general technique for dividing volatile from non-volatile vapors based upon the designated operating temperature of the SAW crystal.
Figure 2. GC/SAW nozzle interface showing interaction of column and acoustic cavity
GC/SAW Fast Chromatography System
By integrating SAW detectors with high speed temperature programmed chromatographic columns, specificity over a broad range of vapors at the part-per-billion level in near real time (<1 minute) has been realized . The GC/SAW provides the benefits of a cost-effective solid state detector and the specificity of a temperature programmed GC column.
Figure 3 shows the key elements of a GC/SAW vapor detection system. The analysis is done in two steps corresponding to the two positions a GC valve. In the sample position (shown) air (stack emissions) to be analyzed pass into an optional inlet filter and via a loop trap. The trap holds absorbent that is specific to the analyte of interest (e.g. Tenax). Choice of sample time and flow rate fixes the total quantity of airborne vapors collected in the loop trap.
The GC valve is rotated to its second position and the loop trap is quickly heated by a capacitive discharge which causes captured vapors to be moved to the GC column. Transfer is helped by a helium carrier gas and these vapors re-condense on the inlet of a chromatographic column contained primarily at low temperature.
A microprocessor then applies a linear temperature ramped heating program to the GC column. The column divides the injected compounds in time and, as they are eluted from the column, they condense on the SAW crystal and are detected as frequency varies.
Figure 3. Schematic of GC/SAW System showing major elements of the system
The speed of the analysis system is established by the analysis time and the sample time. Common analysis times can be less than 1 minute and sample times can be 1 to 5 seconds. Chromatographic peaks created are measured in milliseconds. The ability to detect short duration peaks is feasible because the SAW detector is an integrating GC detector which means it has zero dead volume. All other well-known GC detectors are differential and because of dead volume within the detector cannot function with millisecond duration chromatographic peaks.
The GC/SAW system can concurrently detect and quantify a number of chemical vapors within one environmental sample. A field portable system was designed to fit into a small suitcase. A laptop computer offers a completely integrated user interface in a Windows 95 operating environment. An internal microprocessor, a small helium gas tank, and a gate array controller are housed at the base of the suitcase.
Field testing of prototype systems, using chromatographic columns and Surface Acoustic Wave sensors, has showed the ability to detect a wide range of compounds such as explosives, drugs, polychlorinated biphenyl, volatile organics, and dioxins [4,5]. The Office of National Drug Control, the Department of Energy, and the U.S. Environmental Protection Agency (EPA-ETV) have validated the part-per-billion (picogram) sensitivity and field performance of the latest technology. Certification of the technology by the California Environmental Protection Agency is presently pending.
Test Results for Volatile Organics in Water
Recently, field testing of the GC/SAW was performed using over 30 different volatile organic analytes within water matrices. Figure 4 shows a chromatogram example of a water matrix having VOC analytes with both low and high concentrations. This figure shows retention time and concentration measured in Hertz (Hz) for 11 varied analytes.
Both chlorinated and BTEX solvents are represented in the list of analytes. The water concentration of all analytes with the exclusion of four special analytes - benzene, trichloroethene, 1,1 dichloroethane, and tetrachlorethene - was 100 ppb. The concentration of the four special analytes was 500 ppb for 1,1 DCA and benzene, 1000 ppb for PCE, and 1220 ppb for TCE. Performance of the tool was related to measurement RSD and precision.
Over the range analyzed (10-1000 ppb), RSD of four or more replicate samples was 10-20%. Precision over this range was dependent on analyte and also usually 20% or better. In certain cases, the occurrence of high spikes and other interference could disturb the measurement adversely. For screening operations, this was controlled by requiring a laboratory site characterization to define the analytes present before screening them with the GC/SAW.
Test Results for Semi-Volatile PCBS and Dioxins in Soil
Figure 4. Chromatogram showing VOC screening of Water matrix
For preliminary testing of a GC/SAW with furan, dioxin, and PCB standards, field testing of over 200 PCB contaminated soil samples was carried out as part of an EPA sponsored technology validation program (ETV). A more wide-ranging testing program is planned as part of the recommended stack emissions study.
The commercial GC/SAW presently manufactured by Amerasia was verified at the US Department of Energy’s Oak Ridge National Laboratory in Oak Ridge, Tennessee. The GC/SAW was calibrated with various aroclor standards as an integral part of new GC/SAW EPA field technique.
A common < 1 minute chromatogram acquired by exposing the system to a calibrated quantity of PCB 1260 Aroclor® in Figure 5. Picogram sensitivity permitted the use of a fast liquid extraction in hexane to be confirmed without the need for concentration of the extract. Regulatory levels for PCBs were realized in the field within 5 minutes using the new GC/SAW technique. Hence, for this compound, the scale factor was about 10 Hz/picogram. The GC/SAW had a noise floor of 10 Hz, and therefore the least detection level was 3 picograms (signal to noise ratio = 3).
Figure 5. Analysis of semi-volatile PCB compounds in soil
Similar results have been attained with furans and dioxins. Figure 7 shows a < 1 minute duration chromatogram of 2,3,7,8,Tetrachlorodibenzodioxin, and Figure 6 shows a five-point calibration curve. These results were achieved using direct thermal desorbtion of soil spiked with a dioxin standard solutions.
Figure 6. N point calibration for 2378 dioxin
Figure 7. Chromatogram showing screening for 2378 dioxin in less than 1 minute
Summary and Conclusions
Several conclusions were reached during field testing of GC/SAW systems with volatile land semi-volatile compounds in different soil and water matrices. Generally, the sensitivity to semi-volatile compounds was 1-10 parts-per-trillion (picograms), while the sensitivity of the GC/SAW to volatile compounds was 1-10 part-per-billion (nanograms).
As anticipated, lengthening sample time boosted sensitivity; however, these results were realized with a < 1 minute sample time. The tool worked well as a screening instrument which allowed a site to be speedily assessed and only significant samples collected for off-site laboratory validation. In fact, this excluded the testing of a large number of samples which did not have any analytes beyond the regulatory limit.
- United States Patent No. 5,289,715, Vapour Detection Apparatus and Method Using an Acoustic Interferometer.
- E. Staples, G. Watson, and W. Horton, “Spectral Density of Frequency Fluctuations in SAW Sensors,” 186th Meeting of the ElectroChemical Society, Miami Beach, Florida, October 9-14, 1994.
- Edward J. Staples and Gary W. Watson , “GC/SAW Non-Intrusive Inspection System”, White House Conference, Office of National Drug Control Policy, New Hampshire, October 1995.
- G.W. Watson and E.J. Staples, “SAW Resonators as Vapour Sensors,” Proceedings of the 1990 Ultrasonics Symposium, pp.311-314, 90CH2938-9
- G.W. Watson, W. Horton, and E.J. Staples, “GAS Chromatography Utilising SAW Sensors,” Proceedings of the 1991 Ultrasonics Symposium, pp.305-309.
This information has been sourced, reviewed and adapted from materials provided by Electronic Sensor Technology.
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