Vapor concentration, detection sensitivity

Both sensors also showed high sensitivity to N0X (NO + N02) content in air. N0X with a concentration of 10 ppm in air is readily detected by both SnOx and PdAu/SnOx sensors. While a PdAu deposit on tin oxide enhances alcohol vapor and propylene sensitivity, it depresses the sensitivity to N0X. 

The interference to the hydrogen detection of C-I-S structures caused by varying amounts of water vapor is also summarized in Table II. As seen in that table, high concentrations of H2O vapor lower the sensitivity of Pd/SiOx/Si diodes whereas water vapor, in general, lowers the sensitivity of Pd/TiOx/Si diodes at room temperature. 

Occasionally, one needs to measure vapor concentrations that are below the detection limit of the sensor. In these situations, enrichment of the vapor concentration can provide substantial increases in the sensor s apparent sensitivity. Vapor-enrichment schemes for sensors based on sorbent trapping and thermal... 

When considering toxicity manifestation time and vapor dispersion, the detection sensitivity for vapor concentration is one hundredth of LCtso within 1 min. In the case of GB, this required detection sensitivity is 0.15 mg/m, and at this level there is no odor and humans show no signs of toxicity. In the chemical weapon disposal situation, because the workers stay in one place for a long time, the time weighted average (TWA) values are the monitoring target for allowed operational conditions. These TWA values are approximately 1/100,000 of LCtso. The desired alarm time is to be less than several minutes. There is a trade-off relationship between LOD, alarm time, detection accuracy and... 

The sensitivity of these sensors was defined as a signal change upon exposure to the known concentrations of vapors. Sensitivity of the 2.8-nm CdSe nanocrystals was 0.8 PL counts/Torr of methanol with almost no detectable sensitivity to toluene. The sensitivity of the 5.6-nm CdSe nanocrystals was 2.9 PL counts/Torr of methanol and 8.8PL counts/Torr of toluene. Although this environmental sensitivity was compatible with earlier reported sensors based on polished or etched bulk CdSe semiconductor crystals3940 and polymer-nanocrystals composites,16 the sensor reported here had a more selective response to polar and nonpolar vapors due to the multiwavelength PL from different-size nanocrystals incorporated into the polymer film. The response and recovery kinetics of PL from the 2.8-nm nanocrystals in PMM A upon exposure to methanol were very fast (<0.5 min). However, 5.6-nm nanocrystals in the same sensor film exhibited a much longer response and recovery times upon interactions with methanol, 4 and 20min, respectively. The 5.6-nm nanocrystals had 4-min response and 0.5-min recovery times upon interactions with toluene. 

A reversible optical waveguide sensor for ammonia vapor was introduced more recently [137], consisting of a small capillary glass tube fltted with a LED and a phototransistor detector to form a multiple reflecting optical device. When the capillary was coated with a thin solid film composed of a pH-sensitive oxazine dye, the instrument was capable of reversibly sensing ammonia. Vapor concentrations from 100 to below 60 ppm were easily and reproducibly detected. A preliminary qualitative kinetic model was proposed to describe the vapor-film interactions. 

It also may be observed that the RFS technique is very sensitive. Just a few ppm of particles in the oil give a big response by the spectrometer. This is due in part to the RFS filtration process concentrating particles on the disc surface from a relatively large oil volume for subsequent vaporization and detection. 

The solution headspace approach is applicable to a much wider range of samples than the solid approach. When working with sample solutions, headspace equilibrium is more readily attained and the calibration procedure is simplified. The sensitivity of the solution method depends upon the vapor pressure of the constituent to be analysed and its solubility in the solvent phase. Vinyl chloride, butadiene, and acrylonitrile, are readily transferred from polymer solutions into the headspace by heating to 90 °C. The headspace/solution partitioning for these constituents is not appreciably affected by changes in the solvent phase (namely, addition of water) since the more volatile materials favonr the headspace at 90 °C. Less volatile monomers such as styrene (bp = 145 "C) and 2-ethylhexyl acrylate (bp = 214 °C) may not be determined using headspace techniques with the same sensitivities realised for the more volatile monomers. By altering the composition of the solvent phase to decrease the monomer solubility, the equilibrium monomer concentration in the headspace can be increased. This resulted in a dramatic increase in the detection sensitivity for styrene and 2-ethylhexyl acrylate. 

EC sensors are relatively sensitive, as they react to chemical vapor concentrations at the low parts-per-million level. However, EC sensors are not as selective as colorimetric detectors (see Chapter 10). They may respond to various chemicals simultaneously without differentiation capability. This is because the oxidation-reduction reaction between the chemicals in the sample and the electrolyte controls the detection. Any chemicals contained in the sample that will react with the electrolyte on the working electrode surface will generate electrical current and are detected together with those from targeted chemicals. Using a chemical filter may reduce or eliminate some of the chemical interference potential. The nse of a second working electrode that responds to different sets of chemicals from the first working electrode within the same sensor may also lead to better selectivity. 

The detector has an extremely important role in the overall process ofGC analysis. The current popularity and success of GC as an analytical method is attributable in great part to the early development of highly sensitive and reliable means of detection. In sensing the vapor concentration at the column outlet, the detector provides information on the distribution of individual peaks within a chromatogram (which compound ) as well as their relative amounts (how much ). The area measured under a chromatographic peak is generally related to the quality of the compound. 

Corrosion products and deposits. All sulfate reducers produce metal sulfides as corrosion products. Sulfide usually lines pits or is entrapped in material just above the pit surface. When freshly corroded surfaces are exposed to hydrochloric acid, the rotten-egg odor of hydrogen sulfide is easily detected. Rapid, spontaneous decomposition of metal sulfides occurs after sample removal, as water vapor in the air adsorbs onto metal surfaces and reacts with the metal sulfide. The metal sulfides are slowly converted to hydrogen sulfide gas, eventually removing all traces of sulfide (Fig. 6.11). Therefore, only freshly corroded surfaces contain appreciable sulfide. More sensitive spot tests using sodium azide are often successful at detecting metal sulfides at very low concentrations on surfaces. 

An important demonstrated application of this artificial nose system is the high-speed detection of low levels of explosives and explosive-like vapors. Several sensors, based on Nile Red attached to silica microspheres, show high sensitivity to nitroaromatic compounds (NAC) within a mixture12. Different fluorescence response profiles were observed for several NAC s, such as 1,3,5-trinitrotoluene (TNT) and 1,3-dinitrobenzene (DNB), despite their similar structures. These responses were monitored at low concentrations of the NAC vapors (ca. 5 ppb) and at short vapor exposure... 

---