Sensitivity and Response Time

A great feature of the proposed configuration is the possibility to select the overlay thickness and mode order to meet specific requirements in terms of sensitivity and response time. In addition, with commercially available and low cost spectrometers, a wavelength shift resolution of 0.05 nm is easily available, allowing sub-ppm chemical detection. 

More modern instrumental analysis has been used to improve sensitivity and response time. Shown in Figure 7 is a schematic diagram of an instrument that has been developed to measure NO and N02 (18). The detection scheme is based on the chemiluminescence produced when NO in the ambient air reacts with 1% ozone added as a reagent to the sampled airstream. 

Figure 14. Effect of CO on the hydrogen sensitivity and response time of a Pd/Si02 ( vlOOO X)/n-Si capacitor. Device kinetic response to 2% H2/96% N2 at RT. Curve 1, the device is not exposed to 4% CO/96% N2 and initial ambient is 100% O2 curve 2, the device is exposed to 4% CO/96% N2 prior to the initial ambient 100% O2 and curve 3) the initial ambient is 4% CO/96% l. ...

Figure 15. Effect of different I-layer and C-layer on the device sensitivity and response time to a) 1000 PPM H2 in air at 150°C. The device is a Pd/Top I-layer (Si02. AI2, O3, Ta2 O3, Si3 N4)/ Si02/p-Si capacitor and the initial ambient is air (after Ref. 16, 1984 IEEE) and b) 7 PPM, 25 PPM, and 70 PPM NH3 in air at 150 C, device used here is an ultra thin Pt-layer/Pd/Si02/p-Si capacitor (after Ref. 9, with permission).

Controllable film thickness (compromise between sensitivity and response time of the sensor system)... 

The analyte sensitivity and response time of a PVC film can also be markedly improved with addition of plasticizers such as tributyl phosphate (TBP) or tripropyl phosphate (TPP), since it is well established that the presence of plasticizer in the polymer-encapsulating medium increases gas diffusion rate by increasing the degree of polymer segmental mobility. It is... 

The response times of the three kinds of Na+ ISFETs are also within seconds of one another. The above-mentioned characteristics, such as linear response range, sensitivity and response time ate almost the same in PVC, Urushi and KP-13 matrix ISFETs. 

The permeation of He through quartz glass is exploited in He leak detection. Test leaks have many applications including the calibration of MSLD (Section 4.3) and the assessment of the sensitivity and response time of the system. 

Thin films, to attain enough sensitivity and response time, of oxide materials normally deposited on a substrate are typically used as gas sensors, owing to their surface conductivity variation following surface chemisorption [183,184], Surface adsorption on a Sn02 film deposited on alumina produces a sensitive and selective H2S gas sensor [185]. In addition, a number of perovskite-type compounds are being used as gas sensor materials because of their thermal and chemical stabilities. BaTi03, for example, is used as sensor for C02 [183],... 

White et al. synthesized nanometric La2Cu04 through three techniques auto-ignition, Pechini method, and coprecipitation (White et al., 2008). The NPs were used to fabricate sensing electrodes for NO, and the effect of electrode microstructure on the sensitivity and response time was studied. The response times of the sensors were exponentially dependent on electrode grain size. Sensors with fine-grained electrodes were able to produce a steady-state and consistent voltage at lower temperatures, which improved their response sensitivity. 

In summary, it may be concluded that optimal sensitivity and response time can be achieved by applying high enzyme activity in thin membranes. 

The radioanalytical chemist who is responsible for such monitoring selects or designs procedures that meet the client s specifications of sample type, list of radionuclides, measurement reliability and sensitivity, and response time. The analyst also considers the limits imposed by prescribed sample size, solution volume appropriate for chemical separation, and radiation detection instruments at hand. Potentially applicable procedures are selected for these criteria by literature review and then evaluated in a methods development and testing process. A chemical separation procedure can be devised either by selecting the most applicable published method and introducing any needed modifications or by combining pertinent separation steps. 

Practically all the radiation detection and monitoring instruments were replaced by improved circuits of more recent design. In particular, it was found that the vibrating-reed Monitrons, two of which can be seen in Fig. A2.G, are a decided improvement over the older designs in stability, sensitivity, and response time. 

The sensitivity and response time of a nanojunction-based glucose sensor is reported to be 1 nA/mM and the response time is less than 1 s. The features are essential for an in vivo device for real-time monitoring of glucose levels. 

The sensitivity and response time of thick-film sensors fabricated from M0O3, derived from three different preparatory techniques, to CO gas (14-lOOppm) at 450°C are compared in Fignre 2.11 (a as-prepared, b sample a reduced and oxidized in CO/ CO2 mixtnres and c sample a rednced in CO/CO2 mixture and oxidized in air). The response of the sensor towards CO is defined as the normalized variation of resistance of the n-type fihn, namely, (Rg - Rg)/Ro, where Rg is the steady-state resistance (identified as a time-independent plateau) in the background gas (in the present case, 10% 02-balance N2) and Rg is the steady-state resistance of the sensor film (identified again as a time-independent plateau) when exposed to the gas of interest (i.e. CO). The response time (tgo) is calcnlated as the time taken to reach 90% of the difference (Rg - Rg)/Rg. 

On the other hand, the film prepared from M0O3 powders, derived via precipitation technique, turned out to be the most promising in terms of sensitivity and response time. As can be seen from Figure 2.11, significant improvement in the sensitivity and the response time was observed in this case. The film subjected to redox treatment by CO/CO2 showed improved sensitivity compared to the untreated sample. However, the film reduced by CO/CO2 followed by air oxidation showed an increase of —40% in sensitivity and decrease of —300% in the response time at 100 ppm CO level. This is believed to be due to the unique microstructural artefacts developed in these films (Figure 2.10). Clearly, the third method resulted in sensor films with the most improved sensitivity to all levels of CO and particularly the lowest response time for 100ppm CO. 

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