Sensor cell

The detector responds to an average value of the total amount of solute in the sensor cell. In the extreme, the sensor volume or cell could be large enough to hold two... 

This extreme condition rarely happens but serious peak distortion and loss of resolution can still result. This is particularly so if the sensor volume is of the same order of magnitude as the peak volume. The problem can be particularly severe when open tubular columns and columns of small diameter are being used. Scott and Kucera measured the effective sensor cell volume on peak shape and their results are shown in Figure 13. 

Extra-column dispersion can arise in the sample valve, unions, frits, connecting tubing, and the sensor cell of the detector. The maximum sample volume, i.e., that volume that contributes less than 10% to the column variance, is determined by the type of column, dimensions of the column and the chromatographic characteristics of the solute. In practice, the majority of the permitted extra-column dispersion should... 

A low volume (0.2 pi) Valeo sample valve was employed with one end of the open tube connected directly to the valve and the other connected directly to the sensor cell of the detector. The UV detector was the LC 85B manufactured by Perkin Elmer, and specially designed to provide low dispersion with a sensor volume of about 1.4 pi. The total variance due to extra-column dispersion was maintained at... 

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

Figure 7.5. Feedback diagram for skeletal mechanical regulation. When forces are applied to a whole bone, the stimulus that results is sensed by the bone cells in the tissue. The sensor cells then signal bone-forming and -removing cells to change the geometry and material properties of the bone.

Figure 3.21 — (A) Integrated FET with two hydrogen ion-sensitive FET elements. (B) Structure of enzyme-modified FET sensor S plastic card FET enzyme-modified FET chip lUM, immobilized urease membrane. (C) Flowthrough cell Bl fixed sensor cell block B2 movable sensor cell block SC flowthrough cell EC electrical connector RP silicone rubber sheet AMP amplifier. (Reproduced from [151] with permission of Elsevier Science Publishers).

Bifurcated optical fiber Sensor cell Ion-sensing beods... 

Microbial BOD-sensor Cell-biosensor Mycelia-biosensor ... 

Electrical conductivity detector is commonly use. The sensor of the electrical conductivity detector is the simplest of all the detector sensors and consists of only two electrodes situated in a suitable flow cell. The sensor consists of two electrodes sealed into a glass flow cell. In the electric circuit, the two electrodes are arranged to be the impedance component in one arm of a Wheatstone bridge. When ions move into the sensor cell, the electrical impedance between the electrodes changes and the out of balance signal from the bridge is fed to a suitable electronic circuit. The out of balance signal is not inherently linearly related to the ion... 

Recent Developments in Electrochemical Solid Polymer Electrolyte Sensor Cells for Measuring Carbon Monoxide and Oxides of Nitrogen... 

Electrochemical gas detection instruments have been developed which use a hydrated solid polymer electrolyte sensor cell to measure the concentration of specific gases, such as CO, in ambient air. These instruments are a spin-off of GE aerospace fuel cell technology. Since no liquid electrolyte is used, time-related problems associated with liquid electrolytes such as corrosion or containment are avoided. This paper describes the technical characteristics of the hydrated SPE cell as well as recent developments made to further improve the performance and extend the scope of applications. These recent advances include development of NO and NO2 sensor cells, and cells in which the air sample is transported by diffusion rather than a pump mechanism. 

Sensor Cell Operating Mode. The simplest method of sensor operation is as a galvanic cell, whereby the sensor acts as a fuel cell and generates a current proportional to the gas concentration to be detected (1 ). However, when detecting certain species in air, it is difficult to obtain a counter-reference electrode in an acid system that will maintain the sensing electrode at a predetermined potential of approximately 1.0 V, to minimize interference. Counter-reference electrodes such as Pt/air (Op) or noble metal/ noble metal oxide structures have rest potentials in the 1.0 to... 

Potentiostatic Circuit. The electrical circuit used for breadboard testing of three-electrode sensor cells is shown in Figure 2. Amplifier U1 sensed the voltage between the reference and... 

Figure 1. Sensor cell assembly 1, reservoir housing 2, cap 3, support plate 4, M E assembly 5, base plate 6, gasket 1, contact pin 8, thermistor 9, nylon screw 10, Teflon tape 11, gasket 12, gasket 13, counter electrode 14, sensor electrode 15, reference electrode 16, thermistor.

The circuitry used for the breadboard testing of NO and NOp sensor cells was very similar to that shown in Figure 2 only the applied potential was changed. An applied potential of +1.30 V versus the SHE reference electrode was used for NO oxidation while a potential of 0.75 V versus the same reference electrode was used for N02 reduction. Current measurements were again made by measuring the voltage drop across resistor RA. Three electrode systems were used for both gases. 

Current-Voltage Characteristics of Platinoid and Graphite Electrodes in Air. The SPE sensor cells have been optimized for detection of certain oxidizable or reducible species in air. To achieve this, the sensing electrode is maintained at a voltage when there is minimal interference by the O2 in the air. The two electrodes studied in detail have been platinoid and graphite sensing electrodes. 

Figure 3. CO sensor cell voltage-current relationships (O) air, ( Z ) 40 ppm CO...

Linearity of Response and Reaction Products. The response vs. concentration curves obtained for CO, NO and NO2 gas sensor cells are depicted in Figures 5, 6 and 7 respectively. In all instances good linearity over the range studied was observed between current and partial pressure of each of the above gases (as depicted by equation (k)). The proportionality constants, K, with standard de-... 

The observed sensor cell response linearity with standard deviation makes a one-point calibration of a detector possible for the concentration ranges indicated. 

Experiments were also conducted to measure NO oxidation products and aid in identification of the reaction occurring at the electrode surface. Using an NO2 sensor cell in series with an N0 sensor cell, 70-80 ppm NO2 were detected in the exhaust of an N0 sensor cell which had been exposed to 2k0 ppm N0. 

Two N0 sensor cells in series were used to measure n, the number of electrons involved in the oxidation reaction. The following relationship was derived from Fick s Law, and was used to calculate n ... 

Flow rates were varied from 60 to 5 cc/min. The N0 concentrations which were used varied from 2 3 ppm at low flow rates to 11.9 ppm at higher flow rates, Values of n at higher (k0 - 60 cc/min) flow rates were found to be independent of N0 concentration. Due to the low sensor cell currents measured at low flow rates, N0 concentrations were held constant. 

The detection of NO2 in the exhaust of an N0 sensor cell verifies this reaction. 

Response Time. The response-time curve for oxidation of CO with an SPE sensor cell having a platinoid sensing electrode is shown in Figure 8. Similar curves for the oxidation of N0 and reduction of N02 with an SPE cell having a graphite sensing electrode are also shown in Figure 8. All measurements were made at 25°C at gas flow of 60 cc/min. The current-time response can be estimated from the relationship... 

---