Instant Notes: Analytical Chemistry

visible, ultraviolet and near infrared regions by absorbance, fluorescence or

reflectance. They have inherent advantages over electrochemical sensors in not

requiring an electrode system and providing valuable spectral information over

a range of wavelengths, but may suffer from ambient light interference, deple-

tion of immobilized reagents, and slow kinetics of the reactions between

analytes and reagents.

Thermal sensorsfor oxidizable gases such as carbon monoxide and methane

depend on measuring the change in resistance of a heated coil due to the heat of

reactionresulting from oxidation of the gas by adsorbed oxygen. Thermal

biosensors incorporating thermistors have been developed, which measure

heats of reaction of enzymes in the detection of urea, glucose, penicillin and

cholesterol.

Mass-sensitive devicesare based on piezoelectric quartz crystal resonators

covered with a gas-absorbing organic layer. Absorption of an analyte gas causes

a change in resonance frequency that can be detected by an oscillator circuit and

which is sensitive down to ppb levels.

Sensor arrays Individual sensors are often nonspecific, but selectivity (Topic A3) can be
improved by using groups or arraysof several sensors to monitor one or more
analytes using different instrumental parameters and/or different sensor
elements. Sensors in an array may be operated at different electrical potentials,
frequencies or optical wavelengths. A sensor array for the simultaneous moni-
toring of pH, sodium and potassium levels in body fluids can be constructed
from three ion-selective field effect transistors (Fig. 5), each with an appropriate
sensitivity to one of the three analytes.

Thermal- and
mass-sensitive
sensors

326 Section H – Sensors, automation and computing

Reflector

Sample solution

Light input Light output

Fig. 4. Optical sensor with Y-configuration cell. Reproduced from R. Kellner et al.,

Analytical Chemistry, 1998, with permission from Wiley-VCH.