Principles of Enzyme Thermistor Systems: Applications to Biomedical and Other Measurements - Advances in Biochemical Engineering Biotechnology - page 5

turn in the cycle adds to the overall enthalpy change [15]. A later section pro-
vides further details on chemical and enzymatic amplification.
An inherent disadvantage of calorimetry is the lack of specificity: All enthalpy
changes in the reaction mixture contribute to the final measurement. It is there-
fore essential to avoid nonspecific enthalpy changes due to dilution or solvation
effects. In most cases this is not a serious problem. An efficient way of coping
with nonspecific effects in a differential determination is the incorporation of a
reference column with an inactive filling [16].
The flow injection technique is usually employed for an ET assay. The sample
volumes employed are too small to produce a thermal steady state, but generate
a temperature peak. This is traced by a recorder. The peak height of the thermo-
metric recording is proportional to the enthalpy change corresponding to a speci-
fic substrate concentration. In most instances, the area under the peak and the
ascending slope of the peak have also been found to vary linearly with the sub-
strate concentration [17]. A sample introduction of sufficient duration (several
minutes) leads to a thermal steady state resulting in a temperature change, pro-
portional to the enthalpy change up to a certain substrate concentration [62].

1.3
The Transducer

The instrumentation for fabrication of the ET normally employs a thermistor as
a temperature transducer. Thermistors are resistors with a very high negative
temperature coefficient of resistance. These resistors are ceramic semiconduc-
tors, made by sintering mixtures of metal oxides from manganese, nickel, cobalt,
copper, iron and uranium. They can be obtained from the manufacturers in
many different configurations, sizes (down to 0.1-0.3 mm beads) and with
varying resistance values The best empirical expression to date describing the
resistance-temperature relationship is the Steinhart-Hart equation:
1/T = A + B (ln R) + C (ln R) 3
where T = temperature (K); ln R = the natural logarithm of the resitance, and A,
B and C are derived coefficients. For narrow temperature ranges the above rela-
tionship can be approximated by the equation:

R T =R To e b (1/T-1/To)
where R T and R To are the zero-power resistances at the absolute temperatures
T and T 0 ,respectively,and

is a material constant that ranges between 4000
and 5000 K for most thermistor materials. This yields a temperature coefficient
of resistance between -3 and -5.7% per °C. In our ET devices resistances of
2-100 k

b

have been used. Other temperature transducers employed in enzyme
calorimetric analyzers include Peltier elements, Darlington transistors, and
thermopiles. Of these, the thermistor is the most sensitive of the common
temperature transducers.

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