Enzyme Assays - Measured by changes in solution conductivity
Simple, low cost, reliable
EASY TO BUILD
Applicable for blood & urine clinical tests, industrial, school & college projects etc.
The instrument employs a square wave (27kHz) AC bridge with two electrochemical cells, a sample & a reference. This allows differential operation with increased sensitivity & stability.
Measures in the micromolar range.
The temperature is controlled to 0.02 deg C. COST ca. £200
The site is constructed with long pages. Links are underlined in blue. For sub-pages hover to the RHS of Menu items eg. to RHS of HOME is the sub-page Applications
News & Updates will be found further down this Page
This site gives a clear explanation as to the design construction and use of
the conductivity apparatus
The Menu items describe the range of content (remember the sub-pages!)
RESULTS describes kinetic measurements with urease &
The Basic Introduction can be skipped by those who are familiar with
solution conductivity/enzyme assays.
NEWS & RECENT ADDITIONS
An important addition is the section on low cost electrodes, constructed using pencil leads. These DIY electrodes allow construction of simple easy to make electrochemical cells.
The original design for the bridge used a long-tailed pair differential amplifier to extract signal. This has been replaced with a 741 OPAMP which gives better common mode signal rejection [important since the input square wave has a transient on the rising edge] & lower noise.
The null voltage at balance is not zero but about 25 mV, owing to carbon electrode polarisation. Previously the value was higher but has been reduced by increasing the square wave frequency from 10 kHz to 27 kHz. This decreases the time available for the electrodes to polarise on each cycle and so decreases the impact of polarisation.
Following on from detailed studies of electrode polarisation the perfformance of the instrument has been re-assessed. Guide-lines have been evolved that allow conductivity measurements to be made over a 100,000- fold range with a single set of electrodes.
The energy efficiency of the thermostat system could be improved by perhaps 20-30% if pulsed width modulated (PWM) heating was used in place of continuously variable heating. This is currently being tested but has, as yet, not proved able to control the temperature as accurately as the system currently employed.
At present when the the apparatus thermostat is set to 25 deg C the system can be used at a temperature up to about 23 deg C. Higher than that and the system starts to drift. The solutions to this are :
Larger version can be found under