Short Communications: Short technical description of the MonA and PotLab colorimeters

The MonA and PotLab instruments represent the upper and lower limits of a range of rugged colorimeters designed for use in developing countries based on the concept of using a light emitting diode (LED) as an alternative to the tungsten lamp and filter of the conventional colorimeter. The green LED with a maximum emission around 565 nm and a bandwidth of 30 nm was chosen because it is the only colour applicable to a useful range of clinical chemical tests. Initial testing of LEDs revealed that the light output varies with time until thermal equilibrium is achieved. This problem, coupled with the requirement to keep battery consumption as low as possible, was solved by pulsed operation of the LED.

The MonA and PotLab instruments represent the upper and lower limits of a range of rugged colorimeters designed for use in developing countries based on the concept of using a light emitting diode (LED) as an alternative to the tungsten lamp and filter of the conventional colorimeter.The green LED with a maximum emission around 565 nm and a band- width of 30 nm was chosen because it is the only colour applicable to a useful range of clinical chemical tests.
Initial testing of LEDs revealed that the light output varies with time until thermal equilibrium is achieved.This problem, coupled with the requirement to keep battery consumption as low as possible, was solved by pulsed operation of the LED.

Mona
Light from a single LED, pulsed for a nominal 14 msec, reaches two silicon photodiodes which are connected to the transconductance amplifiers.One photocell 'sees' the LED directly (reference) and the other via the cuvette containing the sample to be measured.The 'zero' control sets the two output voltages to be equal when the sample is reckoned as 100% T, i.e. as reagent blank.
The two amplifiers are connected to what are effectively two sample and hold circuits except that one (the reference) has a controlled exponential droop rate.For the first 10 msec (nominal) of the 'pulsed' time (to to 1) these circuits are in the 'sample' mode and acquire their respective amplifier voltages.The 'sample' gates are opened at time l, and the 'reference' side decays exponentially with the pre-set time-constant.At some point in time (t2) the 'reference' voltage reaches the same level as the 'sample' voltage and this causes a comparator to change its output (hi-to-lo).
The time period from to t2 is a measure, to some scale, of the log of the ratio of the two signal voltages which correspond to the 'reference' and 'sample' light levels seen by the two photocells.The function of the digital circuitry is to measure and display this time which is shown as the number of clock pulses in this interval.Using a simple series of logic gates, an accurately controlled clock and a digital counter, the result is finally displayed on a conventional 3-digit segment LED display.This logarithmic circuit provides good accuracy with economy in components and is very stable to ambient temperature changes up to modest values of absorbance.
While the energising LED is 'on' the display is held 'off', thus maintaining the current drain approximately constant, as each takes about 40 mA from the supply.The LED pulse time is made longer than the acquisition time of the sample- and-hold circuits to eliminate interference due to the switch- off of the flash.
The sequence, commences immediately the circuit is powered by pressing the 'READ' button, and repeats at sec (nominal) intervals until the button is released.The 'counting' only occupies a small proportion of sec, so that the display appears 'fixed' to the operator, updating at sec intervals.A simple timer/processor provides the 10 and 14 msec pulses which control the logic, pulse the LED, reset the counter, and the updating.By suitable selection of the time constant and clock frequency, any appropriate scaling factor can be set.Instrument calibration at two points on the scale is provided by selecting, with push-buttons, a precision resistor attenr uator at the output of the 'test' amplifier-an electrical equivalent of optical absorbance.
The electronic circuits operate with + 3 volt lines derived by a series stabilizer from a single 9 volt battery which can decay to 6.5 V minimum.If each reading occupies only a few seconds, a few thousand readings are possible a probable battery life of some months under typical conditions.
No power is consumed at other times and this mode of operation, without any warming-up period, ensures minimum battery consumption and obviates leaving the battery .'on'accidentally.

PotLab
PotLab represents an attempt to produce a minimum cost haemoglobinometer for use with the haemoglobincyanide method recommended by the International Committee for Standardization in Haematology (ICSH).
The green LED is retained as the light source but it is operated in the continuous, mode at somewhat reduced current, about 20 mA.Coupled with the more sensitive photo-detectors, adequate sensitivity is obtained with low power consumption.Hermetically sealed cadmium sulphide light-dependent resistors (CdS) are used.These devices are more sensitive and somewhat cheaper than silicon photodiodes and can operate with simpler circuitry.At constant voltage the CdS cell produces a current which is a function of ilium- ation with a log current change 'gamma' log illumination change which is usually about 0.8 to 0.9 but which unfortunately is variable from sample to sample.The cells are also more temperature sensitive than silicon detectors.Their peak spectral sensitivity is advantageously near to the emission of the green LED.
While a direct reading instrument is preferable, the cost of using either a rugged sensitive tropicalised meter-indicator or a digital display is relatively high.A null indicator offers the cheapest solution and in the PotLab a pair of LEDs is used to indicate the.direction in which, a calibrated potentiometer control must be rotated to achieve balance.
As the name suggests, PotLab uses a potentiometer method of measurement based on two fight dependent resistors in a bridge circuit.A section of a quad amplifier integrated circuit provides the drive to the d.c.null indicators and readings are taken from the calibrated potentiometer (scaled in haemoglobin, g/100 ml).This, with fixed resistors, and the two light-dependent resistors (test and reference), comprises the bridge network.The output of the bridge feeds the comparator which drives a 'long tailed pair' of transistors, so illuminating one or other LED null indicator.
The remaining section of the quad amplifier is concerned with control of the current through the LED.
A ten-turn potentiometer (set calibration control)is in series with the calibrated potentiometer and is intended to be in mid position for a cell of average gamma with the standard calibration scale.The present potentiometer will take up moderate gamma changes from cell to cell but the degree of error has not been assessed.The circuit functions with supply voltages from 5 to 18 222 The Journal of Automatic Chemistry Pocock & Rideout MonA and PotLab colotimeters volts, normal operation being from a 9 V internal battery with the same economical 'press to read' power 'on' button.
The effect of non-linearity of the calibration scale is minimised by its generous length, over 255 mm, and the expansion of the scale divisions in the more critical lower range.Zero adjustment is by mechanical microadjustment which moves the reference cell from the side of the LED.

Background
The objects of the trial were to field-test, in a variety of climatic conditions, two designs of solid state colorimeter calibrated directly in grams per d haemoglobin and using a haemoglobinocyanide method for total haemoglobin in blood (based on the recommendations of the International Committee for Standardisation in Haematology) [1] and the Swizzlestick technique 2 ].
It was originally intended that the trial would take place in two phases.Three digital MonA (Mark 1)colorimeters and three PotLab colorimeters, with the necessary reagents, standards, controls and protocols would be distributed by WHO, Geneva; two to laboratories in South East Asia, two to the Western Pacific and two to the Eastern Mediterranean.
The recipients of the PotLab colorimeters were to carry out haemoglobin estimations only; the recipients of the MonA colorimeters were to carry out total protein and albumin estimations in addition to haemoglobin.When these evaluations had been carried out and the exercises completed, the instruments were to be returned to the Clinical Research Centre (CRC) for checking.The exercises would then be repeated, but the laboratories which originally had a MonA colorimeter would receive a PotLab and vice versa.
In fact the trial was terminated at the end of the first phase partly because the time scale for the exercise became protracted but primarily because sufficient information was forthcoming to make the second phase unnecessary.