VODS - Versatile Optical Detection System
Similar to Chameleon, VODS consists of a circular baseboard, to which source and detector PCB's are soldered. Once installed, the finished assembly is mounted into a delrin housing, as shown at left. A standard 1cm sample cuvette then mounts between the source and detector boards, while a mini-DIN connector brings power, ground and detector signals to the outside world. The system differs from Chameleon in that no signal processing is provided on-board; instead, VODS is designed for inclusion as part of a larger system (for example, the flow injection analyzer described elsewhere).
The source PCB has a light emitting diode and compensation circuitry designed to minimize LED intensity changes with temperature. This minimizes drift and ensures that absorbances as small as a few tenths of a milliAbs (10-3) can be reliably measured. This ability to measure small absorbances is critical to achieving low detection limits with this instrumentation.
The detector PCB supports two different detector options. A three pin light-to-frequency converter, for example a TSL235 (AustriaMicrosystems), mounted in an in-line position is installed when colorimetric detection is desired, whereas a TSL25x can be mounted in the 90 degree position for fluorometric detection.
VODS Detector Board
This uses a 3-terminal detector IC, such as the TSL235 light-to-frequency converter, or the TSL257 light-to-voltage converter.
The detector PCB has three external connections : V+, Gnd, and Signal out. Vdd for the detector is generated by an LM317 voltage regulator, with a 390 or 720 ohm resistor setting Vdd to 3.3V, or 5V, respectively. Capacitors near the Vdd pin are included to minimise ripple on the power supply rail.
The output signal (either a digital pulse train or an analog voltage) is intended to be processed by an external circuit.
Temperature Dependence of an LED’s Output Intensity
Even though an LED might be operated at constant current, its output intensity still depends on temperature. This is demonstrated in the experiment described here. An LM317 current source drives approximately 22 mA through a high brightness yellow LED. This LED is mounted in an aluminium block that can be heated by applying current to a bank of power resistors.
The temperature of this block is monitored by accurately measuring the resistance of an embedded Pt100 element that is connected in series with a high accuracy 100 ohm resistor (0.005% tolerance). By measuring voltage drops across each resistor and applying Ohm’s law, the Pt resistance can be converted into a temperature using the Callendar van Dusen equation. While these measurements are being performed, the current through the LED, the LED’s forward voltage drop VF and its output intensity are also measured, the latter using a TSL235 light-to-frequency converter.
The data acquisition system in these measurements employed a Linear Technology LTC2448 evaluation board (model #845A) that has 8 differential analog input channels (4 are used here), interfaced to a Propeller activity board. As usual, the system is controlled from a LabVIEWTM front panel. A description of a data acquisition system based on Linear Technology evaluation boards can be found here.
The results of the experiment are seen in the four traces below. At the start of the experiment we are in a cool-down phase (top left trace). When the temperature reaches 30C, the heater is powered up and the temperature of the block (and LED) then rises rapidly (red trace), with an accompanying drop observed in both VF (green trace) and the LED’s output intensity (blue trace). Once the temperature has reached ~57C the block heater is turned off and all three traces slowly return to ambient temperature values. Meanwhile, the black trace shows that the LED current has been controlled to better than 1 part in 104 during the entire experiment. Clearly obvious is the high degree of correlation between VF and the LED intensity , and their anti-correlation with temperature.
The change in LED output with temperature measured here is -0.83%/C. Although the overall temperature change induced in this experiment is far more than would be encountered in any typical laboratory setting, the variation in LED intensity even over just a few degrees would make measurements of small absorbances impossible without temperature compensation. Several strategies are available; one will be described in the next section.
Operation of LED's in Temperature Stabilized Mode
Measurement of extremely low concentrations by single beam colorimetry requires an extremely stable light source. This is obvious by considering the form of the Beer-Lambert Law
Aλ = ελcl = log10(I0/I)
The quantity known as absorbance, A has both a chemical and a physical definition. While absorbance depends proportionally on the analyte concentration c it is measured experimentally by taking the log10 of the intensity ratio (I0/I) - here I0 is the intensity of a beam without analyte present and I is the intensity in the presence of analyte. In the limit as c approaches zero, i.e infinite dilution - the intensity I goes to I0.
In a single beam instrument, let us assume that I0 is initially measured and that there are small source fluctuations thereafter. These intensity fluctuations translate into small absorbance fluctuations that effectively set the instrumental detection limit when trying to measure a very dilute analyte.
Clearly, there is considerable merit in finding ways to minimise source fluctuations and various ways of doing this were explored, initially via simulations carried out using Simetrix, a circuit simulation program.
Since the output of most LED’s decreases as temperature increases, an improved drive circuit (see below) compensates by increasing the current through the LED as the temperature rises. The high stability LED source board does just this using a TL431 regulator, a signal diode and a thermistor to further improve the output performance of the LED against variations in temperature.
High Stability LED Source - Schematic
In operation, the voltage at the reference pin of U1, the TL431 is stable with respect to temperature T so that the current through the LED will be constant (and set by R1).
But the presence of signal diode D1 will result in an increase in LED current with T since the diode’s forward voltage drop decreases with T (by approximately 2 mV/K).
Additionally, the LED current vs T can be affected by placing a negative temperature coefficient thermistor Rt in parallel with R1. This will result in a further increase in LED current with T. Using the circuit simulation tool it was found that further fine-tuning of the LED current vs temperature profile is possible by adding a resistance Rs in series with Rt.
In practice, R1,Rt and D1 are installed and then Rs is adjusted to minimize changes in the LED’s output intensity with temperature.
Typical results are shown in the performance evaluation section below.
Here, the forward voltage drop of the signal diode is 700 mV and this value is assumed to change by -2 mV/K. The voltage at the reference pin of U1 - the TL431 is 2.500V.
To compute the current the resistance value at each temperature is taken from the blue trace immediately above.
Temperature Stabilisation Results
The data at left shows the significant downward drift in the LED's output as measured by a TSL235 detector. If the instrument had been zeroed at the commencement of the run, the drift experienced over the course of this run would amount to an absorbance drift of almost 0.01 absorbance units.
The absorbance drift in the red trace has now been controlled to 0.0008 absorbance units.
VODS - Square Wave Modulated Source Option
In some experiments it may be desirable to modulate the VODS light source using an external reference square wave and to then process the detector signal with a lock-in amplifier as is done in the LIA-ODS instrument, described elsewhere. The resulting DC signal then shows excellent noise immunity to detected light at any frequencies other than the reference frequency.
This can be easily implemented by adding a transistor to the basic current source circuit, as shown in the schematic at right. Square wave modulation applied to the transistor’s base resistor will switch the LED on and off at the modulation frequency.