Sensors & transducers
Figure 8. Turbidity calibration standards. For this experiment, optical path 4 was
used with the 530 nm LED board inserted for testing.
results Using the CN0503 evaluation GUI, measurement results were exported into Excel. The resulting calibration curve is shown in Figure 9.
the equation values are stored, INS1 can be used to measure lower-turbidity samples, and INS2 for higher. To get the noise value, we choose a data
point to take the standard deviation of repeated measurements. The standard deviation is the noise value. One data point is chosen near the bottom of the range because the equation fit is linear.
Table 3. TurbidiTy
MeasureMenT noise Value 12 NTU
RMS Noise Value (NTU) 0.282474
To determine the LOD, the noise value is
measured for a blank or low concentration sample, then multiplied by three to represent a 99.7 per cent confidence interval.
Figure 9. Calibration curve for turbidity. The response curve was split into two
sections because the 90° scattering measurement is less responsive to high turbidities. One section represents lower turbidity (0 NTU to 100 NTU), and the other higher turbidity (100 NTU to 750 NTU). Two linear fits were then made to each section. Even though there are now two equation values, the CN0503 can still be used to quickly show resulting NTU values. This is because each optical path can store its own equation values in INS1 and INS2. One important note is that INS1 and INS2 are dependent. The result of the first equation, INS1, is the input variable for the second equation, INS2. Once
46 Table 4. TurbidiTy
MeasureMenT liMiT of deTecTion
Blank Sample
Limit of Detection (NTU) 0.69204
Measuring fluorescence wiTh spinach soluTions fluorescence background When light is shined into a sample containing fluorescent molecules, the electrons move into
optics The optical path configuration for fluorescence measurements is shown in Figure 10. With the CN0503, absorbance measurements can be made in optical path 1 or 4 only, which is because of the 90° detector. Usually, a fluorescence detector is positioned at 90° from the incident light and a monochromatic or long- pass filter is used to increase isolation between the excitation and emitted light. Fluorescence is a very sensitive low level measurement and is subject to interference, so the reference detector and the synchronous detection methods are used to reduce error sources.
February 2022 Instrumentation Monthly
Figure 10. Optical path for fluorescence measurement.
a higher energy state and then lose some of that energy before emitting light at a longer wavelength. The fluorescence emission is chemically specific and can be used to identify the presence and amount of specific molecules in a medium. In this example, fluorescing chlorophyll was demonstrated by using spinach leaves. Among many applications, fluorescence measurements are common in biological assays, dissolved oxygen, chemical oxygen demand, and detecting if pasteurisation has been successful in milk.
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