Orion Dual Star pH/ise dual Channel Benchtop Meter User Manual
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- Meter Features
- With the Autoblank Function Off
- Determining an Isopotential Point
- USA pH Buffer Set EURO pH Buffer Set Buffer mV Range Buffer mV Range
- Testing a pH Electrode for Automatic Buffer Recognition
- North America
- Singapore
Advanced Meter Features |
Section 10 Automatic Blank Correction Feature The Orion Dual Star meter has a feature known as automatic blank correction. In this technique, the meter decides whether blank correction is the best measurement strategy by analyzing the electrode response during a multipoint calibration. A separate blank does not have to be run. Graphically, automatic blank correction is equivalent to drawing a smooth curve through the lowest three points of the multipoint calibration and extrapolating to zero concentration based on the assumption of Nernstian electrode behavior. Multipoint calibrations at the lower limit of detection are desirable when the response of an ion selective electrode is non-linear and cannot be characterized with a one or two point calibration. This is usually seen as a low electrode slope. Generally, the electrode is behaving in a Nernstian manner but the effect of a blank is being observed. See the figure below.
“mud” value of the electrode. It could also be an interference in the reagents that becomes apparent at low levels of analyte ion, or it could be any combination of these effects. The expanded version of the Nernst equation traditionally used for blank correction is as follows: E = E o
In a multi-point calibration, a set of equations is generated and the relationship between them evaluated. For example the equations generated in a three point calibration would be as follows: E 1 = E o + S x log (C 1 + b)
E 2 = E o + S x log (C 2 + b)
E 3 = E o + S x log (C 3 + b)
Thermo Scientific Orion Dual Star pH/ISE Benchtop Meter User Manual |
Section 10 |
Advanced Meter Features The meter evaluates the relationships between the three potentials E 1 , E
2 and E
3 and the three concentrations C 1 , C 2 and C
3 . If the relationships indicate that blank correction is desired, a blank will automatically be calculated and the non-linearity will be corrected for in a Nernstian manner. If the appropriate conditions are not met, the blank is set at zero and each segment of the multipoint calibration is treated independently. When all three of the following conditions are met, blank correction is invoked. 1. The concentration of the first standard is zero, or the slope of the electrode between the first and second standards is less than the slope between the second and third. 2. Potential differences between points are significant. For example, 3. E
3 - E
1 > 10 mV 4. The blank correction algorithm converges at reasonable blank and slope values. Conditions in steps 1 and 2 prevent failure to converge in most situations. However, the slope will attain any value necessary for convergence to a calculated blank value of 3 x C 3 . When blank correction is implemented, the slope value for the electrode may be outside the range of values normally considered acceptable during an ordinary calibration. For best results, calibration standards should be close in range to the expected sample concentrations and should bracket the expected sample concentration. When conditions 1, 2 and 3 are not met, the calibration data is handled by the multipoint calibration method described earlier. Note: When a calibration standard is defined as zero and the meter cannot calculate a blank, an error code E306 will be displayed. In calibrations with more than three points, a combination of methods are utilized. Automatic blank correction is used if the lowest three points satisfy the criteria and multipoint calibration is used for the other points. The slope displayed after calibration is an average of the slope calculated in the automatic blank correction algorithm and the slope(s) for each additional segment.
This error occurs during a three or more point calibration when one solution is defined as zero concentration and the actual concentration of the blank is substantially greater than the standard with the third highest concentration. Since the blank cannot be calculated, the meter cannot use the autoblank function and cannot use the zero concentration standard in the two point calibration segment. To clear the error, calibrate the meter without using a zero concentration standard.
This error occurs during a one or two point calibration if one standard is defined as zero concentration. To clear the error, calibrate the meter without using a zero concentration standard. 88 |
Orion Dual Star pH/ISE Benchtop Meter User Manual Thermo Scientific Advanced Meter Features |
Section 10 Isopotential Point Feature To take advantage of the temperature compensation feature for concentration measurements in the Orion Dual Star meter, it is necessary to experimentally determine the isopotential point of the electrode in use. The slope of all electrodes changes with temperature. This is due in fact to the slope term, S, in the Nernst equation. The slope is actually 2.3 RT/nF, where R and F are constants, n is the charge on the measured species and T is the temperature in degrees Kelvin. Examples of calibration curves at varying temperatures are shown in the figure below. These curves intersect at the isopotential point, which is the concentration at which the potential of the electrode does not vary with temperature. If the isopotential point is known or can be measured experimentally, and if the meter has the means of adjusting the isopotential point, temperature compensation for an ion selective electrode is possible. Temperature compensation can be performed for one and two point calibrations.
Determining an Isopotential Point 1. Prepare several standards with concentration ranges over the measuring range of the electrode. 2. Measure the millivolt value of the standards at room temperature, about 20 to 25 °C. 3. Measure the millivolt value of each standard at 75 °C. 4. Measure the millivolt value of each standard at 10 °C. 5. On semi-logarithmic graph paper, plot the concentration values on the log axis versus the millivolt values on the linear axis, as any calibration curve would be drawn (see the figure on the previous page). 6. The lines will intersect at the isopotential point. Read the concentration off the graph for this point from the log axis. This value is the isopotential point. 7. See the Setup Menu section for instructions on how to adjust the isopotential point on the Orion Dual Star meter. Thermo Scientific Orion Dual Star pH/ISE Benchtop Meter User Manual |
89 Section 10 |
Advanced Meter Features Automatic pH Buffer Recognition Feature The Orion Dual Star meter is capable of automatically recognizing pH 1.68, 4.01, 7.00, 10.01 and 12.46 buffers or pH 1.68, 4.01, 6.86, and 9.18 buffers during a pH calibration, depending on the pH buffer set that is selected in the setup menu. During a calibration, the meter uses the selected buffer set and the raw mV reading of the pH electrode in the buffer to recognize and display the buffer value at the measured temperature. The raw mV reading of the pH electrode in the buffer must be about ± 30 mV from the theoretical mV reading of the buffer in order for the meter to automatically recognize the buffer.
1.68
+285 to +345 mV
1.68 +285 to +345 mV 4.01
+207 to +147 mV
4.01 +207 to +147 mV 7.00
- 30 to + 30 mV
6.86 +38 to -22 mV 10.01
-207 to -147 mV
9.18 -99 to -159 mV 12.46
-293 to -353 mV
To verify that the raw mV reading of the pH electrode in use is ± 30 mV from the theoretical mV reading of the buffer, and therefore verify that the pH electrode in use is capable of performing automatic buffer recognition, perform the following procedure. 1. Set the measurement mode to mV in the setup menu. Prepare the pH electrode according to the instructions in the pH electrode user guide. Ensure that the pH electrode is connected to the meter input that corresponds with the channel that will be measured. 2. Rinse the pH electrode with distilled water, blot it dry with a lint-free tissue and place the pH electrode into a pH 4.01 buffer at 25 °C. 3. Wait for the measurement to stabilize and record the mV value of the pH 4.01 buffer when the measurement is stable. The read type selected in the EZ Startup menu or setup menu will determine how the mV measurements are displayed by the meter. 4. Remove the pH electrode from the pH 4.01 buffer, rinse the pH electrode with distilled water, blot it dry with a lint-free tissue and place it into a pH 7.00 buffer at 25 °C. 5. Wait for the measurement to stabilize and record the mV value of the pH 7.00 buffer when the measurement is stable. 6. The mV reading of the pH electrode in pH 4.01 buffer should be +207 to +147 mV and the mV reading of the pH electrode in pH 7.00 buffer should be -30 to +30 mV. If the mV readings are in the correct ranges, the pH electrode is capable of performing automatic buffer recognition. If the mV readings of the pH electrode are not in the correct ranges, the values of the pH buffers will need to be manually entered during a pH calibration. 90 |
Orion Dual Star pH/ISE Benchtop Meter User Manual Thermo Scientific
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