In this video, we will perform the Site-Specific Correction for the EXO NitraLED Sensor in Kor Software.
The purpose of the Site-Specific Correction is to help users achieve better results with NitraLED by fine-tuning the sensor to compensate for the specific interferences from the turbidity and natural organic matter, or NOM, present at your site.
We want to make sure when we’re working with the NitraLED sensor that, we have our personal protective equipment on since this sensor emits UV light. I have my UV-rated safety gloves and glasses, so I’m ready to go.
For this process, it’s helpful to have two one-liter beakers, a stir plate and stir bar, a ring stand, a NitraLED wiper brush, and your preferred filtration setup with a 0.45 µm filter.
You need to have a calibrated turbidity sensor and Conductivity/temperature sensor installed in your sonde. And you should have already performed the regular 2-point calibration using YSI nitrate standards for the NitraLED. It’s best practice to have a NitraLED wiper brush installed to prevent bubble formation on the sensor faces. We have other videos that cover calibration and wiper installation. It is also important to have your sensor guard installed for the proper function of the turbidity sensor.
Before calibrating, make sure your sensors, sensor guard, glassware, and wiper are all clean. We want to reduce potential sources of contamination.
Let’s start with the site-specific correction. You will need a grab sample from your site of at least two liters. One liter will be filtered to remove turbidity, and one will remain a raw sample. Using an aliquot of your raw sample, determine the nitrate concentration using an independent method, such as by ion chromatography or a benchtop ion-selective electrode or photometer. You can store your samples in a refrigerator if you are waiting on your analysis. Note that the error associated with the method you choose to determine the reference value will apply to the final calibration of the NitraLED.
We used the YSI Photoflex colorimeter to determine the nitrate value of our sample in mg/L. This value will go in Kor Software.
Filter 1 L of the site sample using a 0.45 μm filter. You can use a vacuum pump with a benchtop setup using flasks and a membrane filter, or, I prefer a peristaltic pump with a capsule filter so that I can do this from the field, or a tailgate, if needed.
Now I have two one-liter beakers, one filled with about 800 mL of my raw sample and one filled with about 800 mL of my filtered sample.
In Kor, under the NitraLED sensor, we will select Corrections. Enter the reference nitrate value in mg/L that we determined earlier. Click Apply. Set up a stir plate and a ring stand to hold the EXO. Place the beaker with the raw sample and a stir bar on the stir plate. Turn the stir plate on to a medium-high speed that keeps the sediment suspended. Place the EXO on the ring stand and submerge it in the raw sample.
Make sure the NitraLED is submerged past the purple tip or the sensing window and that all sensor faces are submerged. Be sure there are no bubbles on any of the sensor faces. If there are, remove the sonde and immerse again. You can also run the wiper brush.
Wait for readings to stabilize. This could take 5-10 minutes. If you see sediment depositing, increase the stir speed. Keep checking the sensor lenses Turbidity and NitraLED for bubbles. If you notice your turbidity value is less than 15 FNU, you can choose to click this button to use the factory default kaolin turbidity coefficients instead of a user-defined correction. We are doing a user-defined correction today. Once the turbidity and absorbance values have stabilized, click Apply. Kor will indicate that the sensor should be placed in the filtered sample.
Now remove the sonde from the raw sample, then rinse with DI water, and dry with lint-free wipes before moving to the next step.
Next, place the beaker of the filtered sample with a clean stir bar onto the stir plate. Turn it on to medium-high speed. Place the sonde into the filtered sample. Again, make sure the sensors are covered and that there are no bubbles.
Check the turbidity reading. It should be less than 1.0 FNU. If It’s not, you should filter the sample again. Once the absorbance values are stabilized, click Apply. For the next step, the sensors should remain in the filtered sample. Keep checking that there are no bubbles are on the sensor lenses.
In this step, we are minimizing nitrate error by adjusting the NOM coefficient. You will see two orange error bar lines. Adjust the blue line until it is between the two orange lines. Once it is, click Apply.
Finally, you will see a summary page with all of the calibration values. Kor shows you a reference sheet on the sidebar to help you determine if the values are acceptable. After confirming the values, close the table. Kor saves this as a separate Site-Specific Correction, or SSC, calibration report.
The EXO NitraLED sensor now has an applied Site-Specific Correction. Please check out our other EXO University videos, and thank you for watching!