Calibrations are comprised of 6 different stages. The first stage is called the “Normalization Phase” and lasts 100 seconds. During this time, the LED strength is adjusted to maintain a baseline electrical current of 20 mA. This is performed on the blank solution, containing range specific electrolyte and COD free deionized water. Once the LED is set, oxidation of the blank solution will occur. This is comprised of 3 stages, visualized by 3 distinct curves. The stages are known as the Burn-In, Pre-Burn, and Oxidation of port B. The blank acts as a zero reference for the calibration based on the charge generated from the DI and electrolyte mixture. The area under the curve is used to quantify charge. Once the Port B stages have completed, the Port A stages will begin. The calibrant and electrolyte mixture is introduced across the TiO2 sensor, where similar Pre-Burn and Oxidation curves are generated. The concentration of the calibrant is determined based on the sample COD/BOD range. Once the specified number of calibrations have completed, the calibrant solution can be run as a sample (referred to as a QC check). It is expected that the COD result will be + / – 5% of the standard COD/BOD value.

A C value is reported after a calibration. It is measured in μC, and indicates the raw charge generated during the blank oxidation. In PeCOD Pro, the C Value can also be referred to as the Zero Charge (Z1). The expected C value ranges depend on the color range you are working in (advanced blue and green ranges have lower C values than yellow and red ranges). The acceptable values are 50-300 µC for advanced blue range, 150-700 µC for green, 200-750 µC for yellow range, and 250-800 µC for the red range.

An M Value is also reported at the end of a calibration. It is a ratio of the expected COD to the charge generated during the reference oxidation (of the calibrant solution). It is expressed as COD/μC. The acceptable M value range for the green, yellow, and red ranges is 0.02-0.06 COD/μC. The advanced blue range has an acceptable M value range of 0.01-0.08 COD/μC.

The Y-axis of the oxidation graph is defined as the Iwork (reported as μA) which is a measure of current. The Iterm is essentially the Iwork at the end of each oxidation curve as it levels off. The acceptable Iterm value for advanced blue and green ranges is >16 μA. For the yellow and red ranges, the Iterm value needs to be >14 μA.

Click ‘File’ in the top left corner, and select ‘Preferences’. The ‘General’ tab will pop up, select the ‘QC Regimes’ tab. Here you can see how many calibrations, QC checks, and recalibrations the PeCOD® will preform for the ‘Startup (Daily) Regime’. To see the other regimes, select the ‘Startup (New Sensor)’ tab, or the ‘QC Routine’ tab in the second row. To edit the number of calibrations or QC checks, click the arrows beside the numbers to choose how many of each the PeCOD® will perform.

For calibration, the PeCOD® Analyzer goes through the following phases:

1. Normalization Phase. This is where the PeCOD® Analyzer is adjusting the LED lamp output, trying to achieve a baseline of 20µA.
2. Burn-In Phase (port B). This is where the pre-mix blank solution is brought into the sensor cell. It is oxidized to remove contaminants from the cell.
3. Pre-Burn Phase (port B). This is where a new aliquot of pre-mix blank solution is brought into the sensor cell. This phase removed more contaminants from the cell and conditions the cell for the pre-mix blank.
4. Oxidation (port B). A new aliquot of pre-mix blank is brought into the sensor cell and is oxidized. This value is used in the peCOD calculation. The area under this curve is used to calculate the ‘blank charge’, which corresponds to the C value determined by each calibration. This is the small amount of COD contributed by the electrolyte reagent, which is then subtracted from all future COD analyses to give the final COD result.
5. Pre-Burn (port A). A new aliquot of pre-mix calibrant solution is brought into the sensor cell where it is oxidized. This removes contaminants from the cell and conditions the cell for the pre-mix calibrant.
6. Oxidation phase (port A). A new aliquot of pre-mix calibrant if brought into the sensor cell. This value is used in the peCOD calculation. The area under the curve is used to calculate the reference charge, which is used with the blank charge to determine the relationship between charge and COD. This corresponds to the M value determined by each calibration.

For sample analysis, the PeCOD® Analyzer goes through the following phases:

1. Burn-In phase (port B). The pre-mix blank solution is brought into the sensor cell and is oxidized. This removed contaminants from the cell.
2. Pre-Burn phase (port A). An aliquot of sample is brought into the sensor cell and is oxidized. This removes contaminants from the cell and conditions the cell for the sample.
3. Oxidation phase (port A). A new aliquot of sample is brought into the sensor cell and is oxidized. This value is used in the peCOD calculation. The area under the curve is used to calculate the charge generated by sample oxidation.
4. The blank value will be subtracted from the sample value to determine the sample COD.

It is possible that a PeCOD will fail its calibration. If this occurs, the PeCOD will notify you of the reason it did not pass. Common reasons are that the M value or C value are out of range.

MANTECH’s recommended passing criteria varies for each colour range. The image below outlines the recommended criteria for each range.

When calibrating the PeCOD, the final step of the QC regime is to complete a QC Check on the pre-mixed calibrant solution. This will analyze the COD value of the pre-mixed calibrant solution to confirm the accuracy of the calibration.

Both Calibrant and Standard Solutions are good for one year after they are made. Electrolyte has a shelf life of two years after it is produced. All labels have the expiry date in the box just above the MANTECH logo.

Samples must be filtered prior to PeCOD analysis to ensure that no particulates greater than 50 micron (um) are primed into the peCOD. Particulates larger than 50um can cause clogging, which can lead to damage of the internal fluidics of the machine.

For pulp and paper and wastewater applications, MANTECH recommends using a 35um polyethylene (PE) syringe filter. These filters can contribute trace amounts of organics, which are negligible for wastewater applications. For drinking and source water applications it’s important to use a filter that does not contribute organics to the filtered sample. One of MANTECH’s research partners has recommended a 0.45um polyethersulfone (PES) filter; however, other filter types may also be acceptable, if no organics are contributed by the filter. Since these applications traditionally see less particulates, having a smaller pore size filter hasn’t shown an impact on the PeCOD results.

MANTECH recommends storing samples in the laboratory fridge vs preserving samples. If you do choose to preserve your samples, use H2SO4 to adjust the pH of your samples to ~2.

The temperature range that PeCOD samples should be analyzed is between 10 to 30°C. For samples that are outside of this temperature range, the addition of electrolyte (which is stored at room temperature) prior to analysis will help to bring the sample solution to an acceptable temperature range.

1. The following steps will outline how to correctly homogenize a water/wastewater sample for analysis with the PeCOD.
2. Homogenize the raw sample for 2 minutes. Ensure you are using at least 50mL or more of sample.
   – Prepare the sample with electrolyte in the range being used. Click here for the correct sample-to-electrolyte ratio for each colour range.
   – For example, if you are using yellow range, take 2mL of sample plus 18mL of electrolyte.
3. If dilution is required, prepare the dilution first with the homogenized sample, then prepare with electrolyte.
4. Homogenize the mixture of sample and electrolyte for 1 minute.
5. Place the PeCOD intake tube (Port A) into the sample and press run the analysis.
6. Rinse the homogenizer in deionized water, turn on for 30s while in DI water.
7. Spray down homogenizer with DI water using a laboratory bottle with nozzle (common lab squeeze bottles).

pH Range: 4.0 – 10.0 (after mixing with electrolyte)

The peCOD method requires that the pH of a sample AFTER being mixed with electrolyte must be between 4 – 10. To determine if a sample must be pH-adjusted, mix the sample with peCOD electrolyte at the proper mixing ratio for your COD range, then test the pH of the mixture.

For example, the sample may have a pH of 3.0, but then after preparing with electrolyte, the pH is in the required range, therefore, it is acceptable for immediate peCOD measurement.
If samples have been preserved in acid, they should be neutralized using sodium hydroxide prior to analysis to avoid a low reading, as well as damage to the sensor. When the sample pH is below 4, the photocatalytic oxidation at the TiO2 sensor is affected, leading to poor reproducibility and charge values below theoretical expectation. Below a pH of 2, the TiO2 displays instability. When the pH is above 10, the charge measured for the reference and sample solution yield lower than expected values, again caused by interference at the TiO2 sensor. Sulphuric acid should be used to lower the pH of samples with a pH of 10 or more.

If only storing the electrode block for a short period of time (less than 4 weeks), rinse DI water through the PeCOD and leave the electrode block inside. Make sure all of the sample has been washed through by priming Port A several times. If storing for more than 4 weeks, put DI water through the PeCOD and then remove the electrode block to store outside the PeCOD. Flush the channels with 20-30 mL of DI water before pushing through about 10 mL of NaCl, leaving the channels filled. Tape the ends of the channels to ensure no leaks or crystallization occur.

Open the top plastic door by pushing down firmly on the front centre of the door until a “click” is heard, then release the door. Open the PeCOD analyzer module by pressing firmly down on the fixed bar, and lifting the front latching bar (should unlatch), then lift up the PeCOD sensor lid. Remove the old sensor by lifting it off of the pins and place the new sensor on the same pins with the “THIS SIDE UP” surface (blue side) facing you.

Sensors are expected to last 150 runs when used consistently for an average of 50 samples per week (samples, calibrations and QC checks). However, should a sensor be intermittently used, it is recommended that it be changed after 3-4 weeks of use regardless of the number of completed runs, or based on consistently failing calibration values. When analyzing higher sample concentrations (especially red range) the sensor life expectancy is likely to be shorter, ranging from 60 – 150 total measurements.

If left unopened and sealed in the package, PeCOD sensors have a shelf life of 12 months. Once opened and in use, sensors can be expected to last for approximately 1 month or 200 samples (whichever comes first).

On a day to day basis, the PeCOD sensor inside the Analyzer head, in the same location as it is placed for operation. At the end of each analysis and calibration, the PeCOD primes the sensor line with blank solution to provide a clean environment similar to operating conditions for storage. An alternative to this is to prime DI water manually through Port A of the PeCOD at the end of a day’s operations.

When the PeCOD is not going to be used for some time, for example a week or longer, it is good to remove the sensor from the unit for long term storage. This is done by first removing the Port A and B tubes from liquid, then priming those lines with air. Then, press on the front latch to open the analyzer head and access the sensor. The sensor can then be lifted off the alignment pins. Once removed, it is important to use a syringe to “dry” the internal channels of the sensor. Fill a 10mL syringe with air, then press the syringe (without a tip) against the holes in the back of the sensor. Push the syringe down to push air through the sensor channels. Perform these steps on each of the holes in the back of the sensor, in the following order:

Once dry, place face down on a dry surface to finish the drying process. Next, place the sensor in a cool, dark place for long-term storage. The sensor packaging works as a suitable package for long term storage after properly drying the sensor.