Update: 4 CRMs run, last three were within +/- 10, but still ~ 30 points higher than the reference. It’s at least consistent in the degree off, but I believe that it can be accounted for with an offset. Hooray?
So Micah and I decided last week that there was a fair amount of drift in the pH probe week to week, potentially being one of the causes of the reference sample variation we see. To combat this, starting today we’ll recalibrate the probe at the beginning of each sample day.
Steps to recalibrate probe:
1. Obtain 4/7/10 buffer solutions. These are in stock bottles on the left hand side of the upper shelf in the lab.
2. Fill a sample cup ~1/3rd full with each solution. These don’t need to be weighed, they just need to cover the pH probe bulb completely.
3. Run the Probe Calibration Method on the Titratior. You’ll load the 4.0 buffer, hit ok, then it will run for a while, then load the 7.0 and then the 10.
4. When the calibration has finished it will show a summary screen like:
and the number we’re after is the second SLOPECAL value, -58.18mV/pH. This is the number that lets us equate the millivolt readings from the pH probe to the pH of the sample. For reference, last week the SLOPECAL value was -58.90mV/pH so a little change, but not much.
5. After we have our slope value, we need to calculate mV values for pH 3.5, where the initial offgas spin starts, and 3.0, where the titration ends. To do this, we just figure out how many pH units we move from 7 (7 has a mV reading of 0) to 3.5, or 3.5 units, and multiply that time the SLOPECAL value, and get 203.63. Then we calculate our end mV reading, a change of 4 units and get 232.72.
6. These numbers we take to the laptop, and go in to the Analysis Tab, then double click on the New TA Titration method. This brings up a graphical flow chart of the entire process, and we want the “Titration (EQP)  method. There we want to check the Predispense Potential, and the Termination Potential values. According to Micah we choose the nearest 10s value to our targets, due to granularity in the dispensing protocol, we won’t actually hit exactly 203 or 232. I chose 200, and 230.
Time to run some CRM’s and see if this helps at all!