WEF Discussion Forums
Laboratory Management and Technical Issues
Total Dissolved Solids
Just curious..... What are other laboratories getting for their TDS blanks and MDL's? We seem to be all over the board on this one.
Our detection limit for TDS is 10 mg/L. We consistently achieve blanks below that amount. For our MDL's we got 10, 12, 12, 10, 11, 11, and 10 mg/L giving us a calculated MDL of 3.4 mg/L. I'm not sure how well we do getting below that number, typically no one will request 'J' flags on TDS. One techniqe that we have used to get consistent results is to monitor the cooling/dessicating time. You need to keep the crucibles in your dessicator for the same amount of time every time they come out of your drying oven. That includes your the time bringing the crucibles out to get the initial dry weight. We store ours in the 180 oven when they are not in use. Then before starting the samples we pull them out of the oven and let them cool for an hour in the dessicator. After that we get the initial weights and proceed to filter and evaporate the samples. When getting our dry sample weights we let them sit in the dessicator for an hour again to cool before weighing. That has helped immensely in getting reproducible weights. All of this pertains to using ceramic crucibles. Hope it helps.
Thank you for the information! We have not been controlling the time that exactly: in particular we always start with dishes which have been prepared and stored in the dessicator. We have also, occasionally, used dishes for TDS that have been muffle furnaced. I"am sure that this is contributing to the error. I would love to see results like that and look forward to trying it the way you are doing it. Fingers crossed!
Curious....What is the correct way to measure % Solids? Zero the scale and weight dish & solids?, get a cup weight and subtract that from the wet & dry weight?, or just subtract dry from wet? I found that different lab techs do it different ways....
For % solids you need to subtract the dish dry weight. You want sample dry weight divided by sample wet weight.
For TDS, the result is highly dependent upon the type of dishes used and the accuracy of the balance.
Using Coors ceramic dishes, for example, the initial clean dry weight is 80 grams, which is weighed on a balance to the nearest 0.1 mg.
This translates to an error requirement, for a blank, of no more than 0.5 mg.
Now, 0.5 mg in a weight of 80 grams is an accuracy of 0.0006%!!
So unless Standard Methods allow us to use disposable aluminum dishes, we have to take drastic measures to ensure we can pass the blank QC.
We use a state of the art balance that constantly recalibrates itself.
We have noticed a temperature effect on our balances, old and new, so we store our cleaned dishes, after baking at 180C, in the dessicators for several hours and preferably overnight to ensure they are totally at room temperature before we take a reading.
Ditto for our baked sample dishes.
You can also maintain a database of the average weight of your dishes to catch any that may still be dirty or may be chipped.