4 Replies Latest reply on Jan 11, 2019 3:40 AM by wydnaa

    Help with sample weight



      I am using a 1220 Infinity LC II with OpenLab CDS 2.3.

      I am having some trouble calculating the appropriate concentration with some of my samples and I think its because of sample weight variances/unknown starting concentrations. I attempt to dilute my cannabis oil samples within a small standard calibration window (0.5 ug/mL- 50 ug/mL) and I usually aim for 25 ug/mL so its right in the middle. However sometimes the samples I receive have unknown concentrations so I must estimate the starting concentration to figure out how much of the sample volume to use to mix with my solvent to get to appropriate dilution (which is especially important because of the small curve). BUT sometimes my estimates are off (even by small values like 5 mg) and it is messing with the total concentration.


      Ex: Unknown sample

      Concentration sample: we will estimate 40 mg/mL

      Volume sample: X

      Final concentration= 25 ug/mL

      Final volume= 20 mL

      so... X= 12.5 ul sample + 19,987.5 uL solvent = 20,000 uL total.

      DF= 20,000 / 12.5 = 1600. Then in my injection table I account for the dilution factor value so the concentration will report the concentration of the sample before it was added to the solvent.


      But then my report will read that I actually measured 33 ug/mL instead of the expected 25 ug/mL (sometimes this value is lower than expected too) which will then mess up my final calculation for concentration. So what I think that what is happening is that the viscosity of the oil is not actually allowing me to pipette the appropriate amount (in this case 12.5 ul) and sometimes I pull up too much and sometimes too little (depending on oil type, concentration, how many times I pipette in/out to clear tip etc.). Obviously the oil is measuring over 40 mg/mL (should be around 43mg/mL according to another lab test) and so I know my starting concentration was off by a little...but how do I correct for this error? I know there are labs that have no idea what concentration they are sampling unknown samples and there must be an easier way to account for this (I understand some labs have larger extended calibration curves so their dilutions do not have to be as stringent, however I am limited to the concentration of cannabinoid standards that are legally available in the US and cannot extend my curve to alleviate this issue). So I want to be able to weigh my oil samples and include this into the calculation, but when I type in how much sample weighs (g) into the "sample weight" column it does nothing to change the concentration value. I feel like I am overlooking a major step here but can't seem to identify it. Any recommendations or help regarding this would be greatly appreciated!




          • Re: Help with sample weight

            Thank you mr.white I did make this mistake earlier when I was first starting testing but unfortunately I do not believe this is my problem any longer and I had already stumbled upon that exact page several weeks ago. My problem lies within sample weigh differences specifically. Basically all my test samples are oil base liquids, but I need to incorporate their exact weight in g instead of uL bc it is not always the same due to different densities and sample error. However, thanks so much for your reply--I appreciate it.


            -Kelsey Rogers

          • Re: Help with sample weight

            Hi Kelsey,


            The Sample Amount column is for mass% calculations ---- Mass% = 100 * Conc/ Sample Amount. If you want to change the amounts used to calculate your calibration curve in OL 2.3 you need to change the Compound Amount for Cal Stds. You can find this in the Sequence table when in Acquisition or Injection List if in Data Analysis. If you can't find the column, make sure it isn't hidden.


            Hope this helps,


            • Re: Help with sample weight

              Hello Kelsey


              Sounds like you need to do a custom calculation to include the actual weight of your samples.


              You could do something like this in an intelligent report, or build yourself a custom calculation that does the same thing, i.e. correct for the difference between nominal and actual weight sampled:


              ActualConcentration = NominalAmount * Compound_Concentration/Sample_Amount (I think that's the right way round!)


              NominalAmount could be a report parameter or simply just a number in the equation, and Sample_Amount could be entered in the sequence table.  Compound_Concentration is calculated in OpenLab.  You could simply add this calculation as a custom column in your results table in your report.


              Does that make sense?