# Help with sample weight

Question asked by kelsey_rogers35 on Dec 7, 2018
Latest reply on Jan 11, 2019 by wydnaa

Hello,

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!

Thanks,

Kelsey