Solid Phase Extraction Technique Question

[ravenwork]

Greetings,

I am running an SPE method with a disk. My SOP tells me to perform a final extraction of the disk with 5 mL of solvent, pipet measured. Then, I am to collect the extract, and bring it to 5 mL in a volumetric flask with additional solvent. This doesn't seem like good practice to me. Since I know that I am actually extracting the disk with 5 mL of solvent, then it seems to me I should collect what I can and proceed forward on the basis of 5 mL volume without the addition of more solvent in the volumetric flask.

Comments?

Evan Cooper

[Mark Tracy]

You will always have some holdup in the apparatus, and perhaps even some evaporation of organic solvent. The instruction to pipet the 5 mL of eluent guarantees that the final volume does not exceed 5.0 mL. Presumably, the given volume is sufficient to elute the analyte and quantitatively wash the apparatus into the receiver.

[dougs]

Hey, that's a great idea. I should try that with my spe method. BTW, perhaps it would be benefitial to add the 5ml of elutant 1 or 2ml at a time.

DS

[Tony_Montanari]

I have blown samples down and then reconstitute into a known volume.

[ravenwork]

Yes,

I blow samples down (rather evocative language, what?), and then reconstituted to a known volume, that's what the SOP says to do. However, Tony, what do you think about it? If, as is the case with my SOP, you extract the disk with 5 mL and then reconstitute to 5 mL, then aren't I diluting the sample slightly? Since I know I put 5 mL on the disk, and I want 5 mL of final volume, then shouldn't I just collect the eluant and go?

Evan Cooper

[Uwe Neue]

I think the redilution to 5 mL is the right thing to do. Now you know the concentration of the analyte. If you do not dilute, you do not know the concentration.

Here is the rational. In most SPE methods, the analyte elutes as a wide peak with a long tail. If the method is designed well, it will eluted the majority of the compound in the early fractions of the collected volume. If you have all your analyte eluted early on, then the remainder of the solvent is just a dilution. Since there is always some solvent left in the SPE device, you will now have a fixed amount of analyte in an unknown amount of solvent. Therefore the dilution back to 5 mL is the right thing to do and gives you true control over the concetration of the analyte in your final sample.

Hope this was clear...