How to Manually Sample

Dissolution sampling is the most difficult step of a dissolution run, and the step that is most likely to lead to false failures.  Doing dissolution testing can sometimes be more of an art than a science, and this step is by far the one where your technique matters the most.  Proper dissolution sampling requires you to take 6-8 aliquots out of the vessel within a small time window and a specific zone within each vessel.  The requirements on time and position are essential, as it is the only way to ensure a consistent sample is pulled from the vessel.  Dissolution is all about consistency - the same environment, conditions, etc. across every vessel.

To properly sample, the following must occur:

-The sample must be pulled within +/-2% of the time point being pulled.  For a 30 minute timepoint (such as the prednisone PVT) this means you can pull a sample 36 seconds before AND after the 30 minute timepoint.  29:24 - 30:36 is the valid sampling window in this case.

-The sample must be pulled 1/2 way between the top of the media and the top of the basket or paddle.  The sample must be pulled no closer than 1cm along the vessel wall.  I would also argue that the sample should not be taken within 1cm of the paddle shaft.  The picture in the header is an illustration of these parameters, and I like to call it "the sampling donut".  The reason for this sampling donut is that this is an area where the vessel is well mixed and is the most homogeneous.  Sampling too high or too close to the shaft will tend to have lower concentrations as they are not mixed as well, sampling too close to the vessel wall can contain high shear forces.

-The sample must be filtered as soon as possible

For a 6 vessel system with a 30 second timepoint, you have 12 seconds to collect each sample properly.  It isn't impossible, but it will take some practice.

Here are some tips to help you sample accurately and with reasonable precision.

1) Find a way to get you reproducible height of the cannula when sampling.  You can do this by holding the cannula in such a way that it will be at the right height for you.  You can practice with a vessel filled with water and see what handhold works best for you.  For me, I hold the syringe between my top 2 fingers and rest it against by thumb like I would with a pool stick.  You can also purchase cannulas that have an adjustable gauge which you can set a height on.

2) When sampling, make sure you can watch what you are doing so that you can position the cannula in the proper orientation.  If you are using a cannula with adjustable gauge, the evaporation cover hole will set your orientation in the proper position.

3) Have a calibrated timer ready so you can start immediately at the `-2% of your sampling window.

4) Have your cannulas, syringes, filters, test tubes or LC vials ready to go and preferably labelled.

5) If possible, use a cannula tip filter as your filter of choice.  This way, you are filtering simultaneously as you are collecting the sample.  If you pull a sample and don't immediately filter, the sample will continue to dissolve and you can get higher and more variable results.

6) If you need more time for pulls, then consider staggering your sample drops.  Make sure if you are staggering that you are not dropping samples into rotating media.

7) If you can't reasonable manually sample the product for whatever reason, automation is available.

Proper sampling timing and technique is essential to get reproducible sampling from your dissolution run.

I hope this short article was helpful, and let me know if there is anything you're interested in hearing more about.