An Overview of Dissolved Gasses in Dissolution Testing

When performing dissolution testing, one of the biggest headaches is dealing with dissolved gasses.?Removing dissolved gasses can be one of the biggest headaches in the lab, and when not done properly is one of the most common causes of dissolution failures.?I’ve written about this topic previously, but I thought it was worth a revisit since the cost of helium is on the rise again and is leading to some labs re-evaluating how they choose to deal with dissolved gasses.?As always, this article is based on my experience and opinions and don’t reflect those of my employer.

First of all, why are dissolved gasses such an issue in dissolution??Dissolution is essentially a sample preparation device, but a sample preparation that occurs under very controlled conditions.?When performing dissolution, one needs to make sure that your alignment and accessories are correct so that you have a reproducible mixing (or hydrodynamics) in the vessels.?The dissolution analyst needs to ensure the right temperature, media, sampling position, filtration, etc. are all right and reproducible as well.?The dissolution also needs to minimize variability occurring from other sources which include vibration and dissolved gasses.?Dissolved gasses can lead to significant variability in dissolution, or skew results high or low.

How can dissolved gasses in the vessel lead to different results??Air bubbles can act on the dissolution environment or your dosage form in several ways which can lead to issues.?Bubbles can form on your basket sides or bottom leading the basket to be clogged with air and decrease the interaction of your product with the dissolution media and lead to lower results.?The same can happen with transdermals.?Bubbles can also form on your dosage form itself and either block dissolution or act as an additional force to erode your product (effervescent tablets are an excellent way to have a rapid dissolution after all).?In disintegrating dosage forms or beaded formulations, the air bubbles can agglomerate particles or pull them to the surface of the media and slow the dissolution.?Air bubbles can form on the vessels, paddle blade, etc. and alter the hydrodynamics in the vessel.?These are just some of the ways that air bubbles can impact a dissolution sample.

Given all the way that air bubbles can impact the dissolution test, it would seem like removing these dissolved gasses through de-aeration would be mandatory, but this isn’t necessarily so.?Some methods and products are not sensitive to dissolved gasses.?If you can show through a validation that this is the case, then you can choose to not de-aerate with that method.?Before we discuss that, let’s talk about the proper ways to remove dissolved gasses.

De-aeration can be done in several ways in the lab.?The primary method to remove dissolved gasses is the method described in USP <711>.?This method involves heating the media to 41-45 C, pulling this through a 0.45um PVDF filter with a vacuum, and then holding this under vacuum for an additional 5 minutes once the media has all been vacuum filtered.?If you’re doing this approach, I prefer heating closer to 41 C so that there is less time needed for the media to cool so that you can start your dissolution run.?So, how does this work to remove air??This approach is decreasing the air pressure so that you are boiling the media and removing those dissolved gasses.?This approach can be time consuming if using a single layer filter, so many labs would prefer not to use this approach.?I have in the past used a multi-layer filter which made quick work of degassing.?

Alternate de-aeration methods that a lab wants to use must be compared to the USP degassing procedure and validated to show that they get similar results.?This comparison can be done through product testing with the USP degassing approach and alternate degassing approach, and showing the results are equivalent through an f2.?The comparison can also be done to show that the alternate degassing approach can get <6 ppm dissolved gasses using a total dissolved gasses meter.?These alternate degassing methods include helium sparging, superheating, media degassing stations, etc.?Helium sparging has been the most common approach for years, however, there are challenges associated with this.?First, helium has become variable in price and there have been issues with supply shortages over the last several years.?Secondly, helium sparging is often not properly defined in methods and I’ve seen many failures due to chemists not sparging for long enough or at a high enough flow.?Media degassing stations are used as well, and I’ve seen labs use these successfully.?Superheating can work as well, basically heating at a higher level without a vacuum, but require a longer cool down time before the dissolution can be started.?Unacceptable means to de-aerate include nitrogen sparging which actually super-aerates the media, and sonication which is ineffective for aqueous only solutions.

Another approach is to not de-aerate the dissolution media.?This is something that I recommend should be evaluated for every new method.?What would be done here is to perform the dissolution with the USP degassing approach and compare those results to media which hasn’t been de-aerated.?This is usually an f2 comparison of n=12 degassed vs. n=12 non-degassed media.?There are some rules of thumb I have found where samples tend to be less likely to need de-aeration.?First are Apparatus 3 (Bio-Dis) runs, the Apparatus 3 never requires media de-aeration as it is such a robust mixing environment.?Another are methods with media containing surfactants are less likely to require de-aeration.?The reason for this is that media with surfactants re-aerate more quickly than media without surfactants – fully re-aerating in as little as 15 minutes. Finally, extended release products that are non-disintegrating also tend to be less impacted by dissolved gasses and worth looking into seeing if dissolved gasses need to be removed.

If your product does require de-aeration, then there are some additional things to be aware of in order for your method to be successful.?Degassed dissolution media is not in a state of equilibrium!?Once your media is de-aerated, then you need to move quickly and carefully to ensure that the dissolution media stays de-aerated.?Pour your de-aerated media carefully so that you don’t re-introduce excessive dissolved gasses back into the media.?Weighing dissolution media directly into vessels can reduce the number of times you pour your media and offer great volume accuracy.?You also want to use your media as soon as possible after it has been de-aerated.?Media with surfactants can re-aerate fully in 15 minutes, media without surfactant may re-aerate fully in 30-60 minutes.?Once media is de-aerated – it isn’t the time to go to lunch, check e-mails, or go to a meeting.?If performing the USP method of de-aeration, the media will typically be about 39-41 degrees when you pour it into the vessels and you should be at 37 C +/- 0.5C within about 5-10 minutes.?Take that time to get your sampling cannulas, filters, etc. together or label your sample tubes/vials and you’ll be ready to drop in no time.

In summary, proper media de-aeration is critical for many products to have a proper and reproducible dissolution test.?The USP procedure of heating to 41-45 C under vacuum is the primary approach, but there are other acceptable de-aeration procedures.?Not all products require de-aeration, and you can check and validate this through comparative testing.?If your method does require de-aeration though, this is a media which is not under equilibrium and proper care must be taken to still have that media adequately de-aerated when starting the dissolution run.

Thanks for reading.?If you’ve got questions, suggestions, corrections, etc. let me know in the comments.