Posting this under the heading it deserves: Lyophilisation and why the cake looks the way it does Everything below is what sits behind that.
I was wrong about lyophilisation in a thread last spring and I would like to correct it publicly rather than quietly.
The error was in the units, which changed the conclusion by an order of magnitude. Setting out the corrected version, and the way I now check for that class of mistake.
Where I part company with the opening post, and it is a narrow parting.
Lyophilisation would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.
Post #2 put the caveat in the right place and I want to underline it.
A collapsed cake indicates the product warmed above its collapse temperature during drying. It is not necessarily a quality failure and it is a real observation about the process.
Genuinely open to being wrong about this one.
Lyophilisation is worth one more sentence than it usually gets, and the sentence is the one about how the number was arrived at.
Residual moisture in a lyophilised product is a stability determinant and is measured by Karl Fischer titration. Above a few per cent, shelf life shortens considerably.
The step people skip is the one I have spelled out.
On lyophilisation, the part that usually goes wrong is that the question is asked as though it has one answer. It has a range, and the width of the range is the interesting bit.
If you can post the two or three numbers you are working from, several people here will check the arithmetic rather than argue about the conclusion.
Where I part company with post #9, and it is a narrow parting.
Bulking agents such as mannitol give the cake structure and contribute to the mass in the vial. Their presence is one reason declared fill and peptide content are different numbers.
One case, stated as one case.
Nothing to add, except that this is the answer I would give if asked.
Adding the measurement that post #12 says would settle it.
Small correction to my own earlier position on lyophilisation. I had the units the wrong way round, which changes the conclusion by an order of magnitude and therefore changes it entirely.
Benzyl alcohol as a preservative is also a solubiliser for some peptides, which is an incidental property that occasionally matters.
Answering the question post #14 raises rather than the one it answers.
The confident answers on lyophilisation and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
The arithmetic in post #16 is right; the assumption feeding it is the part to check.
Buffer choice affects both solubility and stability, and the two do not always point the same way. A formulation is a compromise between them rather than an optimum for either.
Stating my assumptions rather than smuggling them in.
What I would want before treating lyophilisation as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.
My experience of lyophilisation contradicts the reply above. I am posting it as a data point rather than as a refutation, because one person's experience is exactly that.
Adding a note of thanks rather than an opinion. I did not know most of that.
Osmolality of a home-reconstituted preparation is whatever the diluent makes it, and nobody is measuring it. In practice at these volumes it is a comfort question.
Lyophilisation removes water by sublimation from the frozen state, which is gentler than evaporation and is why peptides survive it. The cake structure is a record of how well it was done.
Somebody will have a better source than mine, and I hope they post it.
Excipients and their purposes: mannitol, sucrose, or other sugars in lyophilised formulations protect the peptide during drying and storage. They are not impurities; they are functional components.
It is worth stating the boring hypothesis before the interesting one.
Lyophilisation was covered in the wiki last year and the page has a review date on it, which is a better starting point than my memory of a thread.
The number people quote for lyophilisation is a central estimate presented without its interval, and the interval is wide enough that the estimate is nearly uninformative on its own.
Counter-ion exchange from trifluoroacetate to acetate is a purification step with a cost, and material sold without it retains TFA. That affects mass balance and can affect biological assays.
Not the answer, but possibly the question that gets there.
Coming back to post #25, because the follow-up matters more than the original answer.
Lyophilisation and reconstitution: the powder is stable for extended periods if kept cool and dry. Once reconstituted, stability decreases because the dissolved peptide is more vulnerable to oxidation and other degradation.
I have kept the units in throughout, for the obvious reason.
Aggregation is concentration-dependent, which is why reconstituting to a high concentration is not always the convenient choice it appears to be.
It reads as pedantry until the day it does not.
That reframing is the whole thing. The facts I already had.