If not using any benzyl alcohol – and I prefer to use none – then after the 19 mL total of powder and solubility enhancer in our example, we have 31 mL left to make up our total volume of 50 mL. This is the oil volume. Ethyl oleate can be an excellent choice, as it provides a thinner (less viscous) solution than other oils and can have a little more solubizing ability as well. However, other vegetable oils such as soybean or sesame oil may be used. If acquiring oil at the supermarket, Wesson soybean oil is a proven choice.
If you prefer using benzyl alcohol to have a bacteriostatic agent, I recommend using it at only 2% of the final total volume, and no more than 5%. In this example, if we used benzyl alcohol, we’d use 1 mL (2% of 50 mL) and we’d then need only 30 mL of carrier oil.
Let’s also have the materials we need. We’ll need a sufficiently large vial or vials to put our product into, we’ll need a large syringe such as 30 mL or 50 mL though if you’re patient smaller syringes can be used, we’ll need a large gauge needle such as 23 gauge, and we’ll need at least one submicron syringe filter to attach to the syringe (0.45 micron or smaller.) We’ll also need another syringe and needle to measure out desired volume of solubility enhancer. Preferably we’d also have an insulin needle with the plunger removed, but if this is not available a larger gauge needle can substitute for it.
Now that we have all this taken care of we can proceed.
In a first case, let’s say you don’t have an accurate balance to weigh the powder, but you ordered a particular amount, for example 10 grams. If necessary, it’s usually not a problem to assume that your bag of powder contains about the amount you ordered, or close enough. Is it actually reasonable to trust that the amount received is indeed about what was ordered? Well, if the supplier can’t be trusted to send the amount you ordered, then should you trust he sent you a correct and pure powder either? In for a penny, in for a pound. If you don’t have good stuff, then the least of your problems would be the exact weight. My biggest concern with skipping the weighing step is that you may miss detecting that you have a dishonest or careless supplier, if that’s the case.
If you have a balance, then it’s desirable to have weighing paper as well. This can easily be obtained online. It’s not actually necessary to use weighing paper – anything can be used that is probably relatively free of bacteria and dust and is easy to pour powder from – but weighing paper is convenient and inexpensive. If you don’t have it, freshly unrolled aluminum foil can be used, weighing the powder onto the side which had been inside the roll. However, aluminum foil can be annoying to pour powder from.
To weigh powder, choose an area where the air is as clear and still as possible. Have ready the large syringe, a thoroughly cleaned implement such as a laboratory spatula or a table knife, and an alcohol wipe available to give it a last moment, final cleaning as well. Slowly dispense the powder onto the weighing paper on the balance, either by carefully pouring or using your implement to make transfers. Usually it’s best to use the implement to add the final amounts. If you overshoot, use the implement to take excess off of the weighing paper. Avoid breathing onto the powder while weighing. You don’t have to make this a speed contest – in fact, avoid rushing – but it’s also better to not draw this out into taking many minutes.
When having the correct amount of powder on the weighing paper, or if assuming your bag already has the correct amount of powder in it, now add it to the large syringe by pouring into the back end. The syringe will already have the syringe filter attached to it and a large gauge needle attached to the syringe filter. That needle will have its original cap in place.
Now, after the addition of powder, fill another syringe with the calculated amount of benzyl benzoate. You may find it easier to pour into the back of the syringe than to draw through the needle, but either method will work. Add the benzyl benzoate into the back of the large syringe, and return the plunger to the back of the large syringe. With motion, mix the powder and benzyl benzoate together until everything is dissolved. This may take several minutes.
Some or all of the needed carrier oil is now added to the back of the syringe, bringing the volume up to a visible mark. For example, if using a 30 mL syringe and making 50 mL of preparation, simply add oil to the 30 mL mark and then put the plunger back in place. Then mix the oil with what’s already in the syringe, and place the needle of the syringe into the receiving vial. Insert the insulin needle or the additional large gauge needle into the receiving vial (this is to allow air to escape.) Slowly push the preparation through the filter. This may take many minutes of work: use substantial but not excessive thumb pressure, to avoid breaking the filter.
After pushing through the entire solution, if less than the full amount of oil has been used, now add the remainder to the back of the syringe, and push it through as well.
Your preparation is complete.
Heat sterilization is not necessary when this procedure has been followed, but if you want to do it for peace of mind anyway, use a bath of hot water no hotter than 70º C (158º F). All that is needed is for the preparation to come close to this temperature, so for example even 10 minutes time is entirely sufficient to kill any bacteria that might have been present, if there were any. Do not bake.