The thing about 99.99% is that it is a gas purity figure which is reported in volume% by convention.

That means there are measured impurities of no more than 0.0144% or in different terms, 144 parts-per-million by volume.

IOW up to 144 ppm can be rounded down to 0.01% and you still call it 99.99.

The impurities are usually nitrogen, oxygen, water, and total hydrocarbons (if present usually in the form of trace methane). When you think about it though, all of these molecules are much heavier than helium (or hydrogen the other very light gas) so the actual percentage of impurities by weight is much higher than by volume for a light bulk gas like this.

Therefore a complete tank of 99.99 helium can pass a whole lot more absolute impurities through your scientific instrument than a whole tank of 99.99 nitrogen, if you had the choice.

When the manufacturer requirement is 99.9995 minimum, there are experts that give recommendations for special applications and other types of instruments:

https://community.agilent.com/technical/gcms/w/gcms-wiki/135...

But if you're always doing some final purification of your own anyway, might as well follow the other experts from the same company and use ballon-grade instead after all:

https://www.agilent.com/cs/library/brochures/brochure-reduce...

Not affiliated, I use their instruments but set up higher capacity filtering systems.

Very few tank suppliers have a specialty gas lab on site, not even much routine testing ability. These are purely industrial environments, and when you do order a true specialty gas it usually has to be shipped in from another location where it was certified by their main lab. These are not like the welding gases and propane which fly off the shelf and are always stocked. Helium is actually a commodity though, only the high-purity grades are considered "specialty gases".

When you go to pick up your own helium tanks, everybody has on their hardhats, safety glasses, steel-toed boots and fire-retardant coveralls like any other plant, and the main floor where the trucks pull up to the docks is all covered in rows of organized tanks. But very few of the tanks are chained to the storage locations around the periphery, most of them are in rows like dominoes ready to fall. They don't need to be chained since they are not in storage, they are being moved. 90% of their deliveries are made in their own trucks so when you show up you have to wait for a work order to be generated and handed to an operator who will get your tanks.

The dude comes out with a clipboard, gets me to pull up to the right location, picks the first tanks from the row of heliums, rolls them over to the truck. Writes down the tank numbers, and attached to the paperwork is the little sticker that says what grade of helium the contents were certified to exceed. Puts the stickers on the tanks and I'm on my way.

One time my vice-president got a quote from one of the service vendors capable of helping labs transition from helium to hydrogen when appropriate. Too late I was already a pioneer at this and it's supposed to save you money like it already is, not cost you money like some others are paying. We are going to continue to use some helium, and although I found 90% of the helium he had paid for over 20 years had leaked before I got there, we both knew it was not exorbitantly expensive back then. None was being wasted now, and I had to emphasize any clown can afford helium !

How do they do it? Well they're getting the helium for 50 cents a cubic foot and selling it for $1, the balloon is extra.

We're making at least $5 a cubic foot, give me a break.

I knew something was up when I drove away from the loading dock and the next driver had the baggier-and-brighter-orange-than-usual coveralls, really bushy red hair that barely fit under his hardhat, and some truly hellacious steel-toed footwear.