was stored in the fridge for its whole life. for the past I would say year and a half I left it there as I stopped DIY and just bought pre mades as I got lazy. Will this nicotine still be good to use or should I consider getting a new bottle?
There is no fixed shelf life for it and if it was stored cool, dark and in a bottle without much air, it's probably perfectly fine. About 30 months isn't that much, I know people using nicotine base that's 5+ years old. From a health point of view, it's certainly still fine.
I'd go after the actual impression. Is it colorless to light yellow or already dark brown (the browner, the more oxidation happended)? Does it smell very unpleasant,like fish or is is pretty much odorless ? Is it clear, independant from the color? If it's not brown, but colorless to yellowish, doesn't have a strong smell and isn't cloudy, I'd shake it and then vape a bit of with a lighter flavour you know very well.
Doing a test without flavouring works, too, but if you never vaped pure nicotine base before, it will probably taste strange and you don't really have a comparison. Taste fine = use it, Taste weird, peppery, harsh = trash it.
You're dealing with preservatives. They don't "go bad" unless heated enough to breakdown it's long chain molecule (the veg part, cooking temps hot). The only thing that might happen over time is loss of potency with the nic. I've got an open cup here half full of failed mixing experiments. It's sat here for over 6 months, an aside from the normal flavor darkening, it's as clear as it was when it was bought new. No I wouldn't vape it on a bet, it's been open to dust and God knows what, but nothing growing in it, no bateria, etc..