As @salme.infoes mentioned in the comments, plain @ should work, in math or text mode. I had somehow managed generate a \@.
Every char has the category code from 0 up to 0 - Escape character; this signals the start of a control sequence. backslash \ (Unicode code point U+C) is.
I love Tex but sometimes you just bang your head against a wall because it just doesn't work. Here a little tip, if you need an "at" symbol in your.