They take your matrix & spike with cannabinoid standard.
Then see how much they get back.
Mission critical if lab is clearly missing cannabinoids
YOU know what you put in there.
You’re pretty damn sure they’re only recovering a fraction (less than 50% in some cases).
Having them prove they can(t) recover 100% from your matrix usually involves $$, but a reasonable lab director will waive that if it’s clear they are failing miserably (good luck on finding one of those)
Not an uncommon phenomenon