Pretty much as per the question. Do you have to do this in one playthrough, or can you reload chapters and pick up the missing ones later?
Can you get the Lambda Locator achievement in multiple sessions in Half-Life 2
half-life-2
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Best Answer
Yes (as noted by the Half Life Wikia) you can get them out of order if you miss some and need to go back for them. Unfortunately I don't think the achievement visibly allows you to track your progress, but it does take note if you've ever found one at all.