Imagine a big glass jar full of candy, a colorful mixture of jelly beans. You want to know how rare your favorite green ones are. Specifically, you want to know the number of green ones relative to the number of grams of the whole mixture. If you just pull out a handful from the jar and meticulously count the number of green jelly beans, you don’t know what fraction of the total candy you removed! You don’t know the total weight of candy, sans jar, or the weight you removed for bean counting. Chemists analyzing the trace metal atoms in a solid sample face just this problem. Using a technique known as laser ablation mass spectrometry, they can count atoms removed from the solid sample, but they don’t know how much of the sample was removed and measured, or how it relates to the total mass of the sample.