This is a Gas Chromatograph (GC for short). It separates volatile compounds and measures the concentration of those compounds using various detectors.  Brewing scientists can use a GC to find the amount of various off-flavors present in beer.
One off-flavor that is looked at the most is 2,3 butanedione – more commonly known as diacetyl.  Diacetyl is a vicinal diketone produced from alpha-acetolactate during amino acid synthesis in yeast metabolism. It tastes like movie theater popcorn butter or butterscotch.  Most people can detect diacetyl in beer at a scant 30 parts per billion in beer. 
Usually yeast will re-uptake diacetyl and turn it into 2,3 butanediol – a flavorless compound. Some brewers get into trouble with this off-flavor when they take the beer off of the yeast too early. The GC helps detect this compound and lets the brewer know when fermentation is complete.
Other off-flavors that the GC can detect are acetaldehyde (green apples, green Jolly Rancher), dimethyl sulfide (creamed corn, vegetal), and hydrogen sulfide (rotten eggs)

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