Gas Chromatograph

Overview

A gas chromatograph separates a mixture into its individual compounds and measures how much of each is present. It is the workhorse of analytical chemistry for anything volatile, fuels, flavors, solvents, environmental pollutants, blood alcohol. The principle is simple to state: vaporize the sample, push it through a long thin tube with an inert gas, and because different compounds cling to the tube's inner coating to different degrees, they come out the far end at different times, one cleanly after another.

The output is a chromatogram, a row of peaks plotted against time. Where a peak appears identifies the compound; how big it is says how much there was. Everything in the instrument exists to make those peaks sharp, reproducible, and correctly timed.

How it works

A microliter of sample is injected through the septum of the Injector Port, a heated port that flash-vaporizes it inside a glass liner. The carrier gas sweeps the vapor onto the head of the Capillary Column, a fused-silica capillary tens of meters long, coiled inside the Column Oven. The column's inner wall carries a stationary phase; each compound spends time adsorbed on it and time moving in the gas, and that balance, different for every compound, sets how fast it travels.

The oven is the tuning knob. Rather than hold one temperature, the Column Oven ramps on a program, so early-eluting light compounds separate at low temperature while a rising temperature later drives heavy ones off without smearing. A circulation fan keeps the oven uniform, and a cool-down flap drops it quickly between runs.

As each compound leaves the column it reaches the FID/TCD Detector. In a flame ionization detector the effluent burns in a hydrogen flame, producing ions in proportion to the carbon present; the collector electrode reads that current, and the Electrometer Board amplifies it into the peak. A thermal-conductivity cell covers species the flame ignores. Holding the carrier flow rock-steady is the job of the Flow Controller, and the Control Board runs the temperature programs and streams the chromatogram out while the operator follows the run on the Display & Keypad.