An ultra-fast GC (zNose®) with electronically variable sensitivity controls can be used to quantify the concentration of methyl isocyanate (MIC) in two different vapor streams with MIC vapor pressure spanning 0-50 ppb and 0-1000 ppm. The sum of all instrument temperatures, sampling times, and sensitivity settings is called a GC Method and two different GC methods are used for the two vapor streams. Precision is the ability to repeat a measurement and is equal to the standard deviation of a replicate set of measurements taken of a constant vapor concentration. At a low signal to noise levels, minimum detection level is statistically related (proportional) to the standard deviation.