It combines both the techniques i.e., gas chromatography and mass spectrometry in a single unit to identify different substances present in a sample. It performs 100% specific tests and is hence called as the “gold standard” for forensic substance identification.
a) Principle: The basic principle followed by GC-MS is that different substances in a given mixture get separated when heated. These are carried through a column via an inert gas. As these substances emerge from the column, they enter mass spectrometer where they are identified based on the mass of the analyte molecule.
b) Working: The sample is injected into GC-inlet where it gets vaporized along with an inert gas (such as helium, xenon, argon etc) and then is carried through a stationary column. Sample mixture gets separated in the column based on their molecular weight and relative interactions with the column material. This whole unit is placed in an oven to maintain the temperature.
The separated sample enters the mass spectrometer which acts as a detector. The analyte gets bombarded with high energy electron bean resulting in breaking down of the analyte molecules into ions or fragments. These are then identified based on their mass to charge ratios. The mass spectrometer is connected to a computer data base which has stored information/library of standard chromatograms of different substances present throughout the globe. Identification as well as the abundance of molecule can be known by its chromatogram peak.
Fig. 8: GC-MS schematic representation.
c) Applications of GC-MS:
• In the field of metabolic profiling
• Detection of constituents in diverse plant tissues
• In human dosimetry
• Identification of volatile components
• Analysis of pesticides in food industry
• Environmental and forensic application.
d) Specifications for sample analysis by GC-MS:
? Name of the instrument: Agilent 6890GC with 5973N MSD
? Column: HP 5MS column (30mm length x 0.25mm internal diameter x 0.25mm thickness)
? Oven temperature program
• Initial temperature : 40o C
• Hold up time : 2 minutes
• Rate : 10o C/minute
• Final temperature : 280o C
• Hold up time : 10 minutes
• Total run time : 32 minutes
• Injection temperature : 280o C
• Auxillary temperature : 280o C
• Ionization source : 230o C
• Quadrapole temperature : 150o C
• Software : Agilent chemstation
• Library database : NIST/WILT2Y (IN 1999)
• Carrier gas : helium
• Flow rate : 1 ml/ minute
• Injection mode : splitters
• Split ratio : 10 : 1
• Ionization mode : electron capture
• Mass scan range : 29 to 600 dalton