Question

The standard method for domestic laboratory identification of fire remnant evidence mainly refers to ASTM E1618 gas chromatography mass spectrometry. Please answer the questions related to the mass spectrometer:

(1) The five basic components and functions of the instrument.

(2) Total ion chromatogram.

(3) Mass-to-charge ratio (m/z) of the main two fragmentation ions of alkane (alkane).

Answer 1

1. The GC-MS (Gas chromatography- Mass spectrometry), as the name suggests, is made of two parts: the gas chromatograph and the mass spectrometer. The gas chromatograph has a gas stream (carrier gas, mobile phase), usually an inert gas, which mixes with the liquid stationary phase from the sample injector. The gas-phase then passes through the column, which is located in an oven where the temperature of the gas can be controlled.

The difference in the chemical properties between different molecules in the mixture and their relative affinity for the mobile phase will help in the separation of the sample as the sample passes through the column. The molecules are retained by the column at their retention times and are next passed up to the mass spectrometer where they are captured, ionized, accelerated, deflected and detected using the mass-to-charge ratio of the ionized fragments.

GC-MS is mostly used for:

• Tracking of organic pollutants in the environment.
• Detection of illegal narcotics.
• Detection of any explosives, chemical warfare agents, etc.
• Fire investigation.
• Identification of unknown substances from probe missions.

2. Total ion chromatogram is obtained when all the mass peaks for a scan are added up to obtain a total ion current. When this total ion current is plotted against the scan number or time, we obtain a total ion chromatogram (TIC). TIC does not consider a particular set of peaks rather it considers the entire log.
3. Ion fragmentation of alkanes can be considered for three sets of alkanes:

• Straight chain alkanes:

The largest peak is basically for an alkyl radical cation, C_nH_2n+1. The next peaks are separated by 14 amu (loss of CH₂) and [M-15] peak for a loss of CH₃ radical is mostly weak or absent.

e.g. CH₃-CH₂-CH₃

CH₃-CH₂-CH₃]·⁺ → 44     M+

CH₃-CH₂]⁺ + CH₃]·   → 29    M-15

CH₃-CH₂]· + CH₃]⁺ → 15    M-29

• Branched alkanes:

Bond cleavage mostly takes place at the site of branching with the removal of the largest substituent as the radical. The radical achieves stability by delocalization of lone pair of electrons.

e.g.

(CH₃)₃-C]⁺ + (CH₃)₂-CH-CH₂]· → 57(most stable)   [M-57]

• Cycloalkanes:

A significant peak is observed at M-28 (often the base peak) due to loss of ethene (entropically favorable).

e.g.