a) Consider the GC-MS measurement of a mixture of carboxylic acid methyl-, ethyl- and propyl esters, containing the range from C10 to C18 acids. The measurement was performed in the repetitive scanning mode.
Suggest single or multiple RICs to identify:
- Only methyl esters: m/z 74 and/or m/z 87.
- Only ethyl esters: m/z 88 and/or m/z 101.
- Only palmitic acid methyl ester: m/z 74 or m/z 87 plus molecular ion, m/z 270.
b) Why couldn’t you identify palmitic acid methyl ester using a single m/z trace from this GC-MS data?
Using either m/z 74 or m/z 87 would show all methyl esters, whereas using m/z 270 alone would yield the myristic acid propyl ester, too. Strictly speaking, the combination of m/z 74 or m/z 87 plus molecular ion, m/z 270, could also identify methyl esters of isomeric acids of the target compound.
c) Aliphatic alcohols yield no or extremely weak molecular ion peaks in EI-MS. For GC-MS, they are often converted into their trimethylsilyl derivatives which form a characteristic ion at m/z 73. What is the m/z 73 ion and why shouldn’t it be used as a tracer for alcohols in GC-MS?
The ion at m/z 73 is [Si(CH3)3]+. This ion is also present in the column bleed of most stationary phases of GC columns.