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Module 8: Applying Chemical Ideas
Quick questions on Mass spectrometry of organic compounds explained: HSC Chemistry Module 8
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is how a mass spectrometer works?Show answer
The instrument has five stages:
What is reading a mass spectrum?Show answer
Molecular ion (M+). The peak at the highest $m/z$ (excluding the small isotope satellites) is the molecular ion. Its $m/z$ is the molecular mass of the compound.
What is common fragment losses?Show answer
So a loss of 17 ($OH$) suggests an alcohol; a loss of 45 ($COOH$) suggests a carboxylic acid; a loss of 29 ($CHO$) suggests an aldehyde.
What is isotope patterns?Show answer
Chlorine. $^{35}Cl$ : $^{37}Cl$ = 3 : 1. A compound with one $Cl$ shows M : M+2 = 3 : 1. Two chlorines give M : M+2 : M+4 = 9 : 6 : 1.
What is worked logic for an unknown?Show answer
Suppose a mass spectrum shows:
What is strengths and limits?Show answer
Strengths. Gives the exact molecular mass and structural fragments. Picks out chlorine, bromine and sulfur by isotope pattern. Sensitive to nanograms of sample. Used routinely in forensics, drug testing, environmental analysis (often coupled to gas chromatography, GC-MS).
What is molecular ion?Show answer
The peak at the highest $m/z$ (excluding the small isotope satellites) is the molecular ion. Its $m/z$ is the molecular mass of the compound.
What is base peak?Show answer
The tallest peak, set to 100% relative abundance. Often a fragment, not the molecular ion. The base peak indicates the most stable cation formed in fragmentation.
What is fragment peaks?Show answer
Other peaks. The difference between $M^+$ and a fragment is the mass of the neutral radical lost.
What is chlorine?Show answer
$^{35}Cl$ : $^{37}Cl$ = 3 : 1. A compound with one $Cl$ shows M : M+2 = 3 : 1. Two chlorines give M : M+2 : M+4 = 9 : 6 : 1.
What is bromine?Show answer
$^{79}Br$ : $^{81}Br$ $\approx$ 1 : 1. A compound with one $Br$ shows M : M+2 of roughly equal height. Two bromines give M : M+2 : M+4 = 1 : 2 : 1.
What is carbon?Show answer
$^{12}C$ : $^{13}C$ = 98.9 : 1.1, so the M+1 peak is about 1.1% per carbon atom. A 10-carbon molecule shows an M+1 peak about 11% of M; this can be used to count carbons.
What is strengths?Show answer
Gives the exact molecular mass and structural fragments. Picks out chlorine, bromine and sulfur by isotope pattern. Sensitive to nanograms of sample.
What is limits?Show answer
Destroys the sample during analysis. Cannot distinguish stereoisomers. Cannot always distinguish structural isomers if they fragment similarly (propan-1-ol and propan-2-ol have very similar spectra).
What is calling the tallest peak the molecular ion?Show answer
The molecular ion is the highest m/z, not necessarily the tallest. The tallest is the base peak.