Question 1

Which of the following statements best describes the function of the electron beam in a mass spectrometer?

Accepted Answer

It ionizes the sample molecules to form radical cations, which are then analyzed.

Question 2

A compound gives a molecular ion peak at $$m/z = 86$$ and a base peak at $$m/z = 57 in $$its mass spectrum. Which compound is most likely being analyzed?

Accepted Answer

Hexane ($$\mathrm{C_6H_{14}})$$

Question 3

What does the M+1 peak in a mass spectrum primarily result from?

Accepted Answer

The presence of $$^{13}\mathrm{C}$$ isotopes in the molecule.

Question 4

According to the nitrogen rule, which of the following statements is correct?

Accepted Answer

A compound with an odd molecular ion mass contains an odd number of nitrogen atoms.

Question 5

A mass spectrum of 2-bromopropane shows two molecular ion peaks at $$m/z = 122$$ and $$m/z = 124$$ with equal intensity. What does this indicate?

Accepted Answer

Bromine has two major isotopes of nearly equal abundance.

Question 6

Which fragmentation pattern is characteristic of aldehydes and ketones in mass spectrometry?

Accepted Answer

Alpha cleavage, forming a resonance-stabilized acylium ion.

Question 7

A mass spectrum of an unknown compound shows a molecular ion at $$m/z = 60. $$High-resolution mass spectrometry determines the exact mass as 60.0211. Which molecular formula is most likely?

Accepted Answer

$$\mathrm{C_2H_4O_2} ($$acetic acid)

Question 8

Which of the following best explains why the base peak is not always the molecular ion peak in a mass spectrum?

Accepted Answer

The most stable and abundant fragment ion, not necessarily the molecular ion, gives the base peak.

Question 9

Which of the following statements about gas chromatography-mass spectrometry (GC-MS) is correct?

Accepted Answer

GC separates mixture components, and MS provides the mass spectra for each component.

Question 10

A mass spectrum of an alcohol shows a prominent peak at $$M-18. $$What process most likely caused this peak?

Accepted Answer

Dehydration, resulting in the loss of $$\mathrm{H_2O}.$$

Question 11

Which of the following is a correct step in proposing a molecular formula from a molecular ion at $$m/z = 154$$?

Accepted Answer

Divide 154 by 14 to estimate the maximum number of $$\mathrm{CH_2$}$$ units.

Question 12

Which of the following best describes the information provided by high-resolution mass spectrometry compared to low-resolution mass spectrometry?

Accepted Answer

It allows for the determination of the exact molecular formula by measuring m/z to several decimal places.

Question 13

Which of the following fragmentation patterns is characteristic of amines in mass spectrometry?

Accepted Answer

Alpha cleavage between the alkyl group and the carbon bearing the amine nitrogen.

Question 14

A mass spectrum of 2-chloropropane shows two molecular ion peaks at $$m/z = 78$$ and $$m/z = 80 in a 3$$:1 ratio. What does this indicate?

Accepted Answer

Chlorine has two isotopes, $$^{35}\mathrm{Cl}$$ and $$^{37}\mathrm{Cl}$$, with a 3:1 abundance ratio.

Question 15

Which of the following best explains why the mass spectrum of methane ($$\mathrm{CH_4$}) $$shows peaks at $$m/z = 15, 14, 13,$$ and $$12$$?

Accepted Answer

Fragmentation of the molecular ion by loss of one or more hydrogen atoms.

Question 16

Which of the following is a real-world application of GC-MS as described in the study materials?

Accepted Answer

Detection of tetrahydrocannabinol (THC) in urine samples.

Question 17

Which of the following best describes the base peak in a mass spectrum?

Accepted Answer

The most abundant ion in the spectrum, assigned a relative abundance of 100%.

Question 18

Which of the following statements about the analysis of mixtures using GC-MS is correct?

Accepted Answer

Each component of the mixture is separated by GC and then analyzed by MS to obtain its mass spectrum.

Question 19

Which of the following is a common fragmentation pattern for alcohols in mass spectrometry?

Accepted Answer

Alpha cleavage and dehydration to form an alkene.

Question 20

A compound with the formula $$\mathrm{C_2H_5NO_2}$$ gives a molecular ion at $$m/z = 75. $$According to the nitrogen rule, how many nitrogen atoms does this compound contain?

Accepted Answer

One nitrogen atom.

Mass Spectrometry in Organic Chemistry: Principles, Interpretation, and Applications

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