Mass Spectrometry (Cambridge (CIE) A Level Chemistry): Flashcards
Exam code: 9701
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Define molecular ion.
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Define molecular ion.
A molecular ion (M+•) is formed when a molecule loses one electron during electron bombardment in mass spectrometry, producing a positively charged species with one unpaired electron.
What is the m/e ratio, and how is it used to separate ions in a mass spectrometer?
The m/e ratio is the mass (m) of an ion divided by its charge (e). Ions are separated by deflection in a magnetic field: ions with a smaller m/e ratio are deflected more and detected first.
True or False?
The base peak in a mass spectrum corresponds to the most abundant ion fragment.
True.
The base peak is the tallest peak in the mass spectrum. It represents the most abundant (most stable) ion fragment produced during electron bombardment and fragmentation.
In mass spectrometry, molecules are bombarded with ....... electrons, causing them to lose an electron and form a positively charged ...........
In mass spectrometry, molecules are bombarded with high-energy electrons, causing them to lose an electron and form a positively charged molecular ion.
An ion 54Fe2+ is detected in a mass spectrometer. What is its m/e value?
m/e = 54 ÷ 2 = 27. Ions with a smaller m/e value are deflected more by the magnetic field, so 54Fe2+ is deflected more than a heavier or less charged ion with a larger m/e value.
True or False?
Isotopes of the same element have the same number of protons but different numbers of neutrons.
True.
Isotopes are atoms of the same element with the same proton number (and electron number) but different mass numbers due to differing neutron counts. For example, Cl-35 and Cl-37 are isotopes of chlorine.
Define relative abundance.
Relative abundance is the proportion of a particular isotope in a naturally occurring mixture of isotopes of an element, usually expressed as a percentage. Peak heights in a mass spectrum show the relative abundance of each isotope.
Chlorine has two naturally occurring isotopes: Cl-35 with a relative abundance of .......% and Cl-37 with a relative abundance of .......%.
Chlorine has two naturally occurring isotopes: Cl-35 with a relative abundance of 75% and Cl-37 with a relative abundance of 25%.
Why do only positively charged fragment ions reach the detector in a mass spectrometer?
The fragments are accelerated by an electric field and then deflected by a magnetic field. Only positively charged ions respond to these fields and are directed to the detector. Neutral molecules and radicals are not deflected and do not reach the detector.
relative atomic mass
Relative atomic mass (Ar) is the weighted average mass of the isotopes of an element, taking into account their relative abundances, expressed relative to 1/12 the mass of carbon-12.
State the formula used to calculate the relative atomic mass of an element from isotope data.
Ar = [(relative abundance1 × mass1) + (relative abundance2 × mass2) + ...] ÷ 100
where relative abundances are expressed as percentages.
True or False?
Isotopes of the same element have different numbers of protons.
False.
Isotopes of the same element have the same number of protons (and electrons) but different numbers of neutrons, giving them different mass numbers.
Boron has two isotopes: 10B (19.9%) and 11B (80.1%). The relative atomic mass of boron = [(19.9 × ......) + (80.1 × ......)] ÷ 100 = .......
Boron has two isotopes: 10B (19.9%) and 11B (80.1%). The relative atomic mass of boron = [(19.9 × 10) + (80.1 × 11)] ÷ 100 = 10.80.
How can the relative abundance of isotopes be determined experimentally?
By mass spectrometry. The heights of the peaks in the mass spectrum correspond to the proportion of each isotope present in the sample, giving their relative abundances.
True or False?
The relative atomic mass of an element is always equal to the mass number of its most abundant isotope.
False.
The relative atomic mass is a weighted average of all isotope masses according to their relative abundances. It is close to the most abundant isotope's mass but is not necessarily equal to it.
isotope
An isotope is an atom of the same element as another atom but with a different mass number, due to a different number of neutrons. Isotopes have the same proton number and chemical properties.
Oxygen has three isotopes: 16O (99.76%), 17O (0.04%) and 18O (0.20%). The Ar of oxygen = [(99.76 × 16) + (0.04 × 17) + (0.20 × 18)] ÷ 100 = ...... (to 2 d.p.).
Oxygen has three isotopes: 16O (99.76%), 17O (0.04%) and 18O (0.20%). The Ar of oxygen = [(99.76 × 16) + (0.04 × 17) + (0.20 × 18)] ÷ 100 = 16.00 (to 2 d.p.).
Why does relative atomic mass appear as a decimal rather than a whole number for most elements?
Because relative atomic mass is a weighted average of the masses of all naturally occurring isotopes of the element. The weighting reflects the relative abundance of each isotope, which rarely produces a whole number result.
Define molecular ion peak.
A molecular ion peak is the peak with the highest m/e value in a mass spectrum (ignoring M+1 and M+2 peaks). It corresponds to the intact molecule that has lost one electron and gives the relative molecular mass of the compound.
What information about a compound can be obtained from the molecular ion peak in a mass spectrum?
The molecular ion peak gives the relative molecular mass of the compound. The m/e value of the M+ peak equals the molecular mass because the ion carries a single positive charge.
True or False?
A bromine-containing compound shows two molecular ion peaks of approximately equal height, separated by 2 mass units.
True.
Bromine has two naturally occurring isotopes, 79Br and 81Br, with approximately equal abundance (50:50). This produces two M+ peaks of equal height at m/e values 2 apart.
In the mass spectrum of alkanes, the fragment C2H5+ appears at m/e = ....... and the fragment C3H7+ appears at m/e = ........
In the mass spectrum of alkanes, the fragment C2H5+ appears at m/e = 29 and the fragment C3H7+ appears at m/e = 43.
Why do alcohols often show a peak 18 units below the molecular ion peak in their mass spectrum?
Alcohols readily lose a water molecule (M = 18) during fragmentation. This produces a [M-18]+ peak corresponding to the alkene cation formed after loss of H2O.
True or False?
Fragmentation patterns can distinguish between compounds with the same molecular mass.
True.
Different structural isomers can have the same molecular mass but produce different fragmentation patterns. Comparing the m/e values and relative intensities of fragment peaks allows the structure of the compound to be determined.
Define base peak.
A base peak is the most intense (tallest) peak in a mass spectrum. It corresponds to the most stable (most abundant) fragment ion produced during electron bombardment.
In the mass spectrum of propan-1-ol, loss of a water molecule produces a peak at m/e = ......., and the CH2OH+ fragment appears at m/e = ........
In the mass spectrum of propan-1-ol, loss of a water molecule produces a peak at m/e = 42, and the CH2OH+ fragment appears at m/e = 31.
How does the m/e value of the molecular ion peak compare to the molecular mass of the compound? Explain why.
The m/e value of the molecular ion peak equals the relative molecular mass. This is because the molecular ion carries a single positive charge (one electron removed), so m/e = M/1 = M.
Define M+1 peak.
An M+1 peak is a small peak one mass unit above the molecular ion peak in a mass spectrum. It arises from molecules containing a 13C atom instead of 12C. Its size relative to M+ can be used to calculate the number of carbon atoms.
State the formula used to calculate the number of carbon atoms from the M+1 peak.
n = (100 × abundance of [M+1]) ÷ (1.1 × abundance of M+)
where n is the number of carbon atoms and abundances are relative (e.g. from peak heights).
True or False?
A compound with more carbon atoms will have a larger M+1 peak relative to the M+ peak.
True.
Each carbon atom has a ~1.1% chance of being 13C. The more carbon atoms in the molecule, the greater the probability that one of them is 13C, so the M+1 peak is proportionally taller.
Carbon-13 makes up approximately .......% of all carbon atoms. The ratio of 13C to 12C is approximately ........
Carbon-13 makes up approximately 1.1% of all carbon atoms. The ratio of 13C to 12C is approximately 1:99.
A compound has an M+ peak with relative abundance 85 and an [M+1] peak with relative abundance 3. How many carbon atoms are present?
n = (100 × 3) ÷ (1.1 × 85) = 300 ÷ 93.5 = 3.21
There are 3 carbon atoms in the compound.
True or False?
A compound containing one chlorine atom shows M+ and [M+2] peaks in a ratio of 3:1.
True.
Chlorine has two isotopes: 35Cl (75% natural abundance) and 37Cl (25%). A molecule containing one Cl atom therefore shows M+ (due to 35Cl) and [M+2] (due to 37Cl) peaks in a 3:1 ratio.
Define M+2 peak.
An M+2 peak is a peak two mass units above the molecular ion peak. It arises from the presence of heavier halogen isotopes (37Cl or 81Br) in the molecule and indicates the presence of chlorine or bromine.
A compound with one bromine atom shows M+ and [M+2] peaks in a ratio of ....... because 79Br and 81Br have ....... relative abundances.
A compound with one bromine atom shows M+ and [M+2] peaks in a ratio of 1:1 because 79Br and 81Br have equal relative abundances.
What pattern of M+, [M+2] and [M+4] peaks is observed for a compound containing two bromine atoms, and in what ratio?
Three peaks are observed: M+ (79Br + 79Br), [M+2] (79Br + 81Br) and [M+4] (81Br + 81Br), in a ratio of 1:2:1.
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