INSTRUMENTAL ANALYSIS COMPREHENSIVE EXAM
QUESTIONS AND SOLUTIONS 2025/2026 ALL GRADED A+
✔✔What are the ionizing agents for the gas-phase sources? - ✔✔
✔✔What is required in order to use gas-phase sources? - ✔✔∙the sample must be
converted to the gas phase (vaporized) for the ionization process
∙restricted to thermally stable compounds with boiling points less than about 500°C
✔✔What are desorption sources? - ✔✔with desorption sources, solid or liquid samples
are converted directly into gaseous ions
✔✔What are the ionizing agents for the desorption sources? - ✔✔
✔✔Where does the sample come from before being introduced to the ionization
source? - ✔✔it can be directly introduced to the ionization source or it can come fro
another separation method like GC
✔✔What are the axis variables on a molecular mass spectrum? - ✔✔x= m/z
y=relative abundance
✔✔How is relative abundance measure on a molecular MS? - ✔✔The highest peak is
set at 100% and the rest are compared accordingly.
✔✔How is electron ionization/impact (EI) done? - ✔✔Electrons are emitted from a
heated tungsten (W) or rhenium (Re) filament and accelerated by applying
approximately 70 V between the filament and the anode.
The paths of the electrons and molecules are at right angles and intersect near the
center of the source, where collision and ionization occur.
✔✔What is the primary product of EI and how is it formed? - ✔✔The primary product is
singly charged positive ions formed when the energetic e⁻ approach molecules closely
enough to cause them to lose electrons by *electrostatic repulsion*
M + e⁻ → (M∙⁺) + 2e⁻
M=analyte molecule
M∙⁺= molecular ion
✔✔What happens to the positive ions produced by EI? - ✔✔The positive ions are
attracted through the slit in the first accelerating plate by a small potential difference
(typically 5 V) that is applied between this plate and the repellers
✔✔What happens is the ions do not have the same KE when they reach the mass
analyzer? - ✔✔their travel times will differ, causing interferences that result in a broad
peak
, ✔✔Why must EI be done under high vacuum conditions? - ✔✔1. To ensure the filament
doesn't burn out.
2. To help vaporize the samples.
3. To reduce collision between formed ions and atmospheric gases (might cause
neutralization or formation of other ions).
4. To remove sample from the instrument after analysis (reason instrument is left on for
a while after use).
✔✔How much energy is typically used for EI and what is its effect? - ✔✔EI uses 70eV =
6700 kJ/mol
Since typical bond energies are around 200-600 kJ/mol, this causes extensive
fragmentation
✔✔What filament is used for EI source? - ✔✔typically made of ??
-Source of 70eV electrons
✔✔What is the target of the EI source? - ✔✔anode used in association with the filament
producing electrons
✔✔What is the repeller in the EI source? - ✔✔positively-charged electrode used to push
positive ions out of the ionization source
✔✔What is the lens stack in the EI source? - ✔✔series of increasingly more negative
electrodes used to accelerate the ions to a constant KE
✔✔What are the typical reactions during EI? - ✔✔
✔✔What type of ionization method is EI? - ✔✔EI is a hard ionization source
✔✔What are isotopic peaks? - ✔✔peaks occurring at m/z greater than that of the
molecular ion, attributable to ions with same chemical formula but different isotopic
compositions.
✔✔When are isotopic peaks particularly important? - ✔✔when you have bromide,
chloride or other unique isotopic element (H, C, N, Br, Cl)
✔✔What is a molecular ion isotopic cluster? - ✔✔slide 13
✔✔What are the typical losses (and their values) from the molecular ion? - ✔✔
✔✔What type of ionization method is chemical ionization (CI)? - ✔✔CI is a soft
ionization method
QUESTIONS AND SOLUTIONS 2025/2026 ALL GRADED A+
✔✔What are the ionizing agents for the gas-phase sources? - ✔✔
✔✔What is required in order to use gas-phase sources? - ✔✔∙the sample must be
converted to the gas phase (vaporized) for the ionization process
∙restricted to thermally stable compounds with boiling points less than about 500°C
✔✔What are desorption sources? - ✔✔with desorption sources, solid or liquid samples
are converted directly into gaseous ions
✔✔What are the ionizing agents for the desorption sources? - ✔✔
✔✔Where does the sample come from before being introduced to the ionization
source? - ✔✔it can be directly introduced to the ionization source or it can come fro
another separation method like GC
✔✔What are the axis variables on a molecular mass spectrum? - ✔✔x= m/z
y=relative abundance
✔✔How is relative abundance measure on a molecular MS? - ✔✔The highest peak is
set at 100% and the rest are compared accordingly.
✔✔How is electron ionization/impact (EI) done? - ✔✔Electrons are emitted from a
heated tungsten (W) or rhenium (Re) filament and accelerated by applying
approximately 70 V between the filament and the anode.
The paths of the electrons and molecules are at right angles and intersect near the
center of the source, where collision and ionization occur.
✔✔What is the primary product of EI and how is it formed? - ✔✔The primary product is
singly charged positive ions formed when the energetic e⁻ approach molecules closely
enough to cause them to lose electrons by *electrostatic repulsion*
M + e⁻ → (M∙⁺) + 2e⁻
M=analyte molecule
M∙⁺= molecular ion
✔✔What happens to the positive ions produced by EI? - ✔✔The positive ions are
attracted through the slit in the first accelerating plate by a small potential difference
(typically 5 V) that is applied between this plate and the repellers
✔✔What happens is the ions do not have the same KE when they reach the mass
analyzer? - ✔✔their travel times will differ, causing interferences that result in a broad
peak
, ✔✔Why must EI be done under high vacuum conditions? - ✔✔1. To ensure the filament
doesn't burn out.
2. To help vaporize the samples.
3. To reduce collision between formed ions and atmospheric gases (might cause
neutralization or formation of other ions).
4. To remove sample from the instrument after analysis (reason instrument is left on for
a while after use).
✔✔How much energy is typically used for EI and what is its effect? - ✔✔EI uses 70eV =
6700 kJ/mol
Since typical bond energies are around 200-600 kJ/mol, this causes extensive
fragmentation
✔✔What filament is used for EI source? - ✔✔typically made of ??
-Source of 70eV electrons
✔✔What is the target of the EI source? - ✔✔anode used in association with the filament
producing electrons
✔✔What is the repeller in the EI source? - ✔✔positively-charged electrode used to push
positive ions out of the ionization source
✔✔What is the lens stack in the EI source? - ✔✔series of increasingly more negative
electrodes used to accelerate the ions to a constant KE
✔✔What are the typical reactions during EI? - ✔✔
✔✔What type of ionization method is EI? - ✔✔EI is a hard ionization source
✔✔What are isotopic peaks? - ✔✔peaks occurring at m/z greater than that of the
molecular ion, attributable to ions with same chemical formula but different isotopic
compositions.
✔✔When are isotopic peaks particularly important? - ✔✔when you have bromide,
chloride or other unique isotopic element (H, C, N, Br, Cl)
✔✔What is a molecular ion isotopic cluster? - ✔✔slide 13
✔✔What are the typical losses (and their values) from the molecular ion? - ✔✔
✔✔What type of ionization method is chemical ionization (CI)? - ✔✔CI is a soft
ionization method
