Download Word - ASDL Community

Instructions: Use the textual materials found on the ASDL Mass Spectrometry Toolbox to generate
answers to the following list of questions. In some cases you will need to explore a topic more deeply
using the links provided by the ASDL collection on mass spectrometry.
1. What samples are appropriate for analysis by mass spectrometry?
Most samples are now considered appropriate for analysis by mass spectrometry. Initially, this type
of analysis was limited by the molecular weight and by the volatility of the analyte, but with modern
ionization techniques, analysis of even large proteins is routine.
2. What are the significant differences between chemical ionization and electrospray ionization?
Chemical ionization is a soft ionization technique where a blanket gas is ionized via electron impact.
This ionized blanket gas typically undergoes a reaction to create a super acid (CH4  CH5 +) which can
transfer a proton to the analyte also present in the EI source with the blanket gas.
ESI is also a soft ionization technique, but here charge is imparted to the analyte by application of a
high voltage between a needle with a flowing liquid stream containing the analyte and a cathode. As
the liquid evaporates, droplet size decreases and coulombic forces cause the droplets to break
apart. Once the solvent is removed, the charge remains on the analyte. Depending on the size of the
analyte, one or more charges will remain on the analyte.
3. Predict the potential fragment (daughter) ions produced during the electron-impact ionization of
acetophenone. How will this change if chemical ionization is used instead of electron impact?
The mass spectrum is shown in the textual materials. Using the Lewis structure of the molecule, one
can predict the structure of a number of fragment ions. If CI were used, then the molecular ion + 1
would be the dominant peak.
4. Explain how a quadrupole mass filter differs from a time-of-flight mass analyzer.
The quadrupole applies a series of AC and DC potentials to a set of four rods. Ions with different m/z
will travel through the quadrupole only if the trajectory is correct. Different potentials will cause
different ions to achieve a stable trajectory. The process is based on momentum and electrostatics.
In time-of flight instruments, all molecules are given the same kinetic energy. Since KE = ½ mv2, a
heavier ion will travel a distance L in a longer time.
5. Explain why an exact mass measurement (or a resolution of 2500 or more) is necessary to
distinguish N2 from CO and C2H4.
All three compounds have a nominal mass of 28. Exact masses are 28.0061, 27.99491, and 28.0313,
respectively. Since Resolution = m/m, the resolution necessary to distinguish these ions would be
= 28/(.01119)=2502
6. Explain the term molecular ion and list the isotopic-related ions that would be present in a mass
spectrum of CH2Cl2.
A molecular ion is the charged ion related to the intact molecule. It often has the symbol M+
associated with it. When dealing with elements that have a significant isotopic abundance (like Cl or
Br), molecular ions for species with the different isotopes will also appear. In this case one would
12 1 235 35
see a molecular ion with nominal mass of 85 ( C H Cl Cl) with a smaller peak at 87
(12C1H237Cl35Cl), and an even smaller peak at 89 (12C1H237Cl37Cl). The height of each isotopic peak is
related to the abundance of that isotope.
7. Why is high vacuum necessary for mass spectrometry?
Reduces random noise, false signaling. In addition, collisions of ions with other molecules can cause
the ions to scatter or neutralize which will decrease sensitivity.