CHEM 5181 – Fall 2006
Homework Assignment #2
Assigned 5-Sept / Due 14-Sept
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Please make an effort to produce a clean solution. Use a big “font” and plenty of space for
your diagrams and arguments.
Please briefly explain your answers, and solution procedures. We grade as much on the
thinking process as on the final result.
The current homework will always be posted on the course page in Word to save you the
typing, should you decide to type your homework.
Problem 2.1- Electrospray Ionization Charge States.
Below are two expanded views of an electrospray mass spectrum of an unknown biomolecule
collected with the Orbitrap high-resolution mass spectrometer in the CU-Boulder Mass Spectrometry
Facility. The first spectrum shows a charge envelope between m/z 650 and 1150.
(a) Using knowledge of the m/z values for the peaks of the envelope, assign charge states to all peaks
having relative abundance > 15 and determine the mass of unknown compound (Da).
(b) Electrospray peaks do not always appear in charge envelopes. Determining the charge states of
isolated peaks requires a mass spectrum with resolved isotopic peaks. Using data from the second
mass spectrum, which is an expanded view of the region between m/z 737.2 and 738.8, re-calculate
the charge of the peak at m/z 738.02307.
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Problem 2.2- Ion Source Optics.
The figure below is a schematic of an electron ionization source and its extraction optics. (FYI: this
is the ion source used in the Aerosol Mass Spectrometer used by the Jimenez Lab). Ions are created
inside the box to the left, as electrons are emitted from the filament (small circle below the “cathode”
voltage) they ionize some of the neutral molecules inside the box.
Using the values of voltages from the table below plot the velocity and acceleration vs. position
curves for (a) an H2O+ ion and (b) a Hg+ ion as they are extracted by the ion optics and they enter
the quadrupole mass analyzer (labeled as “rod system”).
Assume that the entire box interior is sitting at the V1 (“ion reference”) voltage and that ions are
formed on the central axis of the ion source. Measure the distances from the graph (please use mm
and not inches) and assume that the graph is drawn on a 2:1 scale (twice as big as real).
Note: Disregard superscripts on the voltages.
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Problem 2.3: Processes that control unimolecular ion decomposition.
As we discussed in lecture 2, the two main processes that control the fragmentation of an isolated
unimolecular ion (under vacuum) once it has been formed (by any ionization technique) are the
amount of energy deposited in the molecular ion and the chemical nature of the molecule (including
its resistance to fragmentation).
Here you have to calculate what would be the observed mass spectrum for acrolein and toluene using
the breakdown curves given below for both molecular ions, and the ionization energy distributions
shown in the figure in slide 25 of the notes (labeled Fig 14.29) for field ionization (FI), 70 eV
electron ionization (EI) and chemical ionization (CE). Assume that the 0 in those three curves
corresponds to the ionization potential of each species. The ionization potential of acrolein in 10.10
eV and that of benzene is 9.25 eV. Discretize all curves on 0.25 eV intervals (e.g. in Excel). For
acrolein use the experimental data (symbols) rather than the model (lines). Note FYI: the breakdown
curve of acrolein was obtained by photoionization with photons of increasing energy (using a
tunable laser). The BC of benzene was obtained by performing chemical ionization with a series of
reactants, each of which deposits a precise amount of energy on the molecular ion.
Breakdown curve of the acrolein molecular ion (CH2CHCHO.+). (From Yue Li and Tomas Baer. The
dissociation dynamics and thermochemistry of the acrolein ion studied by threshold photoelectron–
photoion coincidence spectroscopy. International Journal of Mass Spectrometry Volume 218, Issue
1, 15 June 2002, Pages 37-48. http://dx.doi.org/10.1016/S1387-3806(02)00644-9)
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Breakdown curve of the benzene molecular ion (from
http://uigelz.ece.uiuc.edu/pub/presentations/rajesh_gec99.pdf)
Problem 2.4- Order of Magnitude Calculations for an Electron Ionization Source
Note: these calculations are approximate by nature, so don’t try to get many significant figures.
Make the simplifications and approximations you need to get to a numerical answer. I am most
interested in your getting the order of magnitude right for each step (i.e. within a factor of 2 or 3);
and critically evaluating whether that order of magnitude makes sense in the bigger picture. If you
are missing a number, estimate it (and make clear that you are doing so and how). Also, some of the
later steps can be done even if you can’t do the earlier ones, so try each step independently. Please
explain how you are trying to solve each step, so that I can give you partial credit even if you don’t
get the final answer right.
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(a) In a typical EI ion source, an emission current of 2.5 mA is used. What is the electron flux
(electrons / s ) on this source? What is the speed of the electrons if their energy is 70 eV? What is the
associated wavelength? Can you explain why 70 eV electrons are most effective at ionizing almost
any molecule? (as compared with electrons with different kinetic energies).
Electron path
Effective
Ionization
Volume
Filament
(b) Assume that the actual ionization volume is a cube of 3 mm side, and that 100% of the source
pressure of 10-8 Torr is due to N2. What is the ion current detected by the mass spectrometer?
Assume that ions formed in the ionization volume are transmitted to the detector with 30%
efficiency. Note that the EI cross section of N2 is 2.7 A2 (1A = 10-10 m). Refer to the course notes for
the interpretation of the EI cross section.
(c) What is the probability that a given electron that leaves the filament actually ionizes a water
molecule?
(d) What is the probability that a molecule that crosses the ionizer volume at its thermal speed only
once (at 300K) is ionized? (this is the “ionization efficiency”).
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