CH ENGR 104C Lecture Notes - Lecture 3: Mass Spectrometry, Mass Spectrum, Radical Ion

11 views5 pages

For unlimited access to Class Notes, a Class+ subscription is required.

Mass Spectrometry
Overview
Mass Spectrometry is an analytic technique that utilizes the degree of deflection of charged
particles by a magnetic field to find the relative masses of molecular ions and fragments.2 It is a
powerful method because it provides a great deal of information and can be conducted on tiny
samples. Mass spectrometry has a number of applications in organic chemistry, including:
- Determining molecular mass
- Finding out the structure of an unknown substance
- “Verifying the identity and purity of a known substance”1
- Providing data on isotopic abundance
How a Mass Spectrometer Works 2
* Figure A3
Step 1: The sample is vaporized, and then ionized by being bombarded by a beam of high-
energy electrons (usually at 70 eV). The electron beam knocks out an electron from the molecule
of the injected sample, creating a molecular ion (which is also a radical cation because it has an
unpaired electron and a positive charge). Losing an electron weakens the bond, while the
collision gives it extra kinetic energy. These factors make it more likely for the molecular ion to
break into fragments as it travels through the mass spectrometer.
Step 2: There is a pair of oppositely charged plates in the ionization chamber. The positively
charged one causes the positively charged radical cation to accelerate into an analyzer tube.
1 Chem 14C Course Thinkbook by Professor Steven Hardinger, Summer 2006 Version, Pg. 41
2 Bibliographical source: Pg. 702 of Atkins and Jones, Chemical Principles, 3rd Edition and Pg. 484 of Bruice,
Organic Chemistry, 4th Edition
3 Figure A from http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/1334.gif (Site visited on 8/23/06)
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 5 pages and 3 million more documents.

Already have an account? Log in
Step 3: The analyzer tube is surrounded by a curved magnetic field, which causes the path of the
radical cation to be deflected in proportion to its mass-to-charge ratio (m/z). The flight path of
the ion depends on its molecular mass, its charge, and the strength of the magnetic field. Thus, at
a given magnetic field strength, ions of only one specific mass collide with the detector and are
recorded.
Step 4: The strength of the magnetic field is varied in increments to produce a mass spectrum,
which is a plot of m/z (on the x axis) against relative abundance (on the y axis). If we assume
that all ions have a charge of +1, then the peaks give the mass ratios and their heights give the
proportions of ions of different masses.
Aston and Thomson used a process similar to the one outlined above to acquire the mass
spectrum of neon in 1919. They observed that neon’s mass spectrum had two peaks: one at m/z
20 (90% abundance) and one at m/z 22 (10%), but no peak at the atomic weight of Neon (20.20,
which is the weighted average). Since Neon is inert and lacks chemical bonds, the peaks did not
correspond to masses of fragments of the molecular ion, but to the masses of different isotopes of
Neon. The experiment’s conclusion was that “neon has two natural isotopes: 20Ne (90%
abundance) and 22Ne (10% abundance)”4. This was the first time that an isotope had been
detected for a non-radioactive substance. Note that modern spectrometers actually indicate that
neon has a third isotope.
Interpreting a Mass Spectrum
*Figure B5
4 Quoted from Chem 14C Course Thinkbook by Professor Steven Hardinger, Summer 2006 Version, Pg. 215
5 Mass spectrum from http://www.chem.ucalgary.ca/courses/351/Carey/Ch13/ch13-ms.html (labels have been
modified in Adobe Photoshop); Site visited on 8/24/06
Mass-to-charge ratio (m/z)
Relative ion abundance
CH3CH2CH2Br
Base
Peak
M
M+2
Unlock document

This preview shows pages 1-2 of the document.
Unlock all 5 pages and 3 million more documents.

Already have an account? Log in

Get access

Grade+
$10 USD/m
Billed $120 USD annually
Homework Help
Class Notes
Textbook Notes
40 Verified Answers
Study Guides
1 Booster Class
Class+
$8 USD/m
Billed $96 USD annually
Homework Help
Class Notes
Textbook Notes
30 Verified Answers
Study Guides
1 Booster Class