CHEM 310 MHCC Mass Spectrum for Peak at Retention Time Questions

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CHEM 310 Grignard Reaction POST-LAB ASSIGNMENT
GC-MS DATA:
The Gas Chromatography-Mass Spectrometry (GC-MS) data that we collected looks a bit like this (see Figure 1
below). One printout (called the total ion chromatogram or TIC) is a plot of ions collected by the detector over
time. The x-axis is retention time, which is the amount of time between sample injection by the autosampler and
ion detection in the mass spectrometer. For each retention time, there is a corresponding mass spectrum that
shows which ions were detected at that time. For example, in the GC-MS data shown below, three mass spectra
are displayed: they are the mass spectra for each of three signals (Figure 1). We can use the mass spectra to
identify the chemicals that produced each signal.
Figure 1: example of a total ion chromatogram (TIC) with mass spectra shown for three different times
(a-c). In the TIC, each sample is labeled with its retention time (RT) in minutes and its peak area (AA or
MA). The units of peak area are arbitrary: they are just a big number. Use the AA (or MA) value in your
peak area calculations on the next page.
For each of the mass spectra in the lab data, annotate the spectra as described below. There is an example
of a spectrum annotated this way posted on our Canvas site next to this assignment. On the first page of this
assignment, there is a table that gives instructions about the minimum number of signals that you should label.
1. Label the molecular ion peak as “M+” if it is present in the spectrum.
2. Label the M+1 and M+2 peaks (if they are present) with M+1 or M+2.
3. Label major peaks other than the molecular ion with three things:
a. The mass to charge ratio
b. The structure (best) or formula for that charged fragment
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c. Another formula (in parentheses) to show which fragment was lost by the molecular ion to form
that ion. For example, if an ion is the result of a methyl radical being lost from the molecular ion,
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it should be labeled with (–CH3 )
Use the table below to start your analysis of the GC-MS data. The column Mass Spectrum Instructions has
information about how many signals you should be able to confidently identify (label) on each mass spectrum in
addition to the M+, M+1, and M+2. Use the peak area values from the total ion chromatogram (TIC) to estimate
the composition of the reaction mixture at the end of the reaction (% of product mixture).
Name of GC-MS TIC file used to complete this table: 310-002 bench 2 GC Results
Structure and
approximate retention
time (in minutes)
toluene
(2.8 min)
4-bromotoluene
(5.1 min)
4’-methylyacetophenone
(6.1 min)
Compound 2
(9.0 min)
Compound 1
(9.8 min)
Source
Mass
How was this chemical formed?
Spectrum
How did it end up in the reaction
Instructions
mixture?
Reaction between the Grignard
reagent and water; any water
present in the reactants or
No mass
solvents, on the surface of
spectrum
glassware, or in the air that
reacted with the Grignard reagent.
Reactant that never combined
with magnesium to make a
Grignard reagent
Thoroughly
label one
signal with m/z