(Identification of Compounds in Gasoline, or other Accelerants, by Headspace Analysis) A Summary of this Week’s Lab During your final lab period, you will sample the space above a small amount of gasoline or other potential accelerant and analyze the sample for volatile
components. This technique, called headspace analysis, is often employed to sample the volatiles from the charred remains of a fire as part of an arson investigation. You will collect the air sample on powdered carbon or charcoal. A gas has an electrostatic attraction to activated carbon and sticks to its surface by a process called adsorption. The charcoal is activated by heating it in an oven. This drives off moisture and the immense surface area of the dry charcoal provides sites for vapors to adhere to the surface. The volatiles from the gasoline will be adsorped onto charcoal and then collected by dissolution into liquid carbon disulfide. The resulting carbon disulfide sample will be analyzed by gas chromatography—mass spectrometry. The volatile compounds from the gasoline sample will be separated by the gas chromatograph and their mass spectra obtained as in the
previous two labs. You will compare the mass spectra from your previous two labs (eight spectra in all) with the mass spectra of the compounds found in the headspace of your sample. The object is to find how many of the known compounds are volatile components of your sample. Determining the commonality of mass spectral peaks in a suspect and known liquid is a standard forensics technique.
You will expand your mass spectral interpretive skills by learning some of the common
fragmentation patterns that recur in mass spectra. You will complete your lab report and turn it in when you report for the Final Exam.
Advanced Mass Spectral Analysis
Certain masses recur in mass spectra because, like all properties, a mass spectrum is a function of the structure of the molecules that produce it. Therefore, certain structural features beget specific m/z peaks. In the following sections, we will discuss some of the structural features that give easily recognizable ions.
Aldehydes, Ketones and the McLafferty Rearrangement
The molecular radical-cation of adehydes and ketones may decompose by homolytic cleavage of one of the carbonyl carbon’s single bonds, producing an acyl carbocation (acylonium ion) and a
hydrogen atom-radical or alkyl radical. The resulting acylonium ion can lose carbon monoxide and produce a daughter carbocation 28 mass units lower than the mass of the acylonium ion. In the case of ethanal, we would expect to see a molecular ion at m/z = 44; an M – 1 peak at 43 (acylonium ion);
and an M – 29 peak at 15. The corresponding peaks in propanal are at m/z = 58, 57, and 29. Likewise for 2-butanone, the molecular radical-cation (m/z = 72) might lose either a methyl radical or an ethyl radical. The rule of thumb is that the larger mass group cleaves preferentially. Thus, cleavage of the ethyl group leaves an ethylonium ion with m/z = 43. The actual spectrum shows a base peak of 43 and a tiny peak at 57, which corresponds to loss of the methyl radical. Figure 1 shows the sequence of cleavages that start with an α-cleavage of an aldehyde.
R C H
Figure 2 shows the sequence when fragmentation of a ketone begins with an α cleavage.
R C R'
When R is phenyl in either Figure 1 or Figure 2, significant peaks are observed at m/z 105 for the loss of an H atom or alkyl radical and at 77, corresponding to the loss of CO.
When an aldehyde or ketone contains an alkyl group so that a quasi-six-membered ring such as those shown in Figure 3 can form, the molecular radical-cation eliminates two alkenes. One of the alkenes is a neutral molecule and the other is a new radical-cation that is detectable in the mass spectrometer.
For an aldehyde, the mass of the latter ion is 44. For a methyl ketone, the mass is 58. The rearrangement that produces the new radical-cation from the ring is called a McLafferty rearrangement after the chemist who discovered it.
H O
Alkenes, Alkynes and the Retro-Diels—Alder Reaction
The molecular ions of alkenes are relatively stable, so a strong M+ peak is usually observed in the mass spectrum of an alkene. The molecular ions of terminal alkynes readily lose an H atom, giving a very strong M -1 peak in addition to the molecular ion peak.
Cyclic alkenes that contain six carbon atoms readily undergo a retro-Diels—Alder reaction,
producing a neutral ethane molecule and a butadiene radical-ion, which is detectable. Figure 4 shows an example of the retro reaction.
+
neutral m/z = 54 m/z = 82
+
neutral m/z = 68 m/z = 110
Figure 4 Examples of Retro-Diels—Alder Fragmentations Aromatic Hydrocarbons and the Tropylium Ion
Like simple alkenes, benzene displays a strong molecular ion because of its stability by no other characteristic mass peaks. Alkylbenzenes fragment at the benzyl carbon, producing benzyl carbocations with m/z = 91. A benzyl carbocation rearranges into a more stable seven-membered ring carbocation called a tropylium ion that is aromatic (i.e., it contains 4n + 2 π electrons) and hence very stable. The production of a tropylium ion from an alkylbenzene is shown in Figure 5.
CH2CH2R
CH2
+ CH2R
m/z = 91 tropylium ion
Figure 5 Formation of a Tropylium Ion from an Alkylbenzene Acids and Esters
Esters readily undergo both α-cleavages and McLafferty rearrangements. Figure 6 shows these fragmentation patterns for methyl pentanoate.
O
Figure 6 Fragmentation Pattern of a Methyl Ester (Methyl Pentanoate)
Thus, methyl esters commonly show peaks at m/z = 59 and 74, which are the result of an alpha-cleavage and a McLafferty rearrangement, respectively. Likewise, ethyl esters show significant peaks at 73 and 88, respectively.
Carboxylic acids readily form acylonium ions by the loss of a radical OH from the molecular ion. As we know, the acylium ion can go on to lose CO with the formation of a detectable R+.
Alcohols and Phenols
Primary and secondary alcohols may dehydrate by a mechanism similar to the McLafferty rearrangement. They may also fragment by cleavage of a carbinol carbon—carbon bond with the expulsion of an alkyl radical and production of a new radical-cation. Figure 7 shows this latter kind of fragmentation for 2-penanol.
OH OH
+
m/z = 45 m/z = 88
Figure 7 Cleavage in a Secondary Alcohol (2-Pentanol)
The dehydration mechanism also involves the expulsion of a neutral alkene molecule, as shown in Figure 8 for 2-pentanol.
H OH
m/z = 88
+ H2O +
m/z = 42 isopropenyl ion
Figure 8 Dehydration Mechanism for a Secondary Alcohol (2-Pentanol)
Note that the mass of an isopropyl group is 43; whereas, the mass of an isopropenyl group is one mass unit less (42).
Unlike most primary and secondary alcohols, phenols display a strong molecular ion peak. Phenols also display intense peaks at M – 28 and M-29, which correspond to the loss of CO and CHO, respectively. Subtract the major peaks from the M+ peak in a spectrum to find the 28/29 combination that indicates the presence of a phenol.
Summary
In the discussion, we have covered some of the well-known fragmentation patterns that you are likely to observe in mass spectra. Several important families have been omitted from this discussion, but the power of recognizing acylium ions, tropylium ions, retro-Diels—Alder reactions and
McLafferty rearrangements will become obvious to you as you explore mass spectra.
Ensure the GC-MS is turned on and that the operating system is set to the procedure you used in the previous two experiments. The setup procedure for the instrument is described in the handout next to the instrument.
Procedure
1. Obtain a copper wire that is approximately 10 cm in length and is flattened on one end.
2. Obtain a sample of activated charcoal and grind it into a fine powder with a mortar and pestle.
3. Dip the flattened end of the copper wire into a sodium silicate solution.
4. Immediately dip the wet end of the wire into your powered charcoal, making a fine coating of carbon on the wire.
5. Rest the wire on a beaker inside an oven (70 oC) so that the coated end of the wire is not disturbed.
6. Leave the wire in the oven for 15 min. This reactivates the charcoal.
7. While waiting for the wire to dry, obtain a sample of an accelerant from the instructor. The sample is contained in a medium sized, septum-capped test tube.
8. Set up a water bath on a hot plate. The water bath is a 250-mL beaker that is about ¾ full of water.
9. Clamp your test tube to a ring stand. Place the clamped test tube in the water bath and adjust its depth in the bath so that the liquid in the test tube is below the water line of the water bath.
10. Make sure the test tube firmly clamped in place and heat the water bath to 80oC but not to boiling, because we do not want to create an excess of water vapor.
11. Carefully remove the beaker and wire from the oven. Set the beaker on your bench top with the wire still in place and let the wire cool to room temperature
12. Carefully push the un-coated end of the wire through the center of a small piece of cardboard (6 cm x 6 cm square). Rest the cardboard on a ring stand with the coated end of the wire pointing downward.
13. Remove the septum from your accelerant sample and place the coated end of the copper wire into the headspace of the test tube. Adjust the position of the coated wire so that it is just above the surface of the accelerant. The cardboard square serves as a holder for the wire and rests on the top of the test tube.
14. Leave the wire in the headspace with the water bath at 80oC for about 15 min.
15. Lift the wire out of the sample and quickly rinse the coated end of the wire with carbon disulfide.
Catch the washing in a glass vial that can be capped and sampled by an auto-injector.
16. Cap your sample vial.
17. Make sure your vial has a sufficient amount of carbon disulfide solution for the needle of the auto-injector to sample it.
18. Place your sample vial, marked for identification, into the auto-injector’s rack.
19. Inject the sample into the GC-MS and obtain mass spectra as in the previous experiments with the GC-MS.