Arson/ExplosivesArson/Explosives

All arson labs use a method called gas

chromatography/mass spectrometry (GC/MS) to

determine the composition of the accelerants or

explosives found

GCGCMSMS

MS Spectroscopy

http://www.aist.go.jp/RIODB/SDBS/menu-e.html: ca. 90!000 IR, Raman, NMR, ESR and MS spectra.

The mass spectrometer is the first technique we have discussed that actually destroys a small amount of the sample to obtain a spectrum (only 1µg necessary, high sensitivity). MS looks more complex (HV, operator), but is actually based on simpler principles than NMR.

In the EI (electron impact) mode, a high-energy electron beam

bombards molecules and generates positive ions:

NH2

O

Ph

O

NH2

e-

molecular ion

.-2e

-

The molecular ion in turn produces a series of fragment ions:

- NH2

O

NH2

C6H5 -C=O- CO

m/z 105m/z 77

- C6H5.

.

m/z 44

NH2

O

Ph

O

NH2

e-

molecular ion

.-2e

-

The ions are then accelerated and focused using a combination of

electrical and magnetic fields (high resolution: two fields: double

focusing). The ions are finally scanned by varying either the

magnetic or electrical field applied to them. The important point to

recognize is that we are not observing molecular weights, but a ratio

of mass to charge, or m/z. Since the charge is usually 1, this is most

often proportional to mass, but if either the mass or the charge

changes during acceleration and collection, the spectrum can be

confusing, leading to broad “metastable” peaks and to doubly charge

peaks at half the correct mass number.

Schematic diagram of CEC model 21-103 mass spectrometer, a

single-focusing, 180° sector mass analyzer. The magnetic field is

perpendicular to the page.

The resolution of the MS is given by the difference in mass that can be measured

at a given mass size.

M

!MR =

where two peaks are said to be resolved if the valley between them is less than 10% of their height.

Medium! 5,000/1!!HR ! 10,000/1 to 100,000/1

! At R=25"000 res:!500.00 from 499.98

! Mol. Weight (HR elemental formula, composition) ! Fragmentation pattern ! Fingerprint for structural analysis!

Measurement of Mass:

The very high resolution specifications imply that much more than unit mass

measurements can be made meaningfully, and indeed the difference in mass

between compounds containing different atoms but having the same nominal

molecular weight can be detected.

Presence of isotopes; 13C (1.1%) ! way to deduce # of C atoms

M+1 A = 100%

C1: 1.1

C5: 5.5

C10: 11. ! way to deduce

C20: 22 # of C1 atoms

C50: 55

C100: 110

In the case of halogens, particularly bromine (79Br: 50.5%, 81Br: 49.5%) and

chlorine (35Cl: 75.8%, 37Cl: 24.2%) there are two isotopes having a substantial

natural abundance that leads to some rather distinctive pattern near the M+ peak.

(lower MW = “true” one; higher MW = isotope peak).

Fragmentation

M + e (fast) ! M+!+ 2e (slow)

! radical cation; breaks apart by elimination of a neutral molecule

(! fragment has same parity) or elimination of a free radical

(!fragment has changed parity).

!-Cleavage

(Norrish-Type I in photochemistry)

Bonds that are in an ! position to heteroatoms are cleaved

preferentially (only 1x)

O

O

O

H3C

CH2

O

CH3

CH2CH3

O

O

CH2

CH3

CH3

O

Ex:

72 u

-e-

m/e 72

.

!

15 u

.

or

m/e=57

29 u

m/e=43 the heavier substituent is cleaved off preferentially!

O O

CH2H

O

CH3

C3H7

CH2

O

m/z 98

m/z 55

Benzyl-, allyl cleavage

Similar activating influence as heteroatoms.

CH3

C3H7

H3CCH2

CH3

CH2

C2H2

m/z 91

Tropylium-ion very stable

m/z 65

m/z 77 m/z 51

5741

Recognition of the Molecular Ion Peak:

Quite often, recognition of the molecular ion peak (M) poses aproblem. The peak may be very weak or it may not appear at all.

In addition to the EI method described above, several promising

techniques were developed for obtaining and locating the molecular

ion peak for compounds that give very weak or nonexistent molecular

ions by EI.

CI Chemical Ionization:

! The sample is introduced at about 1 torr pressure with a carrier gas

such as CH4 or . The methane is ionized by EI to CH4 , CH3 and so on.

These species react with the excess methane to give secondary ions

CH4 CH4 CH5 CH3

CH4

C2H5+ + H2 and other ions

The secondary ions react with the sample (RH)

CH5 + RH RH2 + CH4

M + 1 ions (quasimolecular ions)

(M - 1 because of hydride abstraction)

Since the [M + 1] ions are chemically produced, they do not have the

great excess of energy associated with ionization by electron impact,

they undergo less fragmentation.

energy content: CH5 > t -C4H9 > NH4

FAB Fast-Atom Bombardment

Polar molecules (peptides) (MW 10!000 and higher)

The bombarding beam consists of xenon (or argon) atoms of high

translational energy.

Fast atom

gun

Atom beam

M

ion beam

Massanalyzer

sample matrix(in glycerol)

This beam is produced by first ionizing xenon atoms with e- to give

xenon radical cations:

Xee

Xe + 2e-

high-energy xenon atoms (Xe)

After acceleration, the radical cations are passed through neutral Xe.

Ionization by translational energy minimizes the amount of vibrational

excitation and this results in less destruction of the ionized molecules.

adduct ions by interaction with matrix [M+H]+

[M+Na]+

[M+glycerol]+

[glycerol]n+

ESI Electrospray:

ESI MS is a powerful method because it allows the analysis ofmolecular ions on polar and high molecular weight compounds inaqueous solutions.

A solution of a substance is sprayed through a capillary into achamber and met by a dry gas flow from the opposite direction. Theresulting mist produces charged droplets that become smaller as thesolvent evaporates. Driven by an electric field of a few kV, thecharged droplets move through a glass capillary and are focused byan electrostatic lens system and directed into a quadrupole massspectrometer. API (atmospheric pressure ionization) uses a similartechnique.

ESI MS of interleukin 6 (MW = 20903):

deconvoluted spectrum: