Forensic Mass Spectrometry
Distinguished Prof. Eric Block
CHM 450B, W 2006
1
NYSP Laboratory Report
CO2 Me
Me
N
OC(O)Ph
cocaine
mw 303
CO 2 Et
Me
N
OC(O)Ph
cocaethylene
mw 317
Me2N
Ph
cocaine,
cocaethylene,
methadone
Identified
in urine
Ph
O
methadone
mw 309
2
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
82
NMe+
182
Me
CO2Me
N
OC(O)Ph
QuickTime™
and a
Me
N
TIFF (Uncompressed)
decompressor
C=O+
+
Me
N
are needed to see this picture.
OC(O)Ph
CO2Me
303
272
Mass Spectrum of Cocaine
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
3
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
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are needed to see this picture.
Mini Gas Chromatograph/Mass Spectrometer (on right; full sized GC-MS on left)
The Mini GC/MS is a robust, reliable, and field-deployable instrument. With
an ability to analyze samples at sensitivities of parts per billion within 15 to
40 minutes, the portable GC/MS can be used during homeland-defense
activities, incident response, and law-enforcement investigations. For
example, the instrument can precisely identify compounds that indicate the
production of chemical-warfare agents and illicit drugs.
4
Electron-Impact Mass Spectrometry
• Mass spectrometry is a technique used for measuring
the molecular weight and determining the molecular
formula of an organic compound.
• In an electron-impact mass spectrometer (EI-MS), a
molecule is vaporized and ionized by bombardment with
a beam of high-energy electrons.
• The energy of the electrons is ~ 1600 kcal (or 70 eV).
• Since it takes ~100 kcal of energy to cleave a typical s
bond, 1600 kcal is an enormous amount of energy to
come into contact with a molecule. Usually only a
portion of this energy is transferred to the molecule.
• The electron beam ionizes the molecule by causing it to
eject an electron.
5
Chemical Ionization Mass Spectrometry
Chemical ionization mass spectrometry (CI-MS) begins
with ionization of methane, ammonia or another gas,
creating a radical cation (e.g. CH4•+ or NH3•+). This in turn
will impact the sample molecule M to produce MH•+
molecular ions. Some of MH•+ fragments into smaller
daughter ions and neutral fragments. Both positive and
negative ions are formed but only positively charged
species will be detected. Less fragmentation occurs with
CI than with EI, hence CI yields less information about
the detailed structure of a molecule, but does yield the
molecular ion; sometimes the molecular ion cannot be
detected by the EI method, hence the two methods are
complementary.
6
Mass Spectrometry
Introduction
7
For descriptive purposes, an analogy can be drawn between a mass
spectrometer and an optical spectrophotometer. In the latter, light is
separated into its various wavelength components by a prism and
then detected with an optical receptor (such as the eye).
8
Analogy between mass analysis and the analysis of light
Analogously, a mass spectrometer contains an ion source that
generates ions, a mass analyzer that separates the ions according to
their mass-to-charge ratio, and an ion detector.
Components of a Mass Spectrometer
9
Ion Sources and Sample Introduction
• Sample Introduction
– The sample inlet is the interface between the sample and the
mass spectrometer. A sample at atmospheric pressure must be
introduced into the instrument such that the vacuum within
remains unchanged.
10
Introduction and Ionization Components in a Mass Spectrometer
• Sample Introduction (continued):
– A sample can be introduced in several ways, the most
common being with a direct insertion probe or by
infusion through a capillary column.
Samples are often introduced using a direct insertion probe or a capillary column.
The probe and capillary carry the sample into the vacuum of the mass spectrometer.
11
Once inside the mass spectrometer, the sample is exposed to the ionization source
Mass Analyzers
Mass analyzers scan or select ions over a particular m/z
range. The key feature of all mass analyzers is their
measurement of m/z, not mass. The mass analyzers
contribute to the accuracy, range and sensitivity of an
instrument. Six common types of mass analyzers are
quadrupole, magnetic sector, time-of-flight, time-of-flight
reflection, quadrupole ion traps and Fourier transformion cyclotron resonance (FT-ICR).
12
Mass Analyzers (continued)
Quadrupole Analyzer
• Quadrupoles are four precisely parallel rods with a
direct current (DC) voltage and a superimposed radiofrequency (RF) potential. The field on the quadrupoles
determines which ions are allowed to reach the
detector. Quadrupoles thus function as a mass filter.
the B
13
Mass Analyzers (continued)
Quadrupole Analyzer (continued)
•
•
Quadrupole mass analyzers have been used in conjunction with
electron ionization since the 1950’s. EI coupled with quadrupole mass
analyzers are employed in the most common mass spectrometers
today.
Quad mass analyzers have found new utility in their capacity to
interface with electrospray ionization. This interface has three primary
advantages:
– 1. Quads are tolerant of relatively high pressures (~5 x 10-5
Torr), which is well suited to electrospray ionization since the
ions are produced under atmospheric pressure conditions.
– 2. Quads are now capable of routinely analyzing up to an m/z of
3000, which is useful because electrospray ionization of proteins
and other biomolecules commonly produces a charge distribution
below m/z 3000.
– 3. The relatively low cost of quadrupole mass specs makes them
attractive as electrospray analyzers. Thus, it is not
surprising that
most of the successful commercial electrospray
14
instruments thus
far have been coupled with quadrupole
mass analyzers.
Tandem Mass Spectrometry
The new ionization techniques are relatively simple and do
not produce a significant amount of fragment ions, in
contrast to EI which produces a lot of fragment ions. Tandem
mass spectrometry (MS/MS) was developed to induce
fragmentation. In tandem MS (abbreviated MSn where n refers
to the number of generations of fragment ions being
analyzed) collisionally induced fragment ions are mass
analyzed.
15
Tandem Mass Spectrometry (continued)
Fragmentation is achieved by inducing ion-molecule
collisions via collision-induced dissociation (CID) or collision
activated dissociation. CID is accomplished by selecting an
ion of interest with a mass filter/analyzer and introducing that
ion into the collision cell. A collision gas (typically Ar) is
introduced into the collision cell, where the selected ion
collides with the argon atoms, resulting in fragmentation. The
fragments are then analyzed to obtain a daughter ion
spectrum. The term MSn is applied to processes which
analyze beyond daughter ions (MS2) to grandaughter (MS3),
and to great-granddaughter ions (MS4). Tandem mass
analysis is primarily used to obtain structural information.
16
Interpretation of EI
Mass Spectra
17
Important Terminology—
• Amu—atomic mass unit/dalton
• M+˙, molecular ion—the ionized molecule; the molecular
ion peak is the peak representing the ionized molecule
that contains only the isotopes of natural abundance
• Base Peak—the peak in the spectrum that represents the
most abundant ion
• Daughter Ion—the product produced by some sort of
fragmentation of a larger ion
• Isotopic Peak—a peak in the spectrum that corresponds
to the presence of one or more heavier isotopes of an ion
• “A” Element—an element that is monoisotopic
• “A + 1” Element—an element with an isotope that is 1
amu above that of the most abundant isotope, but which
is not an ‘A + 2’ element
• “A + 2” Element—an element with an isotope that is 2
18
amu above that of the most abundant isotope
Things to keep in mind—
• Interpretation is based on the chemistry of gaseous ions.
• Ion abundances of <<0.1% can be measured
reproducibly. The error of non-high resolution spectra is
± 10% relative or ± 0.2 absolute, whichever is greater.
• General spectra are shown with unit mass resolution (xaxis).
• Generally, measured spectra contain additional peaks
due to background in the instrument. This arises from
compounds that are desorbing from the walls of the
instrument or leaking from various sources. Thus, a
background spectrum is usually run before the actual
sample is introduced to the instrument, and subtracted
from the sample spectrum.
• Small peaks with masses above that correspond to the
molecular weight are due to the presence of less abundant isotopes.
19
Mass Spectrometry
Introduction
• When the electron beam ionizes the molecule, the species that is
formed is called a radical cation, and symbolized as M+•.
• The radical cation M+• is called the molecular ion or parent ion.
• The mass of M+• represents the molecular weight of M.
• Because M is unstable, it decomposes to form fragments of
radicals and cations that have a lower molecular weight than M+•.
• The mass spectrometer analyzes the masses of cations.
• A mass spectrum is a plot of the amount of each cation (its
relative abundance) versus its mass to charge ratio (m/z, where
m is mass, and z is charge).
• Since z is almost always +1, m/z actually measures the mass (m)
of the individual ions.
20
Basic Mechanisms of Fragmentation
• Mass spectral reactions are unimolecular; the sample pressure in the
EI source is kept sufficiently low so that bimolecular (ion-molecule)
or other collisions are usually negligible. If sufficiently excited, the
M+• ions can decompose by a variety of energy dependent
mechanisms each of which results in the formation of an ion and a
neutral species (radical). This primary product may have sufficient
energy to decompose further.
• In the MS of ABCD, the abundance of BCD+ will depend on the
average rates of its formation and decomposition, whereas [BC+] will
depend upon the relative rates of several competitive reactions.
There are several types of unimolecular reactions that can take
place:
ABCD
e–
ABCD•+
A+ + BCD•
A• + BCD+
D + BC+
D• + ABC+
A + BC+
AD+• + B=C
21
Unknown 1
m/z
m/z
IntInt.
1 1
16
16
17
17
18
18
20
20
<0.1
<0.1
1.01.0
21.0
21.
100
100.
0.20.2
18
relative
abundance
17
16
Mass
(mass-to-charge ratio)
22
m/z
m/z
121
12
13
13
14
14
15
15
16
16
17
17
Int
Int.
15
3.1
1.0
1.0
8.1
8.1
16.
16.
85.
85.
100.
100.
1.1
1.1
16
relative
abundance
14
13
12
17
Mass
(mass-to-charge ratio
23
Mass Spectrometry
Introduction
Consider the mass spectrum of CH4 below:
• The tallest peak in the mass spectrum is called the base peak.
• The base peak is also the M peak, although this may not always
be the case.
• Though most C atoms have an atomic mass of 12, 1.1% have a
mass of 13. Thus, 13CH4 is responsible for the peak at m/z = 17.
24
This is called the M + 1 peak.
re 2.12: The mass
trum Mass
of neon.Spectrum
of
Neon Showing
Major and Minor
Natural Isotopes
20Ne
90.48%
rel. int
22Ne
21Ne
9.25%
0.27%
m/z
25
Mass Spectrometry
Introduction
• The mass spectrum of CH4 consists of more peaks than
just the M peak.
• Since the molecular ion is unstable, it fragments into
other cations and radical cations containing one, two,
three, or four fewer hydrogen atoms than methane itself.
• Thus, the peaks at m/z 15, 14, 13 and 12 are due to these
lower molecular weight fragments.
26
CH3OH
m/z
m/z
Int.
Int
12
12
13
13
14
14
15
15
15.5
15.5
16
16
17
17
28
28
29
29
30
30
31
31
32
32
33
33
34
34
0.3
0.3
1.7
1.7
2.4
2.4
13.
13.
0.2
0.2
0.2
0.2
1.0
6.3
64.
64.
3.8
3.8
100.
100.
66.
66.
1.0
1.0
0.1
0.1
relative
abundance
CH3 OH + e-
CH3 OH+• (m/z 32) + 2e-
CH3 OH+•
CH2 OH+ (m/z 31) + H•
CH3 + (m/z 15) + HO•
CH2 OH+
CHO+ (m/z 29) + H2
31
32
29
15
27
Mass Spectrometry
Introduction
28
Mass Spectrometry
Alkyl Halides and the M + 2 Peak
• Most elements have one major isotope.
• Chlorine has two common isotopes, 35Cl and 37Cl, which
occur naturally in a 3:1 ratio.
 Thus, there are two peaks in a 3:1 ratio for the
molecular ion of an alkyl chloride.
 The larger peak, the M peak, corresponds to the
compound containing the 35Cl. The smaller peak, the
M + 2 peak, corresponds to the compound containing
37Cl.
 Thus, when the molecular ion consists of two peaks
(M and M + 2) in a 3:1 ratio, a Cl atom is present.
• Br has two isotopes—79Br and 81Br, in a ratio of ~1:1.
Thus, when the molecular ion consists of two peaks (M
29
and M + 2) in a 1:1 ratio, a Br atom is present.
Mass Spectrometry
Alkyl Halides and the M + 2 Peak
(CH3)2CH+
30
Mass Spectrometry
Alkyl Halides and the M + 2 Peak
(CH3)2CH+
31
Elemental Composition—
• With high resolution mass spectrometers, exact mass
measurements provide the number of each constituent
element
• Even with unit mass spectrometers, the presence of
natural abundance isotopes makes possible the
deduction of the elemental composition of many ions
• A chemically pure compound will give a mixture of mass
spectra because the elements that compose it are not
isotopically pure
• Of the common elements encountered in organic
compounds, many have more than one isotope in
appreciable abundance. Thus, characteristic isotopic
ratios can result in easy identification of elements in a
structure
32
“A” elements:
“A + 1” elements:
“A + 2” elements:
Element
AA
Mass
Element Mass
HH
1 1
CC
12 12
NN
14 14
OO
16 16
FF
19 19
Si
28 28
Si
PP
31 31
SS
32 32
Cl
35 35
Cl
Br
79 79
Br
I
127
those with only one natural isotope in
appreciable abundance
those that have two isotopes, the second of
which is one mass unit heavier
are the easiest to recognize and are two mass
units higher
%
%
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
AA++1 1
Mass %
Mass
2 2
13 13
15 15
17 17
AA+ +
22
Mass %
Mass
%
0.015
0.015
1.11.1
0.37
0.37
0.04 18
18
0.04
29 29 5.15.1
30
30
33 33 0.79
0.79 34
34
37
37
81
81
%
0.20
0.20
3.4
3.4
4.4
4.4
32.0
32.0
97.3
97.3
Element
Element
type
type
“A”
"A"
“A
"A++1”1"
“A
"A++1”1"
“A
"A++2”2"
"A"
“A”
"A++2”2"
“A
"A"
“A”
"A++2”2"
“A
"A++2”2"
“A
"A+33+2”2"
“A
"A"
A + 2 Elements —
oxygen, silicon, sulfur,
chlorine and bromine
• A second isotope makes an especially prominent
appearance in the spectrum if it is more than one mass
unit higher than the most abundant isotopic species.
Bromine and chlorine and to a lesser extent silicon and
sulfur are striking common examples
• The presence of these elements in an ion is usually easily
recognized from the “isotopic clusters” produced in the
spectrum
• Because of “Linear Superposition of Isotopic Ions”, the
isotopic patterns are even more striking when more than
one A + 2 isotope is present in an ion. For HBr the
isotopic molecular ions at m/z 80 and 82 (H79Br and H81Br)
are in relative proportions of 1:1. The MS of Br2 shows
prominent molecular ions at 158, 160 and 162,
representing 79Br-79Br, 79Br-81Br, 81Br-79Br and 81Br-81Br.
34
1
:
2
:
1
How many peaks will a molecule containing 3 bromine atoms exhibit?
4 peaks at intervals of 2 mass units in the ratio of 1:3:3:1.
What species are responsible for the “four peaks”?
CHBr3
79Br79Br79Br
= 237
79Br79Br81Br = 239
79Br81Br79Br = 239
81Br79Br79Br = 239
81Br81Br79Br = 241
81Br79Br81Br = 241
79Br81Br81Br = 241
81Br81Br81Br = 243
35
The MS of SO2 contains no A + 1 element, shows m/z 65/m/z
64 of 0.9%, which is close to that expected for one carbon
(but is actually due to 33S16O2+ and 32S16O17O+). This is
another reason to check the M + 2 peak first!
SO2
rel. int.
m/z
m/z
m/z
Int
Int.
6464
6565
6666
100.
100.
0.9
0.9
5.0
5.0
36
Are A + 2 peaks present? No!
What does the fact that the “base peak” = M+. imply? stability
How many carbons are implied by the M + 1 peak? 6.8/100:- 6 carbons
(and 6 hydrogen’s)
What is the molecule’s identity? Benzene
What does the A + 2 peak at m/z 80 arise from?
m/zInt.
m/z
Int Int.
m/z
1237 0.2
13
0.4
39
14
0.4
50
15
1.0
2451 0.4
2552 0.8
2663 3.2
2774 2.6
36
0.9
75
37
3.8
3976 13.
4077 0.4
507816.
5179 19.
528020.
53 3.80.8
60
0.2
13.
61
0.4
16.
62
0.8
6319. 2.9
6420. 0.2
72 2.90.4
73 3.91.0
74
3.9
2.2
75
2.2
76 7.07.0
7715.15.
100.
100.
78
79 6.86.8
80 0.20.2
13C 12C H
2
4 6
78
39
51
63
37
The Molecular Ion
• M+ • Provides the most valuable information in the spectrum.
• Its mass and elemental composition show molecular
boundaries into which the structural fragments indicated
must be fitted.
• Unfortunately, for some compounds, the molecular ion is
not sufficiently stable to be found in appreciable abundance
in an EI spectrum
• By convention, mass spectrometrists calculate the
molecular weight (m/z of molecular ion peak) in terms of the
mass of the most abundant isotope of each of the elements
present. Eg: The molecular weight of benzene, which has
substantial peaks from m/z 78 and m/z 80 has a molecular
weight of m/z 78; Br2 has peaks at m/z 158 and m/z 160, but
has a molecular weight of 158. Within these constraints in
an EI spectrum of a pure compound, the molecular ion, if
present, must be found at the highest value of m/z in the
spectrum
38
The Molecular Ion (continued):
Requirements of the Molecular Ion
• The following are necessary but not sufficient
requirements for the molecular ion in the mass
spectrum of a pure sample free from extraneous
peaks such as those from background and ionmolecule interactions
– It must be the ion of highest mass in the spectrum.
– It must be an odd electron ion.
– It must be capable of yielding the important ions in the
high mass region of the spectrum by loss of logical
neutral ions
– If the ion in question fails any of these tests, it cannot
be the molecular ion; if it passes all these tests, it may
or may not be the molecular ion!
39
The Molecular Ion (continued):
Odd Electron Ions
• The species formed when a sample molecule becomes
ionized by losing an electron, leaving one electron
unpaired is called an odd electron ion. It is designated by
the symbol +.. It should be noted that ions in which the
outer-shell electrons are paired are called “evenelectron” ions. These are designated by the symbol +.
• The ease of ionization of outer-shell electrons is n>>s.
Usually, several canonical resonance forms can be drawn
to approximate the electron distribution of the ion.
40
The Molecular Ion (continued):
Odd Electron Ions
• Note that +• indicates only an ion with an unpaired electron, not an
electron in addition to those the formula represents; i.e. adding
an electron to CH4 would give CH4- •.
• In general, ions containing only paired electrons (EE+) are more
stable, and thus more often the abundant fragment ions in an EI
spectrum. Eg: cleavage of a C-H bond in CH4+. forms the stable
EE+ ions CH3+ and H•.
• Soft ionization techniques such as FAB, ESI and MALDI tend to
give EE+ molecular species such as MH+.
• OE+• ions have special mechanistic significance and should be
identified on the spectrum early in the interpretation procedure.
• The importance of a peak after one has corrected for abundance
for contributions of ions containing less common isotopes,
generally increases with:
– Increasing intensity
– Increasing mass in spectrum
– Increasing mass in peak group, particularly the most or
41
second most number of hydrogen atoms for an OE+• peak.
Mass Spectrometry
High Resolution Mass Spectrometers
• Low resolution mass spectrometers report m/z values to the
nearest whole number. Thus, the mass of a given molecular ion
can correspond to many different masses.
• High resolution mass spectrometers measure m/z ratios to four
(or more) decimal places.
 This is valuable because except
for 12C whose mass is defined as
12.0000, the masses of all other
nuclei are very close—but not
exactly—whole numbers.
 Table 14.1 lists the exact mass
values for a few common nuclei.
Using these values it is possible
to determine the single molecular
formula that gives rise to a
molecular ion.
42
Mass Spectrometry
High Resolution Mass Spectrometers
• Consider a compound having a molecular ion at m/z = 60
using a low resolution mass spectrometer. The molecule
could have any one of the following molecular formulas.
43
Hydrocarbons:
Saturated Hydrocarbons:
• For straight chain compounds, M is always present
but with generally low intensity.
• Fragmentation is characterized by clusters of peaks,
with the corresponding peaks of each cluster being
14 (CH2) mass units apart.
• The largest peak in each cluster represents a CnH2n+1
fragment.
• Fragment abundances decrease in a smooth curve
down to M-C2H5.
• The M-CH3 peak is characteristically very weak or
missing.
• Compounds containing more than 8 carbon atoms
show fairly similar spectra. Thus, identification
depends on the molecular ion peak.
44
n-decane
- 14
- 14
142
45
n-tridecane
184
46
n-pentadecane
212
47
n-eicosane
282
48
Mass Spectrometry
Gas Chromatography-Mass Spectrometry (GC-MS)
interface
Sample introduced into GC inlet
vaporized at 250 °C , swept onto
the column by He carrier gas &
separated on column. Sample
components emerge from
column, flowing into the capillary
column interface connecting the
GC col-umn and the MS (He
removed).
The computer drives the MS, records the
data, and converts the electrical impulses
into visual displays and hard copy
displays. Identification of a compound based
on it's mass spectrum relies on the fact that
every compound has a unique fragmentation
pattern
A large library of known mass
spectra is stored on the computer and may
be searched using computer algorithms to
49
assist the analyst in identifying the unknown.
Mass Spectrometry
Gas Chromatography-Mass Spectrometry (GC-MS)
•To analyze a urine sample for tetrahydrocannabinol, (THC) the
principle psychoactive component of marijuana, the organic
compounds are extracted from urine, purified, concentrated
and injected into the GC-MS.
•THC appears as a GC peak, and gives a molecular ion at 314,
its molecular weight.
To improve GC separations, compounds are
often derivatized, e.g.
as their trimethylsilyl
(TMS) ethers or trifluoroacetate (TFA) esters.
OSiMe3
O
C5 H11
OC(O)CF3
O
C5 H11
THC TMS ether (l) & TFA ester (r)
50
Liquid Chromatography-Mass Spectrometry (LC-MS)
Similar to gas chromatography MS (GC-MS), liquid
chromatography mass spectrometry (LC/MS or LC-MS)
separates compounds chromatographically before they are
introduced to the ion source and mass spectrometer. It
differs from GC-MS in that the mobile phase is liquid,
usually a combination of water and organic solvents,
instead of gas. Most commonly, an electrospray ionization
(ESI) source is used in LC-MS.
51
Electrospray Ionization (ESI):
ESI is a method used to generate gaseous ionized molecules from a
liquid solution. This is done by creating a fine spray of highly
charged droplets in the presence of a strong electric field. The
sample solution is sprayed from a region of a strong electric field at
the tip of a metal nozzle maintained at approximately 4000 V. The
highly charged droplets are then electrostatically attracted to the
mass spectrometer inlet.
Either dry gas, heat or both are applied to the droplets before they
enter the vacuum of the mass spectrometer, thus causing the
solvent to evaporate from the surface. As the droplet decreases in
size, the electric field density on its surface increases. The mutual
repulsion between like charges on this surface becomes so great
that it exceeds the forces of surface tension, and ions begin to
leave the droplet through what is known as a “Taylor cone”. The
ions are directed into an orifice through electrostatic lenses leading
to the mass analyzer.
52
Advantages and Disadvantages of ESI-MS
• Advantages
– Practical mass range of up to
70,000 Da.
– Femtomole to low picomole
sensitivity
– Softest ionization technique
– Easily interfaced with LC
– No matrix interference
– Easily adaptable to triple
quadrupole analysis, conducive
to structural analysis
– Multiple charging, allowing for
analysis of high-mass ions with
relatively low m/z range
instrument
– Multiple charging giving better
mass accuracy through
averaging
• Disadvantages
– Low salt tolerance
– Difficulty in cleaning overly
contaminated instrument due to
high sensitivity for certain
compounds
– Low tolerance for mixtures.
Simultaneous mixture analysis
can be poor. The purity of the
sample is important
– Multiple charging, which can be
confusing, especially with
mixture analysis
53
Forensic Mass Spectrometry
•Analysis of Body Fluids for Drugs of Abuse
•Analysis of Hair in Drug Testing
•Sports Testing
•Analysis of Accelerants in Fire Debris
•Analysis of Explosives
•Use of Isotope Ratios
54
J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
Forensic Mass Spectrometry
•Analysis of Hair in Drug Testing
An important feature of hair analysis is its long-term
information on an individual’s drug use in contrast to
the short-tern information provided by urinalysis. Head
hair grows at ca. 1.3 cm/month. Consequently by
sampling the segment of hair corresponding to a
particular time frame, hair analysis can uncover drug
use from a week to years prior to collection of the
specimen. Hair analysis by GC-MS methods has been
used to establish the presence of Cannabinoids
(marijuana); Cocaine; Amphetamines; Opiates (heroin);
Barbiturates; Phencyclidine (PCP).
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Forensic Mass Spectrometry
•Analysis of Accelerants in Fire Debris
When a fire is extinguished fire debris samples must be quickly
obtained and placed in airtight unlined and unused metal cans
for GC-MS analysis. Forensic chemist need to explain in court
GC/MS principles & methods used to identify accelerant.
Accelerant recovery techniques involve direct headspace
methods with or without adsorbent although such techniques
discriminate against high boiling accelerants. Other methods
include distillation, solvent extraction, thermal desorption.
Accelerants include gasoline, light petroleum distillates,
turpentine & diesel fuel. Matrices from which volatiles are
recovered include carpet, wood, sheet rock, soil, concrete, &
roofing. Identification involves pattern matching between a GC
profile and a series of standard accelerants correcting for solid
matrix contributions.
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Forensic Mass Spectrometry
•Analysis of Explosives
Screening of evidentiary material for explosives (“postblast”) residue as
well as detection of hidden explosives in aviation security present many
challenges. There are three classes of explosives: high explosives,
propellants (“low explosives”) and primary explosives. High explosives
are subdivided into three groups: TNT (trinitrotoluene) and related
aromatic nitro compounds; RDX (cyclotrimethylene trinitramine) and
related nitramines with N–NO2 groups; nitroglycerin, pentaerythritol
tetranitrate (PETN) and related nitrated esters with C–ONO2 groups.
Explosives are most often mixtures of the above, e.g. PETN and RDX are
associated with SEMTEX, a Czechoslovakian explosive. Richard Reid,
the “shoe bomber” was found to have triacetone peroxide, TATP, which
was to be used to set off the more powerful “plastic” explosive PETN.
Difficult to analyze some high explosives because of thermal instability
and weak molecular ions due to easy fragmentation. LC-MS may be
preferable to GC-MS. Negative chemical ionization (CI) MS better than
positive ion CI due to high electronegativity.
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Forensic Mass Spectrometry
•Use of Isotope Ratios
Starting from raw materials with various isotopic compositions and
undergoing various isotope effects all along the synthetic or biosynthetic pathways, molecules progressively acquire a characteristic
isotopic composition that constitutes their own “isotopic signature.”
Measurement of isotopic ratios of each of its constituent elements can
act as the specific isotopic signature. This isotopic signature can be
self-generated during synthetic processes without an intentional
modification. This signature can also be intentionally produced by
adding precursors artifically enriched with stable isotopes in the
reaction medium where the synthesis or biosynthesis takes place.
Therefore, this intentionally modified isotope composition can be
considered as an isotope signature of the commercial property of the
compound. Isotope ratios can be accurately measured using isotope
ratio mass spectrometers, IRMS. IRMS has been used to distinguish
natural vanillin ($2500/kg; from Tahiti) from synthetic vanillin made
from clove oil ($10/kg) and natural honey from honey adultrated with
cheaper high fructose corn syrup.
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Natural Isotopes and Trace Elements in Forensics
Most criminal activities result in the generation of some kind of debris, either at the
scene of the crime and/or on the individual perpetrators. Subsequently this material
becomes available to investigators as physical evidence of the crime. However,
traditionally the generation of analytical and forensic chemical data is often compromised by a number of factors:
• Sample mass may be extremely small, this being especially true as criminals
become more sophisticated,
• The generation of data is often time consuming and costly and increasingly the
budget of police forces is being diminished,
• The range of analytes accessible for analysis is often compromised by a requirement
to retain a significant portion of the sample for corroborative studies.
New developments in instrumentation, especially the combination of Laser Ablation with
either Quadrupole, Time of Flight, Sector or Multi-Collector ICP-MS (Inductively
Coupled Plasma-Mass Spectrometry), have created exciting possibilities for the routine
"non–destructive" isotope and trace-element analysis of small and valuable specimens.
Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) offers the
potential of producing fast, definitive, cost effective elemental and isotopic ratio data for
a wide variety of forensic chemical evidence for use in identifying and comparing
physical evidence and thereby unambiguously relating a suspect to a crime scene.
Forensic investigations significantly benefit from the study of unique natural isotopic and
59
trace-elemental fingerprints in most materials involved.
Inductively Coupled Plasma Mass Spectrometry:
Mass Spectrometry of Ionized Atoms
ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is a highly sensitive
mass spectrometer capable of analysis of metals and non-metals at below one
part in 1012. It is based on coupling an inductively coupled plasma (ICP), which
produces atomic ions (as opposed to molecular ions), with a mass
spectrometer as a method of identifying and detecting the ions.
ICP - a high temperature argon plasma sustained with a radiofrequency electric
current produces ions. The electric current is transferred to the plasma by an
induction coil, wrapped around concentric quartz tubes (the plasma torch).
Operating frequencies are 27.12 and 40.68 MHz; operating power is 800 to 1500
W. The plasma is sustained within a constant, high flow of argon gas and
reaches temperatures of 10,000 °C. Total gas consumption is 14 - 18 L/min.
MS - the ions from the plasma are extracted through a series of cones into a
quadrupole mass spectrometer and are separated according to their m/z ratio. A
detector receives an ion signal proportional to the concentration. Sample
concentration is determined through calibration with elemental standards.
Quantitative determination using ICP-MS requires isotope dilution, a single
point method based on an isotopically enriched standard. Laser ablation
60 (LA)
ICP-MS uses lasers to generate samples for ICP-MS analysis.
J. Agric. Food Chem., 53 (10), 4041-4045, 2005.
Determination of the Country of Origin of Garlic (Allium
sativum) Using Trace Metal Profiling
Ralph G. Smith
U.S. Customs and Border Protection Laboratory, 214 Bourne Boulevard, Savannah, Georgia 31408
Abstract:
A method for determining the country of origin of garlic by comparing the trace metal profile of the
sample to an authentic garlic database is presented. Protocols for sample preparation, high-resolution
inductively coupled plasma mass spectrometry, and multivariate statistics are provided. The criteria used
for making a country of origin prediction are also presented. Indications are that the method presented
here may be used to determine the geographic origin of other agricultural products.
Introduction
U.S. Customs and Border Protection (CBP) Laboratories have been working on laboratory methods to facilitate
scientific identification of the country of origin of various agricultural products. One such method presented here
includes the determination of the trace metal profile of the agricultural product for comparison with an
established database of trace metal profiles of the product from various countries. The uptake of trace metals by
agricultural products from the soil in which they are grown provides a mechanism for identification of their
geographic origin. There are a number of factors such as rainfall, sunshine, temperature, soil characteristics, and
plant species that may play an important role in the uptake of trace metals. It is the combination of these factors
that influence the uptake of trace metals creating a rough snapshot or historical record of the plant's growth. In
most cases, the trace metal profiles of agricultural products from various countries display enough statistical
uniqueness to make a definitive country of origin prediction. The use of trace metal profiling for determining the
geographic origin of agricultural products uses high-resolution ICP-MS with lowered detection limits and
elimination of many of the isobaric interferences prevalent with quadrupole ICP-MS instruments. Garlic samples
were analyzed for 18 elements by high-resolution ICPMS including Li, B, Na, Mg, P, S, Ca, Ti, Mn, Fe,61Cu, Ni,
Zn, Rb, Sr, Mo, Cd, and Ba.
Forensic Mass Spectrometry
•Analysis of Body Fluids for Drugs of Abuse
1) Hydrolyze drug metabolites (acid, base, enzymatic); 2) extract from
biological matrices (liquid-liquid or SPE); 3) derivatize to improve
volatility, separation and analysis; 4) chromatographically separate,
typically by GC using a narrow-bore (0.20 to 0.32 mm i.d.) fused-silica
methyl-silicone or phenylmethylsilicone capillary GC column; 5)
deuterated internal standards to confirm extraction process, GC RT &
fragmentation patterns; 6) analysis by EI- or CI-MS (e.g. using NH3; CI
is more sensitive giving only parent ion; 7) mass analysis using full
scan data acquisition or selected ion monitoring (SIMs; can provide
signal intensities 100-fold better and is less subject to interferences
but can miss significant unsuspected drugs).
Cannabinoids (marijuana); Cocaine; Amphetamines; Opiates (heroin);
Barbiturates; Phencyclidine (PCP); Lysergic acid diethylamide (LSD);
Benzodiazepines; Fentanyl.
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Forensic Mass Spectrometry
•Analysis of Accelerants in Fire Debris
GC methods used include GC-MS, GC-IR and GC-AE. GC analysis
of hydrocarbons is most useful when fragmentation pattern is
unique and particularly if several ions can be found that are
characteristic for a substance class, usually alkanes and alkylbenzenes. Typical fragment ions: 43, 57, 71 [alkanes], 91, 106, 120
[alkylbenzenes], 128, 142, 156 [naphthalenes]; alcohols [31, 45],
ketones [43, 58], esters [43, 73], terpenes [93, 136]. Critical to
discriminate against artifacts (e.g. from matrices, extraction solvents, pyrolysis products from the effect of the fire). GC patterns
may show evenly spaced peaks corresponding to n-alkane homologs (medium and high boiling range distillates and kerosene) but
this is not the case for gasoline. Note also that 90% evaporated
gasoline appears quite different from the original gasoline. Gasoline consists of more than 250 components above the 10 ppm level
and is one of the most complex accelerants.
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