Transcript Document
Forensic Mass Spectrometry Distinguished Prof. Eric Block CHM 450B, W 2006
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CO 2 Me Me N OC(O)Ph cocaine mw 303 CO 2 Et Me N OC(O)Ph cocaethylene mw 317 Me 2 N Ph Ph
NYSP Laboratory Report
cocaine, cocaethylene, methadone Identified in urine O methadone mw 309
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82 NMe + Me N + 182 CO 2 Me Me
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N CO 2 Me OC(O)Ph OC(O)Ph 272 303
Mass Spectrum of Cocaine
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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.
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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.
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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. CH 4 •+ or NH 3 •+ ). 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.
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Introduction Mass Spectrometry
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).
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
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•
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.
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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.
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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 transform ion cyclotron resonance (FT-ICR).
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Mass Analyzers (continued)
Quadrupole Analyzer
•
Quadrupoles are four precisely parallel rods with a direct current (DC) voltage and a superimposed radio frequency (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 instruments thus 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 fragmentation. In tandem MS (abbreviated MS n to induce where n refers to the number of generations of fragment ions being analyzed) collisionally induced fragment ions are mass analyzed.
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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 MS
n
is applied to processes which analyze beyond daughter ions (MS
2
) to grandaughter (MS
3
), and to great-granddaughter ions (MS
4
).
Tandem mass analysis is primarily used to obtain structural information.
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Interpretation of EI Mass Spectra
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• • • • • • • •
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 amu above that of the most abundant isotope
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• • • • •
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 (x axis).
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 abun dant isotopes.
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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.
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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• + ABC + D + BC A + BC + + AD +• + B=C
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Unknown 1
m/z
Int Int.
1 1 1.0
1.0
100.
0.2
0.2
relative abundance 18 Mass (mass-to-charge ratio) 16 17
22
m/z
Int.
12 1 13 14 15 16 17 3.1
1.0
8.1
16.
85.
100.
1.1
relative abundance 15 16 12 13 14 17 Mass (mass-to-charge ratio
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Mass Spectrometry Introduction
Consider the mass spectrum of CH 4 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, 13 CH 4 is responsible for the peak at m/z = 17.
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This is called the M + 1 peak.
Figure 2.12: The mass
Mass Spectrum of Neon Showing Major and Minor Natural Isotopes
rel. int
20 Ne 90.48%
m/z
Copyright © by Houghton Mifflin Company. All rights reserved. 21 Ne 0.27% 22 Ne 9.25% 25 4
Mass Spectrometry Introduction
•
The mass spectrum of CH 4 just the M peak.
consists of more peaks than
•
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.
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CH 3 OH
m/z
Int.
CH 3 OH + e CH 3 OH + • CH 2 OH + CH 3 OH + • (m/z 32) + 2e CH 2 OH + (m/z 31) + H • CH 3 + (m/z 15) + HO • CHO + (m/z 29) + H 2 16 17 0.2
1.0
29 31 32 33 34 1.0
0.1
relative abundance 15
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Introduction Mass Spectrometry
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Mass Spectrometry Alkyl Halides and the M + 2 Peak
• •
Most elements have one major isotope.
Chlorine has two common isotopes, occur naturally in a 3:1 ratio. 35 Cl and 37 Cl, which
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 35 Cl. The smaller peak, the M + 2 peak, corresponds to the compound containing 37 Cl.
•
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 — 79 Br and 81 Br, in a ratio of ~1:1. Thus, when the molecular ion consists of two peaks (M and M + 2) in a 1:1 ratio, a Br atom is present.
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Mass Spectrometry Alkyl Halides and the M + 2 Peak
(CH 3 ) 2 CH +
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Mass Spectrometry Alkyl Halides and the M + 2 Peak
(CH 3 ) 2 CH +
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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
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“A” elements: “A + 1” elements: “A + 2” elements: Element Mass % Element Mass % I F P S F P S 1 12 14 16 19 28 31 32 35 79 79 100 127 100 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 A + 1 Mass Mass % % 2 2 13 15 13 15 17 17 1.1
1.1
A + 2 Mass Mass % % Element type 29 33 29 33 5.1
5.1
"A + 2" "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 (H 79 Br and H 81 Br) are in relative proportions of 1:1. The MS of Br2 shows prominent molecular ions at 158, 160 and 162, representing 79 Br 79 Br, 79 Br 81 Br, 81 Br 79 Br and 81 Br 81 Br.
1 : 2 : 1
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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”?
CHBr 3 79 Br 79 Br 79 Br = 237 79 Br 79 Br 81 Br = 239 79 Br 81 Br 79 Br = 239 81 Br 79 Br 79 Br = 239 81 Br 81 Br 79 Br = 241 81 Br 79 Br 81 Br = 241 79 Br 81 Br 81 Br = 241 81 Br 81 Br 81 Br = 243
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The MS of SO 2 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 33 S 16 O 2 + and 32 S 16 O 17 O another reason to check the M + 2 peak first!
+ ). This is
rel. int.
SO 2
m/z
Int.
65 66
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m/z
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?
What is the molecule’s identity?
Benzene What does the A + 2 peak at
m/z
80 arise from?
6.8/100:- 6 carbons (and 6 hydrogen’s) 13 C 2 12 C 4 H 6 12 13 14 37 39 15 50 24 51 25 52 0.2
0.4
0.4
1.0
0.4
0.8
26 63 27 36 74 37 39 75 76 40 77 3.2
2.6
0.9
3.8
13.
0.4
50 78 16.
51 79 19.
52 80 20.
Int.
53 3.8
60 61 13.
0.8
0.2
0.4
62 16.
0.8
63 19.
64 20.
2.9
0.2
72 2.9
0.4
73 74 75 76 77 3.9
2.2
7.0
1.0
3.9
2.2
7.0
15.
15.
78 79 80 6.8
6.8
39 51 63 78
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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; Br 2 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
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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 ion molecule 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!
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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 “even electron” 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.
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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 CH 4 would give CH 4 • .
In general, ions containing only paired electrons (EE spectrum. Eg: cleavage of a C-H bond in CH 4 +.
+ ) are more stable, and thus more often the abundant fragment ions in an EI forms the stable EE + ions CH 3 + 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 second most number of hydrogen atoms for an OE + • peak.
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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 12 C 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.
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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.
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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 (CH 2 ) mass units apart. • The largest peak in each cluster represents a C n H 2n+1 fragment.
• Fragment abundances decrease in a smooth curve down to M-C 2 H 5 .
• The M-CH 3 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.
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n
-decane
- 14 - 14
45
142
n
-tridecane 46
184
n
-pentadecane 47
212
n
-eicosane 48
282
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 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 separa tions, compounds are often derivatized, e.g.
as their trimethylsilyl (TMS) ethers or trifluor oacetate (TFA) esters.
OSiMe 3 OC(O)CF 3 O C 5 H 11 O C 5 H 11 THC TMS ether (l) & TFA ester (r)
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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.
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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.
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•
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
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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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J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
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.
56
J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
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 –NO 2 groups; nitroglycerin, pentaerythritol tetranitrate (PETN) and related nitrated esters with C –ONO 2 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 .
57
J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
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 biosyn thetic 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.
58
J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
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 compro mised 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 10 12 . It is based on coupling an inductively coupled plasma (ICP), which produces atomic ions (as opposed to molecular ions), with spectrometer as a method of identifying and detecting the ions.
a mass 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 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, Cu, Ni, Zn, Rb, Sr, Mo, Cd, and Ba.
Forensic Mass Spectrometry
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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 NH 3 ; 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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J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995
Forensic Mass Spectrometry
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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 alkyl benzenes. 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 sol vents, pyrolysis products from the effect of the fire). GC patterns may show evenly spaced peaks corresponding to n-alkane homo logs (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. Gaso line consists of more than 250 components above the 10 ppm level and is one of the most complex accelerants.
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J. Yinon, Ed., Forensic Applications of Mass Spectrometry, CRC Press, 1995