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INSTRUMENTAL ANALYSIS CHEM 4811

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INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 10 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university – PowerPoint PPT presentation

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Title: INSTRUMENTAL ANALYSIS CHEM 4811


1
INSTRUMENTAL ANALYSIS CHEM 4811
  • CHAPTER 10

DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
2
CHAPTER 10 MASS SPECTROMETRY II SPECTRAL
INTERPRETATION AND APPLICATIONS
3
SPECTRAL INTERPRETATION
- Structural determination of simple molecules
will be covered - The mass spectrum is a plot or
a table - m/z values are on the x-axis of the
spectrum - Relative abundance (relative
concentration) on the y-axis - Base peak is the
most abundant peak and is assigned abundance of
100 - Others are percentages of the base peak
4
SPECTRAL INTERPRETATION
Two ways to interpret spectra - Compare spectrum
to those in a searchable engine (over 400,000
spectra are available) and - Use
interpretation procedure for evaluating spectra
5
EVALUATION OF SPECTRA
- Involves a lot of educational guess work - The
structure must be confirmed by analyzing the pure
form of the substance identified - Identify the
molecular ion if present - Apply the nitrogen
rule - Evaluate for A2 elements - Calculate
A1 and A elements
6
EVALUATION OF SPECTRA
- Look for loss peaks from the molecular ion -
Look for characteristic low mass fragments -
Postulate a possible formula - Calculate rings
plus double bonds - Postulate a reasonable
structure
7
MOLECULAR ION
- Forms by loss of electron when a molecule is
ionized by EI - The radical cation (M) formed
has the same mass as the neutral molecule - The
m/z value of the molecular ion indicates the
molecular weight of the molecule - Molecular
ion absorbs excess energy which causes it to
break apart into fragments - Fragments may be
ions, neutral molecules, or radicals
8
FRAGMENTATION PATTERNS
- Is the mass and abundance of fragment ions -
Is used to deduce the structure of the
molecule - Ions in the mass spectrum are called
fragment ions - Fragments may break apart to
form smaller fragments - A given molecule will
always produce the same fragments if ionization
conditions remain the same
9
FRAGMENTATION PATTERNS
- The base peak is usually not the molecular ion
in EI - A molecular ion is always a radical
(odd number of electrons and never an even
electron ion) M e- ? M 2e- - Even
electron ions result from fragmentation -
Aromatic compounds and conjugated hydrocarbons
give more intense molecular ion peaks
10
FRAGMENTATION PATTERNS
- Alkanes, aliphatic alcohols and nitrates give
less intense molecular peaks - Highly branched
compounds tend not to give molecular peaks -
Abundant fragment peak typically shows loss of
neutral fragment Alpha Cleavage - Cleavage at
the bond adjacent to the C to which a functional
group is attached
11
ISOTOPIC ABUNDANCES
- The most abundant isotope and the unit atomic
mass are used to calculate the molecular
weights - 13C results in a peak one mass number
greater than the mass of the molecular ion in
all organic compounds - The peak is designated
as M1 CH4 12 4(1) 16 m/z of molecular
ion - A small peak of m/z 17 is also seen on
spectrum because of the isotope 13C which is
also stable
12
ISOTOPIC ABUNDANCES
- Natural abundance of deuterium (2H) is usually
ignored (0.016) - Nominal mass is the integer
mass of the most abundant naturally occurring
isotope - Nominal mass is used in MS
calculations but not the atomic weight or the
exact mass
13
COUNTING CARBON ATOMS
- For a hydrocarbon with only one C atom (M1)/M
1.1 - For a hydrocarbon with two C
atoms (M1)/M 2.2 In general (M1)/M 1.1 x
of C atoms in the molecule If (M1) ltlt 1
implies no C atom is present
14
COUNTING OTHER ELEMENTS
- Assume that only C, H, N, O, F, P, and I are
present - The other elements such as N and S
contribute to the (M1) peak intensity Generally
(M1)/M 1.1( C atoms) 0.016( H atoms)
0.3( N atoms) 0.78( S atoms) . -
Contribution from hydrogen is small and is ignored
15
COUNTING OXYGEN ATOMS
- Oxygen has two important isotopes 16O and
18O - Relative abundance 18O/16O 0.2 -
Number of oxygen atoms in a given molecule is
given as (M2)/M 0.20( O atoms) 1.1( C
atoms)2/200
16
HETEROATOMIC COMPOUNDS
Elements are grouped into 3 categories - A
elements are the monoisotopic elements (F, P, I
and somehow H) - A1 elements are those with
two isotopes whose difference is 1 Da (C, N) -
A2 elements are those with an isotope 2 Da
heavier than the most abundant isotope (Cl, Br,
O, S, Si)
17
RINGS AND DOUBLE BONDS
- The number of rings double bonds in a
molecule with formula CxHyNzOm is given as x
1/2y 1/2z 1 For n-hexane (C6H14) 6 ½(14)
0 1 6 7 1 0 For cyclohexane (C6H12) 6
½(12) 0 1 6 6 1 1 For benzene
(C6H6) 6 ½(6) 0 1 6 3 1 4
18
RINGS AND DOUBLE BONDS
- A triple bond is equivalent to two double
bonds For acetylene (C2H2) 2 ½(2) 0 1
2 - This equation does not distinguish between
double bonds, rings, or triple bonds - It is
thus used together with IR, NMR, etc.
19
NITROGEN CONTAINING COMPOUNDS
- Amines, amides, nitriles, nitro compounds -
Many N-containing compounds give no detectable
molecular ion - Alpha cleavage is seen in
aliphatic amines (RCH2NH2 gives rise to CH2NH2
with m/z 30, 44, 58, .) The Nitrogen Rule -
Used to identify a molecular ion peak - The m/z
value of the molecular ion and hence the
molecular weight is an odd number if the
molecule contains an odd number of N atoms
20
NITROGEN CONTAINING COMPOUNDS
- Amides, cyclic aliphatic amines, aromatic
amines, nitriles, and nitro groups give
measurable molecular ions - Amides have
fragmentation patterns similar to their
corresponding carboxylic acids -,Nitro
compounds usually have NO (m/z 30) and NO2
(m/z 46) - Aromatic nitro compounds have
characteristic peaks at M-30 and M-46 (due to
loss of NO and NO2)
21
ALKANES
- Successive loss of methylene groups (CH2, 14
Da) - CH3 with m/z 15 is seen - m/z 15, 29,
43, 57 .. - Branched chain alkanes are less
likely to show a molecular ion peak than
n-alkanes - Cycloalkanes show strong molecular
ion peaks and characteristic peaks separated by
14 Da
22
ALKENES AND ALKYNES
- Both show strong molecular ion peaks (double
and triple bonds are able to absorb energy) -
Alkenes with C atoms gt 4 often show a strong peak
at m/z 41 (formation of allyl ion) - Alkynes
show strong (M-1) peaks (loss of 1 H atom) - It
is difficult to use MS to locate position of
double or triple bonds
23
ALCOHOLS
- CH2 OH - Aliphatic alcohols usually
fragment with loss of H or H2O - m/z 31, 45,
59, . - Look for M-18 peak corresponding to
loss of H2O - Alpha cleavage is seen -
Molecular ion peak is usually weak in primary and
secondary aliphatic alcohols and absent in
tertiary alcohols
24
ALCOHOLS
- Alpha cleavage plus loss of H2O in primary
aliphatic alcohols Tertiary alcohols tend to
lose OH rather than H2O (M-17 peak) - Alcohols
containing more than 4 C atoms often lose both
water and ethylene simultaneously
25
AROMATIC COMPOUNDS
- Very stable and do not fragment easily - Very
intense molecular ion peak is seen - Very little
fragmentation - Usually show noninteger m/z
values due to doubly charged ions (M) -
Benzene ring with alkyl groups under
rearrangement of benzyl cation
26
ALDEHYDES AND KETONES
- Fragment by alpha cleavage - Aldehydes also
fragment by beta cleavage - For aldehydes m/z
29, 43, 57, 71, . - For ketones m/z 43, 57,
71, .. - Ketones and aromatic aldehydes have
strong molecular ion peak - Aliphatic aldehydes
give a weak but measurable molecular ion peak
27
CARBOXYLIC ACIDS AND ESTERS
- Aliphatic carboxylic acids and small aliphatic
esters (4 or 5 C atoms) have weak but measurable
molecular ion peak - Larger esters show no
molecular ion peak - Aromatic carboxylic acids
give strong molecular ion peak - Acids
typically lose OH and COOH through alpha
cleavage (M-17 and M-45 peaks)
28
CARBOXYLIC ACIDS AND ESTERS
- Characteristic peak for acids is m/z 45 -
Esters undergo alpha cleavage to form RCO
ion - Characteristic peak for esters is m/z
74 - Can undergo McLafferty rearrangement (not
discussed here)
29
Cl AND Br CONTAINING COMPOUNDS
- Chlorine has two isotopes 35Cl/37Cl
100/33 - M2 peak is about 33 of M peak -
Bromine has two isotopes 79Br/81Br 1/1 - M
and M2 peaks are approximately equal - Bromine
compounds fragment by loss of Br - Chlorine
compounds fragment by loss of HCl
30
Cl AND Br CONTAINING COMPOUNDS
- Form isotope cluster patterns - Isotopic
clusters are seen when more than one Cl or Br
atom is present in a molecule - One Cl atom
will exhibit masses of R35 and R37 with
relative abundances 10033 - Two Cl atoms will
have R70, R72, R74 with relative abundances
1006611
31
Cl AND Br CONTAINING COMPOUNDS
- Three Cl atoms will have R105, R107, R109,
R111 with relative abundances 10098323 -
One Br atom will have R79 and R81 with
relative abundances 11 - Two Br atoms will
have R158, R160, R162 with relative
abundances 5110049
32
F AND I CONTAINING COMPOUNDS
- Iodine compounds fragment by loss of I -
Iodine and fluorine do not form clusters since
they are monoisotopic - Fluorine compounds
undergo unique reactions (will not be discussed
here) - F also fragments resulting in (M-19)
peak
33
SULFUR CONTAINING COMPOUNDS
- Thiols (RSH) show stronger molecular ion peaks
than their corresponding alcohols - M2 peak
is enhanced due to 34S isotope - Primary thiols
lose H2S on fragmentation (M-34) peak -
Fragmentation patterns are similar to those of
alcohols
34
APPLICATIONS OF MOLECULAR MS
- For molecular weight determination - Molecular
structure determination - Reaction kinetics -
Dating of minerals, fossils, and artifacts -
Quantitative analysis of elements and
compounds - Protein sequencing (proteomics)
35
APPLICATIONS OF MOLECULAR MS
- Gas analysis - Environmental applications
(holomethanes, PCBs, pesticides, dioxins)
36
LIMITATIONS OF MOLECULAR MS
- Compound must be volatile - Must be able to
be converted into the gas phase without
decomposing - Carboxylic acids must be
converted to the corresponding volatile methyl
esters - MS cannot distinguish between certain
isomers
37
ATOMIC MS
- For determination of atomic weights and isotope
distribution of elements Ionization
Sources GD Spark source ICP
38
INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY
(ICP-MS)
- ICP-MS with quadrupole mass analyzer can be
used to determine most elements on the periodic
table in a few seconds - Sensitivity is very
high - Wide concentration range - Used to
obtain isotope ratios - Ionization efficiency is
almost 100
39
INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY
(ICP-MS)
- Has simple mass spectra (elements easily
identified) - For analyzing inorganic materials
in solution (ash, bones, rocks) - Indium
cannot be identified by ICP-MS - Petroleum
fractions for trace elements
40
APPLICATIONS OF ATOMIC MS
- Aqueous solutions are commonly analyzed by
ICP-MS - Extremely high purity water, acids,
bases reagents are used - Solid samples can be
analyzed by laser ablation ICP-MS or by coupling
graphite furnace to ICP-MS - GDMS and spark
source MS are also used for solid samples (for
analysis of art works and jewelry) -
Chromatography or CE is coupled to ICP-MS for the
determination of halogen oxyanions (IO4-, IO3-,
BrO3-, ClO3-)
41
APPLICATIONS OF ATOMIC MS
- For rapid multielement analysis of metals and
nonmetals at ppm and even ppt levels - Analysis
of environmental samples - Analysis of body
fluids for toxic elements (lead, arsenic) -
Trace elements in geological samples - Metals in
alloys - Ceramics and semiconductors
42
APPLICATIONS OF ATOMIC MS
- Pharmaceutical - Cosmetics samples - Food
chemistry GC-ICP-MS or LC-ICP-MS - For
determination of arsenic compounds in
shellfish - For analyzing breakfast cereal,
peanut butter, wine, beer - Whole blood and
serum for Al, Cu, Zn, blood lead, etc.
43
LIMITATIONS OF ATOMIC MS
- Inefficient introduction system - Matrix
effect - Isobaric interference - Degree of
interference from polyatomic ions
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