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INSTRUMENTAL ANALYSIS CHEM 4811
CHAPTER 10
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
CHAPTER 10
MASS SPECTROMETRY II
SPECTRAL INTERPRETATION AND APPLICATIONS
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
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
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 ‘A+2’ elements
- Calculate ‘A+1’ and A elements
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
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
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
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
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
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 M+1
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
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
COUNTING CARBON ATOMS
- For a hydrocarbon with only one C atom(M+1)/M = 1.1%
- For a hydrocarbon with two C atoms(M+1)/M = 2.2%
In general(M+1)/M = 1.1% x # of C atoms in the molecule
If (M+1) << 1% implies no C atom is present
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 (M+1) peak intensity
Generally(M+1)/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
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
(M+2)/M = 0.20(# O atoms) + [1.1(# C atoms)]2/200
HETEROATOMIC COMPOUNDS
Elements are grouped into 3 categories
- ‘A’ elements are the monoisotopic elements(F, P, I and somehow H)
- ‘A+1’ elements are those with two isotopes whose difference is 1 Da (C, N)
- ‘A+2’ elements are those with an isotope 2 Da heavier than the most abundant isotope (Cl, Br, O, S, Si)
RINGS AND DOUBLE BONDS
- The number of rings + double bonds in a molecule withformula 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
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.
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
- 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
•)
NITROGEN CONTAINING COMPOUNDS
- 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
ALKANES
- Both show strong molecular ion peaks(double and triple bonds are able to absorb energy)
- Alkenes with C atoms > 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
ALKENES AND ALKYNES
- 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
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
ALCOHOLS
- 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
AROMATIC COMPOUNDS
- 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
ALDEHYDES AND KETONES
- 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)
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)
CARBOXYLIC ACIDS AND ESTERS
- Chlorine has two isotopes: 35Cl/37Cl = 100/33
- M+2 peak is about 33% of M peak
- Bromine has two isotopes: 79Br/81Br = 1/1
- M and M+2 peaks are approximately equal
- Bromine compounds fragment by loss of Br
- Chlorine compounds fragment by loss of HCl
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 R+35 and R+37 with relative abundances 100:33
- Two Cl atoms will have R+70, R+72, R+74 with relative abundances 100:66:11
Cl AND Br CONTAINING COMPOUNDS
- Three Cl atoms will have R+105, R+107, R+109, R+111 with relative abundances 100:98:32:3
- One Br atom will have R+79 and R+81 with relative abundances 1:1
- Two Br atoms will have R+158, R+160, R+162 with relative abundances 51:100:49
Cl AND Br 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
F AND I CONTAINING COMPOUNDS
- Thiols (RSH) show stronger molecular ion peaks than their corresponding alcohols
- M+2 peak is enhanced due to 34S isotope
- Primary thiols lose H2S on fragmentation: (M-34) peak
- Fragmentation patterns are similar to those of alcohols
SULFUR CONTAINING COMPOUNDS
- For molecular weight determination
- Molecular structure determination
- Reaction kinetics
- Dating of minerals, fossils, and artifacts
- Quantitative analysis of elements and compounds
- Protein sequencing (proteomics)
APPLICATIONS OF MOLECULAR MS
- Gas analysis
- Environmental applications (holomethanes, PCBs, pesticides, dioxins)
APPLICATIONS 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
LIMITATIONS OF MOLECULAR MS
- For determination of atomic weights and isotope distribution of elements
Ionization Sources
GDSpark source
ICP
ATOMIC 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%
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
INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY (ICP-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-)
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
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.
APPLICATIONS OF ATOMIC MS
- Inefficient introduction system
- Matrix effect
- Isobaric interference
- Degree of interference from polyatomic ions
LIMITATIONS OF ATOMIC MS