INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 10DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciencesClayton 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 INTERPRETATIONTwo 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)

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 COMPOUNDSElements 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 (14) + 0 + 1 = 6 7 + 1 = 0

For cyclohexane (C6H12)6 (12) + 0 + 1 = 6 6 + 1 = 1

For benzene (C6H6)6 (6) + 0 + 1 = 6 3 + 1 = 4

RINGS AND DOUBLE BONDS- A triple bond is equivalent to two double bonds

For acetylene (C2H2)2 (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 DaALKANES

- 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 bondsALKENES 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 alcoholsALCOHOLS

- 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 simultaneouslyALCOHOLS

- 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 peakALDEHYDES 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 HClCl 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 100:66:11Cl 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 51:100:49Cl 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) peakF 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 alcoholsSULFUR 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 isomersLIMITATIONS OF MOLECULAR MS

- For determination of atomic weights and isotope distribution of elements

Ionization Sources

GDSpark sourceICPATOMIC 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 elementsINDUCTIVELY 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 semiconductorsAPPLICATIONS 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 ionsLIMITATIONS OF ATOMIC MS