Chapter 20# 4, 5, 12, 13, 16

Fig. 20-1 (p.551) Mass spectrum of ethyl benzene

Fragment peaks

Unit: amu or dalton

Fragment peaks

- Sample inlet system – vaporize sample- Ion source – ionizes analyte gas molecules- Mass analyzer – separates ions according to m/z- Detector – counters ions- Vacuum system – reduces collisions between ions and gas molecules

Fig. 20-11 (p.564) Components of a mass spectrometer

10-5 -10-8

2.1 Sample inlet

2.1.1 External (Batch) inlet systems- Liquid- Gas

2.1.2 Direct probe- Non-volatile liquid- Solid

Fig. 20-12 (p.564) Sample inlet

2.1.3 Chromatography/Electrophoresis

- Permits separation and mass analysis- How to couple two techniques?

GC/MS,

Fig. 27-14 (p.799) Capillary GC-MS

HPLC/MS, nano flow, ESI

Adapted from http://www.bris.ac.uk/nerclsmsf/techniques/hplcms.html

2.2 Ion sources

Hard ionization leaves excess energy in molecule – extensive fragmentation

Soft ionization little energy in molecule – reduced fragmentation

Fig. 20-2 (p.553) Mass spectrum of 1-decanol from (a) a hard ionization source (electron impact) and (b) a soft ionization (chemical ionization)

2.2 Ion sources

Fig. 20-3 (p.553) An electron-impact ion source

2.2.1 Gas-phase ion source

(1) Electron Impact (EI) Electron bombardment of gas/vapor molecules

(2) Chemical ionization (CI)- EI ionization in excess (105 of analyte pressure) of reagent gas (methane)

to generate CH4+ and CH3

+, then

CH4+ + CH4 CH5

+ + CH3

CH3+ + CH4 C2H5

+ + H2

Ions reacts with analyte

CH5+ + A CH4 + AH+ proton transfer

C2H5+ + A C2H4 + AH+ proton transfer

C2H5+ + A C2H6 + (A-H)+

hydride elimination

- analyte

most common ions (M+1)+ and (M-1)+

sometimes (M+17)+ addition of CH5+ or (M+29)+ (addition of C2H5

+)

Adapted from Schröder, E. Massenspektrometrie - Begriffe und Definitionen; Springer-Verlag: Heidelberg, 1991.

2.2.2 Desorption/Ionization sources (For non-volatile or non-stable analytes)

(1) Electrospray ionization (ESI): explosion of charged droplets containing analyte

- solution of analyte pumped through

charged (1-5 kV) capillary

- small droplets become charged

- solvent evaporates, drop shrinks,

surface charge density increases

- charge density reduced by

explosion of charged analyte

molecules (“Coulomb explosion”)

Soft ionization – transfer existing ions

from the solution to the gas phase,

little fragmentation

Easily coupled to HPLC

Adapted from http://www.bris.ac.uk/theory/fab-

ionisation.html

Fig. 20-9 (p.562) Apparatus for ESI

Fig. 20-10 (p.563) Typical ESI MS of proteins and peptides.

- Important technique for large (105 Da) thermally fragile molecules, e.g., peptide, proteins- produce either cations or anions.- Analytes may accumulate multiple charges in ESI, M2+, M3+ … molecular mass = m/z x number of charges

(2) Fast atom bombardment (FAB)

- Sample in glycerol matrix

- Bombarded by high energy Ar or Xe atoms ( few keV)

- Atoms and ions sputtered from surface (ballistic collision)

- Both M+ and M- produced

- Applicable to small or large (>105 Da) unstable molecule

Comparatively soft ionization – less fragmentation

Adapted from http://www.bris.ac.uk/theory/fab-ionisation.html

(3) Matrix-assisted laser desorption/ionization (MALDI)

- analyte dispersed in

UV-absorbing matrix

and placed on sample plate

- pulsed laser struck the sample

and cause desorption of

a plume of ions,

- energy absorption by matrix,

transfer to neutral analyte

desorption of matrix and neural analyte

ionization via PT between

protonated matrix ions and neutral analyte

Fig. 20-7 (p.560) Diagram of MALDI progress.

Fig. 20-8 (p.561) MALDI-TOF spectrum from nicotinic acid matrix irradiated with a 266-nm laser beam.

MALDI spectrum contains: dimmer, trimmer, multiply charged moleculesno fragmentation, Soft ionization

Matrix:

small MW

absorb UV

able to crystallize

2.3 Mass analyzer (separate ions to measure m/z and intensity)

Resolution:- ability to differentiate peaks of similar mass

R = mean mass two peaks / separation between peaks

= (m1+m2)/2(m1-m2)- Resolution depends on mass

R=1000, able to separate 1000 & 1001, or 100.0 & 100.1, or 10000& 10010

- High resolution necessary for exact MW determination- Nominal MW =2 8- Actual MW C2H4

+ = 28.0313- CH2N+ = 28.017- N2

+ = 28.0061, R > 2570

2.3.1 magnetic sector analyzers

Fig. 20-13 (p.567) Schematic of a magnetic sector spectrometer.

Kinetic energy of ion:

KE = zeV = 1/2m2

Magnetic force:

FB = Bze

Centripetal force:

Fc = m2/r

Only for ions with

FB = FC can exit the slit

m/z = B2r2e/2V

For fixed radius & charge

- use permanent magnet, and vary A and B potential V

- Fixed V, vary B of electromagnet

2.3.2 quadrupole analyzer

Fig. 11-6 (p.283) A quadrupole mass spectrometer

VRFcos(2ft)

UDC

Ions travel parallel to four rodsOpposite pairs of rods have oppositive VRFcos(2ft) and UDC

Ions try to follow alternating field in helical trajectory

Fig. 11-7 (p.288) operation of a quadrupole in xz plane

VRFcos(2ft) + UDC

- Stable path only for one m/z value for each field frequency

UDC= 1.212mf2r02

VRF= 7.219mf2r02

UDC /VRF = 1.212/7.219 = 0.1679

R=0.126/(0.16784-UDC/VRF)

- Harder to push heavy molecule – m/zmax < 2000

- Rmax ~ 500

Fig. 11-7 (p.288) Change of UDC and VRF during mass scan

Fig. 11-10 (p.290) A TOF mass spectrometer

2.3.3 Time-of-flight (TOF) analyzer

Unlimited mass range m/zmax > 100 kDa

Poor resolution Rmax < 1000

Poor sensitivity

ZeV

mL

v

Lt

v

LmmvKE

ZeVKE

2

)(2

1

2

1 22

2.4 Detectors

2.4.1 Electron Multipliers

Fig. 11-2 (p.284) Electron multiplier

2.4.2 Microchannel Plates (MCP)

Fig. 11-4 (p.286) MCP

Identification of Pure compounds

(a) Nominal M+ peak (one m/z resolution) (or (M+1)+ or (M-1)+) give MW (not EI)

(b) Exact m/z (fractional m/z resolution) can give stoichiometry but not structure (double-focusing instrument)

(c) Fragment peaks give evidence for functional groups

(M-15)+ peak methyl

(M-18)+ OH or water

(M-45)+ COOH

series (M-14)+, (M-28)+, (M-42)+… sequential CH2 loss in alkanes Isotopic peaks can indicate presence of certain atoms

Cl, Br, S, Si Isotopic ratios can suggest plausible molecules from M+, (M+1)+ and (M+2)+

peaks13C/12C = 1.08%, 2H/1H = 0.015%

(M+1) peak for ethane C2H6 should be (2x1.08) + (6x0.015)=2.25% M+ peak

(f) Comparison with library spectra

Fig. 20-4 (p.556) EI mass spectra of methylene chloride and 1-pentanol

What about peaks at greater m/z than M+?

Two sources

-Isotope peaks –same chemical formula but different masses

-12C1H235Cl2 m=84

-13C1H235Cl2 m=85

-12C1H235Cl37Cl m=86

-13C1H235Cl37Cl m=87

-13C1H237Cl2 m=88

- Heights vary with isotope abundance 13C 1.08% 12C, 2H is 0.015% 1H, 37C is 32.5% 35C

CH2Cl2,

13C, 1 x 1.08 = 1.08 37Cl, 2 x 32.5%

2H, 2 x 0.015 = 0.030%

(M+1)+/M+ = 1.21% (M+2)+/M+ =65%

One of the most powerful analytical toolsSensitive (10-6 -10-13g)Range of ion sources for different situationElement comparison for small and large MW –biomoleculesLimited structural informationQualitative and quantitative analysis of mixturesComposition of solid surfacesIsotopic information in compoundsButComplex instrumentationExpensive: high resolutionStructure obtained indirectlyComplex spectra/fragmentation for hard ionization sourcesSimple spectra for soft ionization sources