7-1
Atomic Mass Spectroscopy• General Features
• Mass Spectrometers Sources
• Atomic mass spectroscopy determines elements by mass Mass to charge ratio Lower detection limits than optical methods Simple spectra
Based on isotopes of elements Can measure isotopic ratios
• Expensive equipment, drift, and interference effects
7-2
Generalities
• Conversion of element to atom Destruction of molecular information
• Further conversion of atom to ion Need charge for mass to charge ratio
• Separation of ions in magnetic field Based on mass to charge ratio
• Determine number of ions Count ions or current
• Detection of isotopes Based on mass to charge, requires constant charge
7-3
Types of MS
• ICP-MS• DCP-MS• MIP-MS• SSMS (spark source)• TIMS (thermal ionization)• GD-MS (glow discharge)• LM-MS (laser microprobe)• SIMS (secondary ionization)
7-4
Spectrometers
7-5
Mass Analyzer
• Quadruple Four electrodes
Two paired Alternation of
potential, ratios maintained Selects single
mass/charge
* Acts as mass filter
Change m/z by potential variation
7-6
Mass Analyzer
• Time of flight Periodic ionization of sample Ions enter drift tube with sample kinetic
energy Ion velocity inversely proportional to mass
7-7
Time of flight
• KE=1/2 mv2
• KE=1/2 m(L/t)2
• t=sqrt(1/2 mL2)
mi = mass of analyte ion
zi = charge on analyte ion
E = extraction field ti = time-of-flight of ion
ls = length of the source
ld = length of the field-free drift region
e = electronic charge (1.6022E-19 C)
7-8
Double Focusing• Focus ions with 2
methods Electrostatic Magnetic
• Electrostatic section Focus ions
within a narrow KE range
• Magnetic Dispersion of
ions Heaviest
ion least perturbed
7-9
Transducers• Need to convert ions to electrons
Electron multiplier Secondary electrons emission
after interaction of ion with surface* Cu/Be* Electrons emitted when hit by
ion Similar in concept to PMT
• Faraday cup Exiting ion strikes collecter electrode
Electrode surrounded by cage that collects scattered ions and secondary electrons
Results are independent of ion energy
• Photographic plates• Scintillators
7-10
ICP-MS
7-11
Calibration
7-12
Spectrum
7-13
ICP-MS
7-14
Interference
• Spectroscopic interference Same m/z for ions in plasma
• Isobaric interference Elements with isotopes of similar mass
In (113 and 115) with 113Cd and 115Sn 40Ar and 40K 58Ni and 58Fe Ti (62-66) with 62Ni, 63Cu, 64Zn, 65Cu, and 66Zn
* Can measure different isotopes of the element
* Ratios between isotopes are known and can therefore be corrected
7-15
Interference
• Polyatomic interference Formed in plasma due to matrix or
atmosphere Mainly for m/z>82 Can include dimers (O2, N2, Ar2) as ions
Interference Analyte
38Ar1H 39K
40Ar 40Ca
35Cl16O 51V
40Ar12C 52Cr
23Na40Ar 63Cu
40Ar35Cl 75As
7-16
Interference
• Oxide and hydroxide Formation of MO+ and MOH+ species
Interferes with elements 16 and 17 mass units above M
