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