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 àBased on isotopes of elements àCan 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 àTwo paired §Alternation of potential, ratios maintained àSelects 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 mv 2 KE=1/2 m(L/t) 2 t=sqrt(1/2 mL 2 ) m i = mass of analyte ion z i = charge on analyte ion E = extraction field t i = time-of-flight of ion l s = length of the source l d = 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 àHeaviest ion least perturbed

7-9 Transducers Need to convert ions to electrons §Electron multiplier àSecondary electrons emission after interaction of ion with surface *Cu/Be *Electrons emitted when hit by ion àSimilar in concept to PMT Faraday cup §Exiting ion strikes collecter electrode àElectrode surrounded by cage that collects scattered ions and secondary electrons àResults 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 àIn (113 and 115) with 113 Cd and 115 Sn à 40 Ar and 40 K à 58 Ni and 58 Fe àTi (62-66) with 62 Ni, 63 Cu, 64 Zn, 65 Cu, and 66 Zn *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 (O 2, N 2, Ar 2 ) as ions InterferenceAnalyte 38 Ar 1 H 39 K 40 Ar 40 Ca 35 Cl 16 O 51 V 40 Ar 12 C 52 Cr 23 Na 40 Ar 63 Cu 40 Ar 35 Cl 75 As

7-16 Interference Oxide and hydroxide §Formation of MO + and MOH + species àInterferes with elements 16 and 17 mass units above M