Atomic Mass Spectrometry Chap 11 Advantages: very low detection limits (0.02 – 0.1 ppb) spectra that are relatively simple and easy to interpret ability to measure isotopic ratios

General Features: Steps in the analysis Atomization Ionization of analyte atoms Separation of ions by mass-to-charge ratio (m/z) e.g., For Pb+, m/z = 208. For Pb2+, m/z = 104 Counting or measuring ions of each m/z Units: amu or Daltons to 3 or 4 decimal places (5) Data plotted as relative intensity vs. m/z

Mass Spectrum Showing Natural Isotopes of Pb Observed as an Impurity in Brass Variability in isotopic abundances in Pb from natural sources creates a large uncertainty in the atomic mass (207.2 ± 0.1).

Bromine 35 Br 79.904(1) Several definitions of “mass” Atomic mass ≡ weighted avg. of masses of isotopes of an element Units: 1 amu = 1 Dalton = 1 u = 1/12 mass of 12C e.g., 56.69% 79Br with a mass = 78.91834 Da 49.31% 81Br with a mass = 80.91629 Da Gives: 79.904(1) Da for Br Bromine 35 Br 79.904(1)

Exact mass ≡ mass of a particular isotope of an element e.g., 13C with an exact mass = 13.00335 Da Nominal mass ≡ integer mass of the species e.g., 13C with a nominal mass = 13 Da Mass-to-charge ratio (m/z) ≡ “mass” for all singly charged ions

Components of a Mass Spectrometer Fig. 11-1 P = 10-5 to 10-8 torr

Types of Atomic Mass Spectrometers (Distinguished by type of mass analyzer) Quadrupole MS Time-of-flight (TOF) MS Double-focusing MS Ion trap MS (covered in Chap 20)

Discrete dynode Fig. 11-2 (a) Transducers for MS Electron Multipliers Discrete dynode Fig. 11-2 (a) dynodes Typical gain ~ 107

Continuous Dynode Fig. 11-2 (b) Transducers for MS Electron Multipliers Continuous Dynode Fig. 11-2 (b) 1.8 – 2.0 kV Typical gain ~ 105 – 108

Faraday Cup Detector Fig. 11-3 The accelerated particles are stopped inside cup The accumulated electric charge is detected as a corresponding electric current.

Faraday Cup Detector Ion beam

Types of Atomic Mass Spectrometers Quadrupole MS Time-of-flight (TOF) MS Double-focusing MS Ion trap MS (covered in Chap 20)

Most common type of mass analyzer for atomic MS Quadrupole MS Fig. 11-6 Most common type of mass analyzer for atomic MS Rugged, compact, high scan rates ~ 100 ms

The “Mass Filter” Fig. 11-7 Constant (DC) voltage and alternating (AC) voltage applied to rods At a particular DC voltage: Large ions tend to drift into wall Small ions spiral, travel farther, then annihilate on wall “Right size” ions spiral, are pulled back by AC, and complete the path

Quadrupole Mass Analyzer Fig. 11-8 Different ions have different stability regions. As DC is ramped, ions are selected. high pass mass filter low pass mass filter both mass filters in operation

Voltage Relationships During a Mass Scan Fig. 11-9

Types of Atomic Mass Spectrometers Quadrupole MS Time-of-flight (TOF) MS Double-focusing MS Ion trap MS (covered in Chap 20)

Schematic of TOF Mass Spectrometer Fig 11-10 Ions formed by bombardment E-field pulse accelerates ions: KE = ½ mv2 Ions then “drift” down tube (~1 ms) according to mass

Time-of-Flight Mass Analyzer KE = 1/2mv2 Heavier ions in later Light ions in first

Schematic of ICP-MS Fig 11-10