1. 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

2. 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 = 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

3. 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).

4. 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., % 79Br with a mass = Da
49.31% 81Br with a mass = Da
Gives: (1) Da for Br
Bromine
35 Br (1)

5. Exact mass ≡ mass of a particular isotope of an element
e.g., 13C with an exact mass = 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

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

7. 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)

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

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

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

11. Faraday Cup Detector
Ion beam

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

13. 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

14. 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

15. 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

16. Voltage Relationships During a Mass Scan
Fig. 11-9

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

18. 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

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

20. Schematic of ICP-MS
Fig 11-10