Mass Spectrometry > Exam #1 > Flashcards

Exam #1 Flashcards

(41 cards)

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1
Q

In mass spectrometry, samples are converted to _______ , which are then separated by their ___________.

A

gaseous ions ; mass-to-charge (m/z)

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2
Q

A mass spectrum is ________________.

A

a plot of m/z (x-axis) vs. relative ion currents (y-axis)

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3
Q

Advantages of Mass Spectrometry

A
  • Provides MW info and structure info
  • High specificity
  • VERY low limits of detection
  • Minute amount of sample required (1 nanogram is all you need)
  • Good for mixture analysis
  • Fast
  • Can be used for a wide variety of possible samples
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4
Q

__________ requires much lower amount of sample than NMR.

A

Mass Spectrometry

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5
Q

Mass Spectrometry is ______ based, while mass spectroscopy is _______ based.

A

particle ; frequency

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6
Q

Applications of Mass Spectrometry

A

-Structural determination of organics, biomolecules, polymers, catalysts, etc.
-Quantitation
-Mixture Analysis
-Elemental analysis
-Fundamental Chemistry
MW determination
-Quality Control
-Environmental Monitoring

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7
Q

Who discovered the mass-to-charge ratio in the late 1890s?

A

J.J. Thomson

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8
Q

Who developed the mass spectrometer and when was it developed?

A

J.J. Thomson, 1913

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9
Q

Positive Ray Apparatus

A
  • Developed by Thomson between 1912-1919.
  • Cathode rays (e- were ejected from a cathode by the impact of heavy ions) were used to ionize gases.
  • “Positive rays” were passed through an electric or magnetic fields and then detected on photographic plates.
  • Parabolic spectra related to m/z were detected.
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10
Q

_________ is the most common type of mass spectrometry.

A

Electron Ionization

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11
Q

Electron Ionization process

A

1) Volatile sample is vaporized
2) The sample is blasted with electrons and excess energy is absorbed (M+.)
3) Uni-molecular decomposition of M+
4) Produces ions
5) a. mass analysis of all ions.
b. data recording
c. plot of bar graph
6) mass spectrum

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12
Q

M+. is a ___________. Also known as “precursor ion.”

A

Molecular ion

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13
Q

The 4 purposes of operating Mass Spectrometers at low pressures.

A

1) Minimize ion/molecule collisions (which could lead to unplanned dissociations or reactions of the ions, or ion path)
2) Prevent electrical arcing at kV potentials needed for some ion focusing
3) Reduce contamination of the ion source and mass analyzers
4) Prevent filament burn-out due to the presence of O2 (equivalent to dissociation)

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14
Q

What is the purpose of the “rough pump” of the mass spectrometers? All mass spectrometers have this.

A
  • Has a motor associated with it. The motor “pushes” the molecules into the hydrocarbon oil.
  • Compresses molecules into a chamber of hot hydrocarbon oil
  • Molecules are trapped in the oil or pushed out the pump exhaust
  • Lowers pressures to about 10^-2 - 10^-3 Torr
  • Gas throughput of a few liters/minute ( 0.1L/second)
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15
Q

How does a diffusion pump work?

A
  • In mass spectrometers, jets of heat polyphenyl ether (oil) are propelled inside the pump (bottom to top) at supersonic speed.
  • Gas molecules of the sample are diffused into the hot oil, and travel to a cooling chamber. The oil condenses and flows back to the boiler.
  • Trapped gases can’t diffuse back to the high vacuum side( against the vapor stream)
  • The vapor of the samples gas (that are trapped) get compressed into a roughing pump.
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16
Q

Downsides of Rough Pump

A
  • The rough pump oil has to be treated as hazardous waste.
  • The gas has to be exhausted outside the lab.
  • Requires hours to heat/cool oil with starting/stopping pumping.
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17
Q

Downsides of Diffusion Pump

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A
  • Pump oil is very expensive
  • The machine requires a constant flow of water
  • Doesn’t do well with power outages (causes oil to become carbon)
18
Q

Upsides of Diffusion Pump

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A
  • Older machines so they’re usually cheaper
  • Very dependable
  • The machine is hard to break
19
Q

Diffusion Pump has typical speeds between _________. It also has pressures of ________ but with pump oil background.

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A

500-800 L/second ; 10^-9 to 10^-10 Torr

20
Q

Turbomolecular Pump

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A
  • Common type
  • A very fast fan with a rotor. The rotor has approximately 20 blades.
  • The blades move at 50,000 or more rpm. The gaseous molecules are being pushed out of the turbopump into a rough pump.
  • Pumping speeds vary between 50-5,000 L/second depending on the pump size
  • Pressures of 10^-10 Torr or less
  • Very low hydrocarbon background
  • Breaking/ Achieving vacuum is fast
21
Q

Downsides of Turbomolecular Pump

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A
  • Expensive to repair
  • Have to be super careful.
  • Random solids can damage the blades.
22
Q

Upsides of Turbomolecular Pump

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A
  • Dependable

- Cheap to operate

23
Q

Cryopump

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A
  • Gases are frozen (condensed) on surfaces cooled to 10-20 K with liquid nitrogen
  • Works well with gases that freeze
  • Gases that don’t freeze are absorbed w/ activated charcoal traps in the condenser.
  • May be used in conjunction with turbopumps.
24
Q

Downside of Cryopumps

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A

-The cryopump has to stay cold. The nitrogen pump can NOT be emptied. If not, it goes all over the “guts” of the machine.

25
Upside of Cryopumps
- Clean, dry, oil-free vacuum. | - Capable of the ultimate vacuum of 10^-11 to 10^-12 Torr.
26
What does volatile mean?
A sample can go into the gas phase and remain stable
27
What does direct introduction mean?
Sample is placed directly into the ion source region of the mass spectrometer. Interference w/ other analytical techniques is not involved.
28
Direct Introduction for Gases and Volatile Liquids?
1) Syringe, leak valve, or pulsed valve introduction of sample into a reservoir. 2) Volatile liquids HAVE TO BE VAPORIZED. This is done by pumping them alone or by heating the inlet system.
29
Direction for Solids and Low-Volatility Liquids?
1) Sample is placed at the end of a probe (direction insertion probe = DIP). The probe may be heated (up to 350 C) to vaporize the sample or a method involving simultaneous desorption and ionization (DI) may be used. For laser desorption experiments, the "probe" is usually a flat plate ("target"). 2) Sample is dissolved in a solvent and flowed into the source where it is ionized with a spray ionization technique
30
Hyphenated-MS Techniques means
Sample goes from a chromatograph, IR, or ICP through an interface and into the MS source.
31
Electrostatic Focusing
- Ions are moved and focused by placing voltages on metal components (plates, tubes, etc.) - Has no effect on neutral particles that go directly to the pump. - Positive voltages repel positive ions and attract negative ions and vice versa. - The higher the voltage, the faster ions are accelerated away from or towards the voltage. - Can repel ions by using the same charge
32
"Steering Beams" of electron ionization means
beams of varying voltages can be applied to move the ions up, down, left, or right
33
3 things electrostatic focusing can do
- extract ions out of the ionization source - Form a focused ion beam that goes into the mass analyzer - Accelerate ions into the mass analyzers
34
1 eV = ? kJ/mol = ? kcal/mol
96.49 kJ/mol ; 23.06 kcal/mol
35
What is meant by electron volt?
the energy acquired by a singly charged particle in moving across a potential difference of 1 volt.
36
Define (first) Ionization Potential
Minimum energy required to move an electron from the highest occupied molecular orbital of a molecule. Generally 5 - 15 eV. IP = IE
37
Electron Ionization (EI)
- Current is passed through a metal wire (usually rhenium or tungsten) - Electrons "boil" off the wire and are accelerated across the source region toward an anode that is usually 70 V more positive. - Therefore the electron energy is 70 eV - The electron beam ionizes gaseous sample molecules - Exclusively for positive ions. e- are spit off and stay off. - Sample has to be in the gas form originally, it won't change for you - Electrons pass near the sample ions, this distorts the electric field of the atom - If the electron energy is greater than the ionization energy (potential) an electron is ejected from the target.
38
If the electron energy is lower than the ionization potential, then
ions won't be made
39
If the electron energy is too high, it will cause what problem with ions?
The number of ions made and the current plateau off because there is so much energy, you can break any bond you want. Not as dependent on EI energy
40
Why would large Electron Energies (such as 70 eV) be used in routine analysis?
- At high energies, the ionization process is saturated (all molecules come into contact with the electron beam and ionize) - Small changes in energy have negligible effects on the spectra.
41
Features of EI
- Established, simple, cheap. Reproducible EE because EI is a physical process not chemical - MOST COMMON IONIZATION TECHNIQUE FOR VOLATILE COMPOUNDS. - Requires initial ionization - Large commercially available data set (more than 150,000 spectra) - WORKS WITH ALL MASS ANALYZERS (SCANNING AND PULSED) - Can do positive and negative ions but negative ion sensitivity is low for most compounds - Good sensitivity - Interpretable fragmentation patterns - Degree of fragmentation is varied based on the EE that is varied. - May produce molecular ions (gives MW info) but sometimes doesn't (esp. when the size of the molecule increases) - Works with high resolution and MS/MS experiments - Physical location of ion production is known (where the filament is, that is where the ion beam is). Makes ion beam focusing easier - Small amount of kinetic energy is imparted to ions (0.5 eV) - Can withstand pressures to about 10^-5. too much higher and you could burn the filament
Mass Spectrometry
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