Final Exam Study Guide Flashcards
1
Q
List Ionization Techniques
A
1) Electron Ionization
2) Photoionization
3) Multiphotonionization
4) Penning Ionization
5) Surface (Thermal) Ionization
6) Chemical Ionization
7) Negative Chemical Ionization
8) Desorption Chemical Ionization
9) Charge-Exchange Ionization (CE)
10) Atmospheric Pressure Chemical Ionization (APCI)
11) Atmospheric Pressure Photoionization (APPI)
12) Thermospray (TSI)
13) Electrospray Ionization
14) Discharge Sources
15) Desorption Ionization (DI)
16) Californium Plasma Desorption (PD)
17) Secondary Ion MS (SIMS)
18) Fast Atom Bombardment
19) Laser Desorption (LD)
20) Matrix assisted laser desorption (MALDI)
21) Surface-Inhanced Laser Desorption (SELDI)
2
Q
Electron Ionization (EI)
A
- 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.
3
Q
Features of EI
A
- 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
4
Q
Photoionization
A
- Ionization occurs after the sample absorbs light from a monochromatic source
- Similar to EI
- Plus: Photon energy can be accurately and precisely controlled. Energy levels can be obtained.
- Each sample molecule absorbs a single photon
- Wavelengths are on the order of 80-180 nm (neon and helium lines)
- This technique measures ion intensity vs. wavelength
5
Q
Multiphotonionization (MPI)
A
- Similar to photoionization, but instead multiple ions are absorbed by each molecule b/c the energy of a single irridiating photon is not sufficient to cause ionization. Multiple photons are needed to reach ionization energy.
- Light sources are common lasers such as Nd: YAG, excimers, CO2.
- Highly selective; molecules must absorb at the given wavelength before ionization and fragmentation will occur.
6
Q
Penning Ionization
A
- Neutrals in long-lived metastable states transfer energy to sample molecules,forming metastable species which fragment.
- Spectra resembles EI
- Can be used when information on precise energy transfer is important.
- Not widely used because producing a metastable neutral population is tedious
- Primarily used in leak detection and DART (Direct Analysis in Real Time)`
7
Q
Surface (Thermal) Ionization
A
- Sample is coated on a filament.
- Heat and electron beam from filament vaporize the sample and produce ions.
- Useful only for low IP materials, like alkali metal salts.
- Used in ionorganic analysis, geochemistry, isotope ratio measurements for inorganics
- Can tell isotopic contribution to any sample.
8
Q
Chemical Ionization
A
- Primary ionization is by EI on a reagent gas.
- Reagent molecules react with sample molecules (ion/molecule reactions)
- For quads, sector- reagent gas is at “higher pressures” (10^0 - 10^-3 Torr)
- For ICR, QIT- lower pressures and millisecond to second timescales.
- Most common CI reaction is proton transfer
9
Q
Negative Chemical Ionization
A
- With a good match between reagent gas and sample, NCI can be 100-1000 times more sensitive than CI or EI. Very selective.
- A steady-state reagent plasma is achieved at source pressures of 1x10^-2 Torr (Sectors and Quads). Plasmas have equal quantities of positive and negative species.
- By appropriate choice of reagent gas, particular types of negative ions can be generated
- Methane NCI is the most common NCI
- More sensitive than CI
- More sensitive than negative EI, but also more selective so not everything is picked up
10
Q
Features of CI
A
- Requires the samples to be in gas phase ; initial vaporization of the sample is required
- Proton transfer provides MW info
- Usually less fragmentation (“Softer”) than EI; How soft it is is dependent on the ionization reaction
- Amount of energy deposited on the ion can be controlled
- Provides structural info that complements EI
- Spectra are less reproducible than EI (Because pressures are more difficult to control then electron beams.
- No large reference spectra library (Pressure is not the same between lab to lab)
- Dirties the source faster than EI (due to higher pressures)
- Sensitivity (LOD)is as good or better than EI
- Reactions other than proton transfer can be used to increase selectivity
11
Q
Desorption Chemical Ionization (Direct Chemical Ionization)
A
A special name for when the sample is vaporized directly into the source by placing it on a heated direct inserption probe (DIP)
- Combo of DIP and CI
12
Q
Charge-Exchange Ionization
A
- A form of CI
1) Positive ions formed by EI (Positive ions only)
2) Excess internal energy is removed by collisions
3) Reagent ion undergoes an A/M rxn with the sample that involves charge exchange - Allows studies ion internal energy effects on fragmentation
- Much better control over internal energy than EI b/c the energy imparted to the ion is limited by the exothermicity of the CE reaction.
13
Q
Atmospheric Pressure Chemical Ionization
A
- A type of spray ionization
- Ions are produced externally and injected as pulses into the trap by electrostatic focusing
- Actually high pressure CI
- Source is atmospheric pressure
- Samples are introduced via a small orifice between the source and a flowing gas stream
- Corona discharge method: Ionization is caused by an electrical discharge at the tip of a needle (of Ni β-emitter) held at 3-6 kV
- Used in atmospheric monitoring and LC/MS
- Extremely sensitive for some species (ex: low polarity species that are difficult to ionize by other methods
- Fast pumping necessary and source may “dirty” rapidly
- Specifically shows the molecular ion peak
- Sample doesn’t have to be polar
- Requires a nebulizer gas that is heated , N2
- Most common in positive mode
- Has become a common LC-MS interface/ionization in the past decade.
- Source design often couples ESI and APCI sources
14
Q
Important Considerations of APCI
A
- Works best for lower molecular weight (less than 1500 Da)
- Works best for low polarity compounds
- Ionization occurs in the gas phase by CI
- Not as “soft” as ESI (gives more fragments)
- Can work with nonpolar LC solvents
15
Q
Important Considerations of ESI
A
- Works best for higher molecular weight compounds
- Works best for polar compounds
- Requires a polar sample to receive a proton
- Ionization occurs primarily within the solution
- LC solvent is always polar
- Can work with amino acids
- Ions are produced externally and injected as pulses into the trap by electrostatic focusing
16
Q
Atmospheric Pressure Photoionization
A
-Electrospray technique
- Ions are produced externally and injected as pulses into the trap by electrostatic focusing
-Alternative to APCI and ESI but less common than both
-Often used in conjunction with LC
-Liquid effluent (spray) is irridiated with a UV lamp, causing photonionization. Interactions (collisions or ion/molecule reactions) can also occur with excited molecules/ions from the LC mobile phase.
-Dopant molecules can be adding to the flowing liquid stream leading to proton transfer ion/molecule rxns. that form [M+H]+
-Used in analysis of steroids, pharmaceutical compounds, pesticides, and small natural products
UV = ionization source
17
Q
Electrospray Ionization (ESI)
A
- Widely used)
- Most common method for low volatile samples.
- Super good interface w/ LC
- Sample solution can also be flowed at very low rates (less than uL/min.). Sometimes samples is pulled from a glass capillary or other emitter (aka nanospray). B/c of its smaller droplet size, nanospray is less sensitive to salt contaminants than ESI
- Electric field at the needle tip charges the surface of the emerging liquid, dispersing it into a fine spray of charged droplets.
- Droplets are driven by the field toward a capillary. They pass through a flow of bath gas (N2) speeding up the evaporation process
- Droplets decrease in size, yielding quasi-molecular ions.
- Ions in the bath flow reach the glass capillary and pass through two stages of pumping to reach the mass analyzer
18
Q
Considerations of ESI
A
- Requires dry gas and high voltage (drying gas assists in evaporating solvent from the new droplets; w/o drying gas the solvent isn’t pumped away and no ions form)
- Samples are usually dissolved in water, methanol, and (often ) acetic acid
- m/z range is usually between 400-1400 m/z. No more than 2000 m/z
- High resolution ; useful in assigning charge and mass
- FT-ICR and orbitraps are the only mass analyzers that can routinely achieve high resolution under ESI conditions.
- Produces highly charged ions
- Multiple charging (high z) means that even species with very high mass (>1,000,000) result in ions with m/z <2500
- Very soft; gives negligible fragmentation even for fragile species
- Biomolecules may retain their solution-phase 3D conformations and non-covalent interactions (H-bonding can be retained with ESI)
- Negative ESI is possible
- Extensive MS pumping
- An LC/MS interface
- Widely commercially available
19
Q
Generally, the larger the molecule, the greater the number of ____________ that it can sustain.
A
charges
20
Q
Bigger the polymer means, _________ electric charges can be accepted. More places to add ________.
A
more; protons
21
Q
ESI can not be used with ___________ because it gives a mess of peaks.
A
organic polymers
22
Q
Field Desorption Ionization (FD)
A
- Popular method for observing low volatility species until FAB came out.
- Solid sample is applied (coated) to a fine wire (usually graphite) whose surface contains an array of sharp-pointed needles or “whiskers.” A high voltage is applied to the emitters
- Only positive, singly-charged ions form
- Not technically “soft” (low-energy methods), but only ions produced furthest from the high voltage make it to the mass analyzer. These ions have also generally undergone the least fragmentation.
- Used in pharmaceuticals
23
Q
Considerations of Field Desorption Ionization
A
- Spectra aren’t very reproducible and are dependent on emitter shape
- Sample prep is time consuming and difficult.
- Finding and maintaining the correct temp and voltage combination is tedious and requires skill
- High ion KE requires a double focusing magnetic sector mass analyzer or quadrupole
- Highly non-volatile samples can be analyzed
- Relatively uncommon today but still used in the pharmaceutical industry
24
Q
Discharge Sources
A
- Sample is vaporized/ionized in a high voltage discharge (plasma) of 1-60 kV
- Includes Inductively coupled plasmas (ICP), AC (spark source) and DC (Townsend, Corona, Hollow Cathode, Glow, Plasma Discharges) versions
- Hard ionization technique; the sample breaks into atomic ions
- Organics fragment extensively
- Liquid and gaseous samples may be directly injected into the discharge. Solids are often mixed into a graphite electrode.
- Kinetic energy distribution of ions is large. MASS ANALYSIS IS USUALLY EITHER BY QUADRUPOLE OR DOUBLE-FOCUSING SECTORS.
25
Q
Uses of Discharge Sources
A
- Elemental isotopic analysis
- Trace metal analysis
- Semiconductor, insulator, thin film analysis
26
Q
Desorption Ionization (DI)
A
- Sample is in vacuum
- Does not atomize sample
- Sample is placed on a probe tip, inserted into the MS source and bombarded with particles or photons
- Soft technique (sample is not atomized)
27
Q
Considerations of Desorption Ionization
A
- Useful for low (or no)-volatility and/or thermally unstable compounds
- Typical samples: biomolecules, drugs, polymers, metal complexes, catalysts, inorganics
- Different DI techniques yield different spectra
- “Soft” ionization producing quasi-molecular ions with little fragmentation
- Produces positive ad negative ions
- Ions production is often matrix dependent, making quantitation difficult
- MS/MS may be used to obtain structural info
28
Q
Secondary Ion Mass Sprectrometry
A
- Common surface analysis technique
- Bombarding Ion Species (Ar+, Xe+, or Cs+)
- Bigger samples (proteins, peptides) can’t be done as well
- Large kinetic energy spread
- Dynamic SIMS: used in surface analysis. Tool for identification and depth profiling of surface components. The ion beam may be scanned across the surface to map the spatial distribution of components. Involves ultra-low pressures to avoid O2 desorption on sample surfaces.
- Organic SIMS: organic samples are deposited on a surface (sometimes monolayers) and subjected to SIMS. Ion formation is generally low. Bombardment may “eat up” sample quickly. To increase ion formation, samples may be deposited on Ag foil (which readily desorb Ag+ adduct ions) or mixed with “preionizing agents such as HCl or NH4Cl
29
Q
Features of SIMS
A
- All DI features
- Practical sample size down to nanomoles
- Mass range usually below 5000 Da, but >20,000 Da is possible
- WORKS WITH ALL MASS ANALYZERS
- Ion guns can be pulsed or continuous but pulsed is usually better because it increases the sample lifetime. entire SPECTRUM OBTAINED FROM MILLISECOND BURST OF THE ION GUN.
- Requires little sample prep, but SPECTRUM IS MATRIX DEPENDENT
- Experiencing a resurgence in its use because of imaging (TOF-SIMS)
30
Q
Fast Atom Bombardment
A
- Dramatically changed MS, leading to a routine analysis of low volatility species
- Sample is dissolved in a low volatility matrix and bombarded with an atom or ion beam (1 - 10 KeV) Sample and matrix (want a matrix that won’t be pumped away) ions are desorbed and ionized. Solution contains 10-20% of sample
- Common bombarding species: Ar, Xe, Ar+ , Xe+ or Cs+
- Common liquid matrices (low volatility): glycerol, thioglycerol, m-nitrobenzyl alcohol
- Bombarding a drop of sample on the probe
- Different than SIMS because it uses atoms instead of ions and a liquid matrix instead of a solid matrix
- Gives a higher yield of organic ions than SIMS
- Been replaced by electrospray and MALDI
31
Q
Considerations of FAB
A
- All DI features
- Not commercially sold anymore
- Instrumentally simple and cheap
- Was used in the analysis of biomolecules, drugs, environmental samples, peptide sequencing
- Gives more fragmentation than other DI techniques
- Works best for MW < 5,000 Da, however species up to 24,000 Da have been detected.
- Works best on pre-ionized species and on polar molecules which will readily protonate ; poor for non-polar, but works good on molecules that want to polarize
- High sensitivity for some species.
- Spectra are very dependent on sample prep
- Does the best job of analyzing organometallics
32
Q
Laser Desorption
A
- Bombarding species: Laser light
- Except for MALDI, ion formation is generally independent of wavelength but dependent on laser power (which is typically 10^7 to 10^10 Watts/cm^2 , a high power laser)
- Laser is located outside of the MS and is allowed into the system through windows (sapphire or glass) or a fiber optic
- Common LD lasers are pulsed ( Nd:YAG, nitrogen, CO2 or excimers)
- With a pulsed laser, ionization is only for 5-50 ns/spectrum, but a pulsed mass analyzer MUST BE USED. (ToF, ICR, QIT, Orbitrap)
33
Q
Considerations of Laser Desorption
A
- Uses: biomolecules, polymers, inorganic species, and surface studies. Samples can have little to no volatility
- More universal than FAB
- Gives more ions than SIMS
- However, lasers are more expensive than ion/atom guns.
- Frequently used with inorganic samples.
- Abundant positive and negative ions are common
34
Q
Matrix Assisted Laser Desorption Ionization (MALDI)
A
- “Soft” laser desorption
- Laser is used for ionization
- Sample is mixed with a solid matrix and placed on a target (probe)
- Matrix must have low volatility and form solid crystals
35
Q
If sample is too volatile for MALDI, use
A
EI
36
Q
If sample is too volatile for EI, use
A
CI
37
Q
Features of MALDI
A
- Desorption of very large analyte ions (Greater than 1,000,000 m/z has been observed)
- MALDI is very dependent on sample prep (layering, mixing, etc.)
- Additives to the matrix can increase ion formation
- Preferred method for ORGANIC POLYMERS.
- Low fragmentation
- Bigger molecules can give too many isotopic distribution. Causes broader peaks.
- All DI features
- Widely commercially available
- Mass range : >1,000,000
- Common for both organic and inorganic samples
- Amount of sample used = very small
- May not work as well as FAB for some organics less than 5000 Da, but it will ionize heavier and less volatile species that don’t give FAB spectra
- Can be used for surface analysis and imaging
- Easily automated
- No background gas pressure and few matrix ion problems
- RESULTS VERY DEPENDENT ON SAMPLE PREP
- A large number of experimental variables can be controlled, which gives more control over ionization parameters
- MORE TOLERANT OF IMPURITIES THAN ESI OR FAB
- CAN’T WORK WITH A LOT OF MASS ANALYZERS
- MOSTLY WORKS WITH TOF AND FT-ICR
38
Q
Surface-Enhanced Laser Desorption Ionization (SELDI)
A
- A variation of MALDI
- Modified surfaces on the target plate retain specific classes of proteins
- Surfaces may be modified chemically or biologically
- Used in proteomics discovery studies and biomarker research
- NOT GOOD FOR QUANTITATIVE RESEARCH DUE TO LIMITED REPRODUCIBILITY
- Some proteins bind to the plate and others are washed away by a solvent. matrix is then added on top of the bound proteins
39
Q
Direct Analysis in Real Time (DART)
A
- Atmospheric pressure ion source that ionizes the sample at ground potential in open air
- LOW MW WEIGHT MOLECULES (LESS THAN 1000 Da)
- Low voltage at sample inlet (1-5 V); no risk of energy if sample orifice is accidentally touched
- DI where sample is placed under ambient conditions into the path of bombarding particles produced by Penning Ionization
- Has been used to analyze solids, liquids, and gases. Sample requires some volatility
- Requires no sample prep
- Ionization can take place directly on the sample surface
- A form of Penning Ionization
- Sampling Inlet is held at about 80 degrees Celsius to prevent condensation
- NOT AS WIDELY AVAILABLE BUT HAS BEEN SOLD WITH MS
40
Q
Desorption Electrospray Ionization
A
- Atmospheric pressure ion source that ionizes the sample under ambient conditions in open air.
- Similar to DART but bombarding species is the spray from ES
- VOLATILE SPECIES WORK ONLY
- Has been used to analyze solids, liquids, and gases
- Ionization can take place directly on the sample surface
- Combination of ESI and DI
- High molecular weight samples give ESI-like spectra that contains multiply charged ions
- VERY LITTLE FRAGMENTATION
- NEGATIVE MODE IS POSSIBLE
- ADJUSTABLE BASED ON ANGLE, MORE ADJUSTABLE THAN DART.
- NO SAMPLE PREP
41
Q
The ___________ is always the last stage of analysis because it destroys the sample.
A
Mass spectrometer
42
Q
Gas Chromatography- Mass Spectrometry
A
- mature, routine.
- Gaseous output of GC column (silicon oxide based) is passed through the interface and into the ion source.
- Requires volatile compounds that are thermally stable. If the sample decomposes upon heating, use LC.
- Ionization by EI or CI&EI
- No ESI or MALDI because the samples are not volatile enough
- All mass analyzers may be used, but Q and QIT are the most common. TOF can also be used
- Mass spectra is taken continuously but the rates are dependent on the mass analyzer. (1-10 spectra/second for Q and sectors)
- GC-TOF is an excellent choice for fast chromatography (40,000 spectra/sec)
- 1 GC run may yield hundreds of spectra
43
Q
What is Total Ion Chromatogram (TIC)
A
- Equivalent to FID’s output.
- Gives relative abundance of each ion
- Can look for fragment ion and precursor ion
- MS equivalent of a GC detector output
- The ion current from only one ion is displayed
44
Q
Pumping Needed for GC-MS
A
1) Capillary Column
2) Packed Column
