Exam #3 Flashcards
Orbitrap
- First new type of mass analyzer
- Similar to QIT and FT-ICR
- Based on orbital trapping
How does an orbitrap work?
- An outer barrel-like electrode and a coaxial inner spindle-like electrode form an electrostatic field.
- Voltage on the spindle is about 3500V and its polarity is opposite to the ions’ polarity
- Diameter of spindle is 8 - 12 mm
- Diameter of cavity is 20 - 30 mm
- Ions are injected into the electrostatic field with an accelerating voltage between 1 - 2 kV; this frees the ions from external source to the mass analyzer
- Ions are trapped due to their electrostatic forces to the inner electrode, which has a voltage of about 3.5 kV DC. They are balanced by centrifugal force.
- Outer electrode is near ground (a few volts).
- Ions cycle around the central electrode in rings and also move back and forth along the axis of the central electrode.
What is unique about orbitraps?
- No magnetic field (unlike FT-ICR)
- No rf voltage or generators (unlike QIT)
The movement of the ions around the central electrode of an orbitrap are _______ and __________ movement.
- coherent
- reproducible
To minimize ________ and ________, the pressure inside an orbitrap is extremely low.
- collisions (which lower coherent motion)
- electrical arcing
The pressure in the orbitrap is ___________. Making it the ___________.
about 5 x 10^-10 Torr ; lowest pressure of mass analyzers
In relation to “k” of orbitraps, what is it and what term does it relate to?
curvature of the electrical field, does not change. Related to the geometry of the orbitrap
What is ω in orbitrap terminology? Independent of _________. Inversely proportional to the _______.
frequency of the harmonic oscillations ; ion velocity, v ; square root of m/z
ω = ?
ω = [z/(mk)]^1/2
m/z is proportional to ___________.
1/ (frequency)^2
What happens if ions are out of coherent motion in an orbitrap?
- ions do not have the same m/z value
- ions would cancel each other out and an image will not be made
Longer free inductance decay (FID) means___________.
better mass accuracy, better resolution, and more intense signal.
Image current of an orbitrap is _________. This is a function of ________.
free inductance decay (FID) ; time
With orbitrap, the image current (in time) is converted to frequency via ___________. _______ is calculated from this.
Fourier Transform ; m/z
Orbitrap is capable of __________ and ____________. The dissociation event occurs _______ and the orbitrap detects the ______.
ion ejection and MS/MS experiments ; outside the orbitrap; products
Orbitrap most often acts as a _________.
mass detector
Orbitrap is usually sold with _____________ sources, but it is also available with _____________.
ESI/APCI ; MALDI
What is the benefit of using an orthogonal system such as ESI/LIT/C-TRP/Orbitrap?
- Keeps neutrals from entering the orbitrap
- minimize pressure
__________ is the most common form of mass spectrometry.
Collision dissociation
Give the values for Orbitrap:
- Mass Accuracy:
- Resolution:
- Dynamic Range:
- Upper m/z range of:
- Mass Accuracy: 1-2 ppm
- Resolution: up to 200,000
- Dynamic Range: about 5000-10,000 (relatively low)
- Upper m/z range of: about 6000 m/z
_________ has a worse resolution in comparison to FT-ICR, but better the QIT.
Orbitrap
Advantages of Orbitrap
- Small
- Cheap ; cheap to buy and low operating costs
- High resolution ( still lower than FT-ICR but better than the rest)
- Sensitive
- No danger of a strong magnet
- Works well with ESI
MS/MS requires ________ to be controlled.
pressure
MS spectrometers operate at low pressures to: 1) 2) 3) 4)
1) Minimize ion/molecule collisions (would cause unplanned dissociations or reactions of ions or change ion path)
2) Prevent electrical arcing at kV potentials needed for some ion focusing/ ion acceleration
3) Reduce contamination of the ion source and mass analyzers . Reduces chemical noise
4) Prevent filament burnout due to the presence of O2.
Pressure is also important for _______ dissociations (collision-induced dissociations) and reactions (bimolecular reactions).
Planned
For bimolecular dissociations, the pressure of the neutral depends on its _________.
concentration
_______ different gauges to measure pressure. No one gauge will fit all.
Many
Thermocouples (TC) is also known as _________. This is a specific type of TC gauge that uses a _________.
- Pirani gauge
- Pt filament
Thermocouples:
- Covers a pressure range of :
- Used with ________
- Junction between two dissimilar metals causes _________ when the metals are at different temperatures.
- ____________ is fed to a filament.
- Temperature of filament depends on _________.
- Pressure changes lead to changes in the filament’s _____ and the _________.
-Rough pumps
- 10^-3 to 10 Torr
- thermal conductivity difference between the metals, which leads to current.
-Constant current
-thermal losses to the surrounding gases
temperature ; thermocouple voltage
Voltage of a thermocouple is related to
pressure
Penning Gauge
- A type of __________
- Intermediate pressure range of about ______.
- A _______ voltage is applied to a central anode.
- A ______ causes ______ to spiral around the anode until they hit a _______.
- The _________ of the resulting current relates to pressure. A __________________ enables more electrons to reach the cathode, resulting in ________..
- cold cathode gauge
- 10^-3 to 10^-5 torr
- DC
- Magnetic field ; electrons ; cathode
- Magnitude ; lower pressure ; a higher current
What is a benefit of the Penning gauge?
- Very low maintenance
- Parts don’t wear out
Bayard-Alpert (B/A) Ion Gauge is the ____________.
most common ultra high vacuum gauge for surface analysis
B/A Ion Gauge
- Electrons are emitted by a ______________ and travel to a _____, ionizing residual gas molecules along the way.
- The ion current produced reaches the _________ and is measured.
- Current depends on the _______ and on the _________, which must be kept constant.
- hot filament ; grid
- ion collector (anode)
- pressure ; filament emission current
Benefits of B/A Gauge
- Reliable and stable
- linear response over a wide pressure range (10^-5 to 10^-10 Torr)
With pressure gauges, different molecules give ___________ responses. Smaller molecules = _________ ; Larger molecules = __________
- different
- lower readings than true pressure (fewer ions being made)
- higher readings than true pressure (more ions being made)
Most gauges are calibrated with _______ to obtain to pressure. True pressure equation is __________.
nitrogen ; P(true) = P(measured) * Correction Factor (for your gauge) / Rx
Note : Rx is the relative gauge sensitivity for the molecule of interest
What is a nude ion gauge and why might it be beneficial?
- The ion gauge is not enclosed in glass
- Makes it more sensitive and accurate. Also allows the free flow of gas into the gauge head
With B/A gauge heads, the filament is made of and what does it do?
Tungsten ; current is applied to the filament. This causes EI to be conducted on the gas molecules in the vicinity of the gauge head.
What is the purpose of the mesh of the B/A gauge head? What important note about magnets applies to this gaugehead?
creates an electric field for electrons to be trapped; doesn’t work well with magnets.
Where is the pressure read on an ion gauge?
Gauge head
What is Collision Theory? k=?
of collisions = (time in seconds)(pressure in Torr)(3.23 x 10^16 molecules cm^-3 Torr^-1)(k)
k= collision rate constant (cm^3 molecule^-1 sec^-1)
What is polarizability?
Alpha symbol ; a coefficient related to the ability of a species to acquire a dipole moment in an electric field
Units: cm^3 or A
Polarizability is a good approximation of a molecule’s ________.
volume
What is “dipole moment”?
µD with unit: Debye (D); Bonds between two atoms of different electronegativities leave a negative charge (-Q) on the more electronegative atom and a positive charge (+Q) on the more electropositive atom. Distance, r, separates the charges.
Causes ion in the molecule to be more highly attracted
µ0 = Qr
What is MS/MS (tandem mass spect)?
Ion generated in ion source, separated in the 1st mass analyzer, and do something with it in other mass analyzers.
Single MS is used to _________________.
obtain a spectra of a neutral molecule
Main areas (3) of application for Tandem mass spect
1) Qualitative and quantitative analysis (i.e. determining sequences of peptides) - about 90-95%
2) Ion chemistry (structures, reaction mechanisms, etc.)
3) Fundamentals (affinities, breakdown curves, etc.)
Keys (3) to MS/MS
1) Two stages (in time or space) of mass analysis
2) Ion is surrogate analyte (means assume precursor ion or molecular ion)
3) Reaction ( e.g. dissociation) is separate from ionization
For mixtures, MS/MS can take the place of chromatography and be used for ____________.
improved specificity and detection limits, structural information
For pure compounds, MS/MS is used for:
Optimization to achieve the most characteristic spectrum, structural information
MS/MS is very _________. Meaning, a _______ ion or ions is chosen for MS/MS analysis. The _________ never sees the other ions b/c they are already kicked out. This greatly _________ the S/N in complex samples, especially for ________.
specific ; specific ion.
Detector ; increases ; biological molecules
MIKES is the same thing as
MS/MS
Mass spectrometers for “in space” include…… For adding additional MS stage would require ______ and ________.
Sectors, quads, hybrids ; additional pumps and electronics
Mass spectrometers for “in time” include….. All mass analysis dissociation steps occur in ________. For adding additional MS stages would require ______.
- FT-ICR, QIT
- one region of space
- software modifications
Hybrid MS/MS techniques ________.
mix time MS with space MS
For MS, what improves specificity? It also improves what?
- MS/MS stages
- high resolution
- It improves lower detection limits by preferentially removing chemical noise (better S/N , poorer S)
If the noise of a spectra is electronic (which means its constant), MS/MS or high resolution __________.
-Is not advisable ; cutting signal without reducing noise only reduces S/N. The signal would decrease.
What are the 4 types of MS/MS scans?
1) Product-Ion Scan
2) Precursor-Ion Scan
3) Neutral-loss Scan
4) Selected Reaction Monitoring Scan (SRM ; aka “Multiple Reaction Monitoring” or MRM)
Product-Ion Scan:
- ________ and most ______ MS/MS scan. Performed with ____________.
- Only type possible for MS/MS in ________ or _______.
- Simplest , most common ; all mass analyzer
- time (FT-ICR, orbitrap, all types of QIT) or pulsed analyzers (TOF and all traps
Any mass analyzer that can do ________ can do all 4 major MS/MS.
SIM
How does the Product-Ion Scan work?
1) mass analyzer number one is set to pass only the precursor ions of a fixed m/z
2) ions dissociate in a reaction chamber
3) the second mass analyzer obtains (“scans”) the complete spectrum
4) Results in all product ions of a precursor ion can be determined.
Uses for Product-Ion Scan (2)
1) Good for probing structures. Gives molecular formula, not the structure.
2) Frequently used with CI and all DI and spray ionization techniques (ionization gives MW but few fragments)
Product-Ion Scan is not typical with _____________ because ________.
EI, because EI has abundant fragments
What is the precursor ion?
The ion you select for dissociation
What MS/MS sequence makes up about 50% of the mass spect. market?
QQQ
For soft ionization techniques, Product-Ion MS/MS gives____________. The _______ does not matter.
structural information for quasi-molecular ions obtained from spray ionization, CI, and DI techniques. Ion source
Product-Ion MS/MS with ESI is used for __________.
-giving a mass spectrum with ions from a mixture of peptides. Gives the product-ion spectrum with excellent S/N and diagnostic fragment ions.
Product-Ion MS/MS can also be used to distinguish between
isomeric ions
Precursor-Ion Scan is the _______ type of MS scan.
second major
Precursor-Ion Scan
- First mass analyzer passes __________.
- Second mass analyzer is fixed to pass the m/z of __________.
- Correlation between the time ______________ and the m/z of the ________ must be passing through the mass analyzer at the same time needed to make this product.
- Results in __________.
- Used for ________.
- all ions
- the product ion of interest
- that the specified product ion is observed ; precursor ion
- All precursor ions of a specific product ion can be determined.
- mIXTURE CHARACTERIZATION
Precursor-Ion Scan only works when the mass analyzers are ________, which means only for _____________________.
scanning ; linear Q and sectors (B or E)
Precursor-Ion Scan MS/MS requires knowledge of _______ in order to choose an ion (m2+) that is characteristic of a class of compounds.
fragmentation patterns
Neutral-loss Scan
- Can only happen with _____ and _____ because it requires a _______ mass analyzer.
- _________ and ________ are both scanned.
- “_________” between the scans is set so that the neutral loss (m3) is fixed.
- Used mostly for _______.
- sectors and quadrupoles ; scanning
- Analyzer 1 and analyzer 2
- “Stagger”
- Mixture analysis
A good way to determine specific types of compounds in mixtures.
Precursor-Ion Scan and Neutral-Loss scan
Selected Reaction Monitoring (SRM) Scan
- ________ mass analyzers are fixed. They must be _______.
- A ________ is monitored.
- SRM MS/MS gains _______ and increased _____ by reducing chemical noise over MS alone.
- Most commonly used for ________.
- Most often used with _________, where specific analytes are monitored during their retention time window,
- Both ; Q or sector
- specific
- specificity, S/N
- Quantitation, esp. for EPA
- LC-MS and GC-MS
Retention time is ________ in SRM.
an additional conformation of the identity of a compound being quantified.
As the number of mass analyzers increase, the number of __________ increases.
(MS)^n
Precursor-Ion Resolution relates to the ______ with which the final mass analysis can be performed. Aka ________.
accuracy ; Back End Rsolution
Back End Resolution
Depends on last mass analyzer
MS/MS in Space
- 2 or more mass analyzers with reaction regions between analyzers
- Uses depend on the properties of the mass analyzers
Reaction region is
field free regions; do not have electric or magnetic fields.
MS/MS in Space analyzers include:
1)
2)
3)
1) QQQ
2) Sector MS/MS Configurations
3) Hybrids
QQQ
- Major non-hybrid MS/MS in space instrument today
- 1st and 3rd Q are operated in normal manner w/ combo of rf and dc voltages
- Collision quad (center quad) is operated in rf-only mode. Used as reaction region
- Rf frequency and voltage are set in a manner that passes all ions (serves as ion beam guide.
- Collisional dissociation occurs only at low energies.
- Quads do not use high accelerating voltages
Benefits of QQQ
-Rapid scanning
-Small size
-Simple Operation
=Easily couple to LC
-(relatively) low cost
-Can perform all types of MS/MS scans (advantage over QIT, Q-TOF, FT-ICR, orbitrap, etc.)
What was the major motivation for the formation of QQQ?
- Desire to incr. product ion (back end) resolution compared to the BE
- With sector MS/MS, there was such a wide velocity spread and peaks were broad, low resolution. Not used today
Downside of QQQs
- Less flexible in terms of reactions that can be performed
- low precursor ion transmission
Sector MS/MS
- A variety of different combinations
- Higher front end resolution than with QQQ
- When coupled with EB as the last analyzer, high back end resolution is possible
- Combinations of E, B, or V can be scanned (Linked scans)
- W/ collisional dissociation, the high V results in high energy collisions.
Hybrids
- Instruments with a combo of two diff. types of mass analyzers
- Often a rxn region allowing high energy collisions (sector) and low energy collisions (quads) are both present, giving maximum flexibility in dissociation energy range.
For sector/quad hybrids ________ are placed first, allowing higher _______________. Sectors are also placed first because the ion beam of the Q is ___________.
sectors; resolution precursor ion selection; unfocused
Sectors’ can’t take _________ionization techniques.
Spray
Advantages of having TOF as final analyzer over Q
- All m/z of product ions can be analyzed within a few hundred microseconds
- Transmission efficiency is much higher than with Q ( Limits of detection is about 100 times better than QQQ)
- Not as discrimitive as Q. Every ion at the start of the TOF makes it to the end, no matter the m/z value
- If reflectron-TOF is used, QTOF has a higher resolving power (50,000) than a QQQ (1000)
Disadvantage of using TOF as final analyzer over Q
- Highly priced considering their capabilities
- Any system with TOF, CAN ONLY PERFOM PRODUCT ION SCANS. Can’t do neutral loss scans.
What does orthogonal accelerator mean with Q-TOFs?
- Most common setup for QTOFs
- The ion gate’s pulsed voltages are only set for ions to make the 90 degree turn at the start time for the TOF analysis.
- Narrower start time and incr. resolution. ( All ions started at the same time = enter the TOF to give you better resolution.)
- Keeps neutrals from the ESI source from entering the TOF and raising its pressure
Features of Q-TOF
- Medium front end resolution at Q1 (meaning you could select precursor ion within 1 m/z unit)
- Moderately high back end resolution at TOF (about 50,000)
- High transmission of MS/MS product ions through TOF
- Ultrahigh sensitivity.
- In normal operation, Q-TOFs can only perfom product-ion MS/MS scans
