Exam #2 Flashcards
(137 cards)
1
Q
Mass Analyzers can use static/dynamic ________ or _______ fields to separate gas phase ions based on their ___________.
A
Electric ; magnetic ; m/z values
2
Q
Can mass analyzers be used alone or with other mass analyzers?
A
Both
3
Q
Ions are accelerated out of the source to the mass analyzer by ______________.
A
electrostatic focusing
4
Q
Why is it good that ions from the ion source usually have a negligible KE value?
A
Good because some mass analyzers measure the velocity and use it in their calculations. You don’t want to add to that.
5
Q
KE is
A
kinetic energy of fully accelerated ion.
6
Q
V is
A
Potential difference you’re accelerating ions with. You know what this value is and you can control it.
7
Q
With mass analyzers, ions move _______ to the _______.
A
from the ion source ; mass analyzer
8
Q
In mass analyzers, how are the ions moved through the vacuum chamber?
A
Via electrostatic focusing
9
Q
What are the two definitions of resolution?
A
1) The separation of two ions of nearly identical m/z (chromatography)
2) Sharpness of a mass spectral peak (of just one single peak)
10
Q
When talking about one peak’s sharpness (resolution), what is it talking about?
A
Full Width Half Maximum Method
11
Q
What is the Full Width Half Maximum Method?
A
Resolution = m/Δm ; the m/z at the top of the peak divided by the width of the peak at half peak
12
Q
What are wider peaks called?
A
Gaussian
13
Q
What are sharper peaks called?
A
Lorentzian peaks
14
Q
As a good rule of thumb, what m/z value is considered high resolution?
A
100,000 or more
15
Q
High resolution mass spectrometers
A
- Double Focusing Sectors
- Orbitraps
- FT-ICR (100,000,000 resolution)
- Some TOFs (40,000 resolution right now but they are improving)
16
Q
Why do we need high resolution?
A
1) Detection of isobaric ions
2) Exact Mass Measurements
3) Determination of an ion’s charge from m/z using C13 (or C12) isotope pattern
4) Increase selectivity and/or sensitivity (incr. resolution decreases the chemical noise; lower number of ions detected but again, the noise is lowered). Increases S/N
17
Q
Small width of peak means
A
High Resolution
18
Q
What is the highest form of resolution?
A
FT-ICR
19
Q
What is meant by “Double Focusing”
A
electric sector followed by a magnetic sector (EB or BE)
20
Q
What is the mass accuracy with high resolution?
A
+/- 0.0003 m/z
21
Q
What calibration compound is used to calibrate the x-axis?
A
FC-43 (Perfluorotributylamine, aka PFTBA); Used for low volatile analysis (EI or CI)
-Calibrates the x-axis
22
Q
What calibration compound is used to calibrate the y-axis?
A
Decafluorotriphenylphosphine (DFTPP)
-For calibrating ion intensity (y-axis)
23
Q
Why are there so many hydrogens on the calibrants?
A
- Presence of Fluorines significantly decreases boiling point (30-40ºC). More volatility b/c there is less hydrogen bonding
- Because of its larger mass, it creates a larger range of analysis for your sample to peak
24
Q
Time of Flight (TOF)
A
- Simplest mass analyzer (in terms of hardware and understanding it)
- Measures time (t) required for ions to travel down a flight tube (L)
- Not as sturdy (electronics get outdated over time)
- Pulse Mass analyzer
25
How do ions move out of the source / how are ions directed to the mass analyzer?
Ions are accelerated out of the source to the mass analyzer by electrostatic focusing.
26
Write out the kinetic energy equation for an ion fully accelerated from the ion source.
KE = (mv^2)/2 = zV
| *Note: "z" is the charge on ions and "V" is the voltage in electrostatic focusing
27
What is the mass spect. definition of resolution?
Resolution is the sharpness of the spectral peaks.
28
What is the chromatography definition of resolution?
The separation of two ions of nearly identical m/z
29
Where are Lorentzian peaks typically found (what computational conversion)?
Fourier-Transform
30
With mass analyzers, in addition to having high resolution, what parameter also needs to be well-defined?
m/z range
31
Kicking out all chemical noise increases ______ and ______.
Resolution and specificity
32
What chemical is used to calibrate the x-axis for mass analysis? What is it calibrating?
Perfluorotributylamine (FC-43) ; m/z range
33
What chemical is used to calibrate the y-axis for mass analysis? What is it calibrating?
Decafluorotriphenylphosphine (DFTPP) ; ion intensity
34
What is the mass range for the x-axis in mass analysis?
50 to 502 m/z
35
Why does TOF require a high accelerating voltage?
-This is to minimize the KE spread that may occur from the ionization event. When ioniziation event occurs, KE is added to the ions. So you want to add such a large accelerating voltage (V) so that the KE added by the ionization event is negligible.
36
Why does fragmentation within the flight tube, along L, not effect the time of flight?
After the accelerating voltage is applied, it does not matter of there is fragmentation or not because the ions will still travel at the constant velocity. Velocity of the ions won't change.
37
Name two ways to increase resolution by decreasing time spread.
1) Delayed Extraction - where you allow the ionization pulse to complete (200-350 seconds) before applying the pulse to extract the ions from the ion source.
2) Reflectron (or Reflector) - Ions are exposed to an electromagnetic field in the middle of the flight path. This fixes the time spread, not the velocity spread.
38
What is the purpose of reflectrons?
- Reflectrons are used to fix the time spread.
- Ions that are moving at a greater velocity have to travel a longer flight path, which means it takes longer to reach the detector. This is so that it and another ion with the same m/z (but may be moving at a slower velocity) will reach the detector at the same time.
39
What is TOF often coupled with (ionization technique(s))?
Any continuous source of ions. Also works really well with pulsed ionization methods.
- MALDI, LD, & 252CfPD (Good because of their pulsed ionization method)
- TOF/SIMS (more common with newer TOF machines)
- ESI and APCI (ions are made continuously but can be "trapped" and pulsed into the extraction region as needed.
- EI, CI, FAB, GC, LC-MS
40
Reflectrons increase resolution by (2 methods)
1) Decrease the time spread for ions of the same m/z
| 2) Increase the flight path, (L)
41
What detector is used for TOF?
Multichannel Plate (MCP)
42
Describe the detector.
-A circular device that is about 2-3 mm wide in diameter. -Composed of multiple glass capillary tubes that each are 0.5-1.0 mm long and have a 5 micrometer diameter. The capillaries are coated with an electron emissive material such as PbO.
43
Why is the detector a good choice for TOF?
- It is fast
- Can go up or down as the ions hit the detector
- Good multiplication of signal.
44
What is signal averaging?
Also known as ensemble averaging
- Not related to Fourier Transform
- Recorded mass spectrum is usually an average of spectra from 100 or more individual ionization events (i.e. laser shots)
45
Signal/Noise ratio is equal to
The square root of the number of scans (n)
46
Overall features of TOF
- Entire mass spectrum is obtained in 500 microseconds
- Pulsed ; can be combined with MALDI, LD and PD
- Can be used with continuous ion sources as well
- Moderate to high resolution (better than quadrupoles, less better than double-focusing, much less better than FT-ICR)
- Has the widest dynamic and linear range of any technique
- The fastest mass analyzer
- Can be used on large molecules (both volatile and nonvolatile)
- Has very high efficiency of ion transmission, meaning it can detect extremely small amounts of ions
- Very low limits of detection
- Uses signal averaging
47
What feature is needed for a TOF digitizer?
Needs a fast analog to digital converter
48
With a magnetic sector, what two forces must be equal to each other in order for the turn/curve to take place?
Magnetic force and the centrifugal force
49
What is the equation for magnetic force?
Bzv
50
What is the equation for centrifugal force?
mv^2/r
51
How do magnetic sectors discriminate m/z?
Using momentum to charge ratio
52
Derive the BS equation for m/z.
mv/z=Br
| m/z=B2z2/2V
53
What is the special focusing feature of BS?
Directional Focusing-
| ions with the same m/z and velocity but take different paths through B will come into focus at the exit slit.
54
Name two parameters which affect ions reaching the exit slit.
1) Scanning V while B is held constant (needs continuous ion source)
2) Scanning B while V is held constant
- Most common
- Scanning analyzer
- Yields a non-linear mass scan
55
How does a magnetic sector work?
- At given values of B & V, only ions with a particular m/z have a circular path of radius, r, that allows them to pass through the exit slit of the analyzer.
- Ions of other m/z will hit the walls of the analyzer
56
Magnetic Sectors require a __________ source of ions.
Constant
57
With directional focusing, the time required to obtain a complete spectrum depends on _________ and _______.
the scan rate ; m/z range
58
The slower the scan rate the ___________ measurement of B and more accurate measurement of ___________.
more accurate ; m/z measurement
59
The accelerating voltage (V) range of the magnetic sector is __________ with ______ being the more common.
1-10 kV ; 3 kV
60
What are the common sector angles for magnetic analyzers?
60, 90, and 180 degrees
61
With magnetic analyzers, lowering the accelerating voltage will _______ the mass range.
increase
62
How is the accelerating voltage of the magnetic analyzer related to the spread in the ion beam?
- The greater the accelerating voltage causes the initial velocity of the ions (from the ionization event) to minimalized relative to their velocity when accelerated by V.
- This gives more monoenergetic ions
63
How are slit width, resolution, and ion abundance related / how do they effect each other?
Narrowing the entrance and/or exit slit width INCR. resolution but also decreases ion abundance
64
What ion sources can be coupled with BS?
- Literally anything that has a continuous source of ions
| - EI, SIMS and CI
65
What mass analyzers are considered continuous/scanning?
E, B and Quadrupoles
66
What mass analyzers are considered pulsed?
TOF, TOF relfectron, FTICR, QIT
67
Rank the mass analyzers from least resolution to highest resolution
Q, QIT, TOF, TOF R, EB, FTICR
68
What ES forces must be perfectly balanced for ions to make the turn/curve?
Electrostatic and centrifugal forces
69
What is a special focusing feature of ES?
- It filters ions based on energy
| - For ions with the same mass, ESA also provides velocity focusing
70
Why does ESA have sharper spectra peaks?
Because it has velocity focusing and energy focusing
71
How does ESA scan a spectra?
- Scans a mass spectrum by scanning the accelerating voltage (V) and scanning the electric field strength (by scanning V prime)
- This brings the ions of varying KE through the exit slit
- Only ions with a particular KE will pass through the circular path of radius, r, and emerge from the exit slit
- It filters ions based on energy. Ions w/ different KEs emerging on different paths.
72
What order of sectors is considered normal? What order is considered reverse?
EB is normal, BE is reversed
73
Pros of EB
- Gives enhanced energy and direction focusing to ion beam
- Increases resolution (up to 10^5 at 1000 m/z)
- Better detection limits b/c of more intense peak
- Better sensitivity
- Can accept ions with a higher range of velocities
- Sharpens the range of velocities
74
How do the peaks of EB differ from peaks made using B alone?
They are sharper, more narrow peaks. Specifically, it sharpens the velocity range.
75
E sector does what
separates ions based on velocity (or KE)
76
B sector does what
separates ions based on momentum
77
What is normal geometry?
EB ; Used for high resolution. Magnetic sector acts as the actual mass analyzer. The E sector sharpens the peaks by giving energy focusing (velocity focusing). The E sector has a slit to allow ions to enter the magnetic sector.
- Gives a more accurate mass measurement
- The magnetic sector focuses the ions to a point. This is where the detector needs to be placed.
78
What is Mattauch-Herzog geometry?
- All values of m/z are brought into focus along a plane
- B is not scanned
- Instead, a multichannel detector is used.
- Simultaneously detects all ions. All m/z are detected at once
- Uses a curved electromagnetic field
79
What is the significance of SIMS?
- Commonly used with scanning mass analyzers to improve the limit of detection or increase selectivity.
- Only a few m/z of interest are allowed through the analyzer at a time, all others are kicked out.
- Commonly used for Quant. Analysis
80
Describe the shape of the quadrupole (Q) rods and how their shape affects their use.
- Circular or half-moon shaped rods that serve as mass to charge filters.
- Only the curved part of the rods is where ions hit.
81
Describe the potentials applied to each/all rods.
Combinations of dc (direct current) and radio frequency (rf) alternating current (ac) voltages are applied to the rods.
-Adjacent rods have opposite polarities
82
What is the typical length and diameter of a Quadrupole rod?
20 cm and 1 cm
83
What is the radio frequency equation?
RF = Vcos wt
84
What is unique about the quadrupole?
- Has the lowest accelerating voltage of all mass analyzers (0-30 V)
- Only continuous ion sources
- Scans the electric field
- Scan one m/z at a time
85
Why are Q called mass filters instead of mass analyzers?
- Because only one m/z is being scanned at a time
| - Only ions of a specific m/z value are able to travel through the rods and to the detector.
86
How does the position of the scan line on the Mathieu diagram affect detection limits, sensitivity, ion transmission, and resolution?
-Everything in the stability region is stable enough to hit the detector. Anything outside of that is too unstable and will hit the rods.
To have stable trajectories U < V/6
87
Increasing the slope of the scan line on the Mathieu diagram
Incr. resolution, becomes more selective for that one m/z, and lowers sensitivity and detection.
88
Decreasing the slope of the scan line on the Mathieu diagram
Incr. Transmission efficiency (more ions can come in), better sensitivity and detection, but lowers the resolution
89
What is meant by "dynamic" mass analyzers?
Mass analyzers that are dependent on time-varying electric fields or on time-dependent ion-movement.
Ex: TOF, Q, ICR, Orbitrap
-Anything with frequency
90
What is meant by "static" mass analyzers?
Mass analyzers that are dependent on the effects of electrical or magnetic fields and NOT on TIME.
Ex: Sectors
91
Why would someone have to increase the voltage applied to a quadrupole over time
Over time, the Q rods will become dirty due to ions colliding the rods. More voltage will be needed to get through this grime so that the ions traveling will feel it.
92
Compare and contrast a regular linear quadrupole and a QIT
- QIT is a 3D quadrupole device that has 2 dome-shaped end cap electrodes and a central ring electrode.
- It most commonly uses "Mass Selective Instability Mode" (aka rf-only mode).
- AC Voltage in radio frequency range is applied to the ring electrode.
- In MS mode, the end cap electrodes are held at ground potential.
- Voltages are applied to the end cap electrodes to kick out ions from the trap.
- Linear quadrupole just has rods
93
How are the ions trapped in a QIT
-At a fixed value of V, all ions above a certain m/z are trapped within the quadrupole field. Each m/z has a specific oscillating frequency (secular frequency)
At a fixed frequency, the storage voltage determines the range of m/z that are trapped by the field.
94
Describe the motion of the ions once trapped
They oscillate at a frequency related to their m/z and this motion is imperfect. As the RF voltage is raised ions of increasing m/z expand their range of motion and escape from the trap and are detected via an EM
95
How does trap size correlate to resolution
Smaller trap size = better resolution
96
What is the purpose of collisional cooling
Used to collide with the oscillating ions to keep them in the center of the trap. This is so they can lose some of their KE and won't leave the trap until you're ready
97
Where is the filament placed in QIT for EI CI versus ESI APCI and FAB
a. The filament is beside the trap and ions are pulsed in at the electron gate which involves accelerating voltages for EI CI
b. Ions are produced externally and injected as pulses into the trap by electrostatic focusing for ESI APCI and FAB
98
What is the upper m/z range for QIT?
650 m/z
99
In QIT, how long can ions be stored for?
milliseconds to seconds
100
What is the sample pressure in QIT?
10^-6 to 10^-7 Torr
101
Why is the dynamic range of QIT so small (10^3)?
Because only a small number of ions can be trapped. If you increase the trap size, the radius between the rods gets bigger = you lose resolution and performance; Too many ions cause charge effects and causes ions to fly out of the trap
102
Can QIT hold positive and negative ions at the same time
Yes
103
Linear Ion Trap
- 2D form of ion trapping
- Rod-like trap
- Same charge is applied to both ends of the rods to trap the ions in the middle.
- Not good for trapping positive and negative ions at the same time.
104
Advantages of LIT over QIT
1) Bigger than QIT. Can hold more ions, better dynamic range b/c it can hold more. More sensitive. More ions = better detection limits
2) Improved trapping efficiency
3) Improved ion-ejection
4) Faster scan speeds
5) improved detector sensitivity
105
How is a mass spectra obtained using HCT
Detecting the number of resonantly ejected ions as a function of applied RF voltage amplitude
106
In tandem MS^n what are two things resonance excitation can do
1) Cause unwanted ions to be ejected out of the trap to isolate the precursor ion
2) Provide precursor ions with KE for collision induced dissociation
107
What is the upper range of HCT?
6000 m/z
108
What three things of HCT that contribute to it having better detection limits, speed resolving power, and mass accuracy than QIT?
1) geometry of the device
2) rf voltage application
3) ejection methods are optimized
109
What mass analyzers can use EM as a detector
Quads and sectors
110
What magnitude (unit) describes the response time of an EM
nanoseconds
111
What is the purpose of the dynode
Converts the ion beam to an electron beam
112
How many stages does a discrete stage dynode have
14-16
113
What is the difference between a continuous and discrete dynode
- With discrete dynode, there are multiple stages where a different voltage is applied.
- With continuous dynode, it is a horn-shaped that has the same voltage applied to it but the ions reach the detector via a series of "bumps" down the funnel of the horn.
114
Are there any advantages of a continuous dynode over a discrete dynode
The continuous dynode is smaller and requires less power to operate
115
What is the material used to coat photomultiplier detector versus a dynode
-Organic phosphor
| With a dynode, there is only copper beryllium
116
With the Daly photomultiplier, what is the ion path?
- The ions are accelerated with a 20-40 kV into a conversion dynode, which emits electrons.
- The electrons hit an organic phosphor which then produces photons
- The photons pass through a quartz or sapphire window and into a photomultiplier tube.
- Inside the PMT, the photon beam is converted back to electrons. the electron beam is multiplied, and electrical current is detected
117
What is the most common MS detector?
Daly Photomultiplier Detector
118
This it the worst ion current detector
Faraday Cup ; good only really for isotopic ratios
- Simple detector in which the ion beam impinges against a metal collector and the electrical current produced by the ion beam is measured.
- No amplification
119
Discharge sources and plasma sources may still use ______ as their detector
Photographic Plates
120
How does FTICR separate ions
It separates ions based on the cyclotron frequencies at which they travel in the presence of a strong magnetic field. Frequency of circular motion = m/z
121
The cube/cylinder of FT-ICR is made of how many plates?
6
122
6. Ions exposed to a strong magnetic field are constrained in circular orbits _______ to the magnetic field
perpendicular
123
What is the equation for angular frequency?
v/r
124
Ions have trapping times of
milliseconds to hours
125
10's of ___________ are produced per second with FT-ICR.
spectra
126
Name and explain three fundamental modes of motion
1) Cyclotron motion – rotation of an ion about a fixed applied magnetic field. Used in m/z analysis
2) Trapping motion – axial back and forth motion of ions between two plates which have a DC potential. Used to trap ions in cell
3) Magnetron motion – slow circular drift of an ion along a path of constant electrostatic potential. Occurs as a result of the cross radial electric field and axial magnetic field
127
How is an ICR spectrum obtained
1) Scan magnetic field, keep frequency fixed
* m/z sequentially come into resonance, their image currents are detected (conventional ICR)
2) Scan excitation frequency and keep magnetic field constant
* m/z sequentially come into resonance, their image currents are detected (rapid san ICR) as frequency can be scanned more rapidly than magnetic field
3) Excite all m/z’s simultaneously with a very fast frequency sweep, keep B constant
* The image currents of all m/z’s are detected simultaneously in time and FT converts time data to frequency data (FTICR)
128
Considerations of TOF
- The fastest mass analyzer (Good choice to couple with chromatography)
- Unlimited mass range of the mass analyzer itself
- Pulsed (most common with pulsed ionization)
- Resolution is at the low end of the high resolution range (but improving)
- Moderate to high resolution
- Analyzer is relatively inexpensive
- Good mass analyzer for ultrafast chromatography
- Commonly couple with GC-MS and LC-MS
- Most TOFs are MALDI-TOFs
- Single Ionization event gives a complete mass spectrum, so very small samples can be used.
- Very low LOD
- Requires good electronics
- Widely commercially available
- Used primarily with MALDI in the analysis of large molecules such as biomolecules, organic polymers, and inorganics) or with chromatography (GC-MS or LC-MS)
129
Microchannel Plates (MCP)
- Detector
- Based on fiber optics
- Measuring the electrical current
- Very fast response times
- Coupled with TOF
- Entire mas spectrum may be obtained in 500 microseconds
- Very sensitive
- Requires a high accelerating voltage.
- HIGH SOURCE PRESSURES (LIKE LC-MS) MAY LEAD TO ARCING. ADDITIONAL PUMPING MAY BE REQUIRED
- Less rugged and more expensive than quadrupoles, but more versatile because of higher resolution
- WORKS WELL W/ SIMS
- HIGHEST RESOLUTION OF ALL
130
Magnetic Sectors' ion source is
EI or CI
131
__________ will not work with MALDI because it needs a continuous source of ions.
Magnetic Sectors
132
________ are slower than other machines but the slower the scan rate gives a __________ measurement of B and m/z measurement
Sectors ; more accurate
133
Quadrupoles
- Simplest MS in operation
- Any continuous ion source ( DART, DESI, ESI)
- CAN'T USE W/ MALDI
- SCANNING MASS ANALYZER
- Requires DC and AC voltages
- Has the lowest accelerating voltage of all mass analyzers
- Low resolution
- m/z range up to 4000
- Precise rod alinement is key
- GOOD FOR EI, not MALDI
- WORKS WELL WITH SIM
- MORE RUGGED AND LESS EXPENSIVE THAN SECTORS, BUT LESS VERSATILE
- RODS ARE DIFFICULT TO ALIGN AND COSTLY TO REALIGN
- HIGH MAINTENANCE
- CONSTANT CLEANING
- READILY COMPATIBLE WITH "HIGH PRESSURE" TECHNIQUES (LC-MS) B/C IT DOESN'T CAUSE ELECTRICAL ARCING
- M/Z ANALYSIS IS NOT DEPENDENT ON VELOCITY, SO A LARGE RANGE OF KEs ARE ACCEPTED
134
Quadrupole Ion Trap (QIT)
- Best-selling Mass spectrometer of all time
- Pulsed
- Storage voltage determines the range of m/z that are trapped by the field
- Pressures are 10^-6 to 10^-7 Torr
- Very simple to operate
- Relatively inexpensive, versatile, rugged, small, and mobile
- Not normally high resolution
- Can achieve m/z ranges up to 5,000
- Ions trapped by high pressure helium
- Ions are detected by an electron multiplier (EM) detector
- Common with GC-MS
- Can trap positive and negative ions at the same time
135
HCT ion trap
- 10x greater ion capacity
- LOD is 10x better
- Most sensitive ion traps available
- Better resolution
- Upper m/z range of 6,000
- Fast, can scan up to 26,000 m/z per second
- Medium resolution
136
MALDI is a ______________ ion source.
pulsed
137
QIT is not usually coupled with MALDI because ________________.
QIT's low upper m/z range isn't capable with MALDI's unlimited m/z range.
