Chapter 7 - Mass spectrometry Flashcards
In magnetic sector mass analyzers, the radius, r, depends on
the momentum, mv, of the ion.
The magnetic sector is a ______ analyzer rather than a _______ analyzer.
momentum, direct mass
For magnetic sector, m/z =
B2r2e/2Vs
Vs =
Kinetic energy of ions accelerated by a potential difference
In magnetic sector, if r is fixed and Vs is kept constant, for a given value of B, only the ions
with the corresponding m/z go through the analyzer.
In a magnetic sector, changing the value of B as a function of time allows
successive observations of ions and various values of m/z.
The magnetic sector can be used as a mass analyzer provided that
the kinetic energy of the ions is constant.
Magnetic sector is _______ focusing.
direction
An optimized magnetic sector alone can provide resolutions of resolving power
2000-7000
The intensity of the magnetic field in magnetic sector mass analyzers is usually between
0.4 and 0.6 Tesla (T)
The bigger the magnetic field in magnetic sector,
the heavier the ion mass analyzed.
________ limits the resolution of magnetic sector.
Energy dispersion
Magnetic sectors produce an energy dispersion when
a beam of ions with different kinetic energies issues from the source.
In order to carry out single ion monitoring, the magnetic field strength of the magnetic sector may be
‘tuned’ to the required m/z value and only ions of the specified value will be allowed to pass through the magnet, all other ions will be lost to the walls of the analyzer.
A minor of magnetic field strength tuning is related to
the quadratic dependence of m/z on the value of B, resulting in high mass peaks appearing closer together.
At high mass, a small change in b will result in
a much larger shift in the mass calibration.
The electrostatic analyzer (ESA) produces a radial electric field between
two oppositely charged plates extending over the ESA angle.
In ESA, an ion travelling in a direction perpendicular to the electrostatic field will follow
a circular path of radius R that fulfills the condition of equilibrium of centripetal force Fc and electric force Fe.
The electrostatic analyzer is a _______ analyzer if E is kept constant.
kinetic energy
Electrostatic analyzers are _______ focusing.
direction
Electric and magnetic sectors with the same energy dispersion oriented perfectly are called
double focusing.
The double focusing instrument is designed in such a way that ions of
different energies (but of the same mass) converge at the detector.
In double focusing forward geometry
the ESA is placed before the magnetic sector.
In double focusing revers geometry
the magnetic sector is placed before the ESA.
The scan rate of a magnetic sector instrument is limited by
the reluctance (or inertia) of an electromagnet, limiting the scan rate.
Laminated magnets permit
more rapid changes in the magnetic field such that the mass range may be scanned at rates approaching 0.1 s/decade mass (70-700, 50-500 m/z)
Problems with hysteresis (path X-Y not the same as Y-X) of the magnet are minimized by
discharging the magnet completely between scans, adding a settling time between scan cycles, and only scanning unidirectional for data collection.
In double focusing, the ion should experience a __________ field strength during its transit through the magnetic field.
homogenous
If scan speeds are too high in double focusing,
the field strength may change during the transit of the ion through the analyzer, causing the radius of deflection curvature to change.
In TOF, ions are subjected to an accelerating electric field that is typically ______ eV.
3000
In TOF, an ion of m/z 100 will take ________ to travel the distance d as an ion of m/z 25.
twice as long
In TOF, the arrival time at the detector between two ions with unit mass difference becomes smaller as
the mass of the ions increases, so the ions are increasingly more difficult to differentiate.
2 problems with regarding resolution when using TOF
- difference in flight times between ions.
- distribution of arrival times at the detector due to a distribution of kinetic energies, even for ions of the same m/z.
A reflectron is a series of
electrostatic lenses which create a homogeneous electrostatic field at the end of the flight path of ions.
In the reflectron, isobaric ions can bunch together to
reduce the distribution of flight times and greatly enhance the resolving power of the instrument.
The reflectron is often referred to as an
ion mirror.
There is no theoretical upper mass limitation in
linear TOF analysis.
In TOF, it becomes increasingly difficult to discriminate between arrival times when m/z becomes
large.
In TOF, discrimination between unit masses becomes difficult over
3000 m/z
In TOF, at 50,000 m/z the mass overlap may be as much as
50 mass units.
The transmittance of a linear TOF analyzer approaches ______ that leads to very high sensitivity.
90%
In TOF, ion losses are solely caused by
collisional scattering due to residual gas or by poor spatial focusing of the ion source.
The mass calibration of the TOF is easy with only
two reference points.
The disadvantage of using reflectron technology is
a slight decrease in the sensitivity of the analyzer due to ion loss and dispersion of the ion beam (accentuated at high mass).
The recording and storage of TOF data can be problematic as
the data is produced at extremely high frequency.
A TOF instrument with a 2 m flight tube and ion energy of 2 keV may produce a complete mass spectrum from
m/z 1 to m/z 800 in 90 microseconds.
The direct coupling of API sources with TOF analyzers can be problematic because
API techniques yield a continuous ion beam, while TOF analyzer operates on a pulsed process.
Ways to overcome the difference between continuous API sources and pulsed TOF analyzer
- Direct the ion beams past a slit placed at the ion source entrance.
- Collect ions in a storage device perpendicular to the incident ion beam.
Practical constraints of pulsing electrode in orthogonally accelerated TOF
- A practical upper pulse frequency is around 30 kHz
- Slow pulsing speeds can limit sensitivity.
Quadrupole ion rods need mechanical accuracy in the _____ range in both the machining of the rods and their spatial positioning.
micron
In a quadrupole, the desired cross section of the rods is
hyperbolic.
Quadrupoles range in length from
50-250 mm.
Quadrupole diameters range from
5-15 mm
Typical quadrupole tunnel radii (r0) is
5 mm.
Each quadrupole rod is paired with
its diametric opposite.
In a quadrupole, when x and y are less than the radius, the ion will have _____ trajectory.
resonant, non-collisional, or stable
For any quadrupole, ___ and ____ are fixed.
ro and omega
For any quadrupole, the variable parameters are
voltages (U and V)
The DC voltage (U) and RF voltage (V) are altered according to a linear relationship which is often referred to as the
SCAN line.
The slope of the SCAN function is often referred to as the
quadrupole GAIN.
The intersection of the scan line with the U axis is the
quadrupole OFFSET.
To ensure that ions of only one selected m/z value are transmitted, the parameters U and V must be chosen so that
the line representing U/V passes close to the apex of region A but still lies within the stable region.
If the slope of the SCAN line is increased so that it misses the apex of the stability region,
no ions of that m/z value will be transmitted.
To transmit ions of subsequently higher unit mass, the voltages U and V are ________, while the ratio U/V is _________.
increased, fixed
Increasing the mass GAIN (the slope of the scan line) by increasing the voltages U and V in constant proportion will
lead to an increase in spectral resolution but decrease sensitivity.
Changes in mass gain affect higher mass ions to a
greater extent.
When increasing the mass quadrupole DC offset voltage, the resolution will_______, but the sensitivity will________.
increase, decrease
Altering the DC offset voltage affects ions of all masses
to the same extent.
If the DC component of the applied voltage is removed, the scan line will lie
horizontally along the U/V plot abscissa. Acts as RF only ion bridge.
The efficiency of transporting and focusing ions is proportional to
V2
As the RF voltage, V, increases, the transmission of ions with high masses can be increased by a
more efficient focalization.
Quadrupole, Hexapole, and Octapole focusing power.
Quadrupole > Hexapole > Octapole
Quadrupole, Hexapole, and Octapole mass range.
Quadrupole < Hexapole < Octapole
Narrow ⇒ Wide
As the number of poles increases, the focusing power of the ion guide
decreases.
As the number of poles increases, the mass range for simultaneous transmission of ions of the ion guide becomes
wider.
