mass spectrometry for pesticides residue analysis- l4
TRANSCRIPT
Lecture (4)-The last lecture in this series
MASS SPECTROMETRY for
pesticides residue analysis
“The commonly used mass to charge analyzer for pesticides residue analysis”
Contents Lecture 1: Pesticides classification with an introduction to mass spectrometry& vacuum system for GC MS/MS and LC MS/MS
Lecture 2 : Electron ionization and Chemical ionization
Lecture 3 : Electrospray Ionization and Atmospheric pressure chemical ionization
Lecture 4 : The commonly used mass to charge analyzer for pesticides residue analysis
QUADRUPOLE• Different types of mass analyzers are used in mass spectrometers include : Linear quadrupole (stable trajectory) , Linear quadrupole (RF), Time-of-flight (Velocity), Orbitrap (Axial oscillation, FT), Fourier transform ion Cyclotron resonance (magnetic, cyclotron resonance, FT) Magnetic sector (Momentum),
• Commonly used mass analyzers for pesticides residue analysis will be discussed in our presentation. The quadrupole analyzer is the most commonly used mass analyzer especially for pesticides residue analysis.
• A quadrupole consists form four parallel rods of circular or, hyperbolic section with a fixed inter-distance 2r.
• Each parallel pair of rods is connected to both radio frequency (RF) and direct current (DC) generators.
• Ions that will enter (z-direction) the quadrupole will be suffered from a total potential of (applied to the four rods)
QUADRUPOLE
• Form the equations that describe the motions of ions in quadrupole two combined variables a and q are obtained:
RF
DC
U voltage of direct potential V zero-to-peak amplitude of the RF voltageZe number of charges times the charge on one electronm/ Ze = m/mz mass to charge
QUADRUPOLE• Since r is constant and at constant ω, m/z can be varied only With the applied U and V
• a m/z will travel through the quadrupole in a staple motion at x and y directions only at a and q values that doesn’t exceed r.
X
YZ
QUADRUPOLE
• A staple diagram for the trajectory of an ion will be obtained when putting the values of a vs. q in other words U vs. V
a/q=2 U/V
High resolutionLow resolution
m/z=K Um/z= K V
QUADRUPOLE• Each ion (m/z) has its stable diagram (specific U/V ratio). Therefore by a
staple z trajectory for a specific m/z will be obtained by controlling U/V value, all other m/z at this time (dwell time) will be in unstable trajectory motions and will not reach the detector. After that, U/V will be changed to pass another m/z. and so on. This process called Single ion monitoring (SIM)
RF-Only Quadrupoles, Hexapoles, and Octopoles
• As shown in the stability diagram, when setting the DC (U) potential to zero wide range of Ions will have a staple radial dimension (X and Y) by RF. At this condition quadrupole is used as a ion quid (referred as q, lower case).
RF-QAt V2
M1 will lost
M3Low
Focusing
M2High
Focusing
V2
• At Low V a low focusing was obtained for higher m/z
• At high V ions of lower m/z will be lost
V2
RF-Only multipole
the efficiency of focusing (stability along X and Y) depends on the depth of the effective potential well, which is inversely proportional to m/z.
higher-order multipoles has an increasingly steeper potential wells leading to better capabilities for ion-guiding and m/z range
Collision Cell
• A collision cell is a tight closed unit surrounding a RF-quadrupole,hexapole or ocatpole with a needle valve through which gas (He, N2, Ar) of pressure higher than the surrounding vacuum pressure collide the entered ions leading to ions fragmentation.
This fragmentation process is termed as a collision-induced dissociation (CID)
Collision focusing
• Collision of an inert gas will be foxed towards the central axis of the Rf-multipole which in turn increases the transmission efficiency through an exit aperture. This process is known as collision focusing.
•By the collional focusing process foxed ions are separated into layers of different m/z value with higher m/z ions gathering in more outward radial layers.
LIT exit aperture (circle of 0.7 mm aperture hole) for ions of reserpine and Cs14I13 + cluster ions
2D linear trap with axial and radial ejection•In a RF multipole ions are with stable motion along X and Y (radial traped)
•when the motion of these ions are trapped also in the Z direction (axial trapped)this multipole is referred as 2D linear ion trap •Axial trapping (Z) is obtained by means of using smaller electrodes (at both the end and front of the RF-multipole) connected to electric field (DC voltage).
The trapped ions can be selectively (m/z) ejection either axial or radial.
Axially ejection is obtained using soft field effects by applying AC voltages between the rods of the linear trap and the trapped electrode
Radial ejection is carried out through slots in two opposite rods by applying an AC voltage on these two rods.
2D linear trap with axial and radial ejection
Electrostatic trap (Orbitrap)
•The orbitrap is a m/z analyzer, trap and detector without applying RF or using a magnet.
•The orbitrap composed from:
External electrode of a barrel shape divided into two equal parts with a small space (max r 20 mm)
Central electrode of a spindle shape (max r 8mm)
Electrostatic trap (Orbitrap)• Ions are injected through the space between two half's of the external electrode or off-axis through a
hole in the external electrode.
• Once ions (with + Kev) are injected inside the orbitrap it start spirals motion around the central
electrode ( - Kv), the resulted attraction force are equal to the centrifugal force of the injected ions
(trapping around the spindle electrode with applying potentials barriers at end of the external
electrodes).
• From the different frequencies of the ion motion inside the orbitrap, the frequency of axial oscillations
is the most important. Since, it depends only on mass to charge.
Electrostatic trap (Orbitrap)• An C Trap (CIT) was used for ions injections into the orbitrap with
specific injection angular position, velocity and foxed ions (apply N2 gas to cool the trapped ions and fox it around the central axis at the trapping stage)
Time-of-flight (TOF) analyzer
• Time-of-flight (TOF) analyzer separates ions (m/z) depending on their flight along a field-free drift path of known length. Where, ions of low m/z will arrive earlier at the detector than the heavier ones.
QqTOFRF only
(cooling, Foxing)Collision cell
(Fragmentation, cooling, Foxing
Foxing
Ions are accelerated before
and after the collision cell
A pulsed electric field directs ions into accelerating column
compensation for the initial energy and spatial spread of the ions
AB SCIEX 6500
Mode Q1 Q2 Q3Q1 MS Q1 scan RF- only RF- onlyQ3 MS Q3 RF- only RF- only scanMRM (MRM) Specific m/z Fragment Specific m/zPrecursor ion (Prec)
Scan fragment Specific m/z
Enhanced product ion (EPI)
Specific m/z fragment Trap and scan
MS/MS/MS (MS3)
Specific m/z fragment Isolate product ion, fragment, trap, scan
Electrostatic trap (Orbitrap)
image current detection by Orbitrap with
Applying a Fourier Transformer
• Jürgen H. Gross, Mass Spectrometry, Springer-Verlag Berlin Heidelberg (2011).
• E.Hoffmann, Mass Spectrometry Principles and Applications, John Wiley & Sons Ltd, England (2007).
• R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004).
• http://onlinelibrary.wiley.com/doi/10.1002/jms.207/pdf
• http://slideplayer.com/slide/3272371/
• http://www.reliance.co.uk/en/news/66/quadrupole+mass+filters
• http://pubs.rsc.org/en/content/articlehtml/2010/an/c0an00021c
• http://nemc.us/docs/2014/Presentations/MonHigh%20Performance%20Liquid%20Chromatography%20in%20Environmental%20Monitoring-2.4-Winkler.pdf
References
