This is the second lecture in series of lectures on mass spectrometry for pesticides residue analysis. This lecture (2) include: Electron ionization and Chemical ionization

1. MASS SPECTROMETRY
for
pesticides residue analysis
“Electron ionization and Chemical ionization”

2. 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

3. By ionization:
• The position of the produced ions in the space can be controlled by
different ways, e.g. : electric and magnetic fields (Lecture 4).
• Ions can be grouped ( sensitivity )
In mass spectrometry technique m/z of the produced ions are measured
(not the exact molecular mass). m/z = m For ions of z =1.
1 mass unit (u) is equal to 1/12 the mass of the most abundant naturally
occurring stable isotope of carbon, 12C.
Molecular Ionization

4. Molecular Ionization
There are Different ionization techniques that can be used for mass spectrometry. In
these series of lectures , the most commonly used ionization techniques for mass
spectrometry analysis of pesticides residue will be discussed, especially:
• Electron Ionization (EI) as an example for Hard ionization technique.
• Chemical ionization (CI), Atmospheric pressure chemical ionization (APCI)
and Electron spray ionization (ESI) as examples for Soft ionization techniques.

5. m/z and M
GC MS/MS 7000 at QCAP Egypt
Molecular ion beak
(m/z = M+) z= 1
Base Beak

6. 1.0073
J. Throck Watson, Introduction to Mass Spectrometry, willy (2007)
m/z and M
Positive ESI Mass spectrum of lysozyme
CPK model of the lysozyme structure
http://lysozyme.co.uk/lysozyme-structure.php

7. Electron Ionization

8. Electron impact (EI)
• The most commonly ionization technique for GC mass spectrometry is the Electron
ionization (EI). It is applicable for molecules of low to non polarity range.
• In the EI technique electrons are generated from a heated filament (rhenium or tungsten
metal) of energy 70 ev (λ= h/mv)
• The analytes (vapor phase) is bombarded by these highly energetic electrons (hard
ionization) produces a molecular ion with a positive charge, as represented by the M+·
• the excess internal energy usually result in formation of an ionized fragment (loss of
either a radical or a neutral ).
• A huge EI Mass spectral library is available for ten of thousands of compounds, which
reflect the importance of this ionization technique
J. Throck Watson, Introduction to Mass Spectrometry, willy (2007)
Jürgen H. Gross, Mass Spectrometry, Springer-Verlag Berlin Heidelberg (2011)

9. EI- production of electron and ions beams
R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004)
The emitted electrons from the electric heated
filament were directed (passed through a hole of
opposite charged plate that grouped the produced
electrons in a beam form) to an anode (Trap) .
Magnetic field causes the electrons to move in a
tight helical path, which increases the path length
of these electrons
The neutral gas molecules enter the ion source
vertically to the electron beam in its middle
70 ev

10. EI- production of electron and ions beams
R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004)
The new produced positive ions will be pushed
by a repeller (a positive charged plate)
To path through a hole of opposite charged
plates that grouped the produced ions in a beam
form.
Do not forget that the pressure in the ion-source is
maintained at a vacuum of approximately 10E- 5
torr (minimizing ion collisions, increase mean
free path)
70 ev

11. Agilent EI source
5977 Series MSD Lab Operator Quick Reference Guide
Ion focus lens
Entrance lens
Repeeler
Draw out
Filament
Basic dirty parts of EI source after more than 6 months
injection of herbs samples for pesticides residue
analysis (QCAP Egypt)
New
+ve M
The new produced positive ions pushed by repeller
And pulled by draw out plate, cylinder

12. EI

13. Ioniz. V = 15 Bond strength. V = 3-5
Robert_M._Silverstein, spectrometric identification of organic compounds, John Wiley (2005)

14. http://community.asdlib.org/activelearningmaterials/introduction-to-mass-spectrometry/
M+
NCl
Cl Cl
OP(OCH2CH3)2
S
M-HCl, 2CH2CH3
M-HCl, CH2CH3
M-HCl,
M-HCl, PS, 2CH2CH3
Which electron will be ejected from the analyte molecules after its EI ?
• The least tightly bound (Double bond, non bonding pairs of e on hetero atoms
• Compounds with a stable structure (high n. aromatic rings) may stabilize the produced positive
charge, which result in a high abundance for the molecular ion m/z
(Q1 Scan) 5 μg/ml Chlorpyrifos - QCAP Egypt
EI of pesticides

15. Since the sensitivity of the molecular ion using the hard ionization EI is very
low. Different ion source systems have been introduced with a relative soft
ionization include: Field ionization, chemical ionization, Atmospheric
chemical ionization and atmospheric pressure ionization (ESI)
These soft ionization technique generally increase both the sensitivity and
selectivity of the analysis.
Chemical ionization (CI) ion source involve an indirect ionization for the
sample molecules reverse to the Electron ionization.
In CI : A gaseous molecule are ionized by its collision with ionized
molecules of reagent gas that introduced in a large amount in the ionization
chamber. Therefore, the reagent gas molecules collide with most emitted
electrons forming reagent ions. These reagent-gas ions react with each other
and also react with sample molecules to form sample ions.
Chemical ionization (CI)

16. Chemical ionization (CI)
R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004)
J. Throck Watson, Introduction to Mass Spectrometry, willy (2007)
200 ev
In CI: The used energy of the emitted electrons from the filament in CI (200
ev) is much higher than that used in EI (70 ev), to increase its penetration
into the region of relatively high pressure reagent gas.
As the reagent gas entire in a high concentration into the ion source CI, a large
portion of this gas spills out into the much larger volume of the ion-source
housing. So, the pressure in the ion source housing when using CI is much
higher than using EI. However, this situation can be minimized with
differential pumping.
The reagent gas molecules are of low mwt as methane (the most
commonly used reagent gas), isobutane and ammonia.

17. Chemical ionization of reagent gases (methane and ammonia)
Jürgen H. Gross, Mass Spectrometry, Springer-Verlag Berlin Heidelberg (2011)
C3H5 +
C2H5 +
EI CI
EI
CI
CH5 +

18. Positive Chemical ionization (PCI)
Chemical Ionization an be used for production of positive and negative ions
simultaneously.
Positive Chemical ionization: the reagent gas molecules are first ionized, then the
formed ionized gas molecules interact (ionize) with the neutral analyte molecules as
follows:
Jürgen H. Gross, Mass Spectrometry, Springer-Verlag Berlin Heidelberg (2011)

19. R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004)
Mass spectra of ephedrine
at EI and CI
165
M+ H

20. Negative Chemical ionization (NCI)
7000 Triple Quad GC/MS Operation Manual
Jürgen H. Gross, Mass Spectrometry, Springer-Verlag Berlin Heidelberg (2011)
Although PCI introduce a higher intensity for the molecular ion, this technique has a
generally high limit of detection. Since there are a high background from the
reagent gas ions (negative reagent ions).
On the contrary, negative chemical ionization (NCI) have a much lower detection
limits even lower than EI (for compounds that have an electron affinity atoms).
In CI both positive and negative ions are formed (M+H / M-H) it is depend upon the
selection of interest polarity through changing the accelerating voltage
Reagent gas + e ( > 200 ev) reagent ions + e (thermal E)

21. N. Belmonte Valles, Journal of Chromatography A, 1264 (2012) 110-116. 10.1016/j.chroma.2012.09.063
NCI of pesticides

22. Chromatograms of 50 ng/ml Malathion analyzed by fast GC–NCI-MS and GC–EI-MS
in SIM mode at right and left respectively.
R. Huˇskova,Analysis of pesticide residues by fast gas chromatography in combination with chemical ionization mass spectrometry, Journal of Chromatography A (2009) 6326–6334
NCI EI
NCI of pesticides

23. sherif2taha@gmail.com
Sherif.taha@qcap-Egypt.com