mass spectrometry for pesticides residue analysis- l2
TRANSCRIPT
MASS SPECTROMETRY for
pesticides residue analysis
Lecture(2)
“Electron ionization and Chemical ionization”
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
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
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.
m/z and M
GC MS/MS 7000 at QCAP Egypt
2x10
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
+EI Scan (9.464-10.520 min, 124 Scans) Heptenophos_Scan_10ppm.D
124.0
89.0
109.0
77.1215.0141.0 169.051.0 199.9
Counts (%) vs. Mass-to-Charge (m/z)50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310
Molecular ion beak (m/z = M+) z= 1
Base Beak
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 structurehttp://lysozyme.co.uk/lysozyme-structure.php
Electron Ionization
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)
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
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
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
EI
Ioniz. V = 15 Bond strength. V = 3-5
Robert_M._Silverstein, spectrometric identification of organic compounds, John Wiley (2005)
http://community.asdlib.org/activelearningmaterials/introduction-to-mass-spectrometry/
2x10
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
+EI Scan (18.481 min) Chlorpyrifos_SCN_c.D
196.9
313.997.0
257.8
207.8285.8
214.9125.0
169.0 243.7 275.8108.9 179.965.0
Counts (%) vs. Mass-to-Charge (m/z)30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
M+
NCl
Cl Cl
OP(OCH2CH3)2
S
M-HCl, 2CH2CH3
M-HCl, CH2CH3
M-HCl,
M-HCl, PS, 2CH2CH3
Mass SpectrumBase Beak
Molecular ion
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
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)
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.
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 +
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)
R.Martin_Understanding Mass Spectra, A Basic Approach 2nd ed (Wiley, 2004)
Mass spectra of ephedrine at EI and CI
165M+ H
Negative Chemical ionization (NCI)
7000 Triple Quad GC/MS Operation ManualJürgen 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)
N. Belmonte Valles, Journal of Chromatography A, 1264 (2012) 110-116. 10.1016/j.chroma.2012.09.063
NCI of pesticides
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
