Transcript analytical chemistry chem 3811

INSTRUMENTAL ANALYSIS
CHEM 4811
CHAPTER 9
DR. AUGUSTINE OFORI AGYEMAN
Assistant professor of chemistry
Department of natural sciences
Clayton state university
CHAPTER 9
MASS SPECTROMETRY I
PRINCIPLES AND APPLICATIONS
PRINCIPLES
Technique involves
- Creating gas phase ions from the analyte atoms or molecules
- Separating the ions according to their mass-to-charge ratio (m/z)
- Measuring the abundance of the ions
PRINCIPLES
Technique can be used for
- Qualitative and quantitative analysis
- Providing information about the mass of atoms and molecules
- Molecular structure determination (organic & inorganic)
- Identification and characterization of materials
PRINCIPLES
- Instrument is mass spectrometer
- Separates gas phase ionized atoms, molecules, and fragments
of molecules
- Separation is based on the difference in mass-to-charge ratio (m/z)
m = unified atomic mass units (u)
1 dalton (Da) = 1 u = 1.665402 x 10-27 kg
z = charge on the ion (may be positive or negative)
PRINCIPLES
- Analyte molecule can undergo electron ionization
M + e- → M●+ + 2e- M●+ is the ionized analyte molecule called molecular ion
- Radical cation is formed by the loss of one electron
- Computer algorithms are used to deconvolute m/z values of multiply
charged ions into the equivalent mass of singly charged ion
- Permits easy determination of molecular weight of analyte
THE MASS SPECTRUM
- A plot of relative abundance vs m/z
- The most abundant peak is known as the base peak
- The base peak is scaled to 100
- Spectrum shows fragmentation patterns
- The m/z values and the fragmentation pattern are used to determine
the molecular weight and structure of organic compounds
- Provides the accurate mass of a given isotope not the
weighted average
RESOLVING POWER
- The ability of a mass spectrometer to separate ions of two
different m/z values
- Resolving power = M/∆M
- M = mass of one singly charged ion
- ∆M = difference in mass between M and the next m/z value
- The resolving power of ions in the 600 range = 600
- The resolving power of ions in the 1200 range = 1200
RESOLVING POWER
- Two methods used to calculate ∆M
- Full width at half maximum (FWHM) = ∆M
- 10% valley (overlap should not be > 10%)
RESOLUTION
- The value of ∆M at a given M
- Expressed in ppm
INSTRUMENTATION
Main components of the mass spectrometer
- Sample input system
- Ionization source
- Mass analyzer
- Detector
- Vacuum pumps
- Computer based data acquisition and processing system
SAMPLE INPUT METHODS
Gas Expansion
- Useful for gas samples and liquids with sufficiently high
vapor pressures
- The gas or vapor expands into an evacuated and heated vessel
- Sample leaks through holes in a gold foil seal into the ionization
source (termed molecular leak inlet)
- Pressure in ionization is maintained at 10-6 – 10-8 torr
SAMPLE INPUT METHODS
Direct Insertion Probe
- For liquids with high boiling points and solids with sufficiently
high vapor pressure
- The probe (with the sample in a glass capillary at the tip) is
inserted into the ionization source
- The probe is electrically heated to vaporize the sample
- This method has a problem with contamination
SAMPLE INPUT METHODS
Direct Exposure Probe
- Sample is first dissolved in a solvent
- A drop of solution is placed at the rounded glass tip of the probe
- Solvent evaporates leaving a thin film of sample
- The tip is inserted into the ionization source and heated to
vaporize sample
- Less likely to be contaminated
SAMPLE INPUT METHODS
Chromatography and Electrophoresis Systems
- Chromatographic instruments are used to separate mixtures of
gases and liquids
- Separated components are introduced into a mass spectrometer
for detection
- The GC-MS system
- LC-MS system is used for nonvolatile organic compounds
- Capillary electrophoresis (CE) can also be coupled to MS
IONIZATION SOURCES
Electron Ionization (EI)
- Commonly used for analysis of organic samples
- Electrons are emitted from a heated tungsten filament cathode
- Electrons are accelerated towards the anode with a potential
of about 50 – 100 V
- Electrons meet at right angles with the sample molecules
- Interaction with the high energy electrons causes ionization of
sample molecules and fragmentation into smaller ions
IONIZATION SOURCES
Electron Ionization (EI)
- Referred to as hard ionization source due to the high energy
EI source
- Ions are accelerated into the mass analyzer by an accelerating
voltage of ~ 104 V
- Both negative and positive ions are formed by EI
- Negative ions form from molecules containing acid groups or
electronegative atoms
IONIZATION SOURCES
Electron Ionization (EI)
- Collision between ions and molecules may also result in ions
with higher m/z values than the molecular ion
An example is the (M+1) peak
- Reaction between analyte molecule and H+ to form MH+
or (M+H)+ in which charge equals a+1
- Low pressure in the ionization source minimizes reaction
between ions and molecules
IONIZATION SOURCES
Chemical Ionization (CI)
- A large excess of reagent gas (1000 – 10000 times) is introduced
into the ionization region
- Pressures in source are typically higher than EI
- Electrons are allowed to bombard the gas-sample mixture
Examples of reagent gas
- Methane, ammonia, isobutane
IONIZATION SOURCES
Chemical Ionization (CI)
- Reagent gases are much more likely ionized by the electrons
than sample due to large excess
- Sample molecules are subsequently ionized by collision with
ionized reagent gas molecules
- Considered soft ionization source
- Less fragmentation and molecular ion is much more abundant
- Combination of CI and EI spectra provide good interpretation
IONIZATION SOURCES
Chemical Ionization (CI)
- For methane reagent gas
CH 4 and CH 3 are formed upon interactio n with electrons
CH 4  CH 4  CH 5  CH 3
CH 3  CH 4  C 2 H 5  H 2
Proton transfer occurs when sample molecules collide with
CH 5 and C 2 H 5
IONIZATION SOURCES
Chemical Ionization (CI)
M  CH5  MH   CH4
M  C 2 H 5  MH   C 2 H 4
The following may occur if analyte is a saturated HC
M  CH 5  (M - H)   CH 4  H 2
M  C 2 H 5  (M - H)   C 2 H 6
M  C 2 H 5  (M  C 2 H 5 )  with m/z  (M  29)
IONIZATION SOURCES
Atmospheric Pressure Ionization (API) Sources
- Two major types
Electrospray Ionization (ESI)
and
Atmospheric Pressure Chemical Ionization (APCI)
- Operate at atmospheric pressure
- Modified version of ESI is the Ion Spray Source
- Used for mixtures of nonvolatile high molecular weight compounds
IONIZATION SOURCES
Atmospheric Pressure Ionization (API) Sources
Applications
- Pharmaceutical chemistry
- Biochemistry
- Clinical biomonitoring
Electrospray
- Fine spray of positively or negatively charged droplets
IONIZATION SOURCES
Desorption Ionization
- For direct ionization of solids
- Excellent tool for analysis of large molecules
- Solid samples are placed on a support and then bombarded
with ions or photons
- Different types are available
IONIZATION SOURCES
Desorption Ionization
Desorption Chemical Ionization
- Used for nonvolatile compounds
- Sample is directly introduced into the chemical ionization
source on a tungsten or rhenium wire
Secondary Ion Mass Spectrometry (SIMS)
- For surface analysis
- For large molecules
IONIZATION SOURCES
Desorption Ionization
Laser Desorption Ionization
- Uses pulsed laser
- Provides selective ionization by choosing appropriate λ
- Laser is focused on a solid surface to ionize material
Examples of Lasers
- IR laser: CO2 laser
- UV laser: Nd:YAG (yttrium aluminum garnet)
IONIZATION SOURCES
Desorption Ionization
Matrix-Assisted Laser Desorption Ionization (MALDI)
- Matrix disperses large amounts of energy absorbed by the laser
- Minimizes fragmentation of the molecule
- Permits analysis of molecular weight over 10,000 Da
- Used for study of polymers, proteins, peptides
IONIZATION SOURCES
Desorption Ionization
Matrix-Assisted Laser Desorption Ionization (MALDI)
Matrix
- must be stable in vacuum and not react chemically
- must absorb strongly at laser λ (where analyte absorbs weakly)
Examples
- IR region : urea, alcohols, carboxylic acids
- UV region: 3-hydroxypicolinic acid, 5-chlorosalicylic acid
IONIZATION SOURCES
Desorption Ionization
Fast Atom Bombardment (FAB)
- Employs fast moving neutral inert gas atoms (Ar) to ionize
large molecules
- Sample is dissolved in glycerol and spread in a thin layer on
a metal probe
- Probe is then inserted into the mass spectrometer and a beam
of fast moving atoms probe the surface
IONIZATION SOURCES
Desorption Ionization
Fast Atom Bombardment (FAB)
- Used for analysis of surfactants and proteins (MW > 10,000)
- For large and thermally unstable molecules
- Technique works well at room temperature
- Simple and high sensitivity
- Sample can be recovered
IONIZATION SOURCES
Desorption Ionization
Fast Atom Bombardment (FAB)
- Modified technique is the continuous flow FAB (CFFAB)
- Sample introduction is through a fused silica capillary tube
- Solvent flows continuously and sample is introduced by
continuous flow injection
- For analysis of blood, urine, other body fluids, waste water
IONIZATION SOURCES
Inorganic MS Ionization Sources
Solid Samples
- Glow Discharge (GD) and Spark sources
- For sputtering and ionizing species from solid surfaces
- Primarily for atomic mass determination of elements
- GD has better S/N and able to sputter more material from sample
IONIZATION SOURCES
Inorganic MS Ionization Sources
Liquid Samples
- Inductively coupled plasma (ICP)
- Has high ionization efficiency
- Provides very simple mass spectra
MASS ANALYZERS
- Differentiates ions according to their m/z
- Different designs are available
Scanning Instruments
- Only ions of a given m/z pass through the analyzer at
a given time
- Magnetic Sector Mass Analyzer
- Quadrupole Mass Analyzer
MASS ANALYZERS
Simultaneous Transmission Instruments
- Allow transmission of all ions at the same time
- Time-of-flight (TOF)
- Ion Trap
- Ion Cyclotron Resonance Mass Analyzer
- Dispersive Magnetic Mass Analyzer
Tandem Mass Spectrometer (MSn)
- Two or more mass analyzers in sequence
MAGNETIC SECTOR MASS ANALYZER
- Gas phase molecules are ionized by a beam of high energy
electrons
- Electrons may be ejected from molecules (ionization) or bonds
in molecules may rapture (fragmentation)
- Ions are then accelerated in a field (sector) at a voltage V
- Sector can have any apex angle (60o and 90o are common)
- Most modern instruments combine both electric sector
and magnetic sector (double-focusing MS)
MAGNETIC SECTOR MASS ANALYZER
- The electric sector acts as an energy filter
- m/z range is 1 – 1400 for single-focusing and
5,000 – 10,000 for double-focusing instruments
- Energy of each ion = zV
- Kinetic energy depends on charge and voltage but not
on mass of ion
- Ions with small masses must travel at a higher velocity
than ions with larger masses
MAGNETIC SECTOR MASS ANALYZER
- For single positively charged ions
 2zV 
v

 m 
1
mv 2  zV
2
1/2
m = mass of ion
v = velocity of ion
z = charge of ion
V = accelerating voltage
- V changes as m varies such that ½ mv2 is constant
vα
1
m
MAGNETIC SECTOR MASS ANALYZER
- Ions enter a curved section of a homogeneous magnetic
field B after acceleration
- Ions move in a circle with radius r
- Attractive force on magnet = Bzv
- Centrifugal force on the ion = mv2/r
- The two forces are equal if the ion follows the radius of
curvature of the magnet
MAGNETIC SECTOR MASS ANALYZER
mv 2
 Bzv
r
mv
r 
Bz
Substituting for v and rearranging gives
m B2 r 2

z
2V
MAGNETIC SECTOR MASS ANALYZER
- Radius of circular path depends on m/z if V and B are kept constant
- Ions with different m/z travel in circles with different radii
- Basis of separation by m/z
- Ions with the right m/z reach the detector and others hit the
sides of the instrument and be lost
- Which m/z to reach the detector can be selected by varying V or B
- B is varied and V is kept constant in modern instruments
TIME OF FLIGHT (TOF) ANALYZER
- Makes use of a drift tube
- Pulses of ions are accelerated into the an evacuated drift tube
(free of field or external force)
- Velocity of an ion depends on m/z
(depends on mass if all ions have the same charge)
- Lighter ions move faster along the tube than heavier ions
- Ions are separated in the drift tube according to their velocities (v)
TIME OF FLIGHT (TOF) ANALYZER
 2zV 
v

m


1/2
- V = accelerating voltage
- If L is the length of tube (typically 1-2 m) and t is the flight
time of ion, then v = L/t
- Implies mass-to-charge ratio and flight time can be found from
m 2Vt 2

z
L2
m
tL
2zV
- An ion mirror called a reflectron is used to increase resolution
QUADRUPOLE MASS ANALYZER
- Separates ions in an electric field (the quadrupole field)
- Field is varied with time
- Oscillating radio frequency (RF) voltage and a constant DC
voltage are used to create the field
- These are applied to four precisely machined parallel metal rods
- The result is an AC potential superimposed on a DC potential
- Ion beam is directed axially between the four rods
QUADRUPOLE MASS ANALYZER
- Opposite pairs of rods are connected to opposite ends of a
DC source
- Ions follow an oscillating (corkscrew) path through the
quadrupole to the detector
- For a given ratio of DC to RF at a fixed frequency, only ions of
a given m/z value will pass through the quadrupole
- Other ions with different m/z values will collide with the
rods and be lost
QUADRUPOLE MASS ANALYZER
- The quadrupole acts as a filter so is often called the mass filter
- Sample must be ionized and in the gas phase
- m/z range is 1 – 1000 Da
- Has smaller range and lower resolution than magnetic sector
but faster
- Is the most common analyzer
- Rugged, inexpensive, and compact
MS – MS (TANDEM MS) INSTRUMENTS
- Employs two or more stages of mass analyzers
- Example is two quadrupoles coupled in series
- First analyzer selects ion (precursor ion) and second analyzer
selects the fragments of the precursor ion
- Used to obtain more information about the structure of
fragment ions
- Fragment ions may be dissociated into lighter fragment ions or
converted into heavier ions by reaction with neutral molecule
ION TRAP
- A device in which gaseous ions are formed and/or stored
for periods of time
Two commercial types
- Quadrupole Ion Trap (QIT)
and
- Ion-Cyclotron Resonance Trap (ICR)
ION TRAP
Quadrupole Ion Trap (QIT)
- Also called Paul Ion Trap
- Uses a quadrupole field to separate ions
- A 3D field is created using a ring-shaped electrode between
two end cap electrodes
- A fixed frequency RF voltage is applied to the ring electrode
- The end cap electrodes are either grounded or under
RF or DC voltage
ION TRAP
Quadrupole Ion Trap (QIT)
- Ions are stored in trap by moving in trajectories between
electrodes
- This is done by changing signs of electrodes to repel ions as
they approach the electrodes
- Ions of a given m/z pass through an opening to the detector
when the RF of the ring electrode is changed
- m/z range is 10 – 1000 Da
ION TRAP
Fourier Transform Ion-Cyclotron Resonance (FTICR)
- Also called Penning Ion Trap
- Uses magnetic field to trap and store ions
- Consists of six conducting plates arranged as a cube
- Cubic cell is about 100 mm on a side and is located inside
a strong magnetic field
- Sample is ionized by an electron beam
ION TRAP
Fourier Transform Ion-Cyclotron Resonance (FTICR)
- The ions then move in circular orbits
- Path is perpendicular to the applied field
- The operating frequency is called the cyclotron frequency
DETECTORS
- Measure one m/z value at a time (single channel detectors)
- Multiple detectors are used for multiple ion detection
- High resolution magnetic sector instruments use multiple
detectors (called multicollectors)
DETECTORS
Electron Multiplier (EM)
- The most common detector in MS for ions
- Similar to PMT
- Very sensitive and has fast response
DETECTORS
Faraday Cup
- A metal or carbon cup serves to capture ions and store the charge
- Cup shape decreases loss of electrons
- Least expensive detector for ions
- Has long response time
DETECTORS
Array Detectors
- Used in TOF MS instruments
- Employs a focal plane camera (FPC) consisting of an array of
31 Faraday Cup
- Up to 15 m/z values can be measured simultaneously
- Exhibits improved precision compared with single channel
detectors