INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 9 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...

INSTRUMENTAL ANALYSIS CHEM 4811

CHAPTER 9

DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment 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


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


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


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


- 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


- Collision between ions and molecules may also result in ionswith 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 reactionbetween 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


- 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


- For methane reagent gas

electronswithninteractiouponformedareCHandCH 34

3544 CHCHCHCH

25243 HHCCHCH

Proton transfer occurs when sample molecules collide with

525 HCandCH

IONIZATION SOURCES


45 CHMHCHM

The following may occur if analyte is a saturated HC

4252 HCMHHCM

245 HCHH)-(MCHM

6252 HCH)-(MHCM

29)(Mm/zwith)HC(MHCM 5252

IONIZATION SOURCES

Atmospheric Pressure Ionization (API) Sources

- Two major typesElectrospray 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 Chemical Ionization- Used for nonvolatile compounds

- Sample is directly introduced into the chemical ionizationsource on a tungsten or rhenium wire

Secondary Ion Mass Spectrometry (SIMS)- For surface analysis- For large molecules

IONIZATION SOURCES


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


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


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


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



- 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



- 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)


- 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


- For single positively charged ions

m = mass of ionv = velocity of ionz = charge of ion

V = accelerating voltage

- V changes as m varies such that ½ mv2 is constant

zVmv2

1 2 1/2

m

2zVv

m

1αv


- 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


Substituting for v and rearranging gives

Bzvr

mv2

Bz

mvr

2V

rB

z

m 22


- 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

- 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

- An ion mirror called a reflectron is used to increase resolution

1/2

m

2zVv

2

2

L

2Vt

z

m

2zV

mLt

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


- 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


- 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 detectorwhen 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 multipledetectors (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

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INSTRUMENTAL ANALYSIS CHEM 4811 CHAPTER 9 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of...

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