Mass Spectroscopy

CHEM 466

Upali Siriwardane

Marilyn Cox

Jim Plamer

History of Mass spectroscopy

http://www.chemistry.ohio-state.edu/~allen/587%20W04/587%20W04%20130-136%20std.pdf

http://www.cem.msu.edu/~cem333/Week18.pdf

http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch13ms.html

Introduction...

Mass spectroscopy is perhaps one of the most widely applicable of all the analytical tools available to the analytical chemist in the sense that this technique is capable of providing information about

(1) the qualitative and quantitative composition of both organic and inorganic analytes in complex mixtures

(2) the structures of a wide variety of complex molecular species

(3) isotopic ratios of atoms in samples and the structure and composition of solid surfaces.

Uses of Mass Spec

forms ions, usually positive, study charge/mass ratio

very characteristic fragmentation pattern in charge/mass ratio

data easier to interpret than IR and/or NMR

provides accurate MW of sample

used to determine isotopic abundances

Where are Mass Spectrometers Used?

Mass spectrometers are used in industry and academia for both routine and research purposes. The following list is just a brief summary of the major mass spectrometric applications:

Biotechnology: the analysis of proteins, peptides, oligonucleotides

Pharmaceutical: drug discovery, combinatorial chemistry, pharmacokinetics, drug metabolism

Clinical: neonatal screening, haemoglobin analysis, drug testing

Environmental: PAHs, PCBs, water quality, food contamination

Geological: oil composition

http://www.varianinc.com/image/vimage/docs/products/chrom/gcms/shared/ms2200bro_r2.pdf

"In Mass Spectroscopy (MS..."

In Mass Spectroscopy (MS), atomic and molecular weights are generally expessed in terms of atomic mass units (amu).

The atomic mass unit is based on upon a relative scale in which the reference is the carbon isotope ¹²₆C, which is assigned a mass of exactly 12 amu. Thus the amu is rdefined as 1/12 of the mass of one neutral carbon atom. Mass spectrometrists also call the amu the Dalton (Da).

"The chemical atomic weight or..."

The chemical atomic weight or the average atomic weight (A) of an element in nature is given by the equation

A = A₁p₁ + A₂p₂ + .......+ A_np_n

where A₁, A₂, ...... A_n are the atomic masses in Daltons of the n isotopes of the element and p₁, p₂ ...... p_n are the fractional abundance of these isotopes.

Components of Mass Spec

Fig. 20-10, pg. 512

Inlet System

Table 20-3, pg. 505 Natural Abundances of Isotopes of Some Common Reagents

Components of Mass Spec

Fig. 20-11, pg. 513

“Schematic of (a) an external sample introduction system (note the that various parts are not to scale) and (b) a sample probe for inserting a sample directly into the ion source.”

use book & ELMO

Sample Handling

batch inlet: 1-5 L surge tank

gases and volatile liquids

direct probe inlet: non-volatile liquids

gas chromatographic inlet systems

permeable porous material to release carrier gas

Operation of Mass Spec

http://www.colby.edu/chemistry/OChem/

DEMOS/MassSpec.html

Table 20-1, pg. 500
Ion Sources for
Molecular Mass Spectroscopy

Magnetic Sector Analyzers...

Magnetic sector analyzers employ a permanent magnet or electromagnet to cause the beam from the ion source to travel in a circular path of 180, 90, or 60 degrees. Here, ions are formed by electron impact.

"The translational energy of an..."

The translational energy of an ion of mass m and charge z upon exciting slit B is given by

K = Zev = ½ mv² Equation 1

where V is the voltage between A and B, v is the velocity of the ion after acceleration, and e is the charge of the ion.

Double Focusing Instruments...

These type of instruments, unlike single-focusing which simply minimize directional errors, are designed to limit both the errors introduced because ions are initially moving in different directions and also the errors introduced due to the fact that ions of the same mass-to-charge ratio may have different translational energies. A schematic of a double-focusing instrument is shown next.

Slide 18

Quadrupole Mass Filters...

Quadrupole mass spectrometers are usually more compact, less expensive, more rugged than their magnetic sector counterparts. A quadrupole is analogous to a narrow-band filter in that it , set at any operating conditions, it transmits only ions within a small range of m/z values.

Time-of-Flight Analyzers...

In time-of-flight instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions or laser generated photons. The ions produced are then accelerated by an electric field and then made to pass into a field-free drift tube about a meter long.

Computerized Mass Spectrometers...

Minicomputers and microprocessors are integral part of modern mass spectrometers. The figure below is a block diagram of the computerized control and data acquisition system of a triple quadrupole mass spectrometer.

Ion Sources...

The appearance of mass spectra for a given molecular species is highly dependant upon the method used for ion formation.

Gas-Phase Sources...

Gas-phase sources require volatilization of the sample before ionization and thus are limited to thermally stable compounds that have boiling points less than about 500°C.

Mass Spectra

Fig. 20-2, pg. 501

"Mass spectra for 1-decanol:

(a) 70-ev electron impact

(b)chemical ionization with isobutane as reagent gas.”

book & ELMO

Fig. 20-4, pg. 504

Electron-impact mass spectra of:

(a) methylene chloride and

(b) 1-pentanol.

Fig. 20-6, pg. 507

Mass spectra for glutamic acid:

(a) electron impact ionization,

(b) field ionization, and

(c.) field desorption

"Electron-impact ionization is not very..."

Electron-impact ionization is not very efficient and only about one molecule in a million undergoes the primary reaction

M + e^- ® M.⁺ + 2e^-

Electron Impact spectra are very complex due to the high energies possessed by the accelerated electrons which collide with the sample and lead to fragmentation. These complex spectra are very useful for compound identification.

"Advantages of Electron Impact sources"

Advantages of Electron Impact sources:

(1) They are convenient and produce high ion currents.

(2) Extensive fragmentation can lead to unambiguous identification of analytes.

"Disadvantages of Electron Impact sources"

Disadvantages of Electron Impact sources:

(1) The need to volatilize the sample limits this method since it excludes analysis of thermally unstable compounds.

(2) Excessive fragmentation can lead to the disappearance of the molecular ion peak therefore preventing the molecular mass of the analyte to be determined.

Chemical Ionization Sources...

These sources employ the use of a reagent to impart energy to the sample. The reagent is bombarded with highly accelerated electrons and then made to collide with the sample in its gaseous phase.

Chemical Ionization Source

reagent gas 10³- 10⁴ times concentrated as sample

collisions with reagent gas ions causes ionization

less fragmentation, simpler spectra

special modifications to deal with higher pressures

Spark Source

rf spark source, 30 kv

sample part of electrodes, produces gaseous ionic plasma

Field Ionization Source

metallic anode

cathode acts as slit

separation: 0.5 to 2 mm

5 to 20 kv potential applied

produces mainly M and M+1 peaks

Desorption Sources...

In desorption methods, energy is introduced in various forms to the liquid or solid sample in such a way as to cause direct formation of gaseous ions. As a consequence, spectra are greatly simplified and often consist of only the molecular ion or the protonated molecular ion.

Identification of Pure Compounds by Mass Spectroscopy...

Mass spectroscopy can be used to determine the molecular weight of a compound but this involves an identification of a molecular peak and a comprehensive study of a spectrum.

"TANDEM MASS SPECTROSCOPY:"

TANDEM MASS SPECTROSCOPY: This type of spectroscopy simply involves the coupling of one mass spectrometer to another and this hyphenated technique has resulted in dramatic progress in the analysis of complex mixtures.

SECONDARY ION MASS SPECTROSCOPY: This is one of the most highly developed of the mass spectrometric surface methods, with several manufacturers offering instruments for this technique. It involves the bombarding of a surface with a beam of ions formed in an ion gun. The ions generated from the surface layer are then drawn into a spectrometer for mass analysis.

MS/MS instrument

Fig. 20-24, pg. 530

"Schematic of a tandem quadrupole MS/MS instrument.”

Electron Impact Source

bombardment of sample with beam of electrons

Fig. 20-3, pg. 502
"An electron impact source."

Fragmentation Patterns

Table 20-2, pg. 503

"Some Typical Reactions in an Electron Impact Source."

Electron Impact Ionization Process

M + e^------> M⁺ + 2e^-

where M⁺ => molecular ion

Electron Impact Ionization Process

Molecular Ions:

M⁺

(M+1)⁺

(M+2)⁺

Electron Impact Ionization Process

Molecular Ions:

M⁺

reults from removing an electron from a molecule

Electron Impact Ionization Process

Molecular Ions:

(M+1)⁺

results from one atom/molecular of C-13 or H-2

Electron Impact Ionization Process

Molecular Ions:

(M+2)⁺

small for most organics because it requires two heavy atoms/molecule

1 C-13 and 1 H-2

2 C-13s

2 H-2s

sizeable for chlorinated or brominated compounds

Electron Impact Ionization Process

Molecular Ions:

peaks for collision products: function of concentration (pressure)

stability of the molecular ion

stabilized by p e^- systems, cyclic

base peak

Electron Impact Ionization Process

base peak

highest peak

peak height against which all others are measured for use in peak tables

Mass Analyzer

resolution vs price and application

Single-Focusing Analyzers
with Magnetic Deflection

Fig. 20-12

pg. 515

"Schematic of a

magnetic sector

spectrometer."

Magnetic Centripetal Force

F_m = Bzev

where F_m => magnetic centripetal force

B => magnetic field strength

v => velocity of particle

z => charge on particle

e => charge of electron

Centrifugal Force

F_c = mv²/r

where F_c => balancing centrifugal force

r => radius of curvature of magnetic sector

m => mass of particle

Kinetic Energy

KE = zeV = 1/2mv²

where KE => kinetic energy

V => accelerating potential

Mass to Charge Ratio, m/z

F_m= F_c

thus

Bzev = mv²/r

where v = Bzr/m

m/z = (B²r²e)/2V

Mass Analyzer

Double-Focusing Analyzers

higher resolution, need higher amplification

2 magnets or 1 magnet & 1 electrostatic field

Fig. 20-13, pg. 517
Mattacuh-Herzog type double-focusing mass spectrometer.

Double Focus
Mass Spectrometer

Time of Flight Analyzers

non-magnetic separation

detector - electron multiplier tube

instantaneous display of results

Fig. 20-14, pg. 518
Schematic of a time-of-flight mass spectrometer.

Quadrupole Analyzers

4 short parallel metal rods

opposite rods same charge on dc source, AC rf applied ontop

Quadrupole Mass Spectrometer

Ion Trap Analyzer

Variable radio frequency voltage applied to the ring electrode

ions of appropriate m/z circulate in stable orbit

scan rf, heavier particles stable, lighter particles collide with ring electrode

ejected ions detected by transducer as an ion current

Fig. 20-15, pg. 518
Ion Trap Mass Spectrometer

Detectors...

Electron Multipliers: A discrete-dynode electron multiplier is designed for detection of positive ions. Each dynode is held at successively higher voltage and there is a burst of electrons that is emitted when struck by energetic electrons or ions. A continuous-dynode electrons electron multiplier is a trumpet-shaped device made of glass that is heavily doped with lead.

Detectors...

The Faraday Cup detector: This detector functions as follows. When positive ions strike the surface of the cathode, electrons move flow from the ground through the resistor to neutralize the charge. The resulting potential drop across the resistor is amplified via a high-impedance amplifier.

Mass Analyzers...

There are several methods available for separating ions with different mass-to-charge ratios. Ideally, the mass analyzer should be capable of distinguishing between minute mass differences.

Resolution of Mass Spectrometers...

Resolution, in MS, refers to the ability of a mass spectrometer to differentiate between masses and is quantitatively defined as

R = m / Dm

where Dm is the mass difference between two adjacent peaks that are just resolved and m is the nominal mass of the first peak (the mean mass of the two peaks is sometimes used instead).

Measurement and Display of Results

photographic results in double-focus

electron current from well protected electrode

galvanometers with sensitized paper

strip chart recorder

computer display

Fourier Transform
Mass Spectrometer

Usually trapped ion analyzer

ions created by brief electron beam burst

short rf pulse that increase linearly in frequency with time

Computerized Mass Spectrometers

Fig. 20-17

pg. 520

"A trapped

ion analyzer

cell.”

Computerized
Mass Spectrometers

Fig. 20-18, pg. 521

"Schematic diagram showing the timing of

(a) the radio-frequency signal

(b) the transient image signal."

Fig. 20-18, pg. 521

Computerized
Mass Spectrometers

Fig. 20-19, pg. 521

"Time domain (a) and (b) frequency or mass domain spectrum for 1,1,1,2-tetrachloroethane."

Fig. 20-19, pg. 521

Computerized
Mass Spectrometers

Fig. 20-21, pg. 523

"A computer display of mass-spectral data. The compound was isolated from a blood serum extract by chromatography. The spectrum showed it to be the barbiturate, pentobarbital."

Fig. 20-21, pg. 523

Determination of
Molecular Formula

distinguish between compounds of same MW

C₅H₁₀O₄ or C₁₀H₁₄

Determination of
Molecular Formula

distinguish between compounds of same MW

C₅H₁₀O₄

¹³C 5 * 1.08%     = 5.40%

²H 10 * 0.016%   = 0.16%

¹⁷O 4 * 0.04%     = 0.16%

-------

¹³⁵peak/¹³⁴peak 5.72%

Determination of
Molecular Formula

distinguish between compounds of same MW

C₁₀H₁₄

¹³C 10 * 1.08% = 10.8%

²H 14 * 0.016% = 0.22%

-------

¹³⁵peak/¹³⁴peak 11.0%

Determination of
Molecular Formula

Table 20-6, pg. 526

"Isotopic Abundance Percentages and Molecular Weights for Various Combinations of Carbon, Hydrogen, Oxygen, and Nitrogen."

Example 20-5

Calculate the ratios of the (M+1)⁺ to M⁺ peak heights for the following compounds: dinitrogenbenzene and an olefin,

Table 20-6, pg. 526

Nitrogen Rule

organic compounds with even MW, O or even number of N

odd MW, odd number of nitrogen atoms

Fragmentation Patterns

Table 20-2, pg. 503

"Some Typical Reactions in an Electron Impact Source."

Mass Spectrum of Toluene

Common Fragments

Slide 86

Fig. 20-1, pg. 500
Mass Spectrum of Ethyl Benzene

Identification of Compounds from Fragmentation Patterns

Fragmentation Patterns: Rules

C - C bonds weaker than C - H bonds

fragmentation most likely at a branch

positive charge remains with fragment with most branching

Fragmentation Patterns

     CH₃

      |

CH₃ - CH₂ -- C -- CH₃

      |

     CH₃

Fragmentation Patterns

     CH₃

      |

CH₃ - CH₂ -- C -- CH₃ m/e = 71

      +

Fragmentation Patterns

+CH₃ m/e = 15

CH₃

|

+C - CH₃ m/e = 57

|

CH₃

CH₃ - CH₂ + m/e = 29

Fragmentation Patterns

for alkenes: cleavage is favored at second bond away from double bond

CH₃ -- CH₂ - CH = CH - CH₃

----->

CH₂ -- CH -- CH - CH₃

+

m/e = 55

Fragmentation Patterns

for aromatics: cleavage at beta bond from ring, m/e = 91

f - CH₂ --R -----> f - CH₂ +

Fragmentation Patterns

terminal non-carbon group: cleaves non-carbon group

R -- X -----> R +

where R => Cl, Br, I OH, OR, SH, SR, NH₂, NHR, NR₂

Example:
2,2,4-trimethylpentane

CH₃

|

CH₃ - C - CH₂ - CH - CH₃

| |

CH₃ CH₃

Example:
2,2,4-trimethylpentane

     CH₃

      |

CH₃ - C - CH₂ - CH - CH₃

      | |

     CH₃ CH₃

m/e % of base peak Fragment

57 100%

56 33

41 28

Example:
2,2,4-trimethylpentane

     CH₃

      |

CH₃ - C - CH₂ - CH - CH₃

      | |

     CH₃ CH₃

m/e % of base peak Fragment

43 25

29 18

27 11

Example:
2,2,4-trimethylpentane

     CH₃

      |

CH₃ - C - CH₂ - CH - CH₃

      | |

     CH₃ CH₃

m/e % of base peak Fragment

99 5

15 5

55 4

Example:
2,2,4-trimethylpentane

     CH₃

      |

CH₃ - C - CH₂ - CH - CH₃

      | |

     CH₃ CH₃

m/e % of base peak Fragment

42 2

114 1

71 1

69 0.5


	http://www.chemistry.ohio-state.edu/~allen/587%20W04/587%20W04%20130-136%20std.pdf
	http://www.cem.msu.edu/~cem333/Week18.pdf
	http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch13ms.html


	Mass spectroscopy is perhaps one of the most widely applicable of all the analytical tools available to the analytical chemist in the sense that this technique is capable of providing information about
	(1) the qualitative and quantitative composition of both organic and inorganic analytes in complex mixtures
	(2) the structures of a wide variety of complex molecular species
	(3) isotopic ratios of atoms in samples and the structure and composition of solid surfaces.


	forms ions, usually positive, study charge/mass ratio
	very characteristic fragmentation pattern in charge/mass ratio
	data easier to interpret than IR and/or NMR
	provides accurate MW of sample
	used to determine isotopic abundances



	Mass spectrometers are used in industry and academia for both routine and research purposes. The following list is just a brief summary of the major mass spectrometric applications:

	Biotechnology: the analysis of proteins, peptides, oligonucleotides

	Pharmaceutical: drug discovery, combinatorial chemistry, pharmacokinetics, drug metabolism

	Clinical: neonatal screening, haemoglobin analysis, drug testing

	Environmental: PAHs, PCBs, water quality, food contamination

	Geological: oil composition
	http://www.varianinc.com/image/vimage/docs/products/chrom/gcms/shared/ms2200bro_r2.pdf


		In Mass Spectroscopy (MS), atomic and molecular weights are generally expessed in terms of atomic mass units (amu).
		The atomic mass unit is based on upon a relative scale in which the reference is the carbon isotope ¹²₆C, which is assigned a mass of exactly 12 amu. Thus the amu is rdefined as 1/12 of the mass of one neutral carbon atom. Mass spectrometrists also call the amu the Dalton (Da).


	The chemical atomic weight or the average atomic weight (A) of an element in nature is given by the equation
	A = A₁p₁ + A₂p₂ + .......+ A_np_n

	where A₁, A₂, ...... A_n are the atomic masses in Daltons of the n isotopes of the element and p₁, p₂ ...... p_n are the fractional abundance of these isotopes.


	Fig. 20-11, pg. 513
	“Schematic of (a) an external sample introduction system (note the that various parts are not to scale) and (b) a sample probe for inserting a sample directly into the ion source.”

	use book & ELMO


	batch inlet: 1-5 L surge tank
	gases and volatile liquids
	direct probe inlet: non-volatile liquids
	gas chromatographic inlet systems
	permeable porous material to release carrier gas


	Magnetic sector analyzers employ a permanent magnet or electromagnet to cause the beam from the ion source to travel in a circular path of 180, 90, or 60 degrees. Here, ions are formed by electron impact.


	The translational energy of an ion of mass m and charge z upon exciting slit B is given by
	K = Zev = ½ mv² Equation 1
	where V is the voltage between A and B, v is the velocity of the ion after acceleration, and e is the charge of the ion.


	These type of instruments, unlike single-focusing which simply minimize directional errors, are designed to limit both the errors introduced because ions are initially moving in different directions and also the errors introduced due to the fact that ions of the same mass-to-charge ratio may have different translational energies. A schematic of a double-focusing instrument is shown next.


	Quadrupole mass spectrometers are usually more compact, less expensive, more rugged than their magnetic sector counterparts. A quadrupole is analogous to a narrow-band filter in that it , set at any operating conditions, it transmits only ions within a small range of m/z values.


	In time-of-flight instruments, positive ions are produced periodically by bombardment of the sample with brief pulses of electrons, secondary ions or laser generated photons. The ions produced are then accelerated by an electric field and then made to pass into a field-free drift tube about a meter long.


	Minicomputers and microprocessors are integral part of modern mass spectrometers. The figure below is a block diagram of the computerized control and data acquisition system of a triple quadrupole mass spectrometer.


	The appearance of mass spectra for a given molecular species is highly dependant upon the method used for ion formation.


	Gas-phase sources require volatilization of the sample before ionization and thus are limited to thermally stable compounds that have boiling points less than about 500°C.


	Fig. 20-2, pg. 501
	"Mass spectra for 1-decanol:
	(a) 70-ev electron impact
	(b)chemical ionization with isobutane as reagent gas.”

	book & ELMO


	Electron-impact mass spectra of:
	(a) methylene chloride and
	(b) 1-pentanol.


	Mass spectra for glutamic acid:
	(a) electron impact ionization,
	(b) field ionization, and
	(c.) field desorption


	Electron-impact ionization is not very efficient and only about one molecule in a million undergoes the primary reaction

	M + e^- ® M.⁺ + 2e^-
	Electron Impact spectra are very complex due to the high energies possessed by the accelerated electrons which collide with the sample and lead to fragmentation. These complex spectra are very useful for compound identification.


	Advantages of Electron Impact sources:

		(1) They are convenient and produce high ion currents.
		(2) Extensive fragmentation can lead to unambiguous identification of analytes.


	Disadvantages of Electron Impact sources:
		(1) The need to volatilize the sample limits this method since it excludes analysis of thermally unstable compounds.
		(2) Excessive fragmentation can lead to the disappearance of the molecular ion peak therefore preventing the molecular mass of the analyte to be determined.


	These sources employ the use of a reagent to impart energy to the sample. The reagent is bombarded with highly accelerated electrons and then made to collide with the sample in its gaseous phase.


	reagent gas 10³- 10⁴ times concentrated as sample
	collisions with reagent gas ions causes ionization
	less fragmentation, simpler spectra
	special modifications to deal with higher pressures


	rf spark source, 30 kv
	sample part of electrodes, produces gaseous ionic plasma


	metallic anode
	cathode acts as slit
	separation: 0.5 to 2 mm
	5 to 20 kv potential applied
	produces mainly M and M+1 peaks


	In desorption methods, energy is introduced in various forms to the liquid or solid sample in such a way as to cause direct formation of gaseous ions. As a consequence, spectra are greatly simplified and often consist of only the molecular ion or the protonated molecular ion.


	Mass spectroscopy can be used to determine the molecular weight of a compound but this involves an identification of a molecular peak and a comprehensive study of a spectrum.


	TANDEM MASS SPECTROSCOPY: This type of spectroscopy simply involves the coupling of one mass spectrometer to another and this hyphenated technique has resulted in dramatic progress in the analysis of complex mixtures.

	SECONDARY ION MASS SPECTROSCOPY: This is one of the most highly developed of the mass spectrometric surface methods, with several manufacturers offering instruments for this technique. It involves the bombarding of a surface with a beam of ions formed in an ion gun. The ions generated from the surface layer are then drawn into a spectrometer for mass analysis.


	Fig. 20-24, pg. 530
	"Schematic of a tandem quadrupole MS/MS instrument.”