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Mass Spectroscopy

Mass Spectroscopy (Mass Spec)

Applying Atomic Structure Knowledge to Chemical Analysis

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Mass Spectroscopy

Mass Spectroscopy

• Spectroscopy is the study of the interaction of electromagnetic radiation with matter.

• In mass spectroscopy, atoms and/or molecules are exposed to a beam of high-speed electrons.

• The electron beam knocks electrons off the atoms or molecules and thereby changes them into positively charged ions.

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Mass Spectroscopy

Mass Spectroscopy

• If the sample is an atom, the mass spectra will reveal the different isotopes of the element.

• If the sample is a molecule, it is broken into several fragments, each of which becomes ionized in the electron beam.

• After ionization, an applied electric field accelerates the positive ions into a chamber where an applied magnetic field deflects their path.

• Positive ions of different masses and charges are deflected differently in the field.

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Mass Spectroscopy

Mass Spectroscopy Analysis

• The amount of deflection in the magnetic field for each ion depends on its mass and charge.

• The most massive, singly charged ions are deflected the smallest amount.

• The locations where different ions hit the detector plate can be correlated to their atomic masses and charges.

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Mass Spectroscopy

Mass Spectrometer Basics

Selection by magnetic field velocity

++++++++

----------------

+velocity

amplifier

recorder or PC

Ionization by electron beams

+

+

++++ +

(This must be done in vacuum so the ions can move about freely without hitting air molecules)

Accelerating voltage applied

+

(Ions are accelerated through a series of slits with decreasing voltages)

Deflection in a magnetic field

Dete

cto

r

+++

(An ions deflection depends on their mass and charge, but most all have a +1 charge)

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Mass Spectroscopy

sample

slits

positive ions

electron beam

heating device to vaporize sample

accelerated ion beam

magnetic field

least massive ions

most massive ions

ion-accelerating electric field

Schematic of a Mass Spectrometer

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Mass Spectroscopy

Carbon atom with 4electrons in its outer orbit Oxygen atom with 6

electrons in its outer orbit

Six electrons being shared by oxygen and carbon

(or three covalent bonds between oxygen and carbon)

Carbon Monoxide

Note: Carbon Monoxide has 28 units of mass

Note: Carbon has 12 units of mass Oxygen has 16 units of mass

Note:

C O

1) Molecular formation by atomic collisions

2) Ion Formation

Mass Spectrometry Competing Phenomena

C O

C O

Alternate Drawing

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Mass Spectroscopy

Energy

(collection of positive ions)

16 unitNote: This ion has a mass of per unit charge 12 units

Note: This ion has a mass per unit charge

Note: This ion has a mass of per unit of charge 28 units

Note: This ion has a mass of per unit of charge 14 units

Carbon Monoxide

2) Ions formation by electron collisions with atoms and/or molecules

C O

O+

C +

C O++C O

+

1) Molecular Formation 2) Ion Formation

Mass Spectrometry Competing Phenomena

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Mass Spectroscopy

C O

Mass Spectrometry Ionization, acceleration and selection

O+

C +

CO

++

CO

+

Carbon Monoxide sample from a vacuum process chamber

e e

CO

CO

Cold wireHot wire

e

e

e

C O

e e

CO

a detector that counts the ions that pass through the quadrupoles

Quadrupole rods direct select mass to chargeions down middle path depending on magneticfields applied to pole pairs.

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Mass Spectroscopy

Mass Spectra – the QuadrupoleA quadrupole (4 rods) is one configuration used for deflecting ions to separate them by mass. A magnetic field is created by the 4 rods inside the steel tube and can be adjusted to cause different ions to reach the detector as the applied magnetic field is changed.

4 steel rods inside

ionizing filaments

quadrupole mass spectrometer

installed in a vacuum application

electronics for quadrupole mass spec

the quadrupoles

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Mass Spectroscopy

Mass Spectra – the DataA mass spectra is sometimes referred to as a “stick diagram” which shows the relative amounts of the different ions as a function of their mass, expressed as the ratio of mass to charge, m/z. Most of the ions formed in the electric field have a +1 charge.

Rela

tive a

bundance

Mass to charge ratio, m/z

90 92 94 96 98 100 102

Schematic of a Typical Mass Spectra

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Mass Spectroscopy

Mass Spectra – the Data

• The y-axis reflects the relative abundance of a particular ion hitting the detector.

• The ions hitting the detector at each location on the detector produce an electrical current.

• This current is proportional to the number of ions hitting the detector.

• The more ions of a given size that reach the detector, the larger the signal for that ion.

• In this application, the detector acts something like a counter.

Rela

tive a

bundance

, %

Mass to charge ratio, m/z

90 92 94 96 98 100 102

100

80

60

40

20

0

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Mass Spectroscopy

Mass Spectra – the Analysis• The x-axis reflects the mass (m)

of each ion as a ratio to its charge (z).

• Most ions formed in mass spectrometers have a +1 charge.

• For samples that are atoms, the different mass to charge ratios reflect different isotopes. Isotopes of an element have different numbers of neutrons but the same number of protons.

• For samples that are molecules, the ion with the highest m/z ratio is called the “parent” ion. This ion is the original molecule with one less electron, and thus has the same mass weight as the original species.

Rela

tive a

bundance

, %

Mass to charge ratio, m/z

90 92 94 96 98 100 102

100

80

60

40

20

0

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Mass Spectroscopy

Mass Spectra – the Data

•This is the mass spectra of molybdenum, Mo.

•Each of the 7 peaks reflect a different isotope of Mo.

•If they all have +1 charge, they all have masses of 92, 94, 95, 96, 98, and 100.

Rela

tive a

bundance

Mass to charge ratio, m/z

90 92 94 96 98 100 102

Using the spectra, which isotope of Mo is the most abundant in this particular sample?

The tallest “stick” is the one at m/z = 98. This isotope of Mo is the most abundant in the sample analyzed.

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Mass Spectroscopy

Mass Spectra – the Data

• Mo is element 42 in the periodic table, and has an average atomic weight of 95.94.

• If you read the relative abundance of each isotope’s peak, you could calculate the average atomic weight from this spectra and compare the value you get to the published value of 95.94.

Rela

tive a

bundance

Mass to charge ratio, m/z

90 92 94 96 98 100 102

Knowing the atomic number of molybdenum is 42, how may neutrons are in the Mo isotope represented by the peak that is most abundant in the sample?

How many neutrons are in the peak with the smallest abundance?

98 (protons + neutrons) – 42 protons = 56 neutrons

97 (protons + neutrons) – 42 protons = 55 neutrons

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Mass Spectroscopy

Isotopic masses are used: • to determine average atomic mass of

elements.• to identify a compound’s composition and

structure.• for archaeological dating.•to identify particulates in space (when

mounted on a satellite or the space station).• to assure safe environments in nuclear

powered vessels.• to monitor process conditions when

fabricating computer microchips.

Mass Spec Applications

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Mass Spectroscopy

Many disciplines use mass spectroscopy for chemical identification.

Astronomy: analysis of astronomical components of the solar system

Electronics: analysis of microchipsEnvironmental: detection of toxic chemical, monitoring of

nuclear facilities, analysis of petroleum products, etc.Forensics: toxicology, trace metals, biological materials,

etc.Medical: drug abuse diagnosis, analysis of

pharmaceuticals and products of genetic engineeringMilitary: mobile mass spectrometers are used to detect

liquid chemical warfare agents

Mass Spec Applications

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Mass Spectroscopy