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mass spectroscopy POGIL, Study Guides, Projects, Research of Chemistry

Chemistry

pogil for AP CHemistry students or college students. kfjnrognro'gi'orgi

Typology: Study Guides, Projects, Research

2023/2024

Uploaded on 11/02/2023

amy-jacobs
amy-jacobs 🇺🇸

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Mass Spectroscopy 1
Mass Spectroscopy
How do we know isotopes exist?
Why?
When John Dalton proposed the first formal atomic theory, he stated “Atoms of the same element are
identical.” Today we know that is not true—many elements contain several different isotopes, or atoms
that differ in mass. Mass spectroscopy is the principle technique used to study isotopes. It is used to both
“count” and “weigh” atoms in a sample, just not in the traditional sense.
Model 1 – Sorting by Mass
1. According to Model 1 what four processes occur inside a mass spectrometer?
2. Consider where the sample is introduced into the mass spectrometer in Model 1. Which one of
the four processes from Question 1 is the first process?
3. Match the four processes from Question 1 to the following descriptions.
Ions collide with a metal plate. Electrons are transferred from the metal to the
ion, producing a current and thus a signal to a computer.
Ions are attracted to the negative side of an electromagnetic field causing
separation of the mixture based on mass and charge.
Electrons are knocked off sample particles to form (mostly) +1 ions.
Ions move through a series of charged plates to form a narrow beam of high
speed particles with equal kinetic energy.
+
−
+−
Ionization Acceleration Deflection
Magnesium Atoms
(mixture of isotopes)
Magnesium Ions
(mixture of isotopes)
Detection
To vacuum
pump
Mg Mg+
24Mg+26Mg+
25Mg+
pf3
pf4
pf5

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Mass Spectroscopy 1

How do we know isotopes exist?^ Mass Spectroscopy

Why? When John Dalton proposed the first formal atomic theory, he stated “Atoms of the same element are identical.” Today we know that is not true—many elements contain several different isotopes, or atoms

that differ in mass. Mass spectroscopy is the principle technique used to study isotopes. It is used to both “count” and “weigh” atoms in a sample, just not in the traditional sense.

Model 1 – Sorting by Mass

  1. According to Model 1 what four processes occur inside a mass spectrometer?
  2. Consider where the sample is introduced into the mass spectrometer in Model 1. Which one of the four processes from Question 1 is the first process?
  3. Match the four processes from Question 1 to the following descriptions. Ions collide with a metal plate. Electrons are transferred from the metal to the ion, producing a current and thus a signal to a computer. Ions are attracted to the negative side of an electromagnetic field causing separation of the mixture based on mass and charge. Electrons are knocked off sample particles to form (mostly) +1 ions. Ions move through a series of charged plates to form a narrow beam of high speed particles with equal kinetic energy.
  • Ionization^ −Acceleration^ Deflection (mixture of isotopes)Magnesium Atoms (mixture of isotopes)Magnesium Ions

Detection

To vacuumpump

Mg (^) Mg+ (^24) Mg+ (^26) Mg+ (^25) Mg+

2 POGIL™^ Activities for AP* Chemistry

  1. When a sample is injected into the mass spectrometer, do the atoms or molecules turn into positive or negative ions? Justify your answer with at least two pieces of evidence from Model 1.
  2. According to Model 1, what causes the sample mixture to become separated?

Read This! The key to mass spectrometry is that all of the particles go into the deflection chamber with the same kinetic energy. They do not, however, have the same mass/charge ratio (m/z). Although most of the ions

formed are electromagnet is different. The strength of the electromagnet can be varied so only particles with a particular mass/charge ratio can make it to the detector. Other particles collide with the metallic sides of the instrument, +1 ions, their masses are different. Therefore, the amount of deflection they experience by the are neutralized, and then removed by the vacuum pump. The machine is calibrated using carbon-12 isotopes which are, by definition, exactly 12 amu (12.0000000…amu).

  1. Consider the following ions formed in a mass spectrometer. Rank the ions in terms of their de gree of deflection by the electromagnet from least to greatest. Greater deflection means a tighter turn towards the negative pole of the electromagnet. Make sure all group members are able to- explain the ranking. (^19) F 1 + (^16) O 1 + (^17) O 1 +
  2. Why is it necessary to have the mass spectrometer chamber under vacuum (very low pressure) for it to work properly?

4 POGIL™^ Activities for AP* Chemistry

Read This! The mass spectra you have been looking at in this activity used percent abundance on the however, the spectra use relative intensity. The ions from the sample are sorted by mass/charge ratio by the y -axis. Typically,

mass spectrometer. The ion that hits the detector most often is assigned a relative intensity of 100. The other ions are given proportional relative intensities based on their abundance in the sample. An example of magnesium’s mass spectrum shown both ways is given below.

  1. Consider the two mass spectra in the a. The sum of all percent abundances for magnesium is equal to 100. Explain why this is reasonable. Read This! box. b. The sum of all relative intensities for magnesium does not equal 100. Explain why this is reasonable.
  2. Imagine that the relative intensities on the mass spectra in the number of particles in a sample. a. Theoretically, how many magnesium ions were detected by the mass spectrometer? Read This! box represent the b. What percentage of the ions were answer. 24 Mg ions? Show mathematical work to support your c. Show mathematically how a computer might translate the 13 peak in the relative intensity graph to 10% for the percent abundance graph.

Percent Abundance Mass/Charge Mass/Charge

(^) 100 50 79%10% 11% 24 25 26

100 Relative Intensity^1314

(^) 100 50 24 25 26

Mass Spectroscopy 5

Read This! The process of ionization inside of a mass spectrometer is quite violent. There are several methods of ion ization used in industry, but many of them remove electrons from the atoms or molecules by high energy-

particle bombardment. In other words, the electrons are knocked off the atoms or molecules by high speed particles colliding with them. Occasionally this process will break apart a molecule. This is called fragmentation. The pieces are analyzed by the mass spectrometer along with the whole molecules.

  1. The following information was gathered by mass spectroscopy for the element fluorine. Fluorine has only one natural isotope, but it does form diatomic molecules naturally. Propose an explana tion for the two lines on fluorine’s mass spectrum. -
  2. The element chlorine has two natural isotopes: dance). The mass spectrum of chlorine has five lines. a. Three of the lines in the mass spectrum are from diatomic molecules of chlorine. List the 35 Cl (76% abundance) and 37 Cl (24% abun- three possible combinations of the two isotopes and their total mass number. b. Explain the remaining two lines in the spectrum. c. Draw a mass spectrum that would result from diatomic chlorine. Include the mass/charge number and estimate the relative abundance of each ion. (Assume only Although the heights of the peaks are difficult to predict you should be able to determine +1 ions are formed.) which will be taller or shorter based on the abundance of each chlorine isotope.

Percent Abundance (^19) Mass/Charge 38

Mass Spectroscopy 7

  1. What would the mass spectrum of magnesium look like if a small portion of atoms were ionized to 2+ (^) ions?
  2. Mass spectroscopy is also used to study large organic molecules. When a sample of a pure com pound is analyzed in the instrument, some of the molecules get ionized whole (molecular ions), while some are fragmented and ionized. The fragmentation occurs in predictable patterns allow-- ing scientists to propose chemical structures for unknown substances. Consider the mass spec trum of pentane (CH 3 CH 2 CH 2 CH 2 CH 3 ) shown below. -

a. Match the molecular ion and fragment ions below to each of the lettered peak clusters in the mass spectrum. CH 3 CH 2 CH 2 CH 2 CH 3 + CH CH CH 333 CHCHCH 222 CHCH+ 22 CH+ 2 + b.^ CHPeak cluster A has a small peak at 14. What may have fallen of the CH^3 + 3 +^ ion to form this peak?

Relative Intensity 15 A 29 43 57 72

B

C

D (^) E