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Mass Spectrometry of Elements

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Mass Spectrometry of Elements

John Dalton once said "atoms of the same element are identical". After many years, scientists have found that this statement is not true! Some elements have different isotopes that differ in mass. To study these isotopes, we can look at the mass spectroscopy of elements!

  • First, we will define mass spectrometry.
  • Then, we will talk about atomic mass spectroscopy and mass spectrum.
  • After, we will learn how to calculate relative atomic mass of an element based on information from mass spectroscopy.
  • We will also look at some element's graphs and examples of exam questions.

Define Mass Spectrometry

Let's start by defining mass spectrometry.

Mass spectrometry (or mass spectroscopy) is a method used to determine the atomic mass of atoms/molecules in a sample by ionizing a chemical species and sorting the ions based on their mass to charge ratio.

Most mass spectrometers use a technique called electron impact ionization (EI). This technique uses a beam of electrons to remove an electron (or electrons) from a molecule, forming a radical cation. This radical cation is also called the parent ion or the molecular ion.

A radical cation has both a positive charge and an unshared electron.

Insert formula_1

For example, consider the molecule methane (CH4). This molecule will form a radical cation, according the above equation, in the following manner:

Curious about how a mass spectrometer works? Let's take a look!

Suppose that you have a sample of methane (CH4). First, the gas sample will be inserted into the spectrometer. Then, electrons are beamed at the gas molecules, which might break into fragments and create radical cations. These charged particles travel to an ion detector after a deflection by a strong magnetic field that splits the ions based on their mass-to-charge ratio (m/z).

As the magnetic field increases, the ions of heavier isotopes move toward the ion detector. This ion detector is connected to a computer, showing the mass spectrum of the sample. The ion detector can detect their mass-to-charge ratio (which is their atomic mass), and the number of atoms that arrive at the detector for each m/z value.

Did you know that mass spectroscopy has other uses besides determining atomic masses? For example, it is also used to detect the use of steroids by athletes, and to monitor the breath of patients that are under anesthesia!

Atomic Mass Spectroscopy

When the computer reads the collected data from the ion detector, it creates a mass spectrum that tells us the number of components in the sample, the relative molecular mass of each component, and their relative abundance.

A mass spectrum is referred to as a graph showing the signal intensity of the detector versus ion atomic mass.

Let's look at the mass spectrum of natural copper. This mass spectrum shows us the existence of two isotopes of copper. Specifically, it tells us the fractional abundances of both isotopes. So, by looking at the mass spectrum, we can tell that elemental copper is 69.17% 63Cu and 30.83% 65Cu.

Mass Spectroscopy of Elements Mass Spectrum of Copper Atomic Mass SpectroscopyMass Spectrum of Copper (Cu), StudySmarter Originals.

In the mass spectrum graph, m/z could also be written as atomic mass (amu) per charge or mass-to-charge ratio.

If you are still feeling confused, don't stress! We will look at more examples involving mass spectrum graphs in a bit.

Based on your knowledge of mass spectrometry, which of the following statements is true?

a) Data obtained from mass spectrometry tells us the common oxidation states of elements.

b) Mass spectrometry shows the atomic size trends within the periodic table.

c) Mass spectrometry supports the existence of isotopes.

The correct answer choice is C. Mass spectroscopy is used to determine the mass of individual atoms of an element.

Mass Spectrometry Equation

Mass spectrometry explains the relationship between the mass spectrum of an element and the masses of the isotopes of that element. Remember: elements occur in nature as mixtures of isotopes.

Isotopes are atoms of the same element with the same number of protons (atomic number) but different mass numbers (differing numbers of neutrons).

We can use the information from mass spectroscopy to calculate the relative atomic mass of an element, using the equation below:

Inserted formula_2

Let's look at the mass spectrum of neon. Notice that it shows three peaks, or three isotopes of neon: 20Ne (90.9% abundance), 21Ne (0.3% abundance), and 22Ne (8.8% abundance).

Mass Spectroscopy of Elements Mass Spectrum of Neon Mass Spectrometry Equation StudySmarterMass Spectrum of Neon (Ne), StudySmarter Original.

We can use the equation above to find out the relative atomic mass of Neon.

Inserted formula_3

Did you know that mass spectrometry can also be used to identify organic compounds in organic chemistry? During the process of mass spectrometry, the sample gets bombarded with high-energy electrons, causing the fragmentation of the compound. In problems like this, the mass spectrum is a little different. The molecular ion causes the peak at the highest m/z ratio. This peak is known as the parent peak, and it tells us the relative molecular mass of the sample!

For example, the mass spectrum of propanone (CH3COCH3) has a parent peak at a mass/charge ratio of 58.

Mass Spectroscopy of Elements Mass Spectrum of Propanone Mass Spectrometry Equation StudySmarterMass spectrum of propanone, StudySmarter Original.

Mass Spectroscopy: Graphs of Elements

Before looking at some examples of questions related to mass spectrometry that you might see in your exam, let's look at the graphs (mass spectrum) of some elements!

Fluorine and Iodine only have one peak on their mass spectrum. So, both 19F and 127I have 100% abundance. On the other hand, Elemental bromine has two peaks, so it is 51% bromine-79 and 49% bromine-81. Chlorine also has two peak on its mass spectrum: 76% 35Cl, and 24% 37Cl

Mass Spectroscopy of Elements Mass Spectra of Halogens Mass Spectroscopy Graphs of Elements StudySmarterMass Spectra of Halogens, StudySmarter Original.

The mass spectrum of selenium is shown below. Can you guess how many isotopes of selenium (Se) exist? If you said 5, then you are right!

Mass Spectroscopy of Elements Mass Spectrum of Selenium Mass Spectroscopy Graphs of Elements StudySmarterMass spectrum of Selenium, StudySmarter Original.

In your AP exam, you will not be asked to interpret of mass spectra of samples containing multiple elements!

Mass Spectrometry Examples

Now that we know how to analyze a mass spectrum, let's look at some examples of exam questions.

The mass spectrum of a sample of a pure element is shown below. What is the identity of this element?

a) Ga

b) Br

c) Ge

d) W

Mass Spectroscopy of Elements Mass Spectrum of an unknown pure element Mass Spectrometry Examples StudySmarterMass spectrum of an unknown pure element, StudySmarter Original.

Look at the atomic mass possessing the highest relative abundance. In this case, the highest peak is at the atomic mass 184. Then, find an element with an atomic mass closest to 184. Tungsten has an atomic weight of 183.64. So, this mass spectrum corresponds to a sample of Tungsten (W).

Let's solve a different question.

The mass spectrum of a sample of a pure element is shown below. Which of the following statements is true?

a) The most abundant isotope of the element has an atomic mass of 64 u.

b) The most abundant isotope of the element has an atomic mass of 68 u.

c) The avg. atomic mass of the element is 64 u.

Mass Spectroscopy of Elements Mass Spectrum of an unknown sample Mass Spectrometry Examples StudySmarterMass spectrum of an unknown sample, StudySmarter Original.

The highest peak in the mass spectrum tells us the most abundant isotope. So, answer choice A is correct.

Mass Spectroscopy of Elements - Key takeaways

  • Mass spectrometry (or mass spectroscopy) is a method used to determine the atomic mass of atoms/molecules in a sample by ionizing a chemical species and sorting the ions based on their mass to charge ratio.
  • Mass spectrometry explains the relationship between the mass spectrum of an element and the masses of the isotopes of that element.
  • We can use the information from mass spectroscopy to calculate the relative atomic mass of an element.
  • When dealing with samples of a single element, the highest peak in the spectrum tells us the most abundant isotope of that element.

References

  1. AP ® Chemistry COURSE AND EXAM DESCRIPTION (n.d.). https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf ‌
  2. Shelton, M., Princeton Review (Firm, & Penguin Random House. (2019). Cracking the AP chemistry exam. Penguin Random House. ‌
  3. Winter, M. (2017, December 31). WebElements Periodic Table» Copper» isotope data. Webelements.com. https://www.webelements.com/copper/isotopes.html ‌
  4. Theodore Lawrence Brown, Eugene, H., Bursten, B. E., Murphy, C. J., Woodward, P. M., Stoltzfus, M. W., & Lufaso, M. W. (2018). Chemistry : the central science (14th ed.). Pearson. ‌

Frequently Asked Questions about Mass Spectrometry of Elements

Mass spectrum is not calculated. But, it can be generated from a mass spectrometer and used to determine the existence of isotopes in natural elements!

The mass spectrum of an element is sketched/generated by a computer connected to the ion detector in the mass spectrometer. 

A mass spectrum graph is drawn by a computer connected to the ion detector in the mass spectrometer. But, if you know the atomic masses and abundance of each isotope, you might be able to draw your own mass spectrum graph!

Mass spectroscopy is used to determine the atomic mass of atoms/molecules in a sample. It also tells us the relative abundances of the isotopes of an element.

The relative abundance of ions is indicated by peak intensities.

Final Mass Spectrometry of Elements Quiz

Question

Mass spectrometry is a method used to determine the _______ of atoms/molecules in a sample 

Show answer

Answer

atomic mass

Show question

Question

In mass spectrometers that use electron impact ionization (EI)a beam of electrons is used to remove an electron (or electrons) from a molecule, forming a _______, also called a molecular ion.

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Answer

 radical cation

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Question

True or false: A radical cation has both a positive charge and an unshared electron. 

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Answer

True

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Question

A mass spectrometer separates the ions based on their _______ ratio (m/z).

Show answer

Answer

mass-to-charge

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Question

In a mass spectrometer, as the magnetic field ______ , the ions of heavier isotopes move toward the ion detector.

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Answer

increases

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Question

When the computer reads the collected data from the ion detector, it creates a ______

Show answer

Answer

mass spectrum

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Question

True or fase: a mass spectrum that tells us the number of components in the sample, the relative molecular mass of each component, and their relative abundance.

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Answer

True

Show question

Question

How many isotopes are shown in the mass spectrum of elemental copper (Cu)? 

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Answer

2

Show question

Question

Which of the following is the most abundant isotope of Copper (Cu)?

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Answer

63Cu

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Question

Isotopes are atoms of the same element with the same number of protons (atomic number) but different _______

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Answer

 number of neutrons

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Question

The parent peak tells us the _______ of the sample.

Show answer

Answer

relative molecular mass

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Question

How many peaks does fluorine has in its mass spectrum?

Show answer

Answer

1

Show question

Question

Elements occur in nature as mixtures of ______.

Show answer

Answer

Isotopes

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