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!
- This article is about 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.
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.
Fig. 2: Mass Spectrum of Copper (Cu), Vaia 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:
$$A_{r}=\frac{\Sigma (isotope\ mass\cdot isotope\abundance)}{100}$$
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).
Fig. 3: Mass Spectrum of Neon (Ne), Vaia Original.
We can use the equation above to find out the relative atomic mass of Neon.
$$A_{r}=\frac{\Sigma (isotope\ mass\cdot isotope\abundance)}{100}$$
$$A_{r}\ of\ neon\ (Ne)=\frac{(20\cdot 90.9)+(21\cdot 0.3)+(22\cdot 8.8)}{100}=20.2$$
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.
Fig. 4: Mass spectrum of propanone, Vaia 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
Fig. 5: Mass Spectra of Halogens, Vaia 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!
Fig. 6: Mass spectrum of Selenium, Vaia 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
Fig. 7: Mass spectrum of an unknown pure element, Vaia 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.
Fig. 8: Mass spectrum of an unknown sample, Vaia 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
- AP ® Chemistry COURSE AND EXAM DESCRIPTION (n.d.). https://apcentral.collegeboard.org/pdf/ap-chemistry-course-and-exam-description.pdf
- Shelton, M., Princeton Review (Firm, & Penguin Random House. (2019). Cracking the AP chemistry exam. Penguin Random House.
- Winter, M. (2017, December 31). WebElements Periodic Table» Copper» isotope data. Webelements.com. https://www.webelements.com/copper/isotopes.html
- 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.
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