In 2005, a space probe named Huygens landed on Titan, Saturn's biggest moon. It collected gas samples during its descent through the moon's atmosphere and melted frozen hydrocarbons from the surface after landing. Scientists wanted to learn what elements and molecules were present in the samples. To do this, they used a technique called mass spectrometry.

Mass spectrometry is a way to analyze molecules and determine their mass by measuring the mass-to-charge ratio of their ions. It can also provide information about the molecular mass of a compound and the abundance and masses of isotopes. This process helps scientists better understand the chemical makeup of samples collected from space. If you're interested in space exploration, you might want to learn more about mass spectrometry and its role in uncovering the mysteries of our universe.

What is time of flight mass spectrometry?

Time of flight (TOF) mass spectrometry is a form of mass spectrometry that accelerates positively charged ions to the same kinetic energy. Scientists can calculate the mass of the ions using this kinetic energy and their time taken to travel a fixed distance down the flight tube.

What are the stages of TOF spectrometry?

TOF spectrometry may sound like a complex process, but it actually consists of just four simple stages. These stages are ionisation, acceleration, flight, and detection. Let's take a closer look at each one.

First, in the ionisation stage, the sample is ionised, meaning that it is given a positive or negative charge. This is usually done using a laser or an electron beam.

Next, in the acceleration stage, the ions are accelerated to high speeds using an electric field. This gives them kinetic energy, which is the energy of motion.

Then, in the flight stage, the ions are sent through a flight tube. This tube is usually several meters long and is kept under a vacuum to prevent collisions with air molecules. As the ions travel through the flight tube, they separate based on their mass-to-charge ratio, with lighter ions traveling faster than heavier ones.

Finally, in the detection stage, the separated ions are detected and analyzed. This is usually done using a detector that measures the time it takes for each ion to reach the end of the flight tube. This information can be used to determine the mass-to-charge ratio of each ion, allowing scientists to identify the different components of the sample.

In summary, TOF spectrometry is a process that involves ionisation, acceleration, flight, and detection. By breaking down samples into their component ions and analyzing them, scientists can learn more about their chemical makeup and properties.

Ionisation

When particles enter a mass spectrometer, they are neutrally charged, which isn't useful for scientists. To make them easier to manipulate, the particles are ionized, or given a positive charge. There are two methods for ionizing particles.

The first method is electron impact. This involves vaporizing the sample and firing high-energy electrons at it using an electron gun. This knocks off one electron, forming a +1 ion known as the molecular ion. This technique is used for low mass elements and compounds, but it can cause fragmentation of the molecular ion into smaller particles. The second method is electrospray ionization. In this method, the sample is dissolved and forced through a fine needle attached to the positive terminal of a high-voltage power supply. This causes each particle to gain a proton, forming a +1 ion. This is a gentler technique that rarely causes fragmentation, making it easier to identify the molecular ion. While electron impact can be helpful for determining the structure of the ion, it is a harsh process that can only be used for low mass molecules. Electrospray ionization, on the other hand, is a gentler technique that is preferable for identifying the molecular ion. Understanding these ionization techniques helps scientists better analyze samples using mass spectrometry.

Acceleration

Once the particles are ionized, they are attracted to a negatively charged plate in the mass spectrometer and accelerated to the same kinetic energy. The energy, velocity, and mass of the particles are linked by the equation KE = 1/2 mv^2, where KE is the kinetic energy, m is the mass, and v is the velocity.

By rearranging the equation to make velocity the subject, we get v = sqrt(2KE/m). This means that the velocity is proportional to the square root of the kinetic energy divided by the mass. Since the kinetic energy remains the same for all particles, an increase in mass will result in a decrease in velocity. Therefore, heavier ions will have lower velocities than lighter ions. The ions then enter a flight tube, which is usually several meters long and kept under a vacuum to prevent collisions with air molecules. As the ions travel through the flight tube, they separate based on their mass-to-charge ratio, with lighter ions traveling faster than heavier ones. At the end of the flight tube, the separated ions are detected and analyzed. This information can be used to determine the mass-to-charge ratio of each ion, allowing scientists to identify the different components of the sample. In summary, the kinetic energy, velocity, and mass of the ions are linked. The velocity is proportional to the square root of the kinetic energy divided by the mass, meaning that heavier ions have lower velocities than lighter ions. This principle is used in mass spectrometry to separate ions based on their mass-to-charge ratio and analyze the components of a sample.

Flight

The ions pass through a hole in the negative plate and travel along a long cylinder called the flight tube. During their flight, they spread out according to their velocities and masses.

Detection

At the end of the flight tube in a mass spectrometer, the positive ions hit a negatively charged electrical plate and gain an electron, generating a current. The size of the current is proportional to the number of ions hitting the plate, indicating the abundance of each ion.

Since lighter ions have a greater velocity than heavier ones, they will travel faster and reach the plate before the heavier ions. This allows the ions to be separated based on their mass-to-charge ratio. The entire process of Time-of-Flight (TOF) mass spectrometry is done under vacuum to prevent collisions between the ions and air particles. In summary, mass spectrometry is a powerful analytical technique that allows scientists to separate and analyze the components of a sample based on their mass-to-charge ratio. By ionizing the particles and accelerating them through a flight tube, the ions can be separated and detected based on their mass-to-charge ratio. The resulting data can provide valuable information about the composition and structure of the sample being analyzed. 

What is relative atomic mass?

To calculate the relative atomic mass of an element using the mass spectrum produced by TOF spectrometry, we first identify the peaks on the graph. Each peak represents an ion with a specific mass-to-charge ratio.

Next, we determine the relative abundance of each ion by looking at the y-axis of the graph. We can then calculate the weighted average mass of the isotopes using the formula:

(percentage abundance of isotope A x mass of isotope A) + (percentage abundance of isotope B x mass of isotope B) + ...

This calculation takes into account the abundance of each isotope and its mass, resulting in the relative atomic mass of the element.

For example, let's say we have a sample of carbon with three isotopes: carbon-12, carbon-13, and carbon-14. The mass spectrum would show three peaks, each representing an ion with a specific mass-to-charge ratio.

We determine the relative abundance of each isotope by looking at the y-axis of the graph. Let's say carbon-12 has an abundance of 98%, carbon-13 has an abundance of 1%, and carbon-14 has an abundance of 1%.

Using the formula, we can calculate the weighted average mass of the isotopes:

(0.98 x 12) + (0.01 x 13) + (0.01 x 14) = 12.01

Therefore, the relative atomic mass of carbon is approximately 12.01.

In summary, the mass numbers of elements on the periodic table are not whole numbers because they represent the relative atomic mass of the element, which takes into account the abundance of each isotope. TOF spectrometry can be used to determine the mass-to-charge ratio and relative abundance of ions, allowing us to calculate the relative atomic mass of an element.

Working out relative atomic mass

Let’s go through an example:

A sample of neon gives the following data.

‍

We can see two peaks - one at 20 and one at 22. These represent isotopes that have relative abundances of 90% and 10% respectively.

To calculate the relative atomic mass, we convert the abundance of each isotope to a decimal, multiply it by the isotope’s mass, and add all these values together. So for this sample:

20 x 0.9 = 18

22 x 0.1 = 2.2

18 + 2.2 = 20.2

The relative atomic mass is 20.2.

Mass spectroscopy exam questions

In addition, time of flight spectrometry can also be used to calculate other values such as mass, velocity, time of flight, kinetic energy, and distance travelled of ions or molecules, provided the other values are known. This is done using the equations t = d/v and KE = 1/2mv^2.

When working through calculations, it is important to lay out your workings neatly and follow the process methodically. It is also important to pay attention to units and ensure they are consistent throughout the calculation.

Overall, mass spectrometry is a powerful analytical technique that allows scientists to analyze the composition and structure of a sample. With its many applications and capabilities, it is a valuable tool in a variety of fields, including chemistry, biology, and medicine.

Mass Spectrometry

What is mass spectrometry used for? 

Mass spectrometry is used to find the relative molecular mass of a substance and the abundance of isotopes in a sample. 

How does mass spectrometry work? 

Mass spectrometry works by ionising particles, passing them through a flight tube and detecting their abundance. From there, their mass can be worked out using their speed, the length of the tube and the energy supplied. 

What is MRM in mass spectroscopy? 

Multiple reaction monitoring (MRM) mass spectroscopy is a type of mass spectroscopy, in which a specific molecule is put through the spectrometer twice. The molecular ion first fragments into smaller molecules and some of these molecules are specially selected and then put through the spectrometer again, whilst others are ignored. It is often used to analyse proteins and other biological molecules. 

How do you prepare samples for mass spectrometry?

There are two different methods for preparing samples for mass spectrometry. In electron impact, the sample is vapourised and high energy electrons are fired at it through an electron gun. This knocks off one electron. In electrospray ionisation, the sample is dissolved and forced through a fine needle attached to the positive terminal of a high-voltage power supply, where it gains a proton.

How do you identify compounds in mass spectroscopy?

We can identify compounds by the peaks they produce on spectra produced in mass spectrometry. The peaks show the molecule's mass to charge ratio, which is related to relative molecular mass.

‍