Can ionized particles be collected?

mass spectrometry

In this post you will learn what a mass spectrometer is and the different types used in mass spectrometry.

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Mass spectrometer simply explained

Mass spectrometry is a method for determining the mass of atoms and molecules. You can download the device responsible for this as a mass spectrometry describe.

A mass spectrometer basically consists of three components: the ion source, the analyzer and the detector.

You can determine the mass of the particle if the charge is known about following formula determine:

A mass spectrometer is a device used to determine the mass of atoms or molecules when the charge is known. The result is what is known as a mass spectrum. This is a graphic representation of the intensity of the ions as a function of the mass-to-charge ratio.

Mass spectrometry

With mass spectrometry you can imagine a process with which you can determine the mass of atoms and molecules.

More precisely, you determine that first Mass-to-charge ratio () of particles. The mass-to-charge ratio is the reciprocal of the specific charge ().
At known charge you can now infer the mass.

You can also determine the number of particles with a known mass-to-charge ratio.

Mass spectroscopy is used, for example, in the following areas:

  • archeology: Analysis of isotope ratios in bones. This allows conclusions to be drawn about a person's diet, among other things.
  • biology: In biology, mass spectroscopy is used in so-called proteomics (= research into proteins in living beings) and in metabolomics (= research into metabolic properties).
  • chemistry: Analysis to determine chemical elements or compounds. Qualitative (detection of unknown substances) and quantitative (amount of a substance in a compound) are possible.
  • physics: Measurement of the isotopic composition of chemical elements and determination of the mass of atomic nuclei.

As Result the mass spectrometric Investigations you get a so-called Mass spectrum. You can imagine this as a graphic representation of the intensity of the ions (= number of impacting ions) depending on the mass-to-charge ratio.

The y-axis describes the absolute intensity the ions and the x-axis the mass-to-charge ratio. The measured intensity of the signals are summarized in a line (peak).

Mass spectrometer setup

The measuring device within mass spectroscopy is referred to as a mass spectrometer or also a mass spectrograph or mass spectroscope.

A mass spectrometer basically contains the following three components called ion source, analyzer and detector.

Ion source

The ion source serves to ionize the sample to be examined (= Analyte). Ionization is the process in which electrons are removed from an atom or molecule and usually a positively charged ion (cation) remains. However, negatively charged ions (anions) can also be formed.

A wide variety of methods exist for ionization using the ion source. A method often used in practice is the so-called Electron impact ionization (EGG).

Here the sample molecules to be ionized are first transferred into the gas phase. After that, an electron beam passes through one electric field accelerates and hits the incoming sample molecules.
When the electron beam and the sample molecules meet, electrons are "knocked out" of the neutral molecules (= ionization). The resulting radical cation then usually breaks down into smaller mass fragments, one of which remains charged.

Other important Methods For example, chemical ionization (CI), field ionization (FI), field desorption (FD) or electron spray ionization (ESI) are used to ionize the sample molecules.

After the ionization, the ions are extracted from the ion source with an electric field and are often transferred to the ion source by applying a voltage Analyzer to hand over.

Analyzer

In order to analyze the mass precisely, the ions must have the same direction and speed.

This is what the so-called analyzer or Mass selector responsible. It selects the ions according to their mass-to-charge ratio () and then forwards it. He works on the principle of one Speed ​​filter or also called Wienfilter.

The analyzer often consists of one Plate capacitor and one magnetic field . The capacitor has a positively charged capacitor plate on top and a negatively charged capacitor plate on the bottom. The magnetic field is pointing from the observer path.

When the ion runs through the capacitor, it works Lorentz force () up and the electric force () downward. In order for the particles to fly straight through the velocity filter, these two forces must be equal ().
The ions that are flying now must have one uniform speed and direction have.

detector

Finally, the ions are directed into the so-called detector with the same speed and direction.

Most detectors have a magnetic field that points away from the observer again.

The basic principle of most detectors is based on the fact that the ions fly through a screen into the detector. Here they are in a Circular path distracted and hit the screen again after a semicircle. Positive ions are after above deflected and negative ions after below. The radius of this orbit gives you information about the mass of the ion.

Important Examples for a detector are the Faraday interceptor, the photomultiplier, the Daly detector and the secondary electron multiplier (SEV).

Mass spectrometer functionality and evaluation

The most important requirement for determining the mass of a particle is that you have its charge know

As already said, in order for the particles to fly straight through the speed filter, they must have an equal Lorentz force and an electrical force.

The Lorentz Force stands for the force on moving charges in magnetic fields. It is perpendicular to the direction of movement. The electric force is the force between two point charges in the electric field. It is perpendicular to the plate surfaces.
The Lorentz force is made up of the magnetic flux density, among other things together. It tells you how strong a magnetic field is. It always works in a certain direction and is therefore a vector. stands for the size of the charge on the moving particle and for the speed of this particle.

The electric force is also composed of the charge and additionally from the strength of the electric field together.

The speed of the particles inside the analyzer results from:

To determine the mass of the particles in the detector, you have to use the Lorentz force () with the centripetal force () equate. That is the force that forces a body onto a circular path.

Now you equate these two forces:

To get the specific charge () of the particle, you can use a from the equation and cut through share:

The mass-to-charge ratio would now be the reciprocal of the equation:

Multiplied by you now get the formula for the mass of the particle:

Now you can for Insert the above formula. However, you have to keep in mind that you are looking for speed the value for from the first magnetic field ():

Mass spectrometer types

You can differentiate between the different types of mass spectrometer according to the analyzers used.

Single particle mass spectrometer

The Single particle mass spectrometer examine the analytes in real time.

It consists of an inlet system, a laser pulse, an analyzer and a detector. The particles to be analyzed can be taken into a vacuum chamber through the inlet system. The laser pulse ionizes the particles, whose mass-to-charge ratio you can determine in the analyzer. Finally, you can measure the particle size in the detector based on the airspeed.

Time-of-flight mass spectrometer

The Time-of-flight mass spectrometer can determine the mass-to-charge ratio by measuring the flight time. The ions are accelerated in an electric field and travel the flight path .

When entering the analyzer, all ions have the same energy, which means that light ions move faster than heavy ions. As the name suggests, this so-called time-of-flight analyzer measures the time of flight the analyte. This flight time is proportional to the mass-to-charge ratio ().

Ion trap mass spectrometer

The Ion trap mass spectrometer, which is often related to the Gas chromatography is used.

The ions are collected and stabilized by a cooling gas (e.g. helium). Then an electromagnetic field keeps the ions in a certain area so that they can be analyzed. If a voltage is now applied, ions with a certain mass are made unstable and transported into a detector (electron multiplier).

Quadrupole mass spectrometer

The Quadrupole mass spectrometer has a so-called electric quadrupole as an analyzer. This consists of four parallel rod electrodes that have two equally large positive and negative charges.

The ions are accelerated by an electric field and land in the quadrupole. In the alternating field of this quadrupole, the ions are selected according to the mass-to-charge ratio so that only particles with a certain mass can pass through the field.

Sector field mass spectrometer

At the Sector field mass spectrometer the ions are deflected in electric and magnetic fields.

In the detector you can determine the mass of the particles from the radius of the circular paths, which depends on the momentum in the magnetic field and the energy in the electric field.

Tandem mass spectroscopy

The Tandem mass spectroscopy or MS / MS is an extension of classical mass spectroscopy. You can use it to examine the composition and structure of a sample.

Within tandem mass spectroscopy, the mass spectrometer two Analyzers. The ions are selected in the first analyzer. These ions then undergo a reaction, the products of which are examined in the second analyzer.

The most common analyzers in tandem mass spectrometry are ion traps or quadrupoles, among others.