Spectroscopy Series VOL. 15.4: Mass spectroscopy summary

Greetings friends...

We must all know that it is one of the oldest spectroscopy techniques, but even in spite of great technological advances and more sophisticated and easy-to-use techniques, this technique is still widely used by scientists in different areas. Undoubtedly, this technique is the forerunner of the most current and is still the consent of many.

First of all we must understand the meaning of mass spectroscopy, that is, what is this technique?

It is an analysis technique that allows to identify various materials or compounds of organic, inorganic and biological nature. Through it you can obtain both qualitative and quantitative information. Mass spectroscopy allows to determine the distribution of the molecules of different materials and to be a substance always depending on its mass. It also has the possibility of obtaining very effective information of the molecular mass of the analyzed material and from here it is possible to extract information from the atomic structure of the material, just to detect its presence or quantify its chemical concentration.

What is the physical basis of mass spectroscopy?

In obtaining ions by means of the molecules of the material to be analyzed, these molecules are largely organic in nature, with a phase in the gaseous state that can be moved when they are in the presence of a magnetic field. When obtaining the molecular ions of the material the next step is to separate its mass according to its elementary charge and then go in the direction of a detector where it captures the signal and translates it into a spectrum.

What is a mass spectrum?

The spectrum is a graphic pattern that shows us the mass / charge ratio of the material that we want to analyze by means of chemical analysis in the mass spectrometer. On the other hand, the spectrum gives us two-dimensional information of the ionic phenomenon involved in this process, which is represented by a process that involves certain parameters of these ions that give precise information as a function of the mass and charge of the material.

What can be obtained using mass spectroscopy?

Very important information about the structure of the analyzed material, as well as information about molecular samples that are quite complex to analyze, is the great advantage that this technique possesses with respect to others. Also according to the qualitative and quantitative analysis we can obtain the chemical composition of the material correctly interpreting the mass spectrum.

In each publication referring to this topic I shared a schema that was the fundamental part in the course of each of my articles, it is from this scheme that I took my departure to explain the essential foundation of this technique, where I explained step by step each of the steps to follow in order to perform a complete analysis through the mass spectrometer.

^{Diagram of a mass spectrometer}

It is important to mention the following:

A mass spectrometer must have the ability to vaporize very volatile substances, and in turn must have the ability to create ions from molecules in a gaseous state. After passing this stage and having generated the ions, it must fulfill the most important task that is to separate the ions according to the mass / charge ratio of the material. And finally, after vaporizing the ions and separating them from the previous relationship, detecting and forming them must record the information and translate it into the mass spectrum.

We will summarize each of the stages mentioned in the previous scheme

1. Input system: as its name indicates, its function is to introduce the sample of the material that you want to characterize, something very important that we must keep in mind is that for mass spectroscopy a very small amount of sample must be introduced into the system . It is also important to point out that this technique has certain limitations when it comes to vaporizing the sample, ideally it has an approximate pressure of 10exp-6 mm of mercury to obtain a visible spectrum.

Two methods of sample introduction are used; the first is directly, which consists of introducing the sample of the material that you want to analyze directly in the ionization source, this is done carefully with a kind of metal tube, where the sample is held at the tip.

The second method is to vaporize the sample outside the measuring equipment. It is advisable to use a fully enameled glass container inside to help maintain the indicated temperature, that is, the enamel acts as a powerful thermal insulator.

2. Source of ionization: responsible for converting the sample into ions and by means of the bombardment of electrons or photons towards the material. Another option that is used a lot for the transformation of ions is through thermal or electrical energy.

Some ionization methods for the samples are ionization by means of electronic impact, where the samples are ionized through the bombardment of electrons with high energy, to be able to originate the ionization, electrons are used with an incandescent filament very similar to that of a light bulb, these they emit a thermoelectric energy and they accelerate due to the difference of voltage variable potential.

It is important to note that for this type of ionization to occur, it is necessary to create a parallel magnetic field in the direction of the electron's trajectory, in order that they can focus correctly and can describe a helical path until the anode finally arrives. .

Chemical ionization is another method that is used to be able to ionize the sample in the spectrometer, in this an ionizing material is normally used that can convert the ion and transfer its charge to the molecules of the material due to a molecular chemical reaction.

We must introduce a methane in the source of ionization and produce a determined pressure around 1 to 1.5 mm of mercury, so that the reaction can take place, thanks to the fact that the electrons essentially ionize the methane molecules within the sample.

3. Mass analyzer is one of the most important components of the spectrometer whose function is to separate the ions from the material in relation to their mass and their charge, which can be expressed through the following equation:

R = m/Δm

We can explain this relationship in the following way, for example if we have a mass spectrum and within it we observe each corresponding peak side by side, Δm would be the difference between one peak and another, that is two adjacent peaks where m is the average mass between these two peaks, therefore count all the peaks between the two adjacent ones and add the average of all.

The mass analyzer is the most flexible part of the mass spectrometer. The different types of mass spectrometers vary according to the analyzer they have. In the case of the magnetic section spectrometer, it uses an electric or magnetic field to affect the trajectory or velocity of the charged particles in a certain way. The force exerted by the electric and magnetic fields is defined by the Lorentz force:

it's the electric field vector,

it's the magnetic field vector,

is the particle charge,

is the velocity vector and

symbolizes the product vector.

This means that most analyzers of a mass spectrometer use this equation to be able to determine this charge / mass ratio of a material.

There are several types of analyzers among which we have:

Magnetic field analyzer, work with kinetic energy that is driven at a high speed thanks to the electric field in which the ions are subjected, all this happens after having passed through the ion chamber. They are characterized by having an electromagnet that disperses each ion of the material to then determine the ratio c / m. They also have a slit that helps isolate the ions so that they can obtain an accurate trajectory that can direct them towards the detector.

Quadrupole analyzer, known mainly for having 4 circular metal bars, said bars are parallel to each other. The function of these bars is that the ions can have an impact directly on the center of the analyzer. Then the ions must be accelerated very quickly in the space between the cylindrical metal bars, making a discard of ions and alone the strongest ones reach the detector.

Flight time analyzer, the ions are produced by the impulse of electrons through a source that by means of an electrical potential generates the ions that are bombarded to the sample, said ions provide energy packages. The speed of these is acquired by each ion that is inversely proportional to its charge and mass. The analyzer must have a length and the time in which the ion takes to cross the analyzer is also measured.

Ion trap analyzer, is the most used for the analysis of gases through a chromatographic detector, is similar to the quadrupole but with some small modifications and one of them is the electromagnetic confinement area that is generated by means of frequency signals . It is also important to say that the anions and cations of the gases are isolated for long periods of time due to the movement of the electric or magnetic field.

4. Detector: we know perfectly well that the function of this component is to capture the signal coming from the analyzer, there is a great variety of detectors that adapt to the needs of the operator or person who wishes to acquire it. The first ones used were the mass electron multipliers that operated at a fairly high vacuum inside the spectrometer chamber.

We can say that when the current transmitted especially from the source ions that leave the analyzer reach the detector, they come with a very small intensity, which is quite complicated when detecting these ions due to their tiny size, as mentioned in my post previous analysis and detection must be done very quickly so that we can take accurate data from the sample sweep.

Some of the types of detectors are the following:

Multiplier of electrons, for its proper functioning it is necessary to provide it with an energy strong enough for the ions of the material to impinge on an initial plate covered with oxidized metal and then go to another plate where the ions emit another type of current directed towards a third plate inside the multichannel detector. Finally emits current again to go to the 4th plate, this process is repeated repeatedly, and is named multiplier for the multiple plates it contains.

Faraday cup, used specifically for the analysis of different types of gases, this detector has a cup or box shape, which is why it bears his name. When the ions coming from the analyzer impact to be able to detect them, they are neutralized due to the transfer of electrons that emit the signal in the electrode, this signal is always analogous to the first current of the ions.

Detector "Channeltron", is composed inside by a material covered by lead oxide, this material has to present the characteristics of a semiconductor for the conduction of current and resistivity of the same.

When detecting positive ions, you must of course apply a negative potential in the upper part of the tube, this in order that the ions can be diverted from their path to a plate that expels the positive potential and then can be attracted to the inside the tube to then interact internally and thus produce an emission of electrons that are directed to the end of the tube, where it presents a low potential near the earth with a continuous gradient that comes from the top to the bottom of the tube.

A considerable advantage that this detector presents with respect to the aforementioned, is that it can be exposed to a high atmospheric pressure, taking into account that voltage is not injected into the component, which allows to break the vacuum easily without presenting any inconvenience. Another advantage is that you need minimum vacuum to make the measurements.

Multi-channel detector, there is a wide variety of this type of detectors, the most used comes with batteries that is made up of several layers, has a small detection system that is also very effective to capture the signal from the analyzer. Its advantage lies in the ability to detect signals simultaneously in a fairly broad spectral range, so it is perfect for us to locate each ion and detail in the spectral line and be able to amplify the signal in an extraordinary way.

5. Vacuum system: for an efficient characterization it is necessary to be able to have an excellent vacuum, and this is not only in the mass spectrometer, but in any measurement equipment that requires a vacuum in order to function correctly. In general, the mass spectrometer requires an approximate vacuum of 10exp-5torr so that the path of the molecular ions is free without any interference in the path that leads to the detector.

We have two types of vacuum system in the mass spectrometer which:

Oil diffuser pump, It is a container in which the bottom contains oil that must be subsequently heated with the help of a multistage injector that provides the appropriate electric current so that it can produce the boiling necessary to be able to ascend through a kind of circular and elongated tubes until finding an exit space. All this process is thanks to the gas molecules in the process are dragged to the inside of the pump due to the impulse of the more dense oil molecules, where it condenses to fall back to the initial tank where the oil was before the boiling process.

Turbomolecular pump, is a turbine that is made up of several blades whose function is to rotate by means of the rotor, they move at very high speeds (80,000 revolutions per minute).

The principle of operation of these pumps basically consists in that the gas molecules when hitting the rotating shaft vanes acquire a high pumping speed, this makes the vacuum effectively maintain inside the spectrometer. The molecular ions that are driven by the pump rotor have to reach the paddles before colliding with the other molecule so that it can deflect its direction.

5. Record of obtained data: in charge of storing all information supplied in the sweep of the sample, these are recorded and must be stored in a computer, to later perform their corresponding analysis of each spectrum peak. There are different specialized programs that have a wide spectrum library with which a qualitative analysis of the spectrum obtained can be made and a comparison can be made with other materials included in the library.

6. Mass spectrum: provides us with two-dimensional information of the ionic phenomenon involved in this process, which is represented by a process that involves certain parameters of these ions that give precise information as a function of the mass and charge of the material.

^{Mass spectrum. Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0).}

With a spectrum it is possible to determine peaks of different phases of the compound, the relative intensity, comparison of one spectrum with another, you can also deduce the proportion of each element contained within the sample.

6. Some applications of mass spectroscopy

Identify and structurally determine pure compounds. All these through their molecular mass, isotopic distribution patterns, determination of molecular structure.

To be able to analyze organic compounds from very complex samples.

Analyze elementally different compounds.

Determine your molecular formula.

Materials through their pattern fragmentation can be identified.

And speaking specifically we can say that it is applied in the pharmaceutical industries, also to identify people's drug abuse through blood and urine tests. Control the gases of people suffering from respiratory diseases at the time of surgery. In the archaeological area to identify different specimens. Identification of polluting substances in the environment, especially in the air, among others.

7. Advantages of mass spectroscopy

It allows to work with samples of different sizes.

Being able to obtain results in a record time is one of the great advantages that this technique possesses with respect to others that take several hours to analyze a material.

Performing qualitative and quantitative analysis is also another great advantage since the technique is very complete and covers a very broad study.

Its simplicity is one of the most outstanding characteristics of this technique.

You want to know more about my spectroscopy series visit the following links:

Vol.1 Vol.2 Vol.3 Vol.4 Vol.5 Vol.6 Vol.7 Vol.8 Vol.9 Vol.10 Vol.11 Vol.12 Vol.13 Vol.14 Vol.15 Vol.15.1 Vol.15.2

If you want more information about the subject you can visit the following links:

Mass spectrometry/ Wikipedia

Mass Spectrometry/ Prem,ier Biosoft

THE MASS SPECTROMETER

Analytical chemistry/ New world enciclopedy

How Does Mass Spectroscopy Work?

Plasma Atomic Emission Spectroscopy

Gas Chromatography

Chemical ionization/ Wikipedia

The Ionization of Atoms by Electron Impact

The Working Principle Of Turbo Molecular Pump

turbo molecular pump

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