Spectroscopy Series VOL. 15.3: Mass Spectroscopy

On this occasion I want to share with all of you the 4th part of my series of mass spectroscopy that is undoubtedly one of the most used techniques throughout history and that is still part of science in general together to many other techniques that have perfected spectroscopy over the years and the advance of science.

In the previous installments I commented that this powerful technique is in charge of analyzing different materials or compounds of organic, inorganic and biological nature, by means of which we can obtain both qualitative and quantitative information through the distribution of the molecules of various materials and to a substance always depending on its mass. At the same time it also presents 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 this, with only detecting its presence or quantifying its chemical concentration.

In this article I will continue explaining in detail the entire process involved that is needed in a mass spectrometer to study different materials.

In order to follow the sequence of the previous publications, it is convenient to make a brief summary so as to be able to understand more efficiently the topic to be discussed.

Mass spectroscopy is based specifically on obtaining ions from different molecules within the sample that we want to study, most of these molecules within the sample are organic matter with a phase in a gaseous state that tend to move in presence of a magnetic field. After obtaining the ions from the molecules inside the material, we proceed to separate their mass and elemental charge, where finally and thanks to a specialized detection equipment we can observe the spectrum.

Now, let's continue with the sequence of the previous publications. In the following diagram we can see how it bases the equipment used for the analysis of materials by means of mass spectroscopy (spectrometer).

^{Diagram of a mass spectrometer}

Let's refresh the following:

The first element or component of the spectrometer is the sample introduction system, whose objective is to cover the amount of sample of the material that you want to analyze, it should be noted that the proportion of sample that must be introduced in the system must be in an order a micro or maybe less. It is also important to mention that 3 methods are used to introduce samples which are: Direct, indirect and from a chromatograph.

The second element is the ionization source, responsible for converting the sample into ions through the bombardment of electrons. Another option that is used a lot for the transformation of ions is through thermal or electrical energy. We have different sources of ions which are: Ionization through electronic impact and chemical ionization.

Third element corresponds to the Analyzer, is responsible for separating the ions based on the mass and load of the material analyzed. Important to note that is expressed by the following equation:

R = m/Δm

There are several types of analyzers which are: Analyzer that uses magnetic field, Quadrupole Analyzer, Flight Time Analyzer and Ion trap analyzer.

And finally in my last article write about the Detector, as everyone knows the function of this component is to capture the signal from the analyzer. In this article, mention some types of detectors used in the mass spectrometer, among which are: Electron multiplier, Faraday cup, Channeltron detector and the multichannel detector.

All this information has been explained in detail in the three previous installments which you can see in the following links:

Vol.15

Vol.15.1

Vol.15.2

Vacuum system

We all know the importance of a vacuum system, since, if we do not have a total vacuum inside a certain measurement equipment, in this case the spectrometer, it is impossible to satisfactorily perform the analysis or characterization of various materials. So that we can carry out a perfect measurement we must have a total vacuum in the mass spectrometer, it is recommended a vacuum of 10exp-5torr so that the path of molecular ions is free without any interference in the path that leads to the detector.

In the mass spectrometer, two types of vacuum systems are used, one of which is the oil diffusing pumps and the other the turbomolecular pumps.

The oil diffuser pump, it is a container where the lower part contains oil that must be subsequently heated with the help of a multistage injector that provides the appropriate electrical current so that it can produce the necessary boiling to be able to ascend through a sort 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 denser oil molecules, where it condenses to fall back to the initial deposit where the oil was before the boiling process .

During the process the molecular ions that are the lower part of the pump called "low pressure zone", until the top of the pump where the molecules are condensed, the oil tends to trap the gas molecules present in the system gradually reducing the pressure. This cycle is repeated repeatedly, then the oil is divided, leaving the most volatile part outside and the densest part inside the container.

This type of pumps has great advantages, apart from its easy handling, they have a very long lifetime, the spare parts are economical. However, it has some drawbacks due to the fact that oil is used for its operation, this can cause certain spills in the spectrometer which immediately causes damage to the components of the equipment and consequently errors in the measurements and interpreter of results.

Turbomolecular pump: this is a kind of turbine composed of several blades which rotate the rotor inside, they are characterized by moving at impressive speeds approximately 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.

Another very important detail in this type of pumps is that the speed of the blades of the rotor axis must comply with the same order as the thermal agitation speed of the molecular ions, that is, the initial pressure must be very low as the speed of travel of the molecules are superior to the distance of the rotor axis, with all this we can reach a good molecular flow and results in an efficient molecular pumping.

The advantages are very good, this type of pump performs an efficient vacuum without any margin of error, is easy to control, and does not cause external damage to the measurement equipment, however this type of vacuum systems are extremely expensive.

^{Turbomolecular pump scheme, courtesy of wikipedia. Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0).}

Data logging system

As its name indicates, it is responsible for storing all information supplied in the sample sweep, these are recorded and stored on a computer, to later perform its 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.

Mass spectrum

This provides important 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 accurate information based on the mass and charge of the material.

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

The peak of the spectrogram that appears with the highest value of m / e corresponds to the ionized molecule without fragment and is called the master mass. This mass pattern allows us to determine quickly and accurately the molecular mass, provided that it is operated with an ionization voltage not excessively high, which would produce the total fragmentation of the molecule.

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.

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?

SpectrophotometryUNDERSTANDING ESSENTIAL OIL DIFFUSERS

The Working Principle Of Turbo Molecular Pump

turbo molecular pump

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