Spectroscopy Series VOL. 15.1: Mass Spectroscopy

Greetings again friends, lovers of science, today I bring you the continuation of my previous delivery where he spoke about mass spectroscopy, one of the oldest techniques and that is still part of science in general, despite the large technological advances that have been presented in recent years.

Let's remember the following: This technique is responsible for analyzing different materials or compounds of organic, inorganic and biological nature, and thanks to these studies obtain qualitative and quantitative information through the distribution of the molecules of various materials and 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 material analyzed 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.

I mentioned in my previous submission that the physical foundation of this technique is based on obtaining ions from different molecules within the material that you want to analyze, these molecules are mostly organic in phase of gaseous state that move in the presence of a field After obtaining the ions of these 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.

To obtain more complete and detailed information on these important aspects of mass spectroscopy, I recommend you read this article Vol.15 and so you can more easily understand this new delivery.

In this case the sequence of the previous article will continue where in the following diagram shows the basis of the mass spectrometer. In this post I will be in the Ion Source, and now I will explain the Analyzer.

^{Diagram of a mass spectrometer}

Continuums talking about the process on which mass spectroscopy is based.

Summarizing a little about the first two components of the mass spectrometer, we said that the input system is responsible for entering a certain amount of sample of the material in the spectrometer. This amount must be very small in the order of a micro or maybe less. And at the same time 3 methods are used to introduce samples that are: direct, indirect and chromatography.

Then we talk about the source of ions, which is responsible for converting the sample into ions already by bombarding electrons or photons to the material. Another option that is used a lot for the transformation of ions is through thermal or electrical energy. And this component uses similarly some ionization methods such as; ionization by electronic impact and chemical ionization.

Now, to explain the next steps on which mass spectroscopy is based, take important information from wikipedia, which helps to understand this technique much better.

Let's start talking about the Analyzer.

This component has the function of separating the ions from the material as a function of their mass and their charge, which can be expressed through the following equation:

R = m/Δm

<p<we can="" explain="" this="" equation="" in="" the="" following="" way;="" if="" we="" have="" a="" spectrum="" and="" observe="" each="" peak,="" <strong="">Δm would be the difference of the masses between two adjacent peaks and m would be the average of the mass between these two peaks, that is to say to count all the peaks between the two adjacent ones and to add the average of all.

It is important to mention that if we have two peaks, these can be considered separated if the height between the valleys of these peaks does not exceed 10% of height.

^{This is a simple demonstration of a mass spectrometer signal, where the peaks do not exceed 10% of their height between valleys.}

In Wikipedia we find the following information regarding the analyzer of a mass spectrometer:

^{Schematic of a magnetic sector mass spectrometer. Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)}

^{Schematic of an elecric sector for a mass spectrometer. Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)}

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.

We can explain this part as, for example, if we have analyzers of a mass spectrometer, most of these components use this equation to determine this charge / mass ratio of a material.

The output of the ion source needs a mixture to be able to separate these ions and thus be able to detect them individually. Different methods of analysis are used for this procedure, such as a quadrupole, magnetic, electrical analyzer, one of the most used that is the chromatography to identify gases and also a special ion trap that allows an efficient detection.

The analyzer that uses magnetic field, these work with a kinetic energy driven at a very high speed due to the electric field in which the ions of the material have been subjected, this happens after having left the ionization chamber.

These are also characterized by using an electromagnet that helps to disperse all the ions of the material, whose purpose is to be able to determine the charge/mass ratio.

The magnetic analyzers have a small slit whose purpose is to isolate the ions so that they can obtain an accurate trajectory that can direct them towards the detector.

^{Diagram of a mass spectrometer analyzer}

The diagram above shows that in order for us to have a fixed magnetic field and potential, the particle must present a curve, otherwise it would be practically impossible to detect the ions of the material. To be precise, the potential must be varied gradually and the intensity of the magnetism so that the masses can create a perfect curve and be able to detect them all.

In most mass spectrometers that use a magnetic analyzer, the operators try to squeeze them to the maximum, that is to say they use all their potential to be able to detect well the masses, it is the only way for these equipment to show an appropriate sensitivity at the time of the sweep of the samples, with this in turn improves its resolution exponentially. As mentioned before, the resolution power of a mass spectrometer is measured according to the ratio of its load / material mass R = m / Δm.The resolution of these analyzers has the ability to measure the nominal mass of an ion.

The Quadrupolar analyzer is another of the analyzers used in the mass spectrometer, it is characterized by having some species of circular metal bars with a radius of approximately 2 cm and 12 cm long. These are parallel to each other, the idea that these bars are hyperbolic or cylindrical is so that the ions of the source can impact exactly on the center of the analyzer, they work with great precision in this circumference. These 4 bars are the heart of the spectrometer, the ions must accelerate very fast in the space between the cylindrical metal bars, making a discard of ions and solos the strongest ones reach the detector.

^{Diagram of a quadrupole analyzer of a mass spectrometer}

This type of devices do not need a magnetic field to be able to perform the sweep and thus obtain a dispersion of c/m.

Since they need electric field to perform their sweeping, they are widely used for various materials since the measurements can be obtained quickly, in an approximate time of 0.01 seconds, however this type of analyzer does not need slits to be able to focus the Ions and this would be a great disadvantage since the team becomes very sensitive to small errors that can distort the spectra.

Although it also has great advantages over other analyzers and can be extrapolated mass with great ease through the spectra of different materials.

And its main disadvantage is that it presents little resolution with respect to other analyzers.

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.

To separate the ions according to their mass, this is produced in the way they go to the detector of the spectrometer, which is at the end of the analyzer tube. Although this type of analyzer presents a small disadvantage and is that the time in which the ions take to reach the detector occurs very quickly, ie in small seconds which requires a high detection speed. However, it also has its advantages and is its simplicity, easy assembly of its components and access to the source of ionization. The analysis is super fast.

^{Diagram of a flight time analyzer of a mass spectrometer}

Finally to conclude this section of mass spectrometer analyzers we have the Ion Trap Analyzer, this is used specifically for the identification of gases, by means of a chromatographic detector. It is very similar to the quadrupole analyzer only presents small modifications such as the creation of an electromagnetic confinement zone that are generated through different frequency signals.

This type of analyzer the anions and cations of the gases are isolated for long periods of time due to the movement of the electric or magnetic field.

^{Diagram of an ion trap analyzer of a mass spectrometer}

Its composition is simple, it contains 3 electrodes in a hyperbolic surface, the function of these electrodes is to ionize, fragment and analyze during the whole process of scanning the sample. To these is applied a potential of radiofrequency that give rise to a quadruple tridimensional electromagnetic field in which the ions are enclosed, that is why it is called a trap, since its main function is to trap them without any output.

Do not miss my next installment where I will continue talking about this fascinating world of mass spectroscopy