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HM Mass Spectrometer - Basics

Poster HM Mass Spectrometer Design (pptx)

CHAPTER 1: Basics 

 

Mass Spectrometer Principle

 

 

Ionization Source

The ionization source is a device that can be used in several methods and techniques to ionize molecules. The method chosen depends on the type of the sample, such as gas, liquid, or solid. Since our project focuses on toxic gases, electron impact ionization is the suitable method in our case. [1]

The sample should be in a gaseous phase to enter the ion source. In this situation, we use a heated filament made of tungsten to collide with the gaseous sample molecules and generate positive ions. Since it is much more difficult to remove more than one electron, most of the ions will have a charge of +1.

It is important to note that the ion source must be evacuated to prevent contact with air molecules. [2]

                                          Figure 1

 

Acceleration Zone

The positive ions are pushed out by the ion repeller, which is a positively charged plate (see Figure 1). Subsequently, they pass through the acceleration zone, where they are subjected to an intense electric field specifically designed to accelerate them and increase their kinetic energy.

The acceleration zone is created by three plates that are gradually charged negatively, with the first disc being strongly negative (300 V), generating a powerful electric field that propels the positive ions towards the last disc.

As the ions progress towards the subsequent discs, they continue to be accelerated by the electric field until they reach the final disc, which is neutral and acts as a ground (see Figure 2).

The precise control of the electric field between the discs ensures optimal focusing of the positive ions, preventing any undesired dispersion or deflection. An intermediate disc, also negatively charged (150 V) but at a reduced intensity compared to the first disc, ensures a gradual acceleration of the positive ions. This stabilization in their trajectory allows them to pass to the third disc, which acts as a final barrier, enabling the positive ions to reach the region of the mass spectrometer dedicated to their analysis.

We should note that all positive ions will have the same kinetic energy after acceleration. [3]

                                          Figure 2

 

Deflection Zone

The deflection zone in a mass spectrometer is a crucial step for separating ions based on the mass-to-charge ratio (m/z), allowing us to identify each chemical species in our sample.

The ions emerging from the ionization source enter a region with a uniform magnetic field, causing them to experience a magnetic force that deviates them from their initial trajectory.

Ions with higher m/z ratios will experience less deflection compared to ions with lower m/z ratios. This difference enables us to separate ions based on their masses. Since most ions carry the same charge of +1, the extent of deflection will be determined by the mass of the ions. [4]

The path of the ions is typically curved with a radius R. This trajectory is governed by either Biot-Savart's law or the Lorentz force. [5]

In other words, the heavier the ions, the less they deflect. Furthermore, they will require a stronger magnetic field to deflect them (see Figure 3).

                                          Figure 3

 

Detection of Ions

Within the field of mass spectrometry, the detection phase is crucial as it involves identifying ions that have passed through the mass analyzer's path.

The detector plays an essential role in converting ions into a detectable and usable signal.

Once sorted by the mass analyzer, the ions enter the detection zone where the detector is located. This component is designed to capture the physical characteristics of the ions, including their kinetic energy, electric charge, and mass. Mass spectrometers utilize a variety of detector types, each with unique advantages tailored to the specific requirements of the analytical application. Furthermore, the selection of the detector is influenced by the design of the instrument itself. [6]

When an ion reaches the collector, its charge is neutralized by an electron transferring from the metal to the ion. The flow of electrons generated in the wire is identified as an electric current, which can be amplified and recorded. The level of detected current increases with the increasing number of arriving ions.

We should note that our detector is finished and ready to be tested (see Figure 4).

                                           Figure 4

 

 

Electromagnet

 

Introduction

As mentioned earlier, we cannot deflect ions without a magnetic field, and it must be strong enough to deflect the heavy ions emitted from the toxic gases.

The previous tests indicate that the Helmholtz coil is not effective in generating a strong magnetic field (0.2 T). [3]

Based on the famous formula of the magnetic field 

B=μnI

In the equation, B represents the magnetic field magnitude in Tesla, μ stands for the permeability in Henry per meter, n denotes the number of turns per meter, and I represents the electrical current in Amperes.

 

Magnetic Permeability

 

 

Purification of Iron

The purity of iron is an important factor to consider because pure iron is more effective in magnetic fields due to its high permeability.

So, in order to remove impurities from iron, there are several methods, each with its own advantages and disadvantages, to produce pure iron. [7]

We chose to use the electrolytic refining method, which involves an electrolysis cell. The impure iron serves as the anode, while on the cathode side, we use another iron that will be purified. [8]

 

Needed Materials

 

Electrolysis Cell

To purify iron, we use an electrolysis cell, which is a container capable of handling the electrolyte solution and separating the anode from the cathode. The container is often made of acrylic.

Power Source

We will need a DC power supply to deliver a constant current with sufficient voltage to drive the electrolysis process. The specific voltage and current requirements will depend on the cell setup and the desired purity of iron.

Impure Iron Anode

The iron you want to purify acts as the anode. The size and shape of the iron will depend on the amount you wish to purify.

Iron Electrolyte Solution

This is a crucial component in the process. Iron(II) chloride (FeCl2) is a popular choice due to its good solubility and availability.

The Cathode

The cathode is where the purified iron will deposit. It is typically made of a thin sheet of iron, which is commercially available as high-purity iron.

 

The Electrolytic Refining Process

 

Preparing the electrolysis cell

We fill the cell with an ion electrolyte solution, such as FeCl2 in our case. Then, we submerge the impure iron anode and the cathode into the solution while keeping them separated. To minimize the dissolution of anode material into the cathode compartment, a diaphragm can be utilized.

Applying a DC current

We must connect the power source to the electrodes. The anode will be connected to the positive terminal, while the cathode will be connected to the negative terminal.

Electrolysis

When the current is applied to the cell, chemical reactions occur.

At the anode side, oxidation takes place: Fe → Fe²⁺ + 2e⁻.

The iron atoms from the impure iron anode lose electrons and dissolve into the solution as Fe²⁺ ions.

At the cathode, reduction occurs: Fe²⁺ + 2e⁻ → Fe.

The Fe²⁺ ions from the electrolyte solution gain electrons and deposit on the cathode as pure iron atoms (see Figure 5).

                                          Figure 5

 

Drawbacks

 

Electrorefining is technically feasible for small-scale iron purification, but it is not the most practical option for several reasons. It has low efficiency compared to other methods and requires high energy consumption to move the iron ions. In this situation, we either have to use another method of purification or purchase high-purity iron (99.5%) and undergo the electrolytic refining process to achieve even higher purity (99.9%).

 

Annealing

 

History of annealing

Annealing is a heat treatment process for metals that involves heating to a specific temperature and then cooling at a controlled rate. In history, annealing was discovered around the 12th century when the word originated from the Middle English term "anelen," which means to set on fire or bake. In Europe, it was discovered through advances in blacksmithing that annealing alters the properties of steel and iron. [9]

Blacksmiths discovered that by heating iron to a specific temperature and then rapidly cooling it, the iron would become stronger and more durable.

The metal heat treatment techniques were developed in the Middle Ages when medieval blacksmiths discovered a process that involved reheating quenched metal to a lower temperature and then cooling it slowly. This enhancement made weapons and armor more effective in wars.

The Industrial Revolution in the 18th and 19th centuries led to the development of the annealing process as we know it today. This process involves heating metal to a high temperature and then allowing it to cool slowly. Subsequently, heat treatment methods have continued to evolve up to the present day. [10]

 Annealing & Permeability

 In general, annealing can rise the magnetic permeability of the metals, due to a heat treatment process at high temperatures, annealing could make 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Simulation

 

                                                         Figure 6

 

We used COMSOL Multiphysics to simulate the coil for a solenoid with a cylindrical shape (4 cm in diameter and 5 cm in height).

We used 100 turns, a relative permeability of 3000, and 6 amperes, and that was the result. (see Figure 6).

We repeated the simulation for two solenoids separated by a distance of 14 cm. By applying Biot-Savart's law, we calculated that each solenoid can produce 0.1 tesla, resulting in a combined magnetic field of 0.2 tesla across the entire distance. (see Figure 7).

 

                                                      Figure 7                          

 

As we can see, the magnetic field is uniform only in the middle, which is insufficient for our application. By employing additional solenoids, we can attain an even more uniform magnetic field.

 

 

 

 

References:

[1]: Medhe, S., 2018. Ionization techniques in mass spectrometry: a review. Mass Spectrom Purif Tech, 4(01), p.1000126.

[2]: 4.11: Mass Spectrometry by Pavan M. V. Raja & Andrew R. Barron is licensed CC BY 4.0. 

[3]: Asmaa EL MIR, Évolution de dispositif de spectromètre de masse pour la détection des gaz de combustion des déchets municipaux. Master thesis report

‏[4]: Copy William Harris "How Mass Spectrometry Works" 1 January 1970. HowStuffWorks.com. 22 May 2024  

[5]:University of Calgary - Department of chemistry - Mass spectroscopy Ch13 https://www.chem.ucalgary.ca/courses/350/Carey5th/Ch13/ch13-ms.html

[6]: Medhe, Sharad. (2018). Mass Spectrometry: Detectors Review. Annual Review of Chemical and Biomolecular Engineering. 3. 51-58. 10.11648/j.cbe.20180304.11. 

[7]: Uchikoshi, Masahito, et al. "Metal purification method and metal refinement method." U.S. Patent No. 6,391,081. 21 May 2002.

[8]: Kekesi, Tamas, Kouji Mimura, and Minoru Isshiki. "Ultra-high purification of iron by anion exchange in hydrochloric acid solutions." Hydrometallurgy 63.1 (2002): 1-13.

[9]: Annealing: Definition, Purpose, How It Works, and Stages. (2024, June 5). https://www.xometry.com/resources/materials/annealing/

[10]: A Historical Journey Through the Evolution of Metal Heat Treating. (2024, January 1). https://jfheattreatinginc.com/2024/01/a-historical-journey-through-the-evolution-of-metal-heat-treating/

[11]: Lin Li, "Controlling annealing and magnetic treatment parameters to achieve high permeabilities in 55 Ni-Fe toroid cores," in IEEE Transactions on Magnetics, vol. 37, no. 4, pp. 2315-2317, July 2001, doi: 10.1109/20.951158.

[12]: Improvement of Permeability and Magnetic Shielding Effect of Pure Iron Magnetic Shield Materials.(2005, October 1). JFE TECHNICAL REPORT. https://www.jfe-steel.co.jp/en/research/report/006/pdf/006-06.pdf

[13]: He, Y., Zhang, R., Zhou, T. et al. Effect of Annealing Temperature and Time on the Magnetic Properties and Magnetic Anisotropy of a Temper-Rolled, Semi-processed Non-oriented Electrical Steel. JOM 76, 1050–1065 (2024). https://doi.org/10.1007/s11837-023-06361-w