Mass-Spectrometer Design - Report Example

Mass-Spectrometer Design Report Teacher               Page This is a report where one learns in detail the various design specifications as well as reasoning for the purpose of the construction of the mass spectrometer as it is used to detect illicit drugs at the airport, where these are usually smuggled. The report has the purpose of providing the reader with a basic understanding of how a mass spectrometer works and how it is particularly designed to achieve its purpose. The mass spectrometer detects illicit drugs at the airport through its ability to detect specific ion concentrations found only in the elements that make up drugs, thus making it comparable to other chemical compounds. Moreover, the report also outlines an evaluation of the design specifications, as well as the limitations and recommendations pertaining to the subject in order to achieve better reliability regarding the use of the instrument. Introduction There are three essential functions that a mass spectrometer is expected to perform (as seen on Figure 1). The first of this is to bombard molecules with a high-energy stream of electrons. The result would be the conversion of the molecules to ions, and thus this results in the acceleration of the particles. The acceleration is made possible by a series of polarized accelerating plates and the creation of up to 10 kV of potential difference, which is considered big enough. This also results in the production of a beam of positive ions that rapidly travelling into a uniform beam as directed by the focusing slits. Vacuum pumps connected to the ionization chamber are mainly responsible for the continuous drawing off of the other molecules that remain un-ionized. The ensuing absorption of electrons thus produces negative ions as a consequence, which are in turn absorbed by repeller plates and are consequently accelerated at the same rate as the positive ions. Mass-to-charge ratios of the ions in a magnetic or electric field then become the basis for the separation of all these accelerated ions. The mass spectrometer then detects the approximate number of ions that strike it and have a peculiar fixed-curved radius of curvature brought about by the applied magnetic field. Those ions that have especially large or small mass-to-charge ratios do not reach the detector at all because they strike the sides of the analyzer tube. Thus, the constant variation of the magnetic flux or the accelerating voltage of all the ions is the one that makes them detected by the mass spectrometer. The data detected is then recorded in a graph by the mass spectrometer amplifying the output, determining the mass of the ions through their angle of deflection, and plotting this data proportionately in a graph in order to determine the chemical formula, thereby further determining type of the sample compound. In this way, the mass spectrometer can exactly determine whether it is really illicit drugs that it has detected or not. The output is a mass spectrum, where the number of particles is indicated. Graphic and tabular forms are then stored in the computer. Therefore, mass spectrometers are used in applications where the identity of unknown compounds is determined, and these compounds are quantified. The mass-to-charge ratio of the charged particles is the one that is instrumental in determining the quantity of each type of identified molecule. Section 3 illustrates the final design. Page 2 The figure shows the details and dimensions of the entire instrument as well as the list of all required major components and the details of all major electrical components. The major electrical components of the mass spectrometer includes the required currents and voltages as well as the details pertaining to the utilized electric and magnetic fields and their corresponding subsystems that generate these fields. Section 4 shows two different sets of calculations performed by the mass spectrometer in determining the identity of two different atoms. The good thing about this type of detector is that as calculations proceed along the way, intermediate results are provided, thus there is easy verification of the results later on. Appendix A shows all the formulae used in the preceding calculations. There is also a checklist of comparison of the mass spectrometer vis-à-vis original specifications in terms of characteristics and performance. Lastly, there is a conclusion in Section 6 concerning whether conclusions have been met or not and to which degree they have been met or exceeded. (copy picture) Figure 2. Diagram showing the curvature resulting from a magnetic field and the scatter pattern that results from the detector based on a mass-to-charge ratio Page 3 Specifications What is most necessary in the determination of the specifications of the mass spectrometer is considering the types of substances that need to be identified, such as commonly smuggled illicit drugs. All these illicit drugs have common features like the elements carbon, hydrogen, oxygen, and nitrogen. The mass spectrometer is precisely able to detect these elements especially in substances like cannabis (THC), cocaine, modafinil, opiates, amphetamines, MDPV, tranquilizers, LSD (tryptamine), and psilocybin mushrooms. Moreover, there is a need to particularly consider the electric field, E, especially as applied to the insulating material. The electric field can withstand conduction current until a sufficient amount of voltage applied can actually allow conduction to happen. This mechanism is known as the dielectric breakdown of air, with air acting as the insulation inside the acceleration chamber of the mass spectrometer. One good insulator is air and this is because it has a resistance to breakdown at an electric field strength valued at 3*106 V/m. In this particular device’s design, the electric field that was applied was 8000V/m, and this is the value required to cause the acceleration of ions in the mass spectrometer, where the range is 1000-10000V/m. Another point of consideration here is the presence of the solenoid at the mass spectrometer’s deflection stage. This represents the point of magnetic saturation, or the point at which magnetic material of the coil cannot further increase in terms of magnetism. This results in the leveling off of the total magnetic flux density and a raid drop in the magnetic field. The copper wire used in the design thus will be the one to induce magnetic flux intensity well below the saturation point for copper, which has a value of 1.6T. Compact dimensions are also equally important to the design process, particularly those of the mass spectrometer. These dimensions largely depend on the calculations that involve the previously outlined specifications. The proposed design to form factor has the following dimensions for its length, width and depth, respectively: 0.70m, 0.55m and 0.30m. Page 5 Lorentz’ force equation, Cyclotron theory and Newtonian laws of motion all show the derivation of a relationship in order to predict the mass-to-charge ratio of ions and in order to show the ionization, acceleration and deflection stages of the mass spectrometer. The combination of Newton’s centripetal force equation and Lorentz’ equation will yield: (Copy equation) In order to get the value of V, the left side should be rearranged: (copy equation) The combined equation above is incorporated into the Kinetic Energy Formula for V in order to yield: (copy equation) Finally, the simplification of KE equals: (copy equation) In the above equation, q represents the Charge of Particle (1.602*10-19C), m stands for the mass expressed in kg, and B stands for the Magnetic Flux Density (T). Page 6-7 The following equation is used for specifying the distance between the plates for the acceleration chamber: (copy equation) In this particular equation, V refers to the driving voltage, d is the distance between plates, and E stands for the value of the electric field strength. The value for V as specified and as this is finalized according to Australian standards is V = 8000 X 0.03 = 240 V. The values here have been chosen in order to make sure that all parameters are logically achievable without any unnecessary extra costs, components or sacrifices that have to be made on performance. Moreover, the variation in the magnetic flux is used in order to obtain the desired configuration for the working parameters for the deflection component of the mass spectrometer. This adjustment affected the amounts of deflection that an ion experienced, thus resulting in corresponding changes to the ion trajectories. This means that at the detector part of the mass spectrometer, the ones that can be interpreted are only specific mass-to-charge ratios for a specified magnetic flux. Moreover, in order to determine the appropriate ranges for magnetic flux, one needs to calculate the velocity values through the acceleration chamber for each of the required detected elements. These elements include C, H, O, N and Cl. The equations for determining velocities were obtained through a combination of the Centripetal force equations and some Lorentz’ force: Velocity for Carbon: (copy equation) Velocity for Hydrogen: (copy equation) Velocity for Oxygen: (copy equation) Velocity for Nitrogen: (copy equation) Velocity for Chlorine: (copy equation) In the above equations, one should take note that the radius of the arm was consistent at 0.1m for the design of the whole machine. Moreover, the solenoid in the deflection phase of the mass spectrometer affected the velocities according to the corresponding applied magnetic field, with the r value fixed at 0.1m. Page 8-9 The formula for the magnetic flux of each previously mentioned element (C, H, O, N, and Cl) must be: (copy the Bz formula) This formula was derived from the one that was proposed by the cyclotron theory for the trajectory of the particle: (copy the r formula) Required magnetic flux density for Carbon: (copy formula) Required magnetic flux density for Hydrogen: (copy formula) Required magnetic flux density for Oxygen: (copy formula) Required magnetic flux density for Nitrogen: (copy formula) Required magnetic flux density for Chlorine: (copy formula) In the above formulae and solutions for the magnetic flux density of the elements, the mass directly affects the velocity of the traveling charged particles. This therefore requires different magnetic flux densities applied in order to allow these charged ions to reach the detector. One should therefore also note that each of the magnetic flux values of the elements above are at a safe level, which is below the magnetic saturation limit of 1.6T. In addition, the corresponding values of currents for these solenoid configurations can be determined using (copy B formula) and when rearranged would appear as (copy Iair formula). In these aforementioned equations, µo is the value for the Magnetic Permeability of free space, which is equivalent to 1.2566*10-6H/m. The value for N corresponds to the number of turns the solenoid wire has taken, with I as the Current in amperes (A), and h as the height in meters (m). Experiments prove that a current valued at 1-5A was best for the mass spectrometer, and so this was taken into consideration for in design specifications. The condition, however, is that h and N remain constant, with the magnetic flux density B varying in terms of the element detected. Hypothetically, if this one is tested on Cl and H, it would appear as: (copy all formulae) One should take note that ideal values of the current should be within the range of 5A or even lower. This is achieved by making the solenoid have a smaller height or a greater number of turns, while maintaining minimal values for the height although care should be taken to ensure that it should not be too small to be easily manufactured and controlled. The number of turns is limited to the thickness of the material as well as the availability of the space. Page 10 No matter which way it is solved, the sample calculation for chlorine indicates that 5.637A can actually support 250 turns and 1cm height, and this is perfectly reasonable. For the iron-core, the formula is as follows, considering that µr or the relative permeability of 99.8% annealed iron is 6000: (copy formula, except µr and Note) The advantage of the iron-core mass spectrometer to the air-core is that the former meets the minimum requirements that the design is aiming for. On the other hand, with the air-core, there is difficulty finding a spot where the required current is minimal. The solution is to either reduce the number of turns to a minimum of 20 and to increase the height to around something that can get close to 1A. When it comes to chlorine, it has the highest point required for detection with the mass spectrometer, thus the value of the required current depends on what ion is detected. It is reasonable in the previously specified design that the air-core solenoid is still within reachable currents, valued at 7A. With these specifications, there is no need to sacrifice size, although the problem is that the height of the solenoid iron-core is the main reason for the unwanted design form. (copy formula) The realistic solenoid is 200 turns and 1cm in height. With 7.0459A as the highest current required for the system to detect chlorine, and 1.1881A for the lowest required current, such as one required to produce magnetic fields for hydrogen, the estimated value of the required current ranges from the values of 1A to 7.5A. Page 11 The factors to be taken into consideration when dealing with spectrometer outputs, relative currents used in detection and variations in mass/charge ratio include the elimination of possibilities of unrelated trace elements as well as assuming that only the most common isotopes or ions are considered. In the detection system of the mass spectrometer, the number of ions that can be detected as represented by the ratio n is essential in determining the composition and identity of an unknown compound. As an example, if the system detects that n = 1254:1488:108 for m/q = 14:12:1, and where the relative abundance is represented as R, the nk is the ion ration value for kth value, and ntotal represents the total of all ration values of all ions, the calculations may appear as: (copy R formula, do not copy note) Using basic stoichiometric principles, if one assumes that the given m/q values are only assigned to the masses of the most common isotopes, the m/q is equal to 14:12:1. Consequently, by virtue of their atomic masses, these are nitrogen, carbon and hydrogen respectively. The percentage compositions are: (copy percentage composition only) In the case of a sample of 100g, the following calculations can be achieved in order to arrive at the empirical formula of C7H6N5: (copy table, but change second column heading to “Division by lowest number to determine the number of moles” and the third column heading to “Multiplication by a common factor 5 in order to find the lowest possible whole numbers for ratio”) Page 12 The trial molar mass of the given example is solved as follows: (copy trial molar mass formula) Using basic stoichiometry to solve for the actual molar mass, the number of units of each atom of the molecule can be derived by dividing actual molecular mass by the trial molecular mass. With the number of units N assumed as 2, each of the number of atoms in the empirical formula will be multiplied by two. The molecular formula, however, can only be known if the actual molar mass of the experimental element is known. In this example, it is unknown. The molecular formula is then compared with the standard values of the atomic masses of the known illegal drug compounds, such as the elements carbon, hydrogen and nitrogen. Page 13 Evaluation When it comes to the specifications for the design of the mass spectrometer, it is obvious that that the minimum desirable requirements would be a magnetic strength of 8000V/m and a range of magnetic flux densities from 0.0299-0.1771T in order to avoid magnetic saturation. The current device with these specifications ca actually work most efficiently on a 240V power supply with a current value rating of 1A to 7A without any transformers or other accessories to power the device. For the purpose of safety in the airport security setting, there is a need to keep the design as compact as possible. Initially, one had to contend with bulky size discomforts as well as an air-core solenoid that lowered the efficiency of the device. However, with the air-core solenoid providing reasonable currents, it is deemed more preferable than the iron-core solenoid. Conclusion The mass spectrometer is an ideal device for the detection of commonly smuggled illicit drugs into and out of certain countries. The mass spectrometer is a device that is able to detect the specific known drugs based on the drug spectra database, especially when the device is coupled with software. The mass spectrometer detects the illicit drugs by detecting the unknown materials and traces of drugs found in people and belongings. Furthermore, there is a need to cover only the detection of the basic elements of carbon, hydrogen, oxygen and nitrogen. The detection is subject to specific currents for the magnetic fields to detect such elements, with chlorine as the maximum that can be detected. Based on these aforementioned statements, there is definitely room for the optimization of the design in order to arrive at a more compact size than it actually is right now. One suggestion that will hypothetically increase accuracy is to couple the system with a chromatography portion so as to include detection of gas or liquid elements, which characterize modern synthetic drugs. Moreover, variations of the magnetic flux affect the reliability of detection, and thus it is important that the device be void from magnetic disruptions and be far from possible strong magnetic interference. Read More