The Magnetic Mass Spectrometer

Transcription

1 Exp-9-Mass-Spectroeter.Doc (TJR) Physics Departent, University of Windsor Introduction 64-3 Laboratory Experient 9 The Magnetic Mass Spectroeter Without question, ass spectroeters have been one of the ost useful analytical tools of the twentieth century. Fro the discovery of isotopes to the first observation of fullerenes, these instruents nave enabled scientists to identify a olecule by a quantity that sees uniaginably sall on our scale: its ass. Today, the anufacturing of ass spectroeters is a big industry. They lend theselves to any useful tasks - fro edical analysis to pollution onitoring to bob detection. There are any different types of ass spectroeters. The one used here is based on the design of the earliest versions. It is a "hoeade" single focusing, 90 agnetic sector ass spectroeter. It will be used in this exercise to observe the constituents of air and other gas ixtures (such as car exhaust). In addition, the onset of dissociative ionization will be qualitatively studied. Description The apparatus has all of the coponents of the triode tube used in the ionization potential of N experient; a cathode, a grid (slit), and a collector. But the siilarity ends there. The ost obvious difference is the vacuu chaber, which is no longer a sall gas filled bulb, but a large piece of brass plubing to which is attached an oil diffusion pup. This device aintains a vacuu of about 3x0-6 torr inside the chaber. A needle valve will allow you to adit gas of any type for ass analysis. The chaber pressure is easured by a Bayard-Alpert ionization gauge, which itself is just a triode. Referring to Figure, the instruent is constructed as follows. To the extree left of Figure a 0- A AC power supply provides current to heat a tungsten filaent (a). The resulting therionic electrons are accelerated through a sall opening in the cathode housing (b) by a 0-00 DC supply and pass into a nearly field free region where they collide with gas olecules, creating positive ions. Electrode (b) is connected to a 0-0 k high voltage supply, and the filaent bias "floats" (i.e. its voltage is not referenced to ground) on top of that so that there is always a potential difference between (a) and (b) to accelerate the electrons. Ions created between electrodes (b) and (c) will be pulled through the aperture in (c) as its potential is about 80-90% of the high voltage as set by the resistor chain. For this reason the top hat shaped electrode (c) is also called the extraction electrode. The result is a bea of positive ions which are focused by electrodes (d), (e), and (f) onto slit as they accelerate towards the ground potential. Those last three electrodes together for an electrostatic lens. Deflection plates (g) are provided to direct the focused ion bea through slit should alignent be needed. Once the ions pass through slit they enter a region of unifor agnetic field directed out of the page, and are ade to travel through a path with a 0 c radius of curvature by the Lorentz force. They then pass through slit and into a collector known as a Faraday cup. A sensitive electroeter can be used to easure the resulting current. The repeller slit between slit and the Faraday cup is biased negatively by a DC power supply to prevent stray electrons fro interfering with the current. Electrons

2 are released when ions strike the edge of slit. Shielding of the electroeter cable is very iportant due the sall current easured. Figure. Scheatic diagra of agnetic ass spectroeter. Procedure Be very alert to the high voltage of the ion source which can deliver a lethal shock! Do not operate alone! Do not go near the power sources behind the plexiglass! Discuss safety concerns with the TA or Professor. Only adjust potentioeters and the needle valve via plexiglass rods. Discuss operating the copressed gas in the cylinder with the TA. Be sure the needle valve is closed (fully clockwise). Turn on the rotary vane pup and allow the chaber to evacuate until the pressure on the fore line falls below 00 illitorr (760 torr = atosphere). Turn on the cooling water to the diffusion pup and turn on the diffusion pup. It will take about 30 inutes to reach a working vacuu, then turn on the ionization gauge. Wait until the pressure reaches the 0-6 torr range. (This ay take a weekend, depending on when it was last used!) Check that the power supplies for the ion source have their controls turned down. Plug the in and turn on the ion supply and the high voltage supply. Turn on the agnet supply and set it at 0.50 aps. Turn on the deflection plate supply and set it to zero. Set the filaent bias (this controls the

3 electron ipact energy) to full scale (80 ) and slowly increase the current through the filaent until its eission current reaches 00 μa. Turn the ariac up slowly to ~ 40, as easured on the volteter attached to the resistor chain. The ariac indirectly alters the ion energy (0-0k). Treat this control with the respect it deserves and do not exceed 00 on the volteter! With the gas supply line open to the argon/heliu ixture in the cylinder (or air, or soe other gas source), slowly turn the needle valve. [The reading on the low pressure side of the gas regulator should not exceed.5 bar. Please also ensure the tap on the cylinder head is closed at the end of the lab session.] The pressure as easured by the ionization gauge will clib to soe value below x 0-4 torr. Do not exceed this value. You will probably have to adjust the eission current to at this point to return it to 00 μa. Finally, turn on the electroeter (at the pa scale) and slowly vary the high voltage (via the ariac) within the perissible, safe working, range. It ay take several trials and adjustents, but you should easily produce a ass spectru of argon/heliu, using an X-Y chart recorder. These atoic gases, having very different asses, are excellent for calibration purposes. Obtain ass spectra for several different settings of agnetic field and decide which works best. You ay find other ass peaks too (Why?), try and identify as any peaks as possible. Reeber the scale is non-linear, the ass is inversely proportional to the voltage. Since the agnetic field is unknown, you will have to use one peak (the largest) as a reference for the others. (What gas is that?) One proble when using electron ipact ionization is that olecules ay fragent into saller ions, leading to peaks in the ass spectra that are not directly fro the saple. This fragentation is known as dissociative ionization, and it can be partly avoided by using low electron ipact energies. Obtain ass spectra of air at several different settings of electron ipact energy (this requires patient adjustent of the electron eission current and acceleration voltage) and deterine (roughly) the onset of dissociative ionization of nitrogen and oxygen. It is ipossible to distinguish between a doubly charged diatoic olecule and a singly charged ato fro the sae. However, dissociative ionization will occur uch ore readily than double ionization at electron ipact energies below 00 e. Thus the onset observed in this experient will be for ostly for dissociation. The power supply for the electron ipact energy is not regulated, and you will notice its value drops as the electron bea current increases. This will take soe getting used to during operation, but it should be possible to see ass spectra all the way down to 30 e ipact energy. Using a spectru with the sharpest peaks, easure the full width at half axiu (FWHM) for each peak and calculate the resolution of the instruent for each peak Copare this with the theoretical value (see below). You ay want to agnify the horizontal scale of the chart recorder to be able to easure this well enough. Finally, for fun, why not get a sealed plastic bag of car exhaust and attach it to the gas inlet line and record its ass spectru. What do you observe? Can you identify any ass peaks? Electrostatic Focusing The three disk shaped electrodes with 5 holes cut in their centers (d, e and f) for an electrostatic lens designed to focus the ion bea onto slit, and thus axiize the signal intensity. This particular configuration is known as an Einzel lens. The outer electrodes are connected together, and the inner electrode is bias positively with respect to the others (negatively for focusing electrons), at about 60% of the high voltage. The action of an Einzel lens can be understood in a qualitative way as 3

4 follows. (Look at ray in Figure.) Figure. Ion trajectories in an Einzel lens. a b: The ion oves upwards by the radial coponent ( ) of the electric field, and is slowed down by the axial ( ) coponent of the field. b c: Here the ion spends a longer tie than in the a b region. It is pushed towards the center by the radial coponent ( ) of the field and is further decelerated by the axial coponent ( ).Thus the deflection towards the axis along b c is larger than the deflection away fro the axis in region a b. Siilar arguents hold for the regions c d and d e: The bea of ions is brought to focus at a distance that depends on the ratio of the voltages on the lens eleents. Magnetic Focusing Even though the ion bea passing through slit is diverging, it can still be returned to a focus at slit, as long as both slits and the center of curvature of the central path lie on a straight line (see Figure ). This is known as Barber's rule and you need not derive it. Questions Explain the operation of an oil diffusion pup and an ion gauge. Can this particular pup account for any unidentified peaks in your ass? If the collision cross section for an ion with the residual gas in the vacuu chaber is σ ~ 0-6 c, what is the ean free path between collisions for the operational pressure? 4

5 Derive equation () below, the focus condition for a agnetic ass spectroeter, q B R = () where is the ion ass, q its charge, the voltage it was accelerated by, B the agnitude of the agnetic field, and R the radius of the path the ion took to get fro slit to slit. For a fixed radius and agnetic field one obtains equation () for two different ions: / q / q = () Usually the source produces only singly charged ions (q = q = e) and the result is (3). = (3) Thus the lowest ass is found at the highest accelerating voltage. The ass resolution is liited by the finite slit widths s and s. Two adjacent asses M and M + ΔM occurring at voltages and + Δ are said to be resolved when their centers are separated by the FWHM of the peak at M. The quantity M/ΔM is called the resolution and is a figure of erit for ass spectroeters. Use Eqn (3) to show that: To calculate the resolution, let Δ = FWHM. M ΔM It can be shown fro ion optics that the resolution of a agnetic ass spectroeter is given by: M ΔM R = s + s In this apparatus, s = s =.0, and R = 0.0 c. Does the result fro (5) atch your calculation for several peaks in the ass spectra you obtained? Would this instruent be capable of resolving the isotopes of Xenon? Δ (5) (4) References Mark, T.O, J. Che. Phys. 63, 373 (975) Melissinos, A.C., Experients in Modern Physics Roboz, J., Mass Spectroetery White, F.A., Mass Spectroetry in Science and Technology Note: Inforation on ass spectroeters is exhaustive. Keep your researches to inforation specific to this instruent. Do not trouble yourself with derivations fro ion optics. 5