Download Construction of a Negative Ion Mode Linear Time of

Construction of a Negative Ion Mode Linear
Time of Flight Mass Spectrometer
Tyler Vanover
April 30, 2013
Abstract
The aim of this project was to construct a linear time of flight mass
spectrometer in negative ion mode. In order to construct a negative ion
mode linear TOF it becomes a necessary endeavour to learn about the
inner workings of the TOF including the exploration of the source, flight
tube, detector and electronics. One of the main issues combating high
resolution of the linear TOF is the temporal, spatial and kinetic energy
distributions of ions in the source region. The way to remedy this is
to employ the technique of delayed pulse extraction, which I explored in
detail. Because of a malfunction with an integral part of the delayed pulse
extraction technique, I was not able to employ this method to obtain data.
However, I did get some response from the detector while operating the
TOF in continuous positive ion extraction mode, which proves that ions
are being created and that the detector is working.
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1.1
Introduction
The Linear Time of Flight
The Time of Flight mass spectrometer (TOF) is an instrument that is capable
of measuring the ratio of mass-to-charge m
z of ions, atoms, or molecules (Lu).
Its operation is based on the idea that ions with the same kinetic energy 12 mv 2
, but different m
z ratios can be separated by their flight time over a given field
free path (figure 1) (Fjeldsted). Due to the incredible resolution of the TOF
spectrometer that is available today, and by utilizing the various isotopic mass
differences of ions, it is possible to determine not only the m
z ratio, but also
the charge and the mass separately for those ions. Two advantages of using the
TOF machine are its tremendously fast data acquisition rates and the ability of
detecting trace amounts of ions (Fjeldsted). One attractive feature of the TOF
machine is its conceptual simplicity. It represents an excellent application of
sophomore physics. Ions which are generated at the source region by a nitrogen
laser, for example, are accelerated to an energy q∆V (∆V = P2 − P1 ) by the
electric field Es = ∆V
ds , where Es is the electric field of the source, V2 is the
voltage of the field free region, V1 is the voltage of the repeller plate in the
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Figure 1: Schematic of Linear Time of Flight Mass Spectrometer
source, ds is the length of the source region, and L is the length of the flight
tube as seen in figure 1. Although the time of flight mass spectrometer is
conceptually simple, in practice, it can be quite complicated. The generated
ions are spread over a finite volume in space and they do not start with zero
kinetic energy. Both of these features reduce the mass resolution. Fortunately,
the introduction of what is called the delayed pulse extraction technique corrects
for both features and allows all ions to enter the field free region with the same
m
kinetic energy. The resolution ∆m
of the linear TOF is of the order 300-800
(Guilhaus). Once the ions are in the field free region, they all travel down the
tube with the same initial kinetic energy, which is given by
1
mv 2 = q∆V
2
The ion velocity, v, can be written as
r
v=
2q∆V
m
The time of flight, t, of an ion of mass m along the flight tube L is
r
L
m
L
t= = √
v
q
2∆V
where
t=Drift time
L=Drift length of field free region
m=Mass of ion
2
(1)
(2)
(3)
Figure 2: Flow Chart of Basic Components
q=Charge of ion
∆ V=Potential difference of plates in the source region
1.2
Basic Components of a Negative Ion Mode Linear
Time of Flight
Conceptually, the negative ion linear TOF consists of 4 major parts as outlined
in figure 2. The high vacuum system is necessary because we do not want the
ions to be bumping into other particles such that they can float freely down
the flight tube with as little interference as possible. To acquire such a vacuum
we have used two turbomolecular pumps in combination with two common
laboratory vacuum pumps. Our system has capabilities on the order of 10−8
torr. The ion source plays a major role in the operation of the TOF as this is
where the probe tip is placed and ions are created using pulsed laser extraction.
The ions are then accelerated through a potential difference and thrust into the
mass analyzer. The mass analyzer is simply an elongated tube that has been
evacuated where the ions are given time to separate such that the heavier ions
reach the detector last and the lighter ions first. There are two main types of
analyzers: reflectron and linear. The reflectron offers superior resolution, but
is much harder to construct. Our mass analyzer is of the linear type. The
detection system is coupled with other electronics responsible for the pulsed
laser extraction such that it knows when to detect an incoming ion packet. This
is then fed to a data analysis system where a graph of the intensity of the
counts is plotted against the mass to charge ratio. Now lets take a look at each
component in detail.
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Figure 3: Source Region
Figure 4: Schematic of Source Region
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Source Area
The source area is where the ions are generated and given their initial kinetic
energies where they are then sent into the flight tube of specified length and
lastly received at the detector. Since this is where the ions are generated and
given their initial energies, this results in the area where the main source of error
in the resolution can be attributed. As ions are created in the source region
they will have time, space and kinetic energy distributions that cause error in
the mass resolution (Opsal). Figure 3 is our source which is represented as a
schematic in figure 4. In figure 3 P 1 represents the repeller voltage plate and P 2
represents the acceleration grid. Note the wires coming from the source flange
that carry the current from the power supply. In figure 3 you get a better feel for
what the source plates look like as they are stacked on top of one another. The
probe tip rests in the middle of the repelling plate and is bombarded by the laser
source that is incident at some angle. Since we are interested in constructing
a linear TOF in negative ion mode the repelling voltage should be negative
while the accelerating grid should have a positive voltage applied to it and the
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Figure 5: Kinetic Energy Distribution
distance between the plates as noted in figure 1, ds is the accelerating region
before the ions are hurled into the flight tube. As I mentioned before, these ions
will have temporal, spatial and kinetic energy distribution that interfere with
the mass resolution.
2.1
Temporal Distribution
As you may have guessed this includes the distributions in the ion time of
formation. It also includes limitations of the ion-detection and time-recording
devices, but that isnt so much our concern here as we cannot control that aspect.
Essentially what this means is that the distributions of ion formation results in
ions which enter the drift tube at different times and maintain a constant time
difference as they approach the detector (Opsal). Since the mass resolution is
t
given as 2∆t
these effects can be minimized by using longer flight tubes.
2.2
Kinetic Energy Distribution
From the equations in the introduction you can see that the flight time of the ion
is dependent on the kinetic
p m energy of the ion as it leaves the acceleration region
which is given by t = 2KE
L where the KE now has an error term of U0 such
that the new KE is given to be KE = q∆V + U0 where U0 refers to the initial
kinetic energy from the ions initial components along the flight axis (Opsal). As
depicted in figure 5, the ions with energies of q∆V +U0 that have the same mass
arrive at the detector sooner than ions with an energy of q∆V .This results in
the mass spectrum being skewed to the low-mass side, hence a lower resolution
(opsal).
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Figure 6: Delayed Pulse Extraction
2.3
Spatial Distribution
The spatial distribution of the ions is really an effect on the initial kinetic
energies of the ions. When ions are formed at different locations in the source,
it means that the accelerating region will be shorter or longer for the particular
ion which results in a smaller or bigger initial kinetic energy as the ion enters
the field free region. As these ions enter the field free flight tube there is a region
known as the space focus plane where the faster ions that are formed near the
rear of the source catch up with the slower ions that are formed near the front of
the source, as you can see in figure 5. It turns out that this point is independent
of the mass and is located at a distance of 2ds from the grid where the ions exit
the accelerating region. There is a way in which we can essentially push this
focal plane back to the detector such that the mass resolution is dramatically
increased. This method is called delayed pulse extraction.
2.4
Delayed Pulse Extraction
As I mentioned above, we want to move the space focus plane back to the
detector where the ions are going to be detected to correct for imperfections
found in the spatial, KE, and temporal distributions. The technique of delayed
pulse extraction will be employed to do so. Figure 6 gives a representation
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of how the delayed pulse extraction works which is not so transparent at first
glance. During step 1 one there is no field applied between the two source
plates that we saw in figure 6. The laser is fired at the source tip and ions
emerge from the sample in what is known as a plume very shortly after being
hit with the laser. The ions that you see in step 1 all have the same mass but
have different velocities. The result of having these different velocities means
that they will have different locations in the source area labeled ds in figure 1
when the electric field is turned on (Matrix Assisted...). The electric field in the
accelerating region gets weaker as the ion gets closer to the grid. As a result
the ions with the higher velocities will not feel as much of a push toward the
grid as the ions with slower velocities. In the end the ions reach the grid with
the same velocities hence the same kinetic energy, which plays a key role in the
resolution.
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Flight Tube
The flight tube is one of the most simple components of the TOF instrument, at
least for the linear TOF. The flight tube is the region of the mass spectrometer
that immediately follows the source region. The flight tube is the cavity on the
mass spectrometer that is considered a field free region and the point where
the initial kinetic energies of all the ions entering the tube should be the same.
The ions then traverse the evacuated tube and are received at the detector
depending on the mass to charge ratio. Most flight tubes have horizontal and
vertical deflection plates that range in voltage and are used to direct the ions
onto the detector (Watson).
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4.1
Detection
The Microchannel Plate (MCP)
The components that make up the detector are the mesh grid and the Micro
Channel Plate detector (MCP). The mesh grid is simply a steering mechanism
where the ions can be directed onto the MCP. The MCP is a very thin ( 2mm)
highly resistive material that contains an array of tiny tubes (microchannels)
that are densely distributed across the plate(Fjeldsted). One of the key characteristics of the tubes is that they are not perpendicular with respect to the
surface of the plate, but are rather angled at some angle ( 8o ) (Fjeldsted). As
an ion enters one of the channels, the angle guarantees that the ion will strike
the wall of the channel and this impact releases electrons from the walls of the
channel that then perpetuate themselves as they propagate down the channel.
The purpose of this is to amplify the signal. When the electrons exit the MCP
they are then digitized an output to a device such as an oscilloscope (Fjeldsted).
Figure 7 is a photo of the detector and figure 8 is a schematic of how it works.
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Figure 7: Detector
Figure 8: Microchannel Plate Schematic
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5
Resolution and Calibration
From the equations at the top in the introduction we can see that the mass is
a function of the flight time, m(t) (Fjeldsted). With that in mind, the user will
always try to keep the voltages applied to the plates, the distance between the
plates, and the length of the flight tube constant such that equation 3 can be
rewritten as
m = At2
(4)
In practice, there is a delay from the time the electronics sends a starting pulse
to time that a high voltage is present on the rear ion pulser plate. Unlike delayed
pulse extraction, this delay was not intentional. There is also a delay in the time
an ion reaches the front surface of the ion detector until the signal is generated
that is digitized by the acquisition system (Fjeldsted). Even though these are
short delays they are significant and must be accounted for. Because it is not
possible to measure the true TOF, we must correct the measured time, tm , by
subtracting the sum of the start and stop delay times which will be denoted as
t0
t = tm − t0
(5)
Substituting equation into equation gives:
m = A(tm − t0 )2
(6)
As a result, if we want to find the mass of the ion, we have to have values for A
and t0 and to do this a calibration is performed.
5.1
Mass Calibration
A solution of compounds whose masses are known with great precision are
analyzed and a table is made with the calibrant masses and their respective
flight times, tm (Fjeldsted). Here is an example:
Calibrant Mass
49.99626
68.99466
130.99147
218.98508
263.98656
413.97697
Flight Time
13.2900
15.8984
20.8786
26.6954
29.2101
36.3206
Now that we have m and tm for a variety values across the mass range, we
can have a computer to use nonlinear regression to find the values of A and t0
such that the mass can be as close as possible to the real value for all of the
mass values in the calibration (Fjeldsted).
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5.2
Data Acquisition
The signal that is output by the MCP can be fed to an oscilloscope where
intensity (counts) is plotted against the mass-to-charge ratio. For most analyses,
the tradeoff is made to widen the range of detected TOFs at the expense of
slower data sampling rates. The data can be recorded from the oscilloscope and
transferred to a pc. The data is converted to ASCII format and then imported
into GRAMS spectral analysis software (Watson).
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Getting Started
The first thing I did for my senior research project was get my hands ”dirty.”
I took the entire TOF apart, took an inventory of what we had and attempted
to determine the purpose of each component of the TOF. I started by learning
how to bring the TOF back to atmospheric pressure (Lab Notebook 47). I
then began looking to each component of the TOF starting with the source and
detector as I was able to examine those prior to my proposal. I then began to
examine the deflection region in the flight tube.
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The Deflection Region
The deflection region is rather important as it is what extracts the ions from
the source and collimates them. I took the deflection region off of the TOF
and checked for continuity to ensure everything was connected properly. I then
documented what each pin on the flange corresponded to (Lab Notebook 56).
While working with the deflection region I was also testing the power supplies
making sure that each output registered what it was suppose to (Lab Notebook
53).
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Source Setback
After getting a good feel for all of the components on the TOF, I began to
reassemble it. Before I put the system back under vacuum, I noticed that we
had a problem with the source region. The problem was that when the loading
rod was inserted into the source region it was not aligned so that the probe
tip would rest in the middle of the repelling plate. The interlock is a series of
precisely aligned holes that the loading rod must go through in order to place the
probe tip in the center of the repelling plate. If you look back to figure 3 you will
notice that the plates P 1 and P 2 are held in place using insulating ceramic rods,
which are screwed into the source flange. The ceramic rods showed some signs of
damage, which had presumably lead to the plates becoming askew. To remedy
this, I realigned the source with a thin washer which pushed the repelling plate
in the desired direction just enough such that the probe tip could rest inside the
repelling plate. This raised concerns between Dr. Lahamer and I because the
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Figure 9: Original Bench Setup
ions must take a very precise path through this deflection region and this could
be a source of error in the end. Once this was taken care of I reassembled the
system completely and put it back under vacuum. I then began searching for
an appropriate laser and setting up the optics for the bench.
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Nitrogen Laser and Bench Setup
The nitrogen laser that I found in the laboratory that was not already part
of a system, has a wavelength of 337 nm and a pulse energy of 300µJ. The
reason that we are using that particular wavelength is because that is what has
been shown to be a good wavelength for producing negative ions, according to
Dr. Lahamer. The next order of business was to set up the bench where the
laser and optics would remain. Figure 11 gives you an idea of what the original
bench setup looked like. The bench setup is extremely important in order for the
delayed pulse extraction to perform properly. For the setup you see in figure 10
the laser had its own internal trigger. When the laser fired its beam would reduce
aberrations by passing through the iris, it would then pass (mostly) through the
reflecting lens, be reflected by the mirror and focused onto the sample tip by the
focusing lens. This results in the ionization of the sample material (hopefully).
When light passes through the reflecting lens a small portion of the incident light
is reflected and that reflected light triggers the photo-detector, which triggers
other electronics needed for the delayed pulse extraction. The important thing
to take away from this is that the photo-detector determines the period of the
laser pulse and this is needed because we want to extract ions from the sample
exactly out of phase with the pulse period of the laser. I had a problem getting
the photo-detector to trigger the DG535 pulse/delay generator. For this setup
it is important for the DG535 to be triggered because it creates a delay in the
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Figure 10: Oscilloscope Output using HeNe Laser
pulse received from the photo-detector. This pulse is then used to trigger the
the high voltage supply which is applied to the repelling plate for the extraction
of the ions. If you look back to figure 1 the delay in that diagram should occur
when the high voltage is applied to the repelling plate. Eventually, I discovered
that the problem was not the photo-detector, but rather the laser. I was able
to come to this conclusion by using a Helium Neon laser and chopper while
observing the output on the oscilloscope. What you see in figure 11 is a good
representation of what I should have been seeing using the nitrogen laser. The
output from the photo-detector is the result from the incident HeNe laser and
the output from the DG535 is exactly out of phase with the pulse from the HeNe
laser. Upon seeing this, Dr. Lahamer and I agreed that the photo-detector was
not receiving the proper intensity from the nitrogen laser because the signal
from the photo-detector was very faint and had no width. Since the light that
was reflected from the the reflecting lens was used to trigger the photo-detector,
it makes sense that the beam did not have the intensity needed to establish some
pulse width. For the image you see in figure 11, I did not use a reflecting lens
to obtain that, but rather had the laser directly incident on the photo-detector.
The clarity of the image in figure 11 seems to be good evidence that the photodetector was not receiving the appropriate intensity. I could no longer use the
setup seen in figure 10 because it relies on the reflecting light from the reflecting
lens to trigger the photo-detector, so I had to design a new system.
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Figure 11: Electronics for Delayed Pulse Extraction
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Improved Bench Setup and Electronics
After using the HeNe laser I learned that there was a way that I could trigger
the laser with a pulse generator and use the photo-detector to determine the
pulse width of the laser. I could then use the DG535 to incorporate a delay that
is comprable to the pulse width, such that the high voltage would be applied
during the instances the laser is not incident on the sample. Figure 13 gives
a schematic of what the electronics for my new system looks like. It should
be noted that the bench setup didn’t change much. I just removed the photodetector and reflecting lens. It should also be noted that you need to determine
the pulse width each time you change the repetition rate. To determine the
pulse width of the laser you must use the photo-detector. By using the photodetector you can set the delay to a time similar to that value you found from
the photo-detector. So we still need to use the photo-detector for this setup.
However, it is only needed when you begin the analysis and it can be set such
that the laser is directly incident on the photo-detector. If you decide to change
the repetition rate of the laser, you must use the photo-detector again to find
the pulse width and set the DG535 delay again. However, it is unlikely that the
user will be doing this often. This was one of the biggest accomplishments of
my senior project. Once I had overcome this barrier, I was once again hindered;
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The PVX pulse generator was malfunctioning.
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The PVX Pulse Generator and Remote Pulser
Up to this point I had the pulsing electronics working and everything seemed to
be properly synced. As you noticed in figure 12, the output from the DG535 goes
to the PVX pulse generator. The output from the DG535 is a 5 Volt square wave
that triggers the PVX. When the PVX is triggered it outputs a waveform that
is exactly like the incoming waveform except the amplitude is much larger. In
this case the amplitude would have been on the order of 103 Volts. However, the
PVX seemed to have an issue with what the manufacturer calls an ”overcurrent.”
After making several calls to the manufacturer, they deemed it necessary to send
the PVX in for repairs. I was devestated. It seemed that all hope had been lost
because we had no way of producing the high voltage waveform necessary to
employ the technique of delayed pulse extraction. Dr. Lahamer found a device
manufactured by Jordan called a ”Remote Pulser” that worked essentially the
same as the PVX. However, after searching online for a product description, I
found that the remote pulser is part of a package sold by Jordan and that it
was suppose to be used alongside a specific power supply also manufactured by
Jordan (Jordan). After speaking with Dan and the manufacturer I discovered
that it would be possible to use the remote pulser with our power supplies, but
that did not end successfully. In the end, I scrapped the idea of using the remote
pulser and decided to try to get a signal using continuous extraction.
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Continuous Extraction
For the continuous extraction the setup becomes much less complex. If you take
out the DG535 and the PVX from figure 11 you have the schematic for continuous extraction. However, for continuous extraction, the high voltage power
supply is connected directly to the repelling plate and the electronics in figure
11, with the exception of the DG535 and PVX, simply determine the periodic
pulse of the laser and simultaneously triggers the oscilloscope. When I employed
this method I was basically looking for some kind of signal to let me know that
the detector is working, regardless of the eminent poor resolution. Also, while
operating the TOF in this mode I ionized some C60 that Dr. Lahamer had
already had in the laboratory in hopes of seeing something on the oscilloscope.
I tried both positive and negative ion mode for the continuous extraction, but
only got one reading for the positive ion mode. If you look to figure 13 you
can see the single peak that I got. This peak was the average of 500 sweeps for
500 points, as you can see by looking at the oscilloscope, so it most certainly is
not a fluke. While I know that this may seem relatively unimpressive, it does
tell us that ions are being created and that the detector seems to be in working
condition, both of which is good news because at the beginning of the semester
nobody had any idea what functioned on the linear TOF. Even though we have
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Figure 12: Peak from Positive Ion Continuous Extraction
no way of proving it, Dr. Lahamer thinks that the place where the peak is
located is generally the area where he has found C60 in the past.
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Conclusion
As the semester comes to a close, I realize that I have accomplished a lot. I have
learned how the linear time-of-flight mass spectrometer works and applied that
knowledge to our instrument in hopes of resurrecting it. In doing so, I designed
my own method for employing the technique of delayed pulse extraction, which
was unfortunately hindered by the malfunction of the PVX pulse generator. I
also verified that were were in fact creating ions and that the detector is in
working condition. In the future when the PVX has been repaired, it would be
great if somebody could continue my work and see if they could get some data
from the spectrometer, as the PVX seems to be the only obstace in the way of
being able to employ the technique of delayed pulse extraction.
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Acknowledgments
First, I would like to thank Dr. Amer Lahamer for his assistance throughout
the semester. Your prior knowledge and guidance has helped immensely over
the semester. I would also like to thank Dan Brewer who helped me numerous
times with anything and everyhting that I asked of him. I learned a lot from Dan
over the course of the semester, mostly in the way of electronics. Many times
Dan also helped me locate documents online that helped deepen my sense of
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knowledge and inquiry involving the TOF. I would like to thank Dr. Hodge and
Dr. Veillette for the knowledge that they shared through class and conversation.
I would also like to thank my friends and colleagues for the support and advice.
Lastly, I would like to thank Franceska for all the support and encouragement.
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[3] Guilhaus, Michael. ”Special Feature: Tutorial. Principles and Instrumentation in Time-of-flight Mass Spectrometry. Physical and Instrumental
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