08/06/2004: "Report Number Nine from NASA"

Maine Space Grant Consortium
NASA Internship Program
Summer 2004, JSC
Dr. L.E. Nyquist, Thermal Ionization Mass Spectrometry
Working Title: “Investigations of Chemical Processes on Hot Re Filaments in Relation to Ion Yield in Thermal Ionization Mass Spectrometry”
Report #9 (Week Ending 8/6/04)

   On Monday I continued collecting data on chromium. There was one last filament loaded with Cr together with silica gel and boric acid. This element seems to produce a large signal fairly easily, much like strontium. It has been very helpful to watch the progress of these experimental runs while in control of them myself. I can decide when to change the filament current to add more heat and I can decide to wait and watch what the signal is doing. At times the signal was steady and made a fairly straight line on the analog recorder. As I stepped the current higher there were places where the signal would waver up and down erratically, grow dramatically larger, or diminish slowly. These changes can serve as a warning to me that conditions are changing and that I would soon be unable to use further data, should I collect it.
(click on 'more' to read more)

   The experimental runs that I do are quite different from those done to determine the isotopic ratios, that is, they are different from the way one would run the machine normally. I ramp up the current continually from a very low ion signal through a range including that used for data collection in isotopic ratio determinations and well past. I do this in order to collect data on the energy required for ionization. By continually varying the temperature I can see how the signal grows. The rate of the signal’s growth can be used to find the effective energy of ionization. The way I calculate this can be seen in report number four.
   Also, I am only watching the signal from one of the isotopes using only one of the seven faraday cup ion detectors. When the computer is controlling the run and collecting data out of which to calculate the isotopic ratios the operator simply brings the filament current up to a level where the signal is adequate for taking data.
   Later in the day on Monday I spent some time entering new data into existing graphs and set up a filament loaded with zirconium and tantalum pentoxide to heat overnight at 2.2 amps.
   On Tuesday I worked on collecting data on zirconium. It turns out that adjusting the magnet strength to direct particular masses to the center faraday cup is easy but calculating the correct value ahead of time is not. I spent a long time slowly ramping up the filament current and scanning the magnet up and down around the point where I expected to find a Zr signal. After a while, I thought I might be on the wrong track and I changed the filament to the one loaded with strontium. Zirconium is at mass 90 and strontium at mass 88. When I had calibrated the magnet strength I had done so using chromium, which has a mass of 52. Since this is numerically far away I thought that the calibration might be off. By finding a corrected value for the magnet strength needed to direct ⁸⁸Sr into the center cup I could re-calculate the value for ⁹⁰Zr. It turns out this was a good idea because I was off by 0.3% from the correct value, or nearly a full mass number. Also, I had severely underestimated the temperature at which I could first expect to see Zr ions appearing. I finally found a signal when the filament reached over 1900°C. By the time I had focused the signal and run a mass spectrum to ensure I had the correct peak the signal had decayed to the extent that I could not gather any temperature/ion yield data.
   Later that day I had my first successful run with Zr, this time after heating the filament at 2.8 A for an hour. I was able to take down fourteen data points but the signal decayed the entire time, showing perhaps that most of the sample had evaporated from the filament as neutral particles before they could be ionized. On discussing this with Dr. Nyquist we decided to try running Zr again but with ten times the samples size: 4 micrograms rather than 400 nanograms. I spent the afternoon on Tuesday graphing the some of the experimental results against various elemental properties looking for a pattern. Until this week we had neglected to consider the influence of the ‘K’ factor in the equation:
n⁺/n⁰ = Ke^-(Q/kT)
By taking this number from the linear rearrangement of this equation (which is there the y-intercept of the line) and graphing it against various elemental properties we hope to establish a pattern that will guide further investigations. The number can be referred to as a pre-exponential factor. Since we are not sure exactly what is going on chemically, any guess is worth looking at and future experiments can be designed to look at which of these guesses is correct.
We looked at the average atomic mass, as bigger atoms move more slowly than small ones. We also considered the influence of the electronegativity of the elements and this appeared to show a distinct relationship. Surprisingly the ionization potential appeared to show no correlation whatsoever with the pre-exponential factor. Or perhaps this isn’t so surprising since we believe that the ionization potential plays a role in the value of Q.
   On Wednesday I ran the last Zr load in the magazine by simply ramping it up to a temperature where I found a signal. That is, I did not pre-heat it in the way I had done with the two previous Zr runs. I found a signal but it faded before I could even focus it properly. I spent the rest of the day working on putting things together to write my final report.
   On Thursday I waited for next magazine to be loaded with zirconium and magnesium. Dr. Nyquist has decided he would like to try a new substance as an emitter: scandium trioxide (Sc₂O₃). Mr. Reese loaded several samples of both Zr and Mg with each of the two emitters of interest, scandium trioxide and tantalum pentoxide. He also loaded two samples of Mg with aluminum, as it had been done before in this lab. While I waited I working further on data analysis and my final report. I spent some of this time looking at an article I had found for Dr. Nyquist which inspired him to try the scandium as an emitter. He asked me to graph one of the graphs in the article on different scales to make a comparison to the methods we are using to analyze my data. I found that they are using a similar method to calculate energies.
   On Friday I ran some Zr and some Mg. The Zr loads were 10 times the size of the ones previously loaded but still did not produce large signals. Also, they faded quickly, though not so quickly as the smaller samples. The sample of Zr loaded with tantalum ran better and longer than the one loaded with scandium. I took data on three runs of Mg. The first took the longest period of time because I had to locate the signal and recalibrate the magnet strength. I found a peak for Na (mass number 23), which surprised me. Later I asked Dr. Nyquist about it and he told me that this is normal and is used to help in calibration. I also found a strong peak for Al, which confirmed that I had found the strongest Mg peak at mass number 24. For much of the run the signal grew between each change in filament current until I reached a fairly high value of about 8 V (8,000 millivolts). At that time the signal strength dropped precipitously and stayed in the range of 3 V. This would have been fine for collecting data for isotopic ratios but it meant that I could not gather any further information about the energy of ionization. The next Mg run was loaded with tantalum and it was depleted fairly quickly and before even reaching one full volt of signal. The final run, with scandium, was not much better, not even reaching 200 millivolts. It dropped to almost nothing and could not be brought higher even when I increased the filament current to very high temperatures.
   Next week we will run a few more experiments and I have suggested to Dr. Nyquist that we sit down and look over what the summer has produced and see whether we can come to any conclusions. I’ll start wrapping things up definitively by Wednesday.

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