Rebecca VanWagoner and Dr. Paul Farnsworth, Chemistry and Biochemistry
Inductively coupled plasma mass spectrometry (ICPMS) is a useful tool for trace chemical analysis. However, instrument response to the analyte changes with increasing concentration of other elements in solution. This change in response due to matrix elements lowers the accuracy of ICPMS. Most of the matrix effects occur in the vacuum interface between the plasma torch and the mass spectrometer. My goal in this research project was to map the temperatures and velocities of analyte inside the ICPMS interface and look for changes with changing matrix concentration. Knowing where and how changes occur would point to why matrix effects happen and lead to a way to eliminate them.
During the semester, I learned to use several instruments necessary for spectroscopic research. With a CCD array, I determined the volume of the space sampled by the optical probes and created images of this area. The research group built an interferometer which displays evenly spaced peaks of light as a function of laser wavelength. I set up a rubidium cell with the interferometer to precisely calibrate the Doppler shifts of the moving atoms. I became familiar with a diode laser, oscilloscope, lock-in amplifier, function generator, and computer software. I used these instruments to obtain spectra of metastable argon atoms in an ICPMS and to correlate the spectra with velocities and temperatures.
Argon velocities and temperatures were found at different point within the first vacuum stage of the ICPMS interface. The atoms entering the interface increased in velocity and decreased in temperature as they moved downstream. The absorbance signal decreased as the atoms spread out. Effects of a shock structure could also be seen in the spectra. Behind the sampling cone 10 mm and to the side 8 mm, atoms were moving backward relative to the probe in the barrel shock. At 16 mm away from the sampling cone, the signal became bimodal, denoting the Mach disc. Part of the atoms slowed down and heated up while others were relatively unaffected.
Recently, I have also obtained spectra of calcium fluorescence in the vacuum interface. The calcium data is noisy and the signal must be optimized before calculations can show the temperature and velocity profiles. I hope to accomplish this soon and move on to solutions with different matrix elements and concentrations.
To understand how argon and other components of the plasma expand in the vacuum interface, I read many articles on ICPMS and matrix effects. I learned how repulsions between ions in the plasma cause spreading which decreases signal in the mass spectrometer. These effects become more pronounced with higher matrix concentrations and especially in the presence of easily ionized elements like alkali metals. Matrix effects include decreased ion transport efficiency to the MS detector, lower ion kinetic energies, and inaccurate signals.
With this knowledge and the work I had done so far, I presented my research at the Spring Research Conference. I showed argon spectra, the experimental setup, and the CCD pictures of the probed area inside the vacuum chamber. I talked about the processes of expansion and matrix effects, and shared the goals that I am striving to accomplish as the research proceeds.
As I continue to study the processes in the ICPMS, I expect to see differences in the way that argon atoms and calcium ions are distributed in the plasma with different matrixes. Calcium ions should be strongly repelled by matrix ions and spread more than neutral argon. The velocity may decrease in the center of the stream and be enhanced in the off-axis regions.
The velocity will then be resolved into radial and axial components as I measure the absorbance on opposite sides of the center axis. With the probes angled at 45 degrees, I should be able to tell what portion of the velocity measured is due to motion downstream and what portion is owed to motion away from the center line.
Noting the changes in temperature and velocity that result from changing the matrix composition and concentration, I will be able to tell where and why matrix effects are occurring and thus help eliminate them. I am anxious to correlate the data I receive with the results from research performed by other group members. We hope to correlate matrix effects with plasma potential and the changes in velocity distributions of analytes in the plasma. The combined research will provide a mechanism to illustrate the processes occurring in the ICPMS interface.
The research has not been without problems. Several setbacks have delayed the project. The interferometer was more difficult to align than I expected. Oil from the vacuum pumps flowed into the vacuum chamber so that all the equipment had to be cleaned up, and new procedures had to be developed to prevent the oil from coming in again. A diode laser module died. It lased at the proper wavelength for the argon transition, and the argon experiments are delayed until we obtain a new module. Despite these setbacks, the work continues, and I am still hopeful that the project will be completed and published.