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R. S. Houk
Analytical Spectroscopy, Inorganic Mass Spectrometry.
Our research involves the fundamental study and applications of new ionization techniques for mass spectrometry (MS). The general objective is to devise new methodology for important analytical problems based on the sensitivity and selectivity of MS when combined with an appropriate ionization source. We were the first to extract ions from an inductively coupled plasma (ICP) for analysis and detection by MS. The mass spectrum depicted here shows peaks from an aqueous solution of CdI2 at only 10 ppb. All eight Cd isotopes and the only isotopeof I are readily detected as Cd+ and I+. The ICP-MS approach is a considerable improvement on existing techniques for elemental and isotopic analysis of solutions. It is now being widely used in numerous scientific disciplines including biomedicine, geochemistry, mining, materials science, and the nuclear industry.
Three recent areas are described to illustrate our program:
Inorganic Elements in Biological Molecules (see publications 76, 83, 88, 111 and 117)
This work has been done in collaboration with Finnigan MAT and CETAC Technologies Inc. An ICP-MS device can readily measure unusual elements attached to proteins, DNA or other biological molecules. A chromatographic separation is combined with the ICP-MS device to identify the molecules. The chromatogram shown below is an example. Proteins in human serum are separated by size exclusion chromatography, while the ICP-MS device monitors particular elements of interest, uranium and thorium in this case. The retention time of the chromatographic peaks corresponds to that of large molecules, which shows that these elements are attached almost completely to proteins in serum. Other such experiments have new proteins that contain selenium and have studied the oxidation of DNA by Cr(VI) and the binding of Pt from the anticancer drug cisplatin to DNA.
Analysis of Solids by Laser Ablation ICP-MS (see publications 89, 100 & 115)
Usually, the sample is dissolved before it is analyzed by ICP-MS. Many materials resist dissolution, however, and the analytical results are often needed too rapidly for such pretreatment procedures. There remains a long-standing need for a general method for analysis of solids as is. We are investigating laser ablation ICP-MS for this purpose.
It is easy to identify the elements present but much harder to calibrate the response, since the preparation of solid standard materials is difficult. We have recently shown that dried particulates produced from a nebulized solution can be used to calibrate the response of a laser ablation ICP-MS device. Analytical results for the analysis of steels are accurate to within a few percent, which is comparable to the accuracy achievable by ablating matched solid standards or by dissolving the sample.
Inorganic Electrospray MS (see publications 102 & 109)
The selectivity and sensitivity of ICP-MS derive from the fact that the ICP produces mainly atomic ions. Electrospray is a much softer source that generally provides intact parent ions and allows measurement of molecular weight and structure.
We have recently begun to investigate electrospray MS for identification of unknown inorganic molecules and for distinguishing different forms of the elements. An example is given below. Solutions containing either the cation Cr(III) or the anion Cr(VI) are sprayed with an excess of chloride in the negative ion mode. The Cr(VI) solution produces mainly CrO3-, while the Cr(III) in the other sample can be identified as CrOCl2-. The toxicity and biological role of these two forms of Cr differ markedly; Cr(VI) is a carcinogen while Cr(III) is essential, at least at low concentrations. The observation that both forms of Cr can be distinguished under the same conditions provides a fast, easy way to distinguish these compounds.
The suitability of ICP-MS as an element of selective chromatographic detector that can also determine isoptope ratios is being studied. We also perform fundamental studies in which we measure temperatures, electron densities, and individual reactions paths in the ICP by MS.
Reduction of polyatomic ion interferences in ICP-MS by removing the solvent (publications 105 & 108)
ICP high resolution MS with Fourier transform ion cyclotron resonance (collaboration with J. R. Eyler, Univ. of Florida, publication 107) and new concepts for high resolution with a quadrupole MS (publication 114)
Identification of the reaction products of electrochemical incineration of toxic organic compounds (collaboration with D. C. Johnson, ISU, publications 101, 112 & 116)
Methodology for precise measurement of isotope ratios (publications 87, 100 & 103)
Characterization of conditions in the ICP and the ion extraction process and whether the mass spectrum reflects the properties of the plasma (publications 86, 91, 96, 99, 104 & 106)
Courses taught recently:
- Chem 155, 165 General Chemistry for Engineers
- Chem 211L Quantitative Analysis Laboratory
- Chem 316 Instrumental Analysis (lecture & lab)
- Chem 513 Analytical Spectroscopy
- Chem 577 Mass Spectrometry
Recent innovations include:
Taught Instrumental Analysis to students at Salsbury Chemicals in Charles City via a fiber optic link through the Iowa Communications Network.
Acquired several major equipment items including a triple quadrupole MS and an ICP-MS for undergraduate laboratories.
Implemented lab experiments in ICP-MS that span the last three years of the undergraduate experience. These experiments integrate concepts from course work in analytical, physical and inorganic chemistry that are usually segregated into different courses and illustrate the interconnected nature of chemistry.