Speaker
Description
One of the strengths of Resonance Ionization Mass Spectrometry (RIMS) is its selectivity, that is the ability to ionize only one element in a sample containing many different elements and thereby eliminating the need for chemical purification prior to analysis. However, in cases where backgrounds are unavoidable, such as ultra-trace analysis or when overwhelming excesses of isobaric atoms are present, RIMS has the unique ability to quantitatively measure backgrounds by tuning the laser(s) off-resonance. The resonance signal of the analyte element disappears, leaving behind ions produced by other processes such as secondary ionization due to sputtering, off-resonance ionization of atoms and molecules, and photodissociation of molecules into fragments isobaric with the analyte. RIMS spectra generally take from several to several tens of minutes to collect. Provided that the sample does not change over that time under the influence of the ion beam or laser used to atomize it, off-resonance spectra collected later can be quantitatively subtracted to produce background-free results. However, ion sputtering during analysis can change the ratio of atoms to molecules in the sputtered flux over time, and hence change the contribution of non-resonant background ions caused by photodissociation. Further, in the case of ultra-trace analysis in which lasers are used to desorb material, the elemental composition of the sample changes over time as the more volatile components are preferentially removed in the early stages of the analysis. In these cases the off-resonance spectrum collected many minutes later is not a true representation of the background present in the resonance spectrum which was, in effect, collected on a different sample.
To address this issue we have developed a RIMS method known as blinking, in which the resonance signal is extinguished (or “blinked”) by switching one of the lasers between on- and off-resonance every other (or every third, or fourth, etc.) pulse. In this way, perfect background subtraction is possible, since the on- and off-resonance spectra are interleaved rather than sequential, and both therefore sample the same time-dependent changes in the analyte. We first demonstrated this technique by alternating on- and off-resonance pulses from two lasers and achieved success in measuring 238Pu accurately at concentrations less than 1 ppb in a soil matrix containing a large (~30,000:1) excess of 238U. We later developed a Ti:Sappire laser capable of self-blinking, such that a second laser is not needed, and measured 238Pu against an excess of 238U of ~50,000:1. The new laser has no moving parts, so there is no settling time after switching wavelengths, and can therefore blink at an arbitrary rate (we currently use 1500 Hz). In this talk, we demonstrate this laser for trace analysis of 238Pu, as well direct analysis of ultra-trace fission products in irradiated uranium. LLNL-ABS-858820
| Workshop Themes | Laser design/performance |
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