Isotope-selective detection

In this section special interest is focused on the intersection of the intersystem crossing of the K2 A state with the b 77u state. Applying one-color pump probe spectroscopy with a wavelength of 833.7 nm, the wave packet dynamics of around = 12 is studied. Isotope-selective detection... 

Other mass spectrometric techniques such as RIMS and AMS possess high isotope selectivity for extreme ultratrace and isotope analysis of, in particular, radiotoxic isotopes ( C, "Ca, Sr, Tc, Pb, U and plutonium isotopes) in the environment, in cosmochemistry, radiodating, nutrition and biomedical research. RIMS has become as an nltrasensitive and selective analytical technique for the determination of extremely low isotope abundances. In spite of the excellent analytical features of RIMS (detection limit for isotopes 10 atoms per sample) and exciting applications for the determination of extremely low abundances and isotope ratios of long-lived radionuclides, such as all plutonium isotopes (including Pu), U or " Ca, but no commercial instrument is available on the analytical market. 

Proton (or inverse ) detection of a heteronucleus ( C, N, Sn, etc.) relies on the selective detection of such proton satellites arising from scalar coupling with that heteronucleus, while suppressing all other proton signals. As for direct heteronucleus observation, the Sn isotope is preferable for proton... 

Koletzko S, Haisch M, Seeboth I, Braden B, Hengels K, Koletzko B, Hering P. Isotope-selective non-disper-sive infrared spectrometry for detection of Helicobacter pylori infection with C-urea breath test. Lancet 1995 345 961-2. 

For measurements of cluster-size distributions in cold molecular beams (Sect. 4.3), or for monitoring the mass distribution of laser-desorbed molecules from surfaces, these combined techniques of laser ionization and mass spectrometry are very useful [105, 106]. For the detection of rare isotopes in the presence of other much more abundant isotopes, the double discrimination of isotope-selective excitation by the first laser LI and the subsequent mass separation by the mass spectrometer is essential to completely separate the isotopes, even if the far wings of their absorption lines overlap [107]. The combination of resonant multiphoton ionization (REMPI) with mass spectrometry for the investigation of molecular dynamics and fragmentation is discussed in Chap. 5. 

The selective detection sensitivity for spurious species with absorption spectra overlapping those of more abundant molecules or atoms can be enhanced further by a combination of mass spectrometer and resonant two-photon ionization (Fig. 10.4). This is, for instance, important for the detection of rare isotopic components in the presence of other more abundant isotopes [1374, 1375]. 

Fig. 10.4 Selective excitation of a wanted isotope by a pulsed, amplified single-mode dye laser with subsequent ionization by an excimer laser and mass-selective detection behind a time-of-flight spectrometer...

A further strength of the mass spectrometric detector is its ability to distinguish between analytes labelled with stable isotopes, thereby allowing such compounds to be used as convenient internal standards for quantification of a wide variety of biomedical and environmental compounds. Alternatively, stable-isotope-labelled compounds may themselves be selectively detected and measured in connection with metabolic and isotopic enrichment studies. 

The isotope-selective analysis by optical detection methods is almost impossible unless transitions with sufficiently large isotope shifts as observed with light and heavy elements are available. In contrast to traditional emission or absorption techniques the high-resolution laser spectroscopy enables Doppler-free measurements since the spectral linewidth of tunable CW lasers is commonly less than the Doppler profile... 

The environmentally imponanl radionuclides Sr and -" Pb have been detected by RIMS-VETA. Two-step excitation of Rydberg states by two cw dye lasers, followed by nonresonant ionization by a cw infrared CO2 laser, was used. The solid samples were dissolved by standard methods. The LODs were 60 ag (6x10 g) for Pb and 30 ag for Sr. Absolute Pb concentrations of 0.12-0.14 fg were measured in brain tissue. Isotope selectivity of lO for Sr/ Sr was achieved. 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

With a change of the excitation wavelength (A = 833.7 nm), the influence of the crossing h JJu state could be investigated. The highly mass-resolved detection allowed the isotope-selective and K2) exploration of... 

---