Sensitivity of Ionization Spectroscopy

The following estimation illustrates the possible sensitivity of ionization spectroscopy (Fig. 6.28a). Let Nj be the density of excited molecules in level Ej, Pki the probability per second that a molecule in level Ef is ionized, and Ua the number of photons absorbed per second on the transition E/ . If R/c is the total relaxation rate of level besides the ionization rate (spontaneous transitions plus collision-induced deactivation) the signal rate in counts per second for the absorption path length Ax and for ni incident laser photons per second under steady state conditions is  

With a proper design the collection efficiency 5 for the ionized electrons or ions can reach 5 = 1. If the electrons or ions are accelerated to several keV and detected by electron multipliers or channeltrons, a detection efficiency of rj = can also be achieved. If the ionization probability Pk can be made large compared to the relaxation rate Ru of the level k), the signal S then becomes with 8 = r] = 1  

This means that every laser photon absorbed in the transition Ei Ek gives rise to a detected ion or electron. It implies that single absorbed photons can be detected with an overall efficiency close to unity (or 100%). In experimental practice there are, of course, additional losses and sources of noise, which limit the detection efficiency to a somewhat lower level. However, for all absorbing transitions Ei Ek where the upper level Ek can readily be ionized, ionization spectroscopy is the most sensitive detection technique and is superior to all other methods discussed so far [6.73,6.74]. 

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The following estimation illustrates the possible sensitivity of ionization spectroscopy. Let be the density of excited molecules in level Ej, the probability per second that a molecule in level E is ionized, and n the number of photons absorbed per second on the transition E. If is the... 

The sensitivity of ZEKE spectroscopy is high because, even though a small fraction of the molecules excited by the PROBE laser have n-values within the range capable of being ionized by the electric field detection pulse, the ions produced from these special ZEKE states are detected with 100% quantum efficiency against a perfectly dark background. 

The sensitivity of vibrational spectroscopy allows interactions of polyelectrolytes and surfactants to be monitored in aqueous and nonaqueous solutions. The solubilization and conformational properties of a comb-shaped copolymer of 1-octadecane-co-maleic anhydride in aqueous solution in the presence and absence of SDS depend on the degree of ionization of the copolymer (2i). The C-H stretching region of the Raman spectrum is sensitive to such interactions. Figure 11 illustrates how the C-H stretching band shifts as a function of solvent (in this case water and heptane). 

A more detailed representation of ionization spectroscopy and its various applications to sensitive detection of atoms and molecules can be found in [92-94, 99, 100, 103]. 

The sensitivity of Raman spectroscopy in the gas phase can be greatly enhanced by combination with one of the detection techniques discussed in Chap. 6. For example, the vibrationally excited molecules produced by Raman-Stokes scattering can be selectively detected by resonant two-photon ionization with two visible lasers or by UV ionization with a laser frequency (o ], which can ionize molecules in level Ef but not in E (Fig. 8.8). 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Anbar, Determination of Subprogram Amounts of Chemical Agents in the Atmosphere , Edge-woodArs Contract Rept EC-CR-74028, SRI Proj 3122 (1974) ( A method of mass spectroscopy, employing a silicone membrane and field ionization, which involves other new techniques, is presented which is sensitive to picogram amts of chemical agents in the atm)... 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

Eluate from a chromatography column can be passed through a plasma to atomize and ionize its components and measure selected elements by atomic emission spectroscopy or mass spectrometry. An atomic emission detector directs eluate through a helium plasma in a microwave cavity. Every element of the periodic table produces characteristic emission that can be detected by a photodiode array polychromator (Figure 20-14). Sensitivity for sulfur can be 10 times better than the sensitivity of a flame photometric detector. 

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