Zero background technique

The various techniques for measuring fluorescence decays are described below in more detail. All fluorescence techniques, to varying degrees, have the advantage that they are zero-background techniques and thus are usually more sensitive compared to other methods, like absorption. 

Photoelectron spectroscopy is a zero-background technique, meaning that the detector sees nothing (except a weak noise signal) until there is an interaction between the radiation and the sample. In addition, our ability to measure charged particles such as electrons or ions is usually much better than our abUity to detect photons. 

Widespread interest has developed during the last few years in a new analytical technique which uses a tandem electrostatic acclerator as one element of a double mass spectrometer [1-19]1. With this technique, individual atoms of many nuclear species from most areas of the periodic table can be identified with good efficiency and in the presence of an almost zero background of unwanted atoms and molecular species of the same mass. 

In contrast, the direct detection technique for 14C can count approximately one percent of the 14C atoms that are present in a 200 pg sample with virtually zero background in times that are of the order of a few hours. Thus, for milligram quantities of carbon, the improvement in sensitivity of direct counting over radioactivity is of the order 7.2 x 10s. 

PTIS is not quantitative as the relative intensities of the photoconductivity peaks depend on temperature, but it is very sensitive as can be inferred from Fig. 4.4, since, in the best cases, the peaks emerge from a zero background, and impurity concentrations as low as 107 cm-3 can be detected and identified by this technique. 

Only about 10-15 % of organic compounds are able to fiuoresce naturally and these tend to have certain features in common such as a rigid and planar structure, e.g. polyaromatics, heterocyclics and uranyl compounds (Figure 2.22). However, most molecules can be derivatised if fluorescence is the desired technique. Fluorescence does not have wide applicability but has excellent sensitivity as the signal is measured against a zero background. It can also be a very selective technique when the compound of interest fluoresces and other components in the sample do not or fluoresce at different wavelengths. 

A three-pulse technique using a synchronously pumped mode-locked dye laser together with a modified Michelson interferometer has been described. By this technique, ps fluorescence decay times may be evaluated without the disadvantages of up-conversion or Kerr cell methods. The suitability of the system for the analysis of low. optical quality samples was suggested. An injection mode-locked Nd-YAG ring laser was used as an excitation source for a zero-background fluorescence study of the time evolution of the emission from large hydrocarbons with 12 ps resolution. ... 

There are some elaborate techniques that are extremely sensitive because they are based on zero-background signals. The most important of these are the dichroitic and the holographic techniques. 

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