Fluorescence volume

FIGURE 11.32 Flow profiles in microchannels, (a) A pressure gradient, - AP, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule, (b) In electroosmotic flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness D. On the right is an experimental measurement of the distortion of a volume of fluid in an electroosmotic flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule [165], Source http //www.niherst.gov.tt/scipop/sci-bits/microfluidics.htm (see Plate 12 for color version). 

Furthermore, laser-produced plasmas may also be used as the fluorescence volume [226], which has been shown to be particularly useful for direct trace determinations in electrically non-conducting solids. 

In flame AFS, elements which form thermally stable oxides such as Al, Mg, Nb, Ta, Zr and the rare earths are hampered by insufficient atomization. This is not the case when an ICP is used as the fluorescence volume. Here the detection limits for laser excitation and non-resonant fluorescence are lower than in ICP-AES (Table 18) [663]. ICP-AFS can be performed for both atomic and ionic states [664]. 

The narrow bandwidth can ensure that the resolution limit is defined by the molecule rather than by the light source. The low divergence can facilitate the use of long path lengths to enhance weak absorption, or can be used to concentrate the power into a small volume, providing an ideal linear fluorescent volume of gas which can be efficiently imaged on to a spectrometer slit. 

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