Frans_M_Everaerts_Isotachophoresis_378342.pdf

UV ABSORPTION METER 165
dicular to this hole, a hole is drilled of diameter 0.3 mm with a conical shape at the
outside on both ends, so that the quartz rod (4) can approach the central hole as close
as possible. The diameter of the quartz rods (optical quality) is 3 mm. The distance
between the quartz rods and the narrowbore tube must be made as small as possible,
otherwise some of the UV light is lost by absorption by oxygen. The quartz rods are
fixed by brass holders (5,ll). Before mounting, the outside of the optical quartz rods
and the inside of the brass holders must be cleaned carefully with cyclohexane in order
to remove all UV-absorbing material. After ths procedure, the quartz rods must be
handled with a pair of clean tweezers in order to prevent dirt from sticking on the surface.
An adhesive must not be used to fix the quartz rods in the holders. Any resin on the
surface of the rods absorbs large amounts of W light; when the rods were fixed in the
holders with Cyanolite, the signal was reduced to 1% of its original value. The two brass
holders, with the quartz rods fixed simply with an O-ring, clamp the slit holder (12) by
means of a set of O-rings. The flanges fit the central housing (not shown in Fig.6.30)
and fixes the slit holder at a pre-determined position.
The edges on the ends of the holders of the quartz rods act as an important lock for
daylight, together with components 10 and 12. This lock is not as important at the side
of the W source because the combined interference filter and end fdter is mounted at
the side of the UV detector (8). If this lock is not positioned in front of the photodiode
(R 330) at least a noisy signal may be expected and too much daylight may even destroy
the phototube.
The holder 9 contains the combined interference filter and end filter. In the W source
(14), the UV light is generated by the microwave-powered low-pressure mercury
electrodeless lamp (6). A slide (13) makes it possible for the narrowbore tube not to be
exposed constantly to the UV light, which may damage the PTFE. After a long exposure
time, the quality of PTFE coloured by UV light is poor. For optimal stability of the UV
lamp, it is preferable to have the lamp constantly in the bright mode. After switching on,
it requires at least 1.5 h to attain full optimal stability. The slit holder (12) contains an
extra hole that permits the conductivity detector to be mounted near the W detector
cell. It has proved to be unimportant if the sequence of either the UV detector and the
conductivity detector is altered. Of course, exceptional cases can occur.
6.5.6. Experimental
It does not need a long explanation about the way in which the UV detector will
produce supplementary information about the isotachophoretic separation, because many
ions lack W absorbance and so many successively moving zones that contain ions without
any UV absorbance will not be detected. In some instances, minor UV-absorbing
components (see Fig.6.15) are present in the electrolytes (the leading electrolyte, the
sample and the terminating electrolyte). These impurities can ‘mark’ a zone boundary,
because these components are also concentrated in zones, sandwiched between the
sample zones.
If a mixture of mainly non-UV-absorbing components is available, one can add to
the sample an extra amount of UV-absorbing markers in such a concentration that, if
these markers cannot be separated from the sample ions and form stable mixed zones, they