Frans_M_Everaerts_Isotachophoresis_378342.pdf

164 DETECTION SYSTEMS
IClz is a double-balanced modulator/demodulator. The input 'Dem in' is connected
with the output of the circuit shown in Fig.6.28.
The potential difference between the connection points 7 and 8 of the ICll is a square
wave that is in phase with the modulation of the W source. Synchronous detection is
achieved by multiplying the voltage on the input by this square-wave voltage. If UV light
from the UV source penetrates the W-sensitive R 330 phototube on the input, a squarewave
voltage also results. On the output of the differential amplifier I&, a d.c. voltage
is the result, which is proportional to the amplitude of the square-wave current in the
phototube. Alternating voltages on the input with frequencies that are not identical with
the modulation frequency or the odd harmonics cause alternating voltages on the output.
These voltages are suppressed by the 6.8- and 68-pF capacitors. The time constants that
result from these capacitors in the amplification of the differential amplifier are of the
same magnitude as that of the potential recorder. The output voltage 1 ~ ~ lo9 x 3 i’,
where Pis the peak-to-peak value of the square-wave current in the phototube; vu can
never exceed 100 mV. The peak-to-peak value of the noise voltage on the output is less
than 0.5 mV.
6.5.5. UV cell
A schematic diagram of the UV detector cell is given in Fig.6.30.
The PTFE narrow-bore tube (1) is not interrupted and is pulled through a slit (3),
shown scaled up in order to demonstrate its construction. The slit is made of brass with
an axial hole of 0.7 mm so as to enable the narrow-bore tube to pass through. Perpen-
Fig.6.30. Schematic diagram of the W detector. A = direction towards the injection system
(terminator compartment). B = direction towards the counter electrode compartment. 1 = PTFE
narrow-bore tube; 2 =position free for mounting the conductivity detector; 3 = W slit; 4 = quartz rod
(optical quality); 5 = holder for the quartz rod; 6 = microwave-powered mercury electrodeless lamp;
7 = photodiode (R 330, Hamamatsu); 8 = UV detector; 9 = holder for end fiter and interference
filter; 10 = construction for assembling 11 and 9; 11 = holder for the quartz rod; 12 = construction
for fixing 1, 3, 5 and 11; 13 = construction for mounting 5,14 and a slide; 14 = UV source.