System Description
3
SCD and NCD User Manual
37
Theory of Operation
The Agilent 8255 and 8355 chemiluminescence detectors detect
target molecules by chemically transforming them in several
steps to an excited species that emits light. The light from this
emission is converted to an electrical signal by a
photomultiplier tube (PMT). For each detector, samples undergo
preliminary reaction(s) with an oxidizer (air for SCD, oxygen
for NCD) and hydrogen in a very hot reaction zone (the burner)
at reduced pressure to form either SO or NO in addition to other
products such as H
2
O and CO
2
. The reaction products then flow
to a reaction cell in a separate detector module. In this cell, they
mix with ozone (O
3
) produced from oxygen using an ozone
generator. The O
3
reacts with SO or NO to generate SO
2
* and
NO
2
* respectively. This reaction cell operates at a pressure of
about 4-7 Torr. These high energy species return to ground state
by chemiluminescence. The emitted light is filtered and then is
detected by a PMT. The electrical signal produced is
proportional to the amount of SO
2
* or NO
2
* formed in the
reaction cell. The sample exits the reaction cell, passes through
an ozone destruction trap, and then passes through a vacuum
pump and out to vent.
SCD
The SCD uses the chemiluminescence (light-producing reaction)
from the reaction of ozone with sulfur monoxide (SO) produced
from combustion of the analyte:
The pressure differential produced by a vacuum pump transfers
the combustion products into a reaction cell, where excess
ozone is added. Light (h
) produced from the subsequent
reaction is optically filtered and detected with a blue-sensitive
photomultiplier tube, and the signal is amplified for display or
output to a data system.
NCD
The NCD uses the chemiluminescence of ozone with nitric oxide
formed from combustion. Reacting nitric oxide with ozone
results in the formation of electronically excited nitrogen
dioxide. The excited nitrogen dioxide emits light, a
chemiluminescence reaction, in the red and infrared region of
SO + H
2
O + other products
Sulfur compound (analyte)
SO + O
3
SO
2
+ O
2
+ h
Q
(< 300-400 nm)