Function and design
multi N/C 2100S (duo, pharma)
24
3.1.5
Detection
NDIR detector
The NDIR detector (non-dispersive infrared absorption detector) is behind the right side
wall of the analyzer.
Gases with molecules from different atoms have specific absorption bands in the in-
frared wavelength range. When a light beam is sent through an arrangement of cells
which contains IR-active gases, these gas components absorb a proportional share of
the total radiation on their characteristic wavelengths according to their concentration
in the gas mixture.
The radiation receiver used in the NDIR detector is selective for CO
2
.
The NDIR detector is not provided for the multi N/C 2100S pharma model.
Measurements using the VITA
method
The CO
2
molecules are detected metrologically as long as they remain in the cell of the
NDIR detector. The measuring gas flow can fluctuate during CO
2
measurement, because,
for example, liquid samples evaporate or condense during dosing. For this reason, the
CO
2
molecules are sometimes detected spectrometrically for a longer time (at lower gas
flows) or a shorter time (at higher gas flows).
The VITA method is formally the residence-time-coupled integration for TOC analyses.
The measuring gas flow is determined in parallel with the NDIR signal in the VITA
method. The NDIR signal is normalized via computer control. This compensates for oc-
curring flow fluctuations, ensuring constant gas flow. Integration is only carried out after
this.
A highly precise digital flowmeter detects the gas flow in the immediate area of the
NDIR detector.
Electrochemical NO detector
(ChD, optional)
For TN
b
detection, the electrochemical NO detector can be used. The NO detector is be-
hind the right side wall of the analyzer. It analyzes the nitrogen oxide (NO) content in
the measuring gas.
After thermal oxidation of the sample, the measuring gas enters the detector. In the de-
tector, the nitrogen oxides diffuse via a highly selective membrane in the electrochemi-
cal measuring cell.
The nitrogen oxides are oxidized at the anode. This alters the current flow between the
electrodes in proportion to the concentration of nitrogen oxide. The change of the cur-
rent flow is evaluated as a signal and the nitrogen content of the analyzed sample is de-
termined from this. The electrolyte in the measuring cell only serves as catalyst and is
not used up.
A supply voltage is required for the operation of the electrochemical NO detector (ChD).
Even if the analyzer is switched off, a support voltage must maintain the electrochemical
equilibrium in the ChD. A battery (U9VL) is installed in the right side part of the analyzer
for this.
The optional ChD is not provided for the multi N/C 2100S pharma model.
Chemiluminescence detector
CLD (optional)
Optional addition of a chemiluminescence detector to the analyzer enables TN
b
determi-
nation. The CLD must be positioned next to the analyzer as an external device.
The measuring gas formed by the thermal oxidation of the sample is dried and then en-
ters the reaction chamber of the chemiluminescence detector. There, the nitrogen
monoxide present in the measuring gas is oxidized with ozone into activated nitrogen
dioxide. Emission of light photons (luminescence) returns the molecules of the nitrogen
dioxide to their original state. The luminescence is recorded. The signal is proportional
to the nitrogen monoxide concentration. The total nitrogen content of the sample can
be determined in this manner.
