multi N/C pharma HT
Function and design
23
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
.
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.
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.
Sample digestion for TN
b
detection cannot result in 100 % NO recovery. While the com-
bustion gases are cooled and condensed, nitrogen oxides also form at higher oxidation
levels.
