Teledyne Chemiluminescence Detection in the 9110T Reaction Cell, Photo Multiplier Tube PMT

Model 9110T NOx Analyzer Principles of Operation

NO2* →NO2 + hν1200nm

All things being constant (temperature, pressure, amount of ozone present, etc.), the relationship between the amount of NO present in the reaction cell and the amount of light emitted from the reaction is very linear. If more NO is present, more IR light is produced. By measuring the amount of IR light produced with a sensor sensitive in the near-infrared spectrum (see Figure 13-2) the amount of NO present can be determined.

In addition, sometimes the excited NO2 collides with other gaseous molecules in the reaction cell chamber or even the molecules of the reaction cell walls and transfers its excess energy to this collision partner (represented by M in the equation 12-3 below) without emitting any light at all. In fact, by far the largest portion of the excited NO2 returns to the ground state this way, leaving only a few percent yield of usable chemiluminescence.

Equation 13-3

NO2* + M → NO2 + M

The probability of a collision between the NO2* molecule and a collision partner M increases proportionally with the reaction cell pressure. This non-radiating collision with the NO2* molecules is usually referred to as third body quenching, an unwanted process further described in Section 13.1.5.2.

Even under the best conditions only about 20% of the NO2 that is formed by the reaction described in equation 12-1 is in the excited state. In order to maximize chemiluminescence, the reaction cell is maintained at reduced pressure (thereby reducing the amount of available collision partners) and is supplied with a large, constant excess of ozone (about 3000-5000 ppm) from the internal ozone generator.

13.1.2.CHEMILUMINESCENCE DETECTION IN THE 9110T REACTION CELL

13.1.2.1. The Photo Multiplier Tube (PMT)

The 9110T uses a special kind of vacuum tube, called a photo-multiplier tube (PMT), to detect the amount of light created by the NO and O3 reaction in the reaction cell.

Photons enter the PMT and strike a negatively charged photo cathode causing it to emit electrons. These electrons are accelerated by an applied high voltage and multiplied through a sequence of similar acceleration steps (dynodes) until a useable current signal is generated (see Section 13.5 for a more detailed description). The more light present (in this case photons given off by the chemiluminescent reaction described above), the more current is produced. Therefore the more NO present in the reaction cell the more current is produced by the PMT.

The current produced by the PMT is converted to a voltage and amplified by the preamplifier board and then communicated to the 9110T’s CPU via the A D converter circuitry on the analyzer.

13.1.2.2. Optical Filter

A high pass optical filter, only transparent to wavelengths of light above 645nm, placed between the reaction cell and the PMT (see Figure 13-1) in conjunction with the response characteristics of the PMT creates a very narrow window of wavelengths of light to which the 9110T will respond.

Teledyne Analytical Instruments 320