In streams that contain only one of these compounds (or if a total chloride measurement is satisfactory), the measurement can be made directly without removal of the water. This eliminate the need for the chiller or other driers. In some cases, a dilution type probe may be used.

Theory of Operation
IMS is an ionization based time of flight technique, performed at atmospheric pressure. A description of the IMS cell is seen in figure 1. The sample is drawn over a semipermeable membrane by way of an internal eductor. The membrane serves several purposes. It serves to protect the interior of the cell from particles and high moisture levels, provides a degree of selectivity, and allows various levels of sensitivity based on permeation rate. The molecules of interest permeate through the membrane, and are picked up by the carrier flow, which sweeps the other side of the membrane. The carrier stream delivers the sample molecules to the reaction region of the cell, which contains a small Ni63 radioactive source.

There the sample is ionized as a result of a series of ion-molecule reactions. In most cases, compounds known as dopants are added to the carrier stream. These dopants enter into the ion-molecule chain of reactions to provide a degree of selectivity based on the charge affinity of the analyte. Once the sample has been ionized, the ions begin to drift towards the opposite end of the cell due to the influence of an electrostatic field. A shutter grid is located in the tube which can be biased electrically to either block the ions, or allow them to pass through. This shutter grid is pulsed periodically to allow the ions into the drift region. There, they begin to separate out based on their size and shape while flowing counter to a drift gas flow which is introduced at the end of the drift tube. A collector plate located at the end of the tube detects the arrival of the ions by producing a current. This current is amplified to produce a time of flight spectrum. Ions are identified by their characteristic drift time position in the spectrum. Spectra reflect the charge sharing phenomenon characteristic of IMS. The Ni63 source provides a finite amount of charge. The sample gas is ionized by accepting charge from the dopant ion. Thus, as the concentration of the sample gas increases the sample ion peak height increases, while the dopant ion peak height decreases.

Analyzer and Sample System Description
The analyzer is seen in figure 2 below. Housed in a stainless or painted steel enclosure, the controlling electronics are located on the left hand side and the temperature controlled pneumatics are located in an inner enclosure on the right hand side. The front panel contains the user interface via a flat touch panel and back-lit LCD display. From the front panel the operator can perform calibrations, set alarm levels, set password protection, acknowledge alarms and all related tasks. The front panel also displays the concentration, diagnostics information, as well as analyzer and alarm status. Remote indication of concentration is provided by RS-232 and/or 4-20 mA loop, and contact closures provide alarm status. The analyzer can be supplied with on-board calibration, allowing the operator to periodically check for accuracy. However, stack monitoring requirements typically require calibration through the sample probe, using certified gas cylinders. Utility requirements are 110-240 VAC and clean dry instrument air.