Ion mobility spectroscopy, or IMS, is a technology that analyzes the components in a gas by separating the ions inside. With an ion source that ionizes the gas, the ions are traveling across an electrical field, where ions traveling at different velocities (mainly due to different masses) arrive the destination at different times.

When one group of ions arrives, the detector, typically a simple electrode, collects the charge and results in a current peak. When the other group of ions arrives, another peak appears. The current is amplified and converted into voltage peaks, and a spectrum of voltage vs. time can be plotted, which indicates the identities of the ions, thus the components of the gas.

The diagram below is a sample form of IMS using a Ni-63 radiation source to ionize the gases. The collector is simple a metal disk. One useful design is the gates, which open and close to reset the measurements, allowing repetition that decreases the signal to noise ratio.

Hill Jr H H, Siems W F, St. Louis R H. Ion mobility spectrometry[J]. Analytical Chemistry, 1990, 62(23): 1201A-1209A.

An IMS of 2-pentanone in air:

Ion Mobility Spectrometry (Second Edition): ISBN 0-203-50475-5

The main advantage of IMS over the traditional methods of identifying gas components is that it does not involve vacuum. The carrier gas in IMS can perfectly be air, so the portability and cost of the device is greatly reduced. For this reason, IMS is generally applied in handheld devices like the toxic gas detector for security or military or drug and explosive detector in airports.

ChemPro100 is a popular military chemical detector for warfare agents and toxic gases. It applies an 160uci Am241 radiation source as the ionizer and has a size of barely 880g. I recently got a broken one, so I decided to break it into pieces and reuse the Am241 source for my other projects like the XRF experiment (the source is NRC exempt).

Gas inlet with a replaceable filter:

The first board locates at the upper side of the device. This is probably the IMS module, including the ionizing chamber with the Am241 and the amplifiers.

Below the IMS board, there is a black power supply board: the below picture is the inner side of that board.

Under the power supply board, a small gas pump appears, and it is connected to both the IMS chamber and the gas outlet at the top

Connection to the gas outlet:

Connection to the back side of the IMS chamber. This is the pumping inlet, and the other end of the IMS chamber is connected to the filter of the gas inlet at the top of the detector using the short metal tube in the above image.

Opening the black IMS chamber, one can see a lead cap, and after removing the cap, the radioactive source can be seem, plated on a silver foil and fixed by a screw. Beside the source chamber there is a smaller chamber with a chip inside, but I cannot confirm what is the function (maybe a flow meter?).

The cap:

The cap has an open, connected to the port on the golden board, leading to another golden board below.

The below golden board has many channels. I’m not sure how they work.

The main controller is below the IMS chamber, and there is an antenna on the backside.

Under this control board is the screen module, but due to a bad screw, I was not able to take it out.