This instrument cryogenically concentrates a sample of air to trap certain volatile organic compounds (VOC’s) that otherwise could not be measured. Once concentrated, the sample air is injected into a Gas Chromatograph. Normally, this is done with liquid nitrogen (LN) stored in a separate Dewar. The uniqueness of this instrument is it eliminates the need for LN. This and allows it to be transported and used at remote locations where the use of LN is impractical. Not only does this instrument eliminate the need for LN but, it is a fraction of the price. To date two versions of this instrument have been developed and used in multiple experiments. Version 3 of this instrument is on the horizon. We have a new cryogenic system that should halve the sampling time (~0.5 hours). We designed it to be versatile and easy to use by equipping it with a graphical control interface on a palm pilot. It is designed to run side by side with a Kleemenko cooling unit and a gas chromatograph.

Helium, an inert gas, is pumped through to flush the system at the inlet A. Then the air sample enters and runs through the series of solenoids and the Mass Flow Controller (MFC) at B and is sent up to the stream select valves at C.

The sample then briefly exits the preconcentrator at D to pass through the small Dewar of the MMR. The sample returns to the preconcentrator only to be sent to the large Dewar. In the large Dewar, the air samples pass through a super-chilled loop (-180°C) so that whatever organic compounds it contains condense on the six-micron glass beads packed at the bottom of the loop. Then the heaters turn on in the MMR to vaporize the previously condensed organic compounds and the switcher valves in the preconcentrator rotate to flush the large Dewar with helium and send everything into a Gas Chromatograph.

The circuit board controls the heaters, the stream select valve, the switcher valve, the solenoids, and the MFC. The heaters are regulated via a potentiometer and maintain a steady working temperature. The Palm pilot controls the remaining procedures through the data acquisition device (DAQ). The incorporation of the Palm Pilot gives us user-friendly control over not only the preconcentrator but also the MMR. The Palm controls the cycles in the preconcentrator and the temperatures in the MMR. All the data from the circuit board is relayed to the Palm pilot through the DAQ. It converts the analog electrical signals to digital and vice-versa.

We fitted the machine with a series of solenoid valves to regulate flow in the system and send the samples to the correct destinations. For safety measures we fitted the system with a 50 psi pressure sensor and a blow-off valve incase the pressure spiked or human error allowed too much helium in. MFC regulates the speed of the air samples so that the amount of sample passed through is merely of function of time. The excess sample is re-routed back through solenoids until the proper mass flow is achieved.

The air samples leave the preconcentrator in long heated tubes at the back designed to reduce condensation on the way to the Gas Chromatograph. The heaters are regulated by a potentiometer on the circuit board and are monitored by thermocouples. The metal plate that the switcher valves are mounted on is also heated to prevent condensation inside and near the valves.

Future Work

The third version of this instrument will be much different than the first two. The goal of the third version is to: 1) make the unit independent of an external personal computer 2) eliminate the third party temperature controllers 3) consolidate the instrument to one box. To do this, we will have to design a control system from scratch. This involves considerable hardware and software development