Grantee Research Project Results

A new cost effecting instrument is capable of analyzing the semi-volatile organic and inorganic gas phase precursors.

Starting on September 1st, 2009, Dr. Sonya Collier commenced work to continue where Dr. Zhang left off.  The main issue thus far has centered around the low ion signal making detection of trace compounds more difficult.  Multiple approaches have been implemented to narrow down possible sources for the low ion-current.  Initially the most obvious problems were eliminated beginning with adding a sealing case over the PCB board stack to guard against leaks that would otherwise disturb the laminar flow in the mobility cavity.  The sealing sleeve was implemented along with insulating end flanges that pressed down the whole stack more uniformly.  This also helped eliminate the possibility of imperfect electrical contact between PCB boards which might disrupt the uniform electric field needed for proper analysis.  Another approach was to utilize SimION along with a user-defined program to simulate the passage of ions through the mobility spectrometer and predict spatial distribution of different ions as well as diffusion losses.  One cause for the low ion current that was considered was that an under-estimation of diffusive losses had been made.  The SimION model confirmed that the diffusive losses would be low.  Experimentally the mobility spectrometer was reconfigured in order to test diffusive loss as well as total ion current without mobility separation.  This series of tests confirmed that the diffusive losses were indeed what were expected and these tests also pin pointed the main issue with the low ion current.  The Nickel-63, though theoretically able to produce 30 nA of ion current at the nominal flow rate for the sample input into the instrument, was actually yielding around 1-3 pA.  Two main causes for this have been hypothesized: first the radioactive foil is wedged in a small space where the actual space is much smaller than the average penetration distance for the ionizing beta particles thereby limiting the total current output, and second, the pinhole used to inject the ionized sample into the mobility separation space is a large source of ion losses to the walls.  Due to the geometry requirements related to the second issue, that problem source cannot be eliminated.  Instead a higher ionization production is needed.  One option that is currently being explored is UV photo ionization. 

The main objective for the subsequent reporting period will be to finalize the implementation of the UV photo-ionization source and test the total ion output using the modified configuration briefly discussed above.  The diffusive losses will be retested and once a satisfactory ion current magnitude is achieved then the instrument will be reinstated into this original form and tested with various volatile vapors.  In-laboratory testing will include single compound vapors, mixed-compound vapors, as well as absolute sensitivity tests for various vapors.  Subsequent to the in-laboratory tests deployment of the instrument may follow in areas where known pollutants have been detected for further verification that the instrument is operating nominally.  The final objective will be to design a modified version of this instrument that can be placed in series with a mass spectrometer for a two dimensional compound verification system where mass and mobility are identified.