K 3gaT

E

Electric Field Strength

D

Drift Path Length

Drift Time

e

Unit Charge

m

Ion Mass (analyte)

M

Molecular Mass (dntt gas)

N

Number Density

k

Boltzmann-Constant

T

Temperature

r

Minimum in Potential Curve

a

Collision Integral

A

Correction Term

Figure 5. Relationship of ion mobility to molecular terms.

Figure 5. Relationship of ion mobility to molecular terms.

The use of ion multipliers is problematic at pressures near atmospheric due to secondary electrons colliding with drift gas. IMS researchers have thus been forced to utilize direct electrometer measurements on the very small signals produced by the ion packets. Electrometer measurements use amplifiers with high input impendence to measure the voltage resulting from the flow of current through high-precision, high-value resistors (typically 1010 to 1012 Q, see Fig. 6) or from the accumulation of charge on a small input capacitor.

Figure 6. Conventional instruments use a high input impedance operational amplifier in a current to voltage circuit.

Limitations on the smallest detectable current arise from noise effects in the amplifying feedback resistor, the variable capacitance of the ion collector, the cables (inherent in the implementation of the discrete devices external to the operational amplifier), and from the amplifier itself. High-value feedback resistors are typically used to produce voltages from the small ion currents. For a 1012 Q feedback resistor, the thermal noise arising from thermally induced charge fluctuations amounts to about ± 1 femtoamp at room temperature. Additionally, the current commercially available operational amplifiers have a current noise of over 10 femtoamps at 10 KHZ bandwidths.

The other method for determination of charge is to measure the change stored on a capacitor. In conventional configurations, this approach suffers from the same noise fluctuations and detection limits as are obtained with a standard electrometer. Detection limits of several thousand ions have not yet been realized for portable ion mobility spectrometers which use this conventional technology. The CTIA approach, however, markedly reduces the detection limit.

The CTIA employed for IMS applications was designed by Eugene Atlas and associates at Imager Laboratories (San Marcos, CA), and fabricated using 0.35 micron technology. Feedback capacitors were measured to be approximately 10 femtofarad (high sensitivity/small full well mode) and 990 femtofarad (for a total of 1,000 femtofarad in low sensitivity/large full well mode, see Fig. 7).

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