Ionization mechanisms responsible for the formation of prepeaks and afterpeaks on Ion signals observed In pulsed glow discharges and potential analytical use of pulsed glow-discharge mass spectrometry have been investigated. The formation of a prepeak at the beginning of the pulse Is related to the Ionization efficiency of electrons accelerated across the cathode fall region. The appearance of an afterpeak at the termination of the discharge is associated with the energy, population, and Ionization efficiency of metastable discharge gas atoms. As a result of the differences In pulse profiles, It Is possible to acquire data over specific regions of the pulse that permit discrimination against Interfering signals In the mass spectrum.

We have designed and constructed an atmospheric pressure laser desorption/chemical ionization (AP-LD/CI) source that utilizes a laser pulse to desorb intact neutral molecules, followed by chemical ionization via reagent ions produced by a corona discharge. This source employs a heated capillary atmospheric pressure inlet coupled to a quadrupole ion trap mass spectrometer and allows sampling under normal ambient air conditions. Preliminary results demonstrate that this technique provides approximately 150-fold increase in analyte ions compared to the ion population generated by atmospheric pressure infrared matrix-assisted laser desorption/ionization (AP-IR-MALDI).

A pulsed glow discharge, rather than a conventional constant dc voltage discharge, is used as an ion source for a quadrupole mass spectrometer. Both sputter yield and ion signal are enhanced by using the pulsed system because of an increase in the voltage necessary to maintain a constant average current at the cathode over the pulse period. Irregularities are seen in the pulse spectrum that appear as rapid surges in the ion signal for both sputtered and contaminant gas species. These peaks appear at the beginning of the pulse for gaseous species but are limited to the postpulse period for sputtered species. Differences in the signal forms allow for the discrimination against selected types of ion signals by using narrow data collection gates placed over different portions of the pulse period.

Atom density decreases as the black fades to orange. (a) dc mode. The device consists of a negative cathode and a grounded anode immersed in a low-pressure rare gas, usually argon. A potential (V) breaks down the discharge fill gas, yielding Ar + that is attracted to the cathode-which, in this case, is the sample. The ions collisionally sputter neutral atoms from the surface into the adjacent GD plasma. As these sputtered atoms diffuse through the GD plasma, collisions with electrons, ions, and metastable atoms excite and ionize the sample atoms. (b) Pulsed mode operates on the same principles described for dc mode, but in short, repetitive dc pulses.

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