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摘要
In this work we describe a novel concept for an efficient ion extraction for laser ablation time-of-flight mass spectrometry. The concept combines a sample, placed in a chamber filled with a helium buffer gas, a conventional converging-diverging supersonic nozzle connected to this chamber at short distance behind the sample, an electrodynamic Rf-only ion funnel placed on the nozzle axis in immediate vicinity of the nozzle exit plane and a pulsed laser source (frequency-doubled Nd:YAG, 532 nm, 3 ns pulse duration; 7.6 x 10(8) W power, 10 Hz repetition rate), focused onto the sample surface to a spot diameter of 110 gm after passing through the funnel and the supersonic nozzle.
The primary hot ions generated during ablation are cooled by collisions with buffer gas, accelerated into the subsonic converging part of the nozzle and finally transported through the supersonic diverging part of the nozzle into the Rf-only funnel via gas expansion. Inside funnel ions are again cooled and focused by the converging electrodes and the applied RF field towards the high vacuum stage containing the ion optics and mass analyzer. To evaluate the performance of the instrument, parametric studies were carried out. It was found that ion transmission is strongly dependent on the applied RF amplitude and frequency as well as the buffer gas pressure in the ablation region and funnel chamber. The present configuration achieves an integrated sensitivity approximately 2 orders of magnitude higher than when the ablation is carried out under high-vacuum conditions with the sample placed at the position of the ion funnel exit. The new ion guide leads to significantly longer duration of ion signals from a single laser pulse and thereby would also alleviate space charge effects. The signal profile is mostly dependent on pressure conditions in the ion source. To some extent it is also affected by the RF-operating conditions of the ion funnel. Optimizing for high transmission of low m/Qions caused an intermediate suppression of ion signals of high m/Q isotopes and vice versa. Because the respective suppression is highest during the peak maximum of the highly abundant ions, this behavior is attributed to exceeding the space charge limit inside the ion funnel for high total ion currents. Furthermore, cluster ion abundancies and their transmission were characterized. Molecular oxide ions and water clusters were found to be the dominating molecular ions in this study. Their formation generally increased with buffer gas pressure in the ablation chamber, while otherwise transmission through the ion funnel was similarly affected by the ion funnel operation as by atomic ions of similar m/Q Relative sensitivity coefficients were found to be generally comparable when analyzing standard reference materials of similar matrix composition with laser ablation using a wavelength of 532 nm, nanosecond laser.
- 出版日期2018-8
