Time-of-Flight Secondary ion mass spectrometry (ToF-SIMS) offers new possibilities for surface analysis of various organic samples such as materials science, material defect analysis, semiconductor analysis, and biological analysis. Advances in ToF-SIMS are based on developing novel first-order cluster ion beams for molecular analysis. Au3+ and Bi3+ cluster ions increased the secondary ion yield of high-mass molecular samples by 100-1000 fold compared with conventional atomic ion beams [1]. In particular, Ar cluster ion beams are being studied for both sputtering and analysis.[2] Despite several advantages of Ar gas clusters, the secondary ion yield of molecules sputtered by GCIB decreases with increasing cluster size. In addition, although there is a problem in that spatial resolution is low due to a relatively large beam size compared to a liquid metal ion beam, improvement of the gas cluster ion beam to minimize sample damage in the primary ion beam analysis has been continuously required. Recently, although the Ar mixed gas and CO2 have improved, the change in the characteristics of the cluster ion beam concerning the Ar and CO2 mixing ratio has not been studied in detail. If the gas cluster ion beam can be controlled by the ratio of the gas mixture, more effective cluster ion beam improvement will be possible.

Over the past few years, the Korea Basic Science Institute (KBSI) has been developing cluster ion guns and ToF-SIMS for three-dimensional (3D) mass imaging analysis of organic materials and bio samples [3].

In this study, the characteristic change of the cluster ion beam according to the ratio of Ar and CO2 mixed gas was measured.[4] Also, to improve the ToF-SIMS analysis, we propose the Ar and CO2 mixed gas ratio of the GCIB source. This mixed gas method can improve the efficiency of secondary ions and increase spatial and depth resolution compared to pure Ar gas.

Reference

[1] J.S. Fletcher, J.C. Vickerman, N. Winograd, Label free biochemical 2D and 3D imaging using secondary ion mass spectrometry, Curr. Opin. Chem. Biol. 15.5 (2011) 733–740.

[2] I. Yamada, J. Matsuo, Z. Insepov, D. Takeuchi, M. Akizuki, and N. Toyod, J. Vac. Sci. Technol. A, 14(3), 1996, 781

[3]S.J. Lee, C.M. Choi, B.K. Min, J.Y. Baek, J.Y. Eo, M.C. Choi, Development of an argon gas cluster ion beam for ToF‐SIMS analysis, Bull. Korean Chem. Soc. 40.9 (2019) 877–881.

[4] Sang Ju Lee, Aram Hong, Jinwan Cho, Chang Min Choi, Ji Young Baek, Jae Yeong Eo, Byeong Jun Cha, Woo Jun Byeon, Jin Young We, Sangwon Hyun, Minwoo Jeon, Choelho Jeon, Dong Jin Ku, Myoung Choul Choi*, Applied Surface Science, 572, 2022, 151467

We have been studying a vacuum electrospray droplet ion (V-EDI) beam gun as a new massive cluster ion beam for secondary ion mass spectrometry (SIMS). In the previous studies, the V-EDI gun was installed in a time-of-flight SIMS (TOF-SIMS) system, and the secondary ion spectra produced by the V-EDI beams were measured for several biomolecular samples[1-3]. The fragment ion generation was strongly suppressed and the protonated intact molecules were observed with high intensities. However, the basic characteristics of the V-EDI beams are not enough revealed. In this study, the properties of V-EDI beams produced by capillaries with different inner diameters will be investigated.

The moderately focused (~0.5 mm) V-EDI beams which were generated by vacuum electrospray of a 0.01 M trifluoroacetic acid aqueous ethanol solution (water/ethanol=4:1) were raster-scanned over an area of typically 4.5x4.0 mm² at a scan speed of 0.1 Hz for short (300~1000 s) or long (2000~50000 s) time. For sputtering yield evaluation, primary 8 kV V-EDI beams produced from capillaries with different inner diameters (5, 10 and 15 μm) were irradiated uniformly on polymer or plasticizer film (~100 nm) samples prepared by spin-coating. The irradiated samples were analyzed with atomic force microscopy (AFM) and spectroscopic ellipsometry. The microscopic images of the films irradiated for short periods were observed with AFM, and the sputtering volumes per primary ion were calculated from the diameter and depth of the impact crater. The thicknesses of the films irradiated for long periods were observed with spectroscopic ellipsometry, and the sputtering volumes per primary ion were calculated from the difference in film thickness between irradiated area and non-irradiated area.

The sputtering volume per primary ion depended strongly on the inner diameter of each capillary. The sputtering volumes of the V-EDI beam generated by the capillary with an inner diameter of 5 µm were smaller than those by other capillaries (i.d. 10 and 15 μm). The differences in sputtering volume are assumed to be originated from the droplet ion size included in the V-EDI beams obtained from capillaries with different inner diameters.

References

[1] S. Ninomiya, Y. Sakai, R. Watanabe, M. Sogou, T. Miyayama, D. Sakai, K. Watanabe, L.C. Chen, K. Hiraoka, Rapid Commun. Mass Spectrom. 30, 2279 (2016).

[2] S. Ninomiya, L.C. Chen, K. Hiraoka, J. Vac. Sci. Technol. B36, 03F134 (2018).

[3] S. Ninomiya, Y. Sakai, L.C. Chen, K. Hiraoka, Mass Spectrometry7, A0069 (2018).

Two of the greatest challenges in SIMS research are low ionization yields and matrix effects. Polyatomic water beams demonstrate that primary ion beam chemistry can increase yields while reducing matrix effects. We report a study demonstrating that carrier gas chemistry alters the SIMS characteristics of water cluster beam. The study aims to test if carbon dioxide reacts with water clusters thus increasing the likelihood of clusters acting as proton donors.

By applying water clusters with a reactive (CO₂ containing) carrier gas to drug standards we were able to measure the effect on the secondary ion yield. We show that compared to an inert (Ar) carrier gas, the yield observed with reactive water clusters is up to 50 times higher (Figure 1). This implies a change in primary cluster composition as a result of using a reactive carrier gas.

Matrix affects arise from the often unpredictable interaction of surface species which results in an ionization suppression/enhancement effect. Complex sample environments present a greater obstacle due to a myriad of potential chemical interactions. We show that by applying the newly developed chemically reactive water cluster beam, it is possible to reduce the matrix effects achieving a higher level of quantification over a range of drug concentrations (Fig. 2).

Together these observations indicate a novel methodology promoting the application of SIMS in a more sensitive and more quantitative manner to complex samples.