Abstract
The popularity of argon gas cluster ion beams (Ar-GCIB) as primary ion beams in time-of-flight secondary ion mass spectrometry (TOF-SIMS) has increased because the molecular ions of large organic- and biomolecules can be detected with less damage to the sample surfaces. However, Ar-GCIB is limited by poor mass resolution as well as poor mass accuracy. The inferior quality of the mass resolution in a TOF-SIMS spectrum obtained by using Ar-GCIB compared to the one obtained by a bismuth liquid metal cluster ion beam and others makes it difficult to identify unknown peaks because of the mass interference from the neighboring peaks. However, in this study, the authors demonstrate improved mass resolution in TOF-SIMS using Ar-GCIB through the delayed extraction of secondary ions, a method typically used in TOF mass spectrometry to increase mass resolution. As for poor mass accuracy, although mass calibration using internal peaks with low mass such as hydrogen and carbon is a common approach in TOF-SIMS, it is unsuited to the present study because of the disappearance of the low-mass peaks in the delayed extraction mode. To resolve this issue, external mass calibration, another regularly used method in TOF-MS, was adapted to enhance mass accuracy in the spectrum and image generated by TOF-SIMS using Ar-GCIB in the delayed extraction mode. By producing spectra analyses of a peptide mixture and bovine serum albumin protein digested with trypsin, along with image analyses of rat brain samples, the authors demonstrate for the first time the enhancement of mass resolution and mass accuracy for the purpose of analyzing large biomolecules in TOF-SIMS using Ar-GCIB through the use of delayed extraction and external mass calibration.
Highlights
INTRODUCTIONA number of studies have examined the performance of cluster ion beams with respect to tissue imaging on the brains of rodents. In these studies, various biological molecules such as cholesterols, fatty acids, and sulfatides were detected in the brain tissue, with the intensity of the secondary ions boosted significantly when using argon gas cluster ion beams (Ar-GCIB) than when using a bismuth cluster ion beam
A number of studies have examined the performance of cluster ion beams with respect to tissue imaging on the brains of rodents.12–16 In these studies, various biological molecules such as cholesterols, fatty acids, and sulfatides were detected in the brain tissue, with the intensity of the secondary ions boosted significantly when using argon gas cluster ion beams (Ar-GCIB) than when using a bismuth cluster ion beam.Understandably, GCIB is gaining in popularity as the primary ion source for measuring organic- and biomaterials in TOF-SIMS analyses
Ar-GCIB has poor lateral resolution as well as poor mass resolution caused by difficulties in both spatial and time focusing of large argon clusters
Summary
A number of studies have examined the performance of cluster ion beams with respect to tissue imaging on the brains of rodents. In these studies, various biological molecules such as cholesterols, fatty acids, and sulfatides were detected in the brain tissue, with the intensity of the secondary ions boosted significantly when using Ar-GCIB than when using a bismuth cluster ion beam. 02A321-2 Shon et al.: Improved mass resolution and mass accuracy in TOF-SIMS spectra This approach was utilized by Angerer et al who demonstrated that a 40 keV Ar4000þ beam could be focused to approximately 2–3 lm at the sample surface to successfully produce a chemical image of sphingomyelin on human hair.. We are concerned with increasing the mass resolution and mass accuracy of the spectrum and image in the Ar-GCIB-TOF-SIMS analysis by using delayed extraction and external mass calibration. In TOF-SIMS, delayed extraction achieves time-of-flight compensation in ion energy spread by applying extracting pulse energy onto the extractor of the secondary ions with a specific time delay after the pulsed primary ion beam irradiates the sample surface. The spectrum acquired by using the delayed extraction method has lost the peaks of hydrogen and carbon ions, which causes difficulties in internal mass calibration—the typical method of calibrating mass in TOF-SIMS analysis. As with the delayed extraction method, the external mass calibration method is widely used in various mass spectrometry techniques. these two methods, delayed extraction and external mass calibration, are not new, they will be useful in identifying unknown materials when newly applied to an Ar-GCIB-TOF-SIMS analysis
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