Older TOF-SIMS system upgrades.
Older TOF-SIMS system upgrades.
Many of the laboratory time-of-flight SIMS instruments that were state-of-the-art in the late 80s and early 90s are still prolific producers of valuable materials data. However they suffer from outdated data acquisition systems that in many cases are no longer properly supported because the original instrument manufacturers have moved to the latest technology or changed their emphasis.
In the mean time there has been a revolution in the performance of personal computers with the result that they more than match the workstations that were considered necessary in the past. Many of the users of these older instruments are aware of the potential benefits of moving to PCs and indeed already use them for report writing and some data processing. However they are severely hampered by the difficulties of interfacing to specialist fast timing electronics and by the need to extract data from proprietary format data files on outdated disk formats. In practice this means that they have had to maintain the old equipment in working order alongside any new PCs.
Kore Technology personnel have been involved in the development of time-of-flight SIMS instruments since the mid 1980s in Cambridge Mass Spectrometry and Kratos. In the early years this exclusively involved large laboratory based instruments. However after its foundation in 1991 Kore extended time-of-flight technology into portable and bench-top instrumentation requiring the development of new designs from scratch. The design decisions, taken in this more price sensitive market, have had the happy result that Kore can now offer a relatively inexpensive upgrade path for the users of laboratory instrumentation described above.
The Kore 4GHz time-to-digital converter (TDC) is a compact, bench-top timing unit designed to record a fast stream of event times relative to a single start event (at time=0). It performs this task with a timing resolution of 0.25 ns with minimal dead time, high data rates and the ability to repeat the experiment at very high repetition rates. These features make the unit ideal for use in Time-of-Flight (TOF) mass spectrometry applications. In such an application, the unit issues or receives a cycle start signal at pre-set intervals and waits to receive timing pulses from an ion detector. The pulse times are recorded, buffered and streamed to the PC via a standard USB 2.0 serial interface. Here the standard driver software may process them to spectra ‘live’, or stream them to the hard disk for more flexible processing options.
GRAMS/AI™ is a Windows based software package for spectroscopic and chromatographic processing, plotting and viewing offered by Thermo Scientific. It has extensive facilities and is capable of exchanging and organising information used with other packages, e.g. reports, databases, graphics etc. In addition there are add-on applications that provide specialist processing, such as chemometrics and there is a powerful macro language, Array Basic, to ease repetitive tasks. Because it is used by a wide customer base in many fields, GRAMS™ enjoys the support of an extensive software team who continually improve facilities and ensure that it can take advantage of all the latest operating system features. Kore is an approved value-added reseller for GRAMS/AI™ and has made use of its flexible customization features to provide a friendly graphical user interface for TOF SIMS data collection.
Kore have written a Windows application to collect chemical maps using the Kore TDC described above, for those instruments that have scannable primary beams. An analogue output card provides the scan control while the application records all the ion arrival times. Up to 16 chemical maps may be monitored during the acquisition to confirm correct operation of the instrument. The data is stored in “list-mode”, which allows post processing to use any desired mass range for constructing images, without having to repeat the experiment. Images created during acquisition or post-processing may be saved in portable grey map (*.pgm) format. This is a standard ASCII format which may be easily read for DIY processing. Alternatively various free packages (for example ImageJ) can be used to convert to other image formats, or perform a wide variety of image processing operations.
The application is fully multitasking and therefore happily coexists with other software in a normal Windows environment. The acquisition may be run as a standalone application, but more often is launched from a purpose built interface under GRAMS/AI™ (above) that simplifies the selection of mass windows of interest.
Most imaging systems are fitted with a secondary electron detector (SEM). Typically this has a pre-amplifier and then, on old systems, the signal is fed to a slow phosphor monitor. The resulting image is often rather dim and only transiently persistant, difficult to view in a well lit modern laboratory. In addition, the special monitor required is becoming a very expensive item to purchase should replacment ever be necessary.
Kore has written a very simple Windows application to make use of the analogue input present on the PCI based scanning card. By connecting this to the SEM detector pre-amp, a continuously refreshed SEM image may be written to the computer screen. The image persistance is of course indefinite and the viewing contrast much higher. The scanning rate is fixed at about 1 frame per second, slow enough to get good signal-to-noise in the image and yet fast enough to give good visual feedback whilst setting up the ion gun.
Displayed to the right is a composite SIMS image (several fields of view assembled into a larger image) of loose fibres trapped under a hexagonal stainless steel mesh. The data was acquired with the SIMS imaging module on a Kratos TOF-SIMS, with the Kore upgrade, by the Laboratory for Surface and Interface Analysis, Department of Engineering Materials group at Sheffield University. They have written their own pallette generator program, which they have been kind enough to make available to other Kore users. In the image below, maps for aluminium (yellow), chromium (blue) and Carbon (pink) have been overlayed. This image demonstrates nicely how the choice of colour scheme can make the data come alive to the eye.
Kore Technology is a centre of excellence in time-of-flight mass spectrometer technology and has a very strong R&D capability in terms of its personnel, all of whom have been heavily involved in a variety of analytical instrumentation development programmes.