Tough TOF Made Easier Fast speed and high resolution have always been the hallmarks of TOF systems. Now flexibility, ruggedness and reasonable costs can be added to that list.
by Tim Studt
Forensics Manager Kevin Shanks at AIT Labs works with Waters ACQUITY UPLC/LCT time-of-flight mass spectrometer LC-MS for toxicology studies. | Time-of-flight mass spectrometry is known for its high sensitivity, resolution and exact mass measurements. Unfortunately, market expansion has been limited by its bulkiness, high cost and time-consuming sample prep in GC applications. In comparison to standard GC-MS systems, however, GC-TOFMS systems deliver faster results and have a greater sensitivity.
Applications for GC-TOFMS systems are found in environmental and materials analysis labs for organic analysis and large molecule screening. They’re also used in advanced life science research, food and flavor research, forensic studies, and in monitoring very low concentrations of volatile organic compounds (VOCs). For example, arson analysis has historically comprised long GC-MS run times followed by reviewing the mass chromatographic date. The application of GC-TOFMS using fast GC techniques reduces the data acquisition period and, when coupled with an automated processing method, reduces the overall analysis times.
Even though the overall market for GC-TOFMS systems is relatively low, there are a number of equipment suppliers, including Almsco, Bruker, DANI Instruments, S.p.A., JEOL, LECO, Scientific Analysis Instruments, Shimadzu Scientific Instruments, Thermo Fisher Scientific and Waters Corp. GC-TOFMS systems from these suppliers are sometimes assembled with third-party GC systems, such as Agilent Technologies’ 6890 GC system and a core TOFMS analyzer.
GC-TOFMS applications generate much higher ion rates than double quadrupole TOFMS systems because the chromatography feeds the TOFMS directly.
Recent improvements have expanded the usefulness of GC-TOFMS systems. One of these, atmospheric pressure (AP), delivers the flexibility to analyze volatile and semi-volatile compounds of low and intermediate polarity that were traditionally analyzed by dedicated vacuum-based GC-MS systems.
GC-MS is an established, powerful research tool. These systems are generally used with electron ionization (EI) and chemical ionization (CI), which have large commercial and open source libraries that researchers can use to identify compounds of interest. However, EI and CI are harsh fragmentation techniques and can rend some molecular ions unidentifiable. Atmospheric pressure chemical ionization (APCI) is a softer ionization technique (no vacuum required) and has been considered an alternative to the conventional EI and CI techniques. AP interfaces for GC-TOFMS systems are available from Bruker and Waters.
AP interfaces obviate the need for dedicated instruments and offers good mass accuracy and resolution on a single platform with a simple interchange between LC and GC operation in just a few minutes without the need for any tools.
“The versatility of the AP ionization interface allows researchers to swap quickly and easily from an LC to a GC system for a TOFMS detector,” says Gordon Kearney, TOF product manager for Waters. “This also provides GC capabilities to a much wider audience and allows analysis of many more molecular ions.”
One of the issues with an AP ionization interface to a GC system, however, is that the resulting data doesn’t match traditional NIST GC-MS libraries that were performed with traditional vacuum-based interfaces.
Time-of-flight issues To partially address the relatively high cost issue with TOFMS systems, LECO offers researchers a range of TOFMS systems to choose from. Its TruTOF HT (high throughput) TOFMS was introduced at the 2007 ASMS and 2008 Pittcon. The Pegasus HT TOFMS combines proprietary GC-TOFMS technology with ChromaTOF software for increased lab productivity. And the top-of-the-line Pegasus 4D GCxGC-TOFMS combines two-dimensional GC with the Pegasus HT TOFMS. With the Pegasus 4D, researchers are able to detect thousands of more compounds than researchers were able to obtain with a conventional GC system. LECO also claims that their new TOF systems are rugged and easy-to-use, wh ich have been problems for TOF systems in the past.
A proton transfer reaction TOFMS (PTR-TOFMS) has been used to evaluate the aerosol cloud system during the Arctic summer. PTR is used for analyzing all volatile organic compounds (VOCs) with a proton affinity higher than that of water—compounds with proton affinities lower than that of water can be detected with a switchable reagent ion option.
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To evaluate the chemistry of the Arctic Ocean’s atmosphere, a PTR-TOFMS equipped with a Pfeiffer Vacuum turbopump was employed. Organic trace gases were continuously measured throughout a broad mass range with a temporal resolution of 1 min. Air samples were taken by helicopter at altitudes to 3,000 m and analyzed with the PTR-TOFMS to obtain vertical trace gas profiles. Photos by: Pfeiffer Vacuum
| The TruTOF HT TOFMS offers the power, speed and software features of their mid-range Pegasus HT in a cost-competitive benchtop design. Continuous full-range mass acquisition rates up to 80 spectra/sec on this system accurately identify unknown compounds in complex samples. The TruTOF also offers both EI and CI capabilities and has an EI detection limit (sensitivity) specification for a full-range acquisition of >10:1 for a mass chromatographic peak of m/z 284. Comparatively, the Pegasus HT provides full-range mass acquisition rates up to 500 spectra/sec, as does the Pegasus 4D. The Pegasus HT and 4D models also employ a Dynamic Signal Tracking (DST) feature that enhances their dynamic range, spectral integrity and overall system robustness.
The new challenges for these systems, in particular the GCxGC, are users being able to handle the increased amount of data generated by the systems for complex samples. To accommodate this issue, LECO developers created a new classification system that allows researchers to group compounds with similar structures.
Waters Corp. also offers a range of TOF-based instruments. With a spectral sampling rate of up to 20 spectra/sec, its GCT Premier is a benchtop orthogonal acceleration (oa) TOFMS with high sensitivity, resolution, and exact mass measurements (sub-5 ppm RMS) for GC-MS applications. Waters’ Xevo QTOFMS systems offer exact mass capabilities to 2 ppm RMS, resolution to more than 10,000 FWHM (full wave, half maximum), a 104 linear dynamic range, and a spectal sampling rate to 20 spectra/sec.
And its top-of-the-line Synapt G2 MS is a second-generation Synapt platform with up to 40,000 FWHM resolution, 105 linear dynamic range, 1-ppm RMS exact mass measurement, and 20 spectra/sec sample rate. The Synapt MS systems are next-generation quadrupole oa-Tof systems that combine its Aquity UPLC, API/MALDI, exact mass and Chemically Intelligent Informatics. The Synapt G2 is a step above other systems, and at a higher price point as well. The qTof is not an option on this system.
“More researchers are investing in this level of instrumentation,” says Tim Jenkins, business development manager, Mass Spectroscopy and Chemical Analysis, Waters. “Along with the atmospheric pressure interface, these TOFMS systems have no performance compromises—researchers don’t want compromises. Researchers buy these systems because they don’t want to be limited in their analyses, and they want to increase their labs’ capabilities and productivity.
The APGC is a particularly important enhancement to the TOF systems, according to Jenkins. “Researchers often have to work with fragmented ions, like those in pesticides. These systems can handle those analyses, which are difficult to do with traditional systems with EI interfaces.”
Introduced at the ASMS 2008, Bruker’s microTOF II API-TOFMS has an optional GC-APCI interface for a single MS platform that accommodates LC and GC systems. The microTOF II has a resolution of more than 16,000 FWHM, a mass accuracy to 1 to 2 ppm RMS, and a spectral scan rate of 20 spectra/sec.
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