Mass Spectrometry: Processing Power

Many of the recent developments in Mass Spectrometry (MS) have been focused on promoting the mass spectrometer as both a universal and a highly specific detector. Although these seem to be diametrically opposed objectives, such is the flexibility and ease of use of modern MS instrumentation that these two functions are a reality.

Quality Improvement

The accessibility of modern mass spectrometry is approaching the “black box” level. Many believe that the previous limitations of MS such as speed, resolution, dynamic range and ease of use have all been eradicated. But while progress has been made in these areas, such problems still arise from time to time.

Users who are familiar with Gas Chromatography with Flame Ionization Detection (GC-FID) are now able to use Gas Chromatography Mass Spectrometry (GC-MS) routinely. While the FID detector has a wide dynamic range that can extend over seven orders of magnitude, GC-MS can be limited to four or five.

This limitation arises from a combination of a number of instrument characteristics, including ion source and detector limitations as well as potential limitations in the mass analyzer (such as an ion trap) itself. Different ionisation processes, such as chemical ionization for GCMS and electrospray for Liquid Chromatography Mass Spectrometry (LC-MS) can be even more limited and may also be compound dependant. It is therefore important to be aware of these limitations when developing methods, particularly where quantitation is involved.

The “need for speed” is driven by the progress that has been made in the front-end technologies, such as GC-MS and LC-MS, as well as the requirement to extract as much information from a single sample injection in the shortest period of time.

The provision of MS instrumentation is still seen as a heavy financial investment and it is natural for the buyer to want to see a significant return on that investment – by running as many samples as possible. As chromatographic methods and technology are developed to provide higher resolution and sample throughput, the resulting peak widths are reduced.

Much has been written on the optimum data sampling rate for the accurate determination of peak areas but it is generally accepted that the faster the sampling rate, the more accurate the peak integration will be. Ultrafast GC, Ultra High Performance Liquid Chromatography (UHPLC) and Two-dimensional Gas Chromatography (GCxGC) are examples of front-end techno-logies that are pushing up the data sampling rates that are expected of any mass spectrometer.

Conventional HPLC is able to provide peak widths of the order of 2-3 seconds Full Width at Half Maximum (FWHM) whereas UHPLC can now offer peaks in the order of 0.7 seconds FWHM. Ultrafast GC can produce peak widths of around 0.1 seconds FWHM. In order to maintain the accuracy and precision of peak integration, the mass spectrometer cycle time must be reduced in order to get sufficient numbers of samples across such narrow peaks. Given that many MS instruments, such as the LTQ-Orbitrap and the Quadrupole Time-of-Flight (Q-ToF) technologies are now providing routine high resolution information, there is a requirement for faster data acquisition rates for the MS itself. Data sets from a single injection are approaching Gigabit proportions.

Computing Capability

If the possibility of Tandem Mass Spectrometry (MS-MS) acquisition modes is introduced, the amount of data can be overwhelming. The development of faster electronics, faster and more powerful computers and greater data storage capability is barely keeping pace with this. Without such development, much of today’s MS analyses would be impracticable.

Mass spectrometry has been a powerful tool and it is the development of readily available digital processing capabilities to match, that has made MS accessible. The development of ionization techniques, such as Direct Analysis in Real Time (DART) and Desorption Electrospray Ionization (DESI) coupled with improvements in scope and sensitivity of existing ionization Drug Discovery processes, is expanding the applicability of MS analysis. The development of “ambient” ionization techniques, described by Cooks – where ions are created outside the MS instrument to produce mass spectral data from samples in their native state – can allow real time analysis and reduces possible artefact generation and sample breakdown in the harsher environments of the more common ionization processes. These techniques have been used to perform biological tissue imaging by mass spectrometry as well as a variety of other applications including environmental and forensic studies.

The coupling of Ion Mobility Spectrometry (IMS) techniques, such as with the Synapt HDMS mass spectrometry system, adds another dimension of separation – which allows the differentiation of molecules based on size and shape prior to coupling with mass spectrometry. This increased information content is compatible with Ultra Performance Liquid Chromatography (UPLC) timescales, which allows the greater characterization of samples.

Instrument vendors are keen to provide a “Solution” to a problem as the opportunity for cross selling of products is great. This can include everything from the sample preparation consumables and equipment through to the software to provide statistical reporting of study results. However, the MS instrument may only be a small component in the overall solution. Looking at the field of Metabolomics, the mass spectrometer is an important piece in the analytical jigsaw, but who is buying them? Biologists, zoologists, clinicians, statisticians or accountants? There seems to be a declining focus on chromatography and mass spectrometry professionals. Conversely, it is important for the chemist who is specializing in mass spectrometry to have a firm understanding of the end use of the data that is produced, to ensure that it is “fit for purpose”.

The developments in mass spectrometry have made it more accessible and it is now a powerful tool to many researchers. However, whatever the progress that is made, it will remain as only a part of the solution that requires a well balanced approach from multidisciplinary research groups. Developments are only of benefit to the wider communities when they are readily adopted. While word of mouth and networking are important, the majority of the market still relies on key individuals providing peer reviewed contributions to quality publications. This provides the “proof of concept” that others will be able to adapt and adopt.