The latest advancements in time-of-flight mass spectrometry have allowed the technology to easily tackle day-to-day analysis of complex samples by GC.
On the face of it, time-of-flight mass spectrometry (TOF-MS) ought to be more popular than it is. Operating according to a concept developed in the 1940s and 1950s, TOF-MS has for 20 years been the method of choice for analysis of biomolecules, in conjunction with the ionization technique of matrix-assisted laser desorption/ionization (MALDI), which allows large ions to be formed while avoiding fragmentation. However, as far as gas chromatography (GC) is concerned, TOF instruments have never been anywhere near as popular as quadrupole analyzers.
A quadrupole mass spectrometer essentially acts as a filter, letting through ions of a particular mass/charge ratio. In “scan” mode, the voltages applied to the quadrupoles are changed so that the instrument sequentially lets through ions across the entire mass range, typically over a period less than half a second. This is fine for obtaining a rough-and-ready mass spectrum—which historically has been all that’s needed—but the fact that the vast majority of the ions are discarded results in a lack of sensitivity. The alternative setup, where the quadrupole is set to let through ions of a pre-selected mass (selected ion monitoring or SIM mode), gives much better sensitivity for the ions selected, but has the disadvantage that the analyst needs to know in advance what they’re looking for. This is not a problem when just one compound is being studied, but less useful if a sample is to be screened for a range of compounds, or when carrying out preliminary work on a new sample type.
In many ways, TOF-MS brings together the best of both full-scan and SIM modes of quadrupoles, providing data across the full mass range while retaining good sensitivity. By creating ions in a tight “packet” and sending them through to the detector, ion wastage is minimized. This means that TOF mass spectrometers—in principle—should have sensitivity that’s at least as good as quadrupole instruments.
Unfortunately, inefficiencies in creating the ion packet, the production of distorted mass spectra, their physical size and relatively high cost make TOFs less appealing than quadrupoles for GC applications. However, advances in TOF technology have changed this, and have led to the following benefits for the GC analyst: (1) improvements in moving ions from the source to the flight tube sensitivity now mean that TOFs can match the sensitivities possible in SIM mode on a quadrupole, but across the full mass range; (2) the elimination of spectral distortion allows TOFs to produce spectra that are a close match to those in widely available libraries of (quadrupole-acquired) mass spectra; (3) a reduction in size now makes TOFs more suitable for laboratories where space is at a premium and; (4) manufacturing developments make TOFs less expensive to the point where the added benefits outweigh the additional expense compared to a regular quadrupole.
Together, these advances dispense with the arguments against using TOF for GC and finally make it an appealing prospect for GCMS applications. What this means for the analyst is examined below by considering four examples from across the application range.
Detecting drugs in urine is a good example of the importance of reliable identification of target compounds in a complex matrix, that at the same time makes a clear case of the need for whole-sample screening. Figure 1 shows an analysis of urine taken from a participant in a methadone-substitution program.
An extraction process was first used to remove inorganics and heavy organic molecules from the sample before being sent through the GCMS system. The result is shown in Figure 1, before any background compensation processes had been applied. Although the sample is clearly highly complex, a number of compounds that would have been difficult or impossible to identify from quadrupole data or by using traditional compound identification software are now easily identified. In particular, challenging polar benzodiazepines, such as nordiazepam, have been confidently identified because of the high sensitivity and good spectral quality provided by the TOF instrument.
A striking example of the ability of this method to “pull out” trace compounds is shown by the identification of alprazolam underneath a large peak caused by amisulpride M. Despite the high levels of interference, the mass spectrum obtained is a close match to that in the NIST library. This spectral quality is a key advantage of modern TOF instruments. Older instruments produce spectra that are substantially different from those in spectral libraries, making on-the-fly spectral matching all but impossible.
An added point to notice about this example is the short GC run-time. The narrow peaks that result from such “fast GC” are easily handled by the high data-capture rate of TOF, in this case with clear relevance for rapid screening.
Another example of a complex matrix is beer, which being derived from natural ingredients, contains a vast array of compounds. The combination of these give beer varieties their distinctive flavors, but their concentrations can vary widely, with some having a big impact on flavor even at trace levels. The added issue of large amounts of interferents such as water and alcohol make beer a big challenge for the analyst, but one that can nevertheless be approached by combining TOF mass spectrometry with sorptive extraction and thermal desorption—techniques that both offer a sensitivity enhancement.
In the initial step, a popular bottled beer was sorptively extracted onto a polymer-coated cartridge to selectively extract the less polar compounds, which in general are of greater importance for flavor and aroma. The sensitivity of this extraction method was then enhanced by concentrating the compounds into a narrow band of vapor using thermal desorption (TD), before passing the sample through the GCMS system.
Figure 2 shows the chromatogram of the sample following post-run processing to remove background signals. It is important to note that the availability of detailed mass spectral data across the whole chromatogram means that this compensation process can be done intelligently—removing unwanted signals while retaining signals because of chromatographic peaks.
Examples of the mass spectra obtained are shown for the largest peak (ethyl hexanoate) and a peak more than 11,000 times smaller (tridecanal). In both cases, good matches against the library spectra were obtained. A numerical measure of this is provided by the match factors, which use chemometrics to assign a number to the spectrum, with a maximum of 1,000 for a perfect fit.
Air pollution, crude oil applications
Air pollution remains a global concern because of its impact on human health and the environment. Increasingly low limit levels for hazardous air pollutants such as benzene and toluene (often known as air toxics) demand a system that can detect compounds at less than 1 ppb. Historically, such high levels of sensitivity were only possible using a quadrupole instrument in SIM mode, but analyzing samples for multiple analytes using this technique is laborious. TOF-MS provides clear benefits in such cases, with its combination of SIM-level sensitivity and the full GCMS datasets that allow identification of all compounds in one run, as well as any unknowns (i.e. compounds not in the target list).
The sensitivity of TOF for this sort of situation is demonstrated by the analysis of one liter of semi-rural air for air toxics (Figure 3). Compounds with a wide range of concentrations (and volatilities) were detected, spanning 5 ppt for trichloromethane (chloroform) to about 1 ppb for styrene. This was achieved even though only about one-tenth of the sample was sent to the GC. So in principle, detection limits of 0.5 ppt or lower would be possible if all the sample had been used.
To demonstrate that spectral quality is retained even at the ultra-low concentrations typical of such relatively “clean” air samples, Figure 3 also shows the spectrum of trichloroethene, which has a very good match despite a concentration of only 20 ppt.
Crude oil provides a final example of the power of TOF instruments to handle minor components in complex mixtures. Because the analytes of prime interest are relatively volatile and non-polar, separations are usually performed using GC with a non-polar column. However, the resulting chromatograms are typically highly complex, with some partly resolved peaks being compromised by a background “hump” of unresolved materials. A combination of background subtraction and library searching can help identify some of these compounds, but for many underlain by this matrix, there is little hope of confident identification. Such problems have increasingly become an issue as environmental legislation demands ever-greater attention to the individual components, while at the same time the industry moves to “dirtier” sources of petrochemicals.
Addressing this challenge is comprehensive two-dimensional GC (GC×GC), which involves collecting “chunks” of effluent from the first non-polar column, and re-injecting them onto a second more polar column. The resulting improved separation increases the resolution and allows the identification of many more compounds than with one-dimensional GC. However, the nature of GC×GC means that peak widths are typically only 50 to 200 milliseconds wide. This is too fast to allow good resolution with quadrupole mass spectrometers, which rarely acquire more than 10 scans per second. A faster method of detection is therefore necessary, and this is where TOF—by acquiring tens of thousands of scans per second—has long been recognized as an ideal solution.
Figure 4 exemplifies what can be achieved in the analysis of crude oil using a GC×GC TOF-MS setup. In this case, a group of triterpenoids known as hopanes were of particular interest because of their ability to be used as a chemical fingerprint for oil from a particular location. Hopanes have a distinctive ion at m/z 191 in their mass spectra, and this has been used to filter the data to provide an “extracted-ion” chromatogram, displaying just those peaks with mass spectra containing that ion. The C27–C35 hopanes are well separated and have spectra of sufficiently good quality to allow confident identification.
The above examples demonstrate the latest advances in TOF-MS technology—advances that allow it to provide spectra closely matching those in spectral libraries. Laboratories now have at their disposal a technique that, by combining the sensitivity of quadrupole instruments with the ability to acquire data across the full mass range in a single run, allows the rapid identification of trace-level analytes even in the most challenging of samples.
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