The introduction of recent legislation, such as the EU Environmental Liabilities Directive 2004/35/EC, has led to a demand among analysts for more precise and robust methods for the identification of pollutants in environmental matrices, such as soil, sediment and water. However, meeting this demand has proved extremely challenging, fundamentally because of the large and continually increasing number of toxic compounds that may need to be monitored in a single analysis.
Figure 1 – Fast GC–TOF MS analysis of 92 target compounds (plus three internal standards) at 70 eV in less than 10 min. Peak identities are listed in Table 1. The insets illustrate the excellent peak shape achieved, with basal peak widths of approximately 2 sec. Internal standards are marked by: A: d10-acenaphthene, P: d10-phenanthrene and C: d12-chrysene.
Specific challenges are multiple chromatographic coelutions and the complexity of the background, which often make it difficult to detect and quantify trace-level compounds of interest using conventional quadrupole-based GC–MS methods. Time-of-flight (TOF) mass spectrometers such as the BenchTOF (Markes International, Cincinnati, OH) provide a useful alternative in these cases, generating reference-quality spectra at better sensitivity levels than conventional quadrupoles used in selected ion monitoring (SIM) mode. They also operate without scanning, so the spectra are free from skew and produced at much faster acquisition speeds. The resulting information-rich data sets provide the ideal raw material for data-mining algorithms such as spectral deconvolution, which can then be used to detect trace coeluting or matrix-masked compounds.
Figure 2 –Comparison of the deconvolved spectra for four target analytes (top, red) with the NIST library spectra (bottom, blue). Note that, without deconvolution, the α-endosulfan and trans-nonachlor peaks would most likely have escaped notice.
Another limitation of current GC–MS methods is that several key environmental target compounds—such as the “drin”-type pesticides—exhibit extreme fragmentation and/or weak molecular ions. This limits sensitivity and makes them even more difficult to identify at low concentrations and against a complex background. Soft ionization techniques can be used to reduce the degree of fragmentation, but traditional approaches to this, like chemical ionization, have been cumbersome to implement, because they demand specialized hardware and more frequent system maintenance.
This article describes the use of Select-eV technology (Markes International), which enables low-energy (soft) ionization without compromising sensitivity and without any need for reagents or hardware changes. The authors also show how Select-eV complements the other fundamental advantages of BenchTOF instruments for the fast identification of trace environmental contaminants.
Sample preparation: A mixture of 92 common contaminants was prepared (see Table 1) at a concentration of 1 ppm in dichloromethane. The contaminants covered a variety of chemical classes, including polycyclic aromatic hydrocarbons (PAHs) and chlorinated pesticides. Three deuterated PAHs (d10-acenaphthene, d10-phenanthrene and d12-chrysene) were also included as internal standards.
- GC—Instrument: Agilent 7890A (Agilent Technologies, Palo Alto, CA); column: BP5-MS, 20 m × 0.18 mm × 0.18 μm; carrier gas: He, constant flow at 0.8 mL/min; oven temp.: 40°C (0.7 min), 55°C/min to 240°C, 28°C/min to 330°C (2.0 min); injector: split/splitless injector; liner: 4.0-mm-i.d. liner, 1 μL injection; mode: split, 50:1; inlet temp.: 280 °C; septum purge: on, 3 mL/ min; total run time: 9.55 min
- MS—Instrument: BenchTOF-Select (Markes International); filament voltage: 1.9 V; ion source: 280°C; transfer line: 300°C; mass range: m/z 35–500; data rate: 16 Hz (625 spectral accumulations per data point)
- Software: The TOF-DS software package for BenchTOF was used for data analysis.
Results and discussion
Confident identification of contaminants
Figure 1 shows the fast GC–TOF MS chromatogram obtained at 70 eV for the analysis of 92 environmental contaminants. A split ratio of 50:1 was used for this analysis, meaning that the 1-μL GC injection provided just 20 pg of each target analyte on-column. Peak identities are listed in Table 1.
The spectral quality of BenchTOF and the powerful deconvolution algorithm of TOF-DS are demonstrated by the separation and successful identification of the four target compounds shown in Figure 2.
Despite the chromatographic run time being less than 10 min, all 92 target compounds were confidently identified, with NIST library match factors >700. This level of productivity could allow a sample turnover of approximately 140 samples within a single 24-hr period.
Table 1 – Identities, retention times, match factors and RSD values for the 92 target analytes; RSD values are based on area response ratios resulting from five injections of the standard mixture
Figure 3 – Mass spectral comparisons for five target compounds at 70 eV and 11 eV. In each case, signal-to-noise ratios are given for the molecular ion (red text), illustrating the impact of reduced fragmentation using Select-eV. (Molecular ions: chloroneb,
m/z 206; heptachlor epoxide,
m/z 386; hexachlorobutadiene,
m/z 258; N-nitroso-di-n-propylamine,
m/z 130; 4-nitroaniline,
The repeatability of the method was evaluated by five injections of the standard mixture. The relative standard deviation (RSD) was calculated for each of the 92 target analytes, based on their area response ratios. The RSD range was found to be 0.4–7.2%, with an average value of 2.4%. All RSD values are provided in Table 1.
Improving detection limits with Select-eV
While certain components in the mixture of environmental contaminants, such as the PAHs, exhibit strong molecular ions, many others undergo extensive fragmentation at conventional 70-eV ionization. Extensive fragmentation often makes it difficult to find sufficiently intense quantifier ions for confident speciation and quantitation.
Previously, a supplementary soft ionization technique, such as chemical or field ionization, would have to be used to provide enhanced molecular ions for these challenging compounds. However, neither of these ionization techniques offers improvements in sensitivity, and they actually demand more frequent cleaning of the ion source. Select-eV technology eliminates these drawbacks by providing variable-energy electron ionization without compromising sensitivity; in fact, selectivity and sensitivity are often improved.
The positive impact of Select-eV is demonstrated in the examples shown in Figure 3. At an ionization energy of 11 eV, the improvement in molecular ion intensity, coupled with a reduction in fragmentation, results in overall >fivefold improvements of the signal-to-noise ratios for the molecular ions. Limits of detection are therefore extended, which would allow ultratrace analysis of these challenging environmental contaminants.
The required Select-eV ionization energy is software controlled and requires no change to system hardware or configuration. This means that analyses can be carried out at different ionization energies simply by changing a parameter in the TOF-DS acquisition sequence.
In this article, the authors have shown that fast GC–TOF MS provides rapid detection and identification of common pollutants, such as PAHs and pesticides, in a single run. They have also shown the enhancement in system sensitivity and selectivity that is possible with Select-eV, which offers complementary spectra for enhanced compound identification while minimizing the instrument downtime often associated with soft ionization techniques.
Dr. David Barden is Technical Copywriter, and Dr. Laura McGregor is TOF Product Marketing Manager, Markes International, Inc., 11126-D Kenwood Rd., Cincinnati, OH 45242, U.S.A.; tel.: 866-483-5684; e-mail: [email protected]; www.markes.com