In high-throughput analytical testing laboratories robust streamlined analytical and data processing workflows are key requirements for the accurate and reliable determination of trace level residues and contaminants in our food and our environment. These methods must overcome the challenges of an ever-growing list of compounds and diversity of sample matrices, in addition to ever-demanding sensitivity and identification requirements.
For analysis of volatile and semi-volatile contaminants, gas chromatography coupled to low-resolution, nominal mass triple quadrupole mass spectrometers (GC-MS/MS) has been the system of choice for detection of a wide range of target compounds. However, a GC-MS/MS acquisition method requires at least two precursor ions for product selected reaction monitoring (SRM) transitions to be optimized for selectivity and sensitivity for each analyte. Also, the retention time for each analyte must be pre-programmed into the acquisition method. This initial method development can be a time-consuming process. Only targeted ions are detected and quantified.
Moving toward high-resolution mass spectrometry in GC applications
The development of additional hyphenated GC-MS analytical systems such as high-resolution accurate mass (HRAM) Orbitrap mass spectrometry coupled to GC has proved to be a valuable alternative to triple quadrupole GC-MS. With HRAM mass spectrometry, the default acquisition mode is untargeted (full-scan), meaning that all the ions are acquired with high selectivity at the same time across a specified mass range, making the method setup and data acquisition simple to manage and giving the analyst the flexibility to decide which compounds to focus on. In addition, this can extend into retrospective analysis of data to evaluate for the presence/absence of other contaminants not necessarily of interest at the time of acquisition.
Evolved to exceed the demands of analytical testing, the new Thermo Scientific Orbitrap Exploris GC-MS is developed to simplify operations and deliver consistently confident results. In this blog article I would like to highlight some major benefits of the new Orbitrap Exploris GC and Orbitrap Exploris GC 240.
Unmatched resolving power in GC-MS
Acquiring reliable, accurate mass measurements is critical when detecting low-level contaminants in complex matrices. Through the high mass resolving power of the Orbitrap, discrimination between matrix interferences and target analyte ions can be confidently achieved. When the resolution is insufficient, the mass profile of ions overlaps, which could result in high mass error and incorrect assignment of the mass of the target ion. The problem is demonstrated in Figure 1, where a soil extract in n-hexane was analyzed at resolving powers of 15,000, 60,000, and 240,000 (at FWHM m/z 200). The mass spectra show the pesticide bifenthrin and a background matrix ion at a similar mass creating interference. Excellent mass accuracy was achieved for bifenthrin using a resolving power (RP) of 240,000, with near baseline resolution. However, at a 15,000 RP, bifenthrin was not sufficiently resolved from the matrix interference, resulting in a poorer mass accuracy assignment. (Reference: Technical note 10730)
[caption id="attachment_22945" align="aligncenter" width="605"] Figure 1. Effect of resolving power (RP) on mass accuracy of an analyte in matrix. Mass spectra of bifenthrin (fragment ion m/z 167.08113 used often as confirmatory ion) 10 ng/mL in soil acquired at 15,000, 60,000, and 240,000 RP (at FWHM m/z 200).[/caption]
Fast scan speed
Fast data acquisition, to allow sufficient data points across narrow GC peaks, is critical to achieve accurate and precise compound identification. The new generation of GC Orbitraps can perform up to 40 Hz at resolution setting 7,500 at m/z 200. An example using the Orbitrap Exploris GC is shown in Figure 2, where 15 data points across the 2.3 s wide peak for 2-nitroaniline were obtained in full scan at 60,000 resolution. (Reference: Application Note 10724)
[caption id="attachment_22946" align="aligncenter" width="519"] Figure 2. Extracted ion chromatogram of 2-nitroaniline (m/z 138.04238 ± 5 ppm window) in a 1,000 pg/µL mixed solvent standard. Data acquired in full scan at 60,000 resolution (FWHM at m/z 200).[/caption]
Reliable compound identification
The Exploris Orbitrap GC, with full scan range mass accuracy and sensitivity, enables accurate and reliable commercial library (e.g., NIST/Wiley) matching. Figure 3 shows the NIST library search results achieved using the Exploris Orbitrap GC for the analysis of aldrin in a mixed pesticide standard, with both forward and reverse library match scores of >890 achieved. Similar library search scores were demonstrated for a wide range of pesticides in a mixed pesticide standard in a whole flour matrix. (Reference: Application Note 10724)
[caption id="attachment_22947" align="aligncenter" width="733"] Figure 3. NIST library search mass spectra and match results achieved for the analysis of aldrin in a mixed pesticide standard using an Orbitrap Exploris GC operated in full scan at 60k resolution (FWHM at m/z 200).[/caption]
Wide linear dynamic range
A wide linear dynamic range is essential, especially when dealing with applications where the samples analyzed contain a complex chemical background that could potentially interfere with the analytes of interest. Results using hexachloroethane solvent standards demonstrated a linear dynamic range extending to six orders of magnitude (0.1–10,000 pg on-column) making the Orbitrap Exploris GC mass spectrometry an ideal platform for quantitative analysis. (Reference: Application Note 10724)
Consistent results across daily sample batches
Consistent instrument performance is key to ensure quality of results. This was shown for the analysis of pesticides and PCBs in food using 35-times apple matrix injections over two days of continuous operation by repeatedly injecting an apple extract (10 μg/kg). Response of pesticides and PCBs were consistently stable, illustrated in Figure 4, for a selection of pesticide and PCBs, demonstrating a robust system performance critical for analytical testing. (Reference: Application Note 10728)
[caption id="attachment_22948" align="alignnone" width="868"] Figure 4. Analysis of a selection of pesticides and PCBs in apples and carrots using the Exploris Orbitrap GC.[/caption]
The sensitivity achievable with the Orbitrap Exploris GC-MS was evaluated for the analysis of whole flour spiked with pesticides. For this a whole flour sample extract was spiked with pesticides at 10 pg/µL level, equivalent to the European Union (EU) default maximum residue level (MRL) set at 10 µg/kg. Excellent sensitivity with IDL values ranging from 0.18 to 2.45 pg/µL was achieved, with an average value of 0.7 pg/µL. The results confirmed that the Orbitrap Exploris GC-MS has the sensitivity levels to meet the regulatory analysis of pesticides in matrix matched standards. (Reference: Application note 10741)
To have a high degree of confidence in compound identification, low (<1 ppm) mass accuracy is critical. To test the mass accuracy that can be achieved using the Orbitrap Exploris GC, repeat injections of mixed pesticides standards (10 pg/µL) were carried out in Application Note 10724. Sub-ppm mass accuracy was maintained across compound concentrations using an Orbitrap Exploris GC, as exemplified for hexachloroethane. In all cases, irrespective of the m/z and concentration level, <1 ppm values were observed. This is essential, as any compromise in accuracy of mass measurements can result in false identification and non-detection of toxic chemicals such as pesticides in a screening experiment. It is also necessary to maintain this performance at all concentration levels, as any level can be encountered in real world samples.
The data shown here demonstrate that the Orbitrap Exploris GC-MS mass spectrometer has the potential to deliver exceptional analytical performance for analytical testing and scientific research applications. Orbitrap mass spectrometer coupled to the TRACE 1310 GC represents a suitable alternative to traditional GC-MS/MS approaches for the analysis of trace level contaminants in food and the environment. The smaller instrument footprint and simplified system setup enables operational use while offering powerful gains in quantitative performance, ease of use, simplicity, and productivity in research and analytical testing laboratories.