Flow modulation comprehensive two-dimensional gas chromatography-mass spectrometry with a supersonic molecular beam
Journal of Chromatography A, 1129 (2006) 95-104
A new approach of flow modulation comprehensive two-dimensional gas chromatography-mass spectrometry (GCxGC-MS) with supersonic molecular beam (SMB) and a quadrupole mass analyzer is presented. Flow modulation uniquely enables GCxGC-MS to be achieved even with the limited scan speed of quadrupole MS, and its 20 ml/min column flow rate is handled, splitless, by the SMB interface. Flow modulation GCxGC-SMB-MS shares all the major benefits of GCxGC and combines them with GC-MS including: (a) increased GC separation capability; (b) improved sensitivity via narrower GC peaks; (c) improved sensitivity through reduced matrix interference and chemical noise; (d) polarity and functional group sample information via the order of elution from the second polar column. In addition, GCxGC-SMB-MS is uniquely characterized by the features of GC-MS with SMB of enhanced and trustworthy molecular ion plus isotope abundance analysis (IAA) for improved sample identification and fast fly-through ion source response time. The combination of flow modulation GCxGC with GC-MS with SMB (supersonic GC-MS) was explored with complex matrices such as diesel fuel analysis and pesticide analysis in agricultural products.
Isotope abundance analysis methods and software for improved sample identification with supersonic gas chromatography/mass spectrometry
Rapid Commun. Mass Spectrom. 2006; 20: 2579-2588
We present newly developed isotope abundance analysis (IAA) methods and software which are used to derive elemental formula information from experimental mass spectral data of molecular ion isotopomeric abundances. The software, using a novel method, can also be used to automatically confirm or reject NIST library search results, thereby significantly improving the confidence level in sample identifications. In the case of IAA confirmation of the NIST library results, sample identification is unambiguous, since the confirmation is achieved by two independent sets of data and analytical methods. In the case of a rejection, such as when the molecule is not included in the library's databases, the IAA software independently provides a list of elemental formulae with declining order of matching to the isotopomeric experimental data, in a similar way to accurate mass measurements with costly instruments. IAA is ideally applicable to gas chromatography/mass spectrometry (GC/MS) (and liquid chromatography/electron ionization mass spectrometry (LC/EIMS)) with a supersonic molecular beam (SMB) since it requires a trustworthy and highly abundant true molecular ion that is unique to the SMB-MS systems, plus the absence of self chemical ionization and vacuum background noise, again unique features of GC/SMB-MS. The various features of the IAA methods and software are described, their performance is demonstrated with the analysis of experimental GC-SMB-MS data and the IAA concept is compared with accurate mass alternatives. The combination of IAA and GC/SMB-MS is believed to be superior to accurate mass GC/MS in view of the general availability of trustworthy molecular ions for an extended range of compounds.
A simple and inexpensive "solvent in silicone tube extraction" approach and its evaluation in the gas chromatographic analysis of pesticides in fruits and vegetables
Journal of Chromatography A, 24-57, 2007 (journal)
A novel, simple, and inexpensive approach to sorptive extraction, which we call solvent in silicone tube extraction (SiSTEx), was applied to pesticide residue analysis and its effectiveness and efficiency were evaluated. In SiSTEx, which is a form of open tubular sorptive extraction, a piece of silicone tubing (4 cm long, 1.47 mm ID, 1.96 mm OD in this study) is attached to the cap of a 20 mL glass vial that contains the aqueous sample. The tubing is plugged at the end dangling in the sample solution, and MeCN (e. g., 40 lL) added by syringe to the inner tube volume through a septum in the cap. A stir-bar is used to mix the sample for a certain time (e. g., 60 min), which allows chemicals to partition into the tubing where they diffuse across the silicone and partition into the MeCN. The final MeCN extract is then analyzed for the concentrated analytes. In this study, the SiSTEx approach was evaluated for the analysis of organophosphorus (OP) and organochlorine (OC) pesticides in fruits and vegetables using GC/pulsed flame photometric (PFPD) and halogen specific (XSD) detectors for analysis. The produced samples were initially extracted by a rapid MeCN procedure, and 5 mL of the initial extract was diluted four-fold with water to undergo sorptive extraction for 60 min. The final extract was analyzed by GC/PFPD + XSD for 14 OP and 22 OC pesticides. This simple approach was able to detect 26 of the 36 pesticides at 10 ng/g or less original equivalent sample concentration with average reproducibility of 11 %RSD. For those 26 pesticides, a 44-fold lower detection limit on average was achieved in matrix extracts using SiSTEx despite the four-fold dilution with water.
Identification and confirmation of chemical residues in food by chromatography-mass spectrometry and other techniques
Trends in Analytical Chemistry, Vol. 27, No. 11, 2008
A quantitative answer cannot exist in analysis without a qualitative component to give enough confidence that the result meets the analytical needs (i.e. the result relates to the analyte and not something else). Just as a quantitative method must typically undergo an empirical validation process to demonstrate that it is fit for purpose, qualitative methods should also empirically demonstrate that they are suitable to meet the analytical needs. However, thorough qualitative method validation requires analysis of a great number of samples (possibly more than can be reasonably done), which is generally avoided due to the time and the effort involved. Instead, mass spectrometry (MS) is generally assumed to be the gold standard for qualitative methods, and its results are typically unquestioned. For example, a system was developed by European regulators of veterinary drug residues in food animals (2002/657/ EC), in which the number of identification points given in MS analyses depends on the general degree of selectivity of the MS technique used. This well-defined approach gives a definite answer for decision-makers, so it has grown in popularity. However, the identification-points system is not scientific. The reality is that each situation requires information gathering and careful deductive thinking on the part of the analyst to make MS identifications. Rather than devise arbitrary requirements that need to be met by an unthinking analyst, we remind the analytical community that confirmation can be given only if two or more independent analyses are in agreement, preferably using orthogonally selective (independent) chemical mechanisms. In this article, we discuss the proper use of terminology, highlight the identification power of various MStechniques, demonstrate how MS identifications can fail if precautions are not taken, and re-assert the value of basic confirmation practices, qualitative method validation, information checklists, routine quality-control procedures, and blind proficiency-test analyses.
Isotope abundance analysis for improved sample identification with tandem mass spectrometry
Rapid Commun. Mass Spectrom. 2009; 23: 3668-3672
Tandem mass spectrometry (MS/MS) is widely used for trace level sample analysis in complex mixtures. However, sample identification in MS/MS is challenging and not as trustworthy as with electron ionization (EI) mass spectral libraries. This paper presents a novel method for the combination of isotope abundance analysis (IAA) and EI-MS/MS for improved sample identification even at trace level in complex matrices. Accordingly, the first quadrupole is scanned in a narrow range around the molecular ion group of isotopomers such as MR, [MR1]R and [MR2]R, Q2 serves for collision-induced dissociation to produce product ions while Q3 transfers the major sample product ions with low resolution, thus encompassing and uniformly transmitting all the product ion isotopomers. IAA can then be used to derive elemental formula information from the cleansed experimental data. IAA-MS/MS was experimentally tested with perfluorotributylamine and a very good matching factor of 995 (out of 1000) was obtained for IAA on m/z 502, 503 and 504 (fragment ion isotopomers) while Q3 transmitted the m/z 264 product ion with a mass window of 6 m/z units. The IAA-MS/MS method was further tested with the pesticide diazinon on its molecular ions m/z 304, 305 and 306 whileQ3was locked on its m/z 179 product ion with a mass window of 6 m/z units.Again, very good matching factors were obtained, even for 40 pg diazinon on-column during its GC/MS analysis (match=981). IAA-MS/MS combines the traditional benefits of MS/MS in the removal of matrix interferences with the IAA power of elemental analysis.
Fast mass programming controller for a supersonic gas chromatography mass spectrometer
Meas. Sci. Technol. 22 (2011) 125104 (8pp)
In a gas chromatograph mass spectrometer employing a quadrupole mass filter, molecules are ionized and transferred to a mass analyzer, where their mass to charge ratios (m/z) are measured. After the ionization step, the ions pass through a series of ion lenses that focus and guide them into the mass analyzer. The voltages on these lenses can be optimized for each specific m/z value (as the rest of the system is also optimized) to increase the number of ions reaching the mass analyzer. In certain cases, this dynamic mass-dependent optimization of the lenses can increase the signal by a factor of 2 or more. To implement this dynamic optimization, a digital circuit was developed, based on a digital signal controller and high-voltage (HV) amplifiers, that is able to optimize eight independent HV channels ranging between ±150 V at a rate of 100 us.
Comprehensive Controller for Super Sonic Molecular Beam GC-MS
2012 IEEE 27th Convention of Electrical and Electronics Engineers in Israel
This paper presents a new, comprehensive digital circuit used for the control of a novel Gas Chromatograph Mass Spectrometer (GC-MS) interface that is based on Supersonic Molecular Beam (SMB). The circuit includes a Texas Instruments 150MHz digital signal controller (DSC), high voltage amplifiers for 8 independent channels and 4 independent channels of high resolution PWM. The circuit, along with a sophisticated embedded program and a custom made PC application, control all aspects of the interface: smart filament emission-current stabilization, static and scanning mass-dependent ion-source voltages, transfer-line heater PID controls with thermocouple feedbacks, on/off valves, relays and several peripheral device controls that enable the full operation of a turbo-molecular vacuum pump, and of gas flow and pressure controllers. All aspects of this comprehensive controller were successfully tested and it is already being used by several analytical chemists around the world for the control of the Aviv Analytical 5975-SMB GC-MS with Cold-EI.
Fast Mass Programming and PWM Controllers for Super Sonic GC-MS Using DSP
Proceedings of the 5th European DSP Education and Research Conference, 2012
In a Gas Chromatograph Mass Spectrometer (GC-MS) employing a quadrupole mass filter, molecules are ionized and transferred to a mass analyzer, where their mass to charge ratios (m/z) are measured. After the ionization step, the ions pass through a series of ion-lenses that focus and guide them into the mass analyzer. The voltages on these lenses can be optimized for each specific m/z value (as the rest of the system is also optimized) to increase the amount of ions reaching the mass analyzer. In certain cases this dynamic mass-dependent optimization of the lenses can increase the signal by a factor of 2 or more. In addition, any GC-MS interface requires that the molecules vaporized and separated by the Gas Chromatograph (GC) will be transferred into the Mass Spectrometer (MS) at high temperature in order to avoid condensation. Any GC-MS and especially those with a supersonic molecular beams interface can benefit from the ability to change the temperature of this transfer-line between the GC and MS. To implement this dynamic optimization a digital circuit was developed, based on a digital signal controller and high voltage amplifiers that is able to optimize 8 independent high voltage channels ranging between ± 150 V at a rate of 100 us. In addition, 4 independent channels of high resolution PWM unit within the above mentioned controller was utilized in order to control the temperature.
What Can Be Improved in GC-MS - When Multi Benefits Can Be Transformed into a GC-MS Revolution
International Journal of Analytical Mass Spectrometry and Chromatography, 2013, 1, 31-47
Gas chromatography mass spectrometry (GC-MS) is a widely used central analytical technology. Commercially avail- able GC-MS systems use different types of mass analyzers such as Quadrupole, Ion Trap and/or Time of Flight, but practically all systems utilize the same Nier type electron ionization (EI) ion source and the same standard GC-MS transfer-line interface. Consequently, the various GC-MS vendors characterize their systems by a short list of specifications that relate to improvements in the technology of GC or of MS and not of the interfacing technology and ion source. This article presents a list of 62 ways in which the performance of GC-MS as a whole can be improved by an innovative interface and ion source. Such an interface can possibly lead to a GC-MS revolution in a way that is similar to that which is brought to Liquid Chromatography Mass Spectrometry (LC-MS) by the Electrospray ionization interface and ion source and not by improvements to the technology of LC or MS. These 62 possible GC-MS improvements (grouped into eight main categories) are not merely theoretical as they are provided by the Cold-EI GC-MS interface, which is based on the ionization of vibrationally cold sample molecules in a Supersonic Molecular Beam (SMB) within a fly- through ion source. An explanation and discussion is provided for each of these possible improvements.
Open Probe fast GC-MS - Real time analysis with separation
International Journal of Mass Spectrometry 371 (2014) 47-53
An Open Probe inlet was combined with a low thermal mass (LTM) ultra fast gas chromatograph (GC) and mass spectrometer (MS) of GC-MS for obtaining real time analysis with separation. The Open Probe is based on a vaporization oven that is open to room air while having helium purge flow protection to eliminate air leakage into the oven and MS ion source. Sample introduction into the Open Probe is as simple as touch the sample, push the sample holder into the Open Probe oven and have the results in 20- 30 s. The Open Probe is mounted onto the LTM fast GC that is coupled with the MS of either GC-MS with Cold EI or standard GC-MS. Open Probe fast GC-MS provides real time analysis in combination with GC separation, library identification, absence of ion suppression effects and uniform electron ionization response for improved quantitation, and it uses the low cost MS of GC-MS. The operation of the Open Probe fast GC-MS is demonstrated in the 20 s separation and 40 s full analysis cycle time of heroin in its street drug powder, cockroach repeller liquid residue on tomato in 30 s and trace TNT on human hand in 40 s.
Electron ionization LC-MS with supersonic molecular beams—the new concept, benefits and applications
Journal of Mass Spectrometry, 50 (2015) 1252-1263
A new type of electron ionization LC-MS with supersonic molecular beams (EI-LC-MS with SMB) is described. This system and its operational methods are based on pneumatic spray formation of the LC liquid flow in a heated spray vaporization chamber, full sample thermal vaporization and subsequent electron ionization of vibrationally cold molecules in supersonic molecular beams. The vaporized sample compounds are transferred into a supersonic nozzle via a flow restrictor capillary. Consequently, while the pneumatic spray is formed and vaporized at above atmospheric pressure the supersonic nozzle backing pressure is about 0.15 Bar for the formation of supersonic molecular beams with vibrationally cold sample molecules without cluster formation with the solvent vapor. The sample compounds are ionized in a fly-though EI ion source as vibrationally cold molecules in the SMB, resulting in ‘Cold EI’ (EI of vibrationally cold molecules) mass spectra that exhibit the standard EI fragments combined with enhanced molecular ions. We evaluated the EI-LC-MS with SMB systemand demonstrated its effectiveness in NIST library sample identification which is complemented with the availability of enhanced molecular ions. The EI-LC-MS with SMB system is characterized by linear response of five orders of magnitude and uniform compound independent response including for non-polar compounds. This feature improves sample quantitation that can be approximated without compound specific calibration. Cold EI, like EI, is free from ion suppression and/or enhancement effects (that plague ESI and/or APCI) which facilitate faster LC separation because full separation is not essential. The absence of ion suppression effects enables the exploration of fast flow injection MS-MS as an alternative to lengthy LC-MS analysis. These features are demonstrated in a few examples, and the analysis of the main ingredients of Cannabis on a few Cannabis flower extracts is demonstrated. Finally, the advantages of EI-LC-MS with SMB are listed and discussed.
How enhanced molecular ions in Cold EI improve compound identification by the NIST library
Rapid Commun. Mass Spectrom. 2015; 29: 2287-2292
RATIONALE: Library-based compound identification with electron ionization (EI) mass spectrometry (MS) is a well established identification method which provides the names and structures of sample compounds up to the isomer level. The library (such as NIST) search algorithm compares different EI mass spectra in the library’s database with the measured EI mass spectrum, assigning each of them a similarity score called ’Match’ and an overall identification probability. Cold EI, electron ionization of vibrationally cold molecules in supersonic molecular beams, provides mass spectra with all the standard EI fragment ions combined with enhanced Molecular Ions and high-mass fragments. As a result, Cold EI mass spectra differ from those provided by standard EI and tend to yield lower matching scores. However, in most cases, library identification actually improves with Cold EI, as library identification probabilities for the correct library mass spectra increase, despite the lower matching factors. METHODS: This research examined the way that enhanced molecular ion abundances affect library identification probability and the way that Cold EI mass spectra, which include enhanced molecular ions and high-mass fragment ions, typically improve library identification results. It involved several computer simulations, which incrementally modified the relative abundances of the various ions and analyzed the resulting mass spectra. RESULTS: The simulation results support previous measurements, showing that while enhanced molecular ion and high-mass fragment ions lower the matching factor of the correct library compound, the matching factors of the incorrect library candidates are lowered even more, resulting in a rise in the identification probability for the correct compound. CONCLUSIONS: This behavior which was previously observed by analyzing Cold EI mass spectra can be explained by the fact that high-mass ions, and especially the molecular ion, characterize a compound more than low-mass ions and therefore carries more weight in library search identification algorithms. These ions are uniquely abundant in Cold EI, which therefore enables enhanced compound characterization along with improved NIST library based identification.
A New Pulsed Flow Modulation GC × GC–MS with Cold EI System and Its Application for Jet Fuel Analysis
Chromatographia 2016; 79: 741-754
We designed and operated a new system of pulsed flow modulation (PFM) two dimensional comprehensive gas chromatography (GC × GC) mass spectrometry (MS). This system is based on the combination of PFM–GC × GC with a quadrupole mass spectrometer of GC–MS via a supersonic molecular beams interface and its fly-through Cold EI ion source and applied this system for the analysis of JP8 jet fuel. PFM is a simple GC × GC modulator that does not consume cryogenic gases while providing tunable second GC × GC column injection time for enabling the use of quadrupole based mass spectrometry regardless its limited scanning speed. We analyzed JP8 jet fuel with our new PFM–GC × GC–MS with Cold EI system and found that as the second dimension GC elution time is increased the observed molecular ion mass is reduced. This unique observation that helped in improved sample compounds identification under co-elution conditions was enabled via having abundant molecular ions in Cold EI for all the fuel compounds. We named this type of analysis as PFM–GC × GC × MS. We found and discuss in this paper that PFM–GC × GC–MS with Cold EI combines improved separation of GC × GC with Cold EI benefits of tailing-free ultra-fast ion source response time and enhanced molecular ions and mass spectral isomer and isotope information for the provision of increased sample identification information.