GC-MS with Cold EI

The basic GC-MS instrument modifications for its conversion into GC-MS with Cold EI include: a) The analytical column of a conventional GC is connected to a supersonic nozzle via a heated transfer line, mixed with added helium make up gas, with unrestricted column type (ID), length and flow rate; b) Sampling to the vacuum system is in the form of a supersonic molecular beam, as the organic sample compounds expand with added make up helium gas from the supersonic nozzle into a separately (differentially) pumped nozzle vacuum chamber; c) The electron ionization ion source is modified to allow for unperturbed axial passage of the molecular beam (fly-through) with a higher ionizing electron emission current; d) A suitable 90 degrees ion mirror is added to suppress mass spectral noise and keep the mass analyzer clean. 

Our older GC-MS with Cold EI apparatus is based on an Agilent 6890 GC + 5972 MSD and SMB interface and its fly-through ion sources, and it is described in reference [17]. A more advanced GC-MS with Cold EI named 1200-SMB was built and evaluated [25] with record setting performance [29]. We also integrated the GC-MS with Cold EI technology with the Agilent 7890 GC + 5975 MSD and recently with Agilent 5977 MSD and created our best performing GC-MS with Cold EI named 5975-SMB and 5977-SMB

The 5977-SMB Supersonic GC-MS was available from Aviv Analytical www.avivanalytical.com  

This document describes our continued "Quest for Ultimate Performance GC-MS". This Internet web site only few figures, but the features of the GC-MS with Cold EI are further illustrated and demonstrated in a power point presentation titled "5975-SMB GC-MS with Cold EI" that includes 170 slides. This presentation is available on request.

Please share with us your GC-MS challenges so that we can discuss how GC-MS with Cold EI can help to solve your toughest GC-MS requirements. Please consult with us about the commercial availability of the 5975-SMB GC-MS with Cold EI.    

2. Table of Content

1. Introduction.
2. Table of content.
3. Summary of SMB-MS Advantages and Unique Features.
4. The Supersonic Molecular Beam Mass GC-MS Apparatus.
5. Electron Ionization of Cold Molecules in the SMB (Cold EI).
6. Isotope Abundance Analysis for Improved Sample Identification.
7. Classical EI with SMB.
8. Cluster Chemical Ionization with SMB.
9. Hyperthermal Surface Ionization.
10. The Analysis of Thermally Labile and Low Volatility Compounds.
11. Sensitivity Considerations and Evaluation.
12. Fast and Ultra-Fast GC-MS.
13. ChromatoProbe Direct/Dirty Sample Introduction Device.
14. Applications of the Supersonic GC-MS.
15. Laser Desorption Fast GC-MS.
16. Flow Modulation GCxGC-MS.
17. Supersonic LC-EI-MS.
18. References.

3. Summary of GC-MS with Cold EI Advantages and Unique Features

We consider our GC-MS with Cold EI as a major breakthrough, with improvements of all the central aspects of GC-MS. We consider "Cold EI" namely electron ionization of vibrationally cold molecules in SMB as the ideal ionization method, which in combination with the high column flow rate tolerance enables the provision of highest level of information, for the broadest range of compounds, at the shortest amount of time and for the lowest sample concentration that could be in the most complex matrices.

GC-MS with Cold EI redefines the boundaries of GC-MS performance in sample identification, range of compounds amenable for analysis, sensitivity and speed of analysis for the following reasons:
 

1.   Sample identification and MS information
Cold EI provides the highest confidence level in sample identification and mass spectral information. The unique capabilities of the fly-through Cold EI ion source truly make it the "Ideal Ion Source" for the following reasons:

1. The molecular ion is practically always exhibited.
2. Improved library search identification probabilities and confirmation capabilities due to the presence of M+.
3. Isotope abundance analysis (IAA) method and software is available for the provision of automated confirmation or rejection of library identification.
4. Classical EI is provided if needed simply by lowering the helium makeup gas flow rate.
5. Easy to use cluster chemical ionization is provided for the further confirmation (if needed) of the molecular ion.
6. Elemental formula and isotope information is uniquely provided with the M+ isotopomer peaks with the IAA software.
7. Increased isomer and structural MS information is provided.
8. Tunable fragmentation is achieved by controlling the electron energy.
9. M+ could be the predominant peak exhibited at low electron energies (Soft Cold EI) for increased MS separation power.
10. Unique intra-nozzle deuterium exchange enables OH, NH and SH identification.
11. Tailing-free ultra fast ion source response time is provided, regardless the sample volatility.
12. The ion source is self-cleaned and provides temperature independent mass spectra without vacuum memory effects.

2.   Extended Range of Compounds Amenable for Analysis
Cold EI enables the ultimate range of compounds amenable for GC-MS analysis. The key parameter for this unique capability is the use of very high column flow rate for obtaining significantly (up to 200C) lower elution temperatures, while the enhanced molecular ion compensates for the traded GC resolution. High column flow rate further reduces intra injector liner degradation (lower elution temperature from the liner to the column) and intra ion source sample dissociation is inherently avoided. (details are provided in ref 20)

As a result, Supersonic GC-MS is characterized by:    
a. Significantly extended range of thermally labile compounds that are amenable for analysis.
b. Significantly extended range of low volatility compounds that are amenable for analyzed.
c. Significantly extended range of polar compounds (such as free fatty acids) are amenable for analysis without derivatization. 

3.   Speed
SMB enables the highest capability and fastest fast GC-MS, from the reduction or elimination of sample preparation to the final fast analysis results. Basically, fast GC-MS is achieved based on the trade-off of GC resolution for speed of analysis and compensation for the sacrificed GC separation with enhanced separation power of the MS and/or MS-MS. Fast GC-MS with Cold EI (a few minutes down to a few seconds) is characterized by unrestricted column type, length and flow rate, very high temperature operation capability, thermally labile compound analysis capability, higher sensitivity and enhanced molecular ion peak in Cold EI EI. Cold EI uniquely enables simple syringe based fast splitless injections, ultra-fast ion source response time and compatibility with the scanning speed of quadrupole (or any other) mass analyzers through the use of high flow rate megabore or standard narrowbore columns. The unique capabilities of "extract-free dirty sample introduction" with the ChromatoProbe sample introduction device and Laser Desorption sampling method are also important for fast analysis. Recently we combined the GC-MS with Cold EI with a novel low thermal mass fast GC that enables under one minute full analysis cycle time with up to 2000C/min temperature programming rate and under 12 seconds cooling back time. A novel Open Probe fast GC-MS further supplement and complement the LTM fast GC in providing fast sampling and sample introduction for achieving real time analysis with separation and with the many Cold EI benefits. 

4.   Sensitivity

GC-MS with Cold EI is the most sensitive GC-MS, and the harder the compound for analysis the greater is its gain in sensitivity. Several factors contributed to its superior sensitivity as explained in references [29]: 

1. Enhanced molecular ion.
2. Elimination of ion source peak tailing and degradation.
3. Elimination of vacuum background noise.
4. Elimination of mass independent noise.
5. Significantly increased range of thermally labile, polar and low volatility compounds that are amenable for analysis.
6. Improved compatibility with large volume injections.
7. Lower sample elution temperatures and column bleed.
8. Ghost peaks are reduced through sample elution at lower temperatures.
9. Narrow GC peaks with fast GC-MS
10. Matrix interferences are much lower on the molecular ion.

5.   Response Uniformity

GC-MS with Cold EI is uniquely characterized with having compound independent uniform response. This feature is in marked contrast with Electrospray or any other LC-MS API ionization method and while standard EI also exhibit uniform response for small molecules it is completely eroded due to ion source tailing for large and low volatility compounds. The Cold EI uniform response is very useful in the provision of relative compounds amount in mixtures hence enables the elucidation of chemical reaction yields      

6.   Flexibility  (and ease of use)

1. Any column can be used without restrictions on its diameter, length and flow rate. This feature allows the optimal trade-off of GC resolution, speed and sensitivity and simplifies method development.
2. Two columns can be simultaneously connected for fast screening and confirmation.
3. The columns can be replaced, as in GC-FID, without breaking vacuum.
4. Improved splitless coupling with external thermal desorption and purge and trap inlets can be achieved, without flow rate limitations.
5. A unique ChromatoProbe sample introduction device provides fast probe sampling and instant ChromatoProbe/GC-MS switching. The ChromatoProbe also uniquely enables the injection of very "dirty" samples without any sample preparation.
6. The Cold EI, Classical EI-SMB, low electron energy Soft Cold EI and Cluster CI ion sources are easily and quickly interchangeable by a change of a method without breaking vacuum.
7. The same GC-MS system can also be used as electron ionization LC-MS with minor modifications.

4. The GC-MS with Cold EI Apparatus
The material described in this web site is based on our research work that was performed in the last 20 years mostly with three GC-MS with Cold EI systems 

A) 5972-SMB Supersonic GC-MS [17].

B) 1200-SMB Supersonic GC-MS [25].

C) 5975-SMB GC-MS with Cold EI www.avivanalytical.com

Schematic diagram and photo of the 5975-SMB

The 5975-SMB GC-MS with Cold EI system was designed as shown in the photo above as an add-on modification to a standard Agilent GC-MS (7890 GC + 5975 MSD).
The 5975-SMB design is based on the following basic concepts:
1. The Agilent bench top GC-MSD serves as the base platform for the combination with the supersonic molecular beam technology.
2. The Agilent 7890 GC is unchanged while the MSD is only slightly modified through the elimination of its EI ion source and transfer line. No irreversible modifications are performed.
3. The SMB pneumatics is computer controlled by an Aviv Analytical EPC that controls the helium (or hydrogen) SMB make up gas and access to PFTBA tune compound vial if needed.
4. The transfer-line and supersonic nozzle are temperature controlled by the Aviv Analytical electronics and its control software.  
5. The transfer-line accepts one or simultaneously two column outputs that are mixed near the supersonic nozzle with a high flow rate make up gas (typically 60 ml/min, 31 cm transfer line length). Column replacement is simple, and does not require vacuum opening.
6. The supersonic nozzle is made from Ruby with a nozzle diameter of 100 micron. The nozzle-skimmer position is XYZ controlled and optimized during installation from outside the vacuum.
7. The miniaturized nozzle vacuum chamber enables the reduction of the added bench space to less than 28 cm.
8. A fly-through dual cage Electron Ionization (EI) ion source [18] is positioned at the entrance of the MSD vacuum chamber in place of the original transfer-line. It is powered by the Aviv Analytical electronics and its operational software.  
9. A 90 degrees EI ion mirror replaces the Agilent EI ion source. No change was made in the Agilent ion source house and thus the Agilent ion source can be re-mounted if so desired. Two out of the three original ion optics lenses are used, coupled with a new front lens in the coupling to the MSD.
10. The 5975 MSD quadruple mass analyzer and ion detector are unchanged.
11. A new electrical feed-through is added and placed instead of the MSD front window.
This 5975-SMB GC-MS with Cold EI system brings the SMB technology to a user friendly bench-top system in a design that targets reliability and ease of use as a prime consideration.

The benefits of supersonic molecular beam interface and its related fly-through EI ion source were also combined with the Varian 1200L GC-MS and MS-MS, resulting in a powerful GC-MS platform. This system named 1200-SMB is described in details including in a schematic diagram in reference [25].

In GC-MS with Cold EI in its various forms the column output is mixed in front of a supersonic nozzle with ~60 ml/min helium make up gas that flows through a heated and temperature controlled transfer line to the supersonic nozzle. The helium make up gas can be mixed via the opening of a valve with perfluorotributylamine (PFTBA) which serves for the system tuning and calibration. The helium make up gas can also be mixed with methanol vapor for inducing cluster chemical ionization. The sample compounds seeded in helium make up gas expand from a 100 micron diameter supersonic nozzle into a nozzle vacuum chamber that is differentially pumped by a Varian Navigator 301 Turbo molecular pump having 250 L/s pumping speed. The helium pressure at this vacuum chamber is about 6 Micro Bar. The supersonic expansion vibrationally cools the sample compounds and the expanded supersonic free jet is skimmed by a 0.8 mm skimmer and collimated in the second mass analyzer vacuum chamber into a supersonic molecular beam. The supersonic molecular beam seeded with vibrationally cold sample compounds fly through a dual cage electron ionization ion source [18] where it is ionized by 70 eV electrons with 10 mA emission current. The ions are focused by ion lens system, deflected 90º degrees by an ion mirror and enter the mass analyzer (5975 MSD). The 90º ion mirror serves to keep the mass analyzers clean from sample induced contaminations and it is separately heated. The original mass analyzer (5975 MSD) and its triple axis ion detector are used without a change and data is processed with the original Agilent Chemstation (or MassHunter with 5977) while NIST library search can be enhanced by the Tal Aviv isotope abundance analysis software.  

5. Electron Ionization of Vibrationally Cold Molecules in the SMB ("Cold EI")
Supersonic expansion of gas into a vacuum system results in uniform velocity to all the expanding species. In fact, a simple expansion from about one atmosphere through 100 micron diameter pinhole nozzle results in a supersonic free jet. Accordingly, the supersonic expansion leads to collisions with relatively low velocity of the sample compounds and the carrier gas atoms, resulting in substantial super-cooling of the sample compounds vibrational temperature to well below 50K. This is like having the ion source at an ultra-low temperature but without condensing the sample compounds. For this reason we named this EI of SMB compounds as "Cold EI". As a result of this sample vibration cooling, the level of information contained in the cold EI mass spectra is greatly increased.

Examples of Cold EI, Soft Low Electron Energy Cold EI and NIST library Mass Spectra of the Indicated Hydrocarbons

In the figures above we show at left a comparison of Soft Cold EI mass spectrum (upper trace), Cold EI mass spectrum (middle trace) and NIST library mass spectrum (bottom trace) of n-C24H50. Soft Cold EI mass spectrum was obtained at 18 eV electron energy, 16 mm nozzle-skimmer distance and 84 ml/min helium make-up gas flow while the Cold EI mass spectrum was obtained at 70 eV electron energy, 7 mm nozzle-skimmer distance and 54 ml/min helium make-up gas flow rate.

In the figures above at right we show a comparison of Soft Cold EI mass spectrum (upper trace), Cold EI mass spectrum (middle trace) and NIST library mass spectrum (bottom trace) of the highly branched hydrocarbon isomer squalane C30H62. The middle trace Cold EI mass spectrum was obtained at 70 eV electron energy, 7 mm nozzle-skimmer distance and 54 ml/min helium make-up gas flow rate while the upper Soft Cold EI mass spectrum was obtained at 18 eV electron energy, 16 mm nozzle-skimmer distance

As demonstrated Cold EI provide significantly enhanced molecular ions and amplifies isomer structural mass spectral information yet it retains the standard EI fragments for achieving effective NIST library based identification. Soft Cold EI at low electron energy Cold EI provides the ultimate selectivity in hydrocarbons analysis and approaches a step closer to the ideal of molecular ion only.    

 We consider cold EI with SMB to be the ideal GC-MS ionization method, having an enhanced molecular ion and superior molecular isomer and structural information with the following main features and advantages: [30]

1. The exact molecular ion practically always exists in 70 eV electron ionization (EI) MS with SMB. The relative height of the molecular ion peak is increased by up to three orders of magnitude due to the vibrational supercooling. On the other hand, the conventional EI fragmentation pattern is retained for library identification. Actually, the EI-MS of small molecules is relatively unchanged while for large molecules, due to their large vibrational heat capacity, a substantial increase in M+ is observed. While in standard EI the molecular ion is exhibited in less than 70% of the samples, in Cold EI it is provided in about 99% of the samples, and even in the rare cases that it could be absent, cluster chemical ionization is provided as a complementary ionization method. As a result, while the highest mass peak in standard EI MS can not be trusted, Cold EI provides trustworthy molecular ions.   

2. The level of information achieved in a single EI-SMB-MS scan, is greater than that provided by standard EI and CI combined, without the CI problems, and with the uniform high sensitivity of EI to all molecules. This is one of the reasons for our consideration of "Cold EI" as an ideal ionization method.

3. Fragmentation tunability and fragment order of appearance information can be achieved through the control of the electron energy. Due to the molecular vibrational super-cooling, the electron energy is the only parameter that governs the degree of ion fragmentation. Thus, this control over the degree of fragmentation is achieved with minimal loss of sensitivity since the reduced electron ionization cross section at low electron energies is partially compensated for by the reduced degree of fragmentation so that the relative intensity of the molecular ion is increased and its absolute intensity is only slightly affected.

4. Unique isomer information is provided due to the vibrational super-cooling. Any small isomer mass spectral difference is considerably amplified with "Cold EI" and a novel method of isomer abundance analysis is enabled for hydrocarbon and fuel characterization.

5. Improved library search identification probabilities is obtained with Cold EI since although the obtained matching factors are lower than those obtained with standard EI ion sources, the NIST provided identification probability namely confidence level in compound identification is improved since the molecular ion is the most characteristic and unique ion in the mass spectrum (lower fit but better hit). In addition, identification confirmation is achieved due to the molecular weight information. Note that about 30% of the NIST library compounds have no molecular ion (below 1% normalized intensity) with standard EI. This value is increased to 50% for compounds with molecular weight over 300 amu. We also note  that while in GC-MS with Cold EI the molecular ion is increased, it is reduced in standard GC-MS systems in comparison with the library mass spectra. This observation emerges from the fact that standard EI ion sources are operated at higher temperatures than used to obtain the library under GC-MS analysis conditions of complex mixtures, in order to prevent peak tailing and maintain proper ion source cleanliness. Thus, the obtained ratio of probability of proper first hit to second non sample probability was found experimentally to be superior with the Supersonic GC-MS in comparison with standard GC-MS (19, 46).

6. Elemental analysis is enabled through accurate isotope abundance analysis (IAA) of the relative intensity of the molecular ion group of mass spectral peaks (isotopomers). The features of enhanced molecular ion abundance combined with total lack of residual intra-ion source chemical ionization (self CI) and reduced vacuum background enable an accurate measurement of the intensity ratios of the molecular ion peaks, resulting in elemental analysis capability with unit resolution mass analyzers. A unique IAA software was developed that automatically confirms or rejects NIST library identification and can further provide a list of possible elemental formulas with declining order of matching to the experimental IAA. If the elemental content is known, geochemical and isotope labeling/abundance information is available through the analysis of these molecular ion peak ratios.

7. Deuterium exchange at the sample vial or near the supersonic nozzle can be employed for OH, NH and SH labeling. The on-line mixing of the carrier gas with deuterated methanol or heavy water enables effective and fast deuterium exchange before the supersonic expansion. It provides unique structural and isomeric information.

8. The fly through "Cold EI" ion source is self cleaned, robust and usually does not require cleaning maintenance. Since the long filament is close to the ion cage, the actual ion source temperature is over 400C and sample condensation is avoided. Furthermore, ultra fast ion source response time (sub millisecond) is ensured and peak tailing is completely eliminated regardless the sample volatility due to the background filtration process based on differences in the ion kinetic energy of beam species and vacuum background. Accordingly, almost no vacuum background is observed even after many dirty sample analyses.

6. Isotope Abundance Analysis for Improved Sample Identification

We have developed an isotope abundance analysis (IAA) method and software (ref 26 and USA patent 7,345,275) that converts experimental mass spectral data of molecular ion isotope abundances into elemental formulas information. Our novel method and software can also automatically confirm or reject NIST library search results, and as a result, significantly improves the confidence level in sample identification. In case of IAA confirmation of NIST library results, sample identification is unambiguous in view of its confirmation by an independent set of data and method. In case of rejection, IAA independently provides a list of elemental formulas with declining order of matching to the experimental data, in similarity to costly accurate mass measurements. Our IAA method and software named TAMI (Tal-Aviv Molecule Identifier) is ideally applicable to the GC-MS with Cold EI since IAA requires having a trustworthy and high abundance true molecular ion that is unique to GC-MS with Cold EI, plus absence of self CI and vacuum background, again unique features of our GC-MS with Cold EI. The TAMI software can also improve the mass accuracy of quadruple MS and it is very easy to use with standard centroid data files. We claim that the combination of IAA and GC-MS with Cold EI is superior to accurate mass GC-MS in view of the general availability of trustworthy enhanced  molecular ion in GC-MS with Cold EI for extended range of compounds. We consider the combination of enhanced molecular ion, NIST search and IAA with GC-MS with Cold EI as the best method for the provision of ultimate confidence level in sample identification. The Tal-Aviv Molecule Identifier software  is available by Aviv Analytical    

7. Classical EI with SMB

GC-MS with Cold EI offers a competitive classical EI ionization method with its fly-through ion source, at no added hardware (cost) as a supplementary ionization method to cold EI. We named this mode as classical EI-SMB.

Cold EI-Classical EI-SMB switching is easy, simply by the lowering of the total column and helium makeup gas flow rate to about 5 ml/min, which produces about 100 MilliBar pressure behind the nozzle plus heating the transfer line and nozzle to 300ºC. Under these conditions, the vibrational cooling is ineffective and the sample temperature after its expansion from a 300ºC nozzle is about 200ºC, resulting in the provision of classical EI mass spectra with excellent matching factors to the library (33). In addition to reduced make up gas flow rate, the tune method must be changed since a few other lens voltages have different optimal values. All these changes are programmable and can be executed by a simple change of method, hence in a fast and easy way without any mechanical changes or additional hardware. Classical EI-SMB has similar sensitivity and slightly better library search matching factors in comparison with standard EI of available GC-MS hence it is a viable addition to Cold EI. Furthermore, the fly-through classical EI-SMB ion source offers several important advantages over standard EI ion sources in view of its scattering free operation feature including: a) ultimate inertness (no sample contact with the ion source ); b) tailing free operation; c) robustness; d) no ion-molecule reaction interference hence improved compatibility with IAA; e) increased column flow rate compatibility without affecting the sensitivity. Thus, while Cold EI is the main ion source of GC-MS with Cold EI, there is no need for having an additional standard EI ion source in it in view of availability of a competitive and easy to switch classical EI-SMB ion source as a viable option that serves as a better classical EI ion source.      

8. Cluster Chemical Ionization with SMB

Cluster chemical ionization is an easy to use supplementary ionization methods of the GC-MS with Cold EI. It serves for two main purposes of: a) to provide molecular ion information for about 1% of samples that do not show a molecular ion even in cold EI and; b) to ensure the unambiguous identification of the molecular ion even when it is small.

Upon the supersonic expansion of helium mixed with methanol, various clusters of the solvent with the sample molecules can be formed. As a result of 70 eV electron ionization of these clusters, cluster chemical ionization (cluster CI) mass spectra are obtained through an intra cluster proton or hydrogen atom transfer reaction. Cluster CI mass spectra are characterized by the combination of EI mass spectra of vibrationally cold molecules in the supersonic molecular beam (Cold EI) with CI-like appearance of abundant protonated molecular ion and satellite mass spectral peaks of protonated or non-protonated clusters of sample compounds with one, up to three solvent molecules. Like CI, cluster CI preferably occurs with polar compounds having high proton affinity. However, in contrast to CI, for non-polar compounds or those with reduced proton affinity the cluster CI mass spectrum converges to that of Cold EI. The appearance of a protonated molecular ion and its solvent cluster peaks plus the lack of protonation and cluster satellites for prominent EI fragments enable the unambiguous identification of the molecular ion. In turn, the insertion of the proper molecular ion into the NIST library search of the Cold EI mass spectra, eliminates those candidates with incorrect molecular weights and thus significantly increases the confidence level in sample identification. Furthermore, high confidence level in the correct identification of the molecular weight of unknown compounds is of prime importance in their analysis.

Cluster CI is an effective practical supplementary ionization method of GC-MS with Cold EI due to its ease-of-use and fast conversion of EI into cluster CI which involves the opening of only one valve located at the make-up gas path and having a small vial with methanol. The ease-of-use of cluster CI is analogous to that of liquid CI in ion traps with internal ionization, and it is in marked contrast to that of CI with most other standard GC-MS systems that require a change of the ion source. Cluster CI is described in references [7, 21].    

9. Hyperthermal Surface Ionization

The uniform velocity of all species in the supersonic molecular beam means that the velocities of the carrier gas (hydrogen or helium) and that of the sample molecules are equal. Since the sample compounds are a minor component of the SMB, the velocity of the sample compounds is increased to that of the carrier gas while the carrier gas is only marginally decelerated. Accordingly, the sample compounds are accelerated and their kinetic energy is increased, by about the mass ratio of the sample compound and the carrier gas, to the hyperthermal kinetic energy range of 1-20 eV. Thus, the kinetic energy of the sample molecule increases with its molecular weight and nozzle temperature and is reduced by increasing the carrier gas atomic or molecular weight.
We have found that the surface ionization yield of organic molecules acquired with hyperthermal kinetic energy is increased by many orders of magnitude relative to thermal surface ionization and it can be up to one order of magnitude higher than EI. This phenomenon of hyperthermal surface ionization (HSI) was discovered by Amirav and Danon and studied in detail from its various mechanisms through its analytical applications as reviewed in [3]. HSI is based on a molecule-surface electron transfer process which is promoted by the image potential formed between the ion and the surface. This image potential facilitates the molecule-surface electron transfer and ionization process. The molecular ionization requires the energy difference between the molecular ionization energy and the surface work function (surface ionization energy). When an ion approaches the surface, an image potential is formed between the ion and surface. This image potential reduces its potential energy that can be lower than that of the scattered neutral compound at a given distance from the surface. This critical distance is called the curve crossing distance (Rc). Below this distance a spontaneous electron transfer from the molecule to the surface may occur and can be calculated using a modified Landau Zenner curve crossing equation. If the sample compound has hyperthermal kinetic energy above the thermodynamic energy requirement, it can be scattered as an ion from the surface. Since a portion of the molecular kinetic energy is lost, either to the surface or to internal vibrational degrees of freedom, most of the ionized compounds are reneutralized. As a result, the ionization efficiency is dramatically increased with the molecular kinetic energy, since an increased portion of the scattered ions have sufficient kinetic energy to overcome the image potential in their exit trajectories. Other HSI mechanisms, including negative ion HSI, are briefly described in references 12 and 22 but the mechanism briefly described above is analytically the most significant.
The degree of ionization also depends on the surface work function and molecular ionization energy. Rhenium oxide proved to be an ideal surface for HSI as it combines a high work function with excellent long term stability that is essential for analytical applications. This is achieved by the direct current heating of a rhenium foil to about 1000K while bleeding oxygen on it at a partial pressure of 2-3x10-5 milliBar. As a result, the oxygen catalytically combusts all the organic surface impurities and maintains a steady state of surface cleanliness.
We found that HSI can serve as a semi-universal, very sensitive ion source with tunable selectivity that is ideal for compounds with low ionization energies such as drugs and aromatic compounds. Its tunable selectivity was demonstrated combined with very high HSI yield that is estimated to be over 10% at the surface, and about 2% for the ratio of ions at the surface to nozzle flux (assuming 20% jet separation efficiency).
While HSI provides molecular ions only for polycyclic aromatic hydrocarbons, the HSI mass spectra are usually characterized by a rich and informative fragmentation pattern. The degree of HSI fragmentation naturally depends on the compound but it also depends on the molecular kinetic energy. The HSI fragments usually correspond to those which appear in EI mass spectra albeit with different relative peak intensities. In some cases, such as with cocaine, the HSI MS can be identified by the NIST EI library. In other cases a HSI library must be built and can be effective.

In summary, HSI is characterized by the following two major features and advantages:
A. Increased Sensitivity. Hyperthermal surface ionization is the most sensitive ion source for positive ion formation due to:
  1. Very high ionization efficiencies of over 10%.
  2. The background of the vacuum chamber molecules is reduced or eliminated since they do not possess the required hyperthermal kinetic energy.
  3. For many important classes of molecules such as PAH's and several drugs, only a single molecular or fragment ion appears.
Minimum detected amount of 500 attograms was demonstrated.
B. Tunable Ionization Selectivity. The hyperthermal surface ionization yield depends on the surface work-function, sample molecule and molecular kinetic energy. These parameters can be controlled through the choice of the carrier gas such as helium or hydrogen, the nozzle temperature and/or the choice of surface such as rhenium oxide or molybdenum oxide. Over 1 E 4 anthracene/dodecane selective ionization was achieved, and aromatic selective GC-MS of gasoline was demonstrated. The high selectivity may involve only a minor ionization yield reduction of the selected molecules. Selective ionization can help to simplify complex mixture analysis and opens the door for a much faster GC-MS analysis.
Negative ion HSI generates ions of all the functional groups in the molecule having high electron affinity (I, Br, Cl, F, CN, NO , PO) and enables their selective detection.

10. Extending the Range of Compounds Amenable for GC-MS Analysis - The Analysis of Thermally Labile, polar and Low Volatility Compounds

GC-MS suffers from a major “Achilles Heel” in the form of a relatively small range of volatile, thermally stable compounds that are amenable for analysis. This limited capability with respect to analyzing thermally labile and low volatility large molecules significantly restricts the usefulness and growth potential of GC-MS. 

GC-MS with Cold EI enables the analysis of significantly increased range of thermally labile and low volatility compounds, mostly through its use of degradation free fly-through ion source and in view of its compatibility with high GC column flow rate operation that considerably lowers the sample elution temperatures from the injector liner to the column and from the column. This topic is described and discussed in details in reference [20]. As a result, the use of SMB for sampling and ionization significantly increases the range of compounds that are amenable for analysis in three important areas: 

10.1.  Lowest Volatility Sample Analysis
Every factor of two increased column flow rate and/or shorter column length lowers the elution temperature by ~20ºC. We have demonstrated up to 209ºC lower elution temperatures and as a result the GC-MS analysis of record low volatility compounds such as phthalocyanine, metaloporphirins, reserpine (608 amu polar drug), triglycerides, large aliphatic compounds with molecular weight above 1000 amu and large PAHs with 10 fused rings such as ovalene and decacyclene [17, 20]. These lower elution temperatures contribute to the significant extension of range of low volatility compounds amenable for GC-MS with Cold EI analysis. In addition, tailing-free GC-MS is achieved without any mass spectrometric ion source related temperature limitations due to vacuum background filtration, and the fly-through EI ion source provides enhanced M+ due to the vibrational supercooling regardless the nozzle temperature.

10.2. GC-MS Analysis of Thermally Labile Compounds
The significantly lower elution temperatures encountered with short columns having high column flow rates provides dramatic increase in our capability to analyze thermally labile compounds that are currently considered as difficult or impossible for analysis by GC-MS. The analysis of thermally labile molecules is considerably improved in comparison with conventional GC-MS owing to the following reasons

1. On column (megabore) (or into a standard liner) temperature programmable injection can be coupled with very high carrier gas flow rate to minimize both the injector temperature during the vaporization, and the residence time at the injector. Thus, intra injector degradation can be significantly suppressed.

2. The considerably lower elution temperatures (from the GC column due to high column flow rates) exponentially reduce thermal dissociation inside the column, which is practically eliminated for all "borderline compounds" .

3. The vibrational super-cooling combined with fly-through EI ion source eliminate both molecular decomposition and molecular ion dissociation in the ion source.

 10.3. GC-MS Analysis of Polar Compounds  
Highly polar compounds such as free fatty acids and steroids several free OH groups can be analyzed without derivatization since any intra-ion-source degradation of acids and OH groups at its metalic surface is eliminated in the Cold EI fly-through ion source. 

When these three elements are combined, GC-MS with Cold EI can be considered as equivalent or even slightly superior to APCI-LC-MS, which involves high temperature thermal vaporization. A broad range of carbamate pesticides, antibiotic drugs, underivatized steroids, high explosives and other thermally labile compounds were analyzed (ref 20).

11. Sensitivity Considerations and Evaluation

GC-MS sensitivity is typically defined with octafluoronaphthalene (OFN), and low femtogram limits of detection (LOD) are specified for it by all the major vendors. However, GC-MS LOD's for real samples in common matrices are a few orders of magnitude higher than with OFN, and users seldom encounter 1 pg LOD in the single ion monitoring mode with their samples. This sensitivity difference is defined by us as "the OFN gap".

With the Supersonic GC-MS the OFN gap is significantly reduced! The harder the compound analysis the greater is the 5975-SMB sensitivity gain, and the harsher the matrix the lower is the relative 5975-SMB mass spectral noise. Thus, the 5975-SMB increases the sensitivity particularly when needed, for those compounds that are the bottleneck of the whole analysis, and the sensitivity gain can amount to a few orders of magnitude.

In general, the jet separation losses and reduced ionization probability per electron due to the single x10 times faster sample compound flight through the ion source is more than compensated by x100 times higher ionizing electron emission current and multiple electron paths through the ion source. The sensitivity of the Supersonic GC-MS is improved through the combination of:

1. Enhanced molecular ion

2. Elimination of ion source peak tailing and degradation

3. Elimination of vacuum background noise 

4. Significantly increased range of thermally labile and low volatility compounds that are amenable for analysis

5. Improved compatibility with large volume injections

6. Elimination of mass independent noise

7. Lower sample elution temperatures thus lower column bleed

8. Ghost peaks are reduced through sample elution at lower temperatures

9. Fast GC-MS with SMB generates narrower GC peaks  

10. Matrix interferences are reduced with the combination of enhanced molecular ion and MS-MS

We demonstrated with the 1200-SMB [49, 53] the achievement of <1 fg OFN LOD (SIM, m/z=272). More importantly, we achieved LOD of 2 fg for a more realistic compound such as diazinon and 10 fg for underivatized testosterone that is considered as not amenable for GC-MS analysis. In comparison with standard GC-MS we measured sensitivity enhancement factors of 24 for dimethoate, 30 for methylstearate, 50 for cholesterol, 50 for permethrin, >400 for methomyl and >2000 for C₃₂H₆₆. 

Thus, the harder the compound analysis the greater is the Supersonic GC-SM sensitivity gain for it. 

12. Fast and Ultra-Fast GC-MS

The unique features of GC-MS with Cold EI enable fast GC-MS analysis, which provides a complete solution for all the requirements of an optimized, high performance, fast, high temperature and thermally labile compatible GC-MS.  Currently, the field of fast GC-MS is hampered by a few "Sales Hypes" and false claims. Clearly there is no "free lunch" and fast GC-MS always involves with some trade offs. For example, "Time Compressed" fast GC-MS involves with a significant loss of sensitivity as well as some loss of chromatographic resolution (despite the claims) and microbore column limitations. Fast temperature programming rate is not always needed and could result in reduced chromatographic resolution combined with increased degradation of thermally labile sample compounds. Vacuum chromatography (also known as "Rapid MS") clearly involves with a trade-off of chromatographic separation resolution with speed, with potential compensation through the enhanced separation power of MS-MS for several selected target compounds.
Our general rule is that while using the same column type every factor of two lost GC separation can bring with it a factor of four faster analysis if the right tools for it are available and clearly GC-MS with Cold EI enables very fast GC-MS. 
Fast GC-MS with Cold EI can be uniquely based on the use of increased injector and column flow rate for achieving fast analysis, combined with a compensation for some loss of chromatographic resolution by the enhanced separation power of the mass spectrometer, either through the enhanced selectivity of the molecular ion EI or with MS-MS. Please note that the use of increased flow rate results in speeding up all the elements of chromatography including sample injection, chromatography and cooling back (due to higher initial oven temperature). Furthermore, the enhanced separation power of the mass spectrometer as well as the high injector and column flow rate enables the fast analysis of thermally labile and relatively non-volatile compounds which otherwise are not amenable for analysis by standard GC-MS.
A conventional GC such as the Agilent 7890 or Varian 3800 is connected to the supersonic nozzle (through a short transfer-line) simultaneously with two columns, having no limitations on the column ID, length or flow rate. In this way the conventional GC becomes a fast GC-MS inlet that comprises a new approach for fast GC-MS. Even a very short (50 cm long) megabore capillary column was connected to the supersonic nozzle, and served for obtaining ultra-fast GC-MS analyses [4,8]. Alternatively, we developed a unique low thermal mass fast GC inlet that enables below one minute analysis cycle time including splitless injection, temperature program and cooling back time.   
In contrast to all other approaches, Fast GC-MS with Cold EI method offers a solution to all the requirements of fast GC-MS, from sample preparation to data analysis:

1. Fast splitless injection is achieved with a conventional syringe, even for relatively non-volatile compounds, due to the very high injector flow rate (up to 100 ml/min). This fast injection can be performed at relatively higher GC oven temperatures (due to the elimination of the need for cryo focusing) thus facilitating faster analysis and shorter cooling down time. 

2. A unique extract free dirty sample introduction method and device (ChromatoProbe) enables a true fast analysis including the step of sample preparation. A novel Open Probe further simplifies and speed-up sample collection and introduction [44]. 

3. Fast analysis is achieved with samples having a very wide boiling point range, due to simple and fast column flow programming having a very high/low flow rate ratio of over 60. This unique column flow programming capability enables effective fast GC-MS with standard GC ovens.

4. Effective backflush can be easily achieved.

5. A wide temperature range fast GC-MS is achieved without any ion source related peak tailing.

6. Low thermal mass ultra fast GC is integrated with GC-MS with Cold EI, forming a powerful Fast GC-MS that enables under one minute analysis time with full range temperature programming (up to 2000C/min heating rate and <10 s cooling back) (40).

7. GC-MS of thermally labile compounds is achieved with fast and ultra fast GC-MS for molecules that are usually probed by particle beam or APCI LC-MS.

8. Compatible mass scanning rate is enabled with standard quadrupole mass analyzers. The reduced number of separation plates associated with the use of a high flow rate short columns results in a normal peak width of ~ 1 sec after ~5-10 seconds which does not require TOF-MS. Thus, standard quadrupole mass analyzer can be used.

9. Sufficient overall GC-MS resolving power is provided, even for complex mixture fast analysis. The GC column time separation and MS resolving power are supplemented by the selectivity of enhanced molecular ion abundance in cold EI and/or MS-MS. Thus, many target compounds can be  analyzed in a few seconds in real world complex mixtures including trace level (<10 ppb) pesticides in agricultural matrices.

10. The ultra fast cold EI ion source response time allows the monitoring of fast GC peaks originating even from relatively non-volatile molecules without tailing.

11. Superior low concentration sensitivity is achieved with more than one microLiter fast splitless injections due to the high column flow rate. This is in marked contrast to microbore column fast GC-MS. Fast splitless injections also enable faster temperature programming and GC cooling back time.

12. Simplified sample preparation is achieved due to the superior low concentration sensitivity which allows the injection of smaller amounts of untreated samples. Alternatively, our unique ChromatoProbe and Open Probe devices considerably simplifies sample preparation.

13. Resolution, time and sensitivity trade-off choice is enabled for optimal results. The coupling with a standard GC is allowed without any constraints on the column diameter, length and carrier gas flow rate. Thus, critical parameters such as chromatographic time and resolution can be optimized, with regards to and in consideration of the desired injected sample amount. This is easily achieved due to the practically unlimited column flow allowable. Alternatively a unique low thermal mass fast GC can be used for sub one minute full analysis cycle time.  

14. Megabore (0.53 mm ID capillary column) can be used, having over an order of magnitude superior tolerance for matrix contamination and thus enables increased number of samples to be analyzed as required from fast analysis methods.

15. While fast GC-MS is ideally suited for the fast screening of a large number of samples, confirmation is also needed, preferably with the same GC-MS instrument. GC-MS with Cold EI provides this highly desirable feature, supplementary to fast GC-MS, and can be configured with both a standard and a short columns, which are simultaneously connected to the SMB interface.

In conclusion. Supersonic GC-MS enables faster analysis for a broad range of applications and with it the analysis is practicaly always faster  

13. ChromatoProbe Direct/Dirty Sample Introduction Device

A unique Direct Sample Introduction (DSI) device was developed by us which is especially suitable for use with the GC-MS with Cold EI. This DSI is available by Bruker (now Scion Instruments) under the name "ChromatoProbe" for its 450 GC. The DSI is also available by Agilent under the name Thermal Separation Probe and by FLIR under the name PSI Probe. The Aviv Analytical ChromatoProbe  is also available for the Agilent 6890 and 7890 GC and GC-MS standard split splitless and MMI injectors as described in the Aviv Analytical website.

It serves for three major applications, both with many advantages.

13.1. Probe Sampling for Mass Spectrometry Studies.
The ChromatoProbe device, effectively transforms a conventional temperature programmable GC injector, (preferably a second GC injector in the GC-MS) followed by a short capillary transfer line column (1 m microbore), into a cost-effective alternative to the standard direct insertion probe. It possesses the advantages of faster and easier operation, faster ChromatoProbe/GC-MS interchange, maintaining and preserving the ion source cleanliness, capability of sampling solutions, possible use as a micro-chemical (derivatization) reactor and it is characterized by complete immunity against leaks and thus compatibility with operation by untrained operators. Furthermore, all this is achieved with a low cost.

13.2. Extract-Free Dirty Sample Introduction For GC-MS Analysis.
This new method is based on sampling in a test tube (micro vial) that retains the harmful and non-volatile matrix residue of real world samples. Thus, it eliminates the need for further sample clean-up, while the micro vial is a disposable item. Each analysis begins with gentle solvent vaporization, preferably at a relatively low injector temperature such as 120 C for water/urine (20 C above the solvent boiling temperature), followed by brief injector heating to the temperature required for achieving effective intra injector thermal extraction and sample compound vaporization. The sample semi-volatile compounds are focused on the early portion of the column and are analyzed by the chromatography as usual. This method brings the many known advantages of thermal extraction in an easy to use low cost fashion, combined with the many advantages of GC-MS with Cold EI and best GC integrity. It facilitates extract free analysis of drugs in urine or hair, or pesticides in blended fruit and vegetable items, or in milk, juice and other sludge's. The ChromatoProbe also uniquely allows large volume sample injections of conventional extracts without the associated residues that usually restrict the sample size, and thus lower detected concentration limits can be achieved. The containment of the non-volatile compounds in the disposable test tube also results in faster analysis that can end at a lower column temperature.

13.3 SnifProbe Gas Analysis
SnifProbe is based on the use of 15 mm short pieces of standard 0.53 mm ID capillary or PLOT columns (or micro solid phase extraction tubes) for sampling air born, head space, aroma or air pollution samples. The short (15 mm) column is inserted into the SnifProbe easy-insertion-port and the SnifProbe is located or aimed at the sample environment. A miniature pump is operated for pumping 30 ml/min of air sample through the sample collection short piece of column (or MicroSPE vial). After a few seconds of pumping, the short column is removed from the SnifProbe with a tweezers and placed inside a ChromatoProbe glass vial having a 0.5 mm hole at its bottom. The ChromatoProbe sample holder with its glass vial and sample in the short column are introduced into the GC injector as usual. The sample is then quickly and efficiently vaporized from the short sample column and is transferred to the analytical column for conventional GC and or GC-MS analysis. Thus, SnifProbe extends the ChromatoProbe range of samples that also includes gas phase samples. A photo of SnifProbe can be found in the Aviv Analytical SnifProbe website.  
SnifProbe enables many of the manual SPME, air bags and Tenax tube applications, with a few advantages. SnifProbe is ideal for field or process operation, it is small, enables fast sampling, compatible with the full range of semi volatile compounds and enables low cost sensitive analysis.

For further information on our ChromatoProbe and SnifProbe please visit the ChromatoProbe and SnifProbe Web Site

14. Applications of the GC-MS with Cold EI

 GC-MS with Cold EI excels in a wide range of applications due to its exceptionally broad range of advantages and unique features as above. While it can do all the current standard GC-MS analysis it particularly excels with many applications and can uniquely perform a few types of analyses that cannot be done by any other GC-MS. Accordingly, in the majority of both standard and non-standard applications it can replace the available instrumentation and provide a competitive advantage. A list of a few such major applications includes:

1. Petroleum and hydrocarbon-MS.
Petrochemical analysis benefits from many of the unique features of GC-MS with Cold EI including molecular ion information in alkanes, molecular ion only MS at low electron energies EI, unique isomer information and low boilers large petrochemical compounds GC-MS analysis. A unique isomer abundance analysis method was developed that enables fuel characterization. Hydrocarbon MS is more than just petroleum analysis and includes arson investigations, fuel characterization, fuel adulteration, geochemical applications, environmental analysis and transformer oil analysis. All these areas can significantly benefit from GC-MS with Cold EI as above.   

2. Service GC-MS for Universities, Institutes and Synthetic Chemistry.
The features of: a) provision of trustworthy enhanced molecular ion; b) extended range of thermally labile and low volatility compounds; c) elemental formula information via isotope abundance analysis; d) uniform compound independent response; e) fast analysis; f) enhanced isomer and structural MS information; g) flexible, fast and easy method development; h) easy and fast cold EI-Cluster CI-ChromatoProbe switching, are all important to this application. References 23 and 42 describe our success in these types of applications. Recently we demonstrated the unique ability of GC-MS with Cold EI in the optimization of organic chemical reaction yields via semi online monitoring of their progress and products.