Soft Cold EI – Approaching Molecular Ion Only with Electron Ionization

Aviv Amirav (1,2), Uri Keshet (1) and Albert Danon (3) 

1. School of Chemistry, Tel Aviv University Tel Aviv 69978 Israel

2. Aviv Analytical LTD, 3 Haarad Street Tel Aviv 69107 Israel

3. Nuclear Research Center Negev P.O. Box 9001 Beer Sheva Israel  

Introduction  

GC-MS with Cold EI (Electron Ionization of Cold Molecules in Supersonic Molecular Beams) provides ideal EI mass spectra which combine the usual library searchable EI fragments with enhanced molecular ions for improved library based identification probability, improved confidence in the sample identity and improved sensitivity and selectivity in the sample analysis. However, in some cases such as in the analysis of complex petrochemical matrices a soft ionization method that provides only molecular ions is desirable since it can generate carbon number distributions for petroleum homologues and since mass spectra with molecular ions only add a dimension of separation that can supplement and complement that of the GC. Field ionization is an example of a soft ionization method that can be combined with GC-MS in which the MS adds another independent dimension of information hence it was named GCxMS (F. C. Y Wang, K. Qian and L. A Green, Anal. Chem. 77, 2777-2785 (2005)). However, field ionization is three orders of magnitude less sensitive than EI, it is incompatible with library based identification, its response can be compound dependent and it may exhibit peak broadening/tailing. While 70 eV Cold EI provides the ideal mass spectra one can use the same fly-through Cold EI ion source at low electron energies in an attempt to observe molecular ion alone. Contrary to some perceptions, for most compounds lowering the electron energies does not affect much the observed EI mass spectra due to electron induced excitation of high lying vibrational states of the ions even at low electron energies. However, hydrocarbons are an example of compounds class that their EI mass spectra are relatively largely affected by the electron energy, and in Cold EI the elimination of internal heat amplifies the low electron energy effect. 

We tested low electron energy Cold EI a few times in the past and although the molecular ion was the dominant MS peak we failed to eliminate the fragment ions and get molecular ion only. Thus, we decided to further investigate the reasons for our inability to get molecular ions only. We found that once the sample compound is cooled by the supersonic expansion it can be reheated via reflected scattered helium atoms near the skimmer. Furthermore, once a labile molecular ion is formed it can undergo undesirable collision induced dissociation (CID) the same as in MS-MS, and the magnitude of such CID can be significant for labile molecular ions such as of hydrocarbons. In order to reduce these adverse effects we evaluated the effect of reduction of helium pressure at the ion source and MS vacuum chamber via the increase of the nozzle-skimmer distance. We found out that this increased nozzle-skimmer distance resulted in noticeable increase of the ratio of molecular ions to low mass fragment ions.   

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely improves low electron energy Cold EI and converts it into a Soft Cold EI while further approaching the ideal of molecular ion only ionization method.

Isomer Distribution Analysis – A New Tool for Organic Geochemistry Analysis

Aviv Amirav, Prof of Chemistry at Tel Aviv University and Director - Aviv Analytical 

Saturated hydrocarbons constitute the majority of oils and fuels and they are distributed among a large number of isomers whose number grows in a combinatorial series with size. However, natural synthesis prefers only a small fraction of all possible isomers.

The isomeric content and distribution affects all the major physical and chemical properties of oils and fuels, including: combustion efficiency, octane number, flash point, viscosity, lubrication properties, solubility and solvation power, boiling points and melting points. However, since current instrumentation can not be properly used for isomer distribution analysis this important subject is being ignored, as discussed in another post in this blog.

As a result, organic geochemists currently attempt to characterize oils and lead oil explorations based on information gathered from ppm range biomarker compounds such as hopanes and stearanes instead of using the information content in the majority hydrocarbon isomers.

The Aviv Analytical 5975-SMB GC-MS with Cold EI provides, for the first time, a unique opportunity to explore isomer abundance distributions in order to correlate between them and other various oils and fuels features. In this post we measure and attempt to evaluate the importance of isomer distribution (abundance) analysis in organic geochemistry samples.

For further information on GC-MS with Cold EI and on the possible use of isomer abundance analysis for fuel characterization please refer to the paper "Hydrocarbons and Fuel Analysis with the Supersonic GC-MS - The Novel Concept of Isomer Abundance Analysis", Alexander B. Fialkov and Aviv Amirav. J. Chromatogr. A. 1195, 127-135 (2008).

The main features of the Aviv Analytical 5975-SMB GC-MS with Cold EI include enhanced molecular ions which are observed and can be abundant for all isomers, combined with isomer related structurally informative fragments mass spectral peaks. In addition, extended range of compounds are amenable for analysis to include low volatility and thermally labile compounds that cannot be analyzed by standard GC-MS and unlike ESI-LC-MS it provides uniform response to all compounds for improved quantitative analysis.

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables a new type of oil characterization and analysis for improved organic geochemical information and oil exploration.

Extraterrestrial Allende Meteorite Analysis by the Aviv Analytical ChromatoProbe and 5975-SMB GC-MS with Cold EI

Professor of Chemistry at Tel Aviv University and Director – Aviv Analytical 

The Allende meteorite is the largest carbonaceous chondrite ever found on Earth. It fell on February 8, 1969, near the village of Pueblito de Allende in the state of Chihuahua Mexico. The original stone is believed to have been approximately the size of an automobile traveling towards the Earth at more than 10 miles per second. After breaking up in the atmosphere, it produced thousands of fusion crusted individuals and an extensive search for pieces was conducted which led into what is often described as "the best-studied meteorite in history". The Allende meteorite contains about 4% carbonaceous chondrites. The subject or organic compounds in meteorites was investigated by several authors [1-3] and for example the organic compounds of the Allende meteorite were analyzed by photoionization combined with TOF-MS and some PAHs compounds such as phenanthrene and pyrene were found [3]. 

    

Allende Meteorite sample was provided to Prof. Aviv Amirav by Prof. Richard Zare from Stanford University during the Isranalytica 2010 meeting in Tel Aviv in January 2010. The meteorite small stone was broken with a clean tweezers into small (about 1 mm diameter) stones and it was analyzed via the use of ChromatoProbe sample introduction device for intra injector thermal extraction followed by 5975-SMB GC-MS with Cold EI analysis. 

Keep reading how the Aviv Analytical 5975-SMB GC-MS with Cold EI helps in extraterrestrial matter analysis and provides new data and unique insight into the extraterrestrial matter composition and possible origin.       

The Discovery of New Ant Head Compounds via ChromatoProbe Sampling and 5975-SMB GC-MS with Cold EI

Aviv Amirav (1), Raya Seltzer (2), and Abraham Hefetz (3), 

1. Professor of Chemistry at Tel Aviv University and Director – Aviv Analytical 

2. Graduate student at the department of Zoology at Tel Aviv University 

3. Professor of Zoology at Tel Aviv University   

Introduction

Ants represent an interesting example of social insects and thus the chemical composition of their epicuticle is widely explored including in order to learn about the way they communicate. Much of the chemical ecology research of ants is focused on their wax hydrocarbons structure. In a previous post we described the results of our study of Ants Communication Exploration via their Wax Analysis by GC-MS with Cold EI – The Unique Role of Hydrocarbon Isomer Distributions.

As often the case, when new and powerful equipment is being used new discovery are being found as described in this post. We followed our exploration of the chemical structure of Cataglyphis ant waxes with the Aviv Analytical 5975-SMB GC-MS with Cold EI via sampling with the CPAV7980 ChromatoProbe sample introduction device. The ChromatoProbe allowed us to introduced ants body parts and glands without any sample preparation for their intra injector thermal extraction combined with GC-MS with Cold EI analysis with its features of having significantly enhanced molecular ion abundances, amplified isomer structural mass spectral effects and the ability to have tail free GC-MS analysis of underivatized free fatty acids. 

Keep reading how the 5975-SMB GC-MS with Cold EI in combination with the CPAV7890 ChromatoProbe brings new, untapped and important chemical information about ants wax composition.       

Arson Analysis by GC-MS with Cold EI – Fuel Fingerprinting by Isomer Distribution Analysis

Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director - Aviv Analytical. 

Dan Muller, Ph.D., Arson Investigation Unit, Division of Identification and Forensic Science, Israel Police.

Introduction

Arson is the crime of intentionally and maliciously setting fire with the intent to cause damage. In the USA alone over 25,000 cases of arson are reported per year with estimated civilian death rate of about 300/year and property damage approaching one $Billion per year. Many arsonists use fire accelerants, such as gasoline or diesel fuel to get the fire going. However, arson analysis of the source of Gasoline, Kerosene, Diesel fuel and other accelerants represents a challenging task. We developed a new method of Isomer Distribution Analysis (IDA) and its applications for improved fuel source characterization and fingerprinting, based on the use of GC-MS with Cold EI. For more details please read A. B. Fialkov and A. Amirav "Hydrocarbons and Fuel Analysis with the Supersonic GC-MS - The Novel Concept of Isomer Abundance Analysis". J. Chromatogr. A. 1195, 127-135 (2008). 

In this Blog note we report on the use of the Aviv Analytical 5975-SMB GC-MS with Cold EI system for improved arson analysis via isomer distribution analysis of arson samples. Isomer distribution analysis is based on the fact that fuels and hydrocarbon mixtures are composed from majority of hydrocarbons and each of these hydrocarbons have many isomers that elute before the linear chain alkane, and the greater the degree of branching the earlier is the isomer elution time. Isomer distributions depend on the geographic origin of the oil that served to produce the fuel (accelerant), fuel vendor and method of fuel refining and its catalysts. Thus, isomer distribution analysis provides, as shown in this post, mass chromatograms that serves as a fuel/accelerant fingerprints to correlate them with their sources and arsonists.   

       

Five fuel samples were brought to our lab by the arson team of the Israel central police laboratory. The samples included Gasoline 2, Gasoline 3, Gasoline 4 which is Gasoline 2 after its 50% vaporization, Kerosene which was highly vaporized to be heavier than standard diesel fuel and a local diesel fuel that was contaminated with motor oil. Samples were analyzed as received with injection of 0.2 or 0.5 µL with split ratio 100 (2-5 µg total fuel on column) and analyzed with 70 eV Cold EI (EI of cold molecules in Supersonic Molecular Beams).    

Keep reading how the 5975-SMB GC-MS with Cold EI helps in arson analysis via its unique capability of isomer distribution analysis. 

Hydrocarbon Isomers - Why Aren't They Analyzed

Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director - Aviv Analytical 

Isomer Distribution Analysis - Importance and Need

Fuels and oils are composed mostly from hydrocarbons, both linear chain and branched isomers. In GC and GC-MS analysis of various fuels the aliphatic linear chain hydrocarbons are shown as peaks, while the various branched isomers usually constitute the known unresolved hump together with a multitude of minor peaks that require appropriate mass spectrometry for their proper analysis.  

     

Isomer distributions affect all the major physical and chemical properties of oils and fuels including: combustion and octane number, flash point, viscosity, lubrication properties, solubility and solvation power, boiling points and melting points. However, since current instrumentation can not be used for isomer distribution analysis this important subject is being ignored. In this note we try to explain why GC-MS with standard EI fails to properly analyse hydrocarbon isomer distributions and demonstrate that the 5975-SMB GC-MS with Cold EI can provide this, otherwise missing, vital information.

By how much is the Molecular Ion Enhanced in Cold EI

Aviv Amirav, Uri Keshet, Tal Alon and Alexander B. Fialkov. Tel Aviv University and Aviv Analytical Ltd.

Abstract

Cold EI is electron ionization of vibrationally cold molecules in supersonic molecular beams (SMB) using a fly-through EI ion source. Cold EI is characterized by enhanced molecular ions which are the most characteristic ions for sample identification and quantification. The magnitude of this important enhancement was measured for a series of linear chain hydrocarbons from n-C9H20 to n-C40H82 with staggering results. The measured absolute enhancement of the molecular ion was a factor of 13 for n-C9H20 with pure exponential increase up to a factor of 2500 for n-C40H82. This strong effect is being reported here for the first time.

Keep on reading for the obtained results and the physical principles behind them.

Dramatically Improved Hydrocarbon Analysis with the 5975-SMB GC-MS with Cold EI

Alexander B. Fialkov and Aviv Amirav, Tel Aviv University and Aviv Analytical

In the 2012 IMSC meeting (15-21 September 2012) Dr. Alexander Fialkov and Prof. Aviv Amirav present, among other things, the dramatically improved petroleum-sample analysis performed with the 5975-SMB GC-MS with Cold-EI. A brief abstract of the presentation is given below and we shall be happy to provide our full power point presentation upon request.

Presentation Number PTu-111, Tuesday 9/18, 11:10-14:40, Event Hall
Title: "Dramatically Improved Hydrocarbon Analysis with the 5975-SMB GC-MS with Cold EI"

The Aviv Analytical 5975-SMB GC-MS with Cold EI provides enhanced molecular ions for all hydrocarbons and thus enables their improved analysis including their isomer distributions. In addition, isomers and structurally related fragments mass spectral peaks are exhibited which enables NIST library identification. Furthermore, extended range of compounds are amenable for analysis with short column and high column flow rates, including low volatility large oil and wax compounds that cannot be analyzed by standard GC-MS. In addition, the 5975-SMB GC-MS enables much shorter analysis time and improved sensitivity that could be up to and over three orders of magnitude better on the molecular ions of large hydrocarbons.

Representative results

The figure above compares Cold EI (upper traces) with Standard EI (bottom traces) in the analysis of a Petroleum related hydrocarbon mixture. (60 m column 1.2 ml/min flow rate in Cold EI). While the TIC mass chromatograms are similar, the Cold EI mass spectral content is dramatically improved. The mass spectra are of the isoprenoid isomer of C18H38 with MW=254.3 and while the molecular ion is not observed in standard EI it is the dominant mass spectral peak in Cold EI. A closer look at the standard EI-MS reveals that its molecular ion relative abundance is 0.12%. Such large (three orders of magnitude) sensitivity gain of Cold EI plus its more uniform response enable isomer distribution analysis which is a novel and effective technique for fuel and oil characterization. 

Isomer Distribution Analysis – An Important yet Currently Neglected Analysis

Isomer distributions affect all the major physical and chemical properties of oils and fuels including: combustion and octane number, flash point, viscosity, lubrication properties, solubility and solvation power, boiling point and melting points. However, since current instrumentation can not be used for isomer distribution analysis this important subject is being ignored. With the 5975-SMB GC-MS with Cold EI such vital information can be obtained as above. A few application notes and a paper can be sent upon request on this topic

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