How Enhanced Molecular Ions in Cold EI Improves Sample Identification by the NIST Library

Tal Alon (1,2)  and Aviv Amirav (1,2)

1. School of Chemistry, Tel Aviv University, Tel Aviv 69978 Israel.
2. Aviv Analytical LTD, 3 Haarad Street, Tel Aviv 69107 Israel.

Abstract

Library based compound identification with electron ionization (EI) mass spectrometry (MS) is a well-established identification method which provides sample compounds names and structures 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 is electron ionization of vibrationally cold molecules in supersonic molecular beams. Cold EI provides mass spectra with all the standard EI fragment ions combined with enhanced Molecular Ions (MI) 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 are increased, despite the lower matching factors.

This research examines the way enhanced molecular ion abundances affect library identification probability and the way Cold-EI mass spectra, which include enhanced molecular ions and high mass fragments, typically improve library identification results. It describes the results of several computer simulations, which incrementally modified the relative abundance of the various ions and analyzed the resulting mass spectra. The simulation results support previous measurements, showing that while enhanced molecular ion and high mass fragments 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 of the identification probability for the correct compound. This behavior which was previously demonstrated by analyzing Cold-EI mass spectra occurs because high mass ions, and especially the molecular ion, characterize a compound more than low mass ions and therefore carry more weight in library search identification algorithms. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables the combination of enhanced molecular ion and improved NIST library based sample identification.

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.

Pulsed Flow Modulation GCxGC-MS with Cold EI – The Emergence of Novel Concept of GCxGCxMS

Aviv Amirav, Tal Alon, Uri Keshet and Alexander Fialkov 

Tel Aviv University and Aviv Analytical

Introduction 

Comprehensive two dimensional GCxGC-MS significantly improves the separation of the GC. However, it suffers from complexity and high cost of purchase and maintenance cooling gases. In addition, with standard electron ionization the molecular ions are often weak or absent, particularly in the analysis of complex hydrocarbon mixtures. 

We developed Pulsed Flow Modulation (PFM) for obtaining GCxGC-MS with supersonic molecular beams (SMB) (also named Cold EI) based on quadrupole mass analyzer and without any added cooling gas. For further information on the concept of Pulsed Flow Modulation GCxGC please read M. Poliak, M. Kochman and A. Amirav, "Pulsed Flow Modulation Comprehensive Two Dimensional Gas Chromatography" J. Chromatogr. A. 1186, 189-195 (2008) (also Israel patent number 176724 and USA patent number 7518103). 

In PFM-GCxGC-MS with Cold EI, sample compounds which elute from the first GC column are temporarily stored in a fused silica transfer line and are pulsed periodically injected by ~25 ml/min He gas pulse into the second column. After the pulse, ~20 ml/min He develops the chromatography in the second column for a few seconds (typically 4s). PFM is simple to construct, does not require any added gas with Cold EI and the injection time can be tuned, hence PFM-GCxGC-MS with Cold EI is enabled with quadrupole MS. The increased PFM second GC column flow rate is seamlessly handled by the Cold EI nozzle vacuum chamber. 

We combined PFM-GCxGC with the Aviv Analytical 5975-SMB GC-MS with Cold EI. PFM-GCxGC-MS with Cold EI combines the improved separation of GCxGC with the features of Cold EI of enhanced molecular ions and extended mass spectral isomer and isotope information for the provision of ultimate sample information. Unique to PFM-GCxGC-MS with Cold EI is that as the second dimension elution time is increased the molecular ion mass is reduced for easier GCxGCxMS like identification. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables effective PFM-GCxGC-MS with Cold EI and provides ultimate level of information with an example of the analysis of JP8 jet fuel (with some Biodiesel related FAMEs). This article also describes and exposes the emergence of the novel concept of GCxGCxMS. 

Free Fatty Acids Analysis by GC-MS - Cold EI Versus Standard EI

Aviv Amirav (1)  and David Benanou (2) 

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

2. Veolia Environment Recherche and Innovation 

Gas Chromatography Mass Spectrometry (GC-MS) is known to be limited in the analysis of polar organic compounds such as free acids and bases which require derivatization for their proper analysis. The problem in free acids analysis emerges from extended peak tailing at the GC separation column (and liner) as well as peak tailing and sample compound degradation at the metallic surfaces of the ion source. Recently, a few of the major column vendors introduced new brands of inert columns and liners that alleviates and significantly reduces column related peak tailing by polar compounds. However, the problem of ion source activity remained unsolved and it is currently the main reason why free fatty acids and other polar organic compounds cannot be properly analyzed by standard GC-MS and require derivatization. While derivatization enables the GC-MS analysis of polar compounds such as fatty acids, it requires another step in the sample preparation. Furthermore, derivatization is not always practical since for a large variety of sample types the presence of compounds that require derivatization is not known hence they are analyze without derivatization and in such cases important sample information on such polar compounds is lost.

In this Advanced GC-MS Blog post we describe the analysis of an anti foam agent which in this case is hydrocarbon based oil that also contains free fatty acids. However, the presence of these free fatty acids was not initially known.            

                  

Keep reading to find out how the 5975-SMB GC-MS with Cold EI uniquely enables the analysis of free underivatized fatty acids which could not be properly analyzed by standard GC-MS.  

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.

Advanced GC-MS Presentations of Aviv Analytical in Pittcon 2013

Aviv Amirav and Tal Alon, Tel Aviv University and Aviv Analytical, Emails: amirav@tau.ac.il and polaris@tau.ac.il

In the 2013 Pittcon meeting (Philadelphia 17-21 March 2013) Prof. Aviv Amirav and Dr. Tal Alon will present six advanced GC-MS presentations performed mostly with the 5975-SMB GC-MS with Cold-EI. Brief abstracts of these presentations are given below (we will be happy to provide the full presentations upon request). 

We will be happy to meet you at our posters (240-11, 1210-6, 2180-3) or at the lecture halls after our oral presentations (1430-3, 2080-4, 2330-5) to discuss your advanced GC-MS needs and challenging applications, or any other topic.

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI brings the next GC-MS revolution and largely improves GC-MS performance for a broad range of applications. 

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.       

Ants Communication Exploration via their Wax Analysis by GC-MS with Cold EI – The Unique Role of Hydrocarbon Isomer Distributions

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 are social insects and thus the exploration of their ways of communication is an interesting research area. For example, nestmate recognition is essential for ants for the defense against exploitation of nest resources, and such recognition is largely mediated by chemical signals. When two individuals meet they match the label, the chemical signal it carries on the epicuticle and represents the colony odor, with a postulated neural template present in the brain. If the label matches the template amicable interactions between the individual ensue, (photo in the graphical abstract) otherwise they engage in aggressive interaction. Since the ant wax is composed mostly from saturated hydrocarbons, both linear chain and their branched isomers, we decided to explore the chemical structure of ant waxes with the Aviv Analytical 5975-SMB GC-MS with Cold EI which significantly enhances the molecular ion abundances and amplifies isomer structural mass spectral effects. We studies Cataglyphis ants from 17 different nests in various locations in Israel and found that although morphologically they are practically indistinguishable, isomer distribution analysis found significant differences between cryptic ant species or ants from different nests. Thus, the data strongly supports the hypothesis that ant communication is based on branched hydrocarbon abundances as a recognition fingerprint. 

Keep reading how the 5975-SMB GC-MS with Cold EI helps in ant communication research and to view the important role of isomer distribution analysis in our findings.  
    

Flies Wax Analysis by GC-MS with Cold EI and its Comparison with Standard EI

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

Joanne Yew, Temasek Life Sciences Laboratory and National University of Singapore, Department of Biological Sciences, Singapore. 

Introduction 

Insects such as flies, ants and mosquitoes express species-specific blends of cuticular hydrocarbons (CHCs) on their surface that serve as pheromones and play an important role in their behavior and communication. In view of growing interest and importance of this research area recently several advanced tools were developed for improved CHCs analysis. However, one major limitation of these methods and instrumentations is that alkanes cannot be properly detected via some of the atmospheric pressure ionization ion sources while in standard electron ionization as used with GC-MS the molecular ion is often very weak or altogether missing thereby proper identification is hindered. Furthermore, standard GC-MS is limited to relatively small compounds up to about C40-C44. Thus, we explored the chemical structure of fly and mosquito waxes with the Aviv Analytical 5975-SMB GC-MS with Cold EI which significantly enhances the molecular ion abundances, amplifies and exposes isomer structural mass spectral effects and enables the elution of much larger hydrocarbons via the use of short columns with high column flow rates. In addition, it provides uniform response for accurate quantification of the waxes compounds which is superior to standard EI and in contrast to API ionization methods (APCI, DART MALDI etc). For further information on GC-MS with Cold EI please read the review article A. Amirav, A. Gordin, M. Poliak, T. Alon and A. B. Fialkov "Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams" J. Mass. Spectrom. 43, 141-163 (2008)., which is available on request. In this post we report on the analysis of two fly samples. A) Male Drosophila melanogaster, and B) Male Drosophila mojavensis. 

Keep reading how the 5975-SMB GC-MS with Cold EI drastically improves flies cuticular hydrocarbons analysis and bring novel tools for its characterization. 

               

Pistol Oil Analysis by the Aviv Analytical 5975-SMB GC-MS with Cold EI – A New Forensic Method for the Verification of Pistol Holding

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

Introduction: 

There is an established forensic need to link between a given pistol (or any other firearm) and a suspect, who often denies ownership of that pistol, or claims that it was never touched. Pistols and other firearms are sometimes linked to their holders via their Iron print (induced by iron reaction with human sweat), which can be developed on the suspect's holding hand by special chemicals. However, there is a growing need for alternative forensic methods as some pistols or firearms have chromium coated or plastic based handles which are incompatible with the Iron print technique. In addition, the Iron print method is performed on the human suspect hand, and an alternative method based on field sample collection followed by laboratory analysis would be advantageous. 

The Novel Pistol Oil Analysis Idea: 

Every pistol and fire arm is lubricated by special oil for its improved maintenance and unhindered operation. This oil unavoidably touches the holding hand and consequently must leave its deposit on the holding hand. The oils used typically consist of a range of hydrocarbons, often aliphatic hydrocarbons that are distributed in a range of isomers. Thus, pistol oil analysis on a human hand can serve as an effective new method for linking between a given pistol and/or fire arm to the hand that held it, given that a suitable method and instrument will be found for obtaining a unique pistol oil characterization finger print.       

Keep reading how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables the verification of pistol holding.

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.

The Effects of Enhanced Molecular Ions on NIST's Identification Probability

Aviv Amirav and Tal Alon, Tel Aviv University and Aviv Analytical

   

Cold EI is the Electron Ionization (EI) of vibrationally cold molecules in Supersonic Molecular Beams (SMB) as obtained with the Aviv Analytical 5975-SMB GC-MS with Cold EI. For more details on Cold EI please visit the Aviv Analytical website or read the review article of A. Amirav, A. Gordin, M. Poliak and A. B. Fialkov "Gas Chromatography Mass Spectrometry with Supersonic Molecular Beams", J. Mass. Spectrom. 43, 141-163 (2008) (available on request). 

In Cold EI the abundances of molecular ions are enhanced and while the degree of enhancement is small for small molecules with low internal heat capacity, it can be substantial for large molecules and for those that exhibit low molecular ion abundance in standard (classical) EI such as hydrocarbons and particularly large hydrocarbons (see our previous post). 

In this post we show how enhanced Cold-EI spectra affect the identification results of NIST library searches.

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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