Classical EI-SMB Ion Source and its Comparison with Classical EI with High Efficiency Source

Aviv Amirav, Tel Aviv University and Aviv Analytical Ltd.

Executive Summary

GC-MS with Cold EI includes, among its several benefits, a classical EI mode of operation. Mode changing is uniquely achieved in a few seconds via a click of the mouse method change that involves the reduction of the helium make-up cooling gas flow rate without touching any hardware. Classical EI with the Cold EI contact-free fly-through ion source (named Classical EI-SMB) provides classical EI mass spectra with high NIST library matching factors and identification probabilities. Furthermore, in comparison with the various other standard EI ion sources, Classical EI-SMB excels in having a contact-free fly-through ion source structure and as a result: a) It exhibits uniform compound independent response; b) It has no ion source peak tailing thus preserves the chromatographic separation; c) It is an inherently inert ion source thus it extends the range of compounds amenable for analysis via the analysis of polar and labile compounds such as free fatty acids and/or amides without derivatization; d) It has much lower noise and thus exhibits very high total ion count mass chromatograms signal to noise ratio. Classical EI-SMB is compared in this article with the Agilent 5977B with high efficiency ion source (HES) and all the above written benefits are demonstrated.   

Achieving the Lowest Limits of Identification – GC-MS with Cold EI versus Standard EI with High Efficiency Source

Aviv Amirav, Tel Aviv University and Aviv Analytical Ltd.

Executive Summary  

GC-MS sensitivities are specified with octafluoronaphthalene (OFN). However, for many GC-MS users the most important operational parameter is the sample limits of identification. We compared the Aviv Analytical GC-MS with Cold EI with the Agilent 5977B GC-MS with high efficiency ion source (HES) in sample identification. We found that Cold EI far outperforms the 5977B-HES in both detection and identification limits. Cold EI detected and identified thirteen impurity compounds in a given test mixture while the HES standard EI failed to detect most of these impurities and failed to identify any of them. In this article we demonstrate and discuss several Cold EI benefits of superior sensitivity, better identification capability, greater range of compounds amenable for analysis and faster speed of analysis. The graphical abstract figure above demonstrates the absence of impurity peaks in the TIC with HES versus at least thirteen detected peaks in Cold EI in which each peak exhibits a molecular ion and is amenable for trustworthy identification.   

Permethrin Drug Impurity Analysis with GC-MS with Cold EI and the Road to Failure in Such Analysis by GC-MS with Standard EI

Aviv Amirav, Tel Aviv University and Aviv Analytical

Abstract

A Permethrin impurity was successfully analyzed by GC-MS with Cold EI after it failed to be analyzed by GC-MS with standard EI. In this application note we demonstrate and discuss the ways GC-MS with standard EI analysis of relatively large compounds gradually becomes more difficult as the sample compound size is increased due to reduced total ion count signal, reduced molecular ion relative abundance and increased noise. Accordingly, as the analyzed sample compound becomes larger its GC-MS analysis becomes harder in a gradual fashion until it fails. In contrast, GC-MS with Cold EI can analyze twice larger compounds and thus significantly extends the range of compounds amenable for GC-MS analysis.    

Cold EI Versus Low Electron Energy EI

Aviv Amirav, Tel Aviv University and Aviv Analytical

Executive Summary

Recently, the use of low electron energy electron ionization is claimed to serve as a soft ionization method and it is sometimes referred to as "Soft EI". In this application note, we show and discuss that low-eV EI is not a universal soft ionization method, its applicability is limited to small molecules that exhibit molecular ions in 70 eV EI, and its enhancement of molecular ion abundances is small or non-existent for many compounds. Furthermore, the addition of a 2nd analysis and loss of signal with low eV EI rarely justify its use in real-world applications.

In contrast, Cold EI (electron ionization of cold molecules in supersonic molecular beams) is a far superior "Soft EI" ion source with close to universal applicability. We demonstrate that for squalane (C30H62), the molecular ion is the base peak in the 70 eV Cold EI mass spectrum, which is 10,000-fold higher than its 0.01% relative ion abundance in 14 eV low electron energy EI. Furthermore, Cold EI is the best ion source in all other main performance aspects, and most importantly, it significantly increases the range of compounds and applications amenable for analysis. Thus, Cold EI bridges the GC-MS gap with LC-MS and can increase the total GC-MS market.

Lipids in Human Serum Analysis by the 5975-SMB GC-MS with Cold EI

Aviv Amirav and Svetlana Tsizin, Tel Aviv University and Aviv Analytical, Tel Aviv Israel. 

Gabi Shefer, Ichilov hospital Tel Aviv Israel 

Introduction 

Cholesterol and triglycerides analysis in human blood is among the most widely used chemical medical diagnostics tests. Some estimate the number of such analysis at 1 Billion/year with cost of about $30 for each analysis (three analyses of cholesterol LDL, HDL and triglycerides at $10 each). Thus, the total lipids in blood analysis market is about $30 Billion/year. However, current analysis brings limited information and mass spectrometry can provide far better and more detailed lipids in blood information if an appropriate instrument for such analysis will be available. We used our GC-MS with Cold EI and demonstrated as below the analysis of large range of lipids in blood. Each analysis provided information on the amount of several free fatty acids, cholestadiene, cholesterol, vitamins E and 25 Hydroxy vitamin D3 and many diglycerides, cholesteryl esters and triglycerides. Notably, we can clearly distinguish differences among samples from different people. Each analysis took only 10 min via the use of column flow programing. Our results yielded extensive lipidomics information that may include new diagnostic tools. We feel that this new and advantageous assay for lipids profiling in blood is worthy of further investigation and evaluation.  

Impurities Analysis in Active Pharmaceutical Ingredients Comparison of Cold EI with Standard EI

Aviv Amirav, Tel Aviv University and Aviv Analytical, Tel Aviv Israel.

Introduction

Active pharmaceutical ingredients (APIs) in drug formulations need to have impurity levels < 0.1% according to the FDA or else the impurities need to be characterized via lengthy and expensive clinical toxicology procedures. Current impurities in APIs are typically analyzed by LC-MS. However, such LC-MS analysis is confronted by ion suppression effects for impurities that elute near the API, non-polar impurities are not ionized, those impurities that are discovered exhibit mostly protonated molecular ions without structural information and since Electrospray LC-MS has highly non-uniform ionization yields there is no information on the concentration of the discovered impurities. Thus, those API impurities that are observed need to be fully identified, synthesized and follow compound specific concentration calibration.

GC-MS with Cold EI is ideal for analysis of API impurities because:

• It has uniform compound independent response, thus detected impurity concentrations can be assessed, and those below 0.1% can be neglected 
• It often provides EI-based library identification, which is usually improved by the presence of an enhanced molecular in Cold EI plus structural information from the full display of fragment ions 
• Cold EI ionizes non-polar as well as polar analytes 
• It does not suffer from any ion suppression effects 
• Total ion mass chromatograms in Cold EI often provides greater sensitivity than ESI-LC-MS 
• Cold EI has much greater range of compounds amenable for analysis than any other GC-MS. 

Thus, Cold EI seems ideal for API impurities analysis.

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.

Explosives Analysis with the 5975-SMB GC-MS with Cold EI

Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director - Aviv Analytical Ltd, December 4, 2013.

Introduction

Effective explosives analysis is very important for homeland security and forensic applications. However, explosives analysis is challenging and the number of explosive compounds that require monitoring is growing with time. Furthermore, the growing use by terrorists of peroxide explosive compounds such TATP (triacetone triperoxide, C9H18O6) and HMTD (Hexamethylene triperoxide diamine, C6H12N2O6) implies that modern analytical techniques of explosive analysis must be as effective with peroxide explosive as with the traditional poly nitro explosives. In fact, there is no limit to the desirable LOD and selectivity of explosives analysis but these features must be combined with an analytical system that enables the analysis of the entire explosives family used by terrorists. We note that the actual system sensitivity and selectivity should be measured by its performance with the most difficult to analyze explosives such as the peroxide explosives and not with TNT and alike. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables the sensitive analysis of the full range of explosive compounds including TATP, HMTD, R-Salt, TNT, ETN, PETN, RDX, Tetril, HMX and Urea Nitrate while providing abundant molecular ions or high mass fragments that improve the detection selectivity and identification capability of these labile compounds.

Deuterium Exchange Analysis for Improved Structural Elucidation with the 5975-SMB GC-MS with Cold EI

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

Introduction

Hydrogen-Deuterium exchange (also called H-D or H/D exchange) is a chemical reaction in which hydrogen atoms that are bound to nitrogen or oxygen atoms (possibly sulfur) are exchanged with deuterium atoms of certain deuterated solvents such as deuterated methanol (CD3OD or CH3OD) or heavy water (D2O). Deuterium exchange reactions followed by mass spectrometry analysis provide valuable structural information since these reactions are usually selective: only hydrogen atoms that are bound to nitrogen or oxygen atoms (such as in amines or alcohols) are exchanged with deuterium while other hydrogen atoms are unaffected. Consequently, the mass shift of the molecular ion reveals the number of OH and NH bonds in the explored compound which helps in its structural elucidation. Currently, deuterium exchange experiments are limited to NMR and LC-MS and it is practically ignored in GC-MS since:  

1. most of the small compounds that are amenable for GC-MS analysis can be fully identified by the available extensive EI libraries.
2. large compounds on the GC-MS scale are often incompatible with GC-MS analysis and/or do not exhibit molecular ions in their EI mass spectra.
3. While the compound of interest may exchange its labile hydrogen atoms with deuterium, reversed exchange may occur at the GC liner, long column and particularly at the ion source metal surfaces that quickly react with ever-present water in the mass spectrometer vacuum chamber.

We found that the use of GC-MS with Cold EI uniquely enables the monitoring of deuterium exchange reactions and thus with it deuterium exchange can serve as an additional effective tool for improved structure elucidation of unknown compounds.                       

Keep reading for more information on deuterium exchange monitoring using the Aviv Analytical 5975-SMB GC-MS with Cold EI, including two analysis examples of Quinine and 17β-estradiol that serve for the validation of their structure and number of labile OH hydrogen atoms.

Pesticide Analysis by Pulsed Flow Modulation GCxGC-MS with Cold EI – An Alternative to GC-MS-MS

Uri Keshet (1), Tal Alon (1), Paulina Goldshlag (2) and Aviv Amirav (1)

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

2. Israel Plant Protection and Inspection Services, Beit Dagan, Israel.

3. Aviv Analytical LTD, Tel Aviv, Israel.   

Introduction 

Pesticide analysis in agricultural products is an application area of growing importance which is usually addressed by GC-MS and LC-MS. However, pesticide analysis is confronted by the dominance of matrix interference which severely restricts the achievable limits of detection. As a result, several methods were developed and used for the reduction of matrix interference and the most widely used is MS-MS. While in LC-MS, MS-MS is practically a must in view of absence of libraries and consistent fragments in the mass spectra, in GC-MS the use of full scan possesses a few advantages of unambiguous library identification and its applicability to unlimited number of GC-MS amenable pesticides, provides however that the problem of matrix interference can be properly addressed. In addition to MS-MS at least three other techniques can be used to reduce matrix interference while enabling the use of full scan namely: a) high resolution mass spectrometry; b) GC-MS with Cold EI in which the enhanced molecular ions suppress matrix interference as described in another article in this blog. c) GCxGC-MS with its improved GC separation that can be used for the reduction of matrix interference as described in a few papers with thermal modulation and as discussed in this article.  

We developed Pulsed Flow Modulation (PFM) for obtaining GCxGC-MS with supersonic molecular beams (SMB) (also named Cold EI) based on a low cost single 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 and analyzed with it twelve pesticides in two agricultural matrices. PFM-GCxGC-MS with Cold EI combines the improved separation of GCxGC with the features of Cold EI of enhanced molecular ions and isotope information for the provision of ultimate sample identification information. Unique to PFM-GCxGC-MS with Cold EI is that in contrast to thermal modulation with standard GC-MS the second dimension elution time does not suffer from any ion source peak tailing and/or second column saturation effects and thus it is ideally suitable for pesticide analysis in complex matrices.   

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables effective pesticide analysis in agricultural matrices while using improved PFM-GCxGC separation for obtaining a major reduction of matrix interference by an average measured factor of 32. As a result, the road is open for PFM-GCxGC-MS with Cold EI to serve as an alternative to GC-MS-MS while enabling the use of full scan which is applicable to unlimited number of GC-MS amenable pesticides with improved library identification and the novel use of isotopomer ratios for pesticides identity confirmation

Organo-Iodine Compounds Analysis by the 5975-SMB GC-MS with Cold EI

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

Larisa Panz Ph.D., Ksenia Kulbitski and Professor Mark Gendelman, Schulich Faculty of Chemistry at the Technion Haifa Israel. 

Introduction

Organo-iodine compounds are used in several applications, mostly as intermediates in organic synthesis because of the easy formation and cleavage of the C–I bond. The analysis of organo-iodine compounds is highly challenging since most of them are not compatible with LC-MS and electrospray ionization while in GC-MS their analysis is hampered via their possible decomposition at the GC injector, column and ion source since the iodine-carbon bond is by far the weakest among all halogen-carbon bonds. Furthermore, even if the organo-iodine compounds elute from the GC column their standard EI mass spectra are often void of any molecular ions and as a result their identification becomes impossible. Motivated by the above challenges we explored the analysis of novel synthetic organo-iodine compounds with the Aviv Analytical 5975-SMB GC-MS with Cold EI, developed a method for its successful analysis and compared the results with that obtained with GC-MS with standard EI.

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables the effective analysis of synthetic organo-iodine compounds and provides unambiguous identification of all the synthetic organo-iodine reaction mixture compounds and as a result helps optimizing the synthesis reaction conditions and yields.

What Can be Improved in GC-MS – When Multi Benefits are Transformed into a GC-MS Revolution

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

GC-MS vendors typically describe and characterize their systems using a small set of specifications that include octafluoronaphthalene (OFN) signal to noise ratio (SNR), mass range and scan speed. As a result, improvements in GC-MS are often focused on the incremental increase of these few specifications. However, GC-MS is characterized by many additional features and operational parameters that contribute to its performance, and their improvements can make a big impact on the GC-MS analytical capabilities. Many such GC-MS aspects are improved by incorporating the new Cold-EI GC and MS interface and ion source technology and by using "out of the box" thinking. In LC-MS, the biggest revolution was brought not by LC or MS improvements but rather by the development of a new interface and ionization method namely Electrospray. Similarly, Cold EI with its supersonic molecular beams interface and fly-through ion source brings multiple benefits and improvements into GC-MS which can initiate a new GC-MS revolution. This blog post lists 62 GC-MS improvements brought forth by the Cold EI interface and ion source, encompassing any and every important aspect of GC-MS, and explains how the unique features of GC-MS with Cold EI enable these benefits. 

Keep reading to find out how the 5975-SMB GC-MS with Cold EI improves GC-MS not by one or few but by 62 different aspects. When these multiple benefits are combined they are destined to be transformed into the next GC-MS revolution.  

Does this Pill Contain its Claimed Active Ingredients? Finasteride in Male Baldness Treatment Pills

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

Introduction

Finasteride (C23H36N2O2, MW = 372.2777, structure at right) is an approved drug for the treatment of male baldness. It acts as an inhibitor of the enzyme that converts testosterone to dehydrotestosterone. Finasteride pills are available by Merck and other companies but at a high price. They are also available from a relatively unknown Indian company at over three times lower price. Thus, a colleague friend approached us with a request to analyze a Finasteride pill from the Indian company to ensure the presence (or absence) of this active pharmaceutical ingredient and determine if it is a major or minor constituent in their pills. In fact, the topic of fake drugs is serious and requires attention thus we bring our advanced GC-MS experience with it into this post.

Keep reading how the 5975-SMB GC-MS with Cold EI enables Finasteride in male baldness treatment pills.
          

Enhancing the Identification Capabilities of EI GC-MS - How Quadrupole GC-MS can compete with High Resolution TOF

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

Electron Ionization (EI) mass spectra contain a vast amount of information that can be considered during an identification procedure. It might be surprising to know that most of us examine and utilize only a part of that information and as a result decrease the probability of a correct identification and in many cases cannot identify the analytes at all. It might also be surprising to know that it is rather easy to consider more of the data and improve identification results. 

When we analyze an EI mass spectrum we usually relay on library identification, which provides us the most resembling spectra from the library (and with almost no effort on our part). Library identification is an important tool but it has its shortcomings, and it often fails. 

Keep reading to find out how you can easily increase the probability of a correct identification and identify more compounds, all by using more of the information already found in you spectra. 

Tetryl Analysis using the 5975-SMB GC-MS with Cold EI

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

Introduction

Tetryl, (2,4,6-Trinitrophenylmethylnitramine, C7H5N5O8) is a sensitive explosive compound used to make detonators and explosive booster charges. Tetryl is produced by slowly mixing dimethylaniline with concentrated nitric acid in the presence of sulfuric acid and due to its simple synthesis it is prone for attempted synthesis by terrorists. Tetryl is a thermally labile explosives that is known to be difficult to analyze by GC-MS due to its degradation at the GC injector and at the GC column to N-methylpicramide through a loss of NO and a gain of hydrogen from the column PDMS film. As a result, the analysis of Tetryl is challenging. 

   

We received a sample of Tetryl that was synthesized by A.I. Explosives, Inspection & Services Company (www.aiexplosives.com) that attempted to follow terrorist's synthetic roots and characterize it via the tetryl synthetic impurities.   

In this post we present the uniquely successful analysis of Tetryl perform with Aviv Analytical's 5975-SMB GC-MS.    

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.

Sample Identification – Which is a Better Method and Instrument, GC-MS with its Library Search or LC-HR-MS with the Provision of Elemental Formula?

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

Introduction

A common method of sample identification involves the use of GC-MS with its electron ionization mass spectra and automated library search and identification software. However, a growing portion of sample compounds is not amenable for GC-MS analysis thus the use of LC-MS, particularly with high resolution time of flight MS is growing. The identification method used with such LC-HR-MS instruments is based on the provision of elemental formula. Consequently, interesting questions with practical implications are which of the above identification methods and instruments is better and whether LC-HR-MS can fully replace GC-MS or the later has intrinsic value that LC-HR-MS does not provide.  

In this post we discuss these questions and try to provide answers.