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.   

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.

Linearity, Sensitivity and Response Uniformity Comparison of the Aviv Analytical 5975-SMB with Cold EI and the Agilent 5977A GC-MS with Standard EI

Aviv Amirav (1,2), Uri Keshet (1) and Bogdan Belgorodsky (1) 
1.   Tel Aviv University 
2.   Aviv Analytical Ltd 

Introduction 

Standard GC-MS analysis of low volatility and polar compounds is known to be plagued by ion source related peak tailing and degradation (see our post on peak tailing here). We found that these intra ion source tailing and sample degradation effects strongly depend on the sample concentration. Consequently, when these compounds are analyzed, the response of the standard EI ion source of GCMS is both non-uniform and non-linear. In this article we evaluate and compare the Aviv Analytical 5975-SMB GC-MS with Cold EI with the new Agilent 5977 Extractor GC-MS in the analysis of a test mixture containing five components, using the same on-column amounts. In contrast to the Agilent 5977 the Aviv Analytical 5975-SMB GC-MS with Cold EI exhibits uniform, linear, compound independent response. We concluded that the harder the compound analysis the greater is the sensitivity gain of the 5975-SMB over 5977 extractor with standard EI, up to and over a factor of 100.

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables uniform and linear ion source response with a substantial improvement of sensitivity in comparison with a state of the art GC-MS with standard EI.

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.  

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.  

Drug Impurities Analysis by the Aviv Analytical 5975-SMB GC-MS with Cold EI

Aviv Amirav, School of Chemistry, Tel Aviv University and Director – Aviv Analytical 

Drugs are typically analyzed by LC-MS. When active pharmaceutical ingredients (API) are analyzed for the presence of impurities, the general requirements are that every impurity found should be at concentration below 0.1% of the API or it must be either cleaned or identified and toxicologically characterized which is a time consuming and expensive procedure. However, the ionization yields of Electrospray and/or APCI in LC-MS are highly non-uniform and compound dependent, and as a result when an impurity is found in a given LC-MS mass chromatogram its actual concentration is not known. Consequently, regardless of its actual concentration that can be well below 0.1% every impurity that is found must be isolated, fully identified, synthesized (or obtained) and serve for the provision of a calibration curve in order to measure the actual concentration of that small impurity peak. In addition, several types of impurities such as non-polar compounds are not properly ionized and are not detected by LC-MS. Thus, clearly drug impurity analysis represents an unmet analytical challenge.  

Alternatively, GC-MS with its standard electron ionization (EI) can be used and has, for volatile compounds, approximately uniform, semi-quantitative ionization yield. However, GC-MS with standard EI is compatible only with a limited range of thermally stable drugs and it provides useful molecular ions only for a portion of the analyzed compounds. Furthermore, ion source degradation and peak tailing severely erode the standard EI response uniformity for semi-volatile compounds. 

We used the Aviv Analytical 5975-SMB GC-MS with Cold EI for the analysis of drug impurities. Cold EI is the electron ionization of cold molecules in supersonic molecular beams (SMB). The technology of GC-MS with Cold EI is reviewed by A. Amirav, A. Gordin, M. Poliak, and A. B. Fialkov, J. Mass Spectrom., 43, 141-163. (2008). The later is available upon request. 

GC-MS with Cold EI as implemented with the Aviv Analytical 5975-SMB uniquely extends the range of drugs that are amenable for GC-MS analysis, including thermally labile drugs that are not compatible with standard GC-MS analysis. Furthermore, it is characterized by having uniform response to all compounds due to its contact-free fly-through ion source that eliminates any ion source peak tailing or losses. In addition, it provides trustworthy enhanced molecular ions that can be converted with the Tal-Aviv Molecule Identifier software into elemental formulae. Thus, the high sensitivity and uniform response of Cold EI uniquely enables the detection, identification and quantitation of low level impurities in drugs without lengthy calibration procedures. 

In order to evaluated the above attributes of the 5975-SMB we analyzed impurities in two Active Pharmaceutical Ingredient (API) samples (powder form) of Haloperidol and Ramelteon (Structures are given below).   

      

 

Keep reading to find our how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables quantitative drug impurities analysis and helps in their identification. 

Organic Chemical Reaction Yields Optimization by GC-MS with Cold EI and Walk-by GC-FID

Aviv Amirav, School of Chemistry, Tel Aviv University and Director – Aviv Analytical 

Tal Sela and Arkadi Vigalok, School of Chemistry, Tel Aviv University.  

Introduction

Chemical reactions are typically followed by lengthy and tedious products separation and purification for their identification by NMR and high resolution mass spectrometry (such as with QTOF). However, with this procedure no information is obtained about the synthesis yield, products purity, availability of isomers and on the reaction mechanism, since the already-purified compounds are analyzed and since ESI has non-uniform, highly compound dependent ionization yields. 

In order to enable semi-on-line monitoring and optimization of organic chemical reactions we used the Aviv Analytical 5975-SMB GC-MS with Cold EI as described in a previous post in the blog. The 5975-SMB GC-MS with Cold EI (EI of cold molecules in supersonic molecular beams) was used with reaction mixtures without their prior separation and purification as required for NMR analysis, thereby saving time and effort. GC-MS with Cold EI was demonstrated to significantly extend the range of compounds amenable for analysis, practically always giving molecular ions, enabling effective fast GC-MS analysis and providing elemental formulas via isotope abundance analysis with unit mass resolution quadrupole MS. In addition, it uniquely provides uniform response to all compounds, a feature which is vital for the measurement of chemical reaction yields. 

We are aware of the fact that a system of this kind is typically placed in a separate laboratory and operated by a dedicated operator thus may not be fully available for the simultaneous repetitive monitoring of several reactions. Our goal is to convert the concept of semi-on-line reaction monitoring for its optimization into a widely used concept and technology that is shared and used by students and non-expert analytical chemists. As a result, we refined and further improved the concept of reaction monitoring by the 5975-SMB GC-MS with Cold EI to be followed by the use of a walk-by GC-FID for enabling frequent semi-on-line monitoring of the reactions by the organic chemistry students. Accordingly, the 5975-SMB provides the identification of all the reaction mixture components and their order of elution and elution times while the GC-FID can have the same column, column flow rate and other related method parameters and later serve for the quantitation of the reaction products and for its optimization.       

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI followed by a standard GC-FID can change the way organic synthesis is performed. 

Measurement and Optimization of Organic Chemical Reaction Yields by GC-MS with Cold EI

Aviv Amirav, School of Chemistry, Tel Aviv University and Director – Aviv Analytical 

Alexander Gordin, Bogdan Belgorodsky, Boaz Seemann, Michael Gozin and Alexander B. Fialkov, School of Chemistry, Tel Aviv University.  

Introduction

Chemical reactions are typically involved with mixing the reactants together in an appropriate solvent, often with addition of a catalyst, and allowing them to undergo a reaction, which may take from couple of minutes to few days. The progress of the reaction is either assumed or monitored most-commonly by thin layer chromatography. However, little or no information is obtained on-line on the reaction actual progress and its yield. At the perceived end of the reaction, the products are separated and purified by preparative chromatography, distillation, sublimation, selective precipitation or crystallization, processes which may take several hours or even several days. Subsequently, the purified products are analyzed by 1H and 13C NMR and by high resolution mass spectrometry (often Quadrupole Time of Flight (QTOF) mass spectrometer), typically via flow injection electrospray ionization (FI-ESI). Yet, while FI-ESI-QTOF provides elemental formulas information, it does not provide information about the synthesis yield, products purity, availability of isomers and on the reaction mechanism, since the already-purified compounds are analyzed and since ESI has non-uniform, highly compound dependent ionization yields. 

In order to alleviate the above mentioned shortcomings of organic synthesis, we used the Aviv Analytical 5975-SMB GC-MS with Cold EI for semi-on-line monitoring of organic chemical reactions for obtaining information on the reaction products identity and purity and for mechanism elucidation and reaction yield optimization. The 5975-SMB was used with reaction mixtures without their prior separation and purification as required for NMR analysis, thereby saving time and effort. Our unique 5975-SMB GC-MS with Cold EI is based on GC interface with the MS with supersonic molecular beams (SMB) and on the ionization of the sample molecules during their axial flight through an open electron ionization ion source as vibrationally cold molecules (hence the name Cold EI). GC-MS with Cold EI was demonstrated to significantly extend the range of compounds amenable for analysis, practically always giving molecular ions, enabling effective fast GC-MS analysis and providing elemental formulas via isotope abundance analysis with unit mass resolution quadrupole MS. In addition, it uniquely provides uniform response to all compounds, a feature which is vital for the measurement of chemical reaction yields. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI can change the way organic synthesis is performed via enabling semi on-line monitoring of the progress of organic reactions and via the provision of information on the reaction mechanism, products identity and purity and the chemical reaction yield for its optimization.