The Multiple Benefits of Cold EI – Leading the Way to the Future of GC-MS

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

GC-MS is characterized by many performance aspects and most of them are not mentioned or discussed by the GC-MS vendors and/or by any user's paper. The purpose of this paper is to list 72 such performance parameters and discuss how Cold EI improves the vast majority of them. GC-MS with Cold EI is based on a GC and MS interface with a Supersonic Molecular Beam (SMB) and on electron ionization of sample compounds while they are vibrationally cold in the SMB (thus named Cold EI) in a contact-free fly-through ion source.

Cold EI Improves all the central GC-MS performance aspects, including: identification, mass spectral information, range of compounds that are amenable for analysis, sensitivity (detection limits), speed of analysis, and response uniformity.

Eight categories of GC-MS performances are listed below and in each of them we have several sub categories (given in the number in bracket):    

1. Improved sample identification (13).
2. Extending the range of compounds amenable for GC-MS analysis (3).
3. Speed – faster GC-MS analysis (9).
4. Sensitivity (12).
5. Uniform, compound independent ion source response, quantitation and reproducibility (4).
6. Improved compatibility with GC-MS enhancement technologies (11).
7. Improved GC-MS maintenance, flexibility, ease of use and price (15).
8. Improved utilization of mass analyzer specifications (5).  

Quoting Aristotle "The whole is greater than the sum of its parts", this combination of so many improvements (>60) creates a new and qualitatively superior technology that actually improves every type of analysis. While GC-MS with Cold EI enables new type of analyses and significantly improves challenging analyses, it does not impede on any simple method of analysis (compared with standard EI). Consequently, GC-MS with Cold EI is leading the way to the future of GC-MS. 

 Click here to download the full article 

GC-MS with Cold-EI Demonstration Video

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

In this video we demonstrate a live analysis of a test mixture, showing how the Cold EI technology improves all the central performance aspects of GC-MS. The test mixture includes 1 ng on-column each n-C16H34, methylstearate, cholesterol and n-C32H66 plus 10 pg OFN, and via its analysis we demonstrate twelve unique benefits of GC-MS with Cold EI.

Have a happy new year

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.   

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.

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.

Open Probe Fast GC-MS - Real Time Analysis with Separation

Uri Keshet (1), Tal Alon (1,2) , Alexander B. Fialkov (1), 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 

An Open Probe inlet was combined with a low thermal mass (LTM) ultra fast gas chromatograph (GC) and a mass spectrometer (MS) of a standard GC-MS for obtaining real time analysis with separation. The Open Probe is based on a vaporization oven that is open to room air with an addition of helium purge flow protection that eliminates air leakage into the oven, column and MS ion source. Sample introduction into the Open Probe is as simple as; touch the sample, insert the sample holder into the open probe oven and have the results in 30 s with under a minute ready for next analysis. 

The Open Probe Fast GC-MS revolutionizes the field of real time analysis as it provides several major benefits in comparison with DART, DESI and other ambient ionization methods or MS probes:

• GC separation,
• Library identification
• Absence of ion suppression effects
• Uniform electron ionization response for improved quantitation
• Uses a low cost quadrupole MS (of GC-MS). 

Watch the short video below and keep reading to find out more about the new Open Probe fast GC-MS which was combined with Agilent 7890B GC and 5977 MS and learn about its several demonstrated application and benefits. 

If the video doesn't show, you can click here to see it on Youtube.

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.

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

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. 

Extending the Range of Compounds Amenable for GC-MS Analysis

Aviv Amirav, Alexander B. Fialkov and Tal Alon 

Tel Aviv University and Aviv Analytical. July 22, 2013.  

Introduction

GC-MS is a powerful technique yet it suffers from a major Achilles Heel of limited range of low volatility and thermally labile compounds amenable for analysis. However, it was discovered and investigated by A. B. Fialkov, A. Gordin and A. Amirav, J. Chromatog. A.  991, 217-240 (2003) that this range can be significantly extended via two major changes in GC-MS:

1. The use of short columns with high column flow rates and preferably with a temperature programmable injector which lowers the elution temperatures from the injector liner to the column and from the column itself (slower temperature programming rate and thinner films also help) and; 
2. The use of a fly-through ion source in combination with sample compounds in supersonic molecular beams (Cold EI) eliminates any ion source peak tailing and/or sample degradation on the hot metallic ion source surfaces. 

The use of GC-MS with Cold EI opens the door for significantly increased range of compounds and applications that are amenable for GC-MS analysis. Consequently, GC-MS with Cold EI bridges the gap with LC-MS, helps to analyze the bottleneck hard to analyze compounds in current methods and opens the door for new and untapped opportunities in GC-MS analysis.                

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI uniquely enables significant extension of the range of compounds amenable for GC-MS analysis and as a result enables new types of GC-MS applications.

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.  

Nitrogen as an Alternative for Helium as a Carrier Gas for 5975-SMB GC-MS with Cold EI

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

Introduction

Recently the subject of helium shortage and the possible use of hydrogen or nitrogen as alternative carrier gases in GC-MS became a hot topic in various forums, blogs and GC-MS vendor seminars. We published our opinion on this topic in another post called "Helium Shortage and Hydrogen as a Carrier Gas for GC-MS" a few months ago.

       

In short, we believe that the helium shortage problem is not a long lasting one as alternative natural gas wells are being found and they are likely to contain helium due to long time radioactivity in deep gas wells, albeit the helium price might increase. Due to the advantages of using Helium our advice is to continue and use it as the carrier gas for GC-MS while purchasing additional cylinders "just in case" a temporal shortage will occur.

If and when the helium supply is disrupted, hydrogen or nitrogen can serve as natural alternative carrier gases, but they do come with their own set of problems. Hydrogen is a reactive gas and it degrades certain compounds without a simple indication to what these compounds could be. Note that this hydrogen related reactivity is mostly indirect, via the promotion of surface activity at the liner, column and standard EI ion source metallic surfaces. For further explanations please read the above linked blog post. Hydrogen also somewhat reduces the sensitivity. While nitrogen is inert, its main downside is that it significantly reduces the GC-MS sensitivity. 

This post presents a viable approach to avoid the Helium shortage problems through using the 5975-SMB GC-MS with Cold EI while eliminating any sample degradation issue and providing similar or almost similar sensitivity as with helium. 

Keep reading to find out how the 5975-SMB GC-MS with Cold EI enables helium-free unperturbed operation with nitrogen as its column carrier gas and hydrogen as the Cold EI make up gas.  

         

How Does MS-MS Fail

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

Introduction 

The scientific literature and vendor advertisements are full of reports and examples of success stories of techniques and methods, but their limitations and shortcomings are not reported for obvious reasons. One such technique is tandem mass-spectrometry, or MS-MS, as vendors seldom describe, explain or demonstrate the technique's limits and most importantly – why and how it fails. 

In GC-MS there are at least five ways to increase the detection selectivity against matrix interference and improve sample identification and quantification: a) MS-MS; b) GCxGC (or GC-GC); c) High resolution MS such as TOF; d) A selective ion source such as NCI, and; e) Having enhanced molecular ions which are far more selective than lower mass fragments. In another post in this blog we discussed the unique selectivity of the molecular ions which is much higher than that of any fragment ion. Better understanding of the selectivity enhancement factors of these methods and their performance aspect trade-offs is important for any decision about which system to purchase and optimally use.  

The short answer to the question asked in the title of this post is that MS-MS fails when the selectivity improvement is offset by too large signal loss.    

Keep reading to find out how and why does the MS-MS technique fails (with examples), the reasons for MS-MS signal loss and the factors that affect its selectivity in order to better understand its advantages and limitations.

    

Peak Tailing – Like an Iceberg, it Hides Signal More than Commonly Perceived

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

Peak Tailing is something that every chromatographer faces yet its adverse impact is usually underestimated and not properly perceived. A useful discussion with advices regarding peak tailing is found in Restek's ChromaBlography website.

In this Advanced GC-MS Blog post we focus on GC-MS peak tailing and demonstrate how it is eliminated or reduced using the 5975-SMB GC-MS with Cold EI. We found that peak tailing, like an iceberg, is much more harmful than commonly perceived. In GC-MS, peak tailing can originate at the GC column but the main source of the phenomenon is the ion source (an under-heated transfer line induces more peak broadening than asymmetric peak tailing). Column related peak tailing is a known problem, and recently the major GC capillary column vendors significantly improved the column inertness to polar compounds such as bases and free acids via the availability of inert or ultra inert brand columns. However, ion source peak tailing remains a major problem in GC-MS analysis, and like an iceberg it hides behind a minor chromatographic tailing several serious problems including:

1. Reduced GC separation. 
2. Largely reduced peak height and signal, far more than could be guessed from the slightly deteriorated tailing peak shape. As shown in the figure above, slight peak tailing results in about six times loss of peak height. 
3. Increased noise. The lost signal is converted into a long (and in many cases very long) tail that is transformed into noise as can be seen in many RSIM traces after the elution of a tailing peak.
4. Increased RSD. The analysis reproducibility is adversely affected and the RSD is significantly increased even due to a slightly tailing peak. 
5. Sample identification via library search is adversely affected by peak tailing and particularly from the increase of ion source temperature that is needed to reduce the peak tailing. 
6. While the peak tailing can be reduced via the increase of the ion source temperature such temperature increase exponentially reduces the relative abundance of the molecular ion, reduce the sensitivity in RSIM on the molecular ions and increases the chances of sample degradation at the ion source. 
7. A few classes of compounds such as free fatty acids require laborious derivatization for their GC-MS analysis as otherwise their analysis becomes impractical, and these derivatization procedures aren't always practical themselves.             

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI helps in the complete removal of ion source peak tailing and degradation while providing enhanced molecular ions and superior signal to noise ratio particularly for polar and difficult to analyze compounds.       

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. 

Selectivity Enhancement for the Reduction of Matrix Interference in GC-MS – Cold EI as an Alternative to MS-MS

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

Introduction 

Matrix interference is often the bottleneck in achieving low limits of detection (LOD) when the sample compounds are in complex matrices such as in the analysis of pesticides in various agricultural products. As a result, advanced ways of selectivity enhancement for the reduction of matrix interference were developed, such as MS-MS and high resolution MS. In MS-MS, the selected parent ions undergo collision induced dissociation and one or two daughter ions are transmitted and detected in the third quadrupole MS. We estimate that MS-MS involves with a signal loss of about a factor of 10 while the matrix signal is reduced by an average factor of 200 thereby the selectivity against matrix interference is improved by a factor of about 20. In high resolution MS the selectivity gain is about the resolution/1000 while for compounds with negative mass defects such as dioxins and other multi halogen compounds it can be much higher. Nevertheless, high resolution MS also suffers from reduced signal compared to single ion monitoring with a simple single quadrupole MS instrument. The topics of selectivity gain in MS-MS and high resolution MS will be separately discussed in future blog posts. 

In this post we wish to illuminate another option to reduce matrix interference which is the use of Cold EI (EI of cold molecules in Supersonic Molecular Beams (SMB)) with its enhanced molecular ions, and compare it with MS-MS. 

It is known that matrix interference is reduced with mass. We found that the matrix interference reduction factor is about twenty per one hundred amu higher mass. (M. Kochman, A. Gordin, P. Goldshlag, S. J. Lehotay and A. Amirav "Fast, High Sensitivity, Multi-Pesticide Analysis of Complex Mixtures with the Supersonic GC-MS" J. Chromatogr. A. 974, 185-212 (2002)). Thus, the molecular ion is by far the most selective ion in the EI mass spectrum against matrix interference. As a result, we claim that SIM on the enhanced molecular ion in Cold EI is about as selective against matrix interference as MS-MS on a fragment ion as a parent ion. Since most pesticides are analyzed by MS-MS with standard EI on their major fragment ions as parent MS-MS ions (since their molecular ions are weak or absent), we claim that the Aviv Analytical 5975-SMB GC-MS with Cold EI is as selective as GC-MS-MS with standard EI. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables significant reduction in matrix interference while unlike MS-MS it further enables simultaneous full scan and SIM analysis for achieving universal pesticide screening.  

Cold EI – Approaching the Ideal GC-MS Ion Source

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

Gas chromatography mass spectrometry (GC-MS) is typically operated with a Nier type electron ionization (EI) ion source which is sometimes complemented by a chemical ionization ion source. Most GC-MS users are highly familiar with this "standard" EI ion source and are pleased with its performance and thus it is often accepted without questions about its limitations. 

An interesting question is what are the features of an ideal GC-MS ion source? In other words, what can be improved in the GC-MS EI ion source? A surprising aspect of the list of the ideal ion source features is that it is longer than commonly perceived, and in this note we attempt to discuss those features. 

Cold EI is an advanced EI ion source that performs electron ionization of vibrationally cold molecules in supersonic molecular beams and, as demonstrated, it takes the EI ions-source technology a big step forward towards the ideal. You can read more on Cold EI at the Aviv Analytical website of its 5975-SMB GC-MS with Cold EI or ask us for a review article (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)). 

Keep on reading for the full list of the ideal ion source features and operational properties, and for the improvement introduced by Cold EI which brings it closer to the ideal.  

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.