Thursday, June 14, 2018

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.

Tuesday, June 12, 2018

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.  

Tuesday, June 5, 2018

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.

Monday, November 7, 2016

Triglycerides in Oils Analysis by the 5975-SMB GC-MS with Cold EI


Aviv Amirav, Tel Aviv University and Aviv Analytical, Tel Aviv Israel.
Hans-Gerd Janssen, Unilever R&D Vlaardingen and University of Amsterdam, Amsterdam The Netherland

Introduction 

Triglycerides analysis is challenging since these large compounds do not elute from standard GC columns under the conditions used in GC-MS and furthermore their standard EI mass spectra do not exhibit any molecular ion, as shown in the NIST library. In addition, their analysis by LC-MS is also challenging in view of their poor proton affinity. Thus, and in view of the importance of triglycerides analysis we decided to analyze three oil samples that were previously analyzed by GC-FID in order to evaluate these oils analysis by GC-MS with Cold EI. The challenge was to properly elute these large compounds and demonstrate the availability of abundant molecular ions plus useful and informative high mass fragments. Triglycerides analysis is very important for the food industry and such analysis can also be potentially important for human medical diagnostics in the form of cholesterol and triglycerides analysis in blood. Currently triglycerides are hydrolyzed and methylated to form FAMEs (fatty acid methyl esters) that are analyzed by GC-MS. However, in such analysis vital information is lost on the actual structure and concentration of the various parent triglycerides and GC-MS with Cold EI is challenged to exhibit such information.     

Wednesday, May 27, 2015

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.

Wednesday, May 13, 2015

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.

Monday, December 15, 2014

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. 



Thursday, May 8, 2014

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.

Wednesday, December 18, 2013

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.

Wednesday, November 20, 2013

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.