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.

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

A Universal Method for Pesticide Analysis with the 5975-SMB GC-MS with Cold EI

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

Introduction 

An estimated 2000 chemicals are being used as pesticides worldwide, including banned pesticides and other hazardous chemicals. In view of international food trade this large number implies that pesticide analysis should not be treated as target compounds analysis. Thus, current mass spectrometry instrument development is challenged to provide a one system and method that will be capable of analyzing as many pesticides as possible in full scan mode, in the needed instrumental sensitivity and selectivity in complex agricultural matrices, and in a short amount of time for effective (and lower cost) pesticides screening.   

In order to advance towards meeting the above challenges, a GC-MS with Cold EI (EI of Cold Molecules in Supersonic Molecular Beams) was evaluated in the past with pesticide analysis in agricultural products as described in 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). Recently, we further evaluated pesticide analysis with the Aviv Analytical 5975-SMB GC-MS with Cold EI (GC-MS with supersonic molecular beam (SMB) interface and its fly through ion source) and developed a method to meet the above challenges. 

Keep reading to find out how the Aviv Analytical 5975-SMB GC-MS with Cold EI enables a truly universal method of pesticide analysis in agricultural matrices with improved range of compounds amenable for analysis, reduced matrix interference, lower LOD particularly for difficult to analyze pesticides, improved pesticide identification and shorter analysis time.   

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.