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.   

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.  

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.       

Signal versus Signal to Noise Ratio in Mass Spectrometry

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

Introduction

Based on simple computer simulations, we found that even in the limit of sparse ion noise, signal to noise ratio and not signal alone is the important parameter for sensitivity and chromatography quality evaluation. For the same total ion count (TIC) signal to noise ratio, surprisingly lower signal enables superior sample identification. We recommend the use of a mixture with few compounds for the evaluation of relative GC-MS systems sensitivity via the measurement of their relative TIC signal to noise ratios.

In this short post we explain how we reach these conclusions, and provide you with the (free) simulating software we have used for this purpose.

OFN Sensitivity Specifications – Are they of any Value or Just a Game

Aviv Amirav, Professor of Chemistry at Tel Aviv University and Director – Aviv Analytical
August 15 2012

Introduction

Sensitivity is a prime specification of GC-MS which is often used to decide which system "is better". Usually GC-MS sensitivity is specified with 1 pg OFN RSIM on its molecular ion m/z=272 and currently the quadrupole GC-MS sensitivity specifications of all the major vendors are in the range of 400 to 800 in RMS units.

However, these “sensitivity” specifications should not be used for GC-MS sensitivity comparisons or evaluations since the vendors employ misleading procedures and OFN is a non representative “easy to analyze” sample.  

In this post we discuss the "tricks" vendors probably use to increase their OFN specifications.