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Gas chromatography–mass spectrometry (GC–MS) with Cold EI is based on interfacing GC and MS with supersonic molecular beams (SMBs) along with electron ionization of vibrationally cold sample compounds in SMB in a fly‐through ion source (hence the name Cold EI). Cold EI improves all the central performance aspects of GC–MS, and in this paper, we focus on its improvement of signal‐to‐noise ratio (S/N) and limits of detection (LODs). We found that the harder the compound for analysis with standard EI, the greater the Cold EI gain in S/N and LOD. The lower LOD and higher S/N of Cold EI emerge from a few reasons: (a) similar ionization yield as standard EI, (b) enhanced abundance of molecular ions, (c) elimination of vacuum background noise, (d) elimination of ion source‐related peak tailing and degradation, (e) ability to lower the elution temperatures via the use of high column flow rates, and (f) greater range of thermally labile and low‐volatility compounds that can be analyzed. We demonstrate the superior S/N and lower LOD of Cold EI versus standard EI in a range of compounds, from the simple‐to‐analyze octafluoronaphthalene all the way to reserpine and an organo‐metallic compound that cannot be analyzed by standard EI. These compounds include methyl stearate, cholesterol, n‐C32H66, large polycyclic aromatic hydrocarbons, dioctyl phthalates, diundecyl phthalate, pentachlorophenol, benzidine, lambda‐cyhalothrin, and methidathion. The significantly lower Cold EI LODs that can be over 1000 times better than in standard EI further result in far superior response linearity and greater measurement dynamic range. [ABSTRACT FROM AUTHOR]

Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70-eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that 1. Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions. 2. Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real-world applications. 3. Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library. 4. Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the "Extractor" exhibits relatively abundant molecular ions compared with the "Inert" ion source, while the "High efficiency source" (HES) provides mass spectra with depleted molecular ions compared with the "Inert" ion source or NIST library mass spectra. These conclusions are demonstrated and supported by experimental data in nine figures and two tables. [ABSTRACT FROM AUTHOR]

We explored the use of pulsed flow modulation (PFM) two-dimensional comprehensive gas chromatography (GCxGC) mass spectrometry with supersonic molecular beams (SMB) (also named Cold electron ionization (EI)) for achieving universal pesticide analysis in agricultural products. The use of GCxGC serves as an alternative to MS-MS in the needed reduction of matrix interference while enabling full-scan MS operation for universal pesticide analysis with reduced number of false negatives. Matrix interference is further reduced with Cold EI in view of the enhancement of the molecular ions. Pulsed flow modulation is a simple GCxGC modulator that does not consume cryogenic gases while providing tuneable second GCxGC column injection time for enabling the use of quadrupole-based mass spectrometry regardless its limited scanning speed. PFM-GCxGC-MS with Cold EI combines improved separation of GCxGC with Cold EI benefits of tailing-free ultra-fast ion source response time and enhanced molecular ions for the provision of increased sample identification information and reduced matrix interference. Consequently, PFM GCxGC-MS with Cold EI also improved NIST library identification probabilities of the spiked pesticides. PFM GCxGC is further characterized by largely increased second column sample and matrix capacity that as a result performs much better than thermal modulation GCxGC-MS with standard EI in the suppression of matrix interference. In a comparison with standard GC-MS, we measured with PFM GCxGC-MS with Cold EI an average total ion count matrix interference reduction factor of 32 for 12 pesticides in two matrices of baby leaves mixture and lettuce. In addition, Cold EI further increases the range of pesticides amenable for GC-MS analysis and its response is relatively uniform hence with it the need for pesticides specific calibration is reduced.Pulsed flow modulation GCxGC-MS with Cold EI significantly reduces matrix interference and improves sample identification. [ABSTRACT FROM AUTHOR]

GC-MS with Cold EI improves all of the central GC-MS performance aspects, but it is known mostly for its provision of enhanced molecular ions. This occasionally leads to the misconception that, like chemical ionization, Cold EI is a supplementary ion source to standard EI. However, Cold EI is a highly superior replacement ion source to standard EI. While Cold EI mass spectra are the most informative and fully compatible with mass spectral library (such as NIST) identification, in some cases, the Cold EI mass spectra with their enhanced molecular ions result in a "picture" that is not as one is used to seeing. In this paper, we describe the "Cool Classical EI" mode, which produces classical EI mass spectra like standard EI. The change of Cold EI into the Cool Classical EI mode is software-based, requires no hardware change, and can be achieved even during the analysis. Several mass spectra that were obtained in the Cool Classical EI mode are presented and compared with standard EI and Cold EI mass spectra. In this paper we further demonstrate and discuss several benefits that Cold EI brings that are retained while using Cool Classical EI, including (a) much faster speed of analysis, (b) uniform response, (c) extended range of compounds amenable for analysis, (d) improved sample identification, (e) elimination of ion source related peak tailing, (f) elimination of intraion-source degradation, and (g) better signal-to-noise ratio of the sample compounds.

We report the finding of doubly charged molecular ions in a range of relatively large molecules including hydrocarbons upon their electron ionization as vibrationally cold molecules in supersonic molecular beams (SMB) (also named as Cold EI). Furthermore, we also report the detection by mass spectrometry of triply charged molecular ions in large PAHs such as decacyclene and ovalene upon their cooling in SMB. We found that the relative abundance of doubly charged molecular ions strongly depends on the internal vibrational cooling. While after some vibrational cooling the fragmentation pattern became cooling independent, the relative abundance of the doubly charged molecular ions was noticeably increased upon further cooling via increasing of the cooling make-up gas flow rate. In addition, the relative abundance of the doubly charged molecular ions was strongly increased with the compounds' size, and its electron energy threshold was lower than expected. These observations indicate a new mechanism that involves two separate electron ionization processes in the same compound, most likely with the same electron but at two separate atoms (places) in large molecules, to reduce Coulombic repulsion energy that can lead to fragmentation into two singly charged ions. These findings are shedding new light on electron ionization mass spectra. Accordingly, electron ionization mass spectra are the result of three separate mechanisms with relative magnitudes that depend on the compound size: (a) single electron ionization; (b) double electron ionization; and (c) single electron ionization with subsequent internal excitation by the same ionizing electron in another place.