# Comparison of 3 ionisation methods - electron ionisation, chemical ionisation and atmospheric pressure photoionisation for the characterisation of volatile organic compounds (VOCs)

Géraldine Lucchi, Jean-Luc Le Quéré, Karine Gourrat, Marine Crépin

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Géraldine Lucchi, Jean-Luc Le Quéré, Karine Gourrat, Marine Crépin. Comparison of 3 ionisation methods - electron ionisation, chemical ionisation and atmospheric pressure photoionisation for the characterisation of volatile organic compounds (VOCs). The 16. Weurman Flavour Research Symposium, May 2021, Virtual meeting (Dijon), France., 2021, Proceedings of The 16. Weurman Flavour Research Symposium. 10.5281/zenodo.5541277. hal-04176200

HAL Id: hal-04176200

https://hal.inrae.fr/hal-04176200

Submitted on 9 Aug 2023

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# Comparison of 3 ionization methods - electron ionization, chemical ionization and atmospheric pressure photoionization - for the characterization of volatile organic compounds (VOCs)

Géraldine Lucchi¹, Jean-Luc Le Quéré², Karine Gourrat¹, Marine Crépin¹

¹ChemoSens Platform, UMR1324 CSGA, INRAE, Dijon, France

²FOP team, UMR1324 CSGA, INRAE, Dijon, France

ChemoSens

## Introduction

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Gas chromatography - Mass Spectrometry (GC-MS) is the method of choice to identify and quantify VOCs in food. The main ionization method is the Electron Ionization (EI): high energy exchanges occur, causing reproducible molecular fragmentations. Chemical Ionisation (CI) is another ionization method where a reactive gas (i.e. methane or ammonia) is ionized to form reactant ions. These ions react with the analytes by proton transfer or charge transfer to produce quasi-molecular ions, sometimes accompanied by characteristic adduct ions. Atmospheric Pressure Photolysis (APPI) is the most recent source [1]. Emitted photons give rise to an odd-electron radical cation; hydrogen atom abstraction frequently occurs during in-source collisions, and produces a large quantity of protonated molecules, MH⁺.

In our research platform, we recently coupled a GC Trace 1310 to a High Resolution Mass Spectrometer (HRMS) Orbitrap Fusion (ThermoScientific) with the APPI source developed by Mascom (Bremen, Germany). In this work, first, we present a general overview of the technical developments carried out on 13 VOCs with the GC-APPI-HRMS hyphenated technique. Secondly, we compare the ionization methods listed above. For this purpose, we used 6 VOCs of different chemical classes to determine the Limit Of Detection (LOD) for each source.

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## Methodology

<table><thead><tr><th>Compounds name</th><th>Chemical classes</th><th>MH+</th></tr></thead><tbody><tr><td>Butanoic acid</td><td>Acids</td><td>88</td></tr><tr><td>2-Methylbutanoic acid</td><td>Acids</td><td>102</td></tr><tr><td>Heptan-2-ol</td><td>Alcohols</td><td>116</td></tr><tr><td>2-Methylbutan-1-ol</td><td>Alcohols</td><td>88</td></tr><tr><td>3-Methylbutan-1-ol</td><td>Alcohols</td><td>88</td></tr><tr><td>2-Phenylethanol</td><td>Alcohols</td><td>122</td></tr><tr><td>Linalool</td><td>Alcohols</td><td>154</td></tr><tr><td>Heptanal</td><td>Aldehydes</td><td>114</td></tr><tr><td>2,3-dimethylpyrazine</td><td>Pyrazines</td><td>108</td></tr><tr><td>Heptan-2-one</td><td>Ketones</td><td>114</td></tr><tr><td>Diacetyl</td><td>Ketones</td><td>86</td></tr><tr><td>Ethyl butanoate</td><td>Esters</td><td>116</td></tr><tr><td>Isoamyl acetate</td><td>Esters</td><td>130</td></tr></tbody></table>

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In blue, optimized parameters

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Methodological
development in
GC-APPI-HRMS

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* ✓ Standard solutions in triplicate (0.1 to 200 ng/µL)

* ✓ 4 to 7 points in the compound linearity domain

* ✓ Selection of 1 to 3 ions for each ionization method

* ✓ Σ abundances of these ions for each triplicate and each solution

* ✓ Average and standard deviation for each solution

* ✓ LOD calculation

* ✓ Calculation of minimal detectable concentration

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LOD comparison
according to the
ionization method

## Results

### Methodological development in GC-APPI-HRMS

<figure><img src="image_13.png" /><figcaption>Figure 1 – MS profile in GC-APPI-HRMS for 3 VOCs.</figcaption></figure>

<table><caption>Table I – Optimized parameters for VOCs analysis.</caption><thead><tr><th></th><th>Optimum</th></tr></thead><tbody><tr><td>MS</td><td></td></tr><tr><td>Detector</td><td>Pos</td></tr><tr><td>AGC</td><td>2e5</td></tr><tr><td>Resolution</td><td>15000 (0.006 Da à 89 m/z)</td></tr><tr><td>% RF Lens</td><td>20 - 40</td></tr><tr><td>MS2</td><td></td></tr><tr><td>HCD</td><td>20 % (best fragmentation)</td></tr><tr><td>CID</td><td>22 %</td></tr><tr><td>AGC</td><td>5e4</td></tr><tr><td>Resolution</td><td>15000</td></tr><tr><td>Sheath gas</td><td>2</td></tr><tr><td>Source T°C</td><td>150°C</td></tr></tbody></table>

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* ✓ Fragmentation occurs in MS mode for several VOCs (Figure 1)

* ✓ MS parameters have been optimized (Table I) to limit these fragmentations and to enhance the sensitivity

* ✓ In-source fragmentation remains important for several chemical classes: in fact, the radical cation of the linear oxygenated aroma compounds is subject to rearrangements that require very little activation energy. Ionization energy of oxygenated molecules is relatively low, then, fragmentations occur.

### LOD comparison

<table><caption>Table II – LOD (ng/µL) of different VOCs according to the ionization method.</caption><thead><tr><th rowspan="2">Compound names</th><th rowspan="2">Chemical classes</th><th colspan="4">LOD (ng/µL)</th></tr><tr><th>El</th><th>CI (CH<sub>4</sub>)</th><th>CI (NH<sub>3</sub>)</th><th>APPI</th></tr></thead><tbody><tr><td>2-Methylbutanoic acid</td><td>Acids</td><td>0.029</td><td>0.038</td><td>2.706</td><td>0.331</td></tr><tr><td>Heptan-2-ol</td><td>Alcohols</td><td>0.006</td><td>0.028</td><td>0.719</td><td>0.165</td></tr><tr><td>Heptanal</td><td>Aldehydes</td><td>0.023</td><td>0.008</td><td>6.872</td><td>0.052</td></tr><tr><td>2,3-dimethylpyrazine</td><td>Pyrazines</td><td>0.002</td><td>0.002</td><td>0.005</td><td>0.004</td></tr><tr><td>Heptan-2-one</td><td>Ketones</td><td>0.003</td><td>0.004</td><td>0.052</td><td>0.020</td></tr><tr><td>Isoamyl acetate</td><td>Esters</td><td>0.003</td><td>0.009</td><td>0.064</td><td>0.223</td></tr></tbody></table>

* ✓ Great disparity in sensitivity according to the chemical classes and the ionization method (Table II)

* ✓ EI and CI (CH₄) are the most sensitive ionization methods for the studied acid, ketone and ester

* ✓ For the alcohol, EI is the best one while CI (CH₄) is most appropriate for the aldehyde

* ✓ Relatively high background noise in CI (NH₃) spectra compared to CH₄ is observed, which makes data processing very complex, and consequently, a higher LOD

* ✓ The studied pyrazine is the better-detected molecular species for the 4 considered methods

* ✓ LODs in APPI are better than those described in the literature, where the limit of detection in GC-APPI-MS is between 1 and 100 ng/µL [2]

## Conclusion & Perspectives

These preliminary results allowed to set up optimized parameters to better characterize VOCs by GC-APPI-HRMS.

LOD were calculated for different chemical classes and compared to other ionization methods. EI remains the most appropriate one to identify chemical compounds in databases, while CI (CH₄) could provide supplementary information for molecular characterization. APPI, a promising technique to resolve co-elution problems, has to be improved, especially to reduce in-source fragmentation. The use of a dopant gas (acetone) should be tested in the near future.

## Contact

Centre des Sciences du Goût et de l'Alimentation
UMR1324 INRAE - 6265 CNRS - Université de Bourgogne - Franche

Comté - AgroSupDijon, Dijon, France

https://www2.dijon.inrae.fr/csga/ contact: geraldine.lucchi@inrae.fr

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INRAE

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UBFC
UNIVERSITE
BOURGogne FRANCE

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AGRO SUP
UNIVERSITÉ
BOURGogne FRANCE

cns

[1] Raffaelli A. et al. (2003). Mass Spec. Reviews, 22, 318.

[2] Revel'skii, I. A. et al. (2019). Journal of Analytical Chemistry, Vol. 74, No. 2, pp. 192-197.

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16th Weurman
Online

May 4-6, 2021