Determination of Carbonyl Compounds in Cork Agglomerates by

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Determination of carbonyl compounds in cork agglomerates by GDMEHPLC/UV: Identification of the extracted compounds by HPLC-MS/MS Pedro Francisco Brandão, Rui Miguel Ramos, Paulo Joaquim Ferreira de Almeida, and Jose António Rodrigues J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b05370 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017

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Journal of Agricultural and Food Chemistry

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To be submitted as an original Research Article to Journal of Agricultural and Food Chemistry

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Determination of carbonyl compounds in cork agglomerates by

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GDME-HPLC/UV: Identification of the extracted compounds by

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HPLC-MS/MS

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Pedro Francisco Brandão, Rui Miguel Ramos*, Paulo Joaquim Almeida and José António Rodrigues REQUIMTE/LAQV – Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, no. 687, 4169-007 Porto, Portugal

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* Corresponding Author:

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Departamento de Química e Bioquímica

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Faculdade de Ciências

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Universidade do Porto

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Rua do Campo Alegre, 687

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4169-007 Porto, Portugal

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(tel.) +351 220 402 646; (fax) +351 220 402 659; (e-mail) [email protected]

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ACS Paragon Plus Environment


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Abstract

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A new approach is proposed for the extraction and determination of carbonyl

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compounds in solid samples, like wood or cork materials. Cork products are used as

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building materials due to its singular characteristics; however, little is known about its

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aldehyde emission potential and content. Sample preparation was done by using a gas-

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diffusion microextraction (GDME) device, for the direct extraction of volatile aldehydes

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and derivatization with 2,4-dinitrophenylhydrazine. Analytical determination of the

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extracts was done by HPLC-UV, with detection at 360 nm. The developed methodology

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proved to be a reliable tool for the aldehydes determination in cork agglomerate samples

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with suitable method features. Mass spectrometry studies were performed for each

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sample, which enabled us to identify, in the extracts, the derivatization products of a

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total of 13 aldehydes (formaldehyde, acetaldehyde, furfural, propanal, 5-methylfurfural,

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butanal, benzaldehyde, pentanal, hexanal, trans-2-heptenal, heptanal, octanal and trans-

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2-nonenal) and 4 ketones (3-hydroxy-2-butanone, acetone, cyclohexanone and

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acetophenone). This new analytical methodology simultaneously proved to be

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consistent for the identification and determination of aldehydes in cork agglomerates

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and a very simple and straightforward procedure.

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Keywords

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dnph, building materials, mass-spectrometry, volatile extraction, volatile organic

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compounds

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INTRODUCTION

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Cork and cork-based products are extensively used for many applications, such

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as cork stoppers, flooring, floaters and other products. Its wide range of applications

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arises from its intrinsic characteristics, such as thermal and sound insulation, elasticity

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and lightness 1, 2. Cork agglomerates, produced using cork waste from the production of

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stoppers or lower quality cork, are one of the products with higher economic value and

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can be classified as: (1) expanded cork agglomerates, that are produced using high

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temperature steam that promotes the expansion of cork particles and the release of

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suberin, which will act as natural adhesive 3; and (2) cork agglomerates, that are

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produced using an adhesive agent, normally a phenolic or formaldehyde based resin 3, 4.

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The industrial use of these resins, high temperature processes, application of varnishes

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and UV curing are commonly associated with the occurrence of volatile organic

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compounds (VOCs) on the final product 5, 6. Their presence can impact the indoor air

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quality and potentially affect human health. One particular group of VOCs associated

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with cork products is formed by aldehydes. Furfural, as an example, is one aldehyde

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commonly found in cork products and is generally associated with the use of high

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pressures and temperatures during manufactory, as a result of carbohydrate degradation

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7-9

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formaldehyde is connected with the use of formaldehyde based resins, such as

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melamine resins 5, 6, 12. The production of these resins is based on the reaction between

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melamine, urea and formaldehyde; depending on the molar ratio of these three

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compounds 13, small amounts of formaldehyde could fail to react, which explains the

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free formaldehyde content that can be released and potentially affect the indoor air

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quality. Furthermore, high temperatures and humidity can also increase the

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formaldehyde released due to hydrolysis of the resin 6.

; the presence of benzaldehyde has been linked with the UV curing process 10, 11;



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One of the standard methodologies used for the formaldehyde determination in

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wood based products is the test chamber, EN 717-1 14. This method measures the

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formaldehyde released from wood products in a closed vessel, under defined climatic

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conditions, and has a maximum test duration of 28 days (or until equilibrium inside the

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chamber is reached). Another method for the analysis in wood based products is the EN

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120 standard methodology, also known as the perforator method 15. This methodology

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assesses the formaldehyde content on small wood panels by extracting formaldehyde

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with boiling toluene.

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Other techniques for the extraction of aldehydes and other VOCs have been used

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in cork and wood based samples, such as solid-phase microextraction 16-19, dynamic

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headspace 7, 20, simultaneous distillation and extraction 9. Although the most common

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analytical techniques used are spectrophotometry 21, 22, liquid chromatography 23, 24 and

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gas chromatography 7, 9, 16, 17, other techniques such as cyclic voltammetry 18, have

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successfully been used for the analysis of VOCs in cork and wood based samples.

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Gas-diffusion microextraction (GDME) is a technique which was developed

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recently for the extraction of volatile and semi-volatile compounds 25. It was initially

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created for the extraction of analytes from liquid food samples such as wine and juices

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26

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microextraction with classic membrane-aided gas-diffusion techniques and has already

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been applied in the extraction of several analytes in different samples 27-29. The

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extraction procedure is built on the analytes transfer from the sample (donor phase)

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through a gas-permeable membrane into an acceptor phase, usually liquid. The

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membrane embodies a thin air space inside its pores, and the mass transfer occurs by

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diffusion of the analytes in the gas form across the gas layer separating the two phases.

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Since microextraction is used, the extraction is not exhaustive allowing monitoring the

, but has been recently applied to solid samples as well 27. This technique combines


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concentration of analytes without significantly changing the studied sample. Using a

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derivatization agent in the acceptor solution alongside with GDME increases the

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extraction efficiency and consequently the possibility of enhancing the chromatographic

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detection.

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2,4-Dinitrophenylhydrazine (DNPH) is a well-known derivatization reagent for

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aldehydes, since these compounds cannot be directly detected by HPLC-UV. It is

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relatively inexpensive, reacts rapidly with various chemical species and its reaction

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products are stable 30. Thus, it is generally accepted as one of the best derivatization

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reagents for the determination of aldehydes in different samples, such as food 31,

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beverages 26, wood based materials 11, 24 and in the air 32. The derivatization reaction

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occurs when the carbonyl group of the aldehyde reacts with the amino group of the

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DNPH, generating the respective DNPH derivative. This reaction, which can take place

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in an aqueous solution, is selective for aldehydes and ketones and can occur at room

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temperature; the formation of the derivatives is pH dependent and should take place in

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an acidic medium, since the reaction is reversible and less extensive at high pH. The

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DNPH derivatives can be studied by several analytical techniques such as

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spectrophotometry 33, mass spectrometry 34, voltammetry 35, gas chromatography 36 and

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liquid chromatography combined with different detectors 26, 31 .

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Herein, in this work, GDME was used for the extraction of aldehydes in

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different cork agglomerated composites. The method comprises the derivatization of

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aldehydes with DNPH, followed by the chromatographic separation and detection at

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360 nm. Furthermore, the DNPH derivatives of the analysed samples were identified by

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LC-MS/MS.

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MATERIALS AND METHODS



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Chemicals and samples. All reagents were of analytical grade and were used without

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further purification. Ultrapure water (resistivity not lower than 18.2 MΩ cm at 298 K)

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from a Direct-Q® 3UV water purification system (Millipore) was used for all chemical

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analysis and glassware washing. HPLC grade acetonitrile was from Fisher, USA. All

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eluents were filtered through a Nylon filter (0.45 µm pore size, (Whatman, USA) prior

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to use. Hydrochloric acid was from Merck, Germany. DNPH (99%), sodium acetate,

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ammonium acetate and all aldehydes were purchased from Sigma-Aldrich, Germany.

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The DNPH derivatizing solution (0.25%, w/v) was weekly prepared in a 200 mL

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mixture of water:acetonitrile (1:1); 4 mL of hydrochloric acid, 1 mol L−1, were added

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since the derivatization reaction is optimized at a pH near 2 37, 38.

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Working standard solutions of aldehydes used in this work were daily prepared

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from stock solutions (1.0 g L-1). Formaldehyde standard stock solutions were prepared

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in water and stored in the fridge at 4 ºC. Other stock solutions were prepared in

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methanol and stored at -10 ºC.

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Chromatographic analysis. A PerkinElmer HPLC system (San Jose, USA) model

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S200 with a S200 UV detector (San Jose, USA) was used for all chromatographic

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analysis. TotalChrom Navigator version 6.3.2 (PerkinElmer) was used for data

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acquisition and processing. Chromatographic separation was performed in a Knauer

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Eurospher 100-5 C18 (250 x 4.0 mm; 5 µm) column in gradient mode. Mobile phase

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consisted of acetonitrile and acetate buffer 0.01 mol L-1. Initial conditions consisted of

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50% acetonitrile and 50% acetate buffer; the gradient begins with 50 % of acetonitrile

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and increased linearly to 100% in the following 25 min, then returned to the initial

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conditions in 5 min; an additional 10 min step was used for conditioning, before the

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next injection. The flow rate was 1.0 mL min-1, the injection volume was 20 µL and the


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UV detection was performed at 360 nm. All separations were made at room temperature

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(approximately 20 ºC).

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HPLC–MS/MS studies. The HPLC system (Thermo Electron Corporation, USA) is

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composed by a low pressure quaternary pump with auto-sampler (200-vial capacity

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sample) and a DAD detector (Finnigan Surveyor Plus). A Gemini C18 column (150×4.6

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mm; 3 µm particle size) and a guard column with the same characteristics was used at

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room temperature. Separations were achieved in the same conditions used for HPLC–

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UV but with a flow rate of 0.4 mL min−1 with injection of 25 µL. A quadrupole ion-trap

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mass spectrometer (Finnigan LCQ Deca XP Plus) equipped with an electrospray

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ionization (ESI) source in the negative ion mode was used in the following conditions:

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capillary temperature, 325 °C; source voltage, 5.0 kV; capillary voltage, -15.0 V; sheath

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gas (N2) flow at 60 arbitrary units and auxiliary gas (N2) flow at 23 arbitrary units. The

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mass detection was performed in the range 0–1000 m/z. Xcalibur software Version 1.4

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(Thermo Electron Corporation) was used for data acquisition and processing.

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Recommended experimental procedure. The GDME extraction principles have been

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described elsewhere 25, and a basic representation of the system can be found in Figure

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1. Analytes were extracted from the sample by a gas-diffusion process through a gas-

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permeable hydrophobic membrane (Mitex® 5.0 µm pore size, Millipore) to an acceptor

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solution containing the derivatization reagent, DNPH.

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Except when mentioned otherwise, the following procedure was adopted: (i) 1.0

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g of cork were grinded and added to a thermostatic cell set at 50 °C; (ii) on the top of

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the cell was placed the extraction module, with 1.0 mL of acceptor solution (DNPH)

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inside the GDME; (iii) after 15 min of extraction the acceptor solution was collected to

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be analysed by HPLC-UV. When required, standard additions were performed by



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spiking cork samples with small volumes of standard solutions of aldehydes directly in

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the cell containing the sample.

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Insert Figure 1 here, please.

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Studied samples. The samples used in this work were cork agglomerated composites

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and were supplied by a local producer. Table 1 shows some of the features of each of

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the six samples. The major characteristic that distinguishes them is the type of treatment

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applied after the agglomeration process. Thus, A, C and E are samples without any type

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of treatment, while samples B and D have a varnish treatment. F is the only sample with

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a PVC coating. All samples were bonded using a melamine-urea-formaldehyde resin

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(MUF). Two different formulations for this resin were used in the industrial production

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of the samples (MUF1 and MUF2). The only significant physical difference between

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the two formulations was the gel time, which was higher in MUF2. Higher gel time has

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been linked with increased concentration of melamine in MUF resins 5. Other

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differences include the thickness of the board and its production site. Samples B, D and

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F undergo a process of UV curing after the varnish treatment.

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Insert Table 1 here, please.

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RESULTS AND DISCUSSION

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Extraction optimization. It has been previously shown that extraction efficiency using

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GDME is mostly affected by the time and temperature of extraction, as well as with the

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volume of acceptor solution 25, 27. The analytical response increases almost linearly with

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increasing extraction time and temperature. In contrast, with high volumes of acceptor

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solution the analytical response decreases. In this work, an optimization of the

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extraction parameters was performed (data not showed), and it was consistent with the


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above mentioned. Thus, for the studies performed throughout this work, an extraction

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time of 15 min, a temperature of 50 °C and 1.0 mL of acceptor solution were used.

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Analytical parameters. The proposed methodology performance was evaluated

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considering a calibration curve obtained with the analytical information of extracts of

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aldehydes standard solutions (n=5) prepared in water. Analytical parameters in terms of

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linearity (n ≥ 5), limit of detection (LOD), limit of quantification (LOQ) and intraday

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precision (expressed as relative standard deviation, RSD) are summarized in Table 2.

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After adjusting the experimental data using a linear regression, the coefficient of

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determination obtained for all aldehydes was above 0.996. LOD and LOQ were

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calculated as three and ten times the standard deviation of the intercept divided by the

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slope, respectively 39; intraday precision was assessed by analysing five samples (n = 5)

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on the same day. The influence of the sample matrix on the extraction was evaluated by

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analysing spiked samples at four different concentration levels, which were chosen

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according to the concentration of formaldehyde in the sample, in order to give a signal

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increase between 50% and 200% of the signal of the non-spiked sample. The standard

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additions method was used for the quantification of formaldehyde since the slopes of

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the standard addition curves were statistically different from the calibration curves

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(Student's t-test, 99% confidence interval), due to matrix interferences. Furthermore, it

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was shown previously that the obtained results are not changed by the weight of the

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sample used in the extraction 27.

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Application to samples. The developed methodology was used to quantify five

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aldehydes extracted from six different cork agglomerated boards. Samples were



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analysed with no further treatments and the standard addition method was used for the

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quantification process, in order to account for possible matrix effects, such as

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adsorption factors. Results are summarized on Table 3.

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As these cork agglomerates were manufactured using formaldehyde-based

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resins, formaldehyde was the aldehyde with the highest content found. The emission of

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formaldehyde and its natural presence on cork and solid wood has been vastly studied

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and has been linked to thermal degradation of polysaccharides 40. However,

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formaldehyde content in wood based products, such as plywood, greatly increases with

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its production, due to industrial processing. For the production of wood based materials

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and other agglomerates, the use of a resin is a common necessity. Urea-formaldehyde

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resins are among the most used, considering its rapid cure and low price 5, although

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these resins have been known to emit small amounts of formaldehyde into the

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atmosphere. The difference in the determined formaldehyde content for each sample can

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be related to different manufacturing procedures and characteristics of the resin used in

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the production process. This content was below 8.3 mg kg-1, which is comparable with

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the ones found in previous works 21. A small difference was observed before and after

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the UV curing process. Thus, samples D and F had higher formaldehyde content than

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samples D and E respectively, which could potentially be explained with secondary

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emissions resulting from the UV curing or the presence of ozone 11. A smaller

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formaldehyde content was found in sample C, which might be connected to the different

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formulation of the resin used (MUF2). A high gel time has been linked to a high

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melamine content in the final resin 5, something that has been connected to lower

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formaldehyde emissions 13.

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Benzaldehyde content on samples B, D and F may be correlated with the UV curing process, since it has been shown before 10, 11 that benzaldehyde is one of the


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possible degradation products of some photoinitiators, used in the UV curing process.

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Other aldehydes found and determined in the analysed samples, in low concentrations,

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have been known to appear naturally on wood and wood based products or as a result of

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microbial activity 9, 41, 42.

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Identification of DNPH derivatives by HPLC–MS/MS. DNPH derivatives were used

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to determine carbonyl compounds by mass spectrometry, providing additional

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information and allowing the identification of unknown compounds extracted from the

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samples. The extracts were obtained according to the procedure described previously,

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and the LC-MS/MS analysis was performed in the negative ion mode. In the negative

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ion mode, the mass spectra showed the base peaks as pseudo-molecular ions, i.e.

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molecular ions that have lost one proton [M-H]-, from the carbonyl-DNPHs. Besides the

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initial considered aldehydes, other carbonyl compounds were extracted and were able to

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be identified (Table 4). Identification was possible by comparing the retention time and

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MS spectral information of the DNPH-derivatives with those reported in the literature.

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Furthermore, for confirmation purposes, they were compared with those observed for

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standard solutions.

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DNPH derivatives with fragment ions at m/z 163 and 179 are typical for

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aldehydes, while ketones are characterized by a higher relative abundance of the m/z

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179 fragment in comparison with m/z 163. Other typical ions could be observed at m/z

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152 and 122 as a result of multiple MS/MS fragmentations as [M-H] → [M-H-30] →

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m/z 152 → m/z 122 43.



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A total of 13 aldehydes and 4 ketones were identified in the extracts of the cork

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agglomerate samples. Formaldehyde, acetaldehyde, butanal, pentanal and hexanal, all

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show the characteristic fragmentations for aldehydes saturated in the α-C, meaning that

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besides the typical m/z 163 and m/z 179 they show fragments of [M-H-30] and [M-H-

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45/46/47]; benzaldehyde, on the other hand, exhibits the fragmentation ions of [M-H-

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47] (m/z 238), [M-H-164] (m/z 121) and [M-H-93] (m/z 192).

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3-hydroxy-2-butanone, also known as acetoin, was identified as the Z-isomer of

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the acetoin-DNPH molecule, as small differences in the UV maximum absorption of

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isomers can be used to distinguish between them 44. Acetone is a known impurity

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usually found in DNPH 45.

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The furfural-DNPH derivative is characterized by two daughter ions for [M-H-

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47] (m/z 228) and [M-H-164] (m/z 111) as well as by the absence of the ion [M-H-93]

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(m/z 182). 5-methylfurfural exhibits the same fragmentation pathway as furfural, with

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characteristic fragmentation ions at m/z 242 and m/z 125. Furfuraldehydes have been

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linked with Maillard type reactions and acid-catalyzed sugar degradation of natural

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products 46. Their presence in wood or cork-based agglomerates may be a consequence

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of different thermal treatments performed during the industrial process, which may

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cause the degradation of cell wall polysaccharides and other carbohydrates, leading to

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the production of furfuraldehydes 8.

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Cyclohexanone is a saturated ketone that was detected in samples using the

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proposed methodology. The most representative fragment of this ion was m/z 247,

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corresponding to a loss of a nitrogen oxide molecule. Acetophenone was identified as it

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produced a strong daughter ion of m/z 254, corresponding to a loss of 45 u [NO2 + H],

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and a UV maximum absorption of 382 nm. These compounds, together with


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benzaldehyde, have been reported as possible fragmentation products of photoinitiators

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used in the UV-curing process 10.

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In conclusion, in this work a new simple and straightforward approach for the

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extraction and quantification of carbonyl compounds in cork samples is presented. This

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methodology, which may also be used with other wood or solid samples, is based on a

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gas-diffusion microextraction process with simultaneous derivatization with DNPH and

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HPLC-UV analysis at 360 nm. Furthermore, several carbonyl compounds were

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successfully identified in the extracts of the studied samples, as they may be a result of

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several industrial processes or degradation products of natural components in cork.

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ACKNOWLEDGEMENTS

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This work received financial support from the European Union (FEDER funds

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POCI/01/0145/FEDER/007265) and from FCT/MEC through national funds and co-

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financed by FEDER (UID/ QUI/50006/2013 - NORTE-01-0145-FEDER-00011) under

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the Partnership Agreement PT2020, which includes a studentship to PFB. RMR

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(SFRH/BD/88166/2012) wish to acknowledge FCT for his PhD studentship. The

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authors would like to thank Dr. Zélia Azevedo for the HPLC–MS/MS analyses



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REFERENCES

(1) Şen, A.; Van Den Bulcke, J.; Defoirdt, N.; Van Acker, J.; Pereira, H., Thermal behaviour of cork and cork components. Thermochim. Acta 2014, 582, 94-100. (2) Pereira, H., Cork : Biology, Production and Uses. Elsevier Science: Amsterdam, 2007. (3) Jardin, R. T.; Fernandes, F. A. O.; Pereira, A. B.; Alves de Sousa, R. J., Static and dynamic mechanical response of different cork agglomerates. Mater. Des. 2015, 68, 121-126. (4) Demertzi, M.; Garrido, A.; Dias, A. C.; Arroja, L., Environmental performance of a cork floating floor. Mater. Des. 2015. (5) Mao, A.; Hassan, E. B.; Kim, M. G., Investigation of low mole ratio UF and UMF resins aimed at lowering the formaldehyde emission potential of wood composite boards. BioResources 2013, 8, 2453-2469. (6) Dunky, M., Urea–formaldehyde (UF) adhesive resins for wood. Int. J. Adhes. Adhes. 1998, 18, 95-107. (7) Horn, W.; Ullrich, D.; Seifert, B., VOC emissions from cork products for indoor use. Indoor Air 1998, 8, 39-46. (8) Rocha, S. M.; Coimbra, M. A.; Delgadillo, I., Occurrence of furfuraldehydes during the processing of Quercus suber L. cork. Simultaneous determination of furfural, 5hydroxymethylfurfural and 5-methylfurfural and their relation with cork polysaccharides. Carbohydr. Polym. 2004, 56, 287-293. (9) Rocha, S.; Delgadillo, I.; Correia, A. J. F., GC-MS study of volatiles of normal and microbiologically attacked cork from Quercus suber L. J. Agric. Food Chem. 1996, 44, 865-871. (10) Salthammer, T.; Bednarek, M.; Fuhrmann, F.; Funaki, R.; Tanabe, S. I., Formation of organic indoor air pollutants by UV-curing chemistry. J. Photochem. Photobiol. A: Chem. 2002, 152, 1-9. (11) Kagi, N.; Fujii, S.; Tamura, H.; Namiki, N., Secondary VOC emissions from flooring material surfaces exposed to ozone or UV irradiation. Build. Environ. 2009, 44, 11991205. (12) Ahamad, T.; Alshehri, S. M., Thermal degradation and evolved gas analysis: A polymeric blend of urea formaldehyde (UF) and epoxy (DGEBA) resin. Arabian J. Chem. 2014, 7, 1140-1147. (13) Tohmura, S.-i.; Inoue, A.; Sahari, S., Influence of the melamine content in melamine-urea-formaldehyde resins on formaldehyde emission and cured resin structure. J. Wood Sci. 2001, 47, 451-457. (14) EN 717–1 (2004) Wood-based panels - Determination of formaldehyde release Part 1: Formaldehyde emission by the chamber method. European Committee for Standardisation, Brussels, Belgium. (15) EN 120 (1992) Wood-based panels; Determination of formaldehyde content; Extraction method called the perforator method. European Committee for Standardisation, Brussels, Belgium. (16) Moreira, N.; Lopes, P.; Cabral, M.; Guedes de Pinho, P., HS-SPME/GC-MS methodologies for the analysis of volatile compounds in cork material. Eur. Food Res. Technol. 2016, 242, 457-466.


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449


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450


FIGURE CAPTIONS

451 452

Figure 1 – A schematic representation of the gas-diffusion microextraction system,

453

including a thermostatic cell, the GDME extractor device and a lid to seal the system.



Table 1 - Main characteristics of the six cork agglomerated samples used in this study, in terms of board thickness, PVC coating, varnish treatment and type of resin used in the agglomeration process. sample

thickness (mm)

PVC coating

varnish treatment

resin

A

4.0

-

-

MUF1

B

4.0

-

yes

MUF1

C

4.0

-

-

MUF2

D

4.0

-

yes

MUF2

E

≈ 3.3

-

-

MUF1

F

≈ 3.3

yes

yes

MUF1


Page 18 of 23

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Table 2 – Analytical parameters of the proposed methodology for the determination of aldehydes in cork samples.

compound

regression equation a

r2

formaldehyde

y = (22.3 ± 0.2)x + (12.5 ± 0.8)

butanal

LOD

LOQ (mg kg )

precision (%)b

0.999

0.17

0.57

6.1

y = (209.1 ± 4.3)x + (0.09 ± 1.0)

0.998

0.026

0.086

11.9

benzaldehyde

y = (64.4 ± 2.2)x + (-0.3 ± 0.6)

0.996

0.042

0.14

7.3

pentanal

y = (317.2 ± 6.0)x + (0.8 ± 1.5)

0.998

0.024

0.080

7.6

hexanal

y = (278.3 ± 4.6)x + (1.4 ± 1.2)

0.999

0.021

0.069

14.1

b

-1

y is the peak area in mV; x is the concentration of the aldehyde (mg kg ) expressed as relative standard deviation, RSD


-1

intraday

(mg kg )

a

-1


Table 3 - Concentrations of formaldehyde, butanal, benzaldehyde, pentanal and hexanal determined using the proposed methodology (GDME) on the six different samples.

determined concentration / mg kg-1 a sample

formaldehyde

butanal

benzaldehyde

pentanal

hexanal

A

8.1 ± 0.3

0.24 ± 0.01

0.82 ± 0.03

Determination of Carbonyl Compounds in Cork Agglomerates by

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