Characterization of resin extracted from guayule (Parthenium argentatum): A dataset including GC–MS and FT-ICR MS

Guayule (Parthenium argentatum), a shrub native to the arid region of the U.S. southwest and Mexico belonging to the Asteraceae family, is a source of high quality, hypoallergenic natural rubber with applications in pharmaceutical, tire, and food industries. Production of rubber results in a substantial amount of resin-containing residues which contain a wide variety of secondary metabolites (sesquiterpene esters, triterpene alcohols, fatty acids, etc.). In order to enhance the economic viability of guayule as an industrial crop, value-added use of the residues is needed and has the potential to reduce gross rubber production costs. The main objective of this research is the characterization of guayule resin using rapid and accurate analytical techniques to identify compounds of potential commercial value. Guayule resin is inherently complex and includes many high-molecular-weight and non-volatile compounds that are not easy to observe using traditional chromatographic techniques. The combination of two mass spectroscopy techniques: gas chromatography mass spectroscopy (GC–MS) and high-resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS), were used to characterize the composition of the extracted resin from guayule (Parthenium argentatum). FT-ICR MS was used to characterize hundreds of compounds with over a wide range of molecular weights and degrees of aromaticity at higher levels of mass accuracy than other forms of mass spectrometry. GC–MS was used to identify volatile compounds like mono- and sesquiterpene compounds.

tracted resin from guayule ( Parthenium argentatum ). FT-ICR MS was used to characterize hundreds of compounds with over a wide range of molecular weights and degrees of aromaticity at higher levels of mass accuracy than other forms of mass spectrometry. GC-MS was used to identify volatile compounds like mono-and sesquiterpene compounds.
© 2020 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY license.
( http://creativecommons.org/licenses/by/4.0/ ) Value of the Data • This dataset represents a comprehensive characterization of guayule resin using several complementary analysis methods.

Specifications
• Researchers working on natural resin characterization, processing, and utilization can benefit from this data. • Quantitative and qualitative composition data for guayule resin can be used to select future separation techniques for analysis and to identify potential applications of separated resin fractions based on their expected compositions. • Data from GC-MS is commonly available for resin samples and provides a common basis for comparison between this data and previously collected data for other resin samples. • The FT-ICR MS data provides information about a wider range of molecular weights than GC-MS data due to the wider range of molecules that can be detected using the ionization method.

Data
Data here includes: a table of terpene molecules identified within the guayule resin sample by GC-MS with the parameters used for identification ( Table 1 ); a table of compounds identified by negative-ion APPI FT-ICR MS of guayule resin with the mass-to-charge ratios and assigned molecular formulas for each compound ( Table 2 ); a table of compounds identified by positiveion APPI FT-ICR MS of guayule resin with the mass-to-charge ratios and assigned molecular formulas for hydrocarbon-containing compounds with > 5% relative abundance ( Table 3 ); a figure showing the experimentally collected GC mass spectra (top) and library mass spectra (bottom) for the terpene compounds listed in Table 1 Table 3 ( Fig. 2 ), a figure of color-coded isoabundance plots of the compounds in the hydrocarbon (HC) and oxygenated molecule classes from positive-ion APPI FT-ICR MS of guayule resin corresponding to the compounds in Table 3

Experimental design, materials, and methods
Guayule resin from pilot-scale bulk (solvent) rubber extraction was acquired from the Bridgestone Americas Biorubber Processing Research Center (Mesa, AZ), and characterized as received. The guayule plants were harvested at 24-36 months old, field dried for 1-7 days (varies seasonally) to 10-15% moisture, and milled to pass a ¼ in. (6.4 mm) screen. A miscella of rubber and resin was extracted from the whole ground guayule using a mixture of acetone and hexane. Rubber was precipitated from the miscella with addition of excess acetone. Resin was concentrated by evaporation of the solvent from the miscella; after cooling to room temperature, the resin was nearly solid. The resin was collected into barrels, and stored at ambient temperature and humidity ( < 36% annually) for up to two weeks prior to shipping for analysis. Upon receiving, resin samples were stored in plastic containers at room temperature.

Gas chromatography mass spectroscopy (MS)
Guayule resin was analyzed by GC-MS to characterize terpene composition. A 10% (w/v) solution was prepared by dissolving resin sample in carbon disulfide. The analyses were performed using a GC-MS system (7890A, Agilent Technologies) equipped with a DB-5 MS capillary column (30 m × 0.25 mm, 0.25 mm, Agilent Technologies). The injection volume of 1 μL was made in a   ( continued on next page ) a Error ( m/z ) = difference between theoretical and observed mass.

High-resolution Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS)
Fourier transform ion cyclotron resonance mass spectroscopy (FT-ICR MS) is currently the only analytical technique providing the required resolving power (m/ m 50% ≥ 40 0,0 0 0) and mass accuracy (ppm) for detection and identification of thousands of compounds within a single mass spectrum. This technique typically used to analyze complex natural organic mixtures such as petroleum, biofuels, dissolved organic matter, lipids, and proteins [3] . Guayule resin was analyzed with a custom-built 9.4 T FT-ICR MS at the National High Magnetic Field Laboratory. Atmospheric pressure photoionization (APPI) was used to ionize both polar and non-polar compounds, especially aromatic species, for detection by mass spectrometry. Guayule resin was dissolved in toluene (HPLC grade, JT Baker, Phillipsburg, NJ) to create 1 mg/mL stock solutions.
Stock solutions were diluted to a final sample concentration of 10 μg/mL in toluene for positiveand negative-ion atmospheric pressure photoionization. Samples were introduced to the source through a capillary at a rate of 50 μL/min. Nitrogen was used as a sheath gas (60 psi) and auxiliary gas (4 L/min). Inside the heated vaporizer of the source ( ∼300 °C), the sample was mixed with a nebulization gas (N 2 ) and is passed under a krypton VUV lamp producing 10 eV photons (120 nm). Toluene was used to increase ionization efficiency through dopant-assisted photoionization.  Ions generated at atmospheric pressure were introduced into the mass spectrometer via a heated metal capillary. Ions were guided through the skimmer region and quadrupole (mass transfer mode) for accumulation in the second quadrupole. Finally, ions were collisionally cooled with helium gas ( ∼4 − 5 × 10 −6 Torr at gauge) before optimized passage [4] through a transfer quadrupole to the ICR cell. Multiple (50) individual time-domain transients were coadded, Hanning-apodized, zero-filled, and fast-Fourier-transformed prior to frequency conversion to mass-to-charge ratio [5] to obtain the final mass spectrum. The time domain signal acquisition period was 4.1 s. The obtained FT-MS spectrum contained approximately 7200 and 3500 peaks in negative and positive ionization mode respectively, in the m/z range of 150-800.
Data collection was facilitated by a modular ICR data acquisition system (PREDATOR) [6] . Mass spectral lists were generated with PetroOrg software [7] . Internal calibration of the spectrum was based on homologous series whose elemental compositions differ by integer multiples of 14.01565 Da (i.e.,CH 2 ) [ 8 , 9 ]. Data are visualized by relative abundance histograms for heteroatom classes with > 1% relative abundance, and from isoabundance-contoured plots of double bond equivalents (DBE = number of rings + double bonds to carbon) versus carbon number for members of a single heteroatom class. The relative abundance scale in isoabundance-contoured plots is scaled relative to the most abundant species in the mass spectrum.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.