Fast analysis of volatile compounds from Lippia citriodora with nanoporous aluminum wire as solid-phase microextraction fibres

Abstract In this study, the efficiency of nanoporous aluminium wires as fibres for headspace solid-phase microextraction (HS-SPME) of volatile compounds from Lippia citriodora was investigated and compared with two anodised methods. The prepared fibres are durable with very good chemical and thermal stability which can be coupled to GC and GC/MS. A one at-a-time optimisation strategy was applied for optimising the important extraction parameters such as extraction temperature, extraction time, sample mass and added water. Compared with hydrodistillation (HD), HS-SPME provide the advantages of a small amount of sample, time-saving, simplicity and cheapness.


Introduction
The genus Lippia (Verbenaceae) includes approximately 200 species of herbs, shrubs and small trees. Lemon verbena Lippia citriodorais is indigenous to South America and was introduced in Europe at the end of the seventeenth century. It is cultivated mainly due to the lemon-like aroma emitted by its leaves that are utilised for the preparation of herbal tea, which is reputed to have antispasmodic, antipyretic, sedative and digestive properties. Lemon verbena has a long history of folk uses in treating asthma, spasms, cold, fever, flatulence, colic, diarrhoea, indigestion, insomnia and anxiety (Meshkaalsadat et al. 2010;Carvalho Nilo Bitu et al. 2014).
Solid-phase microextraction (SPME) is based on the distribution of analytes between sample solution and a fibre coated with a stationary phase. Commercially available SPME fibres have a number of drawbacks including relatively low operating temperature (generally in the range of 240-280 °C), mechanical fragility, low stability in acidic or alkaline samples, less selectivity and swelling in organic solvents. Consequently, in recent years, the preparation of tailor-made fibres has been the focus of interest. This technique integrates easy, rapid, sensitive and safe sampling, extraction, concentration and sample introduction procedure (used) in GC and HPLC into a single step. SPME uses a small (1-cm) piece of fused silica, on which a liquid phase, similar to a GC stationary phase, has been coated in order to absorb or adsorb the analytes and concentrate them on the fibre. Some of these coating materials, such as poly(dimethylsiloxane) (PDMS), have been widely used. Some others, such as polyacrylate (PA), polyimide, poly (dimethylsiloxane)-divinylbenzene (PDMS-DVB), Carbowax-divinylbenzene (CW-DVB), Carbowax-TR (template resin-DVB with uniform pore sizes), Carboxen-PDMS, graphite, activated charcoal, poly-(3-methylthiophene) and Nafion, are specialised to remove substances such as metal ions, proteins and polar compounds (Djozan et al. 2001). The use of metal wires as fibre supports with high mechanical stability makes this technique more robust for routine analysis. Several studies have been developed using different material coatings over the traditional fused silica support, such as platinum, anodised aluminium, gold, stainless steel, copper, titanium and NiTi alloy wires (Abolghasemi et al. 2015).
This work is focused on the effectiveness of headspace solid-phase microextraction (HS-SPME) using nanoporous aluminium fibres by different methods for the analysis of the volatile compounds in Lippia citriodora. Using HS-SPME followed by GC-MS, 38 compounds were separated and identified in L. citriodora, which confirmed the obtained results of hydrodistillation method.

Results and discussion
This study evaluates the reliability of SPME nanoporous aluminium fibres with anodised aluminium for extraction of the volatile compounds from the headspace of medicinal plant that was heated in the absence of any solvent by three methods.

Effect of temperature
The extraction temperature was varied from 30 to 75 °C and the results are shown in Figure  S2 (a, b) showing the effect of extraction temperature on the total peak areas of all the obtained compounds and the individual peak areas of the four compounds from L. citriodora. It was found that either the total peak areas or the individual peak areas increased with the temperature up to 70 °C for fibre b and 60 °C for fibre c.

Effect of extraction time
The extraction time varying from 5 to 50 min was investigated and the results are shown in Figure S3(a, b). The profile for the total and individual peaks area shows highest peak area at 35 min for fibre b and 25 min for fibre c.

Sample mass
Generally, signals of analytes increase with the sample amount but extremely large amount of the sample affects extraction efficiency. On the other hand, this does not mean that larger the sample amount, better the results. In the present work, the sample amount varied from 1.0 to 3.0 g and the results of the effects of sample amount on the total peak areas and the individual peak areas of the four target compounds are shown in Figure S4(a, b). just as expected, the total and individual peak areas increased with sample amount up to 2.5 g for fibre b and 2.0 g for fibre c.

Added water
Due to the alumina structure, water vapour (humidity) may affect the fibre's ability to adsorb the volatile compounds of L. citriodora. Therefore, the effect of humidity was studied by the addition of different amounts of water to the samples in the optimised conditions. The results are shown in Figure S5(a, b) for the two fibres. It means that the water molecules can deactivate the fibre surface by blocking the active sites; therefore, the proposed fibre is a good adsorptive fibre for sampling from the dried samples.

HS-SPME of L. citriodora
The optimised HS-SPME conditions were applied to the extraction and concentration of the volatile constituents in L. citriodora with two fibres of aluminium.
The volatile compounds of L. citriodora were extracted and concentrated by HS-SPME, and then the analytes that were extracted on the anodised aluminium fibres were desorbed and analysed by GC-MS. Components were identified, and are listed in Table S1.

Conclusions
In this study, the efficiency of two anodised aluminium wires as fibres for headspace solid-phase microextraction (HS-SPME) of volatile compounds in a L. citriodora was investigated. Compared with conventional HD method, HS-SPME/GC-MS is a simple, rapid, solvent-free and efficient method for the analysis of essential oils in L. citriodora with low sample amount. The proposed methods are environmentally friendly, because no toxic solvent is used. Low cost, high temperature resistance, firmness and long durability are the main advantages of these fibres that were used for analysis of volatile compounds of L. citriodora.

Supplementary material
Experimental details relating to this paper are available online, alongside Table S1 and Figures S1, S2, S3, S4 and S5.

Disclosure statement
No potential conflict of interest was reported by the authors.