# Current Trends in Fully Automated On-Line Analytical Techniques for Beverage Analysis
## 4 In-Tube Solid-Phase Microextraction (In-Tube SPME)
### Capillary Devices  
Open tubular (OT) columns are an important type of column widely employed in GC for many years. These columns have a thin layer of stationary phase on the inner wall of the capillary and parts of them can be used as extractive devices for in-tube SPME. OT columns are classified into two types: Wall-coated open-tubular (WCOT) and porous-layer open-tubular (PLOT). In the first case, a non-porous film is coated into the inner wall of fused silica tubing, while in the second case a porous material is utilized as the coating phase. In the analysis of beverages, WCOT-type coatings like 100% polydimethylsiloxane, 5% phenyl polydimethylsiloxane, 14% cyanopropyl phenyl methyl silicone and polyethylene glycol are usually employed, while PLOT-type coatings like divinyl-benzene, Carboxen molecular sieves, and CP-PoraPLOT amine have also been used [68; 69; 70]. Some of these capillary columns showed efficiency in the extraction of PHAs and patulin, which depends strongly on the stationary phase coating, analyte chemical structure, and matrix complexity [68; 69; 70].  
In order to improve the extraction selectivity and efficiency, novel OT coating materials have been developed. Polypyrrol (PPY) is an electrochemically generated coating that offers ionic, \(\pi\)-\(\pi\) and hydrogen bonding interactions, and its selectivity can be modified by manipulation of the film thickness, which makes it a wide range useful material. PPY has been successfully used in the analysis of catechins and caffeine in tea, wine, and grape juice samples [44]. Another OT microcolumn coating group involves graphene-based materials. [PERSON] et al. [71] covalently bonded a coating containing graphene oxide (GO) monolayers to a fused silica capillary, exhibiting great absorption capacity, stability and high recovery rates between 80-120% for the extraction of triazines in water samples.  
Another alternative to preparing in-tube SPME columns involves the use of filled microcolumns containing a sorbent material in either a particle bed or a monolithic format. Microcolumn drivers may be capillary tubes of fused silica, stainless steel, poly (ether ether ketone) (PEEK) [72] and other hybrid materials like silica-steel tube [63]. The main characteristic of each material varies in its flexibility, mechanical strength, and high pressure and solvent resistance [72]. The sorbent material comprises solid particles with regular or irregular shape, commonly used as SPE or HPLC columns, offering a wide range of commercially available stationary phases and dimensions [72]. [PERSON] et al. [73] packed C18 spherical particles (dp = 10 \(\mu\)m; 100 A pore size) in a PEEK tube (50 mm length; 0.508 i.d) showing adequate results for the analysis of ochratoxin A in wine samples.  
Regarding the monolithic columns, organic polymeric or silica polymeric-based are the most common. These columns can be obtained in situ by a free-radical polymerization method by either thermal or photo-initiation approaches, allowing getting several porous structures and morphologies  
Figure 4: Schematic diagram illustrating the automated on-line in-tube SPME/HPLC technique. Flow-through extraction system (**a**) load position (extraction) and (**b**) inject position (desorption); and draw/eject extraction system (**c**) load position (extraction) and (**d**) inject position (desorption). Adapted from Reference [5]. Reprinted by permission from Elsevier.  
by modifying the energy supplied and the polymerization time. Several monolithic capillaries based on methacrylates, such as poly (methacrylic acid-ethylene glycol dimethacrylate) (MAA-co-EGDMA) and poly (octadec methacrylate-co- ethylendimethylacrylate) (OMA-co-EDMA) have been studied [74; 75]. Furthermore, other modifications in the monolith use carbon nanotubes [64], metallic compounds [65], inorganic oxides [66] and ionic liquids [76].