# Current Trends in Fully Automated On-Line Analytical Techniques for Beverage Analysis
## 2 On-Line Solid-Phase Extraction (On-Line SPE)
### Automated SPE  
Sample preparation techniques coupled with automated analytical techniques have become popular and have been widely used in recent years, obtaining less contamination and more reliable analytical results. The SPE purpose is mainly to retain the analytes of interest in the sorbent material. Once the solid support retains the analytes, these are further eluted with an appropriate solvent to be detected later; alternatively, they can also be measured directly on the support, which provides better detection limits by avoiding the dilution phenomena involved in the elution process utilizing an optosensor [30]. Automated SPE devices are usually classified as either homemade [30] or commercial [31]. Most on-line SPE procedures consist of SPE columns coupled to liquid chromatography (LC) systems with mass spectrometry (MS) or spectrophotometric detection. In the case of coupling SPE with gas chromatography (SPE-GC), a derivatization process and application of small injection volumes are usually required. Several systems have been developed for on-line SPE-GC utilizing six-port rotary valves as an interface, usually called the column switching approach [32]. Several sample preparation units are commercially available for fully automated for SPE-GC and SPE-LC.  
When SPE is coupled to LC, the system configuration consists of extraction and analytical columns, switching valves, and auxiliary pumps (Figure 1). When the valve is in the load position, the sample is introduced into the SPE column, which has been preconditioned with appropriate solvents; then, the valve is changed to the elution position and the analytes are transferred to the analytical column, through the mobile phase, to be separated and detected. The analytes can be eluted in two modes: Straight-flush or back-flush. In the first mode, the analytes are eluted in the same direction in which they were loaded, and in the second mode, the analytes are eluted in the opposite direction to avoid band-broadening [33].  
Therefore, on-line SPE when compared to off-line SPE offers a number of advantages such as: (1) Reducing sample preparation time as conditioning, washing and elution processes are automatically performed; (2) reduces the risk of sample contamination and analyte degradation; (3) reduces analyte loss by evaporation; (4) cartridges can generally be reusable; (5) improves the accuracy, increases the detector sensitivity and makes it possible to work with much less than 1 mL of sample; (6) decrease solvent consumption and waste disposal costs, reducing operator risk of exposure to infectious or toxic solvents [34; 35; 36; 37].  
However, on-line SPE methods still present some challenges such as optimizing the pre-concentration and elution procedures to achieve satisfactory precision in a single step for different classes of compounds. Additionally, SPE on-line coupled with ultra-performance liquid chromatography (UPLC) presents disadvantages because of the elevated back pressure generated by the high flow rates used in small particle size columns (<2 \(\upmu\)m) [34]. On-line SPE has other drawbacks including a demand for expensive equipment; exhibits low portability; coeluting of matrix interferences; absence of extracts to repeat the analysis; sorbents contamination; among others [35].