However, in most cases, offline modes of SP-UPLC have to be adopted to avoid peak broadening and distortion caused by excessive volume of eluent [191,192]. In an offline mode, only a small portion of eluent can be injected, which would nevertheless counteract the benefit of enrichment. There are two major obstacles to the online hyphenation of SP techniques to UPLC. Firstly, the overloading problem is much more serious for UPLC due to its narrower and shorter columns compared with traditional LC systems. The pretreated sample solution such as eluents and extracts containing
strong solvents would result in broad and flat-top shaped peaks with low plate counts in the following chromatographic analysis. Recently, there have been a few studies related to the online SPE-UPLC [161,193–195] and SFE-UPLC methods.