Nowadays, online SP-LC techniques have been widely applied to many fields, such as environmental analysis, food analysis, clinical chemistry, pharmaceutical and bioanalysis, etc. They exhibit the obvious superiority to offline methods in the aspects of reagent and sample consumption, speed, sensitivity and precision, as confirmed by some comparison studies. A multi-residue analytical method based on online SPE-LC method has been recently developed and validated for 40 multi-class pharmaceuticals and metabolites for tap and surface waters. This method was very convenient and
efficient compared to classical off-line SPE method because of its shorter total run time and smaller sample volume (1 mL vs up to 1 L) [154]. Two alternatives, off-line and online SPE-LC methods, were proposed for rapid simultaneous quantification of sulfonylureas in natural water, which enabled the 0.1 gL−1 and 1 ngL−1 LOQ
(limit of quantification) levels to be reached, respectively [155]. It indicates that the online method is much more sensitive. Another comparison study shows that the automated online SPE-LC method used approximately 54-fold less solvent than the automated offline SPE method, which required shorter time (107 min vs 372 min) and lower cost for sample collection ($3.00 vs $50.00) [156]. An online SPE-UPLC method has been developed to allow quantification of a 15-mer unmodified DNA oligonucleotide in human plasma, suggesting that the online mode improved assay precision at lower
concentrations [157].