The Bioanalysis in combination with liquid chromatography is utilized for the fast and efficient quantitation of small molecules, peptides, and proteins in complex matrices such as plasma, blood, urine, feces, and tissue. This combination can analyze compounds that lack a specific chromophore. Bioanalysis Journals involve the concept of mass spectrometry in bioanalysis. The field of bioanalysis has matured significantly from early studies in drug metabolism using simple colorimetry. With the proliferation of sophisticated hyphenated techniques linking advanced separations with mass spectrometry and NMR as detection systems, automation and robotics, today’s bioanalyst is well equipped to deal with the modern challenges of analyzing xenobiotics in biological matrices much faster and with a higher level of confidence. Furthermore, bioanalysts are now involved with the discovery, measurement and qualification of pharmacogenomic profiles and biomarkers and, subsequently, the development of diagnostic kits to individualize patient characterization and treatment. Bioanalysis delivers essential information in concise, at-a-glance article formats. Key advances in the field are reported and analyzed by international experts, providing an authoritative but accessible forum for the modern bioanalyst. A regular Bioanalytical Challenges feature provides practical advice and troubleshooting to laboratory based problems from world renowned experts. Identification and quantification of the metabolites of drugs and drug candidates are routinely performed using liquid chromatography-mass spectrometry (LC-MS). The best practice is to generate a standard curve with the metabolite versus the internal standard. However, to avoid the difficulties in metabolite synthesis, standard curves are sometimes prepared using the substrate, assuming that the signal for substrate and the metabolite will be equivalent. We have tested the errors associated with this assumption using a series of very similar compounds that undergo common metabolic reactions using both conventional flow electrospray ionization LC-MS and low-flow captive spray ionization (CSI) LC-MS. The differences in standard curves for four different types of transformations (O-demethylation, N-demethylation, aromatic hydroxylation, and benzylic hydroxylation) are presented. The results demonstrate that the signals of the substrates compared with those of the metabolites are statistically different in 18 of the 20 substrate-metabolite combinations for both methods. The ratio of the slopes of the standard curves varied up to 4-fold but was slightly less for the CSI method.