Letter to the Editor regarding “A quantitative and comprehensive method to analyze human milk oligosaccharide structures in the urine and feces of infants”

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1. The authors claim the reproducibility and precision of the nano-LC MS to be higher than those of MALDI MS, but only show precision data from MALDI measurements (Table 1). 2. Although quantitation is described to be done by means of deuterated HMO standards, no concentrations or H/D ratios, but only ESI-TOF MS abundances, are shown (Table 3). Using the expression “quantitation” in this context might possibly bear the risk that wrong conclusions are drawn by some readers. 3. Owing to the well-known phenomenon of ion suppression in ESI-MS, which is not compensated by the use of deuterated standards (cf. Duncan [2]), the quality of calibration should be addressed in addition to precision data from the quantifying instrument. 4. The issue of strongly varying water content and, thus, HMO concentrations in urine and feces is not addressed. Therefore, the authors should comment on the practicability of the presented method to obtain true concentrations. 5. As milk, urine, and feces are very complex and distinct sample matrices and, furthermore, numerous preparation steps are required, it seems necessary to present data on analyte recovery, confirming the comparability of the three sample matrices.