Protein phosphorylation is one of the most important posttranslational modifications that is critically involved in many significant cellular processes.[1] It is estimated that one third of all proteins in eukaryotic cells are phosphorylated at any given time.[2] Moreover, a single protein can be phosphorylated and dephosphorylated by different kinases and phosphatases, respectively, on different sites at different times. These phosphorylation variations, that is, resulting from a targeted perturbation, can only be detectable if quantitative information is available. Unfortunately, quantification of protein phosphorylation is a very challenging task that is often hampered by low relative amounts of phosphoproteins and a lack of adequate analytical methods.[3] ESI and MALDI molecular mass spectrometry have been successful in identifying and measuring relative changes in quantity of a particular (phospho)protein.[4] Element mass spectrometry (inductively coupled plasma, ICPMS) has also been reported to compute protein phosphorylation stoichiometry by using the relative measurement 31 P/ 34 S and the protein sequence information obtained by ESIMS. [5,6] As this latter approach requires the presence of S-containing residues (cysteine or methionine), it mostly provides the phosphorylation degree of the whole protein studied. Furthermore, sample-preparation steps, such as reduction and alkylation, may strongly affect the P/S ratio obtained, leading to biased protein phosphorylation results.Absolute quantification of phosphoproteins at given phosphorylation sites is much less commonly addressed, and so far reported methods require chemical synthesis (preferably with incorporated stable isotopes) of each individual phosphopeptide, which must already be known. [7,8] In fact, the main limiting factor to obtain absolute and reliable phosphorylation quantifications is the lack of the phosphopeptide and phosphoprotein standards required. Interestingly, the elemental response by ICPMS, when operated under certain conditions, could be directly proportional to the absolute amount of the element introduced (P in this case).[9] Therefore, in contrast to molecular MS techniques, the signal is independent of the species and sample matrix. However, a problem arises when ICPMS is used as an elemental detector in reversed-phase gradients in which the organic content (mostly acetonitrile) of the mobile phase strongly influences the ionization efficiency in the plasma, even at capillary (4 mL min À1 ) [10] and nano (300 nL min À1 ) [11] flow rates. Herein, we describe the addition of a postcolumn sheath flow with a constant acetonitrile content that is able to buffer gradient composition changes. This leads to a constant 31 P sensitivity along the mHPLC-ICPMS gradient, which is required to separate the different tryptic phosphopeptides originally present in the sample digest and the spiked Pcontaining standard. We then investigated the accuracy and precision that is attainable by using commercially available phosphopeptides. Moreover, th...

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Gut und günstig: Die Verdünnung von Kohlenstoffisotopen mit 13CO2 nach dem Säulvorgang zusammen mit gekoppelter Gaschromatographie, Verbrennung und EI‐MS (Ventil in Position B; siehe Bild) ist eine allgemein anwendbare Methode zur Quantifizierung organischer Verbindungen ohne Einsatz spezifischer Standards. Ein GC/MS‐Apparat lässt sich kostengünstig aufrüsten, ohne dass dabei massenspektrometrische Strukturinformationen verloren gehen.

We appreciate the perspective offered by the authors [Aggarwal and You, 2016] on our paper. [Diez Fern andez et al., 2015] Regarding the impact of the uncertainty in the relative abundance of the 46 Ca isotope, the authors suggest that the uncertainty has to be included on all the data presented in the paper. We clarify that such uncertainty does not apply to the reported B/Ca ratios. The reported B/Ca were quantified not using the relative abundances of each isotope but using a set of external calibration standards of known B/Ca ratio, as described in section 3.4 and illustrated in supporting information Figure S3. This is identical to the approach used in other studies of ratios of trace metals/Ca which have also employed 46