You may perform quantitation other than stable isotope dilution strategy. Deuterated molecules are more affordable than carbon isotopes. But if you cannot find any alternatives you may consider analog molecules which behave like vitamins. I mean solubility (Log P) and pKa value are very close to target analyzing molecules. By spiking this analog to sample at known concentrations, the extraction recovery and any systemic error can be compensated. Surely it cannot be confident and absolute as isotope dilution but for matrix-matched calibration internal standardization is necessary and this could be solved by this approach. Just make a recovery comparison test to the applicability of the candidate molecule...

Considering the header: are standards needed: yes. ESI is a chemical source and its results will depend on the chemistry of the sample, thus quantitation without internal standards by comparing the signals from a sample to one with a standard can be considerable (factor 10) off. The closer the IS is to the analyte the better it will compensate. Thus 13C labelled compounds are good as long as they have more than three labeled atoms in the molecule (to ensure to be outside normal C isotorpe clusters). Deuterated molecules are also fine (as long as it is ensured that the D is having a stable C-D bond and it is not exchanging such as the H in and OH bond.

If you want to have the accurate method, you need to minimize the error caused by the sample matrix to the ionization of the target compound at the ion-source. Internal standard (C13 or deuterated) is the easy way to fix that if you can find it and it is affordable. The alternative method is using standard addition (please google it for the definition and how to do it), but of course it is time consuming and need more work. You can choose whatever works for you.

In fact, not only the degree of accuracy and precision of the expected composition in the quantification in copper LCMS analysis is important, but also this factor should be considered in addition to the standard samples. it should be compared with the drawing the calibration curve graph, too. The second item has been done in some research studies.

Regards

INTERNAL STANDARD NOT REQUIRED ALLWAYS, IF ANY DIFFICULT IN METHOD THAT TIME USING INTERNAL STANDARD FOR REFERENCE PURPOSE ONLY, OTHER WISE NOT REQUIRED

LC-MS is an analytical chemistry technique that combines the capabilities of physical separation by liquid chromatography (or HPLC) with the capabilities of mass spectrometry (MS).