This work is devoted to the quantitative analysis of pharmaceuticals using MS detector connected to HPLC. A theoretical model for constructing a calibration curve for MS quantitative analysis using electrospray ionization has been developed. The model's practical applicability is illustrated with examples. The applicability of the proposed model was tested for both cations and anions. The influence of the composition of the mobile phase, which consisted of water-methanol solutions of three modifications, was considered: neutral, acidified with formic acid, and alkalized with ammonium hydroxide. The test results are summarized in tables. The practical application of quantitative MS analysis is demonstrated using the example of determining inositol in pharmaceutical products.