Echo-peak Technique
The echo-peak technique is an option in case isotopically labeled standards are not available.
Advantage
In this case a fixed concentration of the analyte is injected time-delayed as internal standard onto the same HPLC-column and therefore the analyte-echo elutes at a different time point than the analyte. Since the analyte of interest is used as internal standard, it behaves in the same way as the analyte during the ionization. The peak ratio of analyte and analyte-echo is used for quantitation.
Disadvantage
Even though the echo-peak elutes closely to the analyte of interest different matrix effects can occur which might cause an impaired ionization efficiency. This should be tested for different matrices. In addition, the echo-peak standard is added by a second injection during the HPLC analysis, therefore no losses during sample preparation are corrected.
Literature Example