First of all I am a biotechnologist so... I'm not the most skilled person with HPLC. I was running some standards of organic acids (mostly TCA intermediates) and I can't wrap my head around malic acid, which seems to form two peaks, one of which is only detected at the UV detector (210 nm), and not at the RID detector.

Does anyone have any explanation of what is happening? I would also like to mention that, yes, the calibration standard was prepared from pure DL-malic acid. My first thought was that each peak is one of the two forms but if this is the case, then why does one form give such a low signal at the RID?

Details about the analysis Separations were performed at 45°C and the mobile phase for isocratic elution was 5 mmol/L H2SO4. The flow rate was 0.6 mL/min. Column: Aminex HPX-87H Column (300 x 7.8 mm). Injected 10 μL of a 10 mg/mL solution of DL-malic acid