Final answer:
The student may be unable to detect nystose and kestose due to suboptimal HPLC conditions or the need for a different detection method, such as LC-MS/MS, which provides higher sensitivity. Adjusting the mobile phase, column type, or detection method could potentially solve this issue.
Step-by-step explanation:
The inability to detect nystose and kestose using HPLC with a refractive index detector (RID), as opposed to being able to detect glucose, sucrose, fructose, and raffinose, might be attributed to several factors. The mobile phase used here is 60% acetonitrile at a column temperature of 25 degrees, which may not be optimal for the separation of nystose and kestose.
Fructooligosaccharides like nystose and kestose often require a different type of analytical method or conditions for detection, due to their distinct physicochemical properties. For example, the use of water or a mix of water with organic solvents such as methanol, as well as different column types like C18, can markedly improve the separation and detection of different saccharides in HPLC methods.
Additionally, higher sensitivity detection methods like LC-MS/MS might be necessary for compounds with low refractive index changes or when refractive index detection is not sufficient due to the low concentrations or poor RI responses of the analytes in question.