Technically, phosphate buffers can be used. However, a primary benefit of HILIC as a separation mode is mass spec detection of polar analytes, and considering phosphate buffers are non-volatile, this significantly reduces their applicability in a HILIC separation mode.
Furthermore, solubility needs to be considered. Buffer salts can precipitate in high organic solvent concentration and some precipitate earlier than others. For example, ammonium phosphate is known for solubility issues as low as 70% Acetonitrile.
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