I would dry the solvent by passing it through a column of activated alumina then purify as suggested below and store over 3A molecular sieves.
Follow the link to page 143 of Purification of Laboratory Chemicals By W. L. F. Armarego, Christina Li Lin Chai which includes a section on Diethylene glycol.
"Diethylene glycol [111-46-6] M 106.1, f -10.5°, b 244.3°, [cut] Fractionally distil it in a vacuum (b 133°/14mm, 2.5cm x 1.3m heli-grid column), then recrystallize it by partial freezing. [Feldman et al. J Am Chem Soc, 73 4341 1951, Beilstein 1 III 2090, 1 IV 2390.)"