How do I determine cytotoxicity?

Oligonucleotides are usually shipped in dry form. After you receive your custom oligonucleotides, you will need to first resuspend them in aqueous solution before being able to use them.

Centrifuging your oligos is first step to resuspending them. To prepare them for shipping, the oligos are put through a dry-down process wherein they form a white flaky pellet that settles at the bottom of the tube. While they are being transported, the pellet can get dislodged from the bottom of the tube. When you open the tube, the dislodged oligo pellets could fly out of the tube because of electrostatic attraction. This could result in significant loss of yield. Centrifuging oligos before opening a tube for the first time will help the pellet to settle down so you can open the tube safely without any loss.

Oligos should ideally be resuspended in TE Buffer (10mM TrisHCl pH 8.0, 1 mM EDTA). This helps to maintain a constant pH that supports oligo stability. 

If you are using water to resuspend oligos, you must make sure to use nuclease-free, sterile water, pH 7.0. For long-term storage, do not resuspend oligos in water from deionizing systems or diethylpyrocarbonate (DEPC)-treated water. Their pH could be as low as 5. Resuspending oligos in an acidic solution may cause DNA degradation over a period of time.  

Oligos should not be exposed to highly basic or acidic conditions at any time during resuspension.