How to save privacy sensitive data separately?

Hello, I am constructing a drug survey at the moment. As a reward my participants can take part in a lottery in order to win a voucher. My problem is that I am not allowed to have data about drug use combined with their Name or E-Mail adress (for teb lottery). So my question: Is there any possibility to have a question at the end of the survey that asks for data which is then kept separately from the rest of the survey, so that one cannot retrace the answers via the e-mail of the respondant. Thanks!