This happens usually after you are booked. If you think you are going to be arrested, you should contact a criminal defense attorney to see if there is anything they can do to prevent you from being arrested. Use Avvo to help. Review my web page below to see if it answers any questions you may have.
After they book you, fingerprint you, get your information, see if you have warrants in other places, see if you have any medical consitions so that you are not going to get sick or die in the jail, after shift changes for the guards, after people are fed...they will take care of their stuff first.
That is usually done after your have been booked into the facility. This can take hours as there are often numerous persons in holding cells waiting to be booked in.