One of the most common problems I see in a client built SharePoint environment is multiple site collections comingled in the same content database. While this isn’t the end of the world, it’s a problem because SLAs, security, and even back strategies are often assigned to specific databases based upon the needs of the users that will be using the site collections hosted in each database. If a site collection is not hosted in the database that the admin intended because SharePoint simply picks the first available content database with room the administrator may unwittingly incorrectly back or, even worse, fail to backup a particular site collection because it does not exist in the proper database. This is why I am a proponent of backing up at the Native SQL level but also at the site collection level.
On to the point of this article, how do I use PowerShell to create a site collection and assign the appropriate database
Here are the steps to how you can create Content Database Per Site Collection In SharePoint 2010 using Windows PowerShell