Split arrays and assign them to Variables

Hello, thanks for your help, I believe this code is very close but still having errors on the following line, any help I will appreciate.
strAB[i] = selectedItems[i].Substring(","); Error
I believe   strAB[i] should be like this strAB[2,i]  .......ok  because I always expect pair values example:  {4,23}, {2,18},{4,6} etc
selectedItems[i].Substring(",");   //ERROR IN HERE 
The best overloaded method match for 'string.Substring(int)' has some invalid arguments
 
 
METHOD:
protected void lnkDeleteSelected_Click(object sender, EventArgs e)
{
     if (!string.IsNullOrEmpty(Request.Form["chkSelectedItems"]))
    {
               
       string[] selectedItems = Request.Form["chkSelectedItems"].Split(',');
       string[,] strAB = new string[2, selectedItems.Length];
       for (int i = 0; i < selectedItems.Length; i++)
      {
          strAB[i] = selectedItems[i].Substring(",");
 
      }
   }