How to SUM two SUMmed SQL result fields

I have a simple table structure holding records of reagent_bottles and reagent_aliquots.

Aliquots are created from bottles, and an aliquot record got a foreign key, bottle_id, to a bottles id.

To get the total volume of a particular reagent (counting both the volume in the bottles and the volume in the aliquots), the following query does the job, partly:

SELECT  
      COALESCE(SUM(rb.volume), 0) AS bVolume, 
      COALESCE(SUM(ra.volume), 0) AS aVolume
FROM reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;

returns for the above reagent id (408):

bVolume | aVolume
       2|       6

Adding bVolume and aVolume, is the correct answer, i.e. 8.

Question is, how do I re-write the above query to create a new field, total_volume, summing bVolume and aVolume within the query?

I tried this:

SELECT  
      COALESCE(SUM(rb.volume), 0) AS bVolume , 
      COALESCE(SUM(ra.volume), 0) AS aVolume, 
      SUM(bVolume + aVolume) AS total_volume
FROM 
      reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;

but getting the error, Unknown column, bVolume in field list..!

Answer

Sum the original columns:

SELECT  
      COALESCE(SUM(rb.volume), 0) AS bVolume , 
      COALESCE(SUM(ra.volume), 0) AS aVolume, 
      SUM(COALESCE(rb.volume, 0) + COALESCE(ra.volume, 0)) AS total_volume
FROM 
      reagent_bottles AS rb
LEFT JOIN 
      reagent_aliquotes AS ra
ON rb.id = ra.bottle_id
WHERE reagent_id = 408;