Hi Fruitjuice,
It is my opinion only; perhaps it is time to "Cut Bait" with the 4.6mm * 250 mm Altima 5um C8 column. From what you said above, you are observing peaks for malic (3.272min) and ascorbic (3.186min) acids that are far less than at retention factors of 2 (my calculation is that the values are 0.16 and 0.13 respectively), thus you are not separating either malic or ascorbic acid from the column void volume. Chilling the column may help, but it is not likely that it will help enough, so-to-speak.
Perhaps trying a Bio-Rad Aminex HPX-87H ion-exchange HPLC column at 60 degrees Celsius with 5 mM sulfuric acid as mobile phase would help--I've run this method myself for a subset of the analytes you're working with.
Another possible set of conditions to start with are here, from Agilent:
https://www.agilent.com/cs/library/appl ... 8720EN.pdfI found this also, interesting pdf file:
http://www.cfs.gov.hk/english/programme ... s_2370.pdfon page 12 is suggested using, in series, a C18 mm guard column, a 150 mm C18 analytical column, a 250 mm C18 analytical column and a Dionex OA (Organic Acids) column. Perhaps this is nearly the same as AOAC 986.13, I cannot ascertain this for myself. Seems a bit crazy to me to connect four columns to accomplish this separation, so I kept on looking for other alternatives...
...Restek's Allure Organic Acids phase seems also to be promising:
http://m.restek.com/pdfs/59530.pdfhttp://www.restek.com/Technical-Resourc ... AR2062-UNVThese last two options seem to me to be best, though perhaps the Agilent column could be worked with a bit as well. Best Wishes in your continued method development!