Calculation Of Mobile Phase Compositon (Premix + online)

[Billie Smith]

I performed a chromatographic run using the following:

88% Reservoir A
Reservoir A was a premix of water, acetonitrile and phosphoric acid ( 72:27:1 )

12% Reservoir B
Reservoir B contained water

This produced good chromatography. I would like to premix this mobile phase and avoid using online mixing so that I can run the analysis on an older HPLC system.

Could someone please help me with the math needed to determine the new composition. I am horrible at algebra.

Thanks Billie

[tom jupille]

No algebra required. Think of it this way:

For 1 liter of mobile phase, you would have:

• 880 mL of A
  120 mL of B

In your A reservoir:

• 72% was water, so you had (0.72 x 880 mL = ) 634 mL of water (I'm rounding to the nearest mL).
  27% was acetonitrile, so you had (0.27 x 880 ml = ) 238 mL of acetonitrile
  1% was phosphoric acid (0.01 x 880 ml = ) 9 mL of phosphoric acid.

In your B reservoir, 100% was water, so you had 120 mL of water.

Therefore your totals are:

• (120 mL + 634 mL = ) 754 mL of water
  238 mL of acetonitrile
  9 mL of phosphoric acid.

This adds up to 1001 mL, the extra 1 mL is roundoff error (I would make the water 753 mL).

[Billie Smith]

Fantastic. Thanks. You have just made my week and you have saved me from a good deal of embarrassment.