Help on: Problem with the first injection from HPLC vials

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Question: “When I use a single vial for a series of standard injections the area for the first injection is often lower than that for the subsequent injections. What could cause this?"

Answer:
“There are a variety of potential reasons for injector repeatability problems but the most likely explanation when the first injection from an individual vial gives a lower response than subsequent injections is that the vial was overfilled. This creates a vacuum when the needle pierces the cap and withdraws the test solution and thus the syringe does not withdraw the correct amount. On the next injection the vacuum effect is no longer present and thus the syringe withdraws the correct amount. To prevent the problem, take care to leave a small headspace when filling a vial rather than completely filling the vial.”