HPLC Injection Options

[adam]

Hello All

I wanted to ask the forum about HPLC injection techniques. I am familiar with the standard "full loop" vs "partial loop" techniques. However, I came across something in an old publication called "timed injection", where the volume delivered is a function of the time the valve is left in the inject position. For some reason it doesn’t seem like most vendors are offering this option. But it seems to me that it would offer some advantages, including allowing the absolute least amount of dispersion/mixing with the mobile phase. Has anyone seen this, or know of any HPLC vendors that offer this option.

We are working on a unique application where we want to minimize, as much as possible, the extent to which the sample plug mixes with the mobile phase during the injection process. Any other thoughts about how to minimize this would be much appreciated.

Thank You for any thoughts, suggestions, or if you know where I might further inquire about this!!

[bunnahabhain]

Have you thought about an air pillow before and afer your sample? Create an autosampler program which draws 1 µL air, then 50 µL sample and again 1 µL air. No contact with mobile phase before the sample reaches the column.

[AuroraES]

Hi, I used this option on Chromeleon on a Thermo Scientific Nano Ultimate 3000 system. It was calculated in percentages, so in order for me to deliver a 0.12 uL sample volume, I could use the 1 uL loop, and close the valve after 0.12 % of the loop volume had been delivered. Hope that helps

[lmh]

Very many modern injection systems automatically include the air-gaps at either end of the sample plug to avoid dispersion. It may or may not keep the sample safe all the way to the column though. The moment the valve puts the sample (with its air-plugs) in the stream from pump to column it will be brought up to full pressure, hundreds of bar, which means that a 1uL air-gap will instantly shrink to 0.01uL or less and start to dissolve in adjacent solvent. Ideally 0.01uL is still about 1mm of gap in narrow-bore HPLC tubing, but how quickly it dissolves and vanishes is beyond my guesswork!

I can understand why manufacturers might feel nervous of metering sample by timing of the injection valve. It would make accuracy and precision entirely dependent on pump flow, which would make it impossible to quote the autosampler's precision in its own specification sheet! It also makes you dependent on a very stable flow, which might be true now, but hasn't always been a guaranteed certainty.