multi-draw

[MK]

Hi,
We are purchasing a HPLC for GMP/GLP use and it should be cabable of 10-500 ul injections. The instrument would be typically used for analytical purposes (injection 10-100 ul) and rarely for small scale preparative purposes (100-500 ul).

Problem is that standard autosamplers are typically for <100 ul injections and being a GMP/GLP instrument we would like to avoid changing around loops/needless. I'we found atleast following choices:

-Big loop and needle of 500 / 1500 ul
-small 100 ul needle, multi-draw option and dual loop system (100 and 500 ul)

Anybody having experience of the multi-draw? Does it work? I'm having doubts of having a big needle and loop and mainly use it for small volumes?

[Tim]

We have to use Multidraw loops for a few of our methods - mainly for low level analytes, e.g. in cleaning validation samples. For this they work fine - it does what it suggests, draws up multiple quantites of sample and ejects them into the larger seat capilary (400 or 1400ul, giving a total possible volume of 500 or 1500ul).

The only issues are that it takes quite a bit longer to perform each injection, since the 1100 has to pull up 100ul, move the vial, eject the volume into the capillary, move the vial back, draw up the next volume, move the vial, etc. This only really becomes an issue where you have a really short run time, since this would increase you injection-injection time.

The other issue, which must be considered is that you need to remove the seat capillary again after you have completed your large volume injections, unless you are going to dedicate a system to just doing large injections. If you don't, you get a large dead volume which causes increased extra-column band broadening and can lead to failing system stuitability parameters (N and T typically).

I don't have any experience of the large loop/syringe option, but guess it would be faster for each injection, possibly give similar problems with smaller injection volumes and also there is a risk of decreased accuracy/precision on low volume injections, since the syringe would only be pulling up a small volume, so would be subject to greater error from the stepper motor/gearing, etc.

[tom_mizukami]

I would recommend you evaluate the 900uL syringe and larger loop. The Agilent web site has some comparative data showing precision at low volumes. As long as you stay above 5uL (or evel less maybe)I don't think you will have a problem with the 900uL system. We have used it routinely in a QC lab for several years.

Both setups will add dwell volume to the system, but not equally. With the 900uL setup all of the added volume is upstream on the sample slug. With the larger seat cap the added volume is between the sample and the column, not a good setup for smaller injection volumes.