I'm a new graduate of Chem, and doing some laboratory works before taking my MS. My problem is this:

I was assigned to do plant / herbal extract analysis. What are the general rules whether I use GC or LC in analysis like this?

What do you want to look at in your analysis? There are a million compounds in a plant extract...
If it's not volatile, use LC! If it is, use GC! If you don't know, use LC...

First look the bibliography related with plants (compounds that you should find in a plant extract)

What are your extraction methods? What are you looking for? How much starting material (in grammes) are we talking about? If you start off by doing an organic (eg DCM/CHCl3) extract then MeOH then H2O you should pick up most of the available compounds. You will find that Chlorophyll and tannins interfere with the other compounds on LC and LC-MS as the extinction co-efficient for chlorophyll is very high. You are going to have to remove these interferences before you do other work. LC or LC-MS using reverse phase with a C8/C18 column with a gradient from 95/5 H2O/ACN with 0.1%TFA to 100% ACN with 0.1% TFA (over 10 minutes) will give you a reasonable separation. Some Flash clean up might also be an idea!!

To make your story complete...

For the polar components you may suggest Hydrophilic Interaction Chromatography (HILIC) by Solid Phase Extraction (SPE) or high pressure separation....

Good luck with your studies!

This has nothing to do with an extaction, but I am looking for guidelines to adapt a HPLC method on my system. I have a gradient HPLC method, wich was developed on a system with a zero dead volume of 0.5 ml. My system has a zero dead volume of 5 ml. Is there a rule of thumb wich I can use to adjust the gradient profile?
You comments and suggestions are much appreciated.
Gilbert

Hi Gilbert,
Telling us a bit more about the problem might make it easier to solve.
Where did you get the method from? What are the conditions? Do you have a copy of a testmix solution analysed under the 0.5ml dead volume column. Transfer the method with the same testmix to your system and run it as it is. See what the results are and modify accordingly. Are you using a prep column and scaling up from an analytical. You may need to modify the flow rate to accomodate the change in column.

Gilbert,

I recently read an article from an old issue of LC/GC Volume 18 Number 10 October 2000 p. 1034

The article is titled: "Scaling Gradient LC Methods to LC-MS"

Included is a general discussion of many LC parameters and the mathematical formulas associated with them. Check it out it might help you.

Mike M.

I assume that you are talking about the delay volume between the point where the pump mixes the gradient and the column inlet. This is called the system delay volume, and also sometimes the gradient delay volume.
To convert a gradient from a 0.5 mL delay volume system to a 5 ml delay volume system is impossible unless your system allows you to do delayed injections. If it does, you can make the injection at a time when the gradient is 0.6 mL away from the column. This will work only, if you are working with a low volume injector. If a substantial part of your delay volume is in the injector, the best approach is to reduce the size of the injector loop.
A lot can be done with appropriate tinkering, if your system lets you do this.

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