Dissolving Limonene and Citral

[Tammy]

So I am doing Headspace sampling using a gas-tight syringe. My advisor had originally told me to dissolve limonene and citral into water, until we realized we weren't getting consistent results because they are both insoluble in water. Anyway I told my advisor this and she told me just to use neat compounds, but she wants me to put very little like 100 microliters of citral or limonene into a vial (until we have a chance to figure out an emulsion recipe) into 4 ml or 10 ml vials. I am worried that the sample size is way too small. Would that sample be to small? Would dissolving into something like methanol be a better idea? Or some other solution? Any help or guidance is appreciation

[Peter Apps]

Hi Tammy

100 ul is way too much unles you are working at low temperatures, or injecting very small volumes - i.e. a few microliters. If you dissolve it in methanol you will get a huge methanol peak that will cause problems of its own. You could use a non-volatile solvent, but how you proceed at this stage depends on what the ultimate aim of the study is.

Peter

[Tammy]

Thanks Peter.

The ultimate aim of our study is to look at the degradation products of limonene and citral. Can you recommend any nonvolatile solutions?

[AICMM]

Tammy,

I set up a method to measure menthol and camphor using limonene as an internal standard. To be honest, it did not work very well when I tried headspace, but probably due to the concentration and the matrix it was in. (I also tried purge and trap but that was a mess...) I ended up extracting it in hexane which worked really well. In that case, the limonene was dissolved in methanol to add to the sample. Using a Nukol column (Supelco) gives you really good separation of the hexane from the other three components.

So, one question is, how volatile do you think your breakdown products are going to be, such that you are required to stay with headspace? Do you think the breakdown products will be much more water soluble?

Best regards.

[Tammy]

So, one question is, how volatile do you think your breakdown products are going to be, such that you are required to stay with headspace? Do you think the breakdown products will be much more water soluble?

The breakdown products are volatile, and they will be in the headspace, at least from the literature reading I have been doing. From the expected breakdown products in general they are not more soluble in water; there is one or two products that are more soluble in water, but for the most part they are not.

[Peter Apps]

Hi Tammy

The breakdown products are likely to be as difficult or more difficult than the parent compounds - probably at lower concentrations at least initially as well. Therefore I would suggest that you develop a method for the breakdown products rather than the parents.

If you are going to use headspace, the matrix that you use for method development needs to be the same as (or at least compatible with) the matrix that the citral and limonene are going to be in while they are decomposing.

When you say that you are using a gas-tight syringe, is this on an autosampler ?. If you are trying to do headspace with manual injections bear in mind that it is very difficult indeed to get good quantitation.

Peter

[Consumer Products Guy]

I wouldn't use headspace for this. Dissolve in organic solvent such as methanol or DMF and use non-polar capillary. We quantitate D-limonene this way.

[Tammy]

When you say that you are using a gas-tight syringe, is this on an autosampler ?. If you are trying to do headspace with manual injections bear in mind that it is very difficult indeed to get good quantitation.

No I'm using manual gas-tight syringe. I am a student, so unfortunately we do not have a headspace autosampler at the University.

[Peter Apps]

Hi Tammy

You (and your supervisor) need to rethink this from the beginning. Is there a reason why you need to use headspace, and not a more straightforward and repeatable liquid extraction ? What matrix will you ne workng with ?

Peter

[Tammy]

Hi Tammy

You (and your supervisor) need to rethink this from the beginning. Is there a reason why you need to use headspace, and not a more straightforward and repeatable liquid extraction ? What matrix will you ne workng with ?

Peter

Hey Peter,

I am a graduate student and we decided on headspace because the breakdown products were volatile. There really wasn't any other reason than that. In terms of matrix we'd like to do model citrus drinks, though we haven't finalized the matrix (most likely an emulsion will be used). We'll also be adding natural antioxidants such as rosemary or oregano extract to attempt to slow down the degradation products.

[Peter Apps]

Hi Tammy

Headspace for this analysis has (at least) two major drawbacks; first it will need the samples to be heated, which might well accelerate the degradation that you are studying, second you do not have a headspace sampler and doing it manually is probably not gong to give you the repeatability that you need.

Try liquid extraction - you have to cut your analytical coat according to your instrumental cloth.

Peter

[AICMM]

Tammy,

To answer your original question, methanol should be a good solvent for dissolution but it will show up in the early part of your chromatogram where you might be interested in seeing other things. Depends on your column.

Next, if you are going with headspace, it would be to your advantage to find a good surrogate standard to track your recoveries. You might also consider SPME as an option, there are some papers out there on recovery of components from drinks using SPME.

Purge and trap can still be considered an option if are good with your hands. It would be tedious but manageable. I can provide more info. if you are interested in this route.

Finally, solvent extraction will probably still be the best option.

Best regards.

[Ron]

Just because the compounds are volatile does not mean that headspace or purge and trap is a better sample introduction method than liquid injection.

Your sample in not soluble in water, so you will have to use some percentage of organic solvent to dissolve the samples. The use of a mixed solvent system often leads to partitioning issues and lower sensitivity in headspace analysis. You should at least try liquid injections and see how they work before you spend a lot of time working on a method that may not work.

[Tammy]

Thanks everyone for the information and advice. I am sitting down with my advisor and another teacher on Monday and I will mention these things to them and see from there.