Sample Dilution and SPME

[chemie1]

I am new to using the SPME sampling technique and have a very basic question about how best to dilution a sample in order to get a better response. I am using the SPME technique to analysis for 1,4 dioxane via GC-FID with 1,4 dioxane-d8 as an internal standard. In a few of the samples I am looking at it appears that some sort of dilution is needed. The approach that has been suggested is to vary the sample amount used. The standard set-up is to pipet 5 ml of internal standard into a vial with 5 g of sample then mix, heat and analyze. I was going to reduce the sample size in half, but I was wondering if it would be necessary to add 2.5 ml of water to the vial in order to make up the volume. Or is there a better way to do this? I am looking at shampoos.

Thanks for your help.

[DSP007]

Here, here is some nuance - pipette to 5 ml of metrological described for the volume of 5 ml. It is for the volume of 5 ml pipette has the smallest error. Which is known (about 0.2 ml).
When taking away 2.5 ml volume measurement accuracy will be worse and the precision error will not be known.
Is it critical? If you do not attempt to get into a narrow range of convergence of the results of the analysis - can be uncritically.

As the saying goes, " Free = Will, Rescued =>paradise", but "Seven measure, before cut ."

[Peter Apps]

Do you put the SPME fibre into the sample, or into the headspace ?

What is your internal standard dissolved in ?

Why do you think that you need dilution ?

Replacing 2.5 ml of shampoo with 2.5 ml of water will almost certainly change the partition of analyte into the fibre.

Peter

[chemie1]

Peter -

To answer your questions -

The fiber tip goes into the headspace.

Internal standard is in water.

Dilution is needed in opinion for several reasons - peak resolution, to bring in-line with calibration, to minimize interference and it was what was suggested by power greater than me. So it must be done.

Thanks

[Peter Apps]

So I am presuming that you plan to run samples, and if the peaks are too big, to do another preparation at a higher dilution and run them again.

This uses up a lot of time, and still cruds up the system with interferences.

Do you have any spare sensitivity when you run samples with low concentrations of target analyte - in other words do you have nice big analyte peaks with the samples that have the lowest level of analytes ? If you do you can dilute ALL the samples more than you do at the moment, and run them all once, with one calibration.

If you have to shoot and then dilute, I would dilute with water and run a separate calibration for the diluted samples. This takes into account the changes in partirion between sample and fiber.

Peter

[chemie1]

As I do not know what each sample is going to look like prior to running them, I have to "shoot then dilute" as you so aptly put it. On the lowest level samples that I have ran so far, there is no room for dilution. I would lose the analyte if I tried.

So, I gather the prep the diluted samples would look like this:

2.5 g sample
2.5 ml water
5 ml IS

Then mix and heat as normal....Of course, I may have to adjusted the level dilution as needed for the sample being ran.

I still have one more question....It was suggested that I just add 2.5 g sample & 5 ml IS. I would think that would not be a great approach. Could the sample be "diluted" that way? I am sorry that I do not understand that implications of prepping the sample in that manner. Would I have to alter how I run my calibration curve for either of these approaches?

Thanks so much....I realize there is some much more I have to learn. I guess it truly is a never ending process.

[DSP007]

Hello. Given the strength and foaminess shampoo - even then it is better to weigh in the balance.

[Peter Apps]

I do not see any major problems with using more IS solution and less sample to achieve the dilution. You MUST have a separate calibration for diluted and undiluted samples.

Peter