Chromatography Forum

Hi all,

I am presently looking at butyltin/TBT in sediments and water. my samples are extracted in sodium acetate/acetic acid mixture and derivatize with sodium tetraetyhyl borate.

The problem is that I cannot detect TBT and MBT in my CRM. I am using PACS-2 ans SOPH-1 CRMs.

I am analysing my sample extract using a Varian Saturn 2000 GC/MS.

Any advice on sample preparation and analysis would be appreciated.


thanks

Ruben

Hi Ruben,

What extraction conditions do you use when you try to extract from the sediments and how much sample do you take?

From water samples you can directly derivatize with NaBEt4 (after adjusting the pH) and then extract into hexane.

If you give me your email I can send you some extraction procedures.

J

I am constantly looking for ways to improve my organotin method, so would love to exchange ideas.

Do you perform a leach before your extraction step?

I use methanol/acetic acid and tumble for two hours before the derivatisation.

Also - what pH are you performing the derivatisation at?

Hi all,

Thanks for the replies.....

My method is nasically leaching with sodium acetate/acetic acid mixture. filtering the extract, into a volumetric flask containg sodium chloride, and pH adjusting the sample to about 5-6. I then add the sodium tetraethyl borate to the sample and then 2ml of hexane such that the hexane is in the neck of the vol flask. I then sjake the sample for 12 hours; remove as much of the 2ml hexane as possible, and inject 1ul onto the gc/ms. This method is for sediment samples.

Any views/advice on this?

I don't know how much leachate you are derivatising, but i suspect you might get improved results using more solvent, or something with a greater exchange coefficient. I have seen iso-octane used where there is a low leachate volume used. To use a larger solvent volume you would need to separate the solvent and leachate using a sep funnel.

Also, do you agitate after addition of STEB and solvent? A short tumble should help, or a quick shake by hand (if you are not too worried about RSI).

Finally, you may want to try a fresh batch of STEB. It is sensitive to air (it is liable to spontaneously combust), and if it is beginning to yellow then it has probably begun to oxidise, and will not be much use.

Ruben, your method is quite time consuming and I am not sure if you do the derivatization at the correct pH, it should be below 5.4.

Here is how I do the extraction:

I use a mixture of acetic acid/methanol (3:1) for leaching the butyltin compounds from the sediments/biota. I usually take 0.2 g of sample and add 4 ml of the mixture. I perform the extraction in a waterbath at 37ºC for 1 h, shaking the vial from time to time.

For the derivatization I take 0.2-0.5 ml of the extract and add 3-4 ml of the buffer solution (sodium acetate/acetic acid at pH 5.3), 0.2 ml of STEB (2% in NaOH) and 1 ml Hexane (I do this in 7 ml clear vials) and shake for 8 minutes.

The I remove the hexane and do a preconcentration under a gentle gas stream (Ar, N2 or He) and inject into the GC-MS.

I should mention that I do Isotope Dilution Analysis, that means I don't need to do a calibration of the GC-MS, nor do any extraction efficiencies or dilution/preconcentration factors affect the final result. There is an application from Agilent explaining a bit more about the method.

http://www.chem.agilent.com/Library/app ... 7001EN.pdf

I hope this will help you.

J

I have been thinking of using isotope dilution for a while. I have been focusing on using deuterated organotins. I have found ions suitable for quantitation in TBT-d27 that are not present in TBT, but I am concerned about exchange of the deuterated groups with non-deuterated groups. The tin-alkyl bonds should be quite stable but we have always avoided mixed spikes because we thought there was possible exchange of groups.

With the approach you use, can the contribution correction be automatically applied in the agilent data analysis software?

Maybe some clarification first, you can't do Isotope Dilution Analysis with Deuterated compounds. The term Isotope Dilution is used wrong by most people who use deuterated compounds or compounds labelled with C13, in those cases these compounds are just used as an internal standard and they do a calibration to do the cuantification.

In Isotope Dilution Analysis you don't do a calibration, you can cuantify directly. The compounds I use at labelled with Sn119 and you can do Isotope Dilution like you do for example in ICP-MS, but instead of measuring the Sn isotopes you measure a molecular ion.

I think the correction equations could be applied automatically in the data analysis software, they are very simple equations. Though, I don't have much experience with the report options, I usually export the peak areas to an excel sheet.

So do you not create a calibration curve of response factors? By response factors I mean relative ratios of (corrected) abundances

So do you not create a calibration curve of response factors? By response factors I mean relative ratios of (corrected) abundances

No, you don't need to. You measure the relation between natural analyte in the sample and the labelled analyte added to the sample and then you can calculate directly the concentration of your analyte in the sample.

I'm quite busy today, but I can explain in more detail the principle of Isotope Dilution this weekend.

Sorry for the little delay, but as promised here is a short introduction to Isotope Dilution Analysis.

We start very simple. Imagine you have a box with an unknown amount of red marbles, all marbles have the same properties (color, size, etc.) and behave

equally. Now you want to determine the number of marbles in the box. One way would be to count all marbles, but you can commit many errors and very time

consuming, especially if you want to know the uncertainty of your counting and have to repeat it several times.

The easiest way to determine the number of red marbles is to resort to a process of "dilution". In order to do that we take a small bag with a known number

of blue marbles. The only difference between the red marbles and the blue marbles is the color. We mix the blue marbles with the red marbles until we have a

homogeneous mixture.



Once we have a homogeneous of red and blue marbles we grab a handful of marbles and count the red marbles and the blue marbles and we calculate the ratio

between them. Since we know the number of blue marbles we added to the box we can easilly calculate the initial amount of red marbles in the box.



It is important to point out that once the mixture is homogeneous, the ratio between red and blue marbles won't change, even if we lose marbles. Furthermore,

the ratio will be constant independent of the total number of marbles we grab, providing always the same result.

If we consider now that the marbles are an element, the red marbles would represent one isotope of the element and the blue marbles another isotope of the

same element. Applying the same principle described above we are already talking about Isotope Dilution Analysis.

Isotope Dilution Analysis is based on the intentional alteration of the isotopic abundances of an element in a sample by adding the same element which

enriched in one of its isotope, called spike, to the sample.

The initial concentration of the element in the sample can be calculated directly by the Isotope Dilution Equation. As you can see in the equation there is

no parameter that depends on the instrumental sensitivity. Further, the only parameter you have to determine experimentaly is the isotope ratio in the

mixture.



As Isotope Dilution Analysis only requires isotope ratio and mass measurements, the advantages compared with other methods become apparent:

As mentioned before, there is no parameter regarding the instrumental sensitivity, therefore any variation of this parameter due to instrumental

instabilities such as signal drift or matrix effects will have no influence in the final value of the element concentration in the sample.

After equilibration, losses of analyte do not affect the accuracy of the analytical result, because the measurand, the mixture isotope ratio, is equal in all

sub-samples of the mixture, therefore there is no need to know the preconcentration or dilution factors of the sample or to take into account any

non-quantitative seperation, derivatization, extraction or evaporation step.

I hope this small introduction to Isotope Dilution Analysis will help you a bit.

Many thanks for your explanation! I will no doubt have many more questions regarding the intricacies of ID but this will give me something to think about for now

thanks again - i just got home and I can now see the pictures that were blocked on my work computer.

So with organotin analysis, to calibrate you would spike a known amount of sample with a known amount of spike. This would give you the isotope ratio for a known concentration of sample. Then in an unknown sample the ratio will be greater or less depending on the analytes concentration.

Just a few practical questions:

Is the ratio measured from the mass spec at apoint in the GC peak or are the areas of the extracted ion peaks compared?

You extract the masses and integrate the whole peak, then you take the ratio of the areas to calculate the concentration of your analyte.

For example for TBT you would extract the masses 290 and 291 (corresponding to the Sn isotopes 119 and 120) and the integrate the peaks. Your Isotope Ratio would be the relation of the areas 291/290 (corresponding to the 120/119 Sn isotope ratio).

Could you provide me withe equations required for quantitation or direct me to a useful site?

thanks