I have a question, whether i could use freeze drying the contaminated water sample instead of using SPE cartridge for pre-concentration, when preparing water samples for LCMS analysis. In order to reduce the cost involve in sample preparation (as the commercially available SPE cartridges are very expensive and single use only) this idea came to my mind as PFASs are non-volatile compounds. I appreciate your advice with scientific reasoning.
Both freeze-drying and SPE will enable you to concentrate your samples, However, you need to remember that the SPE will also be removing contaminants and analytical interferences. Whether this is required for the PFAS assay is something you will have to test for by doing a back-to-back analytical comparison.
It is also important to remember that SPE tubes are cheaper than guard cartridges and HPLC columns as without the SPE you are going to be making the guard cartridges and column work harder and potentially shorten their lifespan. A more thorough column wash step after each sample is run may also be required and may cost you in terms of additional solvent and run time. Again this is something you will need to test using your samples.
Finally, changing or modifying your analytical will also require you to re-validate your PFAS method and demonstrate it is still fit for purpose.
Charles H Hocart
Thanks Charles for your detail information and i certainly agree with you. However, my application is with synthetically contaminated water (laboratory reagent water spiked with 2 or 3 PFAS compounds) is for my project requirement. As such, i can freeze dry above samples and continue my analysis. Only problem that i have to work out is, how to remove sediments (adsorbent) in the sample as i add adsorbent material (ie. activated carbon ) in to this water as part of my experiment. I am trying to avoid filtration step as the PFASs could stick onto filer and will give false negative results. how can i overcome this issue?
After freeze drying the water sample (even if it is contaminated with other inorganic contaminants/salts), if i dissolve the residue with 100% MeOH, i could easily minimize inorganic contaminates enter into the guard and analytical column as inorganic contaminates very poorly dissolve in MeOH. However, other organic contaminants certainly be a problem if i use water samples contaminated with other organic compounds. under such circumstances i may have to use SPE i guess.
It is important to remove particulates prior to LC, so rather than filter you could centrifuge and inject an aliquot of the supernatant. How long and how hard you need to centrifuge will depend on the size of the particles. With respect to freeze drying, there is one cautionary note and that is your analytes should be involatile.
Yes after freeze-drying you could change solvent and leave the salts behind and then centrifuge. What are the functional groups on the PFAS of interest?
Charles H Hocart
They are Carboxilate and sulfonates.
I would say they are not volatile compounds.
Depends on the size - the small chain carboxylic acids are quite volatile. eg trifluroacetic and heptabutyric acids.
There are volatile sulfonates as well. We run them quite frequently. Again it depends upon the size.
I would have to question using laboratory grade water for your experiment as it would really provide a proof of concept or viability. Spiking samples of the sort you expect would give a better result.
SPE cartridges are reusable. Unless your eluent is 100% MeOH or close to it your chromatography may suffer a bit by dissolving the sampe in anything other than the starting eluent of your gradient.
Solid-phase extraction of water or wastewater sample contaminated with PFAS is required mostly to avoid analytical interference. A simple experiment with Milli-Q water and drinking water spiked with PFAS will give different peak areas for the same concentration of PFASs. Concentrate sample is an added advantage of SPE that helps to measure concentration of PFAS at trace level in the water sample.
Being a surfactant, PFASs strongly interfere with the LC column through the adsorption and desorption process and PFASs are very sensitive to additional components present in the water. Specifically, the sulfonate group of PFASs gives fluctuating results mostly due to the interaction of PFASs with compounds present in the water or due to adsorption of PFASs on the surface of the materials it interacts.
The freeze-drying method must be aided with a method that removes additional components from the water sample. It will not only help to increase the accuracy of the results but also safeguard LC/MS/MS components such as LC column, Mass spectrometer chamber. Thus, will save cost.
Thank you Takshak for your detail information.
What analytical method do you use in your PFAS experiments and how you overcome analytical vulnerabilities?
According to your experience what would be the better analytical technique that you suggest (LC, GC or any other ). Have you tried IC method ?
The details of the analytical method for PFASs determination is available here Article Kinetic model for sonolytic degradation of non-volatile surf...
To overcome analytical vulnerabilities, the use of PFASs labeled internal standard is must during i) sample preparation for LC/MS/MS analysis and, ii) the solid-phase extraction process. Note that the PFOA (13C8) labeled standard should be used for PFOA analysis and PFOS (13C8) labeled standard for PFOS analysis.
LC/MS/MS detects most of the PFASs having a Carbon chain length of more than 3 if the LC/MS/MS method is correctly developed. We were unable to detect a range of PFASs using GC/MS/MS, maybe due to the non-volatile nature of PFASs compound. I am not aware about any Ion-Chromatographic method which detects PFASs compounds.