foaming in IC samples - inaccurate reading vol. flask

[Arne]

Hi all,

we are experiencing some issues with sample prep for IC. Our protocol involves weighing in an amount of sample and then filling a volumetric flask to the line with water. With a new type of sample we are getting severe foaming and thus the determination of the added volume of water is inaccurate.

What should we do? Is there a defoamer used in sample prep that is non-ionic and won't interfere with IC? Trace cations/anions are ok since we are looking at %-level anions/cations.

Thanks a ton in advance!

[Peter Apps]

Take your pick; https://www.google.com/search?q=laborat ... s-wiz-serp

[Arne]

Thanks Peter,

we were not sure about compatatbility with the column an such. I talked to metrohm and they warned us to use a pre-column if we use a defoamer.

[DR]

Sonicate samples briefly to break up foam, or apply a mild vacuum briefly when flasks are nearly full, or ignore the foam and dilute to mark. This may introduce a slight bias, though, so you have to be consistent about it.

If defoamers and the above fail, use an internal standard.

[Consumer Products Guy]

Our protocol involves weighing in an amount of sample and then filling a volumetric flask to the line with water. With a new type of sample we are getting severe foaming and thus the determination of the added volume of water is inaccurate.

I worked in soap and detergent manufacturing, and we had to deal with this pretty much every day.

I would suggest weighing in the sample, adding a small amount of water and gently swirling, then gently add water to the mark before mixing.

Another way might be to add a small stir bar and gently stir, then use an external magnet to "walk" the added stir bar above the volumetric mark, and gently add water to the mark; then lower the stir bar and stir.

[LALman]

If it will not be adverse to your column how about a spritz of isopropyl alcohol in the neck of the volumetric? When I was running ICP-MS metals I was using BRIJ 35 as a surfactant to keep the ICP-MS spray chamber running smoothly. When it came time to neutralize the waste in a poly tub; it foamed something fierce. But a spritz of isopropyl from a spray bottle cuts that foam right away and keeps it down.

[Peter Apps]

Add a measured weight of water, correct for density at your room temperature if you want to be really accurate.

Peter

[Consumer Products Guy]

If it will not be adverse to your column how about a spritz of isopropyl alcohol in the neck of the volumetric? When I was running ICP-MS metals I was using BRIJ 35 as a surfactant to keep the ICP-MS spray chamber running smoothly. When it came time to neutralize the waste in a poly tub; it foamed something fierce. But a spritz of isopropyl from a spray bottle cuts that foam right away and keeps it down.

We did similar as well.

[Consumer Products Guy]

Add a measured weight of water, correct for density at your room temperature if you want to be really accurate.
Peter

We had at least one assay where standards and samples were made up weight/weight. But I cannot remember which assay(s) offhand.

[Peter Apps]

Add a measured weight of water, correct for density at your room temperature if you want to be really accurate.
Peter

We had at least one assay where standards and samples were made up weight/weight. But I cannot remember which assay(s) offhand.

In a previous incarnation I converted a few methods from volumetric to gravimetric, with actual weights entered into a spreadsheet as correction factors. All the gravimetric versions were faster, and more precise and accurate than the volumetric versions. In a world with fast electronic balances and computerised calculations I cannot understand why volumetric methods are still the norm.

[Consumer Products Guy]

In a previous incarnation I converted a few methods from volumetric to gravimetric, with actual weights entered into a spreadsheet as correction factors. All the gravimetric versions were faster, and more precise and accurate than the volumetric versions. In a world with fast electronic balances and computerised calculations I cannot understand why volumetric methods are still the norm.

Good point. We "trusted" volumetric flasks to be exactly accurate no matter how many wash/heat/cool cycles they had endured.

For many cGMP assays (and others) I changed sample preparations to use a weighed sample in a 20ml disposable vial and then a precise volume dispensed in automatically; then the vial was capped and mixed, much faster than using volumetric flasks, saving on solvent costs and disposal cost, and saving labor to wash flasks. When a pointy-aired supervisor was brought in, he couldn't come to grips that since our sample weights could vary a little and be within the required amount to be weighed, and a fixed amount of solvent accurately added, that TECHNICALLY the volumes of the preparations would be a tiny amount different - but negligible. He assigned three employees the task of preparing and running numerous blind samples both ways so he could evaluate the data and prove me wrong. But instead he found my way was both more accurate and more precise.

I won't even go into boss's opposition to me preparing buffered HPLC mobile phases and other buffers by weight after such had been validated (he wanted to stay with standardizing pH meters, sticking electrodes in and adding buffering chemical dropwise until required pH was attained) - because he'd never heard of that !

Boss likely wanted us to document in a logbook every time we sneezed.....

[Peter Apps]

I had a real struggle explaining a multi-step gravimetric method to a client's chief chemist.

[DR]

"How many zeroes do you thing a 50mL VF is good for?" - usually a decent place to kick off a conversation about the merits of going gravimetric.
Some will ascribe way too many to those things, assume perfect technique etc.

The other benefit of going gravimetric is that reading a meniscus properly is removed from among the analyst to analyst variables.

I've encountered several younger chemists who couldn't get more than a couple of 9s in their r² values as a result of poor use of volumetric flasks and pipets. For a UV method, the difference between a good curve and a marginal one (for me) is whether I get more than five 9s in my r².

[Consumer Products Guy]

I've encountered several younger chemists who couldn't get more than a couple of 9s in their r² values as a result of poor use of volumetric flasks and pipets.

I had to train quite a few lab analysts at our QC facilities as well...."fun"/challenging informing someone who has done such techniques incorrectly for decades that his/her techniques needed improvement....

[DR]

I had to train quite a few lab analysts at our QC facilities as well...."fun"/challenging informing someone who has done such techniques incorrectly for decades that his/her techniques needed improvement....

True story from the UV vault - excited new tech told his mentoring chemist "I got five 9s on my standard curve!!"; she replied "You'll do better next time."

Savage.