volumetric vs. gravimetric dilution

Discussions about GC and other "gas phase" separation techniques.

17 posts Page 1 of 2
Hello, I am wondering why (GC) standards & calibration curves always seem to be created with volumetric measurements rather than gravimetric. For example, preparing standards as [mg analyte / mL solvent] instead of [mg analyte / mg solvent]. Is there any reason not to prepare standards gravimetrically, as long as it is done consistently? Is there error introduced due to differences in solvent densities?
Couple of reasons that I can think of...
1) Volumetric is just faster as labs are setup so that you can quickly get solutions appropriate for running a curve that should bracket a given sample.
2) Autosamplers and injectors are not gravimetric. For those of us who do not use the same injection volume for everything, this matters.
Thanks,
DR
Image
Hey Bink,

Take it from a guy that has done his share of gravimetric standard preparations, also the number of proper semi-micro balances in the lab is a factor...nothing like waiting in line for the only one of those in the lab to be "open" for use. The line(s) could be on the long side.
MattM
By gravimetric analysis you are 'doomed' to weighing dilutions rather than using a volumetric pipette.
I suspect that it is largely traditional - older chemists will remember when analytical (four figure) balances were literally that - mechanical devices where the weight of a sample was balanced on a beam against reference masses. They demanded a fair bit of skill and were slow to use, and both the balances and the reference masses were expensive. Making up a volume in a volumetric flask was easier and quicker.

We now have electronic "balances" that are the most precise and accurate devices in the lab. Comparing them to the old beam balances is like comparing a modern digital camera to a 1950s full plate camera. The precision and accuracy of a bench-top 4-figure balance is at least 10 - 100 times better than volumetrics. APPS, P.J. and ARCHER, M. 2010. Evaluation of the source of bias caused by losses of solvent vapour during sample preparation. Journal of Accreditation and Quality Control 15: 171–180.

For nearly all applications you do not even need to weigh to a precise target - you get within 10% (say) of the target and record the actual weight, then enter all the actual weights (actual weight of solvent, actual weight of solute for stock standard, actual weight of diluent, actual weight of stock added etc) into a spreadsheet and calculate actual mass fractions. Take actual weights of samples, dissolved in actual weights of solvent, etc. If standards and samples are all done gravimetrically then volumes never enter into the calculation.

Peter
Peter Apps
In our lab we also work with volumetric measurements for samples and standards. In my opinion, working gravimetrically is less phrone to errors but the workload would increase too much.

Bink wrote:
For example, preparing standards as [mg analyte / mL solvent] instead of [mg analyte / mg solvent].


In fact working with volumetric glassware you are not making a [mg x / mL solvent] solution but a [mg X / mL total]. This distinction is negligible when you weigh, for instance, 10mg of standard and add solvent to the 100mL mark. But if you, for instance, weigh 5g and add solvent to the 100mL mark, you do not have a 5g / 100mL solvent solution, you have a 5g / 100mL total solution.

This can give rise to errors, especially if people think that adding to the 100mL mark is the same as adding 100mL! Or what if your glassware is, for whatever reason, damaged or has a wrong mark? What if analyst X interpretation of a volumetric mark is different from analyst Y?

Working gravimetrically, you don't have to care about this. You don't even have to work with volumetric glassware. Like Peter said, you add a +- volume and you work with the numbers you have. However, these numbers have to be recorded throughout the procedure.
I agree with Peter Apps' assessment, remember having those big Mettler balances when I started, with the vernier lines.

I initiated a bunch of assay procedures for the company where a specific range of mixed liquid product was weighed into a disposable vial, then used an accurate dispenser to dispense the appropriate amount of volume of solvent to make a certain total amount, like 25.00 ml; then this was capped and readily mixed. This was faster than using volumetric flasks, and pretty much dummy-proof as the operator only had to press a button, and not worry about whether stuff above a volumetric line got dissolved, if a stopper leaked, if the volume was exactly at the line.

This saved us MILLIONS of dollars over the decades compared to using larger amounts of solvents and then their subsequent disposal costs, as well as reduced labor costs, and cost of our department washing all that glassware. Our cGMP and GLP methods were all developed and validated using these techniques.

I did have a pointy-haired boss who believed that intrinsically that the total volumes would be minutely different if one weighed sample near the bottom of the range compared to sample weights near the top of the range, and I agreed with him in theory, but in reality there was no difference. He sought to "prove me wrong", had some techs assay a whole bunch of samples using this technique and with volumetric flask technique, and the dispenser technique was both more accurate and more precise, and he fumed about that but grudgingly accepted it.

I did develop an assay for some of our liquid laundry detergents for which samples were prepared gravimetrically (solvent added to a total weight of say 10.0 grams on a top-loading balance) because the sample viscosity was too thick to readily get past the neck of a volumetric flask, or slow to dissolve. We just weighed sample within the range and added solvent to get total weight within the window of 9.9 - 10.1 grams and didn't record the weight of the solvent.
We always use volumetric, for one reason all of the EPA methods are written as volumetric measurement units, and also I can purchase several cases of volumetric flasks for the price of a single 4 place balance.

If you inject one microliter into the instrument and your initial standard was prepared as mg/g solvent then you need to correct for the density of each type of solvent you are injecting to know the exact mg on column, but if prepared in a volumetric flask as mg/ml, then it doesn't matter if you inject water or methanol, or methylene chloride, 1ul will always have the same mg of analyte in it on the instrument. Also if you do weight/weight standards you will never have exactly the same weight of solvent in each standard and in your calibration software you will have to change the actual standard concentration each time you make a new calibration. Doing it by volume you can control the actual concentration better and it is less work doing conversions on down the line.

Loss of solvent due to evaporation is also negligible since most lab temps are around 20-25c and the volumetric is open to the air for maybe one minute if you are doing it efficiently. Only something like Freon will evaporate so fast it might be a problem as a solvent. If your 4 place balance is not constantly compensating for changes in lab temperature and vibrations then you will have just as much uncertainty in gravimetric dilutions as you will volumetric dilutions.
The past is there to guide us into the future, not to dwell in.
This is a molarity vs molality discussion. Solution chemistry is well enough characterized that volume or mass or moles per total dissolved volume is most commonly used, especially for dilute solutions where you don't need to worry about activity coefficients and partial molar volumes to back out gravimetric results. You get into molal situations when you are adding solids to solids and have no useful data to predict the volume of the final solid after mixing or melting. Fire assay is one example of an all gravimetric method. As a geochemist, I once fiddled around with my trace and major element rock data to see if molality gave me any insights versus ppm.
James_Ball wrote:

If you inject one microliter into the instrument and your initial standard was prepared as mg/g solvent then you need to correct for the density of each type of solvent you are injecting to know the exact mg on column, but if prepared in a volumetric flask as mg/ml, then it doesn't matter if you inject water or methanol, or methylene chloride, 1ul will always have the same mg of analyte in it on the instrument. Also if you do weight/weight standards you will never have exactly the same weight of solvent in each standard and in your calibration software you will have to change the actual standard concentration each time you make a new calibration. Doing it by volume you can control the actual concentration better and it is less work doing conversions on down the line.

Loss of solvent due to evaporation is also negligible since most lab temps are around 20-25c and the volumetric is open to the air for maybe one minute if you are doing it efficiently. Only something like Freon will evaporate so fast it might be a problem as a solvent. If your 4 place balance is not constantly compensating for changes in lab temperature and vibrations then you will have just as much uncertainty in gravimetric dilutions as you will volumetric dilutions.


James, surely for a given method (with its calibration) you prepare all the samples and the standards in the same solvent - and so solvent density is the same for all runs of that method. Even if you make a mass/volume solution, to know the mass on column you have to know the volume injected, which is accurate only for full-loop injections in HPLC. For all other injections the inaccuracy in injection volume falls out of the calculation as long as you inject the same volume of sample as of standard - which you should be doing - and if you prepare both samples and standards gravimetrically and inject the same volume for both then volume falls out completely.

Solvent evaporation poses the same problem for gravimetric and volumetric preparations - it increase solute fraction after the solutions are prepared. Using micro-syringes and septum-capped containers gets around the problem.

The tightest tolerance on volumetric glassware, even at the smallest volumes for flasks are equivalent to 10 mg or more of water. You can get ten times better than that with a open-pan three-figure balance.

Peter
Peter Apps
In the "old days" of mechanical analytical balances, we most often weighed liquids in disposable syringes with caps, to get constant weights.

Once we got 4-place electronic analytical balance (I pushed for this, we had the first in the whole company) we were able to weigh much quicker, so just used a dropper into a volumetric flask. Sometimes, we would actually add some solvent to the volumetric flask, tare that, then add some sample.

For stuff that was volatile but inexpensive or there was plenty of sample and water was to be the solvent, weighing 10 grams on a top loading electronic balance worked, 4 significant figures.
Hi again, Bink,

I find these of interest:

http://q-more.chemeurope.com/q-more-art ... focus.html

http://www.americanlaboratory.com/913-T ... spectives/

I think it is safe to say that both gravimetric and volumetric standard/sample preparations work and that gravimetric preparation has an advantage in terms of precision and accuracy. (also long-term expense, it seems)

It's a matter of where your firm wants to put its money and time...

This looks like an interesting device:

Quantos

https://www.mt.com/dam/non-indexed/po/l ... ric_EN.pdf

You don't need the fancy gizmo, nor do I work for Mettler-Toledo. Just looks neat.
MattM
I put my paper on gravimetric liquid handling on ResearchGate; https://www.researchgate.net/publicatio ... reparation

it does not compare volumetrics with gravimetrics, but it shows the precision you can get with gravimetrics, even with small quantities of solvent.

Peter
Peter Apps
Thanks to all who replied to this. In my case, I am not so much worried about the increased precision of gravimetric vs. volumetric - I would prefer to do gravimetric dilutions because it uses less solvent and there is no glassware cleaning necessary.

James and Peter discussed the area I am concerned with - whether or not gravimetric dilutions can introduce error based on solvent density. For example, there should be less mass of analyte per unit volume in (1mg analyte / g acetone) solution than a (1mg analyte / mL acetone) solution because acetone's density is less than 1g/mL. Assuming that the same volume of standard and sample are injected, does volume 'fall out' of the calculation?
Hi Bink,

Not to step on James' and Peters' toes, the answer to your question is Yes, volume does "fall out".

Any difference in density between calibration standards will be negligible.

The linearity study seems to clearly indicate this in the chemeurope reference I submitted above. In any case, I've done calibration standard preparation by both gravimetric and volumetric means...both work consistently and well.

Enjoy not having to wash volumetric flasks and saving money on solvent/standard materials!
MattM
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