Chromatography Forum

Figured it out.

figured.

Hi Sandra

ppm (parts per million) is not a proper unit (and I am surprised that you are being asked questions in that form), but your thinking that in mass fraction terms it is 1 mg/kg is correct.

Having got to using mass fractions, why then not do everything by mass instead of having to use densities to convert back and forth between mass and volume ?

I have to warn you though that most chemists are so programmed to do dilutions volumetrically that you might have to argue your case with whoever posed the question.

Whenever you are aiming for a very dilute solution the clever part is in designing a dilution scheme that provides sufficient accuracy while not consuming huge volumes of solvent. Using your suggestion as a starting point; 0.1 mg is very difficult to weigh accurately (and more especially so since you have volatile liquids to contend with). Depending on skills and balance resolution, 100 mg would be a better starting point, but that would need 1 kg of hexane to dilute in one step. 100 mg dissolved in 10 g, then a portion of that diluted 100:1 would get to the same concentration using about 1/10 of the volume of hexane.

Peter

Hi Sandra
Having got to using mass fractions, why then not do everything by mass instead of having to use densities to convert back and forth between mass and volume ?

Glad you pointed that out. I woke up remembering that I forgot to mention that we'd be weighing the mass to make the solution.

Whenever you are aiming for a very dilute solution the clever part is in designing a dilution scheme that provides sufficient accuracy while not consuming huge volumes of solvent. Using your suggestion as a starting point

Yes, I planned to start at higher concentration and dilute to the needed 100ppm to be analyzed in GC. In the process of checking whether my calculation procedure was the way to go, I forgot to mention that.

0.1 mg is very difficult to weigh accurately (and more especially so since you have volatile liquids to contend with).

Thanks for the point about volatility. In fact, I have a question. Would preparing the starter solution and kept in the fridge a couple of days before analyzing be a bad idea? This is due to instrument unavailability on the day of our lab hours. The professor has asked me to analyze it by coming to the lab during off class hours since my work schedule is flexible.

Whoa - this started off as a question about calculations, now suddenly you have to make up solutions in real life !

What balance do you have ?, with what readability ?. Is it in a temperature controlled balance room ? What accuracy are you aiming for (does your prof expect) ?. Weighing volatile liquids is tricky. How good are you at weighing solids and non-volatile liquids ?

Time for some more lateral thinking:

Since ppm is not a proper unit anyway it does not have to be mg/kg, it can just as well be microlitres per litre, or micromoles per mole (that's why it is not a proper unit). So you can do the whole excercise volumetrically without worrying about density. It might be aninteresting excercise for different people to do it by the different routes and compae the results. And to anticipate a protest that I said that gravimetrics was the way to go - you were the one that introduced units of mass.

You should mention to your prof that real chemists use SI units, not ppm. And for sound reasons of safety do not do lab work after hours on your own.

Peter

Whoa - this started off as a question about calculations, now suddenly you have to make up solutions in real life !

Yup, I was trying to figure out the calculation in order to make the solution. I was having a flu - still am - and so didn't have energy to elaborate that in those posts last night.

What balance do you have ?, with what readability ?.

I don't remember the brand name but it looks like the top one at this link: http://tinyurl.com/6fvnx3m

Is it in a temperature controlled balance room ?

I am not sure whether the room where the balance is in is temperature controlled.

What accuracy are you aiming for (does your prof expect) ?.

As accurate as possible since we are using FID in this GC analysis to determine the utility of FID as a carbon counter.

Weighing volatile liquids is tricky. How good are you at weighing solids and non-volatile liquids ?

When I did graduate research way back, I did have to weigh solid (powder) A LOT.

Time for some more lateral thinking:

OK

Since ppm is not a proper unit anyway it does not have to be mg/kg, it can just as well be microlitres per litre, or micromoles per mole (that's why it is not a proper unit). So you can do the whole excercise volumetrically without worrying about density.

Sounds good to me.

It might be an interesting excercise for different people to do it by the different routes and compare the results.

I doubt that other students would be cooperative. They're more worried about their grades.

And to anticipate a protest that I said that gravimetrics was the way to go - you were the one that introduced units of mass.

I would have preferred doing it volumetrically but the others are doing it with mass, making me think that's what the prof. expected. I can ask him about that.

You should mention to your prof that real chemists use SI units, not ppm.

Can I wait till the semester is over?:)

And for sound reasons of safety do not do lab work after hours on your own.

I won't. When I do lab work outside class hours, it'd still have to be during day time when the prof. around or stockroom is open.

Time for some more lateral thinking:

Since ppm is not a proper unit anyway it does not have to be mg/kg, it can just as well be microlitres per litre, or micromoles per mole (that's why it is not a proper unit). So you can do the whole excercise volumetrically without worrying about density.

It seems that density is to be used in calculating for ppm while the solution is to be made volumetrically.

I did entertain that thought initially before I posted what I did.

Hi Sandra

There are a number of ways of skinning this particualr cat. I thnk that the first thing that you have to do is to find out exactly what the assignment is.

You only need to use to density to convert volumes into masses if you want the solution to have a particular mass fraction composition. ppm in volume terms is just as valid as ppm in mass terms - but the two solutions will have different compositions, which is why ppm is not a useful way of expressing composition.

NB if you do use density, you have to have figures for density at the temperature of the solution - these organic liquids have quite large coefficients of thermal expansion.

The important thing about the balance is its resolution (I am assuming here that it is properly maintained and calibrated) - how many decimals does it read to ? Your link takes me to the Google home page in isiZulu !

I wish good luck to your colleagues who are going to do this by weighing.

Since you want to examine the FID as a carbon counter what you actually need is a solution with a known and roughly equal mole fraction of the two test compounds. You could do this volumetrically if you knew their molar volumes.

I don't want to spoil the experiment - but an FID is not a carbon counter although it might look as if it is for the two compounds that your prof has selected - there was a recent post on exactly this with some very solid data from the 1960s (?).

Peter

Peter, sorry to hijack a thread, but if you have a link to the discussion on FID and what it's actually detecting, I'd be very grateful.

(background: I have a long-running fight going on here about comparing GC-FID and GC-MS results, and the need/otherwise for calibration. It relates closely to what Sandra is trying to do: client some while back got coworker to analyse some standards from a major chemical supplier (mixed standards quoted in mass percent); results did not match standards; I suggested calibration; our boss vetoed this, put on a cunning grin, and asked how I knew the standards were more reliable than peak area percent by GC-MS from coworker's runs. Chemical supplier unsurprisingly vigorously defended their ability to make and check mixed standards. Boss now tells me he will put it in writing in my annual appraisal that I failed to take action. I am hopping mad. Nothing to do now, but I want to be better informed should it happen again.).

Peter, sorry to hijack a thread, but if you have a link to the discussion on FID and what it's actually detecting, I'd be very grateful. This was the one that I was thinking of: viewtopic.php?f=2&t=15747&p=75878&hilit ... er#p75878- the paper that Oleg links to is clear (and very old) evidence that an FID does not just count carbons. There is a lot of other literature that supports this - I did an intensive review in my previous job but I am pretty sure that I left all the literature behind. There are lots of other threads as well.

(background: I have a long-running fight going on here about comparing GC-FID and GC-MS results, and the need/otherwise for calibration. It relates closely to what Sandra is trying to do: client some while back got coworker to analyse some standards from a major chemical supplier (mixed standards quoted in mass percent); results did not match standards; I suggested calibration; our boss vetoed this, put on a cunning grin, and asked how I knew the standards were more reliable than peak area percent by GC-MS from coworker's runs. Chemical supplier unsurprisingly vigorously defended their ability to make and check mixed standards. Boss now tells me he will put it in writing in my annual appraisal that I failed to take action. I am hopping mad. Nothing to do now, but I want to be better informed should it happen again.).

It sounds as if your boss and Sandra's professor have a lot in common As far as I know a mass spectrometer is even more selective than an FID, and unless the nature of the compounds was such that they could all be expected to give the same response per mole or per unit mass, then relative peak areas on a TIC would not reflect the composition of the mixture. To determine whether or not the response factors are equal you have to do a ...., what is it now ? ..... ah yes, a calibration !

Peter

Peter, thanks so much for that reference and link. Calibration!? Strewth!

Sandra, good luck in making your standard solutions. Making an accurate solution of a volatile liquid is surprisingly challenging, and there should be no shame in admitting it.

....the first thing that you have to do is to find out exactly what the assignment is.

This lab experiment introduces us GC. The detector used is FID. In brief, the experiment is this: separate a mixture of C7to C12 aliphatic hydrocarbons dissolved in n-hexane. (Students will make this mixture solution using pure liquids.) Then investigate the response of FID towards various organic functional groups.

The standard sample is to be analyzed 3 times. To be able to average the duplicate runs, all the peak areas are to be normalized using dodecane peak area, i.e. use dodecane as an internal standard. We are to convert the ppm of compounds in the solution to ppm of carbon for each compound and then divide the average normalized peak areas by the mixing ratio (ppm). Once convinced that FID responds consistently per carbon atom, we are to go to stockroom and look through the list of chemicals in their inventory and find aliphatic H/C that has one or more alcohol, ketone, or aldehyde functional group, making sure that the bp of the chosen compound is > than heptane and < than dodecane. Then, make a solution containing this chosen compounds and the two straight-chain H/C that brackets its bp (but don't use heptane since it volatilizes easily), all in that 100ppm compound range. Analyze it adjusting oven temp so that all the peaks are well resolved. Find out whether the new compound elutes where expected. Make three replicate analyses and determined normalized area/ppm carbon using one of the two straight-chain H/C as the normalizing compounds. Compare the response of the functional group to that of straight-chain H/Cs. Also determine effective carbon number for the functional group compound chosen. Based on the result of the first part of the experiment, is the ECN statistically different than for a straight-chain H/C with the same number of carbon atoms.

Now about preparing the standard mixtures, here is the instruction: Make a solution containing 100ppm (by mass of compound) of each of the 6 straight-chain H/C. (Since separation corresponds to boiling points, no need to make individual solution of each compound for identification). Use gas-tight syringe ( at least 100ul) to measure the volume of each pure compound. ........... It is suggested that you start by making a concentrated solution then dilute to the appropriate concentration for the GC analysis.

You only need to use to density to convert volumes into masses if you want the solution to have a particular mass fraction composition.

That seems to be the goal of the prof.

ppm in volume terms is just as valid as ppm in mass terms - but the two solutions will have different compositions, which is why ppm is not a useful way of expressing composition.

Got it.

NB if you do use density, you have to have figures for density at the temperature of the solution - these organic liquids have quite large coefficients of thermal expansion.

So, to make a "certain" ppm (by mass of compound), I calculate how much (in ul) of each compounds I need to use and hence need to use density values, right?

The important thing about the balance is its resolution (I am assuming here that it is properly maintained and calibrated) - how many decimals does it read to ? Your link takes me to the Google home page in isiZulu !

Sorry. It is 4 decimal values.

I wish good luck to your colleagues who are going to do this by weighing.

I am sure that they would change their strategy soon if not already.
[/quote]

Since you want to examine the FID as a carbon counter what you actually need is a solution with a known and roughly equal mole fraction of the two test compounds. You could do this volumetrically if you knew their molar volumes.

I made it simple in the post by stating only 2 test compounds. It's actually 6.

I don't want to spoil the experiment - but an FID is not a carbon counter although it might look as if it is for the two compounds that your prof has selected - there was a recent post on exactly this with some very solid data from the 1960s (?).
Peter

I will read up on that. Thanks a lot for pointing this out.

Peter, thanks so much for that reference and link. Calibration!? Strewth!

Sandra, good luck in making your standard solutions. Making an accurate solution of a volatile liquid is surprisingly challenging, and there should be no shame in admitting it.

Thanks for the good luck. And yes, I am nervous about making accurate solution of a volatile liquid.

You only need to use to density to convert volumes into masses if you want the solution to have a particular mass fraction composition.

Sorry about the typos and redundant words. Flue always makes me cat funny. So after the correction, here it is:

Hi Peter,

To make 100ppm (by mass of compound) of heptane in hexane:

Since 1 ppm = 1mg/L for an aqueous solution with density of 1.00 g/ml, I would convert 1L to Kg.

Since I am using 25 ml Vol Flask, first redefine 1 ppm to be .025 mg/ .025L

Since hexane is the solvent, use its density value to do convertion: 0.025 L = .0167Kg

So 1 ppm = .025 mg/.0167Kg
Then 100ppm = 2.5 mg/ .0167Kg

Now convert 2.5 mg of heptane to equivalent volume using its density (0.684g/ml) = 3.65 ul.

But because 3.65ul is too small to handle, use 10 times to make 1000ppm.

What I will work on is to get the actual ppm of the final solution to be as close as possible to 100ppm.

BTW, the prof does want us to use the balance to get accurate weight. It can measure with one decimal place.

You only need to use to density to convert volumes into masses if you want the solution to have a particular mass fraction composition.

Sorry about the typos and redundant words. Flue always makes me cat funny. So after the correction, here it is:

Hi Peter,

To make 100ppm (by mass of compound) of heptane in hexane:

Since 1 ppm = 1mg/L if you are aiming for a mass fraction then this is wrong - L is not the unit of mass, Kg is the unit of mass. This then follows through the rest of the calculations for an aqueous solution with density of 1.00 g/ml, I would convert 1L to Kg. you are making this way too complicated, don't worry about aqueus solutions, you are working with organic liquids

Since I am using 25 ml Vol Flask, first redefine 1 ppm to be .025 mg/ .025Lunits, as above

Since hexane is the solvent, use its density value to do convertion: 0.025 L = .0167Kg no, decimal point has moved, and this carries into the rest of the calculations

So 1 ppm = .025 mg/.0167Kg now the decimal is back where it should be
Then 100ppm = 2.5 mg/ .0167Kg sanity check; is 0.167Kg 10 000 times 2.5mg ?, for sure not, you are probably confusing mass:mass and mass:volume, and this is why ppm should never be used

Now convert 2.5 mg of heptane to equivalent volume using its density (0.684g/ml) = 3.65 ul. as a sanity check whenever doing dilutions 1ul = 1mg within an order of magnitude

But because 3.65ul is too small to handle, use 10 times to make 1000ppm.

What I will work on is to get the actual ppm of the final solution to be as close as possible to 100ppm.

BTW, the prof does want us to use the balance to get accurate weight. It can measure with one decimal place.

most likely (I hope) this is 1 decimal of mg, in other words 4 decimals of grams. This would be an ordinary analytical balance. The precision of the balance in rough volume terms is 0.1 ul, which is probably about a hundred times better than you can get with a gas-tight syringe. But let's take it that since the prof does not want you to use the balance, then you have to do this by volumes. You have a 25 ml volumetric flask, what is the mass of solution that it will contain when at the mark ? (hint, the densities of hexane and the other alkanes are similar enough, and there is little enough of the solutes, that you do not need to worry about the impact of solutes on solution density). Given the known densities of each of the solutes, what volume of each of them do you need to have in the 25 ml final solution to have a mass fraction of 100 mg/Kg ?. Can you measure that volume accurately ?, if not you need an extra dilution step

Keep it simple.

Peter