Discussion:
Calculating ppm (by mass of compound) for pure compounds (liquid at room temp) dissolved in a pure compounds (liquid at room temp)
(too old to reply)
manda
2011-03-04 06:32:33 UTC
Permalink
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things. Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight

In the definition of

1 ppm (by mass of f compound) = 1 mg/1 Kg,

1 Kg is the weight of total sample, and hence, it is the combined
weight of hexane + (weight of each H/C dissolved in hexane), right?

Thanks in advance for for conformation or correction.
Bob
2011-03-04 21:00:31 UTC
Permalink
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things. Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point. If you are weighing them, then you
are measuring the actual weight.

Density converts between volume and mass (weight).
Post by manda
In the definition of
1 ppm (by mass of f compound) = 1 mg/1 Kg,
yes
Post by manda
1 Kg is the weight of total sample, and hence, it is the combined
weight of hexane + (weight of each H/C dissolved in hexane), right?
yes


bob
Post by manda
Thanks in advance for for conformation or correction.
manda
2011-03-05 05:41:23 UTC
Permalink
Post by Bob
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things.   Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.

My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C. So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Post by Bob
Post by manda
In the definition of
1 ppm (by mass of f compound) = 1 mg/1 Kg,
yes
Post by manda
1 Kg is the weight of total sample, and hence, it is the combined
weight of hexane + (weight of each H/C dissolved in hexane), right?
yes
bob
Thanks.
Post by Bob
Post by manda
Thanks in advance for for conformation or correction.
nailer
2011-03-05 08:29:23 UTC
Permalink
Hexane is too volatile and is withdrawn due to cancerogenity.
Recommend isooctane.
Is the micropipette calibrated for solvents other than water?
What is an error delivering very small volume of volatile solvent?
Any though about an internal standard?
Post by manda
Post by Bob
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things.   Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C. So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Post by Bob
Post by manda
In the definition of
1 ppm (by mass of f compound) = 1 mg/1 Kg,
yes
Post by manda
1 Kg is the weight of total sample, and hence, it is the combined
weight of hexane + (weight of each H/C dissolved in hexane), right?
yes
bob
Thanks.
Post by Bob
Post by manda
Thanks in advance for for conformation or correction.
Bob
2011-03-05 20:49:48 UTC
Permalink
Post by manda
Post by Bob
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things.   Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C. So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Your logic seems ok.

How much more than 34 mg? (You weighed it on an analytical balance??)

As another person noted, there are some issues in measuring a volatile
hydrocarbon. If you are doing this for a class, I would assume that
proper procedures were presented.


The other poster also worried about toxicity. It is not carcinogenic
(I think), but is weakly neurotoxic (due to metabolism). It is not a
big problem, for occasional use. But it is certainly good to avoid
unnecessary inhalation, and use in hood as much as possible.

bob
Bodo Mysliwietz
2011-03-06 16:31:35 UTC
Permalink
Post by Bob
Post by manda
Post by Bob
Post by manda
other things. Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
because of impurities? You most likely use solvents with a concentration
somewhere below 100.0% (v/v)?
Post by Bob
Post by manda
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C. So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Your logic seems ok.
How much more than 34 mg? (You weighed it on an analytical balance??)
I'm a little bit surpised.

First off all, density of n-hexane is just 0,66g/ml, therefore the
aspirated mass should be less - excactly just 33mg. Additionally, vapor
pressure of n-hexane is much higher than that of water, therefore
aspirated volume should be lowered again.

In fact i would expect a weight less 33mg!

I recommend checking the accuracy of your micropipette using water and a
balance.
Post by Bob
The other poster also worried about toxicity. It is not carcinogenic
(I think),
so far I know it is (weakly) carcinogenic.
Post by Bob
but is weakly neurotoxic (due to metabolism). It is not a
big problem, for occasional use. But it is certainly good to avoid
unnecessary inhalation, and use in hood as much as possible.
ACK.
--
Glück Auf - Bodo Mysliwietz
----------------------------------------
http://www.labortechniker.de/
manda
2011-03-10 15:14:52 UTC
Permalink
Post by Bodo Mysliwietz
Post by Bob
Post by manda
Post by Bob
Post by manda
other things.   Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
because of impurities? You most likely use solvents with a concentration
somewhere below 100.0% (v/v)?
Post by Bob
Post by manda
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C.  So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Your logic seems ok.
How much more than 34 mg? (You weighed it on an analytical balance??)
I'm a little bit surpised.
First off all, density of n-hexane is just 0,66g/ml, therefore the
aspirated mass should be less - excactly just 33mg. Additionally, vapor
pressure of n-hexane is much higher than that of water, therefore
aspirated volume should be lowered again.
In fact i would expect a weight less 33mg!
I recommend checking the accuracy of your micropipette using water and a
balance.
Thanks. I will do that when I get a chance. I didn't trust those
micropipette.
Post by Bodo Mysliwietz
Post by Bob
The other poster also worried about toxicity. It is not carcinogenic
(I think),
so far I know it is (weakly) carcinogenic.
 > but is weakly neurotoxic (due to metabolism). It is not a
Post by Bob
big problem, for occasional use. But it is certainly good to avoid
unnecessary inhalation, and use in hood as much as possible.
ACK.
--
Gl ck Auf - Bodo Mysliwietz
----------------------------------------http://www.labortechniker.de/
manda
2011-03-10 15:12:32 UTC
Permalink
Post by Bob
Post by manda
Post by Bob
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things. Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C.  So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Your logic seems ok.
Thanks for confirming.
Post by Bob
How much more than 34 mg? (You weighed it on an analytical balance??)
Here is the weight of each H/C form C7-C12.
C12: 8.4 mg
C11: 87.4mg
C10: 85.0 mg
C9: 84.1 mg
C8: 81.4 mg
C7: 75.7 mg
Post by Bob
As another person noted, there are some issues in measuring a volatile
hydrocarbon. If you are doing this for a class, I would assume that
proper procedures were presented.
Well, we were told to use gas-tight syringe but the syringe was giving
problem with air trapped at different spot and so some of us use
Micropipet.
Post by Bob
The other poster also worried about toxicity. It is not carcinogenic
(I think), but is weakly neurotoxic (due to metabolism). It is not a
big problem, for occasional use. But it is certainly good to avoid
unnecessary inhalation, and use in hood as much as possible.
bob
Good to know. The analytical balance is not inside the hood. Bummer.
manda
2011-03-10 15:18:35 UTC
Permalink
Post by manda
Post by Bob
Post by manda
Post by Bob
Post by manda
I am making a solution of some hydrocarbons in hexane trying to
eventually get to (via dilution) 100ppm (by mass of compound) to be
analyzed by GC-FID and do method development to later analyze some
other things. Since densities of these H/C are around .7 g/ml,after
weighing them, I use density value to convert to the actual weight
Don't understand that last point.
I understood the professor wrong because he said the weight needs to
be adjusted in making the solution.
Post by Bob
are measuring the actual weight.
So I just use the actual weight do ppm (by mass of compound) for the
starter solution. Than do dilution to get to 100ppm.
Post by Bob
Density converts between volume and mass (weight).
Right.
My calculation (to figure out how much to use) showed that about 25ul
would give 17mg of these H/C.  So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
Your logic seems ok.
Thanks for confirming.
Post by Bob
How much more than 34 mg? (You weighed it on an analytical balance??)
Here is the weight of each H/C form C7-C12.
C12: 8.4 mg
Correction:
C12:88.4 mg
Post by manda
C11: 87.4mg
C10: 85.0 mg
C9: 84.1 mg
C8: 81.4 mg
C7: 75.7 mg
Post by Bob
As another person noted, there are some issues in measuring a volatile
hydrocarbon. If you are doing this for a class, I would assume that
proper procedures were presented.
Well, we were told to use gas-tight syringe but the syringe was giving
problem with air trapped at different spot and so some of us use
Micropipet.
Post by Bob
The other poster also worried about toxicity. It is not carcinogenic
(I think), but is weakly neurotoxic (due to metabolism). It is not a
big problem, for occasional use. But it is certainly good to avoid
unnecessary inhalation, and use in hood as much as possible.
bob
Good to know.  The analytical balance is not inside the hood. Bummer.
Bodo Mysliwietz
2011-03-10 20:20:48 UTC
Permalink
Post by manda
Post by manda
Post by Bob
Post by manda
So when I took 50ul (used micropipet)
of each of these H/C and weighed it on the balance, each of these
weighed way more than 34 mg.
How much more than 34 mg? (You weighed it on an analytical balance??)
Here is the weight of each H/C form C7-C12.
C12:88.4 mg
Post by manda
C11: 87.4mg
C10: 85.0 mg
C9: 84.1 mg
C8: 81.4 mg
C7: 75.7 mg
Hey manda use brain 2.0 ;-)

Let's take account all densities and do a retroactive accounting.
Result: The micropipete aspirates 117 µl ± 1 (except C7 with ~ 111µl)
and works reproducible.

Are you sure that you have adjusted the mp for 50µl?
--
Glück Auf - Bodo Mysliwietz
----------------------------------------
http://www.labortechniker.de/
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