Roto evap H2O and vacuum questions

[Bill]

Hi Group,

Trying to rotovap a highly aqueous soln. with high concentration of
ammonium acetate (30:70 H2O 100 mM buffer:MeOH). The oil-less pumps
only pulls 5" Hg so I switched to an Edwards 8 oil pump with a -90C
cold trap in line to keep stuff out of the pump.

With the oil pump, I can pull 20" Hg but still having problem
stripping the water off, even at 60-70C water bath temp. I've got the
condenser of the rotovap at -10C and I'm not getting much in the
receiver flask. Even stripping methyl tert. butyl ether (MTBE) at 5"
Hg and -10C condenser, the condensate leaves the receiver flask and
eventually makes it's way past the -90C cold trap. I wish I had a LN2
cold trap!

So long story short... any tips to rotovap water and ammonium acetate
to dryness? As always, thanks in advance to the group for your
experience and suggestions.

Best Regards,

WB

[d***@gmail.com]

Post by Bill
So long story short... any tips to rotovap water and ammonium acetate
to dryness?  As always, thanks in advance to the group for your
experience and suggestions.

At 70 C, it's still going to take awhile. Don't make the condenser too
cold. It will block with ice.

Raising the temp may not help. Don't be surprised if some of the
ammonium acetate comes over with the water. AmAc is often used where
you want a volatile buffer.

DB

[spm_sux]

Post by d***@gmail.com

Post by Bill
So long story short... any tips to rotovap water and ammonium acetate
to dryness?  As always, thanks in advance to the group for your
experience and suggestions.

At 70 C, it's still going to take awhile. Don't make the condenser too
cold. It will block with ice.
Raising the temp may not help. Don't be surprised if some of the
ammonium acetate comes over with the water. AmAc is often used where
you want a volatile buffer.
DB

Thank you DB! Great info from you as usual. I do want to get rid of
all water and NH4OAc... it was originally there as a volatile HPLC-
mass spec buffer, but now I am cutting some fractions and need to
loose stray protons (H2O NH4OAc) for NMR experiments.

The condenser does ice up at the start, but once solvents start coming
off, seems to be ice-free. I've checked for reduced pressure
calculators but think I may have to crack my old P-Chem books to get
the boiling points at reduced pressure. Again many thanks to you and
others who reply.

[d***@gmail.com]

Post by d***@gmail.com

Post by Bill
So long story short... any tips to rotovap water and ammonium acetate
to dryness?  As always, thanks in advance to the group for your
experience and suggestions.

At 70 C, it's still going to take awhile. Don't make the condenser too
cold. It will block with ice.
Raising the temp may not help. Don't be surprised if some of the
ammonium acetate comes over with the water. AmAc is often used where
you want a volatile buffer.

Thank you DB!  Great info from you as usual.  I do want to get rid of
all water and NH4OAc... it was originally there as a volatile HPLC-
mass spec buffer, but now I am cutting some fractions and need to
loose stray protons (H2O NH4OAc) for NMR  experiments.

If you can use ammonium carbonate instead, it comes off a lot easier.

I don't know what your product is, but consider ion exchange to
concentrate it and perhaps lose the salt, too, before concentrating.

DB

[Borked Pseudo Mailed]

Post by Bill
So long story short... any tips to rotovap water and
ammonium acetate to dryness?  As always, thanks in advance
to the group for your experience and suggestions.

What are you removing the water / AmAc from and how much?
It will take overnight (depending on the volume), but why
not borrow a place on someone's lyophilizer?

[spm_sux]

Post by Bill
So long story short... any tips to rotovap water and
ammonium acetate to dryness?  As always, thanks in advance
to the group for your experience and suggestions.

What are you removing the water / AmAc from and how much?  
It will take overnight (depending on the volume), but why
not borrow a place on someone's lyophilizer?

The composition in the water bath flask is approximately 600 mL of
30%H20 with 100 mM NH4OAc and 70% methanol. The small molecules in
the mix are probably less than 100mg in the total 600 mL mixed
solvents.

Lyophilization came to mind and I tried freezing my samples -70C and
running them in a Savant speed vap, but samples thawed before
evaporating/subliming.

Thank you for the kind reply!

As for DB's suggestion of switching to ammonium carbonate, I'll try
and see how it affects my chromatography. I've played around a bit
with the chromatography using 10 mM versus 100 mM NH4OAc, but the 100
mM needs to be there at least for this project. Personally, I rarely
go over 2 or 5 mM NH4OAc but method was handed to me.

I've tried ion exchange/SPE and it looks promising, I just don't have
the scale to do large volumes. If I had the equipment/setup I would
do wide-bore column chromatography and polish things off with HPLC.
It helps to have the right tools for the task!

Thanks again for all replies!

[Ron Jones]

Post by Bill
Hi Group,
Trying to rotovap a highly aqueous soln. with high concentration of
ammonium acetate (30:70 H2O 100 mM buffer:MeOH). The oil-less pumps
only pulls 5" Hg so I switched to an Edwards 8 oil pump with a -90C
cold trap in line to keep stuff out of the pump.
With the oil pump, I can pull 20" Hg but still having problem
stripping the water off, even at 60-70C water bath temp. I've got the
condenser of the rotovap at -10C and I'm not getting much in the
receiver flask. Even stripping methyl tert. butyl ether (MTBE) at 5"
Hg and -10C condenser, the condensate leaves the receiver flask and
eventually makes it's way past the -90C cold trap. I wish I had a LN2
cold trap!
So long story short... any tips to rotovap water and ammonium acetate
to dryness? As always, thanks in advance to the group for your
experience and suggestions.
Best Regards,
WB

Sounds to me like you have a leak on the rotating seal, and a lot of air is
being pulled in - hence the loss of TBME from the cold trap!

Not a good idea to have a liq N2 trap, you can condense oxygen..... !

[Shankar Bhattacharyya]

Post by Ron Jones

Post by Bill
Trying to rotovap a highly aqueous soln. with high concentration
of ammonium acetate (30:70 H2O 100 mM buffer:MeOH). The oil-less
pumps only pulls 5" Hg so I switched to an Edwards 8 oil pump with
a -90C cold trap in line to keep stuff out of the pump.
With the oil pump, I can pull 20" Hg but still having problem
stripping the water off, even at 60-70C water bath temp.

Sounds to me like you have a leak on the rotating seal, and a lot
of air is being pulled in - hence the loss of TBME from the cold
trap!

I agree there is probably a leak around the seal. A common problem is
wear on the rotating tube which goes through that seal. Take the
rotary evaporator and examine the seal and the tube carefully.

Post by Ron Jones
Not a good idea to have a liq N2 trap, you can condense oxygen...

But it is certainly pretty.

Anyway, the methanol should go off fairly quickly. The bp is only a
bit over 60 C and, with any kind of vacuum at all, it should blow
over. The interaction with water is not enbough to be a major problem
in removing it.

To get rid of the water efficiently use an azeoptrope. If you add
reagent alcohol with no more than traces of water in it you will get a
much lower boiling azeotrope which will take off a good bit of water.
If you use ethanol and benzene you will remove close to 15% water in
the distillate. Benzene has its own toxicity issues, of course.

Once the volume is small enough, just adding acetone and rotovaping
will get rid of modest amounts of water rather quickly.

If you can, find a geek organic chemist to look at your rotovap.
Oraganic chemists use rotary evaporators all the time and get very
comfortable diagnosing problems.

- Shankar (geek organic chemist turned geek analytical chemist. The
last time I diagnosed a rotovap problem was this afternoon.)

[L***@aol.com]

Post by Shankar Bhattacharyya

Post by Ron Jones

Post by Bill
Trying to rotovap a highly aqueous soln. with high concentration
of ammonium acetate (30:70 H2O 100 mM buffer:MeOH).  The oil-less
pumps only pulls 5" Hg so I switched to an Edwards 8 oil pump with
a -90C cold trap in line to keep stuff out of the pump.
With the oil pump, I can pull 20" Hg but still having problem
stripping the water off, even at 60-70C water bath temp.

Sounds to me like you have a leak on the rotating seal, and a lot
of air is being pulled in - hence the loss of TBME from the cold
trap!

I agree there is probably a leak around the seal. A common problem is
wear on the rotating tube which goes through that seal. Take the
rotary evaporator and examine the seal and the tube carefully.

Post by Ron Jones
Not a good idea to have a liq N2 trap, you can condense oxygen...

But it is certainly pretty.
Anyway, the methanol should go off fairly quickly. The bp is only a
bit over 60 C and, with any kind of vacuum at all, it should blow
over. The interaction with water is not enbough to be a major problem
in removing it.
To get rid of the water efficiently use an azeoptrope. If you add
reagent alcohol with no more than traces of water in it you will get a
much lower boiling azeotrope which will take off a good bit of water.
If you use ethanol and benzene you will remove close to 15% water in
the distillate. Benzene has its own toxicity issues, of course.
Once the volume is small enough, just adding acetone and rotovaping
will get rid of modest amounts of water rather quickly.
If you can, find a geek organic chemist to look at your rotovap.
Oraganic chemists use rotary evaporators all the time and get very
comfortable diagnosing problems.
- Shankar (geek organic chemist turned geek analytical chemist. The
last time I diagnosed a rotovap problem was this afternoon.)

Even water aspirator vacuum should be adequate to remove water with no
trouble. With good seals propionic acid bp about 150C can be readily
stripped too. I'd recommend that you lube the steam duct at the points
where it contacts/enters the seal. Solvents can affect the seal too.
Clean it up and put it in a drying oven for a time.Relube and put
back in.

[Daddy Tadpole]

Post by Shankar Bhattacharyya

Post by Ron Jones

Post by Bill
Trying to rotovap a highly aqueous soln. with high concentration
of ammonium acetate (30:70 H2O 100 mM buffer:MeOH). The oil-less
pumps only pulls 5" Hg so I switched to an Edwards 8 oil pump with
a -90C cold trap in line to keep stuff out of the pump.
With the oil pump, I can pull 20" Hg but still having problem
stripping the water off, even at 60-70C water bath temp.

Sounds to me like you have a leak on the rotating seal, and a lot
of air is being pulled in - hence the loss of TBME from the cold
trap!

I agree there is probably a leak around the seal. A common problem is
wear on the rotating tube which goes through that seal. Take the
rotary evaporator and examine the seal and the tube carefully.

Post by Ron Jones
Not a good idea to have a liq N2 trap, you can condense oxygen...

But it is certainly pretty.
Anyway, the methanol should go off fairly quickly. The bp is only a
bit over 60 C and, with any kind of vacuum at all, it should blow
over. The interaction with water is not enbough to be a major problem
in removing it.
To get rid of the water efficiently use an azeoptrope. If you add
reagent alcohol with no more than traces of water in it you will get a
much lower boiling azeotrope which will take off a good bit of water.
If you use ethanol and benzene you will remove close to 15% water in
the distillate. Benzene has its own toxicity issues, of course.
Once the volume is small enough, just adding acetone and rotovaping
will get rid of modest amounts of water rather quickly.
If you can, find a geek organic chemist to look at your rotovap.
Oraganic chemists use rotary evaporators all the time and get very
comfortable diagnosing problems.
- Shankar (geek organic chemist turned geek analytical chemist. The
last time I diagnosed a rotovap problem was this afternoon.)

You need a vaccum gauge - not so easy since mercury went out of fashion. A
corrosion resistant bourdon gauge will show up any obvious faults.

You need a good vacuum for ammonium acetate. As someone else remarked,
carbonate is better. It needs to be fresh though, or you'll get a lot of
carbamide which needs heat to decompose. Bubble CO2 into ammonia just befor
use and keep checking the pH as both components are volatile.

In the good old days biochemists used pyridine acetate. If you redistil both
of the components through a Vigreux column, and your water is pure enough,
you can get good NMR spectra from a milligram of compound recovered from 100
mL or more of solution.

Finally, for water - solvent mixtures, empty and rinse out the receiving
flask when all the solvent has gone. This applies both to rotary and
centrifugal evaporators.

Regards

[spm_sux]

Thank you to all for the great information! I've certainly got some
things to try. I've got a chepo vacc. gauge and I'm pulling 30" Hg
with Edwards oil pump and 15" with Teflon vane pump. Nomograph shows
that the water should come over at 100 torr and 40 C. We've got some
orgo types in the building and I'll hit them up.

My water bath ran dry on the Buchi and I found that I had to manually
'reset" the thermal protection switch... it's kinda spring loaded, one
way. Opened up the temp safety sensor and manually reset and back up
and running. I set up the drip and drain so hopefully won't go dry
again. Thanks again group for the great info.

B