I greatly appreciate all the responses received from my query about MAS
air-tight rotors, ampoules, and end-caps (specifically for PENCIL rotors). In
particular, thanks to Sophia Hayes, Ben Chew, Alan Ronemus, Sandy Chudek, Fred
Morin, and Jim Frye. This email is to post a summary of what I learned.
Below is a summary of methods/ideas for sealing rotors, for the record.
Seals:
(1) Pyrex constricted inserts / ampoules (Wilmad)
Seal with a torch (constriction aids in centro-symmetric sealing, crucial for
spinning)
(2) Glass ampoules (Wilmad)
Sealed with epoxy (less reliable, occasional breakage of seal)
Glass blower sealing (more reliable)
Spinning kept down to 3 kHz
(3) Sealing end caps (Varian)
5 mm and 7.5 mm sizes in packs of 10 pairs
Part numbers: 5 mm rotors = MSPA005064, and 7.5 mm = MSPA005063
(4) Kel-F ampoules (Varian: 7.5 mm or home-made: Kel-F rods can be bought at San
Diego Plastics, Inc. fax 619-477-4874 or www.sdplastics.com/kelf.html)
(5) Kel-F endcaps (room temperature studies)
Check that they are tight fitting
(6) Teflon top and bottom spacers with multiple ridges (fins)
Sold by jim_work_at_comcast.net
Or in-house, score end caps about half-a-dozen times, the raised ridges act as
o-rings. This holds out moisture for at least an hour.
(7) Silica tubing (o.d. tube = i.d. rotor)
Rounding of one end of tubing by the glass shop and fit into a preparation
line.
Powder then poured in and the silica sealed. Spun up to 4 kHz.
(8) Alumina Ceramic inserts + caps (ceramic with o-ring or boron nitride)
Inserts were scored around the middle to allow for sample removal at end of run
(broken open in a glove box)
Inserts were made by the machine shop
Caps were bought from Wilmad or Doty
(9) End caps with two o-rings per cap
Use a threaded brass wand to insert to end cap past the first o-ring in a hood
after it’s filled. There’s a small ridge under the first o-ring parallel with
the rotor to allow a very small opening.
Place the sample into a home-made glass “pocket” with vacuum line connections
that remove any residual air/gases/water from the rotor. Screw down on the top
of the glass pocket which applies downward force to seal the second o-ring.
Tests:
(1) Spin a weighed rotor containing acetone for 1 day; no weight loss = no
evaporation = good seal
(2) Known changes in chemical shifts of observed nuclei on air/water exposure
(e.g. Li-intercalated carbon)
Shalom,
Yael
--
Yael S. Balazs
NMR Facility Manager phone: +972-4-829-3748
Faculty of Chemistry fax: +972-4-829-5703
Technion - Israel Institute of Technology email: balazs_at_tx.technion.ac.il
Haifa, Israel 32000 web: http://www.technion.ac.il/~balazs
Received on Mon Nov 22 2004 - 17:28:47 MST