- September 2007
Lab Tools - Current
tools and techniques for the lab
keep the effects of environmental bounce out of your data.
By Ewen Callaway
Whether it's an NMR or a two-photon microscope,
scientists love toys - at least when they work. Sometimes
the most mundane things bungle technology: environmental vibrations
from cars driving by, central air conditioning, the voices
of the operators, and even the ocean. As instruments become
more sensitive, subsonic rumblings become more insidious,
particularly for nano-technology applications. With many instruments,
such as atomic-force and electron microscopes, cutting down
on vibration is essential to collecting good data. "You
could spend a million or two million on a microscope and have
it rendered useless because of vibration," says Kurt
Alberline, an anatomist at the University of Utah School of
Medicine, who runs an electron microscopy lab.
When researchers suspect vibration is wreaking havoc on their
data, they should identify the origin of the noise or get
an environmental engineer to find it, say scientists who regularly
deal with vibration. For example, Vicki Colvin, a chemist
at Rice University in Houston, noticed images moving around
in a circle on her transitional electron microscope. "It
was like a ghost," she says. Colvin discovered that an
air duct was causing the problem and spent $1.20 on a shield
to divert air away from the scope. "The easiest way to
get rid of vibrational noise is to stop it at its source."
says Larry Cohen, a neuroscientist at Yale University.
The design of a building is critical to the vibration that
reaches an instrument, says Ahmad Soueid, senior vice president
at HDR Architecture in Omaha, Neb., which has designed more
than a dozen nanotech laboratories. Isolating air-handling
equipment from laboratories and using special joints that
redirect vibration to the ground are some of the fixes his
firm uses. Recently, concerns over vibration plagued a $250
million NIH facility under construction in Baltimore. Initial
reports indicated the building's quivers could render confocal
microscopes useless, although later measurements suggested
most instruments will work with proper dampening (www.the-scientist.com/news/display/23495).
There's no universal fix, says David Platus, president of
Minus K, a company that makes high-end vibration-isolation
tables (see How It Works, p. 76). Solutions vary, from cheap
rubber pads that rest under instruments, to the air-cushioned
tables that have been around for 50 years, to tables that
sense vibration and cancel it out. "The more sensitive
the instrument, the better isolation you need." he says.
Selected Vibration Isolation Tables
(3' X 4' table that can bear 200 lbs)
|*Suitable for most jobs and locations, from
AFM to fluorescence microscopy
*Dampens down to -1 Hz
|*Low Cost Requires air/nitrogen tank or
*Vibrations can still be transmitted through the air or
via electrical cords
|Passive mechanical tables
|Minus K Technology
(table that can bear 200 lbs)
|*Dampens down to 0.5 Hz
*No air canisters compressors needed
|*Tables are "tuned" to a specific
weight (Salzburg solves that problem by tuning to a higher
mass, the removing lead weights as new equipment is added.)
*Vulnerable to vibrations that come through the air or
(Table than can bear 200 lbs)
|*Dampens down to 0.5 Hz
*Compensates for vibrations transmitted via air and electrical
*Can connect to a coputer to troubleshoot a vibrations
|*Costs twice as much as air or passive mechanical
User: Kurt Albertine, an anatomist at the University
of Utah School of Medicine
Project: Electron microscopy of lung tissue samples
Problem: Low-frequency vibration causes double and
Solution: Albertine says the best solution for a scanning
or transmission electron microscope is to put the machine
on at least a one foot-thick slab of concrete. Most EM labs
should be on the first floor or in the basement, he says.
To further dampen vibration, users can load their microscopes
on special dollies with thick rubber feet Albertine recommends
consulting with the microscope's manufacturer to find a solution.
"They know their instruments, and they know how to optimize
them despite a poor environment." he says.
|Transmission electron micrographs
of lung tissue: A sharply focused image (left) is unacceptably
blurred when vibration is not controlled (right).
User: Andrew Minor, a materials scientist at the National
Center For Electron
Microscopy, part of Lawrence Berkeley National Laboratory
Project: Imaging ceramics, polymers and nanowires
at a resolution of a few Angstroms
Problem: 'The vibration from your voice will make
your image worthless," Minor says.
Solution: Minor's microscope rests on a floating,
two-story concrete block. In September, his lab will begin
using the most sensitive microscope in the world, able to
resolve half an Angstrom. Such high-resolution microscopes
are specially designed to limit vibration.
User: Larry Cohen, a neuroscientist at Yale University
Project: Imaging calcium- and voltage-dependent fluorescent
dyes in live neurons with two-photon or epifluorescence microscopes
Problem: Even the slightest shaking can compromise
the millisecond-timed measurements.
Solution: Cohen was among the first light microscopists
to tackle vibration, beginning in the early 1970s. He started
with air tables, which remain the industry standard and cost
several thousand dollars, depending on size. The tables dampen
up-and-down vibration well, says Cohen, but they have more
trouble with side-to-side movements. On the advice of an engineer,
Cohen mounted his microscope on top of motorcycle inner tubes
resting on an air table. That solved most of the problems,
but vibration still got through. He next bought a $5,000 table
from Minus K that uses springs and flexors tuned to an instrument's
precise weight, and it has solved his problems.
Brian Salzberg, a neuroscientist at the University of Pennsylvania
who did a postdoc with Cohen in the 1970s, has gone even further
to quell vibration in his lab. "The engineers, bless
them, cleverly located all the air-handling equipment on the
roof, so the building shakes." he says. As a fix, he
rests his microscope on a $10.000 active isolation table produced
by Halcyonics. The table electronically senses vibrations
then mechanically cancels them out. To dampen vibration transmitted
through the air, Salzberg shrouds his set-up in a soundproof
curtain. "That combination works very well for us."
Noise can come from sources other than vibration, Cohen cautions.
Shot noise, related to the movement of photons, can overshadow
vibrational noise. Moreover, when imaging live animals, breaths
and heartbeats add still more noise. Those are problems isolation
tables won't resolve.
User: Vicki Colvin a physical chemist at Rice University
Project: While at Bell Labs, Colvin did holography,
storing mages and data onto pieces of plastic.
Problem: The lasers encoding the images were extremely
sensitive to vibration; slight changes in temperature could
compromise the process.
Solution: "As any physical scientist will tell
you," Colvin says, "you get your best data in the
middle of the night." At 1 a.m. temperatures in the lab
steadied, and she was able to conduct her holography experiments
Atomic Force Microscopy
User: Holger Schmidt, an electrical engineer at the
University of California, Santa Cruz
Project: Imaging nanoscale magnets used for data storage
with an atomic-force microscope (AFM)
Problem: Vibration makes images blurry and useless.
Solution: Schmidt's microscope came with a floating
air table that has worked well to stop vibration. AFM users
demanding better isolation have been known to place their
instruments in pits filled with sand or rubber chips. Another
effective solution is to suspend the microscope in the air
with bungee cords, says Colvin, who also does atomic-force
microscopy at Rice.
Nuclear Magnetic Resonance
User: Stephen Lynch, a chemist at Stanford University
who runs an NMR core
Project: Analyzing proteins and other macromolecules
on a 600 MHz NMR instrument
Problem: The 14-Tesla magnet in his most powerful
NMR is sensitive to even the slightest movements.
Solution: The machine is located in a basement; Lynch
cautions against trying to set up an NMR on higher floors.
Air-cushioned legs included with the NMR help to stop vibration
from reaching the magnet. Slight changes in temperature can
also create vibration, says Mark Kelly, who runs an NMR lab
at UCSF. Even the sun shining through a window will cause
problems, he says.
How It Works:
Passive Vibration Isolation
By Alla Katsnelson
Passive vibrational isolation tables offer the most Vibrational
noise reduction for the price. They work on the same basic
principle as the suspension of a car - though the wheels move
up and down rapidly as you drive over a bumpy road, the spring
supporting the mass of the cab keeps passengers from feeling
the vertical bounce. Unlike air tables, in which air pumped
into the system acts as the spring, and active tables, which
use sensors and actuators to electronically correct for positional
information, passive isolators are entirely passive, as the
The idea is simple, explains David Platus, president of Minus
K Technology, which manufactures such systems. "A passive
isolator can be a piece of cork - as long as it provides a
much lower frequency than the frequency of the vibration you
want to attenuate." The lower the natural frequency of
your isolation system, the lower the frequencies it will be
able to cancel; the bigger the gap between your system and
the noise it's combating, the better the isolation. (Minus
K tables operate at about 0.5 Hz, and start isolating at 0.7
Hz.) Minus K tables combine a stiff spring with a "negative
stiffness mechanism," which effectively loosens the spring
while maintaining its load supporting capacity. For example,
if a 10-pound load would normally deflect the top of the spring
downward by an inch, that same deflection might take just
a single pound.
|1 - Vertical vibrations are isolated by the spring's
interaction with four pairs of flexures. The weight of
the instrument compresses the pre-loaded spring, floating
the isolator and aligning the flexures.
|2 - A squeeze force from another spring, controlled
by the knob O. is applied to the outside of the flexures
via a screw. The "squeezed" flexures constitute
a "negative stiffness mechanism" (NSM) that
acts like the negative of a spring, reducing the stiffness
of the system.
|3 - Four beam-columns connecting an upper and
lower column plate act as a horizontal spring to isolate
the horizontal motion. The beam-columns are vertically
very stiff, but bend slightly in response to horizontal
vibration. The weight on the deflected beam-columns reduces
the stiffness of the spring, making the system behave
like a spring with an NSM.
||4 - The crank moves the base of the spring up
and down to compensate for changes in the weight of the
payload and to keep the flexures in their straight aligned
position. If you increase the weight on the spring (by
swapping a lighter microscope for a heavier one, for example),
its base must be raised by turning the crank clockwise.
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