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Hello, I'm Kristie Diebold, cleanroom manager and senior research microscopist at McCrone
Associates, and I'm also an instructor at Hooke College of Applied Sciences.
Particle picking is the free-hand manipulation of single particles down to less than 1 micrometer.
This technique is used when a single particle needs further preparation for solubility studies,
electron microscopy, x-ray diffraction or infrared analysis.
The tungsten needle is a specialized needle essential for particle picking.
Three different tip diameters are used for different purposes. A large needle has a short
blunt tip and is very sturdy. It is used mainly for scribing on metal surfaces. Medium needles
have a finer tip, used mainly for cutting particles from collodion. Needles with fine
tips are used most frequently for single particle manipulation. Because the tip is rather fragile,
it bends or breaks frequently. It is helpful to keep several different needles on hand.
Some people color-code their needle holders and needle cases based on the size they contain.
To repair or make a tungsten needle, mount a short length of 22 gauge tungsten wire in
a needle holder. Tungsten is used because it can be etched, chemically or electrochemically,
down to a point having atomic dimensions.Ours will be 1-10 micrometers in diameter.
Wear goggles to protect your eyes.
To create the exothermic reaction that etches the end of the wire to a point, the sodium
nitrite has to be molten, or close to molten, and the tungsten needle has to be red hot.
If the needle is held in the lowest part of the flame, it won't become red hot. You have
to find the hottest part of the flame.
Once the needle is red hot, if you hold the sodium nitrite stick away from the flame,
the needle will cool before you can draw it across the nitrite stick. The stick has to
be held close to, or even slightly within the flame.
While delicately striking the sodium nitrite, roll the needle holder in your fingers while
you pull it back so that you do not bias the erosion. If you pull the needle straight,
you will create a micro-chisel.
The angle of the needle relative to the sodium nitrite stick determines the taper. If the
needle strikes parallel to the stick, you will just make the wire thinner. If the angle
is too great, you will shorten the wire without tapering it.
When you are finished, check your needle tip under a 10X objective.
On behalf of Hooke College of Applied Sciences, I hope this video has been helpful.
What techniques would you like to see demonstrated? Let us know!
Expand your range of microscopy and microanalysis skills in our scheduled and custom courses.
Please visit us at hookecollege.com, or phone us at 630-887-7100,
or email us at education@hookecollege.com.
Thank you for watching!