Instruction manual
14 World Precision Instruments
The most important of these steps is cleaning the glass. Small amounts of dust or grease
can easily clog micron-sized tips. Cleaning with chromic acid solutions before pulling
the electrode is commonly performed, but care must be taken to thoroughly rinse the
pipettes to remove all traces of the chromic acid, which has some affi nity for glass. Some
researchers prefer hydrochloric or nitric acid.
Silanization of the glass is also recommended for small tips. With 1μm and smaller tips,
capillary action becomes prohibitively large, and the hydrophilic surface of the glass
greatly limits the fl ow of fl uid through the tip. Silanization decreases the surface tension
and allows the fl uid to fl ow smoothly through the tip. For similar reasons, we don’t
recommend use of a capillary with an internal fi lament. Some of the many papers on the
art of silanization are listed in the bibliography.
When using the vacuum line to hold and manipulate individual cells, a large tip (about
10–20% of cell size) is recommended. To prevent damage to the cell, this tip should be
fi re-polished.
Volume Calibration by Measuring Droplet
For ejected volumes greater than 1nL, visual inspection using a microscope can be
an accurate gauge of volume. A single pulse deposits a drop of fl uid on the tip of the
micropipette. The volume of this drop may be calculated by measuring the radius of
the drop and assuming the drop to be spherical. Fig. 12 may be helpful in determining
the volume for a given radius. See “Appendix B: Droplet Volume” on page 23 for a
comparison of spherical and cubical volumes.
1 pl
10 pl
100 pl
1 nl
10 nl
100 nl
1 μl
10 μm 100 μm 1 mm
Volume of a Spherical Droplet
V =
Sd
3
6
Volume
Diameter
Fig. 12— (Left) The volume of a droplet increases as the diameter increases. See “Appendix
B: Droplet Volume” on page 23.
Fig. 13—(Right) WPI’s MicroFil is helpful in backfi lling glass pipette tips. Syringe fi lters
(available separately) help prevent clogged micropipette tips.
The following table is useful for converting between different units of volume.
Cubic Measure 1cm
3
1mm
3
(100μm)
3
(10μm)
3
1μm
3
Volume 1mL 1μL 1nL 1pL (10
-12
L) 1fL (10
-15
L)