Agriculture Reference
In-Depth Information
forceps, Hand Lens and Magnifiers
Two essential items for the purity analysis are forceps and hand lenses or other magnifying glasses. Many
types of forceps are used, both to systematically pick through the sample and to remove seed and inert
matter components. Some are sharp-pointed; others are blunt or rounded. Most analysts soon develop a
preference for a particular type, depending on the size and nature of the species being separated; be sure the
points of forceps are not magnetized. A straight tipped scalpel or a glass microscope slide with a straight
edge makes an excellent tool to manipulate seed on the purity board as a supplement to forceps.
Hand lens selection also depends on the size of seed being analyzed and analyst preference. For small-
seeded types such as bluegrass and bentgrass, a lens which magniies up to ten power may be needed. For
most seeds, a 5-7X lens should be adequate. For large seeded types, no lens should be necessary except for
identiication of small incidental seeds found in a sample. However, a 10-15X hand lens can be helpful for
small seeds, with the ield of vision limited to a few seeds at a time.
To prevent eyestrain, the lens should be held in the hand with the back of the index inger against the
forehead so the lens is directly in front of one eye. Both eyes should be left open and the lens shifted back
and forth between eyes to give both equal exercise.
use of Microscopes
Although the hand lens is more traditional, many analysts ind low power binocular microscopes to be an
eficient means of examining seeds for purity analysis. Microscopes should be selected to give an adequate
working distance between the lens and the sample. This enables the analyst to sit erect and, at the same time,
provides room under the lens for hand manipulation of the seed. Magniication should range between ive
and ifteen power. Zoom microscopes are very desirable, but turret type power selectors are also satisfac-
tory. Of utmost importance are high quality optics.
Microscopes should be mounted on the table or desk to provide a lat, uninterrupted surface for the
seed sample. One popular arrangement is to mount the microscope in a post receptacle which allows it to
swivel out of the way, providing maximum lexibility for the work area. Another popular arrangement is to
mount the microscope over a diaphanoscope which is mounted lush with the working surface.
Semi-Automatic Inspection Stations
Many innovations have been developed to automate and speed up the purity examination or to decrease
analyst strain and boredom. Most utilize some method of magniication such as a binocular microscope
to facilitate identiication. Various modiications of semi-automatic inspection stations (Figure 4.4) have
achieved routine use, especially for certain species. Microscopic inspection stations increase eficiency and
reduce neck and back pain, especially when analysts test seeds for long hours every day. Intermediate-sized,
free-lowing seeds such as cereal grains, alfalfa, or ryegrass are ideally suited for this method of examina-
tion. Some move the seed under the ield of view by means of vibration, while others use innovative ways
of seed movement into and out of the ield of view, allowing the analyst to remove contaminating seed or
inert matter. Although many laboratories still perform purity tests of small seeds using the manual method,
semi-automatic inspection stations may be very useful for conducting the larger-sized noxious weed seed
examinations.
Scales and balances
Nowhere is accuracy and precision more important than in properly weighing and recording results of the
purity test. Balances (Fig. 4.5) must be available that will accurately weigh small samples to four decimal
