Agriculture Reference
In-Depth Information
SubdIVIdInG - obTAInInG THE SAMPLE for TESTInG
Subsampling methods were discussed in detail in Chapter 3. It is important that dividers be thoroughly
cleaned between each sample regardless of the type of divider used. This is best done by a compressed air
system, which should be available in all laboratories.
Analysts should make certain that the submitted sample is thoroughly mixed prior to the dividing
process. To make certain that thorough mixing is attained, samples should be run through the divider at
least
three times.
One part should then be selected and returned through the divider two or three times until
the required amount of seed is obtained. The amount to be tested should
never
be obtained by adding or
removing seeds by hand. Tables in the ISTA and AOSA rules specify the weight of submitted and working
samples for different species. In general, purity tests are performed on 2,500 seeds, regardless of the kind
or weight of the seeds; bulk exams such as the noxious exam (AOSA) and other seed determination (ISTA)
require a sample size of 25,000 seeds.
THE PurITy SEPArATIon
The working sample should irst be placed on the work area and checked to make certain it has been prop-
erly labeled as to kind or kinds of seeds and has been subdivided to the correct weight. Then the seed should
be examined to determine if the analysis can be aided by blowing or sieving procedures prior to beginning
the purity examination by the purity analyst.
Blowers and sieves are used in purity analysis to facilitate the separation of inert matter and weed
seeds (as well as other crop seed) that are different in density (blowers) and size (sieves) than the kind of
seed being examined.
Separation of samples can often be made easier and faster without risk of losing inert matter if irst
subjected to a blowing process. Several types of seed blowers are in common use (Figure 4.6). By proper
adjustment of blowing force, much light weight inert matter and incidental seed contamination can be
removed if their speciic gravity or resistance to airlow is different than that of the crop being tested.
After blowing, it may be helpful to remove small (or large) weed and crop seed contamination by
using hand sieves (Figure 4.7). Each of the components removed by both the sieving and blowing proce-
dures should be placed in a separate container and held for later examination.
Following rough separations by blowers and/or sieves, the main portion of the working sample should
be placed on the work area (e.g., purity board) and examined by use of forceps, microscope slide or similar
device having a straight scraping edge. With one of these devices, draw a few seeds at a time from the
main pile and toward the front of the board. This permits careful examination of a smaller portion of seed
and allows inert matter or incidental contaminating seeds to be removed and placed in a nearby container.
This process should be repeated until the entire working sample has been analyzed and placed into separate
containers in one of four categories:
pure seed
,
inert matter
,
other crop seed
and
weed seed
(AOSA). ISTA
recognizes three categories: pure seed, inert matter and other seeds. After the purity components have been
separated, each should be re-examined carefully to make certain the separations are accurate. The percent-
age of each of these fractions should then be determined by weighing each component and dividing by the
sum of the weights of all four components if the working sample weight is 25 grams or less. If the working
sample is more than 25 grams, the percentages are based on the original working sample weight.
A permanent record of each sample should be kept, showing all results of the purity examination. This
should include weights of each component as well as percentages of each. It should also show the character
of inert matter, kinds and numbers of other crop and weed seeds, name, initial or identiication number of
the analyst, and date of the test. Reports of the analysis should be made from this record.
