Picking Cotton — and Nothing Else
Before cotton becomes the fabric of our lives, it must be picked. The problem, however, is that more than cotton ends up being harvested.
According to industry figures, 22 percent of the cotton bales gathered worldwide are either seriously or moderately contaminated with such foreign material as plastic from bags blown into cotton fields, grease and oil from harvesting machinery, pieces of rubber, bits of plants, insects and even excess moisture. Moreover, close to half of all bales have cotton seed-coat fragments in them.
Any part of a harvested cotton plant aside from the boll — such as the leaves (A), hulls (B), bract, secondary leaves (C), seed coats (D) or stems (E) — may contaminate the final fiber product during processing. Reproduced with permission of the Journal of Agricultural and Food Chemistry. ©2006 American Chemical Society.
Now there is a potential solution for this contamination that involves attenuated total reflectance/Fourier transform infrared spectroscopy. David S. Himmelsbach of the US Department of Agriculture’s (USDA) Richard B. Russell Research Center in Athens, Ga., and his colleagues at Hewlett-Packard Co. in Corvallis, Ore., and at the USDA’s Cotton Quality Research Laboratory in Clemson, S.C., used the technique to develop a spectral database to identify several types of foreign matter in cotton.
Such material costs the cotton industry millions of dollars annually in the US alone, and the Memphis, Tenn.-based National Cotton Council of America has made eliminating contaminants a priority. Himmelsbach noted that the technique and database could play a role in this effort. “I see it being used to pinpoint the source of the contamination to enable prevention or to permit remediation.”
At one time, cotton was picked by hand — a slow and labor-intensive process. Today, mechanized harvesters churn through fields and spit out 500-lb bales with greater efficiency and lower cost than the manual method. One of the mechanical techniques, stripper harvesting, takes in not only the cotton lint, but also quite a bit of plant matter, which is later discarded. The other technique, spindle picking, uses rows of rotating fingers to remove the seed cotton without as much waste.
Researchers have devised a technique combining Fourier transform infrared spectroscopy with attenuated total reflectance that enables them to identify nonmetallic foreign matter in cotton samples.
However, when compared with mechanized methods, the traditional approach does have one advantage. “When cotton is picked by hand, there is very little contamination,” Himmelsbach said.
Mechanical harvesters rend contaminants into small pieces that are hard to identify. A complete determination of a foreign substance’s nature once required the use of any of a variety of methods, including visual inspection and chemical analysis. These techniques are difficult to apply in the field, where the contamination occurs and where corrective action should take place.
Several years ago, Himmelsbach conceived the idea of using attenuated total reflectance combined with Fourier transform infrared spectroscopy for impurity analysis. In this technique, a sample is pressed against a crystalline internal reflection element. When a beam of infrared light strikes the crystal, it generates an evanescent wave that extends a few microns into the sample. The wave picks up the molecular fingerprint of the substance, which can be compared with a spectral database. Besides the ability to identify most foreign matter, advantages of the technique include quickness and no required sample preparation.
So as to pursue this concept, Himmelsbach had to wait until small enough devices were developed that could perform the technique. Particularly important was the use of thin diamond for the internal reflection element. Using a Fourier transform infrared spectrometer from Thermo Nicolet (now Thermo Electron Corp.) and an attenuated total reflectance sampling device from Smiths Detection Inc. of Danbury, Conn., the investigators collected spectra over the range of 650 to 4000 cm–1 at a resolution of 8 cm–1 for a variety of samples, including plant matter, soils and sugars. They also measured the spectra of various components of the cotton plant.
After analyzing the data, they found that much of the foreign matter could be identified by characteristics in a fingerprint region that ran from 650 to 1800 cm–1. The contaminants that required additional information for identification could be handled by adding the region from 2700 to 3700 cm–1.
According to Himmelsbach, these results show that the database was usable as is, although additional samples still must be collected and analyzed. For example, cotton plant parts from various locales and harvests must be analyzed and added to the database because the spectral signature could vary with changes in environmental conditions. Once ready for field operation, the technique will enable cotton growers to counteract contamination.
“If oil contamination is identified,” he said, “this might reflect improper maintenance and cleanliness of harvesting equipment. Remediation might include washing the cotton.”
Although promising, the technique does not work for every potential contaminant. Metallic objects, for instance, do not have a telltale mid-infrared spectrum and so cannot be identified by this approach.
The researchers, in conjunction with others around the world who are adapting the method, are extending the database to make it more definitive. They also are working with companies to ensure that the equipment is made widely available soon.
Journal of Agricultural and Food Chemistry, Oct. 4, 2006, pp. 7405-7412.
Contact: David S. Himmelsbach, US Department of Agriculture; e-mail: firstname.lastname@example.org.
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