Now, a Purdue University (West Lafayette, Ind., U.S.A.) team has figured out a way to detect trace levels of a harmful contaminant with devices that many law enforcement laboratories already own.
Purdue graduate student Alona A. Chernyshova works on her lab’s near-infrared spectrometer.
In the past two years, pet foods and milk products contaminated with the organic chemical melamine have harmed children in China and domestic animals in multiple countries. Now, a Purdue University (West Lafayette, Ind., U.S.A.) team has figured out a way to detect traces of melamine powder with devices that many law enforcement laboratories already own.
Lisa J. Mauer, an associate professor of food science, and her colleagues determined that three types of near- and mid-infrared spectrometers could find melamine in powder at the level of 1 part per million (ppm), the upper limit set for infant formula by the U.S. Food and Drug Administration (FDA). The testing required only a few minutes to perform in all cases (J. Agric. Food Chem. 57, 3974).
The FDA’s standard method of testing food for melamine involves liquid chromatography and triple-quadrupole tandem mass spectroscopy—a process that requires labor-intensive sample preparation. Mauer wanted to find out if a less time-consuming method, and one possibly better suited as a sample screening tool, could meet the FDA’s 1-ppm threshold. (The highest amount of melamine permitted in pet food is 2.5 ppm.)
Mauer, a postdoctoral fellow and three graduate students used three instruments to test samples of infant formula powder with and without melamine (which was deliberately added to the formula for the experiments). In the mid-infrared region (4,000 to 650 cm–1), the group used two sampling techniques: diffuse reflectance and multi-bounce attenuated total reflectance. The former requires powders to be mixed with a background material such as potassium bromide; the latter does not. The researchers also obtained reflectance spectra in the near-infrared range (12,500 to 3,500 cm–1).
The spectrum of melamine has two absorption peaks in the mid-infrared from the stretching and bending vibrations of its amino groups, and a third mid-infrared region of absorption from complex vibrations. Melamine has broader spectral features in the near-infrared, but the Purdue team was still able to perform a quantitative analysis on the data.
The researchers found that they could distinguish between spectra of samples laced with melamine traces and those without. Processing times ranged from less than 2 min. for the near-infrared spectrometer to 5 min. for the diffuse reflectance technique, because of the powder blending it required.
All three spectroscopic techniques might be able to detect melamine in lower concentrations than the FDA standard, but researchers could face lower signal-to-noise ratios, according to Mauer. In addition, each brand of powdered formula would require separate calibration.
University chemistry departments, public health departments, police laboratories and even some high schools have at least one infrared spectrometer already on hand. Some non-U.S. governments have also expressed interest in the infrared-spectroscopy method, which could be used as a first-pass screening for imported foods.
email@example.com is the senior writer/editor of Optics & Photonics News.