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Table II: Measurement time averages (rounded to nearest second).
When authenticating material, the user selects the appropriate method from a menu on the instrument's display and enters the
material identification number with the barcode reader or keypad. The operator directs the unit's laser aperture toward the
sample and initiates the measurement. Once data collection begins, the instrument optimizes settings including exposure time,
number of accumulations, and laser power to achieve acceptable SNR as quickly as possible. Table II shows the average measurement
times required for each material. As demonstrated by the information in Table II, the average measurement times vary considerably
depending on the characteristics of the material (Raman cross section, etc.). Specifically, the times range from 1 s to more
than 6 min; however, the majority of materials were measured in less than 1 min. The individual measurement times (not shown)
for the three units differ to some degree, as well, primarily because of environmental conditions during the measurement (sample
positioning, ambient light, etc.).
Figure 6
As indicated in the experimental section, a p-value for each unknown-method pair was generated for measurements across all three test devices. The p-values, which indicate whether the measured spectrum is statistically different from the reference spectrum, were examined
individually and were averaged for presentation purposes. Figure 6 shows data for each material– method pair. Note that the
values shown range from p <10–15 to p >0.1. In normal application of the instrument, the result is a simple pass–fail determination. The unit's default threshold
for "pass" is p ≥0.05. This indicates that the measured spectrum is statistically consistent with the reference spectrum given the uncertainty
of the measurement.
The diagonal elements of Figure 6 represent cases in which the material was tested against the corresponding method, whereas
the off-diagonal entries represent materials being tested by the "wrong" method. The dark-green squares on the diagonal show
where the measured spectrum matched the corresponding reference spectrum with a p-value >0.1, which indicate that these materials easily pass as being consistent with the method reference spectrum. Two materials,
ethyl cellulose and hydroxypropyl cellulose, resulted in average p-values in the range of 0.01 to 0.1 when tested against themselves. As also shown in Figure 6, examination of individual results
from each of the three instruments revealed that these two samples, numbers 10 and 11 (indicated by daggers), were the only
ones that resulted in differences in pass–fail decisions among the three test instruments. For ethyl cellulose, the p-values were 0.03, 0.06, and 0.04, and for hydroxypropyl cellulose, the p-values were 0.02, 0.05, and 0.11 across the three test units, respectively. Thus, both materials were very close to the borderline
threshold of 0.05 and either just passed or just failed.
Robert L. Green is a research scientist at Ahura Scientific, Inc., 46 Jonspin Road, Wilmington, MA 01887, tel. 978.657.5555, fax 978.657.5921, bgreen@ ahurascientific.com.
Articles by Robert L. Green
Christopher D. Brown
Christopher D. Brown is a director of system analytics and applications, both at Ahura Scientific, Inc.
Articles by Christopher D. Brown
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