Multilayer film analysis for glucose in 1-microL samples of plasma

With the ultramicroanalytical system described here we can measure glucose in 1 microL of plasma or serum. The sample is placed on a dry, multilayer film element (Eastman Kodak), where a colored spot about 3.5 mm in diameter develops. The reflectance of these spots is measured with a reflectance digital matrix photometer that was conceived, designed, and constructed in our laboratory. The spot is illuminated with monochromatic light and its image is projected by a camera lens onto the photosensitive surface of a linear photodiode array containing 512 individual photodetectors. The photodetector signals are processed by a computer to obtain the reflectance and diameter of the spot. The latter is proportional to sample volume. Because the reflectance of the spot does not depend greatly on sample volume, accurate pipetting is not required. The coefficients of variation of repeatable glucose analyses were, for 400, 3000, and 5120 mg/L, 1.7, 2.3, and 2.8%, respectively. The correlation coefficient (r) between glucose analyses by our method (y) and with the Ektachem 400 (x) was 0.9918; the regression equation was y = 1.07x - 94.3 mg/L.