Surface-Enhanced Raman Scattering Detection of Amphetamine ...

More documents

Recommendations

Info

$Math 211 Sets Practice Worksheet--Answers 1. Shade the region of ...$

FIG. 2. Overlaid SERS spectra for AMNA (a) and the mean-centered subtraction spectra for AMNA (b). plished by measuring the peak intensity of a Raman band of the amide molecule relative to the peak intensity of a Raman band of an internal standard and demonstrated with chemometrics. PCTP was added to solutions of varying amide concentration as the internal standard for quanti® cation purposes. A Raman band associated with PCTP was used as the reference for the calibration curve of the peak intensity ratio as a function of concentration formed for each amide. Figure 2a shows the overlaid spectra for the compound AMNA co-coated with PCTP onto SERS substrates for the concentration range of 47.2 ppm to 472 ppm. The Raman bands indicated are attributed to the phenyl ring breathing mode (998 cm 2 1 ), 19 ±21 the pyridinyl ring breathing mode (1055 cm 2 1 ), 22 the asymmetric C±N±C stretch (1125 cm 2 1 ), 23 the phenyl±C stretch (1205 cm 2 1 ), 19 ±21 the pyridinyl±C stretch (1223 cm 2 1 ), 19,20 the amide C±N stretch (1389 cm 2 1 ), 2 4 and the ring stretch (1569 cm 2 1 ). 19±21 Also shown (Fig. 2b) are the mean-centered FIG. 3. Overlaid SERS spectra for MMNA (a) and the mean-centered subtraction spectra for MMNA (b). subtraction spectra for AMNA. The spectra were averaged and the mean spectrum subtracted from the original spectrum for the concentration range of 47.2 to 472 ppm. This ® gure highlights the analytical response of the SERS substrate from high to low concentration. An overlay of the SERS spectra for the compound MMNA co-coated with PCTP onto SERS substrates is shown in Fig. 3a. The concentration range of the amide is 60.9 to 2435 ppm. The Raman bands of interest include the phenyl ring breathing mode (1000 cm 2 1 ), 19 ±21 the pyridinyl ring breathing mode (1058 cm 2 1 ), 22 the asymmetric C±N±C stretch (1126 cm 2 1 ), 23 the pyridinyl±C stretch (1219 cm 2 1 ), 19,20 the amide C±N stretch (1389 cm 2 1 ), 24 and the ring stretch (1572 cm 2 1 ). 19 ±21 Figure 3b is an overlay of the mean-centered subtraction spectra for MMNA. A relatively strong band at 1514 cm 2 1 may be noted in the spectra of both amide compounds and corresponds to the ring stretch of PCTP. 25 Also noted, in the spectra for MMNA, is a strong Raman band at 1047 cm 2 1 that is APPLIED SPECTROSCOPY 957
FIG. 4. Calibration curve formed by plotting the peak intensity ratio (I 998/I 1514) as a function of AMNA concentration (ppm). attributed to residual nitrate 26 on the silver foil surface as a consequence of the etching process. This band was eliminated in subsequent experiments by a more thorough rinsing of the etched silver foil prior to surface coating. Peak intensities of the 998 and 1000 cm 2 1 Raman bands of the AMNA and MMNA amide compounds, respectively, were measured relative to the peak intensity of the 1514 cm 2 1 band of PCTP. The peak intensity ratios I 998/I 1514 and I 1000/I 1514 were plotted as a function of concentration of the respective amide (Figs. 4 and 5). Average detection limits (DLs) of 19 ppm for AMNA and 17 ppm for MMNA were determined. Detection limits were calculated by using the point at which the analyte peak is three times the noise level of the background. The error bars in Figs. 4 and 5 represent the spectroscopic noise expected from the measurement. The variation around the best-® t line indicates that the precision of this method is not limited by the instrument, but rather by the reproducibility of the SERS substrates. We have also demonstrated the use of chemometrics for quantitation. 27 Figure 6 illustrates development of a partial least-squares (PLS) model for AMNA. Prior to developing the model, we normalized the spectra by using the PCTP peak. Figure 6a shows the standard error of calibration as a function of the numbers of factors used. In this case, one factor produced the lowest error. Figure 6b shows the results of a cross-validation plotted as predicted vs. actual concentration. The R 2 of 0.987 shows excellent correlation. Figure 6c shows the matrix representing the ® rst factor; comparison with Fig. 2a shows that this factor contains most of the spectral in- FIG. 5. Calibration curve formed by plotting the peak intensity ratio (I 1000/I 1514) as a function of MMNA concentration (ppm). 958 Volume 53, Number 8, 1999 FIG. 6. PLS quantitation of AMNA data: (a) calibration of standard error, (b) cross-validation plot, and (c) ® rst factor matrix. formation. Using this method we found a DL of 18.4 ppm, which corresponds well with the 19 ppm value we obtained by using a simple ratio. The utility of using PLS is that it is easily automated and can solve for more than one component. CONCLUSION This work represents an extension of the use of speci® c surface coatings in SERS detection of illicit drugs. We developed a procedure for the derivatization of amphetamine and methamphetamine with 2-mercaptonicotinic acid to form the amide compounds N-(1-methyl-2-phenylethyl)-2-m ercaptopyridine-3-carboxam ide (AMNA) and N-methyl-N-(1-methyl-2-phenylethyl)-2-mercaptopyridine-3-carboxam ide (M MNA ). The derivatized amines can be identi® ed and quanti® ed from SERS spectra. The appearance of the C±N stretch of the amide is evidence of the new compounds. The DCC coupling reaction employed in this procedure is selective for primary and secondary amines; thus, alkaloids such as morphine, cocaine, and their respective derivatives would not form an amide bond with 2-MNA. ACKNOWLEDGMENTS The authors would like to acknowledge support from Detection Limit, Inc. through a subcontract to the University of Wyoming from HHS STTR Grant Number 1R41DA10670-01. 1. R. G. Schlaadt and P. T. Shannon, Drugs of Choice (Prentice-Hall, Englewood Cliffs, New Jersey, 1986), 2nd ed.
Page 1 and 2: Surface-Enhanced Raman Scattering D
Page 3: NaHCO 3 (3 3 20 mL), followed by a

Surface-Enhanced Raman Scattering Detection of Amphetamine ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?