Daran A. Sadler

Novel SERS-active optical fibers prepared by the immobilization of silver colloidal particles

A novel sensor based upon surface-enhanced Raman scattering (SERS) has been constructed by immobilizing colloidal silver particles onto the distal end of an optical fiber. This same single fiber was then used to both transport the exciting laser radiation and collect the Raman scattering from analytes sorbed onto the colloidal particles. The colloidal particles were immobilized by functionalization of the end of the optical fiber with (3-aminopropyl)trimethoxysilane prior to immersion of the fiber in silver colloid. Spectra were obtained from both 4-(5′-azobenzotriazol)3,5-dimethoxyphenylamine and crystal violet. The within-batch variation of a set of five fibers has been measured as approximately 10%. Raman imaging experiments demonstrated that the effects due to spatial variations in the intensity of the SERS recorded over the distal end of the fiber are removed by the use of a multimode fiber.

Quantitative assessment of surface-enhanced resonance Raman scattering for the analysis of dyes on colloidal silver

Factors that affect quantitative analysis by surface-enhanced resonance Raman scattering (SERRS) have been investigated using azobenzotriazol and reactive dyes. Preaggregation of the silver colloid was the most effective method to obtain repeatable and reproducible scattering. Aggregation by poly(l-lysine) or spermine provided better precision than aggregation by sodium chloride or nitric acid. Repeatable quantitative analysis was achieved with the azobenzotriazol dyes. A linear calibration graph was obtained over different concentration ranges below 10(-)(8) M, depending on the nature of the colloid. Calculations estimate that 10(-)(8) M is the concentration at which monolayer coverage of the dye on the silver colloid is achieved. Above 10(-)(8) M, there was only a minor increase in the scattering intensity from the azobenzotriazol dyes. In contrast, the reactive dyes did not give a response proportional to concentration over the range studied. The different responses obtained for the two types of dye are believed to be caused by differences in the nature of the interaction of the molecules with the silver surface. The conclusion reached is that control of the colloid preparation, aggregation process, and surface chemistry are essential for successful quantitative analysis of dyes on colloidal silver by SERRS.

Preparation of Stable, Reproducible Silver Colloids for Use as Surface-Enhanced Resonance Raman Scattering Substrates

A flow system for the production of stable, reproducible batches of silver colloid for use as substrates for surface-enhanced resonance Raman spectroscopy (SERRS) is described. The colloids were prepared by borohydride reduction of silver nitrate and subsequent stabilization was achieved by adding trisodium citrate. The batches of colloid produced were analyzed using UV-visible spectroscopy and their suitability for use as SERRS substrates was assessed using 3,5-dimethoxy-4-(5′-azobenzotriazole)phenylamine as the analyte. SERRS analysis was carried out using a flowcell. Using the method described, batches of silver colloid were prepared that were stable for at least five months and when used as SERRS substrates resulted in a relative standard deviation in SERRS intensity of 3,5-dimethoxy-4-(5′-azobenzotriazole)phenylamine of 6.6% between colloid batches. The robustness of the system for production of stable, reproducible colloids was assessed using experimental design. The final method proposed enables reproducible, time-stable colloid to be made in a simple manner, thus eliminating one of the major problems associated with the use of SERRS detection in analytical procedures.

Use of multiple emission lines and principal component regression for quantitative analysis in inductively coupled plasma atomic emission spectrometry with charge coupled device detection

A procedure for the automatic selection of spectral lines, based upon principal component regression (PCR), is described. The procedure analyses the consistency of the pattern of emission signals from a number of lines in both a series of standard solutions and the sample solution. From the calibration experiment the principal component associated with the largest eigenvalue is shown to be linearly dependent upon concentration and hence can be used to determine the unknown analyte concentration in the sample solution. The detection limit for this principal component is shown to be lower than the detection limit of any of the individual spectral lines. For example, a set of five Fe lines has a detection limit of 5.4 ng ml–1 for the first principal component, compared to the lowest detection limit of 11 ng ml–1 for Fe II 260.709 nm. In addition, the remaining principal components are shown to be independent of analyte concentration, but dependent upon the presence of unknown, additive spectral interferences. The procedure was used to determine 0.5 µg ml–1 of Cr, Mn and V in a matrix containing Ce, La, Dy, Fe, Mo and Co. Six Cr lines, three Mn lines and seven V lines were used for quantitative analysis. Spectral interferences were correctly predicted for 2 Cr lines and 3 V lines, whilst the predicted concentrations for the three analytes were 0.51 ± 0.01, 0.49 ± 0.01 and 0.51 ± 0.01 µg ml–1, respectively.

Automatic wavelength calibration procedure for use with an optical spectrometer and array detector

Echelle spectrometers with cross-dispersion are often used in emission spectroscopy owing to their high spectral resolution and good light throughput. The resultant two-dimensional dispersion plane is ideally suited to array detectors such as the charge-injection device (CID) or charge-coupled device (CCD). The successful coupling of an echelle spectrometer with an array detector permits true simultaneous spectroscopy to be performed over a large spectral range. In order to correctly identify spectral features it is necessary to have an accurate wavelength calibration function which maps the CCD/CID pixel co-ordinates to wavelength. A new approach to the wavelength calibration of optical spectrometers with array detection is proposed that does not involve a direct modelling of the spectrometer dispersion. Instead, the difference between an ideal conceptual spectrometer and the physical instrument is modelled. The procedure is able to compensate for the effects of manufacturing tolerances and local temperature and pressure conditions. Preliminary results, obtained by simulation with a computer-modelled echelle spectrometer, has shown that a sub-pixel accuracy in the predicted position of spectral lines can be achieved over a temperature range 5–35 °C.

Maintenance of a calibration model for near infrared spectrometry by a combined principal component analysis–partial least squares approach

Abstract A novel strategy for building and maintaining calibration models has been developed for use when the future boundaries of the sample set are unknown or likely to change. Such a strategy could have an impact on the economics and time required to obtain and maintain a calibration model for routine analysis. The strategy is based on both principal component analysis (PCA) and partial least squares (PLS) multivariate techniques. The principal action of the strategy is to define how “similar” a new sample is to the samples currently defining the calibration dataset. This step is performed by residuals analysis, following PCA. If the new sample is considered to have a spectrum “similar” to previously available spectra, then the model is assumed able to predict the analyte concentration. Conversely, if the new sample is considered “dissimilar”, then there is new information in this sample, which is unknown to the calibration model and the new sample is added automatically to the calibration set in order to improve the model. The strategy has been applied to a real industrial dataset provided by BP Amoco Chemicals. The data consists of spectra of 102 sequential samples of a raw material. The strategy produced an accurate calibration model for both target components starting with only the first four samples, and required a further 17 reference measurements to maintain the model for the whole sampling sequence, which was over a 1-year period.

Tutorial guide to the use of wavelet transforms to determine peak shape parameters for interference detection in graphite-furnace atomic absorption spectrometry

A procedure to quantify the shape of the absorbance profile obtained during graphite-furnace atomic absorption spectrometry is presented. The procedure is based upon the use of wavelet transforms to produce a number of quantitative measures of the signal variation, called Lipschitz regularities, whose values depend upon the shape of the absorbance profile. The number, position and value of the Lipschitz regularities allow a quantitative description of an individual absorbance profile to be made. A comparison of absorbance profiles is then made by comparing the set of Lipschitz regularities that describe the absorbance peak shape. Changes in the shape of the absorbance profile, between a standard solution and the sample, may be indicative of interference effects that are modifying either the atomization of the analyte, or the removal of the analyte atoms from the observation volume. The Lipschitz regularities can then be used to detect the presence of interference effects. In this paper, the background to wavelet transform theory is given, along with a description of the meaning and calculation of the Lipschitz regularity. An example of the calculation of the Lipschitz regularities for an atomic absorption profile of Cu is presented. In addition, the use of the Lipschitz regularities to determine changes in the absorbance signal profile, due to the presence of a concomitant matrix species, is demonstrated for Pb in the presence of NaCl.

Use of signal-to-root background ratio as the optimization parameter for inductively coupled plasma atomic emission spectroscopy with charged-coupled device detection

The use of the signal-to-root background ratio (SRBR) of a spectral line as the measurable parameter chosen as the criterion for single-element optimization of an inductively coupled plasma with charge-coupled device detection is described. The theoretical background to the choice of the SRBR, rather than the more usual signal-to-background ratio (SBR), is given. Single-element optimization of the carrier gas flow rate and viewing height using both atomic and ionic lines from ten elements is described. The improvement in the detection limit by using the SRBR over the SBR varies by a factor of 1.0–4.8. For example, the detection limit for the Mn II emission line at 257.610 nm is improved from 13.4 ng ml–1 by SBR optimization to 2.8 ng ml–1 by maximizing the SRBR.

Application of wavelet transforms to determine peak shape parameters for interference detection in graphite-furnace atomic absorption spectrometry

Abstract A procedure to quantify the shape of the absorbance–time profile, obtained during graphite furnace atomic absorption spectrometry, has been used to detect interference effects caused by the presence of a concomitant salt. The quantification of the absorption profile is achieved through the use of the Lipschitz regularity, α 0 , obtained from the wavelet transform of the absorbance–time profile. The temporal position of certain features and their associated values of α 0 provide a unique description of the shape of the absorbance–time profile. Changes to the position or values of α 0 between standard and sample atomizations may be indicative of uncorrected interference effects. A weak, but linear, dependence was found of the value of α 0 upon the analyte concentration for Cr and Cu. The ability of the Lipschitz regularity to detect interference effects was illustrated for Pb, Se and Cu. For Pb, the lowest concentration of NaCl added, 0.005% m/v, changed both the values of α 0 and the peak height absorbance. For Se, no change in the peak height and peak area absorbance signals was detected up to a NaCl concentration of 0.25% m/v. The values of the associated Lipschitz regularities were found to be invariant to NaCl concentration up to this value. For Cu, a concentration of 0.05% m/v NaCl reduced the peak height and peak area absorbance signals by approximately 25% and significantly altered the values of α 0 .

Multi-element optimization of the operating parameters for inductively coupled plasma atomic emission spectrometry with a charge-transfer device detection system: a study of the effects of different response functions

A new response function for ICP-AES is proposed in which the resultant compromise plasma conditions are independent of the concentration of the analytes in the test solution. The response function is designed to be used in situations where the concentrations of the analytes in the sample are unknown, or, when many samples, of varying composition, are to be analysed. In addition, the use of the signal-to-root background ratio (SRBR) as the measurement of the analytical performance of an element for multi-element analysis by ICP-AES, with a charge-coupled device detector, is described. The use of the SRBR was found to give better detection limits than those obtained with the more commonly used signal-to-background ratio (SBR). For instance, the detection limit for Mn II at 257.610 nm is improved from 13 ng ml–1 when using the SBR and the proposed response function, to 2.8 ng ml–1 when using the SRBR. The compromise operating parameters, and hence detection limits for the analytes, were shown to be independent of the composition of the test solution for the new function. In comparison, the detection limit of Mn II varied between 2.9 and 40 ng ml–1, depending on the test solution composition when optimizing with a previously reported response function. Furthermore, biasing the proposed function for a particular analyte by increasing a weighting parameter is demonstrated. For instance, the detection limit of Pb I at 280.199 nm improved from 54 to 41 ng ml–1, by increasing the weighting parameter for Pb from 1 to 10.