Frank Pfeifer

New Developments and Applications of Handheld Raman, Mid-Infrared, and Near-Infrared Spectrometers

Abstract: Recently, miniaturization of Raman, mid-infrared (IR), and near-infrared (NIR) spectrometers has made substantial progress. Though mid-infrared systems are based exclusively on attenuated total reflection (ATR) measurements, near-infrared spectrometers operate in the diffuse reflection or transmission mode. The reduction in size, however, must not be accompanied by deterioration in measurement performance, and portable instrumentation will only have a real impact on quality and process control if Raman, IR, and NIR spectra of comparable quality to laboratory spectrometers can be obtained. In the present communication, a short overview on the building principles of novel handheld systems will be provided and the results of qualitative and quantitative analyses of selected liquid and solid sample systems obtained with these Raman, Fourier transform infrared (FTIR), and NIR spectrometers will be evaluated in terms of their comparability with laboratory instruments and their suitability for on-site and field measurements.

Qualitative and quantitative pharmaceutical analysis with a novel hand-held miniature near infrared spectrometer

Although miniaturisation of vibrational spectrometers began approximately a decade ago, only within the last couple of years have real hand-held Raman, infrared and near infrared (NIR) scanning spectrometers became commercially available. On the customer end the development of portable instrumentation was driven by the request for more flexibility of on-site measurements and on the manufacturer side it was supported by the potential and advantages of micro-electromechanical systems (MEMS) production and the implementation of new technologies. With reference to NIR spectroscopy the expectations for a real hand-held system (<100 g) have been recently realised by a pocket-sized spectrometer with a linear variable filter technology (LVF) as monochromator principle and the additional benefit of significantly reduced costs compared to other portable systems. For a real breakthrough and impact of this instrument, however, it had to be demonstrated that competitive analytical results can be achieved. In this respect, the present communication has put to test the performance of this micro-NIR system with reference to selected qualitative and quantitative pharmaceutical applications.

Miniature Near-Infrared (NIR) Spectrometer Engine For Handheld Applications

While substantial progress has been made recently towards the miniaturization of Raman, mid-infrared (IR), and near-infrared (NIR) spectrometers, there remains continued interest from end-users and product developers in pushing the technology envelope toward even smaller and lower cost analyzers. The potential of these instruments to revolutionize on-site and on-line applications can only be realized if the reduction in size does not compromise performance of the spectrometer beyond the practical need of a given application. In this paper, the working principle of a novel, extremely miniaturized NIR spectrometer will be presented. The ultra-compact spectrometer relies on thin-film linear variable filter (LVF) technology for the light dispersing element. We will also report on an environmental study whereby the contamination of soil by oil is determined quantitatively in the range of 0-12% by weight of oil contamination. The achieved analytical results will be discussed in terms of the instruments competitiveness and suitability for on-site and in-the-field measurements.

Technical Note: Near infrared spectroscopic authentication of seafood

Near infrared (NIR) spectroscopic investigations of whole fish and fish fillets with a miniaturised, hand-held instrument were performed to demonstrate the feasibility of discriminating high-quality, expensive from lower-quality, less expensive, substitutes and responding to the increasing concerns regarding fraud and deception in seafood marketing. Generally, such problems can occur due to the mislabelling of products in the harvesting and processing system or species substitution at the restaurant level. To test the possibility of distinguishing superior from lower quality fish species, NIR spectra were measured in diffuse reflection from the skin and meat of the investigated fish. Subsequently, the spectra were evaluated by principal component analysis and further classified by soft independent modelling of class analogies. In the present communication, the results obtained with respect to the authentication of two different species of mullet, cod and trout, respectively, will be discussed in some detail.

Molecular orientation relaxation in binary blends of poly(methyl methacrylate) by rheo‐optical Fourier‐transform infrared spectroscopy

Rheo-optical Fourier-transform infrared spectroscopy, a technique combining simultaneous mechanical measurements and infrared dichroism spectroscopy, has been employed in order to investigate the molecular chain orientation and orientation relaxation behaviour in uniaxially stretched films of binary blends of long perdeuterated and short undeuterated chains of linear poly(methyl methacrylate) (PMMA). The solution cast film samples have been stretched at constant strain rate up to 100% elongation and subsequently allowed to relax at constant strain at various temperatures above the glass transition temperature. The results reported here show that for the short-chain component a threshold value of molecular weight exists below which a drastic decrease of the long- and short-chain orientation and a change in the relaxation behaviour of the short chains are observed.

Spectra Transfer Between a Fourier Transform Near-Infrared Laboratory and a Miniaturized Handheld Near-Infrared Spectrometer.

The aim of this contribution is to demonstrate the transfer of spectra that have been measured on two different laboratory Fourier transform near-infrared (FT-NIR) spectrometers to the format of a handheld instrument by measuring only a few samples with both spectrometer types. Thus, despite the extreme differences in spectral range and resolution, spectral data sets that have been collected and quantitative as well as qualitative calibrations that have been developed thereof, respectively, over a long period on a laboratory instrument can be conveniently transferred to the handheld system. Thus, the necessity to prepare completely new calibration samples and the effort required to develop calibration models when changing hardware platforms is minimized. The enabling procedure is based on piecewise direct standardization (PDS) and will be described for the data sets of a quantitative and a qualitative application case study. For this purpose the spectra measured on the FT-NIR laboratory spectrometers were used as “master” data and transferred to the “target” format of the handheld instrument. The quantitative test study refers to transmission spectra of three-component liquid solvent mixtures whereas the qualitative application example encompasses diffuse reflection spectra of six different current polymers. To prove the performance of the transfer procedure for quantitative applications, partial least squares (PLS-1) calibrations were developed for the individual components of the solvent mixtures with spectra transferred from the master to the target instrument and the cross-validation parameters were compared with the corresponding parameters obtained for spectra measured on the master and target instruments, respectively. To test the retention of the discrimination ability of the transferred polymer spectra sets principal component analyses (PCAs) were applied exemplarily for three of the six investigated polymers and their identification was demonstrated by Mahalanobis distance plots for all polymers.

Evaluating the Molecular Interaction of Organic Liquid Mixtures Using Near-Infrared Spectroscopy

The near-infrared transmission spectra of two organic liquid three-component systems of variable compositions were investigated in detail. To evaluate the interaction of the different components in the two systems the experimental spectra of the pure components were compared to mathematically constructed “pure component” spectra. Though usually the correlation coefficient (CC) and Manhattan distance (MD) are used to measure the similarity of spectra, in the present investigations principal component analysis (PCA) was found to be a more effective tool to investigate the difference between these spectra and derive parameters characterizing the interaction between the different components. Thus, PC scores for the two types of spectra established some distinct patterns which clearly expressed their differences. For a three-dimensional coordinate system of selected principal components, the Euclidean distances between the mathematically constructed and the experimental spectra of the pure components were calculated. Finally, the mean values of the distances for each component provided indices to rank the interaction of the components in the mixtures. Thus, the results offer a convenient approach that can quantitatively evaluate the molecular interactions of the individual components in organic liquid mixtures by various spectroscopies.

Comparative Variable Temperature Studies of Polyamide II with a Benchtop Fourier Transform and a Miniature Handheld Near-Infrared Spectrometer Using 2D-COS and PCMW-2D Analysis.

The main objective of this communication is to compare the performance of a miniaturized handheld near-infrared (NIR) spectrometer with a benchtop Fourier transform near-infrared (FT-NIR) spectrometer. Generally, NIR spectroscopy is an extremely powerful analytical tool to study hydrogen-bonding changes of amide functionalities in solid and liquid materials and therefore variable temperature NIR measurements of polyamide II (PAII) have been selected as a case study. The information content of the measurement data has been further enhanced by exploiting the potential of two-dimensional correlation spectroscopy (2D-COS) and the perturbation correlation moving window two-dimensional (PCMW2D) evaluation technique. The data provide valuable insights not only into the changes of the hydrogen-bonding structure and the recrystallization of the hydrocarbon segments of the investigated PAII but also in their sequential order. Furthermore, it has been demonstrated that the 2D-COS and PCMW2D results derived from the spectra measured with the miniaturized NIR instrument are equivalent to the information extracted from the data obtained with the high-performance FT-NIR instrument.