View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Grasas y Aceites (E-Journal) Grasas y Aceites Vol. 53. Fasc. 1 (2002), 45-63 45 Spectroscopy: Developments in Instrumentation and Analysis By Vincent Baeten and Pierre Dardenne Quality Department of Agricultural Products. Agricultural Research Centre of Gembloux. Chaussée de Namur, 24. 5030 Gembloux. Belgium. Tel: 32/81/62 03 50, Fax: 32/81/62 03 88. E-mail: [email protected] CONTENTS spectroscopy (NIR), mid-infrared spectroscopy (MIR) and Raman spectroscopy. The theoretical aspects related with these 1. Introduction techniques, the information that can supplied and the main 2. Why spectroscopic techniques features of the instrumentation are presented and briefly 2.1. Analytical characteristics discussed. The last part of the review concerns the application of 2.2. Spectroscopic characteristics the spectroscopy to food analysis, with special emphasis on the 2.3. Instrumental characteristics lipid analysis. The illustrations and examples have been chosen to 3. Some theoretical elements demonstrate the importance of spectroscopic techniques both in 3.1. Classical theory process (on-line) control and in laboratories for the analysis of 3.2. Quantum theory major or minor compounds. 3.3. Absorption and scattering: band position and intensity KEY-WORDS: Analysis - Authentication – Chemometrics - Mid- 4. Selecting the information infrared spectroscopy - Near-infrared spectroscopy - Raman spec- 5. Spectroscopic instrumentation troscopy. 5.1. Near-infrared instrument 5.1.1. Classical instrument 5.1.2. Microscope and camera 5.1.3. Sample presentation 5.2. Mid-infrared instrument 1. INTRODUCTION 5.2.1. Microscope and camera 5.2.2. Sample presentation 5.3. Raman instrument In recent years, spectroscopic techniques have 5.3.1. Microscope and camera come to be regarded as attractive and promising 5.3.2. Sample presentation analytical tools for analyses conducted in research, 6. Applications control or industrial laboratories. These techniques 6.1. Quantitative analysis 6.1.1. Determination of fat content are increasingly considered by analysts as an 6.1.2. Determination of major and minor components obvious solution. This trend stems from instrumental in lipid developments, the extensive use of computers and the 6.2. Qualitative analysis development of appropriate chemometric procedures. References Daily, new applications of spectroscopic techniques in RESUMEN the fields of chemistry, drugs, the agro-food sector, life Espectroscopía: Desarrollo en instrumentación y análisis. sciences and environmental analysis are being demonstrated and published. The objective should be Este artículo de revisión presenta las características, venta- not only to extend the field of application of jas, límites y potencial de tres técnicas espectroscópicas: las es- spectroscopic techniques, but also to use them as a pectroscopias del infrarrojo cercano, del medio infrarrojo y matter of course in control and industrial Raman. Se presentan, y discuten brevemente, los aspectos teóri- cos relacionados con estas técnicas, la información que pueden laboratories and to develop (according to suministrar, y las principales características de la instrumenta- internationally accepted guidelines) fully validated ción. La última parte de la revisión esta dedicada a las aplicacio- spectroscopic methods. nes de la espectroscopia en análisis de alimentos, con especial énfasis en análisis de lípidos. Las ilustraciones y los ejemplos se This paper presents the characteristics, advantages, han elegido para demostrar la importancia de las técnicas espec- limitations and potential of three spectroscopic troscópicas en los procesos en-línea y en los laboratorios en el techniques: near-infrared spectroscopy (NIR), mid-infrared análisis de componentes mayoritarios y minoritarios. spectroscopy (MIR) and Raman spectroscopy. PALABRAS-CLAVE: Análisis - Autentificación – Espectros- Special emphasis is placed on the application of copia infrarrojo cercano- Espectroscopia infrarrojo medio - Es- these techniques to lipid analysis. The illustrations pectroscopia Raman – Microscopio – Quimiometria. and examples have been chosen to demonstrate the importance of infrared (NIR and MIR) and Raman SUMMARY spectroscopies both in process (on-line) control and Spectroscopy: Developments in instrumentation and in laboratories for the analysis of major or minor analysis. compounds. There are many books and papers on the subject; most of the references included here This review presents the characteristics, advantages, limits have been selected for their relevance and clarity. and potential of three spectroscopic techniques: near-infrared 46 Grasas y Aceites Ta bl e I Analytical, spectroscopic and instrumental characteristics of NIR, MIR and Raman spectroscopies FEATURES NIR MIR Raman REFERENCES Rapid analysis -- -(Gerrard 1991) Simple analysis: concentration vs peak area /. -(Pelletier & Davis 1996) Qualitative and quantitative analysis -- -(Baranska 1987) None or reduced sample preparation -. -(Gerrard 1991) Standard calibration and transfert -. / procedures Validated methods (IUPAC, AOCS, ISO) /. / Sample form and sample size -- - No use of pollutant solvents -. - -- - ANALYTICAL Economic method (Scotter 1997; Murray 1999) (e.g. z saving of labour z reduction of samples to analyse) Direct, non-invasive and non destructive -- -(Gerrard 1991) In-situ analysis -. .(Hendra 1997) Emission process na na - (Vickers &Mann 1991) Intensity proportional to concentration na na - (Gerrard 1991) Well-resolved spectra /- -(Gerrard 1991; Wilson 1991) High resolution spectra -- - (Levin & Lewis 1990) Precise spectral frequency measurement -- -(Levin & Lewis 1990) Non-polar group information // -(Terpinski 1987) More significant information .- -(Schrader 1996; Olinga & Siesler, 2000) Structural selectivity / --(Olinga &Siesler, 2000) Signal intensity -- / Trace element determination /. -(Li-Chan 1996) SPECTROSCOPIC Fluorescence-free spectra na na . (Chase 1987) Spectra insensitive to temperature // -(Pelletier & Davis 1996) Sample presentation technique -. - Push-button instrumentation -- .(van de Voort 1994) Works well with aqueous samples ./ -(Vickers & Mann 1991) Suitable for use at-, on- and in-line process -. -(Scotter 1997; Hendra 1997) Compatible with suitable fibre optics -. -(Lewis et al. 1988; Gerrard 1991; Keller et al. al. 1993; Olinga & Siesler, 2000) Process monitoring -. -(Olinga & Siesler, 2000; Ozaki et al. 1992)19 Microscope -- -(Williams & Batchelder 1994) INSTRUMENTAL Camera (Multispectral measurement) -. / 2. WHY SPECTROSCOPIC TECHNIQUES to traditional, tedious and more time-consuming analytical methods. The advantages of NIR, MIR and Raman First, they are rapid, do not require reagents and spectroscopies are numerous and diversified. Table I are non-destructive. The best illustration of this is in shows the major characteristics of these techniques. protein analysis. The protein content of feedstuffs Note that some of these characteristics are the same and foodstuffs is traditionally determined using the as those of other spectroscopic techniques, such as Kjeldhal method. With this method a single analysis Nuclear Magnetic Resonance or Ultraviolet. The of one sample takes 3 hours and requires several characteristics are categorised into three groups: millilitres of different pollutant reagents (e.g., analytical, spectroscopic and instrumental. For each sulphuric acid) (Bertrand, 2000). Currently, with the characteristic and each technique, a diagram has been growing concern about analysts’ safety and added to indicate whether the characteristic is an environmental protection, laboratories try to keep at advantage ( ), a partial advantage (.) or a a minimum the quantity of potentially dangerous disadvantage (). References in which these reagents used in their analyses. In addition to the characteristics are discussed are included in the table. health and environmental aspects of the use of these 2.1. Analytical characteristics products, it is important to add the cost generated by the adequate treatment of the chemical waste. An The analytical characteristics of NIR, MIR and alternative to the Kjeldahl method is the Raman spectroscopies display certain advantages determination of protein content by NIR which takes that make these techniques attractive as alternatives less than a minute and does not require any Vol. 53. Fasc. 1 (2002) 47 reagents. A similar illustration can be provided with common to all three techniques, whereas others are regard to iodine value determination. The time specific to only one or two of the techniques. The required for a single determination using a classical characteristics in common are the high resolution of technique (i.e., the titrimetric or chromatographic the spectra obtained and the precise spectral method) is 30–60 minutes following the procedure frequency measurement that facilitates the chosen, whereas a spectroscopic determination mathematical treatment (e.g. spectral subtraction), takes about 5 minutes. Iodine value can be as well as the transfer of equation between measured by NIR, MIR or Raman spectroscopy instruments. From the spectroscopic point of view, a (Baeten et al., 2000). For the agro-food sector the general advantage of MIR and Raman speed of analysis is a crucial factor in process control spectroscopies is that bands are well resolved and where any delay in obtaining the analytical results can be assigned to specific chemical groups. There may