Technique microsampling

Preparation of very small amounts of essential oils may be necessary if only very small amounts of plant material are available and can be fundamental in chemotaxonomic investigations and control analysis but also for medicinal and spice plant breeding. In the past, numerous attempts have been made to minimize conventional distillation devices. As an example, the modi ed Marcusson device may be quoted (Bicchi et al., 1983) by which 0.2 3 g plant material suspended in 50 mL water can be distilled and collected in 100 pL analytical grade pentane or hexane. The analytical results proved to be identical with those obtained by conventional distillation. 

Microversions of the distillation-extraction apparatus, described by Likens and Nickerson, have also been developed as well for high-density (Godefroot et al., 1981) and low-density solvents (Godefroot et al., 1982). The main advantage of these techniques is that no further enrichment by evaporation is required for subsequent gas chromatographic investigation. 

Headspace (HS) analysis has become one of the very frequently used sampling techniques in the investigation of aromatic plants, fragrances, and spices. It is a means of separating the volatiles from a liquid or solid prior to gas chromatographic analysis and is preferably used for samples that cannot be directly injected into a gas chromatograph. The applied techniques are usually classi ed accord ing to the different sampling principles in static HS analysis and dynamic HS analysis. 

1 Static HS Methods In static HS analysis, the liquid or solid sample is placed into 

Handbook of Essential Oils Science, Technology, and Applications 

5 Stir Bar Sorptive Extraction and Headspace Sorptive Extraction 

Despite the indisputable simplicity and rapidity of SPME, its applicability is limited by the small amount of sorbent on the needle ( 0.5 pL) and consequently SPME has no real opportunity to realize quantitative extraction. Parameters governing recovery of analytes from a sample are partitioning constants and the phase ratio between the sorbent and liquid or gaseous sample. Therefore, basing on theoretical considerations, a procedure for sorptive enrichment with the sensitivity of packed PDMS beds (Baltussen et al., 1997) has been developed for the extraction of aqueous samples using modified PDMS-coated stir bars (Baltussen et al., 1999). 

The stir bars were incorporated into a narrow glass tube coated with a PDMS layer of 1 mm (corresponding to 55 pL for a 10 mm length) applicable to small sample volumes. Such stir bars are commercially available under name Twister (Gerstel, Germany). After certain stirring time the 

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Portland cement is susceptible to corrosion by CO2 and H2S. The chemical attack by CO2 is called carbonation. A microsample technique has been developed to study the CO2 corrosion in cements, because the corrosion is difficult to monitor with common test procedures [264]. This technique is also advantageous as an accelerated testing method. A polymer-modified cement has been tested in field studies [694]. The addition of silica also improves chemical resistance [146], in particular brine corrosion. 

Depth profile of elements in seawater near hydrothermal vents. [From T. Akagi and H. Haraguchi, Simultaneous Multielement Determination of Trace Metals Using W mL of Seawater by Inductively Coupled Plasma Atomic Emission Spectrometry with Gallium Coprecipitation and Microsampling Technique Anal. Chem. 1990, 62.81.]... 

Knshnan K (1988) Characterization of semiconductor silicon using the FT-TR microsampling techniques In Messerschmidt RG, Harthcock MA (eds) Infrared microspectroscopy theory and applications Marcel Dekker, New York, 139-151 Kuo MI, McClelland JF, Luo S, Chien PL, Walker RD, Hse CY (1988) Applications of infrared photoacoustic spectroscopy for wood samples Wood Fiber Sci 20 132-145 Lai Y-Z, Sarkanen KV (1975) Structrual variation in dehydrogenation polymers of comferyl alcohol Cellul Chem Technol 9 239-245... 

Diffuse reflectance has also been used extensively for trace analysis. In such a case, the sample to be studied is dissolved using a volatile solvent and transferred to KBr powder, preferably in microcups. The solvent is evaporated, and the sample has coated the KBr particles. With microsampling techniques, sample loadings lower than 100 ng can give satisfactory spectra. If strict rules for avoiding impurities and contamination are observed, detection limits even at the low nanogram... 

Krishnan, K. Hill, S.L. ET-IR Microsampling Techniques Bio-Rad Digilab Division Cambridge,, MA, 1990. 

The various reflectance methods which are now widely available, such as attenuated total reflectance, specular reflectance, and diffuse reflectance spectroscopy, were also introduced. Photoacoustic spectroscopy, gas chromatography-infrared spectroscopy, temperature studies and microsampling techniques were also described. 

Voellkopf, U., Paul, M. and Denoyer, E.R. (1992). Analysis of solid samples by ICP-mass spectrometry, Fres. Z. Anal. Chem. 342, 907-916 Wei, S.H.Y. (1973). Enamel microsampling technique for assessing fluoride uptake from topical fluoride treatments in vitro, J. Dent. Res. 52,1268-1272 Williams, D.M. (1979). Trace metal determinations in blood obtained in evacuated collection tubes, Clin. Chim. Acta 99, 23-29... 

Microsampling techniques have to be applied when either small amounts or small sizes of analytes have to be investigated. Microsampling techniques may be derived from conventional techniques by miniaturisation. For all such miniaturized sampling techniques a beam condenser (micro-illuminator) is needed. Standard beam condensers are made of a pair of ellipsoidal mirrors. MicropeUets for solids have a diameter of 0.5 and 1.5 mm with sample amounts of 5 to 10 pg in 4 mg KBr. For liquids and solutions, microcells with volume down to 0.3 pi are commercially available. A special case of microsampling is the so-called diamond anvil cell, where a tiny drop of liquid analyte is squeezed between two diamond crystals. Even solidification of sample between the diamonds can be achieved by applying pressures up to 100 bar. Usual dimensions of the diamond surfaces are below one millimeter. This technique is very useful for conformational analysis. 

AU microsampling techniques require very thorough sample preparation. To obtain meaningful results in either transmission or reflection mode of an IR microscope, sufficient skills in microscopic sample preparation are required. 

This article covers the preparation and presentation methods, both traditional and the more recently developed, used in sample analyses by mid-infrared spectroscopy. It focuses more on the so-called macrosampling techniques, and does not cover in detail some of the sample presentation methods, for example diamond-window compression cells (see example in Figure 7), more particularly used nowadays in studies made using an infrared microscope many of the microsampling techniques are, however, merely adaptations of the macrosampling techniques. 

Krishan, K., and S. L. Hill. "FTIR Microsampling Techniques," in J. R. Ferraro, ed.. Practical Fourier Transform Infrared Spectroscopy. New York Academic Press, 1989. 

Fujimori, E., Sawatari, H., Hitose, A., and Har jichi, H. (1994). Simultaneous multielement analysis of rock samples by inductively coupled plasma mass spectrometry using discrete microsampling technique. Chem. Lett.,p. 1467. 

Fujimori, E., Wei, R., Sawatari, H., Chiba, K., and Haraguchi, H. (1996). Multielement determination of trace elements in sediment sample by inductively coupled plasma mass spectrometry with microsampling technique. Bull. Chem. Soc.Jpn. 69(12), 3505. 

Photoacoustic spectroscopy Depth profiting using PAS Emission spectroscopy Microsampling techniques Data processing of digitized IR spectra Elimination of spectral backgrounds... 

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