Metal analysis sample preparation

Table 5.18 Results of comparative study of analysis of 250 ggkg-1 of spiked toxic metals after sample preparation using microwave acid digestion and oxygen bomb combustion. Results in brackets are percentage recovery...

For Raman analysis, sample preparation is much easier than with IR. In fact, the source light is simply focussed onto the solid or liquid sample directly. If a cuvette is used, quartz or glass windows can be used. If a slide or surface is used, a background spectrum should be taken to remove the possibility of any interfering peaks. Glass tubes are often used and since water is a weak Raman scatterer, aqueous samples can be easily analysed. Reflectance measurements, as distinct from transmissive measurements above, can also be made and are useful for studying Aims on metal surfaces or samples on diamond surfaces. Measurements should also ideally take place in the dark to remove ambient light interferences. 

Because of the huge signal enhancement, SERS is particularly useful for trace analysis and for in-situ investigations of various interfacial processes or of mono-layers adsorbed on metals. However, sample preparation is a rather tedious procedure. For this reason, SERS is still more an academic tool rather than a routine analytical instrument. Some applications of SERS are given in [31]. 

G.J. DeMenna and W.J. Edison, Novel Sample Preparation Techniques for Chemical Analysis - Microwave and Pressure, Dissolution, Chemical Analysis of Metals, ASTM STP 994 (F.T. Coyle, ed.), American Society for Testing and Materials, Philadelphia, PA (1987), p. 45. 

Applications Real applications of spark-source MS started on an empirical basis before fundamental insights were available. SSMS is now considered obsolete in many areas, but various unique applications for a variety of biological substances and metals are reported. Usually, each application requires specific sample preparation, sparking procedure and ion detection. SSMS is now used only in a few laboratories worldwide. Spark-source mass spectrometry is still attractive for certain applications (e.g. in the microelectronics industry). This is especially so when a multi-element survey analysis is required, for which the accuracy of the technique is sufficient (generally 15-30% with calibration or within an order of magnitude without). SSMS is considered to be a... 

Gehre, M., Hoefling, R., Kowski, P. and Strauch, G. (1996) Sample preparation device for quantitative hydrogen isotope analysis using chromium metal. Analytical Chemistry 68, 4414 4417. 

Transmission electron microscopy (TEM) is a powerful and mature microstructural characterization technique. The principles and applications of TEM have been described in many books [16 20]. The image formation in TEM is similar to that in optical microscopy, but the resolution of TEM is far superior to that of an optical microscope due to the enormous differences in the wavelengths of the sources used in these two microscopes. Today, most TEMs can be routinely operated at a resolution better than 0.2 nm, which provides the desired microstructural information about ultrathin layers and their interfaces in OLEDs. Electron beams can be focused to nanometer size, so nanochemical analysis of materials can be performed [21]. These unique abilities to provide structural and chemical information down to atomic-nanometer dimensions make it an indispensable technique in OLED development. However, TEM specimens need to be very thin to make them transparent to electrons. This is one of the most formidable obstacles in using TEM in this field. Current versions of OLEDs are composed of hard glass substrates, soft organic materials, and metal layers. Conventional TEM sample preparation techniques are no longer suitable for these samples [22-24], Recently, these difficulties have been overcome by using the advanced dual beam (DB) microscopy technique, which will be discussed later. 

Sample preparation for analysis by hyphenated methods requires some additional planning when compared to nonhyphenated methods. All steps, extraction, concentration, and final solvent selection must take into consideration and be compatible with all the components of the hyphenated instrumentation. For gas chromatographic methods, all the components in the mixture must be in the gaseous state. For liquid chromatography (LC) or high-performance liquid chromatography (HPLC), the samples of the analytes of interest can be solids or liquids, neutral or charged molecules, or ions, but they must be in solution. If the follow-on analysis is by MS, then each of the analytes may require a different method of introduction into the MS. Metals and metal ions may be introduced by HPLC if they are in solution but commonly are introduced via AAS or inductively coupled plasma (ICP). Other analytes may be directly introduced from HPLC to MS [2],... 

Probably the most effective use of XRF and TXRF continues to be in the analysis of samples of biological origin. For instance, TXRF has been used without a significant amount of sample preparation to determine the metal cofactors in enzyme complexes [86]. The protein content in a number of enzymes has been deduced through a TXRF of the sulfur content of the component methionine and cysteine [87]. It was found that for enzymes with low molecular weights and minor amounts of buffer components that a reliable determination of sulfur was possible. In other works, TXRF was used to determine trace elements in serum and homogenized brain samples [88], selenium and other trace elements in serum and urine [89], lead in whole human blood [90], and the Zn/Cu ratio in serum as a means to aid cancer diagnosis [91]. 

Finally, the analytical method should be selected depending on the sensitivity reqnired, the compatibility of the sample matrix with the specific analysis techniqne, and the availability of facilities. Sample preparation, if it is required, can present problems. Significant losses can occur, especially in the case of organometallic complexes, and contamination of environmental sample is of serious concern. The precision of the analysis depends on the metal itself, the method nsed, and the standard nsed for calibration of the instrument. 

In addition, the sample prepared with BMI PF was submitted to Ar sputtering followed by further XPS analysis. After sputtering, the F Is signal was eliminated as well as the Ir—F component in the Ir 4f region (which displays mainly the Ir—Ir component), showing that only the external surface iridium atoms were bounded to F (Figure 15.6). These results indicated strongly that, besides the presence of an Ir—O layer, it was the effective interachon of the IL with the metal surface that may have been responsible for stabilization of the nanoparhcles. 

Automation of sample preparation of foodstuffs for trace metal analysis 123... 

The sample-preparation technique may depend on a number of variables, for example the molecular weight of sample and interferences, the sample volume and analyte concentration, buffer salt (anion and cation) content and metal concentration and type. Other than filtration for particulate removal, most of the approaches are based on the use of chromatographic media for cleaning up samples before analysis. 

AUTOMATION OF SAMPLE PREPARATION OF FOODSTUFFS FOR TRACE METAL ANALYSIS... 

Fig. 4.12 Schematic diagram of the system for automatic sample preparation of foods for trace metal analysis.

Nanospray is a miniaturized version of electrospray. In the original setup of Wilm and Mann (8) it is utilized as an off-line technique using disposable, finely drawn (1 -gm tip), metallized glass capillaries to infuse samples at 10-30 nL/min flow rates. This allows more than 50 min analysis time with just a 1-pT sample. Due to the formation of much smaller droplets and the more effective ionization, there is often no need for LC separation, since the separation is accomplished in m/z or by MS/MS. However, limited reproducibility with respect to quantification and a more complex sample preparation can be seen as drawbacks. An on-line version for hyphenation with capillary and nano-LC as well as CE (slightly modified) is now commercially available. 

---