Direct solid sample analysis

The direct analysis of solid samples, independent of the analytical technique that is used for that purpose, has a number of distinct advantages over procedures that involve an acid digestion or other dissolution methods. Although the relative importance of the various aspects depends very much on the type of sample that has to be analyzed, the analyte that has to be determined and its concentration, and the specific requirements regarding speed of analysis, accuracy and precision, the following features are generally accepted  

Direct analysis of solid samples is faster as only a minimum of sample preparation is required typical sample preparation steps, such as grinding and homogenization of heterogeneous materials, have to be carried out prior to a sample digestion. 

Direct analysis of solid samples is more sensitive, as the analyte content is not diluted during sample preparation. 

The risks of analyte loss and/or contamination are reduced to a minimum as essentially no reagents and only a minimum of laboratory ware is used. 

The use of corrosive and potentially hazardous reagents is avoided, reducing the waste problem tremendously, and contributing to the concept of green chemistry . 

---

A suitable method to determine the degree of homogeneity of an element in a material is by repetitive analysis of a large number of small soUd ahquots by direct solid sample analysis. As shown in Figure 4.3, there is a functional relationship between the sample mass used for analysis and the standard deviation of repetitive analysis. 

ScHRON W, Liebmann A, Nimmereall G 2000) Direct solid sample analysis of sediment, soils, rocks and advanced ceramics by ETV-ICP-AES and GF-AAS. Fresenius J Anal Chem 366 79-88. 

Direct solid sample analysis is still mostly a subsidiary method, confined to specific analytical tasks, rather than truly complementary to traditional analysis via solutions. Solid sampling is not standard in routine... 

Nimmerfall G, Schron W (2001) Direct solid sample analysis of geological samples with SS-GF-AAS and use of 3D calibration. Fresenius J Anal Chem 370 760... 

Figure 4.8 shows an example for this mode of correction applied for the determination of Pb in pig kidney reference material, using direct solid sample analysis. The three-dimensional plot in Figure 4.8a shows that a strong molecular absorption with pronounced fine structure appears short after the atomic absorption signal. Figure 4.8b shows the time-integrated absorbance spectrum of PO,... 

Figure 4.8. Least-squares BC for molecular spectra with rotational fine structure determination of Pb in the BCR 186 Pig Kidney CRM at 217.001 nm using HR-CS ET AAS and direct solid sample analysis (a) absorbance over time and wavelength after correction for continuous absorption (b) reference spectrum absorbance over wavelength integrated over time for NH4H2P04 (the dotted line represents the center pixel) (c) absorbance over time and wavelength after subtraction of the reference spectrum using least-squares BC.

Figure 4.17. Three-dimensional graph for Cr in the NIST 8415 Whole Egg Powder SRM in the vicinity of the 357.87 nm line pyrolysis temperature 700°C, atomization temperature 2500°C direct solid sampling analysis.

Figure 4.19. Determination of Pb in biological reference materials in the vicinity of the 217.001 nm line pyrolysis temperature 700°C, atomization temperature 1700°C Ru permanent modifier direct solid sampling analysis (a) wavelength integrated over time for the NIST 8414 Bovine Muscle SRM (b) wavelength integrated over time for NIST 8415 Whole Egg Powder (c) absorbance over time for Whole Egg Powder, recorded at selected pixels in the vicinity of the analytical line.

Figure 4.20. Three-dimensional graphs for Se in the BCR 186 Pig Kidney CRM pyrolysis temperature 800°C, atomization temperature 2000°C Ir permanent modifier direct solid sampling analysis (a) in the vicinity of the 196.026 nm line (b) in the vicinity of the 203.985 nm line.

Although most samples are commonly presented as liquids for atomic emission spectroscopy, direct solid sample analysis has the advantage that no major pretreatment or dissolution steps are required [44]. This minimises dilution errors or contamination from reagents and reduces the reagent and manpower cost per sample. In addition, improved detection Hmits may be obtained if microsamples or microanalysis are possible without any further dilution. However, the analyst has to ensure that the solid material sampled is representative of the bulk material. ICP-AES has generally a remarkable tolerance for total dissolved sohds compared to ICP-MS or flame AAS so that, depending on the overall matrix, between 2 and 25 % suspended sohds can be coped with. Therefore, most of the sohd sample introduction devices described below are dedicated for ICP-AES. 

X-ray fluorescence XRF is one of the longest established techniques for trace elemental analysis. While XRF is not a very sensitive technique, its main advantages are the capability for direct solid sample analysis combined with multielement determinations. While sample pretreatment of solids can be substantially reduced or even omitted in some cases, perfect matching between standards and samples is required for accurate results, because of severe matrix effects. The main application field of XRF is, therefore, the analysis of solid materials, such as metallurgical and geological samples, where solid standards are readily available. Liquid samples can be analyzed either directly in special cells or by using preconcentration techniques with solid sorbents, which can be directly analyzed after sample loading. More modern methods, like total-reflection X-ray fluorescence, which is a multielement technique mainly for solutions, or particle-induced X-ray emission, which is a micromethod with some spatial resolution, have found limited application in some special areas. For speciation purposes, species separation has to be carried out in front in an offline mode. 

As derived from data included in Table 44.2, most of the published polymer-related ETV-ICPMS applications deal with direct solid sampling analysis (or direct analysis of organic liquids), where the capability to reduce spectral and nonspectral interferences from such a heavy matrix is featured. A judicious selection of the ETV program in order to achieve this goal (including temperature programming and/or addition of modifiers if needed) was in all cases crucial for optimum results to be obtained and often permit the use of aqueous standards for calibration. 

Chapter 8 provides the reader with the basic principles of isotope dilution mass spectrometry used for elemental analysis and also discusses more advanced features of this calibration approach, such as its use in direct solid sample analysis and in elemental speciation work, wherein not the total amount but that of various chemical species of a target element need to be determined. 

Figure 8.34 Time-resolved absorbance spectrum obtained for DORM-1 Dogfish Muscle reference material in the vicinity of the cadmium resonance hne at 228.802 nm pyrolysis temperature 800 °C atomization temperature 1600 °C iridium as permanent modifier direct solid sample analysis...

Figure 8.36 Absorbance-over-time recording for the atomic absorption (AA) and the backgronnd absorption (BG) for DOLT-2 Dogfish Liver reference material at the cobalt absorption line nsing conventional LS AAS (a) solubilized in TMAH (b) direct solid sample analysis...

Figure 8.42 Absorbance over time without (dashed line) and with (solid line) automatic correction for continuous background for cadmium in SARM-19 Coal reference material at 228.802 nm pyrolysis temperature 600 °C, atomization temperature 1600 °C direct solid sample analysis with Ir as permanent modifier...

Ren, J. M., Rattray, R., Salin, E. D., and Gregoire, D. C. (1995). Assessment of direct solid sample analysis by graphite pellet electrothermal vaporization inductively coupled plasma mass spectrometry.J. Anal. At. Spectrom. 10(11), 1027. 

---