Compositional calibration target

Fig. 3.16 Schematic drawing of the MIMOS II Mossbauer spectrometer. The position of the loudspeaker type velocity transducer to which both the reference and main Co/Rh sources are attached is shown. The room temperature transmission spectrum for a prototype internal reference standard shows the peaks corresponding to hematite (a-Fe203), a-Fe, and magnetite (Fe304). The internal reference standards for MIMOS II flight units are hematite, magnetite, and metallic iron. The backscatter spectrum for magnetite (from the external CCT (Compositional Calibration Target) on the rover) is also shown...

An example of a simulated overnight experiment on Mars is shown in Fig. 3.22 for eight temperature intervals using the Compositional Calibration Target (CCT made out of magnetite rock slab) on the rover as the target. In the case of contact-ring temperature sensor failure, the internal temperature sensor would be used (software selectable). 

If high-resolution measurements are performed in order to assign elemental compositions, internal mass calibration is almost always required. The calibration compound can be introduced from a second inlet system or be mixed with the analyte before the analysis. Mixing calibration compounds with the analyte requires some operational skills in order not to suppress the analyte by the reference or vice versa. Therefore, a separate inlet to introduce the calibration compound is advantageous. This can be achieved by introducing volatile standards such as PFK from a reference inlet system in electron ionization, by use of a dual-target probe in fast atom bombardment, or by use of a second sprayer in electrospray ionization. 

As the precision of the ICP-MS isotope ratio is poor compared with the precision using TIMS, the range of applications for ICP-MS have traditionally been limited to measuring induced changes in the isotopic composition of a target element (for example, to calibrate by means of isotope dilution). However, the introduction of multicollector ICP-MS systems to enhance precision and accuracy in isotopic analysis opened up novel applications. 

In EDX experiments on thick samples, for example in SEM, almost all the energy of the incident electron beam is consumed to produce X-rays and the number of atoms in a sample can be calculated from the X-ray intensities in the EDX spectra, by carrying out the ZAF calibration (Z = the atomic number effect A = the absorption effect and F = the fluorescence effect) [18]. However, HRTEM specimens are usually thin, 200nm or less. In this case, most electrons in the incident beam will pass through the specimen and the ZAF calibration cannot be done. In practice, we use some standard specimens (whose compositions are known) as references to obtain a relative composition of a target sample. 

The analytical characteristics of the laser mass spectrometer, as shown above in Table 1, were confirmed experimentally with the help of laser targets of different materials, alloys, sediments and other samples. They were approved in calibration procedures with standard reference materials with certified presence of elements and isotopes and checked by analysis of targets with preliminary unknown elemental composition. 

FIGURE 19.4 Workflow of the PCR/ESI-MS process. Step 1 Extraction and amplification of target DNA with coamplification of calibrant DNA for quality control and semiquantitative analysis of PCR products. Step 2 Mass spectrometric analysis is performed separately on each well in a TOE mass spectrometer with ESI. Step 3 Data processing to determine base composition from mass spectrometric signal and correlation of that base composition to a database of known base compositions for each primer pair to determine organism identification [32]. 

Calibration is typically carried out with solid standards of a composition as similar as possible to that of the target samples, and a suitable internal reference element. This topic will be discussed in detail in Section 39.3.3. Finally, LA-ICPMS also allows for the determination of isotope ratios, which can be a huge bonus for provenancing archaeological samples. The peculiarities of this type of application will be illustrated in Section 39.4. 

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