Solid samples, automatic analysis

Table 4.15 fists the many possibilities for solid sampling for GC analysis. In general, sample preparation should be considered in close conjunction with injection. Robotic sample processors have been introduced for automatic preparation, solvent extraction and injection of samples for GC and GC-MS analyses. Usually, facilities are included for solvent, reagent, and standard additions and for derivatisation of samples. 

Analytical Methods. Liquid scintillation counting (LSC) was done using Packard Models 3375 and 3380 Liquid Scintillation Spectrometers equipped with automatic external standards. Solid samples were combusted in a Packard model 306 Sample Oxidizer prior to LSC analysis. 

These liner exchange systems make feasible yet another analysis mode direct thermal desorption (DTD). Here the liner or an insert is packed with the solid sample. The liner exchange system can then be used in place of a conventional autosampler. The liner is automatically inserted into the PTV and the volatiles thermally desorbed onto the column. Some analysts may feel uneasy about such desorption from the solid phase how does one know that all of the volatile analytes have been released from the sample crystal lattice However, where applicable, this approach may not be as difficult to validate as one might imagine. For instance, the PTV can be cooled after the analyte transfer, and then, at the end of the chromatographic temperature programme, reheated to repeat the process. Ideally all of the analyte should transfer in the first cycle and none in the second, demonstrating that complete desorption occurs in the method. 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

The homogeneity problem is specific to solid samples, as liquid and gaseous samples are considered homogeneous by nature. Thus, for solid samples, an automatic sampling process is recommended to obtain reliable analytical information. To select the most adequate system to obtain a homogeneous sample it is necessary to take into account the sample complexity and the stability of the sample within a certain period of time.37 Further, it is necessary to establish first the nature of analysis that will be used in sample control, especially when the limits of detection are low. One must also be wary of the many contamination risks from reagent impurities, laboratory vessels, laboratory climate, and the operator. 

Synchroton radiation has been employed as a spectral source for a study of the absorption of HCN and DCN in the wavelength range 80—120nm. A vacuum-u.v. spectrophotometer for absorptions in the region 105—200 nm has been described. Solid-, liquid-, and gas-phase samples could be analysed at temperatures from —200 to 100 °C and at pressures between 0 and 150 atmospheres. The absorption spectrum of tra j-di-imide in the vacuum-u.v. has been measured. First-derivative u.v. spectroscopy has been employed in the analysis of Watts nickel plating solutions for trace amounts of saccharin. Impurity levels of 0.1 p.p.m. have been recorded. A wavelength modulated derivative spectrophotometer with a multi-pass absorption cell has been developed for the automatic analysis of atmospheric pollutants. Traces of SOj, NO, and NO2 were detected with limits of 15, 13, and Sp.p.b., respectively. A double-beam single-detector absorption spectrometer has been constructed. Independence... 

In a Hrst arrangement the process control system includes automatic sampUng in the hot zone such as an explosion-proof container, the manual or automatic transfer of the sample to the laboratory, a robot to prepare the analysis, and transfer of the solution to be analyzed to an fiber-optic measurement cell [163]. This architecture is most commonly found today if the laboratory is habitually and directly involved in controlling the various steps in the process. It is JustiHed if the samples must also be prepared in a hot zone, or if the analysis requires semiautomatic operations such as dissolution of solid samples, or sophisticated techniques such as chromatography, mass spectrometry, or emission spec-... 

Special probes have been developed for solid-state NMR that automatically position the sample at the magic angle. Modern instruments with MAS make the analysis of solid samples by NMR a routine analytical procedure. MAS, combined with two RF pulse techniques called cross-polarization and dipolar decoupling (discussed in Section 3.6.4), permits the use of the low-abundance nuclei C and Si to analyze insoluble materials by NMR, including highly cross-linked polymers, glasses, ceramics, and minerals. 

Pumping down the introduction chamber takes usually between half a hour and a few hours depending on the type, composition, and number of samples. High surface area solids and solids releasing water or other volatile compounds may need longer pumping time they may require to reduce the number of samples introduced together and to reduce sample size. Samples are transferred to the analysis chamber with a transfer probe (e in Fig. 7). On recent systems, automatic analysis of series of samples is feasible, thanks to motorized X, Y, Z displacement (f in Fig. 7). 

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...

P T-GC). This technique is now frequently used to detect residues of volatile organic compounds in the environment and in the analysis of liquid and solid foodstuffs. In particular, the coupling with GC-MS systems allows its use as a multi-component process for the automatic analysis of a large series of samples. 

In static headspace sampling [301,302] the polymer is heated in a septum-capped vial for a time sufficient for the solid and vapour phases to reach equilibrium (typically 2 hours). The headspace is then sampled (either manually or automatically) for GC analysis, often followed by FID or NPD detection. Headspace sampling is a very effective method for maintaining a clean chromatographic system. Changing equilibrium temperature and time, and the volumes present in the headspace vial can influence the sensitivity of the static headspace system. SHS-GC-MS is capable of analysing volatile compounds in full scan with ppb level... 

The Gilson Aspec automatic sample preparation system is a fully automated system for solid-phase extraction on disposable columns and online HPLC analysis. The Aspec system offers total automation and total control of the entire sample preparation process including clean-up and concentration. In addition, Aspec can automatically inject prepared samples into on-line HPLC systems. 

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