Sample decomposition step

The slowest step, which determines the speed of the entire analysis, is the sample decomposition step. Digestion of organic materials has generally taken the longest time of all sample types. Although it is true that most of these procedures could be made very efficient in batch operations, efforts to reduce preparation time by modification of procedures could prove invaluable to a routine laboratory(59). Some examples of this as applied to the analysis of bovine liver(60) and orange juice(61) can be found. 

We encounter several sources of error in the sample decomposition step. In fact, such errors often limit the accuracy that can be achieved in an analysis. The sources of these errors include the following ... 

Hydride generation readily allows the power of detection of atomic spectrometric methods to be increased for the determination of elements having volatile hydrides. Moreover, it allows matrix-free determinations of these elements. It should, however, be emphasized that the technique, irrespective of the type of hydride generation used, is prone to a number of systematic errors. Firstly, the hydrideforming elements must be present as inorganic compounds in a well-defined valence state. This may require a sample decomposition step prior to analysis. In the case of water analysis, a treatment with H2SO4-H2O2 may be effective [184]. 

Although the decomposition of a data table yields the elution profiles of the individual compounds, a calibration step is still required to transform peak areas into concentrations. Essentially we can follow two approaches. The first one is to start with a decomposition of the peak cluster by one of the techniques described before, followed by the integration of the peak of the analyte. By comparing the peak area with those obtained for a number of standards we obtain the amount. One should realize that the decomposition step is necessary because the interfering compound is unknown. The second approach is to directly calibrate the method by RAFA, RBL or GRAFA or to decompose the three-way table by Parafac. A serious problem with these methods is that the data sets measured for the sample and for the standard solution should be perfectly synchronized. 

Although sophisticated methods may constitute the core methods for certification it is useful to include good, well executed routine methods. In order to further minimize systematic error, a conscious purposeful attempt should be made to get methods and procedures with wide-ranging and different sample preparation steps, including no decomposition as in instrumental neutron activation analysis and particle induced X-ray emission spectrometry. 

Following RM certificate instructions for material usage and handling, incorporate the RM into the scheme of analysis at the earliest stage possible, i.e. prior to the beginning of sample decomposition. Take it through the entire analytical procedure at the same time and under the identical conditions as the actual analytical samples in order to correctly monitor all the sample manipulation and measurement steps. 

SFE-GC-MS is particularly useful for (semi)volatile analysis of thermo-labile compounds, which degrade at the higher temperatures used for HS-GC-MS. Vreuls et al. [303] have reported in-vial liquid-liquid extraction with subsequent large-volume on-column injection into GC-MS for the determination of organics in water samples. Automated in-vial LLE-GC-MS requires no sample preparation steps such as filtration or solvent evaporation. On-line SPE-GC-MS has been reported [304], Smart et al. [305] used thermal extraction-gas chromatography-ion trap mass spectrometry (TE-GC-MS) for direct analysis of TLC spots. Scraped-off material was gradually heated, and the analytes were thermally extracted. This thermal desorption method is milder than laser desorption, and allows analysis without extensive decomposition. 

Sample decomposition is not frequently automated because of difficulties caused by the long exposure times needed and the corrosive environments they produce. Hawk and Kingston [11] have described recent advances which make this preparative step more suitable for automation and particularly suited to robotic applications. 

The crude sample from Step 1 (100 parts) and 0.05 parts of the decomposition inhibitor, 3-ethyl-3-hydroxymethyloxetane, were distilled at 120 °C at 3 mm Hg through an air cooled condenser containing a 5 cm vigoureux column and the product isolated in 95.3 % yield. 

In addition to the normal flame technique, AAS may be carried out using a graphite furnace. This technique is capable of greater sensitivity than flame operation but usually shows poorer accuracy. The furnace technique requires the same steps for sample decomposition as do the flame techniques. 

The chief methods used for the decomposition of organic and inorganic samples have been evaluated. A brief summary of applications of these techniques to various sample matrices is presented in Table 5.6. The variety of approaches currently available for the decomposition of solid and liquid samples allows the most suitable method to be selected for each application, depending on both the matrix and type of analyte, and subsequent steps to be developed in order to complete the analytical process. In spite of this, sample decomposition should not be looked at as an isolated step, but one that needs to be integrated into the entire analytical process. 

An important capability of Curie point pyrolysers should be that the sample does not suffer any modifications before the pyrolysis step itself. As previously indicated, the housing of the pyrolyser must be heated (commonly with electrical resistances) to avoid condensation or other modifications of the pyrolysate. However, because a waiting time is inherent between the moment of sample introduction in the pyrolyser and the start of the pyrolysis itself, the sample may be heated by radiation from the sample housing. Several Curie point pyrolysers [8b] have the capability to drop the ferromagnetic foil containing the sample from a cool zone into the induction area, which is pre-heated to avoid condensation. The pyrolysis takes place immediately after the sample is transferred into this induction area such that no uncontrolled preliminary sample decomposition takes place. 

Reversible reactions. Many solid-gas reactions are reversible, e.g., dehydration of crystal hydrates, so that rate equations for such processes should include terms for the rate of the reverse reaction. If the rates of contributing forward and reverse reactions are comparable, the general set of kinetic models (Table 3.3.) will not be applicable. The decomposition step in a reversible reaction thus needs to be studied [94] under conditions as far removed from equilibrium as possible (e.g. low pressures or high flow rates of carrier gas) and sensitive tests are required for determining whether the kinetics vary with the prevailing conditions. Sinev [95] has calculated that, for the decomposition of calcium carbonate, the rate of the reverse reaction is comparable with that of the forward reaction even when small sample masses (10 mg) and high flow rates (200 cm s ) of inert gas are used. Interpretation of observations becomes more difficult and the reliabihty of conclusions decreases if local inhomogeneities of kinetic behaviour develop within the reactant mass. 

Non-isothermal kinetic studies [69] of the decomposition of samples of nickel oxalate dihydrate doped with Li and Cr showed no regular pattern of behaviour in the values of the Arrhenius parameters reported for the dehydration. There was evidence that lithium promoted the subsequent decomposition step, but no description of the role of the additive was given. 

Many decompositions are preceded by a dehydration step (see Chapter 7) which has a disrupting effect on the original crystal structure and often yields an amorphous product as the reactant in the decomposition step. Surface textures are observed to change considerably during dehydration. The kinetics of the subsequent decomposition may be very dependent upon the dehydration conditions. If, however, the anhydrous sample recrystallizes before decomposing, the defect content will again be modified. Retention of water may promote recrystallization [20] and rates of decomposition of recrystallized and of amorphous material may be very different. 

In the decomposition step, the sample is heated with sulfuric acid, and the carbonaceous residue is burned off in a muffle furnace. The resultant ash is dissolved in hydrochloric acid, treated with hydroxylamine to reduce the antimony to the trivalent state, and stabilized with tartaric acid. Other ashing aids, such as nitric and perchloric acids or magnesium nitrate, have been used with success in determining antimony (21), but these were not investigated for use because of the observations regarding antimony measurement by heated vaporization atomic absorption which are discussed below. 

Isothermal arrhenius analysis were performed as follows. Measurements were started after several hydriding/dehydriding cycles with samples in the fully hydrided condition and cooled to room temperature. The pressure rise from desorption into a known volume at a given temperature was measured and the temperature was then increased. Desorption rates were determined at each temperature from the slope of the essentially linear increase in pressure with time. This procedure was continued up to 150°C. The sample was held at this temperature until the NaAlH4 decomposition step was finished. The rate data are presented as moles of desorbed hydrogen per mole active sodium per hour as shown in Figure 2. 

Slurry sampling - the preparation of materials by dispersing finely-ground samples in a liquid medium for introduction into atomic spectrometers - has seen much activity in the quest to simplify and accelerate the sample preparation step and avoid, to some extent, sample decomposition in the usual sense. Tan and Marshall... 

Detection of water and C02 with FTIR at low concentrations is unreliable, due to their presence in the atmosphere. These compounds can be detected with MS. The release of water (m/z = 18) and C02 (m/z = 44) during the decomposition of a CPC, system B sample is shown in Figure 6.20. Absorbed water is released during the first ten minutes of the experiment (40°C - 140°C), followed by an amount of chemically bound water at the very beginning of the first decomposition step (the thermal stability of the ammonia ligand is thus higher than that of the water ligand ). 

The iiuroduclion of solids in the form of powders, metals, or particulates into plasma and flame atomizers has the udvanlage of avoiding the oflcn tedious and time-consuming step of sample decomposition and dissoluiion. Such procedures, however, oflcn suffer from severe difficulties with calibration, sample condi-lioning. precision, and accuracy. 

To obtain TRIR spectra with sufficient sensitivity, we typically signal average several thousand laser shots at each IR frequency of interest. A flow cell, therefore, is necessary to prevent excessive sample decomposition, especially when photo-irreversible processes are monitored. A reservoir of solution (typically 10-20 mL) is continually circulated between two calcium or barium fluoride salt plates. To maintain sample integrity for non cyclic systems, one is usually forced in the dispersive TRIR experiment to acquire data in a series of short (e.g., 100-200 cm ) scans rather than in one complete scan. Thus, a substantial amount of sample may be required. Sample integrity is also of significant concern in the step-scan FTIR experiment because data must be collected at each mirror position. To address this concern, very large reservoirs of solution are required alternatively, a sample changing wheel [33] or very focused pump-probe beams in combination with sample translation [34] have been used with thin film samples. 

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