Small sample mass

To maintain the sample at the setpoint temperature during a self-feeding reaction in a power-compensated DSC, small sample mass (e.g. <10 mg) and excellent thermal contact between the sample and its container, as well as the container and the chamber, are required. Figure 3.15 shows the rather unusual effects of using excessive sample masses of glass in... 

Small sample masses, typically less than 0.5 g dry matter (with the ensuing problems of inhomogeneity). 

The approximate relationship between the constitution heterogeneity (CH) and the statistical variance, discussed in the next section, can be used to determine the optimal sample mass when sampling to determine the relative amount of trace constituents in the lot. Two series of samples of large and small sample masses are collected and analyzed. The statistical coefficients of variation are calculated. These, together with the values of the sample mass, can be used to determine the minimum sample mass needed to obtain the desired sampling precision of the estimated critical content of the trace constituent. For details, see Chapter 20 of Pitard (1993). 

A Canberra well-detector system using Fitzpeaks gamma spectrometry software was used to determine caesium-137 activities in all sample fractions. Variations in sample height arising from small sample masses were corrected for where appropriate. The detector efficiency calibration was determined using an NPL mixed gamma standard spiked into a sand matrix (Croudace 1991). 

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. 

Sample mass, volume and form are important for recording accurate and reproducible TG curves. Reliable TG curves rely on minimization of deviation between sample temperature and programmed temperature. Deviation typically results from either endothermic or exothermic reactions in the sample and subsequent heat transfer between heat source and sample. Sample mass is the most important parameter affecting TG curves. In general, small sample mass is better than large mass for minimizing temperature deviation. TG sample mass is usually about several milligrams. The low limit of sample mass depends on the resolution limit of the microbalance. 

Small sample masses sample size may range from several milligrams to several hundred milligrams. 

Relative detection limits are useful figures-of-merit for bulk XRF equipment, where it is usually relevant to know the lowest concentration level at which the spectrometer can be used for qualitative or quantitative determinations. In instalments where very small sample masses are being irradiated (e. g., in the pg range for microscopic XRF (p-XRF) and total-reflection XRF (TXRF)), the absolute detection limit is another useful figure-ofmerit since that provides information on the minimal sample mass than can be analysed in a given set-up. 

The coefficients of the Arrhenius equation, i.e. activation energy E and frequency (pre-exponential) factor A, of pyrolysis reactions may be determined using DSC, as exactly as possible for multicomponent systems when very small sample masses (< 5 mg) are used. Determination of the reaction enthalpy is more difficult since the base line of the recorded signal does not run on equal levels before and after the reaction. Therefore it is not possible to determine the exact integration limits with sufficient certainty. 

Figure 4.193 illustrates in its top graph the fast, explosive decomposition of ammonium picrate. At 475 K the fast, exothermic reaction takes place. The study of explosives by thermogravimetry and DTA is possible because of the small sample masses that can be used. An important branch of thermal analysis is thus the study of chemical stability of compounds that are used industrially. 

Remark Calorimeters that use the quantitative analysis of such AT(t) functions for the determination of the sample heat capacity are called relaxation-type calorimeters. The method became important in particular at low temperatures because it works sufficiently well even with very small sample masses (Collocott, 1999). 

Generally, the advantages of such miniaturized calorimeters compared with a common calorimeter (e.g., the TAM see Section 7.9.2.1) are the very low time constant and the very small sample masses needed. 

Analysis by the RULER technique (Remaining Useful Life Evaluation Routine) studies the overall anti-oxidant additive reserve in fresh and used lubricant samples. It is particularly appropriate for the small sample masses available. The RULER voltammogram scan of a 15 minute ring zone sample for lubricant B shows substantial loss of anti-oxidant additive, as evidenced by the relative decline in the peak intensities in the lower curve of Fig 5, relative to the fresh lubricant, the upper curve. 

The increased sensitivity has many benefits, allowing small sample masses to be used, small transitions to be easily observed, and potentially increased accuracy for measurement of specific heat of materials. The reason for the increased sensitivity is that energy flows more quickly at the higher scan rates. The amount of energy involved remains the same but the time during which it flows is reduced as the scan rate is increased, so the y-axis response of the DSC records the energy flow increases with scan rate. See Section 1.4.3 of Chapter 1 for a full explanation. 

Because of the sensitivity of the DSC technique, a relatively small sample mass can be taken, typically a few milligrams, and the reference may be an empty crucible matched in mass to the sample crucible. These steps mean that (i) the DSC curve corresponds closely to the sample enthalpy changes and (ii) thermal lag between the sample-measuring thermocouple and the sample interior is minimised. 

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