The Sample

In many cases, polymers are insoluble at normal temperatures but may be soluble at elevated temperatures. Commercial spectrometers are normally capable of operation up to temperatures of 150 or 200°C. In extreme cases of insolubility, such as for some fluoropolymers [25], the spectrometer may be 

---

The atomic weight of a particular sample depends on the origin of the sample. 

Gouy balance A balance for the determination of magnetic susceptibility. The sample is weighed in and out of a magnetic field and the susceptibility is calculated from the difference in weights. 

X-ray fluorescence A method of analysis used to identify and measure heavy elements in the presence of each other in any matrix. The sample is irradiated with a beam of primary X-rays of greater energy than the characteristic X-radiation of the elements in the sample. This results in the excitation of the heavy elements present and the emission of characteristic X-ray energies, which can be separated into individual wavelengths and measured. The technique is not suitable for use with elements of lower atomic number than calcium. 

One distinguishes preparatory distillations that are designed to separate the fractions for subsequent analysis from non-preparatory analytical distillations that are performed to characterize the feed itself. For example, the distillation curve that gives the recovered volume or weight as a function of the distillation temperature characterizes the volatility of the sample. 

The sample can vary from 0.5 to 30 liters enabling the following ... 

The sample is distilled at predetermined and precisely controlled temperatures under conditions that give a fractionation equivalent to about one theoretical plate. 

The column is swept continuously by a carrier gas such as helium, hydrogen, nitrogen or argon. The sample is injected into the head of the column where it is vaporized and picked up by the carrier gas. In packed columns, the injected volume is on the order of a microliter, whereas in a capillary column a flow divider (split) is installed at the head of the column and only a tiny fraction of the volume injected, about one per cent, is carried into the column. The different components migrate through the length of the column by a continuous succession of equilibria between the stationary and mobile phases. The components are held up by their attraction for the stationary phase and their vaporization temperatures. 

A correlation between retention times and boiling points is established by calibration with a known mixture of hydrocarbons, usually normal paraffins, whose boiling points are known (see Figure 2.2). From this information, the distribution of boiling points of the sample mixture is obtained. 

Finally, other methods are used to obtain simulated distillation by gas phase chromatography for atmospheric or vacuum residues. For these cases, some of the sample components can not elute and an internal standard is added to the sample in order to obtain this quantity with precision. 

The complexity of petroleum products raises the question of sample validity is the sample representative of the total flow The problem becomes that much more difficult when dealing with samples of heavy materials or samples coming from separations. The diverse chemical families in a petroleum cut can have very different physical characteristics and the homogeneous nature of the cut is often due to the delicate equilibrium between its components. The equilibrium can be upset by extraction or by addition of certain materials as in the case of the precipitation of asphaltenes by light paraffins. 

Before withdrawing a sample it is necessary to agitate it, even if it is a gas, and eventually heat the sample being careful to stay below temperatures which could cause evaporation of the lighter components. 

The sample is burned in oxygen at 1000°C. Nitrogen oxide, NO, is formed and transformed into NO2 by ozone, the NO2 thus formed being in an excited state NO. The return to the normal state of the molecule is accompanied by the emission of photons which are detected by photometry. This type of apparatus is very common today and is capable of reaching detectable limits of about 0.5 ppm. 

The sample is reduced in a hydrogen stream at 800°C in the presence of a nickel catalyst. The ammonia formed is detected by coulometry and the test sensitivity is on the order of one part per million. 

For trace quantities of less than 100 ppm, the most successful method — and the most costly— is neutron activation. The sample is subjected to neutron bombardment in an accelerator where oxygen 16 is converted to unstable nitrogen 16 having a half-life of seven seconds. This is accompanied by emission of (J and 7 rays which are detected and measured. Oxygen concentrations as low as 10 ppm can be detected. At such levels, the problem is to find an acceptable blank sample. 

All these methods begin with combustion of the sample resulting in the sulfur being oxidized to SO2 and SO3. Table 2.3 summarizes the different analytical methods with references to the corresponding standards. 

These are called high temperature induction furnace methods which differ only as to the kind of furnace used and employ the same ASTM procedure. The sample is heated to over 1300°C in an oxygen stream and transformed to SO2 which is analyzed with an infra-red detector. 

Bomb Method the sample is burned in a bomb under oxygen pressures of 30 bar. The sulfur contained in the wash water is analyzed via gravimetry as barium sulfate. 

Lamp Method the sample is burned in a closed system in an atmosphere of 70% CO2 and 30% oxygen in order to avoid formation of nitrogen oxides. This method was to have been abandoned as it takes three hours to carry out, but remains officially required for jet fuel sulfur analysis. 

Quartz Tube Method the sample is burned in a quartz tube and a stream of purified air carries the combustion gases into a hydrogen peroxide solution. 

The sample is pyrolyzed in an 80/20 mixture of oxygen and nitrogen at from 1050 to 1100°C the combustion gases are analyzed by iodine titration or by UV fluorescence. Up to 20% of the sulfur can escape analysis, however. 

Another method which should be cited apart from the others is to pyrolyze the sample in a hydrogen atmosphere. The sulfur is converted to H2S which darkens lead-acetate-impregnated paper. The speed of darkening, measured by an optical device, provides the concentration measurement. This method attains sensitivity thresholds of 0.02 ppm. 

This method can attain, depending on the sample, concentrations on the order of 10 ppm wt. with an error on the order of 20%. 

The sample to be analyzed can be dissolved in an organic solvent, xylene or methylisobutyl ketone. Generally, for reasons of reproducibility and because of matrix effects (the surroundings affect the droplet size and therefore the effectiveness of the nebulization process), it is preferable to mineralize the sample in H2SO4, evaporate it and conduct the test in an aqueous environment. 

The detectable limits are given for samples such as they are introduced into the apparatus they should be previously diluted in order to be nebulized. It thereby is useful to apply a dilution coefficient, usually at least 10. The dilution depends on the sample viscosity. 

The sample should be liquid or in solution. It is pumped and nebulized in an argon atmosphere, then sent through a plasma torch that is, in an environment where the material is strongly ionized resulting from the electromagnetic radiation produced by an induction coil. Refer to the schematic diagram in Figure 2.8. 

To extend the applicability of the characterization factor to the complex mixtures of hydrocarbons found in petroleum fractions, it was necessary to introduce the concept of a mean average boiling point temperature to a petroleum cut. This is calculated from the distillation curves, either ASTM or TBP. The volume average boiling point (VABP) is derived from the cut point temperatures for 10, 20, 50, 80 or 90% for the sample in question. In the above formula, VABP replaces the boiling point for the pure component. 

Where is the temperature at which i% of the sample has been distilled. 

As a consequence, other than its use in the ndM method, the refractive index is very often used in process operations because it can indicate smaii differences in product quality that would be missed by other measurements. The only restriction is that the color of the sample should be less than 5 on the ASTM D 1500 scale. 

Although gas chromatography can give the concentration of each component in a petroleum gas or gasoline sample, the same cannot be said for heavier cuts and one has to be satisfied with analyses by chemical family, by carbon atom distribution, or by representing the sample as a whole by an average molecule. 

If the sample is placed in the path of the infrared beam, usually between the source and the monochromator, it will absorb a part of the photon energy having the same frequency as the vibrations of the sample molecule s atoms. The comparison of the source s emission spectrum with that obtained by transmission through the sample is the sample s transmittance spectrum. 

The sample is placed in a cqnst a nt magnetic field, Bq, and the variation in frequency throughout the t/omain Tieing expfored excites one by one the different resonances. The scan lasts a few minutes. Inversely, one can maintain a constant frequency and cause the magnetic field to vary. 

The sample is again subjected to a constant magnetic field but all the nuclei are excited by a very short radio frequency pulse. The frequency e (e.g., 400 MHz for a proton at 9.4 tesla) is applied over a period of several... 

---