Liquids, sampling

Liquid samples are by far the most frequently involved In automated analytical processes, which is not surprising taking into account the few technical difficulties which they pose. 

Although liquid samples do pose some problems arising from the nature of the whole material, these are easy to solve. Thus, even low sample volumes can be homogenized in an automatic fashion. Large samples are normally handled by taking different representative samples from a given location. Liquids In motion make sampling essentially time-dependent. 

Sample volumes can be measured In any of three general ways (a) by means of a probe aspirating the liquid at a constant rate, (b) by using mechanically or hydraulically actuated, high-precision syringes for a preset time and (c) by weighing the liquid. 

Automatic samplers, which are fairly commonplace In liquid sampling, consist of the following elements  

Obviously, no sampler is required when sampling is performed in a completely continuous fashion. 

FIGURE 8.16 An illustration of a liquid sampling cell placed in the path of the light. 

All three cells utilize neoprene gaskets to cushion the fragile salt crystals from the metal frame. In the case of the sealed and sealed demountable cells, the top neoprene gasket and window have holes drilled in them to coincide with the inlet and outlet ports to facilitate filling the space, created by the spacer, 

FIGURE 8.18 Left, a demountable cell with spacer. Right, a demountable cell without spacer. Note that there are no drilled holes for the sample and no inlet or outlet ports. Both metal frame plates (front and back) have holes for light to pass. 

FIGURE 8.19 Photographs of a sealed (or sealed demountable) cell. 

Liquids can be sampled as either the neat liquid (pure) or mixed with a solvent (solution). Neat liquids are tested when the purpose of the experiment is either identification or the determination of purity. Identification is possible because the spectrum is a fingerprint when no solvent or contaminant is present. Impurities are found when extraneous absorption bands or distortions in analyte absorption bands appear. 

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Now using a hydrocarbon component, say ethane, as an example, let us consider the other parameter, volume, using a plot of pressure versus specific volume (i.e. volume per unit mass of the component, the inverse of the density). The process to be described could be performed physically by placing the liquid sample into a closed cell (PVT cell), and then reducing the pressure of the sample by withdrawing the piston of the cell and increasing the volume contained by the sample. 

It is especially useflil for liquid samples in flat cells, which may extend through tlie entire height of the cavity. In the cylindrical cavity a TEq mode is frequently used because of its fairly high g-factor and the very strong along the sample axis. 

The vibration frequencies of C-H bond are noticeably higher for gaseous thiazole than for its dilute solutions in carbon tetrachloride or tor liquid samples (Table 1-27). The molar extinction coefficient and especially the integrated intensity of the same peaks decrease dramatically with dilution (203). Inversely, the y(C(2jH) and y(C(5(H) frequencies are lower for gaseous thiazole than for its solutions, and still lower than for liquid samples (cf. Table 1-27). 

Although the terms solute and solution are often associated with liquid samples, they can be extended to gas-phase and solid-phase samples as well. The actual units for reporting concentration depend on how the amounts of solute and solution are measured. Table 2.4 lists the most common units of concentration. 

Typical examples of liquid samples include those drawn from containers of commercial solvents beverages, such as milk or fruit juice natural waters, including from lakes, streams, seawater, and rain bodily fluids, such as blood and urine and, suspensions, such as those found in many oral medications. 

Sample Preservation Once removed from its target population, a liquid sample s chemical composition may change as a result of chemical, biological, or physical processes. Following its collection, samples are preserved by controlling the solu-... 

Solid-Phase Extractions In a solid-phase extraction the sample is passed through a cartridge containing solid particulates that serve as the adsorbent material. For liquid samples the solid adsorbent is isolated in either a disk cartridge or a column (Figure 7.17). The choice of adsorbent is determined by the properties of the species being retained and the matrix in which it is found. Representative solid adsorbents... 

Selected Adsorbents for Solid-Phase Extraction of Liquid Samples... 

A technique for separating volatile analytes from liquid samples in which the analytes are subsequently trapped on a solid adsorbent. 

Liquid samples are analyzed in one of two ways. For nonvolatile liquids a suitable sample can be prepared by placing a drop of the liquid between two NaCl plates, forming a thin film that typically is less than 0.01 mm thick. Volatile liquids must be placed in a sealed cell to prevent their evaporation. 

Porro, T. J. Pattacini, S. C. Sample Handling for Mid-Infrared Spectroscopy, Part 1 Solid and Liquid Sampling, Spectroscopy 1993, 8(7), 40-47. 

The liquid sample flows into the nozzle and coats the inside walls. The sample stream arrives at the orifice (the nozzle outlet is about 0.01 cm diameter), where it meets the argon stream and is nebulized. 

The aim of breaking up a thin film of liquid into an aerosol by a cross flow of gas has been developed with frits, which are essentially a means of supporting a film of liquid on a porous surface. As the liquid flows onto one surface of the frit (frequently made from glass), argon gas is forced through from the undersurface (Figure 19.16). Where the gas meets the liquid film, the latter is dispersed into an aerosol and is carried as usual toward the plasma flame. There have been several designs of frit nebulizers, but all work in a similar fashion. Mean droplet diameters are approximately 100 nm, and over 90% of the liquid sample can be transported to the flame. 

This arrangement provides a thin film of liquid sample solution flowing down to a narrow orifice (0.007-cm diameter) through which argon flows at high linear velocity (volume flow is about 0.5-1 1/min). A fine aerosol is produced. This particular nebulizer is efficient for solutions having a high concentration of analyte constituents. 

A liquid sample must be vaporized to a gas or, more likely, to a vapor consisting of an aerosol of gas, small droplets, and even small particles of solid matter. To be examined, the aerosol is mixed with argon gas to make up the needed flow of gas into the plasma and is then swept into the flame. 

Some elements (S, Se, Te, P, As, Sb, Bi, Ge, Sn, Pb) in liquid samples arc conveniently converted into their volatile hydrides before being passed into the plasma, as discussed in Part A (Chapter 15). For some samples, any volatile solvent is first evaporated in a sample holder, which is then heated strongly to vaporize the resulting solid residue, as discussed in Part C (Chapter 17). 

Desorption ionization (DI). General term to encompass the various procedures (e.g., secondary ion mass spectrometry, fast-atom bombardment, californium fission fragment desorption, thermal desorption) in which ions are generated directly from a solid or liquid sample by energy input. Experimental conditions must be clearly stated. 

Figure 8.28 shows how the X-rays fall on the solid or liquid sample which then emits X-ray fluorescence in the region 0.2-20 A. The fluorescence is dispersed by a flat crystal, often of lithium fluoride, which acts as a diffraction grating (rather like the quartz crystal in the X-ray monochromator in Figure 8.3). The fluorescence may be detected by a scintillation counter, a semiconductor detector or a gas flow proportional detector in which the X-rays ionize a gas such as argon and the resulting ions are counted.

Liquid sampling from pressurized lines, using ungrounded or nonconductive containers... 

Field Measurement Conditions Those gathering samples must be aware of the temperature, pressure, flamm ihty, and toxic characteristics of the samples for which they will be responsible. This is particularly important when samples are taken from unfamihar locations. Sample ports will have to be blown down to obtain representative samples. Liquid samples will have to be vented. Temperatures above... 

The role of the nebulizer in ICPMS is to transform the liquid sample into an aerosol. This is carried into the plasma by an ai on flow after passing through a... 

Approximately 70 different elements are routinely determined using ICP-OES. Detection limits are typically in the sub-part-per-billion (sub-ppb) to 0.1 part-per-million (ppm) range. ICP-OES is most commonly used for bulk analysis of liquid samples or solids dissolved in liquids. Special sample introduction techniques, such as spark discharge or laser ablation, allow the analysis of surfaces or thin films. Each element emits a characteristic spectrum in the ultraviolet and visible region. The light intensity at one of the characteristic wavelengths is proportional to the concentration of that element in the sample. 

W. E. Petit and G. Horlick. Spect. Acta. 41B, 699, 1986. Describes an automated system for direct sample-insertion introduction of 10-pL liquid samples or small amounts (10 mg) of powder samples. 

The retentivity relative to solid particles (e.g., spherical particles of polystyrene of definite size) is found from experiments determining the amount of these particles in the suspension to be filtered before and after the filter media. The retentivity K is determined as follows where g, g" =amounts of solid particles in liquid sample before and after the medium, respectively. 

Supercritical fluid extraction (SFE) and Solid Phase Extraction (SPE) are excellent alternatives to traditional extraction methods, with both being used independently for clean-up and/or analyte concentration prior to chromatographic analysis. While SFE has been demonstrated to be an excellent method for extracting organic compounds from solid matrices such as soil and food (36, 37), SPE has been mainly used for diluted liquid samples such as water, biological fluids and samples obtained after-liquid-liquid extraction on solid matrices (38, 39). The coupling of these two techniques (SPE-SFE) turns out to be an interesting method for the quantitative transfer... 

A useful device to have installed in a stirred autoclave is a liquid sampling tube by which liquid samples are withdrawn under pressure through a filter attached to the lower end of the tube. This device is especially useful for analysis of reaction progress and supplements information obtained from pressure-drop determinations. It is much easier to improve a less than satisfactory yield, if it can be determined what is going wrong and when. For academically orientated persons, a study of the rise and decline of various reaction products, as a function of reaction parameters and catalyst, can be a fertile source of useful publications. 

The simplest method to measure gas solubilities is what we will call the stoichiometric technique. It can be done either at constant pressure or with a constant volume of gas. For the constant pressure technique, a given mass of IL is brought into contact with the gas at a fixed pressure. The liquid is stirred vigorously to enhance mass transfer and to allow approach to equilibrium. The total volume of gas delivered to the system (minus the vapor space) is used to determine the solubility. If the experiments are performed at pressures sufficiently high that the ideal gas law does not apply, then accurate equations of state can be employed to convert the volume of gas into moles. For the constant volume technique, a loiown volume of gas is brought into contact with the stirred ionic liquid sample. Once equilibrium is reached, the pressure is noted, and the solubility is determined as before. The effect of temperature (and thus enthalpies and entropies) can be determined by repetition of the experiment at multiple temperatures. 

Figure 4.1-1 Liquid sample cells made from (a) TiZr alloy and (b) vanadium.

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