Instrumentation, SEC

RMS = root mean square noise of the instrument. SEC = standard error of calibration for each constituent. SEA = standard error of analysis or the difference between analysis values from the same samples analyzed by NIR and the reference laboratory. SED = standard error of a difference. Factory SED = standard error of the difference between the same samples analyzed by the master and slave instrument at different times. H = standardized H statistic. SEE = standard error of the laboratory reference values. (This statistic can be either the difference between duplicates in one laboratory or the difference between the same samples analyzed by two different laboratories.) M SEE = standard error of blind duplicates in the master reference laboratory. 

After a short reflection on the choice of laser wavelength for in vivo experiments in Section ILA, a description of various fiber-optic probes (Sec. II.B) and confocal in vivo Raman instruments (Sec. II.C) is given. 

Since the drop volume method involves creation of surface, it is frequently used as a dynamic technique to study adsorption processes occurring over intervals of seconds to minutes. A commercial instrument delivers computer-controlled drops over intervals from 0.5 sec to several hours [38, 39]. Accurate determination of the surface tension is limited to drop times of a second or greater due to hydrodynamic instabilities on the liquid bridge between the detaching and residing drops [40],... 

Wagner and DUlont have described a low-shear viscometer in which the inside diameter of the outer, stationary cylinder is 30 mm and the outside diameter of the inner, rotating cylinder is 28 mm the rotor is driven by an electromagnet. The device operates at 135°C and was found to be free of wobble and turbulence for shear rates between 3 and 8 sec V The conversion of Eq. (2.7) to Eq. (2.9) shows that F/A = (i7)(dv/dr) (instrument constant) for these instruments Evaluate the instrument constant for this viscometer. 

The concentric cylinder viscometer described in Sec. 2.3, as well as numerous other possible instruments, can also be used to measure solution viscosity. The apparatus shown in Fig. 9.6 and its variations are the most widely used for this purpose, however. One limitation of this method is the fact that the velocity gradient is not constant, but varies with r in this type of instrument, as noted in connection with Eq. (9.26). Since we are not considering shear-dependent viscosity in this chapter, we shall ignore this limitation. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

The two principal elements of evaporator control are evaporation rate a.ndproduct concentration. Evaporation rate in single- and multiple-effect evaporators is usually achieved by steam-flow control. Conventional-control instrumentation is used (see Sec. 22), with the added precaution that pressure drop across meter and control valve, which reduces temperature difference available for heat transfer, not be excessive when maximum capacity is desired. Capacity control of thermocompression evaporators depends on the type of compressor positive-displacement compressors can utilize speed control or variations in operating pressure level. Centrifugal machines normally utihze adjustable inlet-guide vanes. Steam jets may have an adjustable spindle in the high-pressure orifice or be arranged as multiple jets that can individually be cut out of the system. 

Generally, size exclusion chromatography is carried out using columns with an internal diameter of 7.8 mm. However, some SEC applications require the use of expensive solvents. For this purpose, size exclusion columns with a smaller internal diameter (4.6 mm) have been developed. Of course one should use proportionally lower flow rates with narrow-bore columns. If the standard column size uses a flow rate of 1 ml/min, then the smaller 4.6-mm columns should be used at a flow rate of 0.35 ml/min. This provides the same linear velocity as 1 ml/min on 7.8-mm columns. The decreased flow rate reduces solvent consumption and solvent disposal cost. The performance of the smaller diameter columns is not compromised if properly optimized instrumentation is used. 

The instrumentation of HdC, including a pump, an injector, a column (set), a detector, and a recorder or computer, is very similar to size exclusion chromatography SEC). The essence of this technique is the column. There are two types of HdC columns open microcapillary tubes and a nonporous gel-packed column. This chapter emphasizes column technology and selection and the applications of this technique on the molecular weight analysis of macromolecules. 

Although the OTHdC has several unique applications in polymer analysis, this technique has several limitations. First, it requires the instrumentation of capillary HPLC, especially the injector and detector, which is not as popular as packed column chromatography at this time. Second, as discussed previously, the separation range of a uniform capillary column is rather narrow. Third, it is difficult to couple capillary columns with different sizes together as SEC columns. 

Another advantage of HdC is its generosity in terms of mobile-phase selection. The polymer size and solution properties of a polymer can be studied using HdC, especially OTHdC, in almost any solvent. In SEC, by comparison, the packing material and mobile phase have to be selected to prevent the nonsize exclusion effect. Because the instrumentation of HdC is similar to SEC, and the packing material and columns have become available commercially, this technique will gain in popularity. 

Figure 6.2 Schematic diagram showing the basic components of (a) SFE and (b) SFC instruments 1, carbon dioxide 2, high pressure pump 3, oven 4, exti action cell (SFE) or column (SFC) 5, collection vial (SFE) or data system (SEC).

Moore and Jorgenson eombined the rapid two-dimensional separation aehieved by LC-CZE with SEC to make the first eomprehensive three-dimensional separation involving an eleetrodriven eomponent in 1995. Size exelusion ehromatography separated the analytes over a period of several hours while the reverse phase HPLC-CZE eombination separated eomponents in only 7 min. A sehematie diagram of the three-dimensional SEC-reverse phase HPLC-CZE instrument is shown in Eigure 9.9 (18). A dilution tee was plaeed between the SEC eolumn and the reverse phase HPLC injeetion loop in order to dilute the eluent from the SEC eolumn, sinee it eon-tained more methanol than was optimal for the reverse phase HPLC eolumn. 

Enantiomeric separations have proven to be one of the most successful applications of packed column SEC. Despite initial reluctance, many analysts now use SEC routinely for both analytical and preparative chiral separations. Additional studies of chiral recognition in SEC and continued improvements in instrumentation will ensure a prominent role for SEC in chiral separations methodology in the future. 

Fiq. 20a. The pulsed Raman spectrum of Mn-doped ZnSe single crystal using a detection interval of 200 nsec. Broad band fluorescence superimposed on a large instrumental scattered light component was observed. Recordings taken with ratemeter time constants (TC) of 1 sec and 10 sec are shown (37). 

The NAA measurements on the paper samples were made at the Breazeale Nuclear Reactor Facility at the Pennsylvania State University with a TRIGA Mark III reactor at a flux of about 1013 n/cm2-sec. Samples were irradiated from 2 to 20 min and counted for 2000 sec, after a 90 min decay time for Ba and a 60 hr decay for Sb, Analyses were performed instrumentally, without radiochemical separation, using a 35cm3 coaxial Ge-Li detector and a 4096-channel pulse height analyzer. With these procedures, detection limits for Ba and Sb were 0.02ug and 0.001 ug, respectively. These sensitivities are comparable to those obtained by GA s radiochemical separation procedure, and are made possible by the use of the higher neutron output from the more powerful reactor and in combination with the higher resolution solid state detector... 

The arenediazocyanides have been known since 1879. They played an important role in the Hantzsch-Bamberger debate on the (Z)/( ,)-isomerism of diazo compounds (see Sec. 7.1). When an aqueous solution of a diazonium salt is added to a solution of sodium or potassium cyanide, both in relatively high concentration, at a temperature below 0°C, a yellow to red (Z)-arenediazocyanide starts to crystallize. Hantzsch and Schulze (1895 a) found that these compounds rearrange into the (ii)-isomers, which have a bathochromically shifted visible absorption (see Sec. 7.1). Under strongly alkaline conditions a 1 2 adduct is formed, to which Stephenson and Waters (1939) assigned the structure 6.36. It was never corroborated, however, by modern instrumental analysis. 

It follows that, at least for SEC, column temperature control can be important. An example of a commercially available column oven is shown in figure 17. The available temperature range varies a little from instrument to instrument but the model shown above has an operational range from 10°C to 99°C. One of the problems associated with the temperature control of ovens is the high thermal capacity of... 

In addition to instrument spreading, which is generally treated as being Gaussian in nature (15, 16), skewing can also be observed in SEC of latices because of entrapment of particles within the porous matrix. This effect generally increases with increasing particle size. 

As shown in Sec. 2.1.1.1, the time constant for the instrument is then given as the time at which 63 percent of the final response is achieved and the instrument response may be described by the simple relationship... 

Assuming that the instrument response is first order, then as shown in Sec. 2.1.1.1, the instrument time constant Xm is then given by the value of time at the 63% point (response to a step-change disturbance), where... 

Supercritical fluid chromatography (SEC) was first reported in 1962, and applications of the technique rapidly increased following the introduction of commercially available instrumentation in the early 1980s due to the ability to determine thermally labile compounds using detection systems more commonly employed with GC. However, few applications of SEC have been published with regard to the determination of triazines. Recently, a chemiluminescence nitrogen detector was used with packed-column SEC and a methanol-modified CO2 mobile phase for the determination of atrazine, simazine, and propazine. Pressure and mobile phase gradients were used to demonstrate the efficacy of fhe fechnique. 

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