Carrier mode separation

Up to the scaling factors An this sum represents a periodic spectrum in frequency space. If the spectral width of the carrier wave Au>c is much smaller than the mode separation ojr, Eqn. 7 represents a regularly spaced comb of laser modes with identical spectral line shapes, namely the line shape of C(u>) (see Fig. 1). If C(oS) is centered at say uic then the comb is shifted from containing only exact harmonics of u)r by uic. The center frequencies of the mode members are calculated from the mode number n [23,24,21] ... 

This equation maps two radio frequencies ay and ay onto the optical frequencies ay. While ay is readily measurable, ay is not easy to access unless the frequency comb contains more than an optical octave, as shown in section 7. The individual modes can be separated, for example with an optical grating, if the spectral width of the carrier function is narrower than the mode separation Au)c ay. This condition is easy to satisfy, even with a free running Ti Saphire laser. 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

Carrier mode chiral separations offer significant advantages and may become a very useful technique for biomedical applications where structurally similar analytes, such as chiral drugs and their metabolites, must be separated and enantiosep-arated simultaneously. Such a rather complex separation problem implies higher enantio- and chemoselectivity requirements to a separation system (Fig. 5) [43]. 

Fig. 5 Simultaneous carrier mode CE separation and enantioseparation of thalidomide and its hydroxylated metabolites (reproduced with permission from [43])...

This section describes equipment for heat transfer to or from solids by the indirect mode. Such equipment is so constructed that the solids load (burden) is separated from the heat-carrier medium by a wall the two phases are never in direct contact. Heat transfer is by conduction based on diffusion laws. Equipment in which the phases are in direct contact is covered in other sections of this Handbook, principally in Sec. 20. 

When columns of the same polarity are used, the elution order of components in GC are not changed and there is no need for trapping. However, when columns of different polarities are used trapping or heart-cutting must be employed. Trapping can be used in trace analysis for enrichment of samples by repetitive preseparation before the main separation is initiated and the total amount or part of a mixture can then be effectively and quantitatively transferred to a second column. The main considerations for a trap are that it should attain either very high or very low temperatures over a short period of time and be chemically inactive. The enrichment is usually carried out with a cold trap, plus an open vent after this, where the trace components are held within the trap and the excess carrier gas is vented. Then, in the re-injection mode the vent behind the trap is closed, the trap is heated and the trapped compounds can be rapidly flushed from the trap and introduced into the second column. Peak broadening and peak distortion, which could occur in the preseparation, are suppressed or eliminated by this re-injection procedure (18). 

An alternative fast GC-MS mode, low-pressure GC-MS, is based on the high optimal velocity for the carrier gas obtained when a separation is performed under reduced pressure. The pressure required for LPGC-MS is 120 kPa, generating a flow of ImLHemin. The main advantages of low-pressure GC-MS are shown in Table 7.29. 

In both cases, GC fingerprint libraries must be built before quantitative analysis can be routinely carried out. In analysis of QTLC by laser pyrolysis scanning (LPS), the TLC plates are placed in a chamber after development, and were irradiated with an IR laser to produce a high temperature at the location of the spot. The analyte is swept by a carrier gas to a GC, and detected with FID or ECD. The technique combines the separation power of TLC and the detection modes of GC [846]. 

Space charge layers and contact potential for efficient charge carrier separation can be achieved with proper semiconductor structure in several ways. When possible semiconductor structures are considered, the charge separation can be attained in an active mode, i.e., by the use of a potential bias in a photoelectrochem-ical cell, or in a passive mode, i.e., with the use of proper contact between different phases. 

A typical extraction manifold is shown in Figure 13.2. The sample is introduced by aspiration or injection into an aqueous carrier that is segmented with an organic solvent and is then transported into a mixing coil where extraction takes place. Phase separation occurs in a membrane phase separator where the organic phase permeates through the Teflon membrane. A portion of one of the phases is led through a flow cell and an on-line detector is used to monitor the analyte content. The back-extraction mode in which the analyte is returned to a suitable aqueous phase is also sometimes used. The fundamentals of liquid liquid extraction for FIA [169,172] and applications of the technique [174 179] have been discussed. Preconcentration factors achieved in FIA (usually 2-5) are considerably smaller than in batch extraction, so FI extraction is used more commonly for the removal of matrix interferences. 

A gas chromatograph is used for the primary separation of the components in gas oil. An automatic unit feeds the chromatograph with samples and a back-flushing unit has heen added in order to remove heavy hydrocarbons, which might otherwise choke the column. TTie carrier gas (nitrogen) is controlled by three electropneumatic valves, as shown in Fig. 4.4. In the normal mode with valve A open and valves B and C closed, carrier gas flows through columns 1 and 2 to the detector. 

The GC/MS-MS analyses were performed on a Varian 3800 gas chromatograph (Varian Chromatography Systems, Walnut Creek, CA) equipped with a 1079 split/splitless injector and a ion trap spectrometer (Varian Saturn 2000, Varian Chromatography Systems) with a waveboard for MS-MS analysis. The system was operated by Saturn GC/MS Workstation v5.4 software. The MS-MS detection method was adapted from elsewhere. PCBs were separated on a 25 m length X 0.32 mm i.d., CPSil-8 column coated with a 0.25 pm film. The GC oven temperature program was as follows 90 °C hold 2 min, rate 30 °C/min to 170 °C, hold for 10 min, rate 3 °C/min to 250 °C, rate 20 °C/min to a final temperature of 280 °C, and hold for 5 min. Helium was employed as a carrier gas, with a constant column flow of 1.0 mE/min. Injector was programmed to return to the split mode after 2 min from the beginning of a run. Split flow was set at 50 mL/min. Injector temperature was held constant at 270 °C. Trap temperatures, manifold temperatures, and transfer line temperatures were 250, 50, and 280 °C, respectively. 

We use a GC Top 8000 gas chromatograph coupled with a PolarisQ ion-trap mass spectrometer and equipped with an AI3000S autosampler (Thermofinnigan www. thermo.com). The steroids are separated on a DB-1 crosslinked methyl-silicone column, 15 mx 0.25 mm i.d., film thickness 0.25 pm (J W Scientific marketed by Agilent). Helium is used as a carrier gas at a constant pressure of about 35 kPa. A 1-pl aliquot of the final derivatized extract is injected into the system operated in splitless mode (valve opened at 2 min). The GC temperature program is the same described before for the quadrupole GC-MS system. The injector and transfer lines are kept at 260°C and 280°C, respectively. The ion source temperature is 225°C. A damping gas flow of helium is applied to the ion trap. 

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