Monomode instrument

For accurate temperature monitoring when conducting a temperature-controlled program, a minimum filling volume of the vessels is crucial. In the case of IR temperature measurement from the bottom of a vessel, only a very small amount of reaction mixture (ca. 50 pL) is sufficient to obtain a precise temperature feedback in a monomode instrument (CEM Discover series). On the other hand, a rectangular mounted IR sensor, as used in Biotage instruments (see Section 3.5) requires a certain minimum filling volume (200 pL for the smallest reaction vials see Fig. 3.21). 

Other microwave-assisted SPOS processes reported in the literature are summarized in Scheme 12.8. The addition of isocyanates to amines bound to Wang resin, for example, was studied both under conventional conditions at room temperature and under the action of microwave irradiation in open vessels by use of a monomode instrument. By monitoring the progress of the addition by on-bead FTIR it was demonstrated that the microwave procedure proceeded significantly faster than the reaction at room temperature (12 compared with 210 min) [38], The temperature during the microwave irradiation experiment was not determined, however, so it is unclear if any nonthermal microwave effects were responsible for the observed rate-enhancements (Scheme 12.8a) [38]. 

Simultaneous external cooling enables more microwave power to be distributed directly to the reaction mixture. Published applications of the simultaneous cooling technique are very rare so far, probably because of the lack of availability of a convenient instrument. This problem was recently overcome by release of the CEM Discover CoolMate monomode instrument for small-scale experiments which can maintain low temperatures in the region —80 to +35 °C [140]. Scaling up of this method has been solved by use of a cryostat in combination with multimode equipment (Milestone, Italy Shikoku Instrumentation, Japan). 

A prototype microwave reaction vessel that takes advantage of bottom-filtration techniques was presented in a more recent publication. The authors described the use of a modified reaction vessel (Fig. 16.2) for a Biotage monomode instrument, with a polypropylene frit, suitable for the filtration/cleavage steps in their microwave-mediated solid-phase Sonogashira coupling (Scheme 16.15) [21]. 

The development of monomode reactors, which focus the electromagnetic waves in an accurately dimensioned wave guide, enables homogeneous distribution of the electric field leading to increased efficiency and reliability. So, the current trend in MAOS is to move away from the multimode MW ovens and use the more dedicated monomode instruments, which have only become available in the last few years [22, 23]. 

The biberty (Fig. 10), a monomode microwave reactor for automated SPPS, was recently introduced by the CEM Corporation [153]. Although this instrument was originally developed for SPPS, it also allows for a broader scale of solid-phase applications. The solid-phase vial is equipped with a polypropylene frit and cap at one end (the entire assembly fitting into the standard 10 mb CEM reaction vessel) to allow the processing of 0.1 to 1.0 mmol quantities of resin attached substrates. An integrated fiber optic probe provides... 

Until 2004, Biotage (formerly Personal Chemistry) offered the Emrys monomode reactor series of instruments (Fig. 3.19). Although no longer commercially available, many instruments are currently still in use. Therefore, this line of products is discussed in detail in this chapter [15]. 

Consequently, which strategy is utilized in reaction optimization experiments is highly dependent on the type of instrument used. Whilst multimode reactors employ powerful magnetrons with up to 1500 W microwave output power, monomode reactors apply a maximum of only 300 W. This is due to the high density microwave field in a single-mode set-up and the smaller sample volumes that need to be heated. In principle, it is possible to translate optimized protocols from monomode to multimode instruments and to increase the scale by a factor of 100 without a loss of efficiency (see Section 4.5). 

Figure 12.7 Monomode microwave reactor Discover (without automation) for use with open or sealed vessels of different volumes (CEM Corp.) [81]. A related instrument with automation (Explorer) has recently been introduced.

The coefficient j8monomodal fibers are used[62]. 

Figure 8.1 Examples of GPC traces showing (a) a broad monomodal molecular weight distribution (PDI = 2.3) and (b) a narrow monomodal molecular weight distribution (PDI = 1.05). The x-axis shows the elution volume for the GPC instrument with molecular weight increasing from right to left.

Fig. 4. Monomode microwave reactor with integrated robotic platform for automated use (left). A liquid handler allows dispensing of reagents into Teflon-sealed reaction vials, while a gripper moves each vial in and out of the microwave cavity after irradiation. The instrument processes up to 120 reactions per run with a maximum throughput of 12-15 reactions/h. The temperature is measured by an IR sensor on the outside of the reaction vessel. Details of the cavity/gripper (top right) and reaction vials (bottom right) are also displayed (Emrys Synthesizer, Personal Chemistry AB). Reprinted with permission from Wiley-VCH.41 (See color insert.)...

MAT 311A instrument operating at 70 eV [electron impact (El) mode] and reported as m/z and relative intensity (%). Field desorption (FD) mass measurements were carried out on a ZAB 2-SE-FDP instrument. Microwave-assisted synthesis was performed using a CEM-Discovery monomode microwave system utilizing an IR temperature sensor and magnetic stirrer in sealed 10 -mL glass vials with aluminum caps and a septum. All reactions were monitored and controlled using a personal computer. 

The crudest way of estimating the particle size (D) as an average number from the breadth of a diffraction line is the widely used Scherrer approximation. It may be applied when the instrumental broadening is much smaller than the line profile (20 > 0.5°) and when a monomodal size distribution results in a homogeneous line profile. An explicit version of the equation determined by using the breadth of the diffraction line at half height (FWHM, pi 72) is given as follows ... 

The DLS and LD instruments yielded monomodal particle size distributions of similar polydispersity. For this reason, the mean particle sizes suffice for quantifying the progress of dispersion. There are distinct and systematic differences between the three instmments. DLS showed a steady reduction of the mean particle size from 220 nm to 160 nm (173°) and 245 nm to 175 nm (90°), while LD measured mean particle sizes between 500 nm and 400 nm. Although delivering a size distribution, the OPC was only used to determine the total particle number concentration above a certain size limit, which provides an additional insight into the dispersion process. The optical spectroscopy, which yields a turbidity spectrum, and the rheometric measurements served as indirect dispersion monitors. In both cases, it was possible to cover most of data variation within one parameter, i.e. in the turbidity at 400 nm wavelength (r4oonm) and in the apparent viscosity at a shear rate of 1 s ( /i/s), respectively... 

Biotage, a company based in Sweden, manufactures a monomode microwave unit called the Initator. Using this instrument, it is possible to run reactions on scales from 0.2-20 mL in sealed tubes. It is possible to automate the unit with a robotic arm, thus allowing up to 60 reactions to be run sequentially. 

We now describe an instrument using monomode fibre and an integrated fibre coupler, that therefore has no breaks in the transmission path due to the insertion of bulk optical components. Back reflection of a portion of the incident beam from the output port means that we detect a mixture of scattered light and a "reference beam", the so-called heterodyne detection scheme in which the first order correlation coefficient g( )(T) is measured directly. The signal is detected by a photomultiplier and processed by a multibit correlator. The fibre end is available to use directly as a probe into a dispersion or may be,mounted in a thermostatted flow cell. Results will be presented showing the performance on model systems as a function of concentration. The use of such an instrument for remote sensing in a variety of applications will be discussed. 

Monomode and Multimode Instruments for large-scale Preparations... 

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