ATR flow cell

Fixed pathlength transmission flow-cells for aqueous solution analysis are easily clogged. Attenuated total reflectance (ATR) provides an alternative method for aqueous solution analysis that avoids this problem. Sabo et al. [493] have reported the first application of an ATR flow-cell for both NPLC and RPLC-FUR. In micro-ATR-IR spectroscopy coupled to HPLC, the trapped effluent of the HPLC separation is added dropwise to the ATR crystal, where the chromatographic solvent is evaporated and the sample is enriched relative to the solution [494], Detection limits are not optimal. The ATR flow-cell is clearly inferior to other interfaces. 

FTIR in multiply hyphenated systems may be either off-line (with on-line collection of peaks) [666,667] or directly on-line [668,669]. Off-line techniques may be essential for minor components in a mixture, where long analysis times are required for FT-based techniques (NMR, IR), or where careful optimisation of the response is needed. In an early study a prototype configuration comprised SEC, a triple quadrupole mass spectrometer, off-line evaporative FTIR with splitting after UV detection see Scheme 7.12c [667]. Off-line IR spectroscopy (LC Transform ) provides good-quality spectra with no interferences from the mobile phase and the potential for very high sensitivity. Advanced approaches consist of an HPLC system incorporating a UV diode array, FTIR (using an ATR flow-cell to obtain on-flow IR spectra), NMR and ToF-MS. 

A dual-channel ATR flow cell has been fabricated and tested in a variety of protein adsorption/desorption studies, permitting the direct comparison of two surfaces or two flowing solutions under otherwise identical conditions. 

To circumvent problems associated with excessive eluent absorptions that prohibit FTIR transmission detection, some remedies are possible. ATR flow cells can be used to inherently reduce the optical path-length. Another option is postcolumn extraction of the LC effluent in order to transfer the analytes to a more IR-transparent solvent. Also, deuterated solvents could be used in which eluent absorption bands are shifted to lower wave numbers and potential analyte absorption bands are revealed. 

A five-compound model test mixture of polymer additives (Irganox 245, BHA, BHT, bisphenol A, Topanol CA) in CD3CN in amounts ranging from ca. 230 to 900 [tg on the column were tested with the hypernated configuration of Scheme 7.12f, using an ATR-FHR flow-cell and on-flow 500 MHz H NMR [673]. In a case of considerable technical overkill, the system was also used to identify a suspected polymer additive as being BHT. 

Short path length flow cells may be used in lower volume, or lower flow situations. The extremely small gap between the optics of fibers limits them to these types of applications. It is possible to make flow cells with sample gaps as small as 25 um, so that they may be used with highly absorbing species. With even higher absorbing species, attenuated total internal reflection (ATR) probes may be appropriate (see below). 

As well as the CIR-type cells discussed above, the ATR technique has also been employed in other types of HP IR cell. An alternative arrangement to a cylindrical crystal inserted through the autoclave body (Figure 3.8) is to embed the ATR crystal in the base or wall of the autoclave. This approach was used in the HP IR cell developed by Wolf et al., illustrated in Figure 3.9 [43]. An ATR crystal is mounted in the bottom of the reactor, which can operate at pressures up to 200 bar and can be adapted to give a high pressure flow-cell. 

Figure 14 shows a series of concentration profiles within an ATR flow-through cell as calculated by a convcclion diffusion model that has been described elsewhere (65) for a small, rapidly diffusing molecule (acetonitrile) and a large, slowly diffusing molecule (hemoglobin). At time t = 0, the concentrations of the molecules at the inlet were switched from zero to non-zero values. The laminar flow profile is established due to relatively low flow rates (low Reynolds numbers), which is clearly... 

Fig. 14. Comparison of the convection-diffusion behavior of acetonitrile and hemoglobin within an ATR flow-through cell as calculated by the convection-diffusion model described in the text. The concentrations of the two molecules were periodically varied between zero and a non-zero value with a frequency of 67 mHz the flow rate was 1.5mL/min. Dark areas represent high concentrations of the solute molecules (65).

Fig. 16. Change of the absorbance signal as a function of time for (a) cyclohexene and (b) TBHP on an uncoated ZnSe IRE (thin line) and on a ZnSe IRE coated with a methyl-modified Ti-Si aerogel catalyst (solid line). At time t = 0, the concentration at the inlet of the ATR flow-through cell was switched from 0 to 3 mmol/L, and at t = 122 s, it was switched back again (50).

The key enabler to using FTIR for BWA detection is to develop selective and robust sampling protocols coupled to a miniaturized, portable FTIR unit. To that end, we have developed front-end liquid flow cells which incorporate electric field (E-Field) concentration methods for spores onto the surface of an Attenuated Total Reflection (ATR) IR crystal. IR spectra are presented which show collection and detection results with BG spores in water. The approaches we have developed take advantage of the fact that all spores are negatively charged in neutral pH solutions. Therefore, E-Field concentration of spores directly onto an ATR sampling element enables low level concentration measurements to be possible. 

New reagentless, E-Field based samphng approaches have been developed and integrated with conventional ATR-FTIR flow cells for the detection and identification of bacterial spores in water. Our results show how E-field concentration of spores onto the surface of an ATR crystal surface can enable low concentrations of spores to be measured. The collection cells are designed for easy interface with virtually any IR spectrometer or detection system, and can prove usefirl in a variety of military, homeland defense, and technological apphcations. 

The TIDAS 11 fibre optic spectrometer from J M uses external fibre optics to connect probes and/or flow cells while at the same time enabling calibration of the instrument through the use of the cuvette holder. The detector is a diode array for fast acquisition of data (12ms/spectrum). The spectral range is 190-1020nm with a wavelength accuracy of 0.3 nm. Probes can be transmission or ATR-based. The dimensions of the unit are 20X48X49 cm. 

Fig. 13. Concentration profiles within an ATR flow-trough cell for a (i) diffusion layer model and (ii) convection-diffusion model (65). The models are described in the text.

ATR flow-through cell within FTIR spectrometer... 

Although ATR has been used to quantify the variation in composition at the surface in TPEs (Sung and Hu, 1980), a related utility is its ability to monitor in situ processes such as reaction injection molding (RIM) (Ishida and Scott, 1986) and protein adsorption onto a polyurethane substrate (Pitt and Cooper, 1986). In the latter, the effect of shear rate on the kinetics of protein adsorption and desorption from phosphate-buffered saline (PBS) was studied in a specially designed flow cell. A very thin film of the commercial MDI-ED-PTMO polyurethane Biomer was cast from solution onto a Ge ATR prism. The thickness of the film was less than the penetration depth so the protein concentration could be monitored after the infrared absorption of the polymer... 

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