Detectors field-flow fractionation

Taylor, H. E., Garbarino, J. R., Murphy, D. M., and Beckett, R. (1992). Inductively coupled-plasma-mass spectrometry as an element-specific detector for field-flow fractionation particle separation. Anal. Chem. 64, 2036-2041. 

Kammer, F.v. and Forstner, U., Natural colloid characterization using flow-field-flow-fractionation followed by multi-detector analysis, Water Sci. and Technol., 37, 173, 1998. 

In thermal field flow fractionation (TFFF), a temperature gradient is applied. The primary potential advantage of this technique is that it can be used to size particles in the range 0.01 pm to 0.001 pm, an order of magnitude smaller than SFFF. Fffractionation market a TFFF polymer fractionator channel module with 286/16 MHz IBM compatible PC, super VGA color monitor workstation to include data acquisition software, hardware and data analysis software. A linear UV detector and single channel high performance pump are optional. 

This book covers some of the significant advances in hyphenated chromatographic separation methods for polymer characterization. Chromatographic separation techniques in this volume include size-exclusion chromatography, liquid chromatography, and field flow fractionation methods that are used in conjunction with information-rich detectors such as molecular size-sensitive or compositional-sensitive detectors or coupled in cross-fractionation modes. 

The understanding of the effects of sample concentration (sample mass) in field-flow fractionation (FFF) has being obtained gradually with the improvement of the sensitivity (detection limit) of high-performance liquid chromatography (HPLC) detectors. Overloading, which was used in earlier publications, emphasizes that there is an upper limit of sample amount (or concentration) below which sample retention will not be dependent on sample mass injected into the FFF channels [1]. Recent studies show that such limits may not exist for thermal FFF (may be true for all the FFF techniques in polymer separation), although some of the most sensitive detectors on the market were used [2]. 

An analytical separation technique requires a detection method responding to some or all of the components eluting from the separation system. The choice of detector is determined by the demands of the sample and analysis. For Field-Flow Fractionation (FFF) techniques many of the detection systems have evolved from those used in liquid chromatography (LC) techniques. 

The dependences of the retention ratio R on the size of the fractionated species (molar mass for the macromolecules or particle diameter for the particulate matter) are presented for various polarization FFF methods in the entry Field-Flow Fractionation Fundamentals. The raw, digitized fractogram, which is a record of the detector response as a function of the retention volume, is represented by a differential distribution function h(y). i can be processed to obtain a series of the height values hi corresponding to the retention volumes as shown in Fig. 1. Subsequently, the retention volumes are converted into the retention ratios Rf. 

Frit-inlet asymmetrical flow field-flow fractionation (FIA-FIFFF) [1-3] utilizes the frit-inlet injection technique, with an asymmetrical flow FFF channel which has one porous wall at the bottom and an upper wall that is replaced by a glass plate. In an asymmetrical flow FFF channel, channel flow is divided into two parts axial flow for driving sample components toward a detector, and the cross-flow, which penetrates through the bottom of the channel wall [4,5], Thus, the field (driving force of separation) is created by the movement of cross-flow, which is constantly lost through the porous wall of the channel bottom. FlA-FlFFF has been developed to utilize the stopless sample injection technique with the conventional asymmetrical channel by implementing an inlet frit nearby the channel inlet end and to reduce possible flow imperfections caused by the porous walls. 

Another separation technique of particular application for proteins, high-molar-mass molecules, and particles is the general class known as field-flow fractionation (FFF) in its various forms (cross-flow, sedimentation, thermal, and electrical). Once again, MALS detection permits mass and size determinations in an absolute sense without calibration. For homogeneous particles of relatively simple structure, a concentration detector is not required to calculate size and differential size and mass fraction distributions. Capillary hydrodynamic fractionation (CHDF) is another particle separation technique that may be used successfully with MALS detection. 

In all of the many forms of chromatography, detection is an inherentiy important final step. The type of detection can aid in the analysis by gathering information that can be used to identify the peaks seen. There can be many peaks that elute from the column of a gas, hquid, or supercritical-fluid chromatograph. Certain detectors are in fact spectrometers that examine each peak for specific information on its identity. This chapter deals with this use of spectrometers as the tail-end detector in chromatography. Other separation techniques, such as field-flow fractionation or capillary electrophoresis, differ in their separation mechanisms, but as far as coupling to spectrometers behave like one of these three types of chromatography. 

The main purpose of the detector in a field-flow fractionation (FFF) system is to quantitatively determine particle number, volume, or mass concentrations in the FFF size-sorted fractions. Consequently, a number, volume, or mass dependent size distribution of the sample can be derived from detection systems applied to FFF [e.g., (UV-Vis) fluorescence, refractive index, inductively coupled plasma ionization mass spectrometry (ICPMS)]. Further, on-line light scattering detectors can provide additional size and molecular weight distributions of the sample. 

Ftg- 2 Elemental size distributions of the colloidal material in a freshwater sample as given from an FLFFF coupled to ICPMS. A UV detector is placed on line prior to the ICPMS and the UV size distribution is included. The signals are plotted as a function of retention time, hydrodynamic diameter (from FFF theory), and molecular weight (from standardization with PSS standards). Source From Determination of continuous size and trace element distribution of colloidal material in natural water by on-line coupling of flow field-flow fractionation with ICMPS, in Anal. Chem. J... 

Oppenheimer, L.E. Momey, T.H. Use of an evaporative light-scattering mass detector in edimentation field-flow fractionation. J. Chromatogr. 1984, 298, 217-224. 

Compton, B.J. Myers, M.N. Giddings, J.C. A single parti- 18. cle photometric detector for steric field-flow fractionation. 

Electrospray mass spectrometry as onhne detector for low molecular weight polymer separations with flow field-flow fractionation. J. Liq. Chromatogr. Relat. Technol. 1997,20, 20. 

Stolpe, B. HasseUov, M. Andersson, K. Turner, D.R. High resolution ICPMS as an on-hne detector for flow field-flow fractionation multi-element determination of coUoidal size distributions in a natural water sample. Anal. Chim. Acta 2005, 535 (1-2), 109-121. 

Kirkland, J.J. Yau, W.W. Quantitative particle-size distributions hy sedimentation field-flow fractionation with densimeter detector. J. Chromatogr. 1991, 550, 799-809. 

Fig. 1 Principle of field-flow fractionation. 1—Solvent reservoir, 2-carrier liquid pump, 3—injection of the sample, 4— separation channel, 5—detector, 6—computer for data acquisition, 7—transversal effective field forces, 8—longitudinal flow of the carrier liquid. A—Section of the channel demonstrating the principle of polarization FFF with two distinct zones compressed differently at the accumulation wall and the parabolic flow velocity profile. B—Section of the channel demonstrating the principle of focusing FFF with two distinct zones focused at different positions and the parabolic flow velocity profile. C—Section of the channel demonstrating the principle of steric ITF with two zones eluting at different velocities according to the distance of their centers from the accumulation wall.

Reschiglian, P. Melucci, D. Torsi, G. A quantitative approach to the analysis of particulate matter in field-flow fractionation with UV-Vis. detectors. The application of an absolute method. Chromatographia 1997, 44, 172. 

Continued improvements in FFF instrumentation, refinements in technique and hyphenation with detectors such as MALLS have broadened the application of this method and effectively elevated it from the status of possibly useful technique to a reliable applied research tool for assessing the size and shape of wheat proteins. For additional reading concerning FFF theory, principles, and applications, the reader is referred to other sections of this encyclopedia as well as to Field-Flow Fractionation Handbook edited by Schimpf et al. A review of the application of FFF to wheat protein analysis can also be found in Preston and Stevenson. 

In field flow fractionation, a sample is injected into a narrow channel where stratification according to particle size takes place under the effects of gravity or centrifugal fields. The coarse particles are eluted from the channel later than the fine ones due to the flow being slower lower down, near the wall, where the coarse fraction prevails. The concentration at the outlet of the channel is monitored over time using a variety of detectors, mostly based on Ught extinction. 

Hydrodynamic chromatography reUes on different particle velocities in laminar flow through capillaries or packed columns. Larger particles move faster with the flow than do fine ones because they are, on average, further away from the capillary wall. The operation and the equipment are the same as in liquid chromatography colloidal particles are injected into a column packed with beads and a suitable detector (ultraviolet light detector or a spectrophotometer) monitors the flow from the column. Both field flow fractionation and hydrodynamic chromatography are most suitable for nearly mono-sized particle systems. 

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