The development of an ultravlolet "on-column" detection technique with Improved signal-to-ndse ratlo allows the study of the separation mechanism in mlcrocapillary hydrodynamic chromatography (HDC). For polystyrene standards, for which data on the molecular slzes in solutions are avallable, the experimental residence tlmes almost exactly follow the modffled theorles after DIMarzio-Guttman and Brenner-Gaydos, at least with good solvents such as tetrahydrofuran. These modlfied theorles use the effective radius of macromolecules, Le., the radius based on the largest size of the chain, rather than hydrodynamic or other theoretical radll. Mlcrocaplllary HDC separatlons are efflclent and dlrectly reflect the size dlstrlbutlon (and hence the molecular weight distribution) of polymer samples. Futthermore, mkrocaplllary HDC is well-suited to studying size effects of polymer molecules In solutlon.Hydrodynamic chromatography (HDC) is a relatively new technique, mainly used for the separation of colloidal particles of different sizes. The pioneering work was done by Small (1-3), who demonstrated its practicability for separating mixtures of polymer latexes and colloidal dispersions using packed beds as the separation column. Pedersen ( 4 ) , Mori et al. (5), and more recently Prud'homme et al. (6) showed that in principle large molecules such as proteins, polystyrenes, and water-soluble polymers can also be separated into fractions of different molecular weights by applying packed bed separation columns. For an extensive review of packed-column HDC, the reader is referred to a recent article by McHugh (7) and a somewhat earlier one by Dodds (8). Mullins et al. (9, lo), Brough et al. ( l l ) , and Shuster et al. (12) demonstrated experimentally that HDC separation of differently sized particles can also be effected in a single open tubular capillary column with a diameter of the order of several tenths of a millimeter. Elie and Renaud (13) succeeded in separating fibrous particles in paper fiber suspensions using even wider open tubes (id. = 1 cm). The particle separation technique called field flow fractionation (FFF), which was developed by Giddings et al. (14), is carried out in open channels with parallel walls. FFF is closely related to open tubular capillary HDC (15) and is also suited to the separation of macromolecules (14).More recently (16) we extended the working range of open tubular or capillary HDC to include microcapillaries of 1-10 pm i.d. for the separation of polystyrene macromolecules from small molecules like toluene. It was found that a qualitative agreement could be obtained between experimental residence ("retention") times and theoretical ones as predicted by an adapted theory of "separation-by-flow", which was originally put forward by DiMarzio and Guttman (DG) ( I 7-19). A test of more quantitative agreement with 'either DG or the more sophisticated Brenner-Gaydos theory (BG) (20) had to await developments in detector technology.The present contribution reports the experiments carried out after an i...

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