Interdetector volume

The effect of interdetector peak broadening on molecular weight disper-sities was described and Mz shown to be independent of interdetector volume.44 The Waters Alliance GPC 2000 series, intended for both room-temperature and high-temperature analysis, has been described.45 The system claims pulseless solvent flow with a precision of 0.075%. 

Interdetector Volume. Because the detectors are placed at different physical positions in the elution stream, the detector signals are... 

In the model there is no interdetector volume difference between the three detector signals. 

Methods that calculate average polymer properties without the DRI are significant because they circumvent complications that arise from the measurement of the interdetector volume between the light-scattering and concentration detectors 10, 11). It is necessary, however, that each local signal,, be divided by the computed particle scattering function, P 6)i. 

The term dn/dc refers to the change in RI of a polymer relative to its concentration. The LS detector responds to M, the VISC detector responds to the intrinsic viscosity ([t]]), which is inversely proportional to molecular density, and the RI detector monitors concentration (C). A single narrow standard is used to determine the offset constants related to the interdetector volume for a given three-detector system [5]. Either C or dn/dc of a polymer sample must be known a priori in order to calculate the other variable using the RI detector [Eq. (3)]. Once both dn/dc and C are known, the LS and VISC [Eqs. (1) and (2)] can be solved to determine M and [77], respectively, for a polymer sample [3]. Structural information, such as chain flexibility, branching, and intramolecular interactions are all related to [7]]. Several key polymer properties related to [iq] are as follows ... 

A different procedure was used by Lecacheux and Lesec (33) for determining interdetector volume for both a viscometer and a light-scattering detector. In this approach, an excluded monodisperse polymer standard is injected. When the correct interdetector volume is selected, the calculated intrinsic viscosity, or molecular weight, is equal to the expected value and remains constant as a function of elution... 

To determine the interdetector delay volume for a viscometer, a broad molecular weight distribution standard can be injected and a Mark-Houwink plot, that is, log [t ] versus log M, generated using universal calibration. The interdetector volume is adjusted until the expected Mark-Houwink exponent is obtained (36). 

Another approach to determining the interdetector volume of a viscometer is first to establish an [t ] versus elution volume calibration curve using a series of narrow polymer standards of known intrinsic viscosities. A broad molecular weight standard is then injected and the interdetector volume is adjusted to obtain superimposition of the intrinsic viscosity calibration curve (37). 

With a light-scattering detector, a log M versus elution volume calibration curve is constructed from a series of narrow molecular weight distribution polymer standards. A broad molecular weight distribution standard is then injected, and an iterative procedure finds the interdetector volume that superimposes the broad MWD standard calibration curve onto the one established by the narrow standards (38). 

Finally, a spectrophotometric method has been proposed in which a low-angle laser light-scattering (LALLS) detector is used as an absorption photometer (35). Interdetector volume is then determined by injecting a solute that absorbs radiation from the LALLS detector. Mourey and Miller (35) used copper cyclohexanebutyrate as the solute and determined interdetector volume using the peak onsets of the LALLS detector and refractometer. 

Consider the BB problem when molar mass sensitive detectors are employed. First, let us analyze the ideal case of a chromatograph fit with perfect detectors and not exhibiting BB, secondary fractionations, or interdetector volumes. In this case, the instantaneous MMD is strictly uniform, and any molar mass detector type would provide the same result ... 

The commercially available molar mass sensitive detectors do include a correction for BB in their software. Unfortunately, the applied correction procedures are not fully disclosed, but they seem to involve an interdetector volume readjustment. For example, the Viscotek Model 200 detector combines a DR in parallel with a specific viscometer. First, the (mass and molar mass) chromatograms of several narrow standards must be measured to determine the interdetector volume and a (uniform EMG) BB function. Then, the MMD is corrected for BB in an unspecified manner. Similarly, Wyatt Corp. has recently introduced a patented BB correction procedure for their triple-detector system (multiangle LS, DR, and specific viscosity sensors). 

The techniques that avoid numerical inversions, correct the MMDs in a single step, and are based on either 1) rotating the linear calibration (when only a mass chromatogram and a linear calibration are available) or 2) modifying the interdetector volume shift (when molar mass sensitive detectors are employed). Their main advantage is that they produce smooth and unique solutions. Their limitation, however, is that they produce only approximate solutions. 

Netopilik, M. Combined effect of interdetector volume and peak spreading in size exclusion chromatography with dual detection. Polymer 1997, 38, 127-130. 

The easiest hyphenated system consists of an LC instrument with a multi-wavelength (e.g. diode-array) UV detector. Such a system is excellent for characterizing copolymers consisting of two or more types of monomeric units, all of which exhibit (different) UV activity. Unfortunately, this is hardly ever the case. A combination of a UV detector and a refractive-index (RI) detector connected in series does in principle provide sufficient information for copolymers (two different monomeric units). However, the interdetector volumes and band broadening are a complicating factor, as are the different background and blank signals (solvent peaks) provided by the two instruments. 

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