Concentrationsensitive Detectors

The concentration detectors most frequently used in SEC of polymers are the UV and the RI detectors. Infrared (IR) detection suffers from problems with the absorption of the mobile phase. Two other detectors are useful in the analysis of non-UV absorbing polymers: the density detector and the evaporative light-scattering detector (ELSD).

The UV detector detects UV-absorbing groups in the polymer, which may be the repeating unit, the end groups, or both. Hence, one has to distinguish two situations. For polymers, in which the repeating units contain a chromophoric group, the response of the UV detector represents the mass eluted in a given volume interval; for polymers with chromophoric end groups, however, the response depends on the number of polymer molecules. Many chromatographers use this assumption, when they derivatize 'nonabsor-bing' polymers with UV-active reagents. Detection wavelengths are in the range of 180-350 nm but many SEC solvents allow detection only above a wavelength of 250 nm. Diode array detectors allow the measurement of an entire UV-spectrum at any point of the chromatogram, which is useful in SEC of copolymers.

Table 1 Column packings for nonaqueous size exclusion chromatography

Producer

Packing

Material

Rockland Technologies

Zorbax PSM

Porous silica microspheres

Shodex

Asahipak GF HQ

Highly crosslinked polyvinyl alcohol

Macherey-Nagel

Nucleogel GPC

St-DVB copolymer

Merck

LiChrogel PS

St-DVB copolymer

Phenomenex

Phenogel

St-DVB copolymer

Polymer Laboratories

PL gel

St-DVB copolymer

Waters

Styragel HR, HT, MW, Ultrastyragel

St-DVB copolymer

Jordi

Jordi GPC

100% DVB polymer

The ELSD detects any nonvolatile components of a sample. In this instrument, the eluate is nebulized and the solvent evaporated from the aerosol. Each droplet containing nonvolatile material will form a particle, which scatters the light of a transversal light beam. The response of this instrument for copolymers is, however, unclear - it depends on various parameters and the nature of these dependencies is rather complex. Moreover, lower oligomers may be strongly underestimated.

The refractive index (RI) detector exists in various modifications. It is the most common instrument in SEC besides the UV detector.

The density detector, which has been developed in the author's group, uses the principle of the mechanical oscillator. The measuring cell of this instrument is an oscillating, U-shaped capillary, the period of which depends on its reduced mass, and thus on the density of its contents. The signal of the density detector is thus inherently digital, and its response is integrated over each measuring interval.

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