Letters
Nomenclature for SizeExclusion Chromatography Sir: One of the four separation modes in liquid chromatography, "steric exclusion", is a column fractionation method based on the molecular sieve effect. There are many names used for this chromatography: Gel Filtration Chromatography (GFC), Gel Permeation Chromatography (GPC), Molecular Sieve Chromatography, Gel Chromatography, Liquid Exclusion Chromatography, and Size-Exclusion Chromatography. The great variety of names applied to the same separation mode causes confusion. Part of this confusion stems from the origin of the chromatography. Most gel filtration experiments have been carried out in aqueous solutions, with hydrophilic gels as bed materials. The term "GFC" is commonly employed today in the field of bio-
chemistry and clinical chemistry, while the term "GPC" is generally accepted in the field of polymer science for the measurement of molecular weight distributions with polystyrene gel in organic media. However, newly developed packing materials such as deactivated porous glass and silica have enabled the separation and the desalting of peptides and the determination of the molecular weight distribution of polymers in the same columns with different solvent media. The use of GFC or GPC, without further qualification, to describe this chromatographic system can be confusing and misleading. Though no comprehensive and completely satisfactory theory for separation mechanisms has yet emerged, most experts agree that these separations are primarily based on the difference in size of solutes in solution and that the larger molecules, excluded from all or a portion of the pores of
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NORTON
PLASTICS ANDSYNTHETICSDIVISION P.O.BOX350 AKRON. OHIO 44309 TEL:(216) 633-3224 CIRCLE 149 ON READER SERVICE CARD
424 A · ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
gels by means of their physical size, elute before the smaller ones, regardless of the types of solutes, the mobile phases, and gels. The term "size exclusion" is a concise word for expressing this separation mode in analogy with the terms "adsorption", "partition", and "ion exchange", which express the other three separation modes. This chromatography cannot be classified as "filtration". The term "permeation" represents neither the separation mechanism nor the separation mode. Since in liquid chromatography every solute must permeate into the pores of packing materials, those that cannot permeate into the pores are not separated. Strictly speaking, every chromatography is permeation chromatography. Porous glass can be used for this technique as packing materials, so that the nomenclature Gel (Permeation, Filtration) Chromatography is not in agreement with this chromatography in the strict sense of
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Letters
first with innovations in plasma spectrometry SMI is widely recognized and respected as both a leader in atomic analysis and innovator of plasma-emission measurement techniques that have become accepted in numerous laboratories and industries worldwide.
Sadao Mori Department of Industrial Chemistry Faculty of Engineering Mie University Tsu, Mie 514, Japan
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SMI
SPECTRAMETRICS INCORPORATED 204 ANDOVER STREET, ANDOVER, MA 01810 (617) 475-7015 CIRCLE 190 ON READER SERVICE CARD
426 A ·
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL
the word. The term "exclusion" includes steric exclusion as well as ion exclusion which belongs to a different technique. It seems reasonable to assume that several proposed theories such as steric exclusion and restricted diffusion take each part of "size exclusion" mechanisms. This situation is similar to the fact that several adsorption or partition mechanisms are proposed for "Adsorption" or "Partition" Chromatography. To the end that we can use the consolidated terminology for the techniques, I propose that the term "Size Exclusion Chromatography (SEC)" be used to describe this chromatography instead of GPC, GFC, Gel Chromatography, and other related names.
1978
Graphite Furnace AA Sir: I would like to clarify a point raised by Ralph Sturgeon in his INSTRUMENTATION article, "Factors Affecting Atomization and Measurement in Graphite Furnace Atomic Absorption Spectrometry" [ANAL. CHEM., 49,1255A (1977)]. On page 1260 A he states, "It is the inseparability of the rate of heating of the atomizer and the maximum temperature which it attains that presents a major problem with commercial ET (electrothermal) atomizers employing threestage heating programs." He reiterates this throughout the text. This problem has been recognised and eliminated by at least one manufacturer of commercial atomizers. The Varian CRA-90, which was released in January 1976 and described [B. R. Culver and D. E. Schrader, Amer. Lab., 8 (3), 59 (1976)] in March 1976, allows independent selection of the heating rate and the maximum temperature. A ramp rate of up to 800 °C/s, a hold temperature of up to 3000 °C, and a hold time of up to 5 s may be independently selected for the atomization stage. I hope this letter will prevent your readers from gaining the mistaken impression that there is no commercial graphite furnace with the desirable characteristics specified by Dr. Sturgeon. P. R. Liddell Varian Techtron Pty. Ltd. 679/687 Springvale Road Mulgrave, Vic, Australia 3170
