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membrane filters & apparatus with description of Membrane Filters used for separation and enrichment in ultrafiltration, microbiology, medicine, chemistry and industry. ,
THIS NEW 16-PAGE BOOKLET gives the following data:
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Filter porosity. Filter performance. Retention efficiency. Special grid data. Resistance to chemicals (with tables). Analytical investigations with constant weight membrane filters. Sterilization of membranes. General filter data. Complete specifications for all kinds of S&S Membrane Filters, with applications and other notes.
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Cellulose acetate membranes for electrophoresis.
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S&S Membrane Filter Apparatus including new Micro Filter Apparatus, Collodion Bag Apparatus for concentration of proteins, and Hellfritz Osmometers. All specifications given.
All text categories are completely illustrated. A bibliography of available literature is described and keyed. A postage paid card is included for your convenience when you request material from the bibliography. For your free copy of this new and complete reference guide, mail coupon below.
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Carl Schleicher & Schuell Co. ^3UE Keene, New Hampshire-Dept. CEN512^^^ Send me the new S&S Membrane Filters • and Apparatus Bulletin No. 88 Name Company Address City State
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DEC.
6,
1965
Chemical Ionization Improves Mass Spectra Less fragmentation with chemical ionization gives higher-molecular-weight peaks A new analyticiil technique in mass spectrometry, based on forming ions by chemical reactions rather than by electron impact, has been developed by Dr. M. S. Burnaby Munson and Dr. Frank H. Field at Esso Research & Engineering, Baytown, Tex. Spectra produced by this method are different from spectra obtained from electron impact mass spectrometry, and are often more useful for determining structure and identifying compounds and mixtures. The advantages of chemical ionization spectrometry increase with molecular weight, Dr. Munson told the Eastern Analytical Symposium, in Xew York City. In the earlier days of mass spectrometry, problems arose concerning vacuum techniques. Pressures of about 10 ,; torr were needed to prevent collisions between ions and molecules. During the past decade, however, interest in studying these collisions has risen; measurements at a few tenths of a torr are now common. It's from research of this kind that the new technique, called chemical ionization mass spectrometry, has been developed. A simple reaction gas is introduced at high pressure (several torr) into the ionization chamber of a mass spectrometer. This gas produces a stable set of ions which do not react with the gas to any appreciable extent. If a small amount of another material is introduced into the medium, the ions of the reaction gas will react with it and produce a spectrum characteristic of the second material. When methane, a convenient reaction gas, enters the ionization chamber, ionization by electron impact produces a set of primary ions—CH4 + , C H , + , C H , + , CH-r, H + , H 2 + . The first two comprise 90 f/c of the total, and CH..+ is the most abundant of the others. These ions react with methane to give a stable set of product ions: CH 4 + goes to CH 5 + , CH 3 + goes to C>H, + , and CHL>+ gives both C 2 H 4 + and CL>H^ + . The only product ion which reacts further with methane is the last one. It goes on to C. i H 5 + . All these reactions have large rate constants and occur without activation energy at almost every collision be-
tween one of these ions and a neutral molecule of methane. At pressures as high as 2 torr, there is no evidence for a decrease in the relative concentrations of the final set of four product ions. Since CH 4 + and CH 3 + represent about 90 (/( of the total primary ionization, CH r ,+ and C 2 H 5 + should comprise about 90% of the highpressure product set of ions, the Esso workers say. The in sensitivity of relative abundance of ions at high pressure means that collisions between the CH 5 + or CL,Hr,+ and methane do not change the nature of the ions, no matter how many collisions there may be. Introduction of another gaseous compound permits the two methane ions to collide with the new compound. Most of the ions in the chemical ionization spectrum of hydrocarbons result from dissociative proton transfer from CH 5 + and dissociative proton or hydride transfer from C,H r , + A comparison of the chemical ionization and electron impact spectra for 1-decanol ( C ^ H . ^ O H ) shows that the former is more useful. The chemical ionization spectrum has a large peak corresponding to the loss of one hydrogen (presumably from a hydride transfer reaction with C 2 H 5 + ). It also has a series of alkyl and alkenyl ions, which probably come from dissociations of C 1 0 H 2 1 " formed by loss of water from the protonated parent ion, C 1 0 H 2 1 OH 2 ^ (dissociative proton transfer from CH 5 + ). The protonated parent ion does not appear in the spectrum for 1-decanol or the other high-molecular-weight alcohols, but it is a major product ion in the spectrum of isopropyl alcohol. By contrast, the electron impact spectrum of 1-decanol gives less than 0.01% ionization corresponding to the molecular weight, and its fragmentation pattern does not show that the compound is an alcohol. The differences between the methods may be due to the lower energy processes in chemical ionization, or they may be due to intrinsic differences in the ionization methods. Chemical ionization gives even-electron ions, and electron impact gives odd-electron ions.
