Report for Analytical Chemists
By any other name it's just a paper
S&S "wet-strength" is a fact Calling a paper "wet-strength" does not make it so. Scientists know that when it says S&S it is, in fact, wet strength. "Ash-Free" or "Ash-Low," genuine wetstrength papers come from S&S. "Ash-Free" for Quantitative Analysis: No. 589-1H. Hardened. Extra Rapid. Thin. For filtration of metallic hydroxides. No. 589-BH. Hardened. Rapid. Coarse and gelatinous ppts pressure filtration. No. 589-WH. Hardened. Medium speed and retention. For gravimetric analysis. No. 589 Red. To prevent collodial dis persion during filtration and washing. No. 507 Hardened. Extra dense. Separa tion of finely divided ppts from corrosive solutions. "Ash-Low" for Qualitative Analysis: No. 410 Extremely rapid. Thin. Gelati nous and coarse crystallines. No. 404 Very rapid. Soft. Loose texture. Coarse and gelatinous precipitates. No. 497 Moderately rapid. Retains pre cipitates that are moderately fine. No. 402 Dense. For filtration of very fine precipitates. No. 576 Extra dense. Hardened smooth. Biological products filtration (serum, in jection fluids). Analytical Filter Papers catalog available on request.
SCHLEICHER & SCHUELL Keene, Ν. Η. Schleicher & Schuell Keene, New Hampshire 03431 Please send Quick Reference Catalog No. 4 to: Name Address City State
Zip AC-169
Circle No. 122 on Readers' Service Card 26 A
·
ANALYTICAL CHEMISTRY
ance in cellular metabolism are well known. These alkali metals also have an ionic strength effect on en zyme systems (which m a y contain other m e t a l s ) . The mode of action of some met als is known, a t least to a certain extent. Probably the most impor t a n t role of trace metals is in the activation (or deactivation) of spe cific enzyme systems. Some are nonenzymatic in function, either in addition to or instead of being en zyme oriented. Examples are vita min, hormone, skeletal, and other controls. Recent evidence has been obtained t h a t trace metals m a y be associated with R N A . M a n y met als show in vitro activity toward enzyme or other systems, but a t present no known in vivo activity or essentiality has been demonstrated. Once the essentiality of an ele ment has been shown, its site of ac tion must be investigated. Suitable isolation and analytical techniques are required for these studies. Those elements for which specific sites of action have already been demonstrated could be found in future investigations to possess ad ditional metabolic functions. While elements are classified as trace or macro, some, notably iron, appear to play the role of both. Iron is present in large amounts, principally in hemoglobin, but traces of iron in tissues function as a component of several enzymatic systems, just as other trace metals in the body. Some elements occur at extremely low concentrations in the body, notably in the blood and, in the author's opinion should be classified as ultratrace elements. These include cobalt, manganese, chromium, nickel, and iodine. This by no means is to imply t h a t these elements should be considered to possess a lesser biological role. Since plants are the basic source of mineral nutrients to both animals and man, their metal content is im p o r t a n t in the maintenance of health. The form in which the met als occur is important, and the min eral content of soils determines the supply and availability to plants. There are over t h i r t y metals and metalloids found in the h u m a n body, most of them in t r a c e quan tities. These are listed in Table I
along with the concentrations found in blood serum, urine, and tissues. The concentrations of several of these are so small t h a t reported lev els v a r y over a wide range depend ing on the analytical method em ployed. These concentrations listed in the table represent a sampling of several values reported believed to to be most reliable. T h e extremely small amounts of some metals dem onstrate the importance of sensi tive analytical techniques. Atomic Absorption Spectroscopy
T h e principles of atomic absorp tion spectroscopy have been amply described elsewhere (7) and will be dealt with here only briefly. Basi cally, the technique involves aspi ration of the sample solution into a flame where the metal ions are con verted into the atomic vapor state. Most of this atomic vapor exists in the ground electronic state and can therefore absorb resonant radiation of an appropriate wavelength. A hollow cathode lamp is used as the source. This is a sharp line source consisting of a cathode made of the particular element in question (or an alloy of it) with a tungsten an ode. T h e lamp is under reduced pressure and is filled with an a p propriate inert gas (e.g., argon, he lium, or neon). When a sufficient voltage is impressed across the elec trodes, the filler gas is positively ionized a t the anode and is acceler ated toward the cathode. As these ions bombard the cathode, they cause the cathode material to "sput t e r " and form atomic vapor which in the process is raised to an excited electronic state. Upon returning to the ground state, the lines charac teristic of the element are emitted. (The line corresponding to the t r a n sition from the ground state to the lowest electronic excited state is known as the resonance line. This is often, but not necessarily, the most strongly absorbed line.) These pass through the flame (along with lines of the filler gas) where certain ones are absorbed. This absorption is measured with an a p propriate monochromator. T h e a b sorption obeys Beer's law. Since, generally, only the test element can absorb this radiation, the method becomes very specific. In addition,
