Report for Analytical Chemists
Matheson Model 3500 Regulator
No adsorption No desorption No inboard leakage An exceptionally well designed regulator featuring a stainless steel diaphragm and packless outlet valve. Delivery pressure range 0-75 p.s.i.g., cylinder pressure gauge 0-3000 p.s.i.g. Recommended for laboratory and processing facilities using ultra high purity gases, corrosive gases; for trace gas analysis, for moisture analysis, doping gases, for crystal growing, etc. See Matheson Catalog, or write for Engineering Report to Matheson, P. O. Box 85, East Rutherford, N. J. 07073. MATHESON GAS PRODUCTS A Division of Will Ross, Inc. East Rutherford, N,J., Cucamonga, Calif.; Glouchester, Mass.; Joliet, III.; La Porte, Texas; Morrow, Ga.; Newark, Calif.; Whitby, Ont. Circle No. 136 on Readers' Service Card
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ANALYTICAL CHEMISTRY
wavelength. If absorption does occur, this is subtracted from all readings a t the analytical wavelength to obtain correct results. As with all analytical methods, a blank should always be run with the method of standard additions. If significant impurities occur in the blank, the actual concentration of these should be determined by the method of s t a n d a r d additions, by adding the standard to a duplicate blank, as with the sample. This is because the flow rate of the sample is often less t h a n t h a t of the aqueous blank solution since it will have a high matrix concentration. When serum or urine samples are simply diluted with water, often no blank correction is necessary if the water is of sufficient purity. If, however, the water does contain significant amounts of the test element, this is an example in which the concentration should be determined as mentioned above since the serum viscosity is greater t h a n t h a t of water. Reporting of Results
A variable found in reporting results for nonfluid samples such as tissues and plants is the form in which the sample is weighed. Some investigators report values based on the fresh (wet) sample weight. Others prefer the dry basis in which the sample is dried by heating a t about 110 °C or is freeze dried, air dried or dried by other means before weighing. Still others report results based on the weight of ash of a sample. These conventions m a k e it very difficult to compare results of different investigators. Reporting concentrations on a fresh basis has more meaning since they can easily be converted to total content of the element in a given sample, organ, plant, etc. and since they represent the concentration in the real sample. Analysis based on fresh weight will be more subject to fluctuations because of differences in water content, water evaporated or water absorbed by handling before weighing the sample. However, these fluctuations m a y be within t h e experimental error of the analysis for some trace elements, and n a t u r a l biological fluctuations of the elemental concentration m a y be as great as or greater t h a n the
uncertainties in weighing. I n addition, it is certainly more convenient simply to weigh the fresh samples t h a n to dry them. T h e weight of dried samples is probably more reproducible t h a n the fresh weight, and for this reason results m a y a p pear to be more reproducible, especially for repeated analysis on a given sample. T h e same is true of the ash content of samples, although this would probably be more variable t h a n the dried sample weight. If samples are dried before weighing, danger of loss of certain volatile elements exists. Because of the above arguments, the author prefers to report concentration on a fresh basis and would like to appeal to other investigators to adopt this as a uniform basis for reporting future results. Or, it would be a simple m a t t e r to weigh samples fresh before drying, and then to report values on both the wet and dry basis for comparison. I n general most tissues or organs contain approximately 70 to 80 per cent water. As an approximation, the ash content of tissues comprises in the neighborhood of 10 per cent of the weight of dried tissue. Determination of Metals in Biological Fluids
Elements which have been determined in urine and blood serum by atomic absorption spectroscopy are summarized in Table I I I . Unless specific limits of determination are given, the described procedures are applicable to determination of physiological levels of the elements as listed in Table I. The specified concentration limits in the Table refer to concentration in the sample. The atomic absorption characteristics of these elements are listed in Table IV. T h e sensitivity is defined as t h a t concentration in ppm (aqueous solution) which gives rise to 1 per cent absorption under the defined conditions. Limits of detection m a y be lower t h a n this and sensitivities are usually improved with organic solvents. The alkali metals can be determined in serum by simple dilution of the sample with water. I n the case of lithium and potassium, the other alkali metal(s) present in serum should be added to standards
