"If
only I'd known about Scott!"
Fe, Mn, Rb, Se, and Zn varied up to 50% with respect to control values (17). The variation of Ca levels was especially high because of the mobilization of residual food from the stomach (18). These observations suggest that there is a need for standardizing sampling and storage conditions for autopsy samples destined for elemental concentration studies. It is important to incorporate these measures into the sampling plan so that the true biological variations of the elemental composition of living tissues may be ascertained. Intrinsic Errors
experiments with yellow mercuric oxide were a m o n g Lavoisier's most painstaking and tedious. What a breeze he w o u l d have had w i t h a cylinder of pure Scott o x y g e n instead! Being able to obtain gases pure and in an infinite n u m b e r of possible mixtures is one of the benefits of practising bench chemistry in the 20th century. You have only to look in the Scott Specialty Gases Catalog t o get an idea of what we mean.
Some Scott custom gas mixtures have as many as 38 c o m p o n e n t s . Others are far simpler but can be made w i t h just about any molecule that possesses significant vapor pressure at room temperature. To name à few: allyl chloride, benzaldehyde, c a r b o n y l sulfide, d i m e t h y l a m i n e and so on d o w n the alphabet to vinylidene chloride, m-xylene and yes, even water. For a c o m p e n d i u m of possibly improbable but certainly possible c u s t o m gas mixtures f r o m Scott, w h y not avail yourself of the 160-page Scott Specialty Gases Catalog? It may prove to be one of the most useful volumes on y o u r reference shelf. Ask for your c o p y today.
S Scott Specialty Gases a division of Scott Environmental Technology Inc. Plumsteadville, PA 18949 · 215: 766-8861 San Bernardino, C A 92411 «714: 887-2571 Troy, M l 48084 · 313: 589-2950 10A
Intrinsic errors are factors inherently present in the sample that falsify the results. These errors, as the definition itself suggests, are difficult to detect, and the analyst has little or no control over them. Yet, one has to anticipate their occurrence and interpret the results cautiously. These errors involve medication, hemolysis, subclinical conditions, and certain inescapable medical restrictions. In addition to intrinsic errors, certain inadvertent errors can also occur. These stem from critically small sample weights, in particular from hygroscopic biological substances and other miscellaneous situations. Certain types of medication (e.g., chelation therapy) are recognized for their role in upsetting the trace element balance. However, there are a few other baffling situations, such as prior exposure to iodine-containing drugs or X-ray contrast media, that greatly elevate the tissue iodine concentrations. Extremely high concentrations of iodine may be retained in extrathyroidal tissues even two years after treatment (19). In blood serum the biological half-life for ingested iodine is 2.5 weeks (20). The widespread use of iodine-containing drugs and X-ray contrast media signals a formidable potential source of internal iodine contamination. Careful evaluation of case histories is necessary to minimize such errors. Yet another example is that of pregnant women taking antinausea drugs and sleeping pills, which interfere with the normal metabolism of essential elements such as Cu, Mn, and Zn. Hemolysis is another source of intrinsic errors. Normal plasma contains much less hemoglobin than serum. Serum may contain 10-20 mg/100 mL, an equivalent of 350 to 700 ng Fe per mL. Below a level of 30 mg of hemoglobin per 100 mL of serum, the naked eye cannot distinguish the presence of hemolysis. Above this level, it may not be noticeable, depending upon the intrinsic color of the plasma. This means that an Fe concentration of up to 1000 ng/mL resulting from hemoglobin can
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be present, but virtually unnoticed. Colorimetric methods commonly used in hospitals for the determination of Fe in serum measure transferrin Fe exclusively; hence hemolysis in this case is of no consequence. However, using modern methods such as atomic absorption spectrophotometry and neutron activation analysis, total Fe is measured, yielding high values (2,21). Subclinical conditions involve marginal deficiencies and excesses from altered input, differences in absorption, and unidentified errors of metal transport. Borderline values obtained in such cases cannot be distinguished from the normal range, which is rather wide for several elements and therefore not well defined. Consistently low or high borderline values should therefore be viewed with suspicion. An example of a medical restriction is seen with surgical specimens (e.g., biopsy), where the surgeon is obliged to use only prescribed tools for acquiring the sample. Failure to determine the accurate weight of critically small samples (e.g.,
