ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
chromate solution. Because the result is well within 1g of the theoretical value, it is apparent that the nitrite-ceric titration is unaffected by chromium(V1) at these levels. This is important in that it indicates that one can determine the sodium nitrite titer without considering the possible chromium(V1) content of the analyte. Also, the accuracy of the technique shows no apparent dependence upon the chromium(V1) to chromium(II1) ratio. For example, the largest and the smallest errors (experiments 12 and 15, respectively) were observed a t virtually the same chromium(V1) to chromium(II1) ratios while the remaining analyses produced fairly constant errors which were near the average error of 46 kg. At these low chromium(II1) levels, precision tended to be somewhat of a problem. When the analyses were performed in triplicate (experiments 10-13), the standard deviation was usually quite large, but the precision was improved markedly by performing the determination with five analyses. Nev-
1577
ertheless, the xror wm always less than 0 . 8 ~ and was typically less than 0 'm. The anr,lytical method presented is shown to be rapid and accurate. The procedure is very well suited for black chromium :dating bath solutions and can be readily adapted for chromium(II1) determination a t the milligram level even in the presence of high chromium(V1) concentrations.
LITERATURE CITED (1) Soweil, R. R.; Pettit, R . 8.Plating and Surface Finishing 1978, 65, 42. (2) Willard, H. H.; Young, P. Trans. Necfrochem. SOC. 1935 67, 347. (3) Monnier D.; Zwahlen, P. Helv. Chim. Acta 1956, 39, 1859. (4) Lingane, J. J. "Electroanalytical Chemistry" 2nd ed.;Interscience: New
York, 1958. p 155.
RECEIVED for review January 29, 1979. Accepted March 22, 1979. This work was supported by the U.S. Department of Energy (DOE), under contract AT(29-1)-789.
Determination of Serum and Blood Densities Lorna T. Sniegoski' and John R. Moody National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234
The apparatus for density determinations with which most people are familiar is the pycnometer, and a good review on its use is given by Bauer (1). Both the accuracy and precision may be very high; however, the sample size requirement is often excessive for use in a clinical laboratory. Reviews of methods of determining the specific gravity or density of physiological fluids have been published by Altman ( 2 ) and by Sunderman and Boerner ( 3 ) . Admittedly, a commercially available digital density meter based upon the mechanical oscillator principle would probably meet most of the requirements for small sample size. Recently, Elder has reviewed the capabilities of this instrument ( 4 ) . However, such instrumentation is expensive and difficult to justify in a laboratory where density measurements are not routinely performed. Thus a need still exists for a simple, inexpensive method for determining serum or other fluid densities. The methods employed for the determination of blood, plasma, or serum density have included gravimetric determination by a weighing tube ( 5 ) ,or by a pycnometer (61, a determination by the falling drop method (7), and a determination by the copper sulfate method (8). Recently, the previously mentioned mechanical oscillator has been applied to the measurement of the density of flowing blood in anesthetized animals (9) and has demonstrated a capability that would have been impossible by classical means. With the exception of the mechanical oscillator method, none of the methods for determining fluid density approach the precision or accuracy obtainable with a pycnometer. For a 10-mL pycnometer, the measurement can be made to an accuracy of within *1 part in 10000. However, the amount of sample required precludes the use of a pycnometer in most practical cases. Therefore, a reliable semi-micro method for determining blood serum densities is needed; the following procedure was developed t o meet this need.
EXPERIMENTAL Materials. Macrodeterminations were made using a 10-mL pycnometer (Type A, Ref. 1) equipped with a thermometer. Semi-micro determinations were made using an 0.5-mL glass Lang-Levy micropipet. A cradle was constructed of sheet aluminum (Figure 1)to hold the micropipet on the analytical balance. For convenience, a micrometer type of pipetting aid was used to This article not
subject to U.S. Copyright.
Table I. Densities of Blood and Serum Samples Density, g/mL
____--_
description of sample
by pycnometer, 23 " C
micropipet, 23 " C
blood serum,
1.0242
1.0235
whole blood, 1.0549 CD C whole blood, 1.0485 1287 whole blood, 1.0505 Pa-Pba porcine blood 1.0584 Sample contained small clots.
1.0543
by
WHO
a
1.0479 1.0502 1.0583
fill the micropipet. All measurements were made to *0.01 mg using a semi-micro analytical balance. Methods. For the macro determination, the clean, dry pycnometer is used in the usual manner. For blood or serum, it is necessary to rinse off the outside of the pycnometer to get reproducible results. For the semi-micro determination, the pipet and its holder are weighed in a similar fashion. The effects of static charge on the pipet are minimized by wiping the pipet with a damp (not wet) lint-free cloth prior to weighing. A stable weight can be obtained usually within 2 min. For calibration, the micropipet is filled with distilled water and weighed. After rinsing out the micropipet, a vacuum line may be used to draw a stream of air through the micropipet in order to dry it between measurements. The room temperature is noted. After calibrating with distilled water, the micropipet is filled with blood or serum and weighed. Care must be taken to avoid aspirating bubbles into the micropipet. The pipet is wiped carefully and weighed. If the sample is first allowed to equilibrate to room temperature for 1 h, and the handling of the micropipet is kept to a minimum, then the sample may be assumed to be at room temperature. Calculations are made in the same manner for both methods. A density of 0.99756 g/mL is used for water at 23 "C. The room temperature was constant over a range of f l "C during these measurements. Minor corrections for temperature are made by the volumetric expansion formula where Vz3is the volume at 23 "C, Vobsdis the observed volume,
Published 1979 by the
American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
of the density is 1 X 10-jg/mL; for the semi-micro method, 2x g/mL. The results agree within 0.1% in the worst case. There may be a slight negative bias (-0.0005 g/mL) for the semi-micro method; however, the limited number of samples analyzed precludes any definite conclusion on a bias this small. Thus, the objective of finding a reasonably accurate (>0.1% accuracy) semi-micro method has been met. The method should be applicable to any body fluid.
LITERATURE CITED
Flgure 1. Cradle of sheet aluminum for holding micropipet on the analytical balance
and Tobsd is the observed temperature in "C. The value for the constant in the formula has been empirically determined for serum and does not differ significantly from the value given for distilled water.
RESULTS AND DISCUSSION The densities of one serum and both porcine and human blood samples were determined. Some of the samples were preserved with EDTA or heparin. Each of the results presented in Table I represents the average of a t least triplicate determinations. All determinations have been corrected t o an observed average laboratory temperature of 23 "C. For the pycnometer method, the standard deviation
Baw,N. ''D8trnbti-m of Density" in "Techniques of Organic Chemistry". Weissberger. A., Ed.; Interscience: New York, 1949; Chapter VI, pp 263-276. Altman, P. L. In "Blood and Other Body Fluids"; Dittmer. D. S.,Ed.; Fed. Am. SOC.Exptl. Bioi., Comrn. on Biol. Handbooks; Washington, D.C.. 1961; pp 12-13. Sunderman, F. W.; Boerner, F. "Normal Values in Clinical Medicine", W. B. Saunders and Co.: Philadelphia, Pa., 1950; pp 93-95. Elder, J. P. "Recent Advances in Fluid Density Measurements"; Am. Lab. 1978, 10 (4). 75. MacLeod, J. "Red-Cell Density in Certain Common Animals": 0.J . Exp. Physiol. 1932, 22, 275. Moore, N. S.,; VanSlyke, D. D. "The Relationship Between Plasma Specific G-avity, phsma Protein Content, and Edema in Nephritis", J . Clin. Invest. 1930, 8 , 337. Barbour, H. G.; Hamilton, W. F. "The Falling Drop Method for Determining Specific Gravity"; J . B o / . Chem. 1926, 69, 625. Philips, R. A.; VanSlyke, D. D.; Hamilton, P. B.; Dole, V. P.; Emerson, K.. Jr.; Archibald, R. M. "Measurement of Specific Gravities of Whole Blood and phsma by Standard Copper Sulfate Soluiions"; J . Bb/. Chem. 1950, 183, 305. Kenner, T.; Leopold, H.; Hinghofer-Szalkay, H. "The Continuous HighPrecision Measurement of the Density of Flowing Blood"; Pflueger's Arch. 1977, 370, 25.
RECEIVED for review December 21,1978. Accepted March 13, 1979.
Improved Accuracy in Atomic Absorption Analysis by Optimization of Optical Cell Parameters Gerald F. Dowd and John C. Hilborn" Environment Canada, Air Pollution Technology Centre, River Road, Ottawa, Canada, K 1A 1C8
Extensive use is made of the atomic absorption detector in the measurement of mercury vapor levels in the atmosphere. The active part of this detector is the optical cell and the air in the optical cell is generally considered to represent the atmosphere being sampled, but only if the mercury concentration remains constant. If it is not constant, the concentration in the optical cell will be changing continuously and will not be equivalent to the concentration in the sampled atmosphere a t any given time. This occurs because there is a finite time associated with achieving a uniform concentration in an optical cell following a step change in input and this mixing time is a function of a number of parameters. These parameters can be selected and used in instrument design to achieve a desired mixing time. We speak of mixing time because time is a parameter which can be measured, but what is really desired is to achieve a certain percentage (usually 99% or 99.9%) of the true atmospheric concentration in given time. If greater than 99% of the final concentration can be achieved quickly in the optical cell, the measured concentration more nearly represents the true atmospheric concentration, i.e., accuracy has been improved. An optical cell can be viewed as a small version of an animal exposure chamber. This fact led us to examine the equations in the literature which have been used to describe the pollutant concentration changes in an exposure chamber following a step 0003-27@0/79/@351-1578$01 .OO/O
change in input concentration (purging) as they apply to optical cells. Silver ( I ) and Nelson ( 2 ) have defined a function that relates the number of air changes in a chamber or cell and concentration. An air change means that a volume of air equal to the volume of the cell has passed through the cell. This function (Equation 1)is called the ideal mixing function as it assumes perfect and instantaneous mixing a t all times.
C1 = COe--N
(1)
C, is the pollutant concentration in the cell before the purging operation begins. C1 is the concentration of pollutant in the cell a t some time during the purging operation. N is the number of air changes. From experimental work, Silver ( I ) has reported a 97% response to a step concentration change with N values greater than 3. Brief and Church (3), working with exposure chambers, have reported that, for safety reasons, a mixing factor K should be applied to the ideal mixing formula.
C1 - Coe-KN
(2)
The mixing factor allowed for nonideality of mixing and was between 0.1-0.3. Our studies of cell purging, using mercury in air concentrations measured by atomic absorption, produced data which did not conform with Equations 1and 2. However, using our 0 1979 American Chemical Society
