I -1
10
MASS 4 0
M A S S 32
-
I
20-
30-
\
W
2
FC
40-
E
50-
a
8 f
60-
I
70
-
BO
-
pot
I
L A
8
A
B i
Figure 3. Comparison of resolution for phenidone developer (A) and Kodak D19 (B) at masses 32 and 40
DISCUSSION The high contrast phenidone developer acts in a manner similar to internal image developers. This is a result of the low reduction potential of phenidone and, therefore, of its low activity. Sodium sulfite is a preservative, which retards oxidation of the developer and dissolves silver halide. Hence, development comprises two reactions; the developer couples with the exposed grains and the sodium sulfite removes any uncoupled grains. However, because of the low activity of phenidone, the sodium sulfite dissolves the surface grains before much of the process has taken place; in effect, the internal image is preferentially developed.
The improved resolution is aided by the fine-grain developing property of phenidone. This reduces clumping since, in the first stage, the developing agent couples onto the exposed silver halide grains and prevents adjacent unexposed grains joining the exposed grains to form larger crystals. Another aid is the adjacency effect which is most apparent in denser lines. With low activity developers,diffusion, at a sharp edge, of fresh developer from low to high density and exhausted developer from high to low density tends to sharpen edges. Together with the coupling action of the developer, this accounts for improved resolution, particularly in the low mass range where instrument focus aberrations are minimal. Hence, germanium metal can now be analyzed more reliably for silicon in the presence of iron because the W i + is adequately resolved from 56Fe2+. The increased latitude of the photoplates enables the range of photometric data to be extended over more exposure levels. The improved slope of the characteristic curve permits more precise measurement of small differences in intensity. The phenidone developer has been in routine use for about a year. It has a reasonably long tank life even though it is susceptible to oxidation. The rate of oxidation is retarded by the presence of the preservative, the use of floating lids to exclude air and the use of nitrogen burst agitation which tends to de-oxygenate the solution. The developer is changed every three weeks or twelve plates. This procedure has resulted in no loss of activity. At each change, freshly prepared developer is used.
ACKNOWLEDGMENT The author wishes to acknowledgevaluable discussions with L. R. Stracchan, Officer-in-Charge, AAEC' Photographic Laboratory. LITERATURE CITED (1) P. R. Kennicott, Anal. Cham., 38, 633 (1966). (2) A. Cavard, Comm. L'Energ. Atom., Report CEA-R-3759 (1969). (3) A. Vance, Dignan Photographics Inc., Newsletter", May, 1-182 (1969).
RECEIVED for review July 2, 1976. Accepted August 18, 1976.
Perchloric Acid-Free Digestion of Blood in Microdetermination of Lead by Anodic Stripping Voltammetry Frank Peter* and R. G. Reynolds Department of Chemistry, Laboratory Services Branch, Ontario Ministty of Health, Toronto, Ontario, Canada
Anodic stripping voltammetry (ASV) is frequently used for the quantitative determination of lead in microsamples of blood. The determination requires the destruction of the red blood cells prior to the analysis, because the lead is contained in these cells. The disruption of the red blood cells is usually carried out with the digestion of the blood specimen. The most commonly used method for the digestion is the boiling of an aliquot of the blood specimen with a mixture of perchloric acid, nitric acid, and sulfuric acid. This method has been recommended by several authors (1-5) and has become a standard technique. Although an individual 100-pl blood sample poses a negligible explosive hazard because of the small volumes, the possibility of buildup of organic perchlorate residues in the digestion of numerous samples requires the use of a special stainless fume hood usually with a separate exhaust system. All this makes the application of perchloric acid
in the digestion potentially hazardous and rather expensive. To avoid these difficulties, a perchloric acid-free digestion has been developed in our laboratory employing a mixture of nitric acid, and sulfuric acid followed by a treatment with hydrogen peroxide. The method involves three steps: 1) charring with the mixture of nitric acid and sulfuric acid, 2) destruction and decolorization of the charred residue with hydrogen peroxide, and 3) elimination of the unused hydrogen peroxide.
EXPERIMENTAL The analysis was carried out with an ESA multiple anodic stripping analyzer, Model 2014 ( 6 ) .One hundred microliters of clot-free blood was pipetted into the polarographiccell. The pipet tip was rinsed twice with 100 p1 of water and the water was added to the blood. Five hun-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
2041
Table I. Lead Concentration in Blood Digested with the Reference Method (A) and the Perchloric Acid-Free Method (B) Blood specimen No. 1 Method n
A
Mean ( d d l )
21 1.0 4.76
fSD RSD, %
No. 3
No. 2
B
A
12
B
A
12 20 0.9 4.50
40 2.0 5.00
B 12
42 1.9 4.52
60 3.0 5.00
59 2.8 4.74
dred microliters of a mixture of 3 volumes of concentrated sulfuric acid (Aristar,BDH) and 5 volumes of concentrated nitric acid (Aristar BDH) was added to the cell. The cell was inserted in an aluminum heating rack and placed on a hot plate. The plate was switched on and set to 400 "C (f20"C). The rack was kept on the hot plate for about 90 min, until the fluid evaporated and only a black, dry residue remained. The rack was taken off the hot plate and when it cooled down 100 gl of hydrogen peroxide (Aristar,BDH) was added to the cell. The rack was returned to the hot plate and after the fluid evaporated, another 100 gl of hydrogen peroxide was added and evaporated again to dryness leaving behind a white residue. Finally, 200 .ul of water was added and boiled to dryness. The rack was taken off the hot plate and, when the cells were cold, 5 ml acidic matrix was added. This contained 1ml perchloric acid (Aristar, BDH) in 11. lead-free water.
RESULTS AND DISCUSSION Three blood specimens with unknown lead concentration were assayed to compare the results of the digestion carried out with and without perchloric acid, respectively. Each specimen was digested twelve times and the lead concentration was determined with ASV. The digestion without perchloric acid was performed as described above. The digestion with perchloric acid was carried out using methodology as follows (6). An acid mixture was prepared containing 24 vol. nitric acid, 24 vol. perchloric acid, and 1 vol. sulfuric acid. Three hundred microliters of this mixture was added to the mixture of 100 pl of blood and 200 pl of water. The tubes were placed on the hot plate at 200 "C (f10"C)and the digestion was completed in 1 h. Five ml acidic matrix were added to the tubes and the plating was carried out in the same manner as with the perchloric acid free digestion. The results are shown in Table I. As seen from the figures, there is an excellent correlation between the two sets of results. It was especially important that the total destruction of the organic matter could be car-
ried out without perchloric acid even at high lead concentration (Blood specimen 3). The digestibility of a blood specimen with elevated lead concentration had to be checked because abnormally high lead concentration alters the chemical composition of the red blood cells leading to an increase in the concentration of a t least two haem precursors: 6-aminolevulinic acid (7) and zinc protoporphyrin (8).The accumulation of these two intermediates might influence the effectiveness of the digestion. When applying the digestion without perchloric acid, the following limitations should be kept in mind: The more sulfuric acid present in the mixture, the longer the charring lasts. Decreasing the volume of the sulfuric acid, on the other hand, decreases the effectiveness of the charring. The greater the volume of the acidic mixture in relationship to the volume of the blood, the more intense is the charring. The increase of the volume over its optimum, however, increases the charring time. The heat treatment in the charring must start with a cold plate, otherwise the nitric acid in the acidic mixture is partially destroyed and does not contribute effectively to the charring. The cell must be cooled before treatment with hydrogen peroxide, because it becomes increasingly unstable a t higher temperatures and self-deterioration decreases its oxidizing power. Elimination of the traces of hydrogen peroxide before the polarographic analysis is very important because any residual peroxide interferes with the plating-stripping and causes erratic response from the instrument.
LITERATURE CITED (1) W. R. Matson, R. M. Griffln, and G. B. Schreiber, "Trace Substancesin Environmental Health, IV", D. Hemphill, Ed., University of Missouri, 1971, pp 396-406. (2) H. A. Schroeder and A. P. Nason, Clln. Chem. ( Winston-Salem,N.C.), 17, 461 (1971). (3) B. Searle, Wlng Chan, and B. Davidow, Clln. Chem. ( Winston-Salem, N.C.), 19, 76 (1973). (4) L. Duic, S. Szechter, and S. Srinivasan, J. flectroanal. Chem. interfacial Electrochem.,41, 89 (1973). (5) N. Ramasamy, S. Parameshwaran, A. Redner, and S.Srinivasan, Trans. Soc. Adv. flectrochem. Sci. Techno/.,8, 50 (1973). (6) "ESA Methodology: Trace Metal Analysis of Blood", Environmental Sciences
Associates, Burlington, Mass.
(7) T. R . Robinson, Arch. fnvlron. Health, 28, 133 (1974). (8) A. A. Lamola, M. Joselow, and T. Yamane, Clin. Chem. ( Wlnston-Salem, N.C.), 21, 93 (1975).
RECEIVEDfor review June 7, 1976. Accepted August 16, 1976.
Detection System for Negative Ions in Mass Spectrometry A. L. C. Smit, M. A. J. Rossetto, and F. H. Field* The Rockefeller University, New York, N. Y. 10021
This paper describes a mass spectrometric detection system for negative ions which is especially useful for mass spectrometers having an electron multiplier ion detection system and a low ion accelerating voltage. This device can be built from commercially available parts and is relatively inexpensive. It may be anticipated from conceptual principles that high pressure negative ion mass spectrometry (negative ion chemical ionization) will be of much practical analytical use, and recent publications (1-12) demonstrate a growing interest 2042
in and use of the technique. However, one major drawback to its wider use is that the electron multiplier and the subsequent ion detection system must float a t a high voltage, and most mass spectrometers do not have this capability. This problem is particularly encountered in quadrupole mass spectrometers (ion accelerating voltage =IO V) and in relatively low ion accelerating voltage magnetic deflection mass spectrometers (ion accelerating voltage 13000 V). The first dynode of the electron multiplier should be a t a potential significantly above ground to attract the negative
ANALYTICAL CHEMISTRY, VOL. 48, NO. 13, NOVEMBER 1976
