CONCLUSION The work presented here demonstrates that use of a stopped-flow absorption microcell system for fluorometric analysis can be readily accomplished without modification of the microcell itself. The front-surface illumination-observation technique can be used by attachment of a reflecting mirror, focusing lens, and a filter. Application of this instrument for reaction-rate analysis of clinical and biological samples is under way, and incorporation of a dualbeam-in-space measurement system will be used to improve the sensitivity and photometric precision of the system.
(3) J. E. Stewart, "Durrum Application Notes No. 7". Durrum Instrument Co., Palo Alto, Calif., 1970. (4) Bulletin B-2437A-J, American Instrument Company, Silver Spring, Md.. 1973. (5) Bulletin 3195-2-3/Rj-4-5M, "The SMAC System", Technicon Instrument Corporation, Tarrytown, N.Y., p 9. (6) J. G. Atwood, Bulletin BL 12/73 1, "The Perkin-Elmer Kinetic Analyzer Model KA-150", Perkin-Elmer Corporation, Norwalk, Conn., 1973. (7) L. Brand and B. Witholt, Fluorescence Measurements, in "Methods in Enzymology", XI, C.H.W. Hirs, Ed., Academic Press, New York, N.Y.. 1967, p 776. (8)G. Winkelman and J. Grossman, Anal. Chem., 39, 1007 (1967). (9) J. McHard and J. D. Winefordner, Anal. Chem., 44, 1922 (1972). (10) S. Ainsworth, Anal. Chem., 37, 537 (1965). (11) T. 0. Tiffany, C. A. Burtis, J. C. Mailen, and L. H. Thacker, Anal. Chem.. 45, 1716 (1973). (12) D. C. Harrington and H. V. Malmstadt, Anal. Chem.. 47, 271 (1975). (13) R. Gabriels. Anal. Chem., 42, 1439 (1970).
LITERATURE CITED (1) K. R. O'Keefe and H. V. Malmstadt, Anal. Chem., 47, 707 (1975). (2) A. C. Javier, S. R. Crouch, and H. V. Malmstadt, Anal. Chem., 41, 239 (1969).
RECEIVEDfor review May 22, 1975. Accepted August 1, 1975.
Geometry Factors and Flux Corrections in Neutron Activation Analysis Stephen R. Piotrowicz,' James L. Fasching,2 Dianne D. Zdankiewicz, and Rudolph W. Karin Department of Chemistry, University of Rhode Island, Kingston, R.I. 02887
Two possible sources of error in neutron activation analysis are variations in the neutron flux over the length, width, and height of a sample and geometry factors during counting of the activated sample. Flux monitors are the main method ( I ) used to evaluate flux variations to make the appropriate activity corrections to samples and standards. Geometry problems in counting have been more severe for beta counting than for gamma counting. Scattering and self-absorption of electrons can introduce significant uncertainties in beta counting. The attainment of a suitable, reproducible counting geometry is therefore necessary in beta counting ( 2 ) .Chemical separations and subsequent precipitation to an identical physical form are the usual methods used to minimize geometry problems in beta counting. The use of a NaI(T1) well detector has helped to minimize geometry problems in gamma spectrometry, but for counting with NaI(T1) and Ge(Li) detectors, geometry problems still exist. The authors have developed a sample preparation, handling, and analysis system designed to minimize these two problems as well as reducing contamination problems. The procedure basically involves four steps: 1)spotting a known volume of sample solution or standard solution on a filter disc and allowing it to dry in a laminar-flow clean bench; 2) sealing the disc between two layers of plastic; 3) irradiation; and 4) counting. The filter discs are punched from Whatman 41 filter paper using a sharpened 2.22-cm arch punch (C. S.Osborne Co., Harrison, N.J., No. 149), a mallet or hammer and a thick piece of leather as a cutting block. A leather cutting block is desirable as it does not dull the cutting edge of the arch punch as rapidly as a wooden cutting block. T o minimize contamination, the filter paper is sealed in a plastic bag and punched while in the bag. All work is done in a laminar-flow clean bench. Four'thicknesses of filter paper can easily be punched in this manner. The punched discs are removed from the center of the punch using Teflon-coated tweezers, the plastic is discarded, and Graduate School of Oceanography, U n i v e r s i t y of Rhode Island, Kingston, R.I. 02881. A u t h o r t o whom r e p r i n t requests should be addressed.
the discs are placed in a clean plastic bag for storage. Using a sheet of filter paper 46 cm X 57 cm folded in quarters and allowing for waste on the edges where handling has occurred (even though all handling is done wearing untalced plastic gloves), over 400 discs can be punched from a single sheet of paper giving a relatively constant blank for a large number of samples. The plastic discs are punched in a similar manner only using a slightly larger arch punch, 3.02 cm, from plastic 4 mils thick. We have begun using 6-mil thick plastic instead of the more commonly found 4-mil thick plastic as it is less sensitive to heat and therefore easier to seal than 4-mil plastic. These dimensions were chosen so as to fit the "rabbits" used in the State of Rhode Island Nuclear Science Center's pneumatic tube sample delivery system; however, any other dimensions for different systems could be used. A solid Teflon sealing plate (20.32 cm X 30.48 cm X 1.27 cm thick) which has been machined out to accept 15 separate discs a t a time is used to prepare the samples for irradiation (Figure 1A). Each disc holder will accept the 3.02cm plastic disc and has a raised island in the center for the 2.22-cm filter disc. A rounded groove has been cut around the raised center to the outside edge of each disc holder. A plastic disc is placed in the disc holder and a 2.22-cm filter disc is then placed on top of the plastic disc. The sample solution or standard solution is then pipetted on this filter disc and allowed to dry. We commonly use volumes of 20 and 50 yl for aqueous solutions and standards. Solutions up to 200 yl have been used; however, this volume takes over 1 hour to dry completely. The pipets used are calibrated gravimetrically. After the solution has dried (usually 10 to 20 minutes), a second plastic disc is placed on top of the filter disc and the two plastic discs are then heat sealed using an ordinary soldering iron fitted with a special, Teflon-coated, aluminum tip (Figure 1B). The method could also be adapted to ion exchange papers or filters from ring ovens where large volumes have been processed. The aluminum tip was manufactured with dimensions such that it would f i t the disc holder and with a rounded edge to fit the groove a t the bottom of the disc holder (Figure IC). The tip is recessed in the center so as not to con-
2328 * ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
1'9""
I
Figure 1. Design of complete sealing equipment ( A ) Teflon sealing plate with arrangement of plastic and filter discs in one sample holder. ( B ) A top and side view of the Teflon-coated aluminum sealing tip. One sample holder with the soldering iron attached to the tip just before sealing a plastic disc.
(C)
Table I. Blank Values Determined by NAA 7-Hour irradiation Isotopes
24Na 38c1
82Br lZ1Te "'Sb 13dCS
Half -life
15.0 h r 37.3 min 35.3 hr 17.0 day 2.8 day 2.05 y r 40.2 hr
Energy, MeV
1.368,1.732 1.643 0.554,0.776 0.574 0.564 0.797 1.597
Elemental concenmation,
g/blank
6.44 i 0.14 2.92 i 0.15" 3.95 x 10-3i: 1.60 x 59.57x 10-3 2 .oo x 10-5i 1 .os x 6.97x 10-4i 0.37 x 3.02 x 10-4 2.75 x
Blanks counted
Detectable blanks
30 3 11 11 11 11 11
30 3 10 0 4 6 7
10-3 10-5 10-4 10-4
Decay time
4 days 20 min 4 days 5 days 5 days 5 days 5 days
35-Hour irradiation
83.9 day 51~r 27.8 day 59Fe 45.6 day 6OCO 5.26 y r 65~n 243 day j5se 120 day 89~r 52.0 day 9 7 ~ r 17.0 h r '**Hf 42.5 day 115 day 18'Ta *9 8 A ~ 64.7 hr a The value for C1 was determined with period. 4°C
10-5 0.39 x 10-5 10 10 21 days 10-3+ 2.16 x 10-3 9 10 5 days i 3.61 x lo-' 9 7 21 days 10-5 1.95x 10-5 4 21 days 10 4 10-3i 8.43 x 10-3 10 5 days 10-3i 1.41 x 10-3 10 10 5 days 10-3 1.62 x 10-3 4 10 5 days 10-3 10 0 5 days 10-5 10 0 5 days 10-5* 1.90 x 10-5 3 10 21 days 10-3 5.83 x 10'3 10 9 5 days a 20 minute irradiation. The average time interval between the end of irradiation and counting
0.889 0.320 1.099,1.292 1.173,1.332 1.115 0.265,0.401 0,910 0.797 0.482 1.221 0.411
2.11 x 4.51x 9.07x 6.18x 9.07 x 3.74 x 5.91 x 55.78x 55.97 x 7.48 x 5.97 x
tact the plastic anywhere except where the seal is desired. The tip has been coated with Teflon to minimize problems of hot plastic adhering to it as well as contamination problems. A 100-watt, 110-120 V ac-dc electric soldering iron is used to heat the tip. The temperature of the constantly heating soldering iron is regulated with a Powerstat variable autotransformer (Superior Electric Co., Bristol, Conn.). A disc can be sealed in 2 to 8 seconds when the iron is a t the correct temperature. With practice, an air- and water-tight seal is obtained over 95% of the time. An excellent method of checking the seal as well as cleaning the plastic disc is to wash the sealed disc with a dilute acid solution and rinse with distilled, demineralized water. The samples can be counted either inside the plastic discs or removed from the plastic and placed in appropriate, nonradioactive containers for counting. Geometry problems in counting are therefore minimized since the flat
11* for One Irradiation
Of
Five Standards
Element
Reproducibility, %
Element
Reproducibility, %
sc Cr Fe co Zn
2.6 12.1 5.4 6.5 6.2
Se Sb CS La Ta
6.5 8.2 5.4 7.O 5.5
disc provides a constant geometry during Ge(Li) counting. Table I lists the average values, with standard deviations, for 18 elements found in 11 different sample blanks. The blanks consisted of two sealed plastic discs with two filter paper discs sealed within them. Table I1 lists the overall reproducibility for various elements in one irradia-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
2327
tion with samples stacked over a 10-cm distance within the rabbit and five standards interspaced through the unknowns. The values range from 1 to 12% with an average of about 5%. The variations include all the sources of error that can occur during neutron activation analysis of a sample. We routinely employ this method for the analysis of sea salt aerosols and water samples for major components and biological fluids for both major and minor elements. In the irradiation of sea salt aerosols and water samples, we have had no sample or standards break open during irradiation. This has involved the irradiation of several hundred discs, some for up to 4 hours. The authors have removed filter discs from the sealed plastic for Na, C1, and Br analysis. The blank values for single Whatman 41 filter paper discs are 1.05 f 0.10 pg, 1.29 f 0.01 pg, and 0.005 f 0.002 pg for Na, C1, and Br, respectively. There is a severe deterioration of the filter paper with 50-11 aliquots of any 2N acid solutions, during a 7-hour irradiation, such that removal of the filter disc from the plastic is not possible. The filter paper can still be handled when neutral or basic solutions are used or if the irradiation time is reduced to 4 hours and care is used in handling. Loss of sample to the plastic occurs when aliquots larger than 50 p1 are used and the filters are removed. Loss of halogens (probably as gaseous HCl and HBr) occurs when acid solutions are irradiated for even short periods.
Blood samples have been irradiated for as long as 35 hours and breakage has occurred only when aliquots of between 0.1 to 1.0 ml have been used. In these cases, failures of sealed discs have occurred between 5 to 10% of the time.
ACKNOWLEDGMENT The authors thank A. C. Bachelder and the University of Rhode Island Engineering Instrument Shop for the construction of the apparatus, the staff of the Rhode Island Nuclear Science Center for space and facilities for analysis, and the University of Rhode Island Computer Laboratory for data processing and reduction.
RECEIVEDfor review June 23, 1975. Accepted July 25, 1975. This research was supported by the Office for the International Decade of Ocean Exploration, National Science Foundation under NSF-IDOE Grant GX-33777 and by the United States Department of Health, Education, and Welfare, Public Health Service, National Institutes of Health Grant No. 1 R 0 1 HD 06675-02.
LITERATURE CITED Jr., Ed., "Guide to Activation Analysis", D. Van Nostrand Co., Inc., N e w York. 1964, pp 14-20. (2) G. H. Friedlander, J. W. Kennedy, and J. M. Miller, "Nuclear and Radio(1) W. S. Lyon,
chemistry", 2nd ed., John Wiley 8 Sons, Inc., New York, 1964, pp 411413.
CORRECTION Errors Caused by Fiowrate Variation In High Performance Size Exclusion Chromatography (GPC) In this articleby D. D. Bly, H. J. Stoklosa, J. J. Kirkland, conclusions on pp 1811-1812. The sketches in the figures iland W. W. Yau, Anal. Chem., 47, 1810 (1975), the tabular lustrate the type of flowrate variation employed while the tabular portion of each curve shows the results obtained. portions of Figures 1-4 were inadvertently eliminated. Figures 1-4 are here reproduced in their entirety. These data are essential for following the discussion and
-
% RANDOM VARIATION IN FLOW RATE
Mn MEAN 112,900
0.0
Rw
2aFOR IO RUNS
2uFOR IO RUNS
MEAN
0
278,100
0
0.5
112,900
700
278,100
1,100
I .o
113,000
1,300
278,200
1,900
3.0
I 12,800
4,000
278, I O 0
6,400
5.0
112,600
7,000
277,400
11,200
I 11,800
10.0
Figure 1. Random variation zero to end of GPC curve
12,700
276,500
n
INCREASING FLOW RATE
_..-..
19,000
in flowrate about a fixed level from time
DEVIATION IN
M,
RELATIVE PERCENT ERROR
00
112,900
0.5
113,800
1.0
l14,W
-
dCviiiioN IN FLOW RATE
-
8,
RELATIVE PERCENT ERROR
0.0
112,900
0.0
0.5
114,800
1.7
1.0
123,400
9.3
3.0
146,700
299
RELATIVE PERCENT ERROR
RELATIVE PERCENT ERROR
00
278,100
0.0
112,900
0.8
278,900
0.3
112,100
-0.7
277,300
-0.3
I 5
279,700
06
111.200
-1.5
276,600
-0.5
0.0
278,100
258,400
-71
-23.8
222,600
-20.0 -31.3
-36.8
191,100
-61.5
128, 7tl)
change in flowrate from time zero to end of
RELATIVE PERCENT ERROR
0.0
PERCENT DEVIATION ,dNRAn
RELATIVE PERCENT ERROR
iin
ii,
-53.7
GPC
DECREASED FLOW RATE RELATIVE PERCENT ERROR
M,
RELATIVE PERCENT ERROR
M,
RELATIVE PERCENT ERROR
0.0
112,900
00
278,100
0.0
112,900
00
278,100
00
0.5
125,030
10.7
305,000
9.1
101,900
-9.7
253,400
-89
1.0
13a.mo
22.4
334,
too
20 J
91,900
-18.6
230, 700
-170
30
2 0 4 , ~
81.0
477.000
715
60,100
-468
157,000
-43 5
672,003
141.6
38,590
-65 9
105, I00
-62 2
II,M)O
-89.7
35,703
-872
107,800
-4.5
273,400
-1.7
104,400
-7.5
270.3W
-2.8
5.0
297,603
1636
-5.7
10.0
716,200
5344
2328
-8.6
86,000
43,500
1.7
262,300
103,203
71,300
2.9
-15 I
-36
42.9
282,900
95,800
0.0
268,100
100.8
286,100
57
278,100
-4.3
397,500
7.6
294.W
7.6 24.2
0.0
108,000
558,xx)
4.6
15.2
299,100
345,300
112,900
53.9
118,100
change in flowrate across the GPC curve
1.6
#,
130.7
121,500
130, 100
00
RELATIVE PERCENT ERROR
ERROR
173,800
3.0
10.0
278,100
282,500
RELATIVE PERCENT
260,KK)
5.0
Figure 2. Uniform
a,,
iNCREASED FLOW RATE
3,
aw
RELATIVE PERCENT ERROR
nw
5.0
DECREASiNG FLOW RATE
Rn
DECREASING FLOW RATE
10.0
Figure 3. Uniform
PSWFNT ,
FLOW RATE
INCREASING FLOW RATE PFRCFNT
Figure 4. Fixed
ANALYTICAL CHEMISTRY, VOL. 47, NO. 13, NOVEMBER 1975
1,500.xIo 4396
flowrate deviation from specified value
