2381
Anal. Chem. 1981, 5 3 , 2381-2383 0 Sunrhlna Gelornster Data X Gel all Data
Q----2-7?--QQ 150
Tempenturn
Figure 4. Gel
‘C
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times via two techniques.
used is in optimizing accelerator concentrations for various polymers. As in Figure 3, it was readily shown that accelerator concentrations in excess of 1.5% did not yield a faster cure
schedule and that, in fact, the polymerization rate after 1.5% accelerator concentration becomes kinetic dependent. Data generated by this technique compares well with data generated with other types of apparatus. Specifically, comparison curves for the gelation of a liquid epoxy and dicyandiamide as the curing agent are shown in Figure 4. The instrumental data were generated by use of a Sunshine Gelometer. Excellent agreement is shown between the two curves for a major portion of the gel curve. A difference does exist between the curves at temperatures in excess of 170 “C.These may be attributed to several factors, such as residual solvent quenching the system during the first minutes of test as it is volatilized off. RECEIVED for review June 16,1981. Accepted September 17, 1981.
Comparison of Two Materials for Storage of Nitrogen Standard Solutions Kathleen A. Mitchell Philip Morris U.S.A., Research Center, P. 0.Box 26583, Richmond, Virginia 2326 1
Most methods for the determination of nitrogen are labor-intensive and numerous references to procedures using features of automation are evidence of the importance of being able to handle large numbers of samples (1-4). One procedure offering high sample throughput is a modified Kjeldahl digestion in a block digestor with an automated colorimetric readout using the Technicon AutoAnalyzer system (5-9). A method modified for this laboratory is based on Wall and Gehrke’s semiautomated procedure (6). The suggestion l(6, 7,9) that standards may be stored and reused until exhausted has been adopted as a labor-saving measure to eliminate daily preparation of the standards. The standards, prepared by the addition of ammonium sulfate to the Kjeldahl digestion matrix where selenium serves1 as the catalyst, are compared to digested samples using a colorimetric readout based on a modified Berthelot reaction. It was observed that the response of 6 week old standards was greater than the response for the freshly prepared standards. This “agiing” phenomenon was evaluated as function of storage conditions. Pyrex glass storage bottles, routinely used for storage of standard solutions, and Nalgene (linear polyethylene) storage bottles are compared for their ability to preserve ammonium sulfate standards prepared in a sulfuric acid digestion matrix. The re3ults presented here describe those conditions which are suitable for storage of nitrogen standard solutions.
prepared as follows: A 1.7688-gportion of dried ammonium sulfate (21.20% N) was weighed and transferred to a 500-mL volumetric flask. The ammonium sulfate was dissolved and brought to volume with deionized water. The working nitrogen standards were prepared in a digestion matrix as follows: 25-mL aliquots of 36 N H2S04were transferred to five 250-mL digestion tubes and 6 g of potassium sulfate and 2 selenized Hengar granules were added to each digestion tube. The digestion tubes were then placed in the block digestor preheated to 200 “C and heated for 15 min. The tubes were allowed to heat to 400 “C for a total “Auto” cycle period of 2.25 h. With caution the digestion tubes were removed from the block digestor and allowed to cool for 30 min. Approximately 20 mL of deionized water was added to each tube. Aliquots of 2, 5, 10, 15, and 20 mL of the stock standard nitrogen solution were pipetted into the separate BD-20 digestion tubes containingthe digestion matrix. The contents of each BD-20 tube were diluted to the 250 mL volume mark with deionized water. The standards were equivalent to 6, 15,30,45, and 60 pg of nitrogen/mL. The total volume of 250 mL was fiitered through Whatman no. 42 fiiter paper and collected in four containers-two Pyrex glass and two Nalgene. Procedure. Each working standard was inverted before a portion was transferred to an AutoAnalyzer sample cup. The working standards were submitted to the AutoAnalyzer system in a random order at various times in a 7-week period. The standard calibration setting of the single-channel colorimeter was held at 775 for the analysis which was equivalent to 0.175 total absorbance units full scale. “Response”, as used in the context of this report, is defined as percent of full scale absorbance units.
EXPERNMENTAL SECTION
RESULTS AND DISCUSSION
Apparatus. The Technicon BD-20 block digestor is used to prepare the digestion matrix for standard8 in the same manner as the digestion of a sample. BD-20 digestion tubes, calibrated to 250 mL are used. The Technicon AutoAnalyzer I1 system includes sampler, proportioning pump, ammonia determination analytical cartridge No. 116-D531-01,AAII single-channel colorimeter, and recorder (available from Technicon Instruments Corp., Tarrytown, NY). Glass storage bottles (Corning Catalog no. 1500-125) and Nalgene bottles (Catalog no. 2104-0004) are available from most laboratory suppliers. Reagents. Reagents for the determination of nitrogen in the BD-20 digests are prepared as described by Technicon (6). Standards. Stock standard nitrogen solution, 750 pg/mL, was
Responses for each concentration level of nitrogen standards stored in four containers were obtained on the AutoAnalyzer system. Paired responses representing the same concentration and same storage material, and their average, are given in Table I for day 1and day 51 of the testing period. Responses for all four bottles a t each concentration are similar on day 1 as shown in Table IA and IB. In Table IC the response of the pairs stored in Nalgene on day 51 continue to show reproducibility. Further, those responses show little differences from the initial responses as seen in Tables IA and IB. Table ID demonstrates the “aging” effect of the standard solutions stored in glass. Large differences in response are
0003-2700/81/0353-2381$01.:!5/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
2382
Table I. Response vs. Concentration for Paired Standards: Nalgene vs. Glass Storage concn, wg/mL
response I1
I
av
concn, d m L
0.082 0.202 0.400 0.602 0.810
6 15 30 45 60
0.082 0.200 0.398 0.598 0.800
6 15 30 45 60
A. Nalgene Storage: Day 1 6 15 30 45 60
0.082 0.202 0.400 0.602 0.809 0.082 0.200 0.397 0.598 0.800
O5
t
1 A
A
0
0
i
0
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-
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ual
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c
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0
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.
0
,
,
1
1
-
0
I
I.
0
1
1
-
TIME( Days)
Figure 1. Ordinate intercept values for the calibration curves for glass storage (A)and Nalgene storage (0). I
I
A
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.
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A
0
1
1
,
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0.083 0.210 0.409 0.605 0.816
0.083 0.210 0.409 0.612 0.820
0.083 0.210 0.409 0.608 0.818
D. Glass Storage: Day 5 1
0.082 0.200 0.399 0.598 0.800
I
response I1
B. Glass Storage: Day 1
0.082 0.202 0.400 0.602 0.811
C. Nalgene Storage: Day 51 6 15 30 45 60
I
1
I
1
1
35
40
45
50
TIME(Days)
Figure 2. SEE values for the calibration curves for glass storage (A) and Nalgene storage (0). seen between pairs at each concentration suggesting a random effect on response as a function of glass storage. Comparing the averages of responses of concentrations for day 1 (Table IB) and day 51 (Table ID) increases of as much as 0.071 response units are observed. The data from day 51 showing the lack of reproducibility between identically glass-stored standard solutions and the large increases between responses on day 1 and day 51 can be described as an “aging” phenomenon. The least-squares parameters of a calibration curve were obtained for each storage mhterial for 11trials over the 51-day period using a simple linear regression analysis of concentration vs. average response. The ordinate intercept and the standard error of the estimate (SEE) are least-squares pa-
0.127 0.281 0.473 0.698 0.859
0.121 0.240 0.487 0.660 0.870
0.124 0.260 0.480 0.679 0.864
rameters of the regression line which provide estimates of systematic and random error. The consecutively obtained calibration curves did not differ from one another in respect to slope. In Figure 1 the ordinate intercept values for the 11 trials are plotted as a function of time. The ordinate intercept values remain close to zero throughout the 51-day period for the Nalgene-stored standards. The ordinate intercept values for glass-stored standards are constantly greater than zero which demonstrates systematic error associated with time. In addition, the magnitude of the ordinate intercept values is large characterizing the “aging” effect. In Figure 2 the SEE values for the 11 trials are plotted as a function of time. The magnitude of the scatter in the points about the least-squares line for the calibration curves for the two storage materials is similar. Low SEE values for calibration curves of standards stored in Nalgene demonstrate consistency of response for the 51-day test period. SEE values for standards stored in glass are of similar magnitude to those stored in Nalgene with important exceptions on days 29,35, and 51 where large SEE values are obtained. These large SEE values demonstrating random error are obtained intermittently, an unacceptable condition for standard solutions. The reproducibility of responses between concentration pairs for Nalgene storage at day 51, the lack of systematic error and little, but consistent random error for a regression of response on concentration associated with time demonstrates the stability of Nalgene storage. By comparison the large differences in responses between concentration pairs; the increases in responses from day 1to day 51 resulting in ordinate intercept values constantly greater than zero; and intermittently large SEE values characterize glass as an unstable storage material for nitrogen standard solutions. The cause of this change in the standards stored in glass is unknown. The suggestion that standards may be stored for reuse is a valid labor saving measure provided preservation and stability are maintained by use of Nalgene high-density linear polyethylene containers.
ACKNOWLEDGMENT The author is indebted to Richard E. Davis for his advice and to Irene Smetena for her assistance.
LITERATURE CITED Law, A. R.; Nicholson, N. J.; Norton, R. L. J . Assoc. Off. Anal. Chem. 1971, 54, 764-760.
Kramme, D. G.; Grlffen, R. H.; Hartford, C. G.; Corrado, J. A. Anal. Chem. 1973, 45, 405-408.
Bietz, J. A. Anal. Chem. 1974, 46, 1617-1618. Nelson, D. W.; Sommers, L. E. J. Assoc. Off. Anal. Chem. 1980, 63, 770-770.
Isaac, R. A.; Johnson, W.C. J . Assoc. Off. Anal. Chem. 1976, 59, 98-100.
Anal. Chem. 1981, 53, 2383
(9) Noel, R. J.; Hambleton, L. G. J . Assoc. Off. Anal. Chem. 1978, 59, 134-140.
(6) Wall, L. L., Sr.; Gehrke, C. W. J . Assoc. Off. Anal. Chem. 1977, 60,
a81-889. (7) Technicon CorDoration, Tarrytown, N.Y., Industrial Method No. 36975A. ( 8 ) Technicon Corporation, Tarrytown, N.Y., Industrlal Method No. 33474w.
2383
for review 13, 1981* Resubmitted and accepted September 18, 1981.
Electrometer Sulbstitute for a Flame Photometric Detector K. W. Michael Siu, James R. Hancock, and Walter A. Aue" Department of Chemistry, Walhousie University, Halifax, Nova Scotia, B3H 4J 1, Canada
Electrometers for gas chromatographic detection are, a t least in our hands, prone to malfunction; and they are expensive to repair or replace. Faced with such a dilemma, we recently assembled a simple amplifier from readily available components. I t served as an inexpensive electrometer substitute for use with our Shimadzu flame photometric detector (FPD). Due to the relatively high current output of the photomultiplier tube, the FPD lends itself well to low-cost amplification. This is possible through the use of inexpensive monolithic field-effect transistor (FET) operational amplifiers that became readily available after the mid 1970s. Futhermore, we are making use of the fact that, nowadays, many recorders have multirange capability. Of course, there is nothing fundamentally new in our approach; however, it is certainly not common knowledge that one can significantly reduce the high cost of a flame photometric detection system in this manner. 'The priice one pays for doing this-in terms of detector performance as well as in terms of electronic components-is minimal.
EXPERIMENTAL SECTION The heart of the electrometer substitute, shown in Figure 1, is a half-dollar FET operational amplifier, TL081CP (Texas Instruments). It converts the photomultiplier current to voltage; in this case a 1 nA current produces a 1 mV output. The operational amplifier is powered by two 9-V alkaline batteries as shown in Figure 2. The bucking is a continuously adjustable 0-2 V source--in this case a lab-made arrangement consisting of a battery, resistors, and potentiometers. The voltage ranges of the recorder (Linear Instruments, Irvine, CA, Model 385, 1 mV-5 V) replace the attenuation setting of a conventional electrometer.
1M
-
Figure 1. Circuit diagram.
electrometer
multi-range
substitute
Figure 2. Schematic setup.
RESULTS AND DISCUSSION To illustrate the stability and sensitivity of the electrometer substitute, Figure 3 shows 3- and 2-ng injections of diphenyl sulfide, monitored with the lab-made circuit and a regular FPD electrometer (Shimadzu Model EM-5S), respectively. The minimum detectable amount is about 2 times higher in the former (sulfur response if3 a quadratic function of the amount injected). Adding a conventional, first-order low-pass filter (lab made; less than $3) reduces the detection limit by nearly half (Le., to a level comparable to that of the regular electrometer). This, however, was not considered necessary for most practical purposes The thermal drift of the operational amplifier was specified as 10 pV/"C. Thus even under the most sensitive experimental sett,ing-a recoirder voltage range of 5 mV full scale-no noticeable drift was observed. The noise level of this electrometer substitute, when measured by grounding the
Figure 3. Chromatograms of 3-ng diphenyl sulfide using lab-made electrometer substitute (left) and of 2-ng diphenyl sulfide using the regular Shimadzu EM-SS electrometer (right).
input through a 1 M resistor, was about 4 p V . Component cost of the substitute electrometer-including chassis, connectors, and heavy-duty batteries-was less than $15 U.S. The less expensive assembly time involved 2 h of graduate student toil. RECEIVED for review June 17,1981. Accepted September 22, 1981.
