to a signal corresponding to kN, where k is a factor chosen to represent the confidence level (e.g., k = 2) and N is the rms noise level (3). The rms noise is approximately the peak-to-peak noise divided by 5 (4, 5). The time, tlim, at which a signal equal to kN results is found directly from the experimental C us. t curve, and the limiting detectable concentration, Clim,is then estimated from tlimand C, using the equation relating C and t. A typical C us. t curve (actually signal us. t but signal, e.g., absorbance, is proportional to concentration) is given in Figure 3. For the case given in Figure 3, the limit of detection for copper (0.026 pg/ml) corresponds to the concentration resulting at 3.0 minutes after the initiation of analyte introduction into the flame. Use of Flask for Determining Analytical Curves. By proper choice of C, (i.e., initial volume and concentration of analyte solution injected into the exponential dilution flask) and by recording the instrumental signal (analyte concentration) as a function of time, it should be possible to rapidly determine an analytical curve (signal us. concentration) without the necessity of preparing a series of standards. Such a technique would allow the analyst to plot an analytical curve with as many points as desired. Of course, the validity of this simple procedure rests upon the instantaneous introduction of sample and mixing of sample with diluent and the exponential change of signal with time. In (3) J. D. Winefordner, W. J. McCarthy, and P. A. St. John, J. Chern. Educ., 44, 80 (1967). (4) V . D. Landon, Proc. Znst. Radio Engineers, 29, 50 (1941). ( 5 ) J. Praglin, “The Measurement of Nanovolts,” Keithley Instruments, Inc., Bulletin 102.1, Cleveland, Ohio, 1966.
Figure 4, analytical curves obtained with the exponential dilution flask (100 pl of 10,000 pg/ml of Cu introduced into flask with flow rate of 175 ml/min) and with individual standards are given. Each point of the analytical curve obtained with the exponential dilution flask is the average of values obtained with four curves (relative standard deviation for all concentrations corresponding to times shorter than 4 minutes was of order of 2 % or less and relative standard deviation for concentrations corresponding to 5 and 6 minutes was 3.0% and 3.8%, respectively). One of the exponential curves used to obtain the data in Figure 4 is given in Figure 5. It is evident that the concentrations obtained with the exponential dilution flask deviate significantly at high concentrations-short times (by about 9 %) and negligibly at low concentrations (by 1-279 An attempt was made by the authors to vary experimental conditions to avoid this deviation but with no success. If the exponential dilution flask is to be used for determination of analytical curves, more experimental studies are needed. Of course, if relative errors of the order of 10% or less are allowable, the present exponential dilution flask can be utilized. ACKNOWLEDGMENT
The authors thank Perkin-Elmer Corp., Norwalk, Conn., for loaning us a Model 303 Atomic Absorption Spectrophotometer for this work. RECEIVED for review July 30, 1970. Accepted August 31, 1970. This work was supported by AFOSR(SRC)-OAR, U. S. A. F., AF-AFOSR-70-1880.
Simplified Version of the Alkali Flame Detector for Nitrogen Mode Operation D. A. Craven Research and Development Center, International Flavors & Fragrances, Union Beach, N. J . 07735
WEHAVE FOUND a simple, improvised version of the thermionic emission detector useful for identifying nitrogen-containing compounds by gas chromatography. The detector consists of a bead of fused RbS04-KBr (1 :1 mixture) placed onto the flame tip of an ordinary hydrogen flame detector. The use of KBr circumvents the difficulty of fusing pure RbS04 and, in addition, makes the detector unresponsive to halogenated compounds. The salt mixture was fused into the shape of a disk, inch thick by 1/2-inch diameter, using a KBr die (Model MK.3, Research and Industrial Instrument Co., England) at 20,000 psi for about one hour. A quarter-section Of this disk was then rounded to about ’/* inch in diameter with emery cloth, bored to 0.02 inch, and counter-bored to fit over the flame tip (see Figure 1). This bead design is suitable for chromatographs with capillary-type flame tips* It has been used successfully On the Hewlett-Packard Model 5750, Perkin-Elmer Model 900, and
-q
t--0.02“
I I
-b
L ”8
Figure 1. Bead design Varian Model 1740 instruments. The operating characteristics in the nitrogen mode agree with those reported by Aue (1, 21, K~~~~~ ( J ) , and others, (1)
w. A. A
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w
c. w.Gehrke, R. c. Tindle, D. L. Stalling, and
C. D. Ruyle, J . Gas Clrromatogr., 5 , 381 (1967). (2) W. A. Aue and S. Lakota, J . Chromntogr., 44, 472 (1969). (3) A. Karmen, Science, 7, 541 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 13, NOVEMBER 1970
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Table I. Response Characteristics of Proposed Detector us. Normal FIDa EnhancementC Enhancementc FID responseb AFD responseb (across detectors) (within AFD) Compound 0.5 N.A. 1 x 10-7 5 x 10-7 Ethyl octanoate N.A. negative N.A. (m) Dichlorobenzene 1 x 10-7 1 x 10-7 1 x 10-0 100 500 2,4,6-Trimethylpyridine 1 x lo-' 3 x 10-13 3 x 105 2 x 108 Triethyl phosphate a Hewlett-Packard Model 5750 Gas Chromatograph, 10-foot X 1i8-inchCarbowax 20M S.S.Column, isothermal at 150 "C., and electrometer range at lo2. b Response in grams per equivalent signal. c Normalized to ethyl octanoate response.
A lean hydrogen flame (15 ml/min) is required to keep background sufficiently low and thus optimize the response to nitrogen compounds. This condition results in a slight decrease in overall detector sensitivity but a hundredfold enhancement for nitrogen compounds was observed. Trimethylpyridine was shown to give a readily calculable peak at the one-nanogram level. The response to phosphorus compounds appeared to be the usual order of magnitude (
