noise were white, the rms noise would only have increased by = 1.35 and so the l/f character of source flicker noise is obvious. The equivalent experiment was run under condition 2. The increase in the rms noise observed between conditions 2 and 2a is that expected if white noise such as shot noise is limiting. Comparison of 2 to 2b data shows that the noise increases by a factor of 4 for a relative increase in A f of 11 which reasonably follows the 4 3 dependence for white noise. Comparison of 2b and 2a data shows no measureable difference, which is expected because Af is little affected by the hp filter since T h p L 10 qp. In the second experiment, the N/S of the analyte emission signal from a 0.5 ppm Ca solution was monitored and the N/S recorded. The effect of three variables, HP pressure, flame height, and slit width, were studied. One variable was varied while the other two were held constant. In each case, the background emission signal was bucked out with a suppression voltage and the analyte emission signal adjusted between 1 and 3 V. The results shown in Table I1 were obtained in about 15 minutes and illustrate the utility of the N/S monitor for optimization of experimental variables. The N/S data take little more time to acquire than is normally needed to measure the signal and background voltages and no calculations were required. The simple experimental design used here does not illustrate the interrelationship between variables. The N/S monitor could be used with a sophisticated experimental design such as Latin Square (3, l l ) ,factorial (3, I I ) , or Simplex (12).
a
CONCLUSIONS A N/S monitor which is relatively easy to build and to use has been described. I t should be useful in any of the applications described below. Optimization. For a given analyte concentration, spectrometric instrumental variables such as slit width, noise equivalent bandpass or time constant, lamp current, flame height, gas pressures can be varied to ascertain quickly their optimum values. Note that the precision of an analysis will be improved by S/N optimization only if the noise in the signals significantly contributes to imprecision. Calibration Curves. When preparing a calibration curve, one can simultaneously obtain a plot of S/Nvs. analyte concentration. This plot will point out the types of precision expected a t one concentration and a t what concentrations the precision is acceptable for a given situation. Error Detection. The output from the N/S monitor or the rms-to-dc converter can be monitored to detect instances of abnormally high noise such as due to transients. Such instances are more difficult to detect on the signal
Table 11. S / N for Optimization of Ca Flame Emissiona H2 pressurc, lbs/m,2
Flame S l i t w i d t h , i~
height,
CTP
N / S x 102
2.0 100 1.o 1.2 100 1.o 1.o 100 1.o 1.3 50 1.o 200 1.o 1.o 1.o 300 1.o 1.1 100 0.5 2.0 100 2.0 0 2 pressure = 15 lb/in.2, PMT voltage = 660-855 V. Signal a t point A adjusted to 1-3 V, g = 10, T , [ , = T~~ = 1sec. Ri = 108 ( 1 ,
readout device where a small amount of noise is on top of a relatively large signal. Additional circuitry could be used to block signal data acquisition a t times of abnormally high noise. Nature of Noise. The nature of the noise can be determined from the fluctuations in the rms noise or N/S voltages. In general, flicker or 1/f noise exhibits a greater number of larger excursions than white noise. The input high pass filter time constant can be varied to change If and thus to differentiate between white and 1/f noise. If the noise is proportional to (If)1k2, then white noise is dominant.
LITERATURE CITED (1) J. D. Winefordner, W. J. McCarthy. and P. A. St. John, J. Chem. Educ., 44, 80 (1967). (2) W. J. McCarthy, "The SignaVNoise Ratio in Spectrochemical Analysis: Its Use in Optimization of Experimental Conditions in Spectrochemical Methods", in "Advances in Analytical Chemistry and Instrumentation", C. N. Reilley and F. N. McLafferty, Ed., Wiley-lnterscience, New York, NY, 1971, pp 493-518. (3) M. L. Parsons and J. D. Winefordner, Appl. Spectrosc., 21, 368 (1967). (4) T. Coor, J. Chem. Educ., 45, A533 (1968). (5) J. D. Ingle. Jr., and S. R. Crouch, Anal. Chem., 44, 785 (1972). (6) G. M. Hieftje, Anal. Chem., 44, (6), 81A; (7), 69A (1972). (7) H. V. Malmstadt, C. G. Enke, S. R. Crouch, and G. Horlick, "Optimization of Electronic Measurements", W. A. Benjamin, Inc., Menlo Park, CA, 1974. (8) V. D. Landon, Proc. hst. Radio Eng., 29, 50 (1941). (9) R. Steinitz, J. Grinberg, V. Bar, and A. Seidman, Rev. Sci. Instrum., 43, 656 (1974). (10) "MP-System 1000, Operation and Applications", McKee-Pedersen Instruments, Danville. CA. 1971, (11) H. A. Laitinen, "Chemical Analysis", McGraw-Hill, Book Co., Inc.. New York. NY, 1960, pp 570-574. (12) S.N. Demingand S. L. Morgan, Anal. Chem., 46, 1170(1974),
RECEIVEDfor review December 9, 1974. Accepted February 10, 1975. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for partial support of this research.
Device for Sampling Headspace from Canned Food R. E. Hurst Fisheries and Marine Service, Vancouver Laboratory, Department of the Environment, 6640 N. W. Marine Drive, Vancouver, B.C., Canada
The can sampling apparatus described here was developed as part of a program to study the volatile compounds in canned salmon using gas chromatography. During the initial stages of this work, a ZAHM air tester was used
(Zahm and Nagel Co. Inc.); however, inclusion of liquid in the gas sample presented a continuous problem. In the operation of this commercial device, the can is punctured and sealed in the center of the can lid which forces the lid down ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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Figure 1. Schematic drawing of the headspace sampling device positioned on a can
Figure 2. The headspace sampling set-up using a collection tube for
sampling
toward the liquid, leaving little or no headspace height. T o overcome this difficulty, a headspace sampling device was designed in which volatiles are collected from the top edge of the lid of a can positioned on its side. This overcomes the problem of moving the lid toward the fluid when the can is punctured and increases the height of headspace above the fluid. For example, the average distance of headspace between the fluid and lid of canned salmon was found to be 0.4 cm. By positioning the can on edge, this distance was increased to 1.5 cm. This is an increase of over 3.5 times which could be made even greater by tilting the can upward a t the sampling edge.
DESCRIPTION A detailed schematic illustration of the headspace sam1222
* ANALYTICAL CHEMISTRY, VOL.
47, NO. 7. JUNE 1975
pling device is given in Figure 1. The components consist of a yoke connecting and supporting two bosses that house inlet and outlet puncture needles operated by pressure caps, and a screw clamp which applies pressure to two ruhber seals. The inlet is attached by means of Teflon tubing to a 50.1111 glass syringe, and the outlet needle is connected in the same manner to a gas sampling syringe or, in this example, to a collection tube the operation of which was described in an earlier publication (I). The sampler is affixed on the top side of a can that is held on edge by first hacking off the screw clamp and pressure caps so that the needles can he withdrawn flush with the inlet and outlet bosses. The device is then brought down over the can so that the yoke rests on the can rim. Now the screw clamp is tightened to compress the rubber gaskets against the ends of the can to make a seal. The plunger of the syringe is withdrawn to a 50-ml volume and connected to the protruding end of the inlet puncture needle which should retain the syringe plunger in the set position, Next, the inlet pressure cap is screwed in which drives the puncture needle through the can into contact with the internal headspace. At this point, if the can is vacuum packed, the plunger will move down displacing a volume equivalent to the vacuum. The collection flask or a depressed gas tight syringe is now connected with tubing to the protruding end of the outlet puncture needle which is then driven through the can by turning in the outlet pressure cap. The headspace gas can now he sampled by either the collection tube procedure or by the gas-tight syringe. The volume of headspace that is removed from the system will he compensated hy the plunger of the syringe. Figure 2. shows the apparatus set-up and Figure 3 is presented as an example of its application.
CONSTRUCTION The yoke and bosses are machined from a solid aluminum bar I s - i n . diameter. The boss a t the inlet has a brass hushing with a rubber seal on the inside face, a Teflon liner through which the puncture needle passes and a %in. N.F. threaded end extending %-in. from the outer face. A second brass bushing is pressed into the boss a t the outlet and has a %-in. N.F. internal thread through its entire length to ac-
Figure 3. Gas chromatographs of headspace taken from canned salmon using the headspace sampler in conjunction with the collection tube technique for concentrating volatiles
commodate the screw clamp. This is a threaded brass tube llh-in. long with a rubber seal on the inner end, a Teflon liner to house the puncture needle and a fixed thumb screw to permit turning. The puncture needles are made from stainless steel hypodermic needles B-D, No. 17 with a disc silver soldered onto the needle to act as a pressure point against the pressure caps as they are screwed in. The device
can be easily adapted to f i t various can sizes by simply adjusting the length of the yolk.
LITERATURE CITED ('I '. E. Hurst,
(London),9'
302 (1974).
RECEIVEDfor review October 29, 1974. Accepted February 3, 1975.
Drop Sampler for Obtaining Fresh and Sea Water Samples for Organic Compound Analysis B. H. Gwnp, H. S. Hertz, W. E. May, S. N. Chesler, S. M. Dyszel, and D. P. Enagonio Bioorganic Standards Section, Analytical Chemistry Division, National Bureau of Standards, Washington, DC 20234
As analytical chemists have become more involved in projects concerning water pollution, environmental impact statements, and base-line studies, the need for a means of obtaining replicate and representative aquatic samples has become apparent. A sampling device that permits the analyst to obtain shallow (51meter) and deeper water samples that are free from contamination by compounds on the surface is necessary. The water sampler shown in Figure 1 was designed to meet the criteria proposed for The Integrated Global Ocean Station System (IGOSS) for such a sampler ( I ) . It utilizes a removable acid-washed ( 2 ) ,3.7-liter (l-gal-
lon) bottle clamped in a stainless steel frame. A Teflon bumper provides some protection of the bottle against impact with the side of the sampling craft. The spring-loaded closure unit, mounted on the top of the sampler, prevents contamination of the inside of the bottle upon passing through the surface of the water. Bottles are cleaned carefully to remove all organic constituents that might contaminate the sample. Each bottle is washed with soap and water, rinsed and immersed in a hot (110 "C) concentrated sulfuric acid bath for 30 min. Following this treatment the bottle is rinsed six times with ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
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