ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
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the tube a t the rate of delivery to the graphite furnace. The results are shown in Figure 4. The broadening is significant, but follows the classical broadening equations fcir laminar flow in a dead volume (5). For narrow peaks this might pose a problem because of loss in chromatographic resolution. This can be avoided by designing the storage tubes to be only slightly larger in interval volume than the peak volume. Likewise the resolution of two peaks can be maintained if the peaks are physically separated by storing each in a separate tube; then even though there is some peak broadening, the peak overlap will not occur.
Abs.
0.008
CONCLUSION T h e technique of Liquid Chromatography-Graphite Furnace Atomic Absorption spectroscopy (LCGFAA) has been applied to the study of environmental samples. In these samples the technique, LCGFAA, aids in the study of metal biotransport and trace organometallic determination a t low levels with concurrent molecular identification. The on-line pulsed mode of sampling gives a single element concentration as a function of retention time. The modification of this technique to an off-line procedure greatly increases the precision of the technique. The off-line storage of individual peaks allows a much higher number of AA data points to be obtained per peak. These additional data more completely describe the concentration profile of the organometallic compound present in the mobile phase. For a sample which contains multiple components, and the identity and retention time of each component is known. The separate component may be stored in different tubes. Each tube may then be analyzed automatically and the peak area for each component determined. For samples in which several organometallics are to he monitored routinely, this technique could be used. We are currently investigating the application of the temporary peak storage mode of sampling for multicomponent mixtures of organometallics of the same metal, and for inultielement multicomponent mixtures.
Abs,
0.008
LITERATURE CITED (1) Vickrey, T. M.; Buren. M. S.; Howell, H. E. Anal. Lett 1978, A l l ,
Figure 4. The UV (254 nm) responses for toluene (upper) and PbPh, (lower) in the peak broadening study
thus lengthening the time required for the experiment. Peak Broadening. The additional AA data for each analyte peak is accompanied by peak broadening due to the dead volume of the storage tube. The possible loss of chromatographic resolution was investigated using toluene and PbPh4. To analyze the effect of analyte storage the samples were chromatographed, passed through the sample site of the UV detector, stored in the tube, and pumped back through
1075-1095. (2) Brinckman, F. E.;Bhir, W. R.; Jewett. D. L.; Iverson, W. F', J. Chromtcgr. Sci. 1977, 15, 495-503. (3) Fernandez, F. J. A f . Absorpt. News/. 1977, 16, 33-37, and references therein. (4) "Heathkit Microprocessor Trainer Manual", Part 2, Sections 7, and 8; Heath Company: Benton Harbor, Mich., 1977. (5) Snyder! L. R.; Kirkland, J. J. "Modern Liquid Chromatography"; Wiley Interscience; New York, 1974; p 34.
RECEIVED for review March 13,1979 Accepted May 14, 1979. We thank the Robert A Welch Foundation For financial support of this work (Grant No. A-694), and the Texas A&M College of Science for the organized Research Funds (1977) to purchase the liquid chromatograph.
Automated Vapor Pressure Osmometer for Determining the Molecular Weight of Polymers Mark E. Myers, Jr.,' Stephen J. Swarln,"' and Byron L. Nellis' General Motors Research Laboratories, Warren, Michigan 48090
T h e vapor pressure osmometer (VPO) instrument is us_ed for determining the number average molecular weight (M,) 'Analytical Chemistry Department. Instrumentation Department. 0003-2700/79/0351-1883$01 O O i O
of organic and inorganic molecules in the molecular weight range of 50 to 20000 ( I , 2). In our experience it is used pJimarily for determinations on synthesized polymers having M , = 1000 to 10000. Such values of M , are too low to be determined by membrane osmometry because the solute IE 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1 1 , SEPTEMBER 1979
vapor Pressure
Osmometer
Ooam Hydraulic
-Mechanical
- - - - Electrical
Figure 1. Block diagram of the automated vapor pressure osmometer Start
7;-; Cbf for 45 s
Repeal
Walt 4 m
6 Times
t R Y L
Stop
E On f o r 4 m
investigation, the VPO will usually give a more accurate result. T h e detailed operation of t h e VPO has been described previously ( 3 , 4 ) . Briefly it involves the measurement of the temperature rise caused by the condensation of solvent vapor onto a droplet of polymer solution in a temperature-controlled cell. A complete determination requires six repetitive measurements on the solvent to establish the instrument zero, followed by six measurements on t h e sample solution and, finally, six measurements on t h e solvent to recheck the zero. Four-minute waiting periods are required between readings so that thermal equilibrium can he established within the cell. A whole determination requires 1.5 h to complete and requires the constant attention of an operator to dispense the solvent and polymer solution, time the equilibrium period, and record the readings. I n this report we briefly describe t h e design of auxiliary devices which automate the operation of a \'PO so that the operator, after preparing the sample and calibration solutions to known concentration, needs only t o load the solutions into the instrument. It will then run for 1.5 h unattended and perform the measurements indicated above. T h e automated instrument has been used in our laboratory for the past three years for the routine determination of the M , of a wide variety of polymer samples in two solvents (2-butanone and ethylene dichloride), with no instrument breakdowns. Detailed descriptions of t h e auxiliary devices needed to automate t h e operation of t h e instrument are available upon request.
EXPERIMENTAL Repeat
E i C Off
6 Times
Wall 4
-
A On for 8 rr
11
A = W a s h Syringe E = Sample Syringe C = Reference S y r , n g e
A Off
Figure 2. Programmer sequence for driving the syringe motors of the automated vapor pressure osmometer
molecules permeate t h e membrane and invalidate the measurement. Gel permeation chromatography (GPC) can be used to obtain an estimate of M,; however, in the absence of a GPC calibration curve for the particular material under
The basic VPO instrument used in this study was an Hitachi Perkin-Elmer Model 115. Instruments utilizing the principle of operation of this instrument are currently available from Wescan Instruments, Santa Clara, Calif. 95050. The incorporation of the automation can be conveniently divided into four areas: (1)sample injection system, (2) servo system, (3)analog to digital readout system, and (4) logic system or programmer. These are shown schematically in Figure 1 and discussed below. Sample Injection System. In all VPO instruments the sample and solvent solutions are injected by turning thumb screws on drive rods which move the plungers of syringes. The automation of this system was accomplished by providing electric motor drives for these syringes. This system was designed so that the syringes could he easily removed for cleaning and filling; and the 1/3-rpm synchronous motor drives were controlled by the logic system. Servo System. The Wheatstone bridge of the VPO is balanced by manually adjusting a precision rheostat on the front panel of the instrument for a null as indicated by a meter on the front panel. Automation of this system was"accomp1ished by providing a servo system for automatic and continuous null of the bridge by the balance rheostat. The servo system also produces an analog
A&C c on T-1
Reset S-2
-n T-2
DC-1
NO
General Pulse
I
Off
T-5
I
0
on
T-3
No Timing Loop X2
Figure 3. Flow diagram of programmer. Notes: (1) T-1 and T-4 are each nominally 45 s and are variable +5 s. ( 2 ) T-2, T-3, T-5 are each nominally 4 min and are variable +15 s. (3) T-6 is nominally 8 min and is variable +15 s. (4) N-1 and N-2 are thumbwheel switches and vary the loop recirculation between 1 and 8 times, (5) DC-1, DC-2, and DC-3 are decade counters (Texas Instruments SN74192N). (6) MC-1 and MC-2
are magnitude comparators (Texas Instruments SN7485N).
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11. SEPTEMBER 1979
voltage output proportional to the position of the rheostat at null. Analog to Digital Readout System. This system converts the analog voltage produced by the servo system to digital form and prints it out on a teletype. Although we used rather sophisticated instrumentation which was available in our laboratory for this function, any simple analog to digital converter capable of accepting a remote "read' command from the logic system (see next section) should be suitable (Le., digital voltmeter). It should also he noted that current models of the VPO instrument incorporate digital readout and can be connected to a digital printer (5),so the servo system and analog to digital readout system would not be required. Logic System (Programmer). The logic system or programmer tells the various syringes when to run and for how long, times the four-minute thermal equilibrium period, and tells the readout system when to print a reading. In short, it is the heart of the automated VPO since it duplicates the functions of the human operator. The programming of the three syringe drive motors through the logic sequence is shown in Figure 2, while Figure 3 depicts the complete flow diagram of the programmer. The programmer was built using integrated circuit timing chips, decade counters, magnitude comparators, and other components so that it could accomplish its tasks reliably, repeatably, and with sufficient versatility to accommodate varying demands. During a standard VPO run, this programmer performs 56 control functions over a period of 1h, 37 min, and 30 s without operator attention. Light emitting diodes in conjunction with a simplified flow diagram on the controller front panel give the operator a visual indication of the status of the test sequence. A detailed schematic of the programmer is available from the authors upon request.
RESULTS A N D D I S C U S S I O N A test of the overall performance of t h e servo and analog to digital readout system was performed by manually running the servo motor to produce various fixed positions of the null
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rheostat. T h e digital output of t h e system at each fixed position (as recorded on the teletype) was plotted against the readings from the null rheostat dial. This plot was linear (correlation coefficient = 1.000). T h e precision and accuracy of the automated instrument, after three years of experience, are considered to be as good as or better than t h a t obtained with manual operation. For example, four determinations on Octoil "S" vacuum pump fluid ( M , = 426.7) gave M , = 425.3 i 6,?(L7). A synthesized polycarbonate was determined t o have M , = 1.400by nuclear qagnetic resonance end group analysis; t h e VPO result was M , = 1450. Of course the automated VPO has the particular advantage of allowing the operator t o do other work in the laboratory. ACKNOWLEDGMENT T h e authors thank R. W. Lietz, R. G. Chimelak, D. R. Smith, W. A. Florance, and W. R. Lee for their valuable contributions to the various aspects of this study. L I T E R A T U R E C,ITED (1) R. E. Dohner, A. H. Wachter, and W. Simon, Helv. Chim. Acta, 50, 2193
(1967). (2) A. H. Wachter and W . Simon, Anal. Chern., 41, 90 (1969). (3) M. E. Myers, Jr., "A ProposedTest Method for Determining Molecular Weight by Vapor Phase Osmometry", presented at P ~ b u r g Conference h on Anaiytical ChemWy and Applied Spectroscopy, Clevehnd, Ohio, March 1, 1971. (4) "1978 Annual Book of ASTM Standards", ASTM Method D3592-77, Standard Recommended Practice for Determining Molecular Weight by Vapor Pressure Osmometry, American Society for Testing and Materials, Philadelphia, Pa., 1978. (5) Wescan Instruments, Santa Clara, Calif. 95050, personal communication. Feb. 6, 1978.
RECEIVED for review February 12, 1979. Accepted May 18, 1979.
Sample Preparation for High-Performance Liquid Chromatography of Higher Plant Pigments Kenneth Eskins" and H. J. Dutton Northern Regional Research Center, Agricultural Research, Science and Education Administration, Illinois 6 1604
Preparation of samples has not kept pace with recent advances in hardware and column technology of high-performance liquid chromatography (HPLC). Grinding, filtration, extraction, phase separation, and evaporative drying have been time-consuming steps in analysis of chlorophylls and carotenoids (1). T h e method of chloroplast pigment analysis used previously in our laboratory ( 2 ) involves three separate washings of an ether solution of pigments with a 10% aqueous sodium chloride. h'ow, by exploiting recently available commercial products, a 2-h sample preparation has been reduced to 20 min or less, and flavonoid pigments which were previously discarded in the wash are retained for analysis. Novel equipment required is a cartridge filled with Cl8Bondapak, which is used to selectively absorb the chloroplast pigments, and a millipore filter to remove cell wall fragments. EXPERIMENTAL The procedure requires a glass tissue homogenizer, a z7 cork borer, a 10-mL glass syringe with Luer-lock, a three-way Hamilton valve with male and female Luer-lock, a Millipore Swinnex-26 filter with type LS 6.O-Fm filters, and Waters Associates Sep-Pak
U.S. Department of Agriculture, Peoria,
I
\
C
D A
E
Figure 1. Assembled apparatus for preparation of chloroplast pigments. (A) Glass tissue homogenizer; (13)glass syringe; (C) three-way valve; (D) millipore filter (5 km); and (E) Sep-Pak e,,-Bondapak
cartridges (reverse phase, C18-BondapakI. Assembled apparatus is shown in Figure 1. The sample is obtained by taking small
This article not subject to U S . Copyright. Published 1979 by the American Chemical Society
