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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
For automatic use, the control of the relay is done remotely, and a control voltage is introduced through SI. The data a t the output is attenuated to 1 mV for recording purposes; this can be changed to suit. T h e potentiostat was used under various conditions and showed exemplary behavior a t small amplitude around zero. For larger transitions, the clamp could cope easily will 1-V steps on 5-pA scale. An example of transient behavior, in the absence of clamping, is shown in Figure 3b where the response of the system is shown a t the highest current range. T h e cell solution used was 0.01 M K2S04 and 0.01 M H2S04and the working electrode was of platinum wire. At 1.2-V square wave produces about 10 ps of rise time with moderate overshoot and no instability. Large transitions a t low current setting are accommodated by clamping. T h e relay used allows about 1 ms of reliable operation which is satisfactory for most applications. In conclusion, the present design, a t low signal amplitude and with small current measuring resistors is able to operate within about 3% dynamic error to about 30 kHz sine and 50-ps rise times. Useful response extends to 60 kHz sine waves and 10-ps rise times. Low frequency operation is error free. When using large voltage steps in dilute solutions and a high current measuring resistor, the effective rise time is maintained to about a millisecond by clamping. The speed of the clamping relay (0.2 ms) is here the limiting factor. F E T switching was experimented with and shown to give faster response, but we prefer t h e relay for reasons of simplicity and reliability.
T h e authors have more detailed construction details available for persons interested in duplicating the construction, as well as some details about preliminary work to program completely automatically the gain of the I / E section.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)
(IO) (1 1) (12) (13) (14) (15) (16) (17)
F. Haber, Z . Phys. Chem., 32, 193 (1900). G. L. Booman and W. B. Holbrock, Anal. Chem., 37, 795 (1965). R . R. Schoeder and I. Shain, Chem. Instrum., 1, 233 (1969). J. E. Mumby and S. P. Pernne, Chem. Instrum., 3, 191 (1971). C. Lamy and C. C. Herrmann, J . Electroanal. Chem., 59, 113 (1975). C. Yanitzki and Y. Friedman, Anal. Chem., 47, 880 (1975). K . S. Stuiick and V. Hora, J . Electroanal. Chem., 70, 253 (1976). J. P. Van Dieren, B. G. W. Kaars, J. M. Los, and B. J. C. Wetsema, J . Electroanal. Chem., 68, 129 (1976). J. Deroo, J. P. Dard, J. Guitton, and B. Le Gorrec, J. Electroanal. Chem., 67, 263 (1976). M. T. Kelley, H. C. Jones, and D. J. Fisher, Anal. Chem., 32, 1263 (1960). See ref. 4 . J. L. Anderson, Chern. Instrum., 7, 25 (1976). K. B. Oidham, J . Electroanal. Chern., 11, 171 (1966). D. K . Roe, Chem. Instrum., 4, 15 (1972). J. E. Davis and E. C. Toren. Anal. Chem.. 46, 647 11974) B. H Vassos and G. W. Ewing, "Analog and Digital Electronics for Scientists", J. Wiley, New York, N.Y., 1972, p 163. T. S. Randhawa and R L. Snthwell. Analyst(London), 100, 726 (1975).
RECEIVED for review September 7, 1977. Accepted December 7 , 1977. Acknowledgement is made to the Office of Coordination of Research and to the Center for Energy and Environmental Research of the University of Puerto Rico for financial support.
Technique for the Prevention of Column Contamination in Pyrolysis Gas-Liquid Chromatography Annabel Mitchell and Manuel Needleman" Victorian College of Pharmacy, 38 1 Royal Parade, Parkville, Victoria, Australia,
There has been a n increasing interest in the past decade in the application of pyrolysis gas-liquid chromatography to the identification of microorganisms. It is surprising, therefore, to observe that the problem of column contamination has been mentioned only recently ( I , 2). T h e contamination, which evidences itself as a tarry deposit on the column after approximately 100 h of column use: results in a loss of resolution, with a concomitant decrease in long-term reproducibility (2), and is most marked when capillary columns are used. Attempts have been made to solve the problem by repacking (3)or removing ( I ) the first section of the column. Alternative proposals have been to use long precolumns ( I ) and backflushing of the column after each chromatographic run (2). Experiments using t h e latter idea showed that high boiling contaminants still built u p insidiously on the column, and a significant fraction, once deposited, was resistant to backflushing. Therefore, the final successful arrangement, shown in Figure 1,incorporated a short disposable precolumn and effectively amalgamated t h e above-mentioned proposals.
EXPERIMENTAL All apparatus and procedures, except for those detailed below, have been previously described (2). The glass precolumn shown in Figure 1was obtained by cutting a 1-m length from a column similar to the main SCOT column. The columns were connected with glass-lined steel tubing, which was also used for the ancillary plumbing. The diagram shows how closing the valve ( M N W needle valve, Scientific Glass Engineering, Melbourne) changes the gas flow 0003-2700/78/0350-0668$01.OO/O
3052 IO cmymi:
DETECTOR
INLET
VALVE 5cm3/mi; INLET
GAS FLOW DURING ELL'TIOS
L
5 c m min
INLET
INLET
Figure 1.
I
t
VALVE CLOSED
GAS FLOW DURING BACKFLUSHING
Backflush plumbing arrangement
pattern from the elution to the backflush mode. The sequence adopted was to raise the temperature to 180 "C, after elution was complete, then to remove the pyroprobe from inlet A, leaving it open, and to backflush for 1 h.
RESULTS AND DISCUSSION This technique has been in operation for over 500 h of column use and no contamination of the main column has been observed. After about 200 h of use, the first 5 cm of precolumn showed some discoloration but, because of the disposable nature of the precolumn, this has not proved t o 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
be a serious problem. The efficacy of the backflush procedure has been shown by the significant deposition of contaminants, similar t o those previously observed, on the glass injection sleeve, which is replaced each time the pyroprobe is removed prior to backflushing. Our experience has shown the advantages of using a precolumn, together with backflushing, as the main column has been kept free of contamination and the replacement frequency of the precolumn has been minimized, thus avoiding t h e regular replumbing involved in a precolumn system that does not incorporate backflushing. T h e relatively low cost
of the valve, allied to t h e advantages outlined above, makes this system an attractive alternative to an automatic backflush arrangement.
LITERATURE CITED (1) P. A. Quinn, J . Chfomatogr. Sci., 12, 796 (1974). (2) M. Needleman and P. Stuchbery, “AnalyticalPyrolysis”,C. E. R. Jones and C. A. Cramers, Ed., Elsevier, Amsterdam, 1976, p 77. (3) M. V. Stack. H. D. Donoghue, J. E. Tyler, and M. Marshall, ref. 2, p 57.
RECEIVEDfor review September 19, 1977. Accepted December 19, 1977.
Gas Chromatographic Determination of Dissolved Hydrogen and Oxygen in Photolysis of Water Steven J. Valenty General Electric Corporate Research and Development, Schenectady, New York 1230 1
T h e use of visible light energy to decompose water into hydrogen and oxygen is a subject of current interest (1-3). T h e principal analytical tool used to detect the small amounts of gaseous products produced in these studies has been mass spectrometry. I n a n attempt t o observe trace quantities of H2and O2 which might be evolved during the photolysis of glass supported monolayer assemblies of surfactant derivatives of tris(2,2’-bipyridine)ruthenium(II)( 2 ) , a simple chromatographic method has been utilized and is reported here. Determination of traces of H2, N2,and O2in aqueous solutions by vapor phase chromatography has been reviewed by Tolk a n d co-workers ( 4 ) . In t h e present method, the aqueous solution is injected directly onto a chromatographic column filled with molecular sieves and the eluting H2 and O2 monitored by a standard thermal conductivity detector. The validity of the hydrogen analysis was tested by using the acidic ferrocyanide/isopropanol system as a reference actinometer (5, 6).
EXPERIMENTAL Materials. Linde purified He and Ar were used as carrier gases. The argon was passed through a 20-in. X 0.375-in.stainless steel tube filled with activated “Ridox” (Fisher Scientific) to minimize the oxygen concentration. Molecular sieve 5A (60/80 mesh) was obtained from Supelco. Potassium ferrocyanide (Baker Analyzed), isopropanol (MCB Spectro), and perchloric acid (Baker Analyzed) were used for the hydrogen actinometry experiments as received with triply distilled water as solvent. Apparatus. A Hewlett-Packard F & M Model 5750 Chromatograph equipped with a thermal conductivity detector and 0.125-in. sleeves in the injector block was utilized. The column (stainless steel, 0.125-in. o.d., 0.010-in. wall) is composed of two shorter columns coupled in series; both filled with 60/80 mesh molecular sieve 5A. The “water adsorbing” precolumn (0.5-m length) is attached to the injector followed by the longer (2.1 m) “gas separating” column. The column was activated by heating in a He stream at 300 “C for 4 h before use. Conditions for the gas analyses are given in Table I. The aqueous solutions are sampled by a 100 p L “Pressure-Lok” series A-2 syringe which has a valve behind the needle such that samples can be transferred (and stored) in the syringe barrel without gas loss. A 4-W germicidal lamp (General Electric G4T4-1, output >210 nm) in a convection cooled housing was attached to a thermostated brass photolysis cell holder such that the plane of the “U” shaped lamp was parallel to and 5 cm distant from the cell’s front window. The light intensity was attenuated with fine copper mesh screen. Of the light incident on the cell face, 88% had a wavelength of 253.7 nm with the remainder not photochemically active. 0003-2700/78/0350-0669$0 1.OO/O
Table I. Experimental Conditions Condition 0, analysis Carrier gas He Carrier gas flowd mL/min 60 Injector temp., C 100 Column temp., “ C 60 Detector temp., C 100 TC filament current, mA 240 TC attenuator setting 1x
H, analysis Ar 40 100 60 100 150
1x
Table 11. Observed Analytical Data for Dissolved Gas Detection 0bserva tion Instrument calibration factor, mol/mm peak height Minimum amount detectable, mol, S / N = 4 Maximum injection volume,
H2
0
8.4 X 10.”
1.3 X
1.7 X 10.’’
2.5
50
2
X
100
P L
Minimum concentration de3.4 X 2.5 X 10.’ tectable, mol, S / N = 4 *lo ? 10 Precision, % 2-100 2- 100 Linearity, % gas saturation 0.9 ? 0 . 1 1.1 f 0.1 Retention time, mina a See Table I for carrier gas and flow rate used. The photolysis cell, constructed of either Teflon or A1 alloy, is demountable and is assembled from four side pieces and two quartz windows using Teflon tape as gasket material around the perimeters of the windows. For these experiments, the light path through the cell is either 0.5 cm (ferrioxalate actinometry) or 0.1 cm (hydrogen actinometry) and the cell’s contents accessed through a septum seal in the top. The cell front window was masked to provide a 2.3 cm X 2.6 cm opening. A Perkin-Elmer Model 575 was used to record all ultraviolet and visible absorption spectra. Procedure. Ferrioxalate actinometry was used for the light intensity determination at 254 nm (7). The light intensities used einstein/s/cm2 (bare lamp in this study are 5.0 f 0.1 X einstein/s/cm2 (Cu mesh attenuated output) and 4.5 f 0.1 X output). Routine calibration was done by injecting known volumes of a H2 or O2 saturated aqueous solution (25 f 1 “C, under 1 atm of pure gas) into the chromatograph. In another method, standard solutions containing less than saturation concentrations of Hz were prepared by 254 nm photolysis ( I = 0.1 cm, 25 “C) of a Nz purged aqueous solution (1.0 mL) containing: 1.0 X M K,[Fe(CN),], G 1978 American Chemical Society
