Simplified Recording Photometer for Flame Analysis Robert H. Dreisbach, Department of Pharmacology, Stanford University School of Medicine, Stanford, Calif.
T
and speed of direct flame photometry are considerably enhanced by the use of a recorder [Solomon, A. K., Caton, D. C., h . 4 ~ CHEW2'7, 1849 (1955)], especially if the recording system does not contribute short-term instability. A recording photometer is also useful for other purposes and for many of these, long-term stability is an important requirement. Sufficient sensitivity and stability for most photometry can be obtained by connecting the output of a high-gain photomultiplier tube directly t o the input of a chopper-amplifier potentiometric recorder which allows high source impedance (Varian Model G-10, 100 mv., 0.5-megohm input). Response to light is linear and sensitivity to light can easily be varied more than a millionfold by changing load resistor and dynode potential. -4stable voltage supply is the only important requirement, because a change in photomultiplier dynode potential of lY0 results in about 7% change in output. HE ACCURACY
.
DESCRIPTION OF APPARATUS
The most satisfactory photomultiplier tubes tried have been the RCA 1P21 and the Dumont 6911, neither of which was especially selected. The lP21 is a 9-stage, side-window, photomultiplier tube with an ultraviolet transmitting envelope and is usable from 200 to 700 mp, with maximum sensitivity a t 400 mp. The 6911 is a 10-stage, end-window, photomultiplier tube with maximum sensitivity a t 750 mp (approximately a t the brightest line
of the K flame spectrum) and has usable sensitivity to 1200 mp. The infrared sensitivity of the 6911 makes the dark current high and sensitive to changes in temperature. The high dark current can be greatly reduced by cooling the tube with dry ice, although this was not done in the present application. Figure 1 shows a circuit diagram of the photomultiplier and control circuit for the 1P21 which has been built into a Beckman D U phototube box. A separate box with an extension was used for the Dumont 6911 tube. Extreme care must be used to prevent stray light from entering the box. Since the recorder has an adjustable 1-megohm damping resistor, no additional damping has been provided in the control circuit. At a load resistance of 2.0 megohms, the full scale response time of the recorder is 4.5 seconds with the recorder damping resistor a t zero. The response time is 4.0 seconds a t a load resistance of 0.5 megohm with zero recorder damping. The dynode voltage supply consists essentially of a transformer, half-wave rectifier, filter condensers. current-limiting resistors, and a number of 85A2 voltage regulator tubes in series across the output sufficient to supply the maximum required voltage. By using a selector switch to connect to any 85A2 tube, the voltage can be varied in 85 volt steps, each step changing the sensitivity approximately 100%. Thus coarse sensitivity control consists of either change in dynode voltage or change in load resistor. Fine sensitivity adjustments are made by changing slit width on the Beckman DU. Flame spectra are analyzed by driving the wave length dial of a Beckman DU directly from the recorder using bevel
gears. For quantitative flame analysis, the recorder is placed in any convenient position, so that sample identification can easily be included in the record. During a series of determinations only the position of the recorder pen needs to be adjusted, and this adjustment is made by means of the zero control on the recorder. The variable resistor in the control circuit (Figure 1) is used initially to adjust the input voltage within the range of the recorder and thereafter the recorder zero control is utilized for pen position adjustments. Since the recorder zero control will shift the pen slightly more than full scale, signals covering a range of 200 mv. can be brought within the 100-niv. recording range. RESULTS
As a n example of application to flame photometry, the recorder can be driven full scale (100 mv.) by the flame emission in acetylene-oxygen from the following concentrations of metals: 40 peq. per liter calcium (422.7 n i w . 0.03-mm. slit, 1P21 tube, dynode voltage 935, load resistor 0.5 megohm) ; 10 peq. per liter potassium (766.5 nip, 0.15-mm. slit, 6911 tube, dynode voltage 1275, load resistor 1.0 megohm); 1 peq. per liter sodium (589.3 mp. 0.01-nim. slit, I P21 tube, dynode voltage T 6 S j load resistor 0.5 megohm). After adjustment of the instrument to give full scale reading (100 mv.) with the above concentration of one metal. spectral interference a t that wave length from concentrations of the other metals 20 tiniea as great was negligible. The standard deviation of sets of ten replicate determinations a t
100
< 30 Sec >
ao e 0
60 SI
I
I
I
0
0
>
f
"
'
y
All
Resistances
In Megohms
Photocathode
IP21
Figure 1.
n
c 0
Circuit diagram for 1 P2 1
SI - 8,. 2-circuit 5-position switch with load rePistances
of 0.08, 0.25, 0.5, 1.0, 2.0 megohms Components all yatt, tolerance 10% 691 1 requires an additional dynode resistor Variable resistor is for flame background or dark current suppressio11
Figure 2. Flame emission from calcium solutions F shows flame background without solution aspiration except hetween arrows--80 peq./l. K a v e length 422.7 mp Slit 0.02 mm. Dynode voltage -835 Photomultiplier tube 1P21 Load resistor 0.5 megohm. Beckman 4030 burner Recorder speed 1 inch/30 sec.
.-.->
I40
20
a Colcium
pEq / L
VOL. 31, NO. 3, MARCH 1959
479
the concentrations indicated (in peq. per liter) were: potassium 10.0 =!z 0.13; 1.0 i. 0.085; calcium 40.0 f 0.42; 4.0 i 0.42; sodium 1.0 + 0.006; 0.1 + 0.004. Thus accuracy was comparable to that reported for a considerably more complex instrument [Solomon, A. K., Caton, D . C., ANALCHEW 27, 1849 (1955)l. Detection limits calculated for sodium (0.0001 p,p.m.) and calcium (0.008 p.p.m.) were approximately one tenth of those reported for another recording flame photometer, while for K (0.0034 p.p.m.) the detection limit was that reported (Khisman, M., Eccleston, B. H., ANAL.CHEK 27, 1861 (1955)]. Figure 2 demonstrates the linearity obtainable with the flame emission from calcium solutions of 20 to 100 peq. per
liter. Flame instability appears to be the principal factor limiting accuracy, since the recording system using the 1P21 is drift-free and random fluctuations at the sensitivities used for flame analysis are less than ==I 1%. During routine flame analyses, sample aspiration time is less than 15 seconds, and can be kept to half this time without sacrificing accuracy. The recorder also reduces the tedium of bracketing each unknown with standard solutions when greatest accuracy is desired. Long-term stability was determined by using a Beckman hydrogen lamp with Model B power supply as light source. Drift and random variations a t maximum sensitivity (dynode voltage 1275, load resistor 2.0 megohms) did not exceed & 37, over a period of 8 hours. At
this sensitivity, full scale reading on the recorder (100 mv.) could be obtained with the light passing through a quartz cell filled with distilled water from the hydrogen lamp and Beckman D U at wave length 200 inw and slit 0.35 mni. KOestimate of the amount of stray light a t this tT-ave length was made. ACKNOWLEDGMENT
The development of this photometer was supported in part by the Institutional Grant of the American Cancer Society. The author is indebted to J. C. Lawrence for assistance in che construction of the apparatus. The apparatus is available from J. C. Lawrence, 733 Florales Drive, Palo *41to.Calif.
Gas-Solid Chromatographic Analysis of Fractions from Air Rectification Columns S. A. Greene,' Aerojet-General Corp., Azusa, Calif. are poorly separated from hydrogen. Elution with hydrogen, using charcoal as the adsorbent at liquid nitrogen temperature, permits separation of helium and neon, but nitrogen again is not eluted. However, nitrogen may be eluted by lowering the Dewar, containing liquid nitrogen, from the column and allowing the column to be warm (Table 11). This must be done cautiously, because hydrogen is rapidly desorbed. With this technique, helium and neon may be quantitatively determined; by raising the column temperature, nitrogen can be determined and hydrogen obtained by differences. Separation of the argon fraction is complicated, because argon and oxygen could not be separated on any of the adsorbents a t room temperature in the apparatus described. Lowering the temperature to that of dry ice did not improve the separations. Ry utilizing argon as the carrier and a Molecular Sieve column at room temperature, oxygen and nitrogen can be determined [Kyryacos, G., Board, C. E., - 4 ~ . 4 ~ . CHEM.29, 787(1957)] and argon is determined by difference.
rare gases are obtained from side T streams which are bled from various sections of air rectification columns HE
(Table I). This paper illustrates how these gas mixtures might be monitored and analyzed by gas-adsorption chromatography. The apparatus has been described [Green, S. A,, Moberg, M. L., Wilson, E. hf., ANAL. CHEhI. 28, 1369 (1956)]. The adsorption columns, 10 feet, 0.25 inches in outside diameter, were filled with 20- to 40-mesh activated adsorbents, and wound into 3-inch coils. Adsorbents were Columbia activated carbon, Davison silica gel, and Linde molecular Sieve 5A. Carrier-gas flow rates were BO ml. per minute. RESULTS AND DISCUSSION
The neon fraction could not be successfully separated at room temperature with this equipment and these conditions. If separation is attempted at 100' K, with argon as the carrier, nitrogen cannot be eluted from the column; helium and neon are eluted together, and 1 Present address, Rocketdyne, Canoga Park, Calif.
Table I.
Gas He H*
Composition of Various Streams from Columns
Neon Fraction 18.6 3.0 44.0 34.4
...
Kr Xe
CHI
480
0
..
.. .. ..
ANALYTICAL CHEMISTRY
Argon Fraction Before After rectification rectification
... ...
.
.
I
... ...
Trice
0.2
84
3.8 96.0
16
Krypton Fraction
... ... ...
...
98.0
Trace 1.0
0.1 1.0
The krypton fraction is separated by either of two adsorbents, depending on the amounts of carbonaceous gas, chiefly methane, present in the stream. On silica gel, krypton and methane could not be separated. When methane is not present (argon and oxygen are not separated), eluting with oxygen on a silica gel column, a t room temperature, ~ i lpermit l separation of argon, krypton, and xenon, and oxygen is determined by difference. When methane is present, it is possible to separate all components by utilizing a lIolecular Sieve 5A column and eluting with oxygen. d t room temperature, argon, krypton, and methane are separated. After methane is eluted, the column is immersed in water a t 90' to 100' C. and xenon is eluted 6 minutes after immersion. Tbis technique does not adversely affect the wave form of xenon, which remains symmetrical. Table II. Retention Volumes of Gases with Various 1 0-Foot Columns and Carrier Gases (60 ml./min. carrier flow, lo-foot columns)
Gas He Ne NP A 0 2
Kr CH, Xe
Retention Volume, Cc. A B C D 180 ... ... ... 360 ... ... ... ... 450 ... ... ... ... 150 160 ... 210 ... ... ... , , . 240 540 ... ... ... 660
. , , 840 960 ... Charcoal column a t 77" K, hydrogen carrier. B. Molecular sieve column a t 23' C., argon carrier. C. Silica gel column at 23" C., oxygen carrier. D. Molecular Sieve column 23-100' C., oxygen carrier. A.
