(4) A. von Zelewsky and 0. Haas, Proceedings of the XVI ICCC, Dublin,

(5) R. R. Ernst, Adv. Magn. Reson., 2, l(1966).

RECEIVED for review June 9, 1975. Accepted September 22, 1975. This work was supported by the Swiss National Foundation for Scientific Research.

1974, 2.25b. LITERATURE CITED

(1) B. Mohos, M. Zobrlst, and A. von Zelewsky, J. Chem. Phys.. 60, 4633

(2) A. Savltzky and M. J. E. Golay, Anal. Chem., 36, 1627 (1964). (3) F. Steiner, Y. Termonla. and F. Dekour, Anal. Chem., 44, 1909 (1972).

(1974).

Simple Cooled Photomultiplier Housing

S. D. Hoyt and J. D. Ingle, Jr.’

Department of Chemistry, Oregon State University, Corvallis, Ore. 9733 1

Cooled photomultiplier housings are used to reduce the dark current and the dark current noise caused by therm- ionic emission. For small light levels, the reduction in dark current noise will improve the signal-to-noise ratio (S/N) if dark current noise is significant. Minimization of dark cur- rent noise is especially critical for tubes that have extended response in the infrared (e.g., S-1, S-20) since they are more susceptible to thermionic emission. Cooled housings can also be used effectively in situations where changes in the temperature of the photomultiplier tube (PMT) environ- ment can cause drift or fluctuation in the photocurrent or dark current.

Only a few basic designs for cooled PMT housings have been used (1-24). The simplest approach (2,22) is to use a two-compartment housing in which the outer compartment is filled with liquid nitrogen or other coolant (e.g., dry ice- acetone). The PMT is placed in the inner compartment and is in thermal contact with the outer compartment ei- ther directly or through air. Cooling may also be achieved by electrical refrigeration (e.g., thermoelectric or Carnot cycle). Finally, a cooled gas may be passed directly over the PMT (1, 2, 4, 7, 10, 12-17). Temperature stability is better (5, 16, 17) when the gas is precooled in a coolant bath rath- er than generated by heating liquid nitrogen.

Commercial cooled PMT housings usually cost between $500 and $1000 and housings described in the literature are often relatively complex to build. Most commercial and home-built cooled housings are designed for use with end- on tubes and are relatively large. Because of the bulkiness of some housings, the distance between the photocathode and the outer wall or viewing window of the housing is large.

In this article, we describe the construction and perfor- mance of a cooled PMT housing which is specifically de- signed for use with common side-on PMT’s and for high light collection efficiency (photocathode close to the hous- ing viewing window). Other important characteristics of the housing include continuous temperature control and temperature measurement between 20 and -60 “C, a shut- ter mechanism, quartz windows for UV response, simplicity of construction, simple replacement of the PMT, and small size. The principle of operation is taken from a paper by Benci et al. (3) in which nitrogen gas cooled by liquid nitro- gen is used to bathe the PMT.

CONSTRUCTION Figures 1, 2, and 3 show the details of construction. Fig-

ure 1 is a side view of a cut down the middle of the housing while Figure 2 is an engineering top view. The scale for both drawings is included so that approximate measure- ments can be taken from the drawings.

The housing consists of five basic parts: a PVC cylinder ( I ) , a spacer-window assembly (16), a shutter assembly

I n

INSTRUMENT I

I 9 I

Figure 1. Side view of housing

(22), a base-socket assembly (9), and an insulation box (8). The PVC cylinder and shutter assembly are connected to- gether with the spacer-window assembly and Allen screws (1-72). Two mounting plates (6, 7) are epoxied to the bot- tom of the PVC cylinder. The top mounting plate (6) is the bottom of the insulation box (8). A h i d e d cover (8) made of 0.040-inch aluminum is attached to this mounting plate and the shutter assembly as shown in Figure 3. The space between the PVC cylinder and outer box is filled with insu- lation (e.g., paper packing material). The bottom mounting plate (7) fits flush against the bottom of the PVC housing. The base-socket assembly (9) is attached to this bottom plate (7) with two 8-32 Allen head screws and an “0” ring seal (10) to keep the coolant gas from leaking out and to allow easy changing of tubes.

The PVC acts as a light tight housing and also provides insulation. Holes for the gas inlet tube (2), gas exit tube (3), and temperature transducer (4) are drilled in the locations indicated and then slots (5) for the spacer-window assem- bly are milled in the PVC housing. The gas tubes (l/d-inch 0.d. copper tubing) are epoxied into the holes and routed as shown in Figure 3a. The temperature transducer holder (15) is made from an IC socket pressed into a %-inch Tef- lon sleeve which can be inserted or removed from the hous- ing.

The base-socket assembly (9) should be built to allow for the large variation in the size of PMT bases. The bottom cap (11) seals the base from room light and is removable so the dynode chain can be reached. A high voltage jack (13) and BNC jack (14) connect the dynode chain to the high voltage supply and the anode to the photocurrent amplifi- er, respectively.

232 ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976

Table I. Dark Current and Dark Current Noise at Different Temperatures for Different Photomultiplier Tubes N, fiowrate, (I./min) 0.0 1.6 2.4 3.2 PMT temperature,'^ 22 -20 -42 -61

PMT voltage, Dark RMS Dark RMS Dark RMS Dark RMS

Tube V current, A noise, A current, A noise, A current, A noise, A current, A noise, A

.Figure 2. Top view of housing

After the dynode resistors are connected to the PMT socket (mica filled Bakelite, Amphenol NO 78511T), the socket (12) is screwed to the aluminum base assembly (9) and wired to the two jacks; lOOK 1% precision dynode resis- tors were used in the standard configuration recommended by the manufacturer (25). The whole base-socket assembly should he washed with detergent, rinsed with distilled water, and dried in an oven to remove oil that would in- crease the leakage current and noise. The pins of the PMT should he cleaned the same way and care taken not to touch them.

The spacer-window assembly consists of a spacer (161, two quartz windows (17). two 0 rings (18), and copper tubes (Yd-inch 0.d.) which are epoxied into holes drilled in the spacer.

The shutter (21) is cut from an aluminum plate and the shutter assembly (22) is milled out of an aluminum block. To provide a sliding surface and to prevent light leads, one felt pad (23) is glued to the face of the shutter assembly while another felt pad (24) is glued to the face of the instru- ment to which the shutter assemhly is attached.

OPERATION After the unit is assembled and attached to the desired

spectrometer, appropriate high voltage supply, and current measurement circuitry, i t should he checked for light leaks with a pen light. Any light leaks can be conveniently sealed by using a synthetic black rubber compound available in tubes a t a hardware store.

The gas flow system is similar to that described by Benci e t al. (3) where the temperature of the housing is controlled by the flow rate of the nitrogen gas. Room temperature ni- trogen from a gas cylinder and regulator passes through a flow meter and then to tube (19) so that the gas flows be- tween the quartz windows to prevent any condensation. From tube (201, the gas flows through a copper coil im- mersed in a Dewar filled with liquid nitrogen (or other coolant) on to the main housing through tube (2) where it

Figure 3. Photographs of housing (a) Assembled and connected to chemiluminascence photometer. (b) Disas- sembled

circulates around the PMT and exits through tube (3). The correlation between the flow rates and the PMT tempera- tures are shown in the Table I.

The temperature of the PMT is monitored with a Na- tional Semiconductor temperature transducer (LX5600) which is a complete temperature measuring unit in a pack- age the size of a transistor. The transducer was wired up according to the manufacturer's specification sheet, and the output voltage, directly proportional to the tempera- ture, was monitored on a chart recorder, The temperatures reported in the table are good to f 2 "C.

RESULTS The cooled photomultiplier housing was tested with the

four RCA photomultiplier tubes listed in the Table. The PMT voltage was supplied by a Keithley model 244 high voltage power supply. Measurements of the dark current were made with a Keithley model 427 current amplifier (300-msec time constant) with the output voltage displayed on a Heath model EU-20A chart recorder.

The dark current and temperature were continually monitored with respect to time with recorders throughout the runs. Table I lists the measured temperature, mean dark current, and rms noise for 4 different gas flow rates. These are the equilibrium values taken about 15 minutes after the flow rate was changed. The rms noise in the dark current was calculated from the charts hy measuring the

ANALYTiCAL CHEMISTRY. VOL. 48, NO. 1, JANUARY 1976 233

peak-to-peak (p-p) values of the dark current for three 1- minute intervals. The three values for the p-p noise were averaged and divided by 5 to convert to the rms noise (26). If the flow rate is not changed, there is no observable change in dark current or temperature over an hour once equilibrium is reached.

DISCUSSION The two sets of data for the 1P28 #1 PMT correspond

to two different days and indicate the dark current and rms noise do vary somewhat from day to day. For all the tubes, the dark current and dark current noise decrease a t lower temperatures. There is considerable variation in the amount of improvement produced by cooling even for equivalent PMT's. Comparison of the data at room tem- perature and -61 "C shows that the dark current is de- creased by a factor of 6 to 45 while the rms noise is reduced by a factor of 2 to 8. This indicates that the cathodic dark current and also the relative importance of dark current from sources other than thermionic emission (e.g., leakage) varies considerably among tubes. This is substantiated by the fact that only for some tubes and temperatures (e.g., 1P28-#2) the noise is proportional to the square root of the dark current as would be expected for shot noise in the thermionic component.

Substantial reduction in the dark current noise is not ex- pected for the tubes tested which inherently have relatively small dark currents. However, the detection limit of a spec- trometric analysis would be improved by a factor of 2-8 by cooling these tubes if dark current noise was the limiting factor at room temperature. Excessively low temperatures can reduce the cathodic responsivity of the tube (18,27).

LITERATURE CITED W. T. Whitney. Rev. Sci. Instrum, 36, 1656 (1965). H. W. Gaudy and J. F. Weller, Rev. Sci. Instrum., 35, 413 (1964). S. Benci, P. A. Benedetti, and M. Manfredi. Appl. Opt., 13, 1554 (1974). C. W. Sherring, J. Physicsf., 3, 1016 (1970). Shardanand. Rev. Sci. Instrum., 43, 641 (1972). P. E. V. Shannon, Rev. Sci. Instrum., 34, 1281 (1963). A. L. Broadfoot. AppI. Opt., 5 , 1259 (1966). C. S. Wiggins and K. Earley, Rev. Sci. Instrum., 33, 1057 (1962). R. J. Smith-Saville and S. Ness, J. Sci. lnstrum., 44, 631 (1967). G. W. Carriveau. J. Phys. E, 3, 929 (1970). R. W. Engstrom, Rev. Sci. Instrum., 18, 587 (1947). A. R. Franklin, W. W. Holloway. Jr., and D. H. McMahon. Rev. Sci. In- strum., 38, 232 (1965). C. J. Bronco. R. M. St. John, and R. G. Fowler, Rev. Sci. hstrum., 29 1145 (1958). R. M. St. John, Rev. Sci. Instrum., 32, 370 (1961). G. G. Harman, Rev. Sci. Instrum., 30, 742 (1959). J. Gerber. Rev. Sei. Instrum., 41, 916 (1970). A. T. Young, Appl. Opt., 2, 51 (1963). A. R. Boileau and F. D. Miller, Appl. Opt., 6, 1179 (1967). A. T. Young, Rev. Sci. Instrum., 38, 1336 (1967). J. P. Keene. Rev. Sci. Instrum., 34, 1220 (1963). R. 8. Murray and J. J. Manning, IRE Trans. Nuci. Sci. NS-7, 2-3), 80 119601.

, F. R. iipsett and R. Horita, AppI. Opt., I, 774 (1962). i23) P. A. Benedetti. S. Benci, and M. Manfredi. Rev. Sci. Instrum., 41, 1336

(19701. (24) J. P. Rodman, AppI. Opt., 2, 165 (1963). (25) RCA Photomultiplier Manual Technical Series PT-61. 1970 RCA Elec-

(26) V. D. Landon, Proc. Inst. Radio Engineers, 29, 50 (1941). (27) M. Cole and D. Ryer, Electro-Opt. Syst. Des., June 1972, pp 16-19.

tronic Components, Harrison, N.J., p 107.

RECEIVED for review July 18, 1975. Accepted September 2, 1975. Acknowledgment is made to the donors of the Petro- leum Research Fund, administered by the American Chem- ical Society, for partial support of this research. Work par- tially supported by NSF Grant No. MPS75-05447.

Stream Combinator and Aqueous Standards in Flame Atomic Absorption Spectrophotometric Analysis of Organic Base Samples

R. S. Tse,* S. C. Wong, and Shirley S. L. Wong'

Department of Chemistry, University of Hong Kong, Pokfulam Road, Hong Kong

Flame atomic absorption spectrometry (AAS) has been widely used in the analysis of trace metal elements in oil and other organic base samples, particularly in lubricating oils (1). The major limitation lies in the difficulty in ob- taining oil-soluble standards. Normally suitable organome- tallic compounds are required (2, 3 ) . The use of easily available inorganic salts as standards must have been in the minds of analytical chemists for some time. Attempts have been made in selecting a suitable solvent ( 4 ) and mixed solvents ( 5 ) in which both the oil sample and the in- organic salt in question are soluble. These solvents are somewhat specific for certain inorganic salts and certain oil samples.

This paper describes the use of a stream combinator to mechanically combine an aqueous stream and an organic stream before admission into the conventional nebulizer. The advantage of such a device is that it allows the use of any water soluble inorganic salt as a standard. The organic stream can be any liquid such as an oil sample diluted with a suitable organic solvent. To illustrate the technique, an analysis to determine trace amounts of iron in a vegetable oil is demonstrated.

Present address, Department of Biochemistry, The Chinese University of Hong Kong, Shatin, Hong Kong.

EXPERIMENTAL Various constructions of the combinator have been attempted

and a satisfactory one is shown in Figure 1. I t was constructed from a Swagelok 1.6-mm (%,j-inch) union. A hole was drilled on the side to allow for the fitting of a 1.6-mm tubing which was used for the aqueous stream. Connection to the nebulizer of the AAS was made through a 0.9-mm 0.d. tubing (a section of a B-D Yale 20G syringe needle) which was mounted on the Swagelok union through a Teflon sleeve and another sleeve made from a section of 1.6-mm tubing. This piece of 0.9-mm tubing was used solely for the purpose of fitting the combinatior to the nebulizer. Another piece of 0.9-mm tubing identically mounted was used for the uptake of the organic stream. The size of this tubing and the variable depth to which this tubing can be inserted into the Swagelok union en- abled a desirable ratio between the uptake rates of the aqueous and the organic streams to be adjusted.

To make up the unknown samples, it was thought that the metal element should be a commonly encountered one and the com- pound used should best be one of the well established standards for trace element determination in oils. Iron caprate was thus cho- sen to make up the samples.

Iron caprate was prepared according to Hearn, Mostyn, and Bedford (3). These authors found the iron content in the caprate to be 9.82%. In the present work, however, the exact iron content was immaterial, since both the unknown samples and the aqueous standards were prepared from the same batch of iron caprate.

The unknown samples were prepared by dissolving known amounts of iron caprate in a solution of 20% v/v vegetable oil in 4-

234 ANALYTICAL CHEMISTRY, VOL. 48, NO. 1 , JANUARY 1976