Universal High-Sensitivity Photometer Fo I Mcas t: r ing Light Scattering, Luminescence, Transmittanee, and Reflectance GERALD OSTEK Polytechnic Institute of Brooklyn, Brooklyn 2, A'. Y. i high-sensiti\itg photometer with readily interchangeable optical components was needed in order to carrj out a wide variety of accurate photometric measurements on a single instrument. .4 photometer has been de\eloped employing a photomultiplier tube whose output is amplified by means of a11 electrometer tube. The instrument is capable o f measuring light intensities down to 20 microlamberts. Colored filters, neutral filters, arid polarizer m a ) be inserted in \ arious combinations for both the iricident and obseried beams. The
I
S SE.UiLY all hranchcs of chemistry there is a nced for
a photometer having a high sensitivity and readily adaptable for different types of photometric measurements. In recent ycars scwcral phototalcctric photometers have been designed cspwially for the nieasurrment of light scattering, employing Q phototube of the multiplier type (3, 7 , 12, 15, 18). The instruincmt dcscribcd nnploys a photomultiplier whose current is
receiter phototube may be rotated about any angle with respect to the incident beam. Cells for liquid samples (including micro quantities) and flat samples ha\ e been constructed. The performance of the instrument is illustrated by twelve distinct types of measurements, which include quantitati\ e measurements of light scattering (turbidity, particle counting, arid diss?ninietry), luminescence (fluorescence, phosphorescence, and chemiluminescence), transmittance of high absorbance sy stems, and reflectance gonionietr?.
amplified bj, an e1ectronietr.r tube. Resides possessing a greater ultimate sensitivity than the other instruments. the instrument has several easily replaceable elements which cnahle i t to be used for a wide variety of photonietric measurernents. I t is characterized by a compactness of design and ruggednces of construction which make for simplicity of operation. ELECTRICAL SYSTEM
I 1-7""f
F 1 ,
i
I/
The instrument operates directly from a 110- to 130-volt line. The light source used is an hH-4 100-watt mercury lamp supplied with a current-stabilized hallast. The lanip reaches its maximum brilliance after operation for about 10 minute?. The photomultiplier employed (RCA 931-h) has been carefully selected to have a high sensitivity and low dark current. The dark current is further reduced hl- placing a drying agent in the proximity of the phototube. Figure 1 is a diagram of the circuit. The filtered high-voltage direct current supply to the phototube is stabilized by means of a corona discharge regulator tuhe (5841). The cathode potential on the phototube (negative with respect to ground) may be varied by means of a resistance (Hi-volt control) from 680 to 900 volts, corresponding to a variation i n relative light sensitivity of the phototube of from 1 to 5 . The output of the phototube is dii,ertly roupled to the electrometer amplifier by a common photomultiplier anode and electrometer grid resistor. The polarities are such that the grid of the electrometer tube (V3 C h 569T) can never become positive and hence the circuit is protected from accidental overloading. Stabilization of the plate and filament voltages of the electrometer tube is provided by a discharge tube (V4,0C3). The amplified current is recorded on a microammeter (0 to 50 Ma.). The microammeter scale is divided uniformly to read intensities and logarithmically to read absorbances (optical densities). Provisions have been made for attachment of a 4-6020 potentiometer-type recorder, the output of the instrument being 10 mv. at full scale. The dark current from the phototube and drift from the electrical components are compensated by altering the negative bias of the electrometer tube by means of a potentiometer resistor. After about 15 minutes of operation the drift is very slight and only the dark current need be zeroed out. The electrometer tube is one element of a Wheatstone bridge with the meter (or the recorder) in the null circuit. Values are so chosen that the meter indicates only the increase in electrometer current from the balanced condition. Thus, the meter indicates relative light intensity. The response was found to be linear with light intensity over the widest range of sensitivity available on the instrument. Resistors inserted in the grid circuit of the electrometer tube provide a variation in sensitivity from 1 to 10,000 in exact multiples of 10. -4continuous variation of sensitivity of a factor of 2 is provided by a variable resistor, which can also be used for calibration of the meter after comparison with an external standard sourre of current. 1165
A N A L Y T I C A L CHEMISTRY
1166
For the instrument used by the author a setting of 35 on the meter calibration with on the photomu~tip~ier tube, a full scale deflection in the microammeter a t the highest decade resistor, corresponds to a phototube output current of IO-' #a. The m i c r o m e t e r deflections can be read to 1% accuracy and hence phototube output currents as low as 10-5#pa. are According to Presented by the manufacturer of the phototube (Tube Division, Radio Corp. of h e r ica, Harrison, N. J.), this current output of thephototuheisequivalent, for light of wave length 420 ma, to a power of 1.25 x 10-15 watt, which for blue light corresponds to roughly 2600 quanta Per second. Further details of the electrical circuit are available (3). OPTICAL SYSTEM
The optical system with its dimensions is shown diagrammatically in Figure 2.
mm. holes and have a positioning pin and groove to keep the long axis of the slit in the vertical position. With the smaller incident nosepiece the divergence is discussed below. Convergent light a t the center of the sample is achieved by placing the converging lens a t position 3. By placing the lens st position 1, a t h r e e to fourfold increase in intensity in the sample chamber is achieved. A light trap is placed outside the sample chamber a t the 0" position. It may be removed for visual alignment of the optical system. Stray radiation is further reduced by blackening the filter holders, nosepieces, etc., and by coating the sample chamber mrfaces with a black rayon flocking. With the chamber lid and shutter closed, the instrument at its highest sensitivity does not record the presence of any stray radiation even in a brightly lit room. The phototube is rotated manually from outside the sample chamber. The detector may be rotated from 0" to 147" in a horizontal plane about the transmitted beam, the angles being read off on a wheel marked in degrees. A photograph of the instrument is given in Figure 3. The over-all length of the instrument, including the electrical system, is 24 inches. CELLS AND SAMPLE HOLDERS
Figure 2.
Optical System
The base of the lamp has three positioning screws with which sityalter On the position of~h~ the lamp lamp to is enclosed obtain maximum in a housing light intenwhich to serves both as a reflector m d to help dissipate heat via a chimney. A metal shield further protects the sample chamber from the heat of the lamp. A removable light stop is located next to theloeked lamp.in A momentarily or theshutter open on a spring ~h~may be opened is twisG ing the knurled cap back to its original direction, whereupon i t will spring np to its normally closed position. A total of six optical components may be added a t the filter ~ ~ n 8 & s F ~ ~~~~~~~~~~~~~~i~~~~~~~ ~ t ~ ; the 16-mm. holes provided and is held in place by retainer rings. Among the optical elements employed for various measurements are polaiizers for polarizing the light vertically and horizontally, an opal glass diffusing plate, converging lens of 40-mm. focal length, neutral filters, yellow glass filter, and a hest filter. Places for five filters in the form of a rotating turret are provided for a t position 2. Five colored glass filters which isolate the 365 405, 436, 546, and 575 m p lines of Fercury ( 4 ) are inserted'iu the turret, so that monochromatic light of any one of these wave lengths is quickly obtained. When the phototube in its housing is at less than 25' (all angles are taken with respect to the transmitted beam), a mechanism attached to the movement of the phototube antomatlcally places a dark neutral filter (ahsorbmce greeter thau 4) in the path of the transfnitted beam. This dense filter protects the phototube from direct transmitted light at 0". When not in position, the neutral filter remains below the optical axis. The filter may be removed for small-angle observations or far transmittance measurements of samples of very high absorbances. The hettm is collimated by means of the incident nosepiece, which consists of two slits in a cylinder. Two incident nasepieces were used. The larger nosepiece has a rectangular entrance slit, 2.5 X 5.0 mm., and rectangular exit slit, 3.5 X 7.5 mm., the distance between the slits being 24.0 mm. The smaller nosepiece bas two square slits 2.5 mm. on an edge, 15.0 mm. apart. The receiver nosepiece a n the phototube housing has two equal slits 2.5 mm. high and 1.0 mm. broad, and 15.0 mm. apart. These nosepieces fit into the standard 16-
f&?j;Ee
Photometric measurements of liquid samples at 0" and 90' are normally carried out in a rectangular sample cell. The cell used in the present instrument is 7.0 om. in height and has a square cross section 2.9 em. on edge. The internal width (path length) is 2.4 cm. The cell fits snugly in a base. The sample should he more than 15 ml. in volume. For angular measurements of liquid samples a semioctagonal cell having flat faces a t O", 45", go", and 135" has been used. However, this cell is limited t o observations of these angles only and is difficult to manufacture. A much more useful angular cell is a cylinder 7.0 cm. high and 4.0 om. in dimeher with an internal diameter of 3.7 cm. The cylindrical cell was tested with a dilute solution of fluorescein and with dusbfree pure solvents m d the radiation envelope was found to be Symmetric ahout 90". The radius of curvature of the cell is large compared with the diameter of the incident beam when the smaller incident nosepiece is used. visual observation of the fluorescent h e m traversing a solution of fluorescein showed that the beam in the cell is very slightly convergent. No internal reflections in the cell nvere observable. The cell is mounted in the same base as that for the rectangular cell. The sample should he a t least 20 ml. in volume, Much smaller sample volumes may be used, however, if a thin-walled
Figure 3. Photometer
1167
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3
(11). The ahsolute turbidity for the Ludox sample a t 436 mp is
determined
a8
follows:
The rectan lar 'cell and the smaller incident nosepiece are emuloved. &h the detector a t 0". the sensitivity of the
;&inlied
m&d
hv 2.303 and divided by the path length (2.4 cm.)
rrreviouslv. Aocdrdina to the inversefourth- power
0.9Cfor 546 and 436 mu. resuectiveiv
(io)."
[The depolarimtion
"." ___, ~ . . . .~ ~ ~ ~ ~ ~ . ~ pared with the scattering for Ludox. For nonaq"ueou8 samples the intensity bf the scattering a t 90" must he corrected by the factor ( x / n 4 z , where n is the index of refraction of the sample and n, is the index of refraction of water (5, 8). ~
Figure 4.
C h a m b e r w i t h Cylindrical Cell in Place
Figure 5.
S a m p l e Holders
test tube is supported a t the center of the cylinder, and surrounded by the solvent of the solution being examined. By these means, volumes as small as 1 ml. can he studied. Another adapter holds test tubes (at least 1 em. in diameter and 7 cm. long) in an upright position at the center of the chamber. Figure 4 shows the chamber with the cylindrical cell in place. A goniometer has been constructed for photometric measurement of flat samples. The sample, 20 mm. square or larger, is held in a clip and may be oriented a t various angles with respect to the incident beam, the angles being given on a graduated scale. The goniometeris inserted by first removing the liquidcell support. Figure 5 shows the cells, test tube adapters, and goniometer. TYPICAL MEASUREMENTS
Light Scattering. The ahmlute turbidity of a light-scattering sample is measured by comparing its intensity of scattering with that of an aqueous suspension of vitreous silica (Ludox) (15). This material a t 8 concentration of about 3% exhibits eonsiderable light scattering yet obeys the inverse-fourth power wavelength relation and shows no appreciable dissymmetry and only a small depolarization (10). Hence, the turbidity of the material is given by 2.303 times its absorbance divided by the path length
By using the ahove calibration technique, the Rayleigh factor a t 90" of dusbfree benzene a t a wave length of 436 mp was found to he 47.1 X 10-6, corresponding to s turbidity (uncorrected for depolarieation) of 7.9 X lo-'. This value is cansidemhly higher than the results of deVaucouleurs ( 1 6 ) hut agrees well with the value found by Brice et al. (5) Icf. Zimm (19)l. A turbidity of lo-' is expected for a solution of macromolecules with a molecular weight of 106 and concentration approximat& 10-4 gram per ml. (cf. 11). Actually, to measure the turbidity of such dilute solutions accurstely, i t is necesmrv to clarify the solution hy ultrafiltration or by
will came the needle of the microammeter to fluotuateerratically. The correct vdueof intensity of scattering of the solution itself is given by the lowest reading on the meter. Conversely, the relative numher of dust particles in a given system can be measured by determining the numher of fluctuations per unit time of the needle of the meter, Hence, the instrument ~ e r v e8.8~ a very sensitive Darticle counter for solution and aerosol samrrles. For particles the size of bacteria, it is best to measure the scattering at small angles (in the cylindrical cell) where the scattering intensity is greatest. By means of the fluctuation technique one can d e teet as few 8.8 10 bacteria per ml. If a suspension is settling, owing to gravitation, the rate of settling can also he followed by this technique. The angular settings are accurate to ahout 1". Figure 6 shows the angular scattering for a sulfur sol measured in the cylindrical cell, the zero angle neutral filter removed. The aeidified sodium thiosulfate solution was made up according to the directions of Johusou and LsMer (9) and allowed to stand for 24 hours. The scattering curve obtained agrees closely with that calculated for sulfur particles 0 6 micron in diameter. The ratio of intensity in Some forward direction to that a t B supplementary angle, the dissymmetry, gives information regarding the shape and largest dimension of high molecular weight particles of refractive index nearly that of the medium (cf. 5, 11). A dilute sample of polystyrene spheres prepared by emulsion polymerization and whose diameter, ZB ohserved in the electron microscope, is 165 mp ( 1 ) was found t o have for green incident light a dissymmetry (extrapolated to infinite dilution) a t 45'and 135"of 2.67. This value agrees exactly with that calculated ( 6 ) for spheres of this Bise, Luminescence. Fluorescence measurements are carried out using incident light of wave length 365 mp and a yellow filter
ANALYTICAL CHEMISTRY
1168 placed before the detector (kept at 90"). As the filter a t the detector transmits none of the incident light, the fluorescence measurements may be carried out even if the solution is turbid. By these means extremely dilute solutions of fluorescing substances may be measured-for example. the fluorescence of a 5 X 10-lo M solution of sodium fluorescein is easily detectable. The fluorescence intensity is directly proportional to the concentration of the fluorescein up to a concentration of about l O - * M , where concentration quenching reduces the fluorescence efficiency and loss in transmittance of the solution n u s t be taken into account. In relative measurements of fluorescence it is not necessary to employ the incident and receiver nosepieces. Their elimination results in an approximately 500-fold increase in sensitivity for a given sample. The fluorescence efficiency of a given substance may be determined by comparing the fluorescence of the solution with a solution of a substance LT ith similar absorption and fluorescence spectra whose fluorescence efficiency is known. Thus, a solution of acriflavine having the s a m e absorbance a t 365 mp as a solution of fluorescein (efficiency SO%), showed half the intensity of fluorescence of the latter and hence has a fluorescence efficiency of 40%. The relative intensity of fluorescence when quenching molecules are added can be used to detect small amounts of the latter material. For example, desoxyribose nucleic acid quenches the fluorescence of acriflavine ( 1 4 ) and is detectable in the instrument at concentrations as low as 10-9 gram per ml. Measurement of the depolarization of fluorescent light gives information regarding the lifetime of the excited state of the molecules and their rotational diffusion constants (cf. 6, 1 7 ) . Such measurements are carried out by insertion of the polarizers at the incident or receiver positions.
04
Ib
210
do i o o:
610
Ib
8
810 90 100
,
IK)
120 I40 ,!I
TIME (SEC.) Figure 7. Luminescence of Copper-.&ctivated Zinc Sulfide Phosphor as a Function of Time Arrow indicates time at which incident light is removed
a w
0
Figure 8.
(546
Angular Reflection of Green Light mp) by a White Bond Paper
Sample setting with respect to incident beam A. 45' B. 30' c. 150
Figure 6.
Angular Scattering of Green Light (546 mp) by Sulfur Sol
Intensity of curve A is ten times that of curve B
The properties of phosphors may be characterized with the instrument. In Figure 7 is shown the intensity of luminescence as a function of time for a copper-activated zinc sulfide phosphor. The incident light is 365 mp in wave length and observations are made a t 90" with the yellow filter at the detector. The phosphor in the form of a powder was pasted on a microscope slide and this was placed on the goniometer sample holder a t 45" to the incident
light. The shutter was kept open until saturation in luminescence was reached (indicated by the arrow in Figure 7) and then the shutter was allowed to spring back to its normally closed s For a position and the decay in luminescence ~ n measured. smaller incident intensity, this phosphor exhibited a slower decay of luminescence. The relative efficiencies of luminescence of phosphors may also be measured, account being taken of any differences in the luminescent spectr a of the various phosphors. The detrctor can easily be removed from thecahamber and used in conjunction with a fluorescent material as a scintillation counter for the detection of x-rays or other ionizing radiations The filter holder at position 5 is covered with a thin aluminum foil and an anthracene crystal is pasted with transparent tape on a filter holder and put at position 6, the crystal facing outward from the phototube. The instrument is sufficiently sensitive to detect chemiluminescence in those reactions emitting such radiation. Thw. with the shutter in the closed position and the nosepiece from the detector removed, the iuminol-hydrogen peroxide reaction and the luriferiri-luciferase reaction can be studied as a function of time. Transmittance and Reflectance. The instrument is well suited for the measurement of transmittance of very opaque samples such as ceramics, paper, cloth, or inks. Transmittance of materials of very high optical densities is determined in the following manner: The nosepieces are removed and the detector is kept a t zero angle. In addition to the dense neutral filter at O", other neutral filters are added a t positions 3 and 4. The latter should be so chosen that for a given wave length of incident light the meter will give a readable deflection when the instrument is set at some
1169
V O L U M E 25, NO. 8, A U G U S T 1 9 5 3 high decade resistance. The neutral filters are then removed and the sample is placed in the chamber (in the goniometer a t 90” to the incident beam in the case of flat samples) and the intensity is determined. The transmittance of the sample is then given by the ratio of the first reading to that of the second and the ratio is multiplied by the combined transmittance of the neutral filters, the transmittance of the neutral filters having previously been determined by an earlier measurement with the instrument. By these means, samples with a transmittance as low as 1O-IO may be determined. Reflectance properties of surfaces are determined by placing the flat sample in the goniometer a t fixed angles with respect to the incident beam and measuring the relative intensities of reflected light a t various angular settings of the detector. The smaller incident nosepiece and the receiver nosepiece should be used in these measurements. The results obtained for a white bond paper are shown in Figure 8. Such measurements should be useful in comparing the optical properties of papers manufactured by various processes or papers to which coatings or filters have been added. The instrument also serves as a glossmeter for comparison of coated glasses, painted surfaces, ground surfaces, sheen of textiles, ctc. ACKNOWLEDGMENT
The author wishes to express his thanks to the American Instrument Co., Silver Spring. Nd., for its cooperation in the design and development of the inqtrument. He is especially in-
debted to Justin J. Shapiro of t’hat company for his many helpful suggestions. LITERATURE CITED
(1) Alfrey, T., Bradford, E. B., Tanderhoff, J . W., and Oster, G., J . Opt. SOC.A m e r . , in press. (2) American Instrument Co., Silver Spring, AIcl., BUZZ. 2202A. (3) Brice, B. A., Halwer, M., and Speiser, R., J . Oyt. Soc. A m e r . , 40, 768 (1950). (4) Corning Glass Works, Corning, X. Y., “Glass Color Filters.” ( 5 ) Doty, P., and Bteiner, R. F., J . Chem. P h y s . , 18, 1211 (1950). (6) Garlick, G. F. J., “Luminescent Materials,” Chap. 8, Oxford,
Oxford University Press, 1949. (7) Hadow, H. J., Sheffer, H., and Hyde, J. C., Can. J . Research, 27B, 791 (1949). ( 8 ) Hermans, J. J., and Levinson, S.,J . Opt. Suc. Attier., 41, 460 (1951). (9) Johnson, I., and LalIer, I-.li., J . Am. Chem. Soc., 69, 1184 (1947). (10) Xommaerts, W.F. H. AI,, J . Colloid Sci., 7 , 71 (1952). (11) Oster, G., Chem. Recs., 43, 319 (1948). (12) Oster, G., J . Gen. Physiol., 33, 445 (1950). 9, 525 (1952). (13) Oster, G., J . Polymer Sei., (14) Oster, G., Trans. Faraday Soc., 47, 660 (1951). (15) Speiser, R., and Brice, B. A . , J . O p t . SOC.A m e r . , 36,364 (1946). (16) Valcouleurs, G. de, Conipt. rend., 229, 35 (1949). (17) Weber, G., Biochem. J . , 51, 146 (1951). (18) Zimni, B. H., J . Chem. Phys., 16, 1099 (1948). (19) Zimm, B. H., J . Polymer Sci., 10, 351(1953). RECEIVED for review February 19, 1953.
Accepted June 1, 1953.
Determination of Mineral Constituents of Rocks by Infrared Spectroscopy JOEY M. HUNT AND DANIEL S. TURNER Research Laboratory, The Carter Oil C o . , Tulsa. Okln. It was desired to develop an infrared spectroscopic method for the qualitative and quantitative analysis of the mineral constituents of sedimentary rocks for use in petroleum exploration research. The method developed consists of grinding rocks to a fine powder and examining the powder as a film on a conventional sodium chloride window. The mineral constituents of the rocks are identified by comparing their spectra with the spectra of pure minerals. Quantitative analyses within 10% of the amount present can be made for minerals which have sharp, well-defined absorption bands such as quartz, kaolinite, orthoclase, calcite, and dolomite. Errors in the analysis are caused by nonuniformity of the sample film and scattering of the infrared radiation. The technique has been used in analyzing oil well cuttings, cores, drilling mud, and surface samples that are obtained in connection with various geological problems related to petroleum exploration.
I
K A previous paper ( 3 ) a powder film technique was described by which it is possible to obtain well-defined infrared spectra of minerals and other inorganic compounds. The present paper describes the application of this technique with some modifications to the determination of the mineral constituents of rocks, both qualitatively and in some cases quantitatively. The importance of grinding mineral samples to a particle size smaller than the wave length of the infrared radiation has been emphasized ( 5 ) . The presence of large particles tends to scatter
the radiation, so that only a small percentage of the incident radiation is transmitted and the absorption bands are very poorly resolved. Several techniques for obtaining small particles of solids have been reported in the literature. Mulling a sample in Sujol or Fluorolube (a misture of completely fluorinated hydrocarbons available from the Hooker Electrochemical Co.) is the most common method and was used by Miller and Wilkins in their excellent and extensive study of the spectra of inorganic chemicals (6). Suspension of a solid with aluminum stearate in cyrbon disulfide has been used for the quantitative analysis of organic chemicals (I). Vnfortunately, neither of these techniques was applicable to the quantitative determination of minerals in rocks, because of the greater hardness of the rocks and their higher densities. Grinding and sedimenting a rock to ohtain a desired particle size have been used for qualitative studies (3-5). They are not satisfactory for quantitative analysis, owing to the separation of the mineral constituents of a rock in a sedimenting column. I n a typical rock the clay minerals will be concentrated in the fine particle fractions, whereas quartz 1% ill be concentrated in the large particle fractions. An unequal distribution of rock constituents can result from differences in particle size, shape, and density. The method ultimately adopted in this work involves grinding all of a rock sample to a particle size of less than 5 microns in a cyclonic type of jet pulverizer known as the Micronizer (obtained from the Sturtevant Mill Co., Park and Clayton Sts., Boston, 22, Mass ). Rock samples ground in this unit are fine enough to run directly as a powder film mount. Quantitative determinations of certain minerals in the rocks are made by comparing absorbances (optical densities) with those obtained from known con-
