(4) Cheng, K. L., Bra>-, R. H., Zbzd., 25, 655 (1953). ( 5 ) Crumoler. T. B.. Ibitl.. 19. 325 (1947). ( 6 j Flaschka,’ H., Soliman, A,,Z.‘ Anal. Chem. 158,254; 159,30 (1957). ( 7 ) Gause, E. H.. Crumpler, T. B., Chetn. Soc. Jonassen. H. B.. J . -47~. 73, 5457’(1951). ( 8 ) Gauss, W.. Moser, P.. Schwarzenbach, . G., HelL:. Chim. A c h 35, 2359 (1952). (9) Gorog, S.,Beck, M. T., Acta Uniu. Szeged., Acta Phys. Chenz. 3, 91 (1957). ( I O ) Hoste, J., Anal. Chiti,. Acta 4, 23 ( 1950). (11) Jonassen, H. B., Hurst, G. G., I,eBlanc, R. B., Meibohm, 4.W ; J . I’hys. Chein. 56, 16 (1952). (12) Jonassen, H. B., LeBlanc, R. B., llribohm, A. \Ir.! Rogan, R. M., J . -4m. Chem. Soc. 72, 2430 (1950).
(13) Jonassen, H. B., Meihohni, A. W., S. Phys. & Colloid Chem. 55, 726 (1951). (14) Peterson. R . E.. Bollier. M. E.. ‘ AXAL. CHEM., 27, 1185 (i955j. (15) Prue, J. E., Schwarzenbach, G., Helv. Chim. Acta 33, 985 (1950). (16’ Reilley, C. K., Srhmid, R . W., i2~a1,. CHEM.30, 953, 947 (1958). (17) Reilley, C. S . , Sheldon, V. V., Chemist-Analyst 46, 59 (1957); l‘alanta 1, 127 (1958). (18) Reilley, C. N., Vavoulis, A., h . 4 ~ . CHEM.31, 243 (1959). (19) Schmid, R. W., Reilley, C. X.,J . Am. Chem. SOC.78, 5513 (1956). (20) Schwarzenbzch, G., Chimia (ilaiazc) 3, 1 (1949). 121) Schwarzenbach. G.. Helv. Chini. ’ Acta 33, 947, 9174 (1950), ilnal. Chzm. Acta 7 , 141 (19521, Analyst 80, 713 (1955).
(22) Schwarzenbach, G., “Die komplexometrische Titration,” pp. 2-6, E’erdiri:trd Enke Verlag, Stuttgart, 1955. (23) Sedivec, V., Vasak, V., Coliection Czechoslov. Cheni. Conimun. 15, 260 (1950). (24) Smith, G . F., .\SAL. C I f E h f . 26, 925 (1954). (25) Smith, G. F., McCurdy, R.II., Jr., Zbztl., 24, 371 (1962). 1266) WetlPsen. C. G.. Gran. G . , Scensk Papperstad. 55, 2 I2 (19j2 1. (27) Whealy, R D., Colgnte. S. O., ANAL.CHFU.28, 1897 (195G).
RECEIVEDfor re\iew March 29, 1962. Accepted July 13, 1962. Division of Analytical Chemistry, 142nd Meeting, ACS, Atlantic City, N . J., September 19G2.
Film Thickness Determination with a Scintillation Counter H. R. LUKENS, Jr.’ Shell Development Co., Emeryville, Calif.
b A long-lived luminescence induced in organic lubricant films b y ultraviolet light has been employed to determine the thickness of the films. The luminescence is measured and recorded with a multiplier phototube and scaler, and has been found to be proportional to film thickness. The phenomenon, long-lived luminescence, has been observed in many organic materials, which indicates that the method should b e widely applicable.
A
for determining the thickness of thin lubricant films has been developed that makes use of a multiplier phototube and scaler to measure a long-lived luminescence of the films. The luminescence is induced b y irradiating the films with ultraviolet light (2537 A.) and is proportional in intensity t o film thickness. The method enables one to use scintillation equipment to measure many substances not susceptible to the radiochemical method of Golden (1). J17hile other t’echniques for obtaining thickness exist, as descrilied by Greenland (21, many of these require specialized equipment. The simplicity of equipment and technique give the present method some advant’age in speed and cost, METHOD
luminescence intensity and sample weight. Each sample was mounted on a stainless steel surface and covered an area of 1 sq. cm. The sample weights n-ere obtained on a Mettler B-6 semimicrobalance (precision 0.01 mg.). The lubricants tested were Cello-Seal (Fisher Scientific Co.), Lubriseal (Arthur H. Thonias Co.), and a polybutene mith n molecular weight of about 1500. Care was taken to achieve even coatings, and small irregularities disappeared upon standing. The mounted samples were irradiated with 2537 A. light for an accurately timed 5 minutes by placing them on a Mineralight Model SL2537 lamp, a 4-watt filtered lamp (Figure I). Subsequent to an irradiation, a sample was placed a t the light-sensitive end of a n EM1 Type 9526B multiplier phototube, the output of which was amplified and fed to a scaler ( 3 ) . The samplcs were counted with the photoSTAINLESS STEEL PLANCHET
2 . 5 CM.
w
SAMPLE MOUNTED
1 7 ~ ON ~ . BOTTOM
RESULTS AND DISCUSSION
S S PLANCHET (SAMPLE ON B ALUMINUM RING (NO I)
EXPERIMENTAL
A number of standard samples of several lubricant materials were prepared to test the relationship between Present address, General Atomic, San Diego, Calif.
1396
ANALYTICAL CHEMISTRY
tube during the time interval of 0.5 to 5.0 minutes after termination of the irradiation. The background count rates were obtained by counting the samples prior to ultraviolet irradiation. The background consisted of instrumental noise and luminescence of the samplrs due to exposure to room light. Thc latter component mas zero for the polybutene, about 100 c.p.m. for CelloSeal, and about 30’30 c.p.m. for Lubriseal. Although the background luminescence decayed very slowly, it mas counted over the same time i n t e n d s as the irradiated samples. The phototube was operated a t such a voltage (1120 volts) and the signals Mere amplified sufficiently (25,000) to provide a counting efficiency of about O.Olyo for individual photons (4). An aluminum ring with a hole 1.7 em. in diameter in the center was used to separate a sample from the surface of the ultraviolet lamp filter in order to avoid changing the sample configuration or weight. A second ring of identical dimension was used to separate ssniple and phototube. Sample irradiation and counting arrangements are shown in Figure I .
ALUMINUM RING (NO 2 ) U V LAMP
iRRADi ATlON
MULTIPLIER PHOTOTUBE COUNTING
Figure 1. Irradiation and counting arrangements
Results are given in Table I. The count rates shown are the average values over the counting period. Absorption by the sample of 2537-A. light and sample luminescence, as given by the absorption coefficients I.( and cy, respectively, influences the luminescent intensity. Other factors of importance, which may be combined into a constant K , are ultraviolet intensity, quantum efficiency for luminescence, lumines-
t
h
2 c > VI
a z
104
u w 0 W W
z 3 0
IO
20
30
40
50
60
T I M E AFTER I R R A D I A T ~ O N , M I N U T E S
Figure 3. cence
Decay of Lubriseal lumines-
Table I.
Count Rates of Irradiated Lubricants
Count rate, c.p.m. (with I
I
10
I
Weight Lubricant Polybutene
Figure 2. thickness
Luminescence as
c m e growth and decay, and luminescmce detection efficiency. Of course, only careful maintensnce of constant experimental procedure permits the expression of the latter factors by a single constant. It is easily shown that measured luminescence intensity, L . is related to samplc thickness, 5, by thp expression,
L
=
Kp(1
- e-(r + dz)/(w
+ a)
(1)
Where I o is intensity of ultraviolet light and I is the intensity a t distance z in the sample, then, of course, I = lo esp(--/*z). Also -dl = pldz. Thus, -dI = -,do[exp ( - p z ) ] d z . Luminescence will be given by dL = CdI, where the constant, C, includes all the terms of K cxcept Io. Emitted light is :tttenuated by the factor, exp (-CY.). Then,
The data given in Table I have been plotted as points in Figure 2, after conversion of sample weights t o film thicknesses. The plotted points show a useful relationship between luminescence intensity and film thickness. Curves obtained according to Equation 1 have been fitted to the data points in the figure. From the goodness of fit, i t appears that there are no serious factors that modify the validity of the equation. -45 [I - exp -(p a ) z ] approaches
+
a
function
of
film
unity, luminescence intensity bccomcs relatively insensitive to changes in thickness. Care$l duplication of the counting period is far more simple than trying to correct for differences in counting intervals, because the decay of luminescence is complex and is apparently the consequence of more than one process. The luminescence decay of a sample of Lubriseal is given in Figure 3 . This type of luminescence has been found to be excited similarly by ultraviolet light in many solid organic materials (hydrocarbons, ket,ones, wood, synthetic polymers, etc.), as iyell as in numerous viscous materials such as the lubricants described. The long - lived luminescence has also been observed in toluene solutions and iso-octane solutions of several ketones, such as Michler's ketone, benzophenonc, methyl n-undecyl ketone, and acetophenone ( 5 ) . Thus, although there are substances th:it do not luminesce appreciably, the method is applicable to a large nuniher of inaterials. The luminescence intensity - film t'hickness relationship has been used successfully to measure various films on steel surfaces. The method has proied to be rapid and, providing care is taken to use exactly the same material for standardization ns is desired to measure, i t is relatively free of interferences. The care in rhoice of standard material is important, for sometimes a
mg.
0 0 0 0 0 0
01 058 18 25 26 27
29 3 9 Crllo-Sen1
110 ( 1 8 ) 610 ( + 1 3 ) 1,900 ( 1 2 2 ) 2,500 ( + 2 5 ) 2,570 ( 1 2 5 ) 2,750 ( * 2 6 ) 27,000(180) 23,000 ( +YO)
6 6
48,000(1110)
0 10 0 23 0 53
68 1 42 I 96
165 ( 1 9 ) 315 i f 1 1 1 710 (i14) 750 ( f l 4 ) 1 , 32 14 5 ((+18) 118) 2,275(524)
0.01 0 0" 0 13 0 18 0 39 0 35 0 52 1 26
366 i5 1 2 ) 1,287 (k18, 3 , 8 4 7 ( f30) 7,107(140) 7,767 ( 1 4 2 ) 11,827 ( 1 5 1 ) 11,327 (&SO) 15,983 ( 5 5 9 )
3 99 Luhriseal
standard deviation)
lubrirant, for example, is altered with use; and in such a case i t is necessary to employ some of the used lubricant as a standard. LITERATURE CITED
( 1 ) Golden, J., Iancaster, P. R . , R o w ,
G. R., Intern. J . ..4ppl. Radiation lsotopes 4 ( 1 / 2 ) j 30 (1958). ( 2 ) (;reenland, I
