Nature of Nonspecific Interactions in Gas-Solid Chromatography James N. Gerber and Donald T. Sawyer Department of Chemistry, University of California, Riverside, Calif. 92502 The gas chromatographic retention of the inert gases and methane has been studied on salt-modified aluminas and porous silica to elucidate the nature of nonspecific interactions in gas-solid chromatography and to test the theory of King and Benson. The latter, which suggests that the energy of interaction is directly proportional to the polarizability of the sorbate molecule, fits the present experimental results with the exception of methane adsorption on porous silica. To account for such an anomaly, an “effective polarizability” for an adsorbed molecule is postulated which is dependent on the nature of the adsorbent surface. The results also provide substantial support for the postulate that the statistical mechanical relationship for mobile adsorption is a good model for gas-solid chromatography.
KING AND BENSONhave proposed a theory for electrostatic interactions at gas-solid surfaces ( 1 , 2) in which the energy of interaction, &t, is given by %tt
=
a
Ceff
L
& -
- ; -14
or
carried out with salt-modified Graphons (12, 13). The total free energy of interaction for a gas-solid system can be represented by AG,
=
EAGapecifio
+ AGnompocific
(3)
where the specific interaction summation includes all possible sources of interaction other than nonspecific dispersive forces. The specific interactions have been studied thoroughly and are well documented by previous data (6-13). However, the nonspecific interaction, which can be represented by the free energy of adsorption for paraffinic hydrocarbons, has not been characterized for salt-modified supports. The present investigation has been directed to the elucidation of the factors that contribute to AGnonspecifie. Such experiments also provide the basis for a further test of the theory of King and Benson. The studies have been accomplished through the use of molecules which can only give nonspecific interactions, i.e., the inert gases and methane, on salt-modified alumina and porous silica beads. To obtain a measurable interaction, the temperature of the columns had to be controlled below room temperature. ADSORPTION THERMODYNAMICS
where a is the polarizability of the adsorbed molecule, C,ff the surface charge, 2 the distance of the molecule from the surface, and Ez the electric field normal to the surface. The theory predicts that the inert gases and methane will interact with the adsorbent in direct relation to their polarizability. This has been confirmed experimentally for alumina at room temperature and above (2). The applicability of the theory to other gas-solid interactions has been suggested (2). Because of a continuing interest in the controlling factors for gas-solid chromatography, a test of the King and Benson theory as it relates to salt-modified aluminas and silicas has been undertaken. Scott and Phillips first suggested the use of salt-modification of adsorbents in gas-solid chromatography (3-5). Since then, the adsorption of numerous organic molecules on salt-modified aluminas and porous silica beads has been studied extensively (6-13). Some preliminary studies also have been (1) J. King, Jr., and S . W. Benson, J. Chem. Phys., 44,1007 (1966). (2) J. King, Jr., and S. W. Benson, ANAL.CHEM.,38, 261 (1966). (3) C. G. Scott, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworths, Washington, D. C., 1962, p 36. (4) C. G. Scott and C. S . G. Phillips, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965, p 266. (5) C. S . G. Phillips and C. G. Scott, “Progress in Gas Chromatography,” J. H. Purnell, Ed., Interscience Publishers, New York, N.Y., 1968. D 121. (6) D. J. Brookman and D. T. Sawyer, ANAL.CHEM.,40,106 (1968). (7) D. T. Sawyer and D. J. Brookman, ibid., p 1847. (8) D. J. Brookman and D. T. Sawyer, ibid., p 2013. (9) Ibid., p 1368. (10) A. F. Isbell, Jr.. and D. T. Sawyer, ibid., 41, 1381 (1969). (11) R. L. McCreery and D. T. Sawyer, J. Chromatogr. Sci., 8 , 122 (1971). (12) J. P. Okamura and D. T. Sawyer, ANAL. CHEM.,43, 1730 (1971). (13) D. F. Cadogan and D. T. Sawyer, ibid., 43, 941 (1971).
The corrected retention volume, VR, can be measured directly and converted to a specific retention volume, VsT,by dividing V , by the total surface area of the column, A . VsTis equivalent to the distribution coefficient, K , which is related to the free energy of adsorption, AGtads,by the relation AGtada= - R T In K =
-RT In VsT
(4)
where R is the universal gas constant and T i s the temperature in “K. Therefore, the free energy of adsorption can be evaluated directly from gas-solid chromatographic measurements. Also, from basic thermodynamics,
where A H t a d s is the enthalpy of adsorption and AS’,,, is the entropy of adsorption. Equations have been developed (7)to obtain standard state free energies and entropies of adsorption @Goadsand ASoads)by using the standard state for the adsorbed phase as suggested by de Boer and Kruyer (14); that is, surface concentrations equal to a two-dimensional standard state perfect gas.
EXPERIMENTAL The solid adsorbents included 100-120 mesh Analabs Type H-151 activated alumina and 100-150 mesh porous silica beads (Porasil C, Waters Associates, Framingham, Mass.). (14) J. H. de Boer and S . Kruyer, Kon. Ned. Akad. Wetensch., Proc. Ser. B, 55, 451-63 (1952). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, J U N E 1972
1199
Table I. Thermodynamic Parameters for H-151 Alumina Coated with Various Salts (10 % wtjwt).
Ar, 0
-0.44 -0.12 0.23 0.34 1.11
2
Nz Kr
CH4 Xe a
2.28 3.54 2.92 3.41 4.50
10.0 13.4 9.8 11.2 12.4
-0.73 -0.55 -0.18 -0.07 0.61
1.72 2.54 3.03 2.87 3.68
9.0 11.3 11.8 10.8 11.3
cal in - . hGoadarat 0.0 deg '
AGCsdaand A H 3 d s ,in kcal/mole;
-0.35 -0.15 0.37 0.47 1.42
2.31 2.85 3.16 3.56 4.85
9.7 11.0 10.2 11.3 12.6
-0.43 -0.25 0.29 0.40 1.35
2.47 3.47 3.58 4.00 5.52
10.6 13.6 12.0 13.2 15.3
"c; a.w., acid-washed
Table 11. Thermodynamic Parameters for Porasil C Coated with Various Salts (10 % wt/wt)" Saltmodifier Gas Ar, 02, NZ Kr
CH4 Xe a
-0.39 0.03 0.02 0.73
2.64 3.16 3.09 4.39
AGoadS and AH'ada, in kcal/mole;
AS'ads,
11.1 11.5 11.2 13.4
-0.63 -0.05 -0.08 0.65
13.3 12.5 12.5 15.0
-0.42 -0.12 -0.13 0.66
2.91 3.43 3.37 4.61
12.2 13.0 12.8 14.5
cal in - ;AGOada,at 0 . 0 "C. deg
For most of the studies the alumina was acid-washed in 6F HC1 overnight to remove iron contamination. The acidwashed alumina was washed with distilled water, dried at 120 "C, and the 100-120 mesh portion separated by ASTM sieves. The alumina was then ready for salt coating; Porasil C was salt-coated without pretreatment. A weighed portion of adsorbent was added to an aqueous solution containing a weighed amount of reagent grade salt to form a heavy slurry. The slurry was quickly dried by rotary evaporation and the resulting material was sieved to the proper mesh size. All of the salt coatings were 10% by weight because previous experience has established that both the nonspecific and specific interactions become essentially independent of amount of coating above this weight (15). With this level of coating, peak tailing was not observed for any of the adsorbents; elimination of this was the main purpose of salt modification. The dried adsorbent was packed in a 3-foot length of 1/8inch 0.d. (thin wall) stainless steel tubing which previously had been rinsed with both polar and non-polar solvents. After each increment of packing was poured into the tube, the tube wall was tapped with a metal rod while the end of the column was repeatedly tapped on the floor. Several of the columns used were prepared in the same manner by other workers for previous studies. Columns were activated before each experiment for at least one hour at 230 "C for Porasil and at 300 "C for alumina in a Varian Aerograph 1200 gas chromatograph with a small flow of reactor grade helium passing through the column. Samples of inert gases were obtained from the J. T. Baker Company in borosilicate glass bottles which were modified by the addition of a stopcock and septum for easy sampling. Methane was taken directly from the natural gas line in the laboratory; its retention volume was identical to that for 99.9% pure methane from a lecture bottle. Oxygen and nitrogen were injected as air from the atmosphere. The sample sizes ranged from 0.5 p1 to 3 p1, depending on the gas and temperature of the column, and were injected using a 10-111 Hamilton syringe (Model No. 701). A gas chromatograph was developed in which the temperature of the column could be maintained at subambient tem(15) G. L. Hargrove and D. T. Sawyer, ANAL.CHEM., 40,409 (1968).
1200
2.99 3.36 3.33 4.74
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
peratures. To accomplish this, an aluminum rod was mounted in an aluminum box (10-in. by 12-in.) so that a dewar vessel of the appropriate cryogenic slush could be raised to immerse the chromatographic column. The injection port and a Carle Model 100 microdetector system were mounted on the inside walls of the aluminum box. The system was made as compact as conveniently possible to keep the extra-column dead volume at a minimum. The gas chromatograph also included a port for measuring the inlet pressure; reactor grade helium was used as the carrier gas. A Leeds & Northrup Speedomax Model H recorder was used to record the detector response. The isothermal slushes were prepared by slowly adding liquid nitrogen to the appropriate liquid until a thick two-phase consistency was achieved. The temperatures of the resulting slushes were, water, 0.0 "C; carbon tetrachloride, -22.9 "C; chlorobenzene, -45.2 "C; chloroform, -63.5 "C; and ethyl acetate, -83.6 "C (16). Using the B.E.T. equation and low temperature NBadsorption data (7), the surface areas of the salt-coated adsorbents were measured. The specific surface areas (m2/g) for the acid-washed H-151 aluminas were, NaCl, 267; LaCl,, 279; and NazMo04, 353. For NaC1-coated, non-acid washed H-151 alumina, the surface area was 471 m"g. The areas of the various Porasil C adsorbents were, NaC1, 63.4; LaCl,, 68.3; and NazMo04,60.5 mz/g. The retention volumes were measured by injecting each adsorbate at least three times on a given column. The apparent retention time, f R ' , was measured directly from the recorded chromatogram. The apparent retention volume, V a t ,was then calculated from the relation
where T, and Ta are the column and flowmeter temperatures (OK), respectively, Fa is the volume flow-rate at ambient temperature and pressure, and f is the James-Martin gas compressibility correction factor, The apparent retention volumes were converted to corrected retention volumes, VR,by
VR
=
VR' -
v,
(10)
(16) W. L. Jolly, "Synthetic Inorganic Chemistry," Prentice-Hall, Englewood Cliffs, N.J., 1960, pp 182-3.
0.c
0,o
-1.c
-1.c
I
I
I
I
/
-
N
E
N
-E
E
-1
\
E
~
c
-2.(
I-
" -2.0
>
>"
0
0
01 0
-
-3i
-3.0
-4s IO
20
POLARIZABILITY,
30
40
50
cm x
Figure 1. Logarithm of specific retention volume for several gases as a function of their poiarizability for a 10% (wtiwt) NaC1-coated, acid-washed H-151 alumina column at three different temperatures where Vdis the system dead volume (void space) as determined by injection of neon at the column temperature. All calculations of intercepts and slopes were accomplished by a computer-aided least squares refinement.
RESULTS The retention volumes of oxygen, nitrogen, the inert gases, and methane have been measured for columns packed with salt-coated H-151 alumina and Porasil C adsorbents. With salt-modified alumina columns, oxygen and nitrogen are separated at room temperature; salt-modified Porasil columns must be cooled to -83.6 "C to obtain a comparable separation. Such behavior indicates that the aluminas exhibit stronger nonspecific interactions than the porous silicas. For this reason, the thermodynamic data for alumina columns have been measured at higher temperatures (0.0, -22.9, and -45.2 "C) than those for Porasil columns (-22.9, -45.2, -63.5, and -83.6 "C). Although oxygen and argon cannot be chromatographically separated at any of the temperatures used, there is some indication at - 83.6 "C that they begin to be separated. King and Benson ( 2 ) have predicted that this pair of molecules would be difficult to separate because of the similar values of their polarizabilities (02, 16.0 X 10-25 cm and Ar, 16.2 X lO-z5 cm) and the resulting similar interaction energies at the surface. Measurement of the retention volumes of the adsorbates at three different temperatures allows their thermodynamic parameters to be calculated from Equations 5-8. These results are summarized in Table I for salt-modified aluminas and in Table I1 for salt-modified Porasils. The more negative the enthalpy of adsorption, the greater the interaction is between the adsorbate and adsorbent. From the data for sodium-chloride coated normal H-151 alumina and acid-washed (a.w.) H-151 alumina, acid washing
-4.0
0
I
I
I
I
IO
20
30
40
50
P O L A R I Z A B I L I T Y , cm x
Figure 2. Logarithm of specific retention volume for several gases as a function of their polarizability for a 10% (wtjwt) NaC1-coated Porasil C column at three different temperatures gives a greater negative enthalpy for all adsorbates and therefore an increase in gas-solid interactions. Also, the data of Table I indicate that of the three coating salts, lanthanum trichloride causes the greatest nonspecific interaction between adsorbate and adsorbent. The Porasil data also indicate that a lanthanum trichloride coating increases the nonspecific interaction of this adsorbent. With Porasil columns, oxygen and nitrogen are inseparable and are eluted with Ar. The value of AHoadsfor methane is almost equal to that for krypton. This is in contrast to the results for alumina where methane interacts more strongly than does krypton. Figure 1 illustrates a plot of log VaTus. polarizability for the inert gases, oxygen, nitrogen, and methane, on a 10% sodiumchloride coated column of acid-washed H-151 alumina. Plots for other salt-modified alumina columns have the same general appearance. The data of Figure 1 indicate a direct correlation at three different temperatures, which is in accord with the theory of King and Benson (2). Nitrogen is above the lines because of its specific interaction due to the triple bond; the deviation increases as the temperature is reduced to give an increased interaction. The characteristics of the salt-coated Porasil columns are illustrated by a plot of log VST L'S. polarizability for a 10% NaC1-coated column (Figure 2). This plot gives a straight line for argon (oxygen and nitrogen), krypton, and xenon, but methane is off the line. This also is true for preliminary studies on a graphitized carbon (Graphon) column. Thus, methane exhibits less interaction than it should according to the theory of King and Benson ( I ) .
DISCUSSION AND CONCLUSIONS The results of the present investigation indicate that the inert gases are adsorbed in a fashion that is consistent with the King-Benson theory ( I ) . However, the adsorption of methANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
1201
Table 111. Comparison of Experimental Entropies of Adsorption with the Theoretical Entropies for Mobile Adsorption (14) Column Ar Kr CHI Xe A. Experimental 9.8 11.2 12.4 10% NasMo04on a.w. H-151 10.0 9.0 11.8 10.8 11.3 10% NaCl on H-151 10.2 11.3 12.6 9.7 10% NaCl on a.w. H-151 10.6 12.0 13.2 15.3 10% hC13 on a.w. H-151 11.1 11.5 11.2 13.4 10%Na~Mo04onPorasilC 12.5 12.5 15.0 13.3 10% NaCl on Porasil C 12.2 13.0 12.8 14.5 10% LaC13on Porasil C B. Theoretical 11.3 12.1 10.5 12.5
The consistency of the intercept values for the two supports is particularly impressive when it is recognized that they result from five separate studies (7-11) for a diverse group of columns over a temperature range from -83.6 O C to 250 OC. With such convincing experimental evidence, these constant intercept values must be related to some basic phenomenon of the gas-solid adsorption process. Because effective gas-solid chromatographic separations depend on a rapid equilibration during the adsorption process, a relationship that has been proposed for mobile adsorption (20) may provide insight to the significance of the constant intercept values. x’CT)
=
amkT/h2 sinh (hv/2kT)
[
kT
This relation can be recast in the form ane on Porasil appears to be an exception to their theory. To fit methane onto the straight line for Ar, Kr, and Xe in Figure 2 would require that it have a polarizability of 24.0 X cm, which is significantly below the measured value of 26.0 X 1 0 - 2 5 cm. One way to explain this anomaly is to characterize the surface interaction by an “effective polarizability.” The latter represents the polarizability of a molecule in relation to the surface to which the molecule is adsorbing. The gas-solid chromatographic data of Figure 2 provide a means of evaluating such a quantity. The diminished interaction of methane implies that the acidic surface of Porasil (17) exerts a repulsive force to the hydrogen atoms of methane to give an “effective polarizability” which is less than its measured polarizability. The basic surface of aluminas (18) does not exert such a repulsive force and, therefore, the “effective polarizability” of methane on salt-modified aluminas is equal to its polarizability. The inert gases have no acidic or basic character and therefore their “effective polarizability” always is equal to their polarizability. Graphon has a slightly acidic surface (19) and preliminary experiments indicate that methane’s “effective polarizability” on Graphon is 24.6 X cm. Therefore, methane is repulsed less by the Graphon surface than the more acidic Porasil surface. The concept of “effective polarizability” allows one to describe the interaction of methane with the acidic surface of Porasil and Graphon and still be consistent with the basic ideas of the King and Benson theory, Thus, the nonspecific interaction of sorbate molecules with salt-modified aluminas and Porasils can be described quantitatively as the free energy of adsorption due to the “effective polarizability” of a molecule on the surface. Figures 1 and 2 also provide illustrative examples of the temperature independent intercepts which are found in this type of plot for each of the two supports. These intercepts are - 3.6 i 0.09 for salt-modified aluminas and - 3.2 f 0.07 for salt-modified Porasils. Analogous intercept values also have been observed for log VsT us. carbon number plots in previous investigations (7-11). In the present investigation log VsTis plotted against polarizability, which appears to give a more accurate intercept than the carbon number plots. Carbon number plots only extend as close to the intercept as ethane because methane has been used to measure the dead volume of the column. All of the points for the present log VsT us. polarizability plots come between ethane (with a polarizability of 44.7 X 10-25 cm) and the intercept (i.e., Xe has a polarizability of 39.9 X lO-Z5 cm and Ar has a polarizability of 16.2 X loez5cm). (17) L. R. Snyder, “Principles of Adsorption Chromatography,” Marcel Dekker, New York, N.Y., 1968, p 163. (18) Ibid.,p 167. (19) Zbid.,p 168. 1202
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
log x’(T) = log
mnkT h
- log sinh (hv/2kT) +
Mom
~
2.303kT
-
_
‘(O)
2.303kT
_ (12)
where x ’(T) is the surface concentration, m the molecular weight of the sorbate, k the Boltzmann constant, Tthe absolute temperature, h Plank’s constant, v the frequency of the molecular vibrations of the sorbate while adsorbed, p o ( T )the chemical potential as a function of temperature only and U(o) the potential energy at the equilibrium position. The constant intercept values (see Figures 1 and 2) can be mathematically represented by an equation which is similar in form to Equation 12 log VsT = ai
+ bcy
(13)
where ai is characteristic of the adsorbent, b is a function of temperature, salt-modifier, and support, and cy is the polarizability of the sorbate. In Equation 12 the only quantity that is a function of polarizability is
‘(O) Therefore, in 2.303kT‘ relating Equation 13 to Equation 12, the intercept value does
not include
~
‘(O) as a part of aiin Equation 13. 2.303kT To relate the intercept values to Equation 13 requires that the dimensions of VsTbe converted from ml/m2 to atoms/cm2, the units of x’(T). Also log VSTmust be converted to standard state conditions by Equations 5 and 6, because log x’(T) already is for such conditions. This will add 12.95 to the intercept values. Substitution of these standard state (log V S T ) O intercept values for the left hand term of Equation 12 permits the equation to be solved for v when U(o)/2.303kTis taken as zero. v is the frequency of vibration for the sorbate molecule while adsorbed to the surface and is usually about 10l2 Hz (21). Taking the corrected intercept values for alumina and Porasil C columns and solving Equation 12 yields v values of 2.88 X l o x aHz for alumina and 1.98 X l O I 3 Hz for Porasil C. Preliminary work with Graphon gives an intercept value of -3.4 which results in a v value of 2.43 X 1013Hz. Because all of these values are high, some further consideration of Equation. 12 is in order. The treatment to this point has assumed that U(o) is directly related to the polarizability of the adsorbed molecules. ~
(20) E. L. Knuth, “Introduction to Statistical Thermodynamics,” McGraw-Hill, New York, N.Y., 1966, p 129. (21) Zbid.,p 121.
the entropy values for the inert gases and methane have been calculated. These are summarized in Table I11 together with the experimental entropy values for salt-modified aluminas and Porasil C, and indicate close agreement. In contrast, the calculated entropy values for localized adsorption are more than twice as large as the mobile values. Therefore, the theory of de Boer and Kruyer also indicates that the interactions for gas-solid chromatography result from mobile adsorption. The present results establish that salt-modified aluminas are effective adsorbents for the separation of the inert gases, nitrogen, oxygen, and methane at room temperature. The development of analytical separations for other inorganic gases on salt-modified aluminas, Porasils, and Graphons currently is under study.
However, if a Lennard-Jones potential is taken as a typical function for U (22),
U= Cr6
+ Br-”
(14)
then C is a function of polarizability, while B is not. Hence, use of Equations 12 and 13 causes the B value to be part of the intercept. As a consequence, the calculated values for v will be too large (as is observed). However, the v values calculated from the intercepts are close enough to the expected quantities to conclude that the constant intercepts of Figures 1 and 2 are characterized by Equation 11, the statistical model for mobile adsorption. Hence, this infers that gas-solid chromatographic interactions at alumina and porous silica can be classified as a mobile adsorption process. de Boer and Kruyer (14) also have derived equations to calculate the standard state entropies of molecules undergoing either mobile or localized adsorption. Using their equation for mobile adsorption
AS,”
=
R In (mal2T6‘*)- 1.522 log T
ACKNOWLEDGMENT
We are grateful to H. H. Schmidt of this department for helpful discussions concerning the statistical mechanical model of gas-solid adsorption chromatography, and to Takanobu Hiramatsu for the use of the data from his studies of Graphon.
+ 2.28
RECEIVED for review October 12, 1971. Accepted February 16, 1972.
(22) D. M. Young and A. D. Crowell, “Physical Adsorption of Gases,” Butterworths, Washington, D.C.,1962, p 20.
On-Line Computer-Controlled Electrical Detection in Spark Source Mass Spectrometry G . H. Morrison, B. N. Colby,l and J. R. Roth Department of Chemistry, Cornell University, Ithaca, N . Y. 14850 The design and operation of a versatile on-line computer controlled electrical detection system are described for use in spark source mass spectrometry. This system involves computer control of the mass spectrometer in either a scanning or peak switching mode with simultaneous acquisition and reduction of data using a PDP-11/20 dedicated computer. The instrumental error of the electrical detection system in the peak switching mode is shown to be 2,3% relative standard deviation.
SPARKSOURCE MASS SPECTROMETRY is generally considered to be one of the most sensitive and comprehensive techniques for the determination of trace elements in solids. The ion sensitive photographic plate which is capable of integrating the entire mass spectrum simultaneously has been used almost exclusively in the past as the ion detector. Although the photoplate has shown itself well suited for trace survey analyses involving a large number of elements, the inherent problems associated with nonlinear response, fixed gain, and time consuming data reduction procedures have recently caused a number of investigators to experiment with electrical detection systems. In 1965 Conzemius, Capellen, and Svec ( I ) reported the Present address, Materials Research Laboratory, University of Illinois, Urbana, 111. (1) R. J. Conzemius, J. Capellen, and H. J. Svec, 13th Annual Conference on Mass Spectrometry, ASTM E-14, S t . Louis, 1965.
first functional electrical detection system in conjunction with the RF spark source mass spectrometer which they later expanded upon (2,3). This system included a means of ratioing the electron multiplier output to a representative part of the total ion beam while mechanically step-scanning the electrostatic analyzer field. Bingham, Brown, and Powers (4-6) have offered an alternate system which is capable of operating in two modes: first, a continuous magnetic scan for rapid semiquantitative analysis, and second, electrostatic peak switching for quantitation of selected elements. This system requires the manual presetting of the ion accelerating voltage for each element before going into the peak switching mode. Evans, Guidoboni, and Leipziger (7) have shown how a similar peak switching system may be used for the routine analysis of metals. ~~
~~
(2) H. J. Svec and R. J. Conzemius, Fourth International Conference on Mass Spectrometry, Berlin, 1967. (3) R. J. Conzemius and H. J. Svec, Talanta, 16,365 (1969). (4) R. A. Bingham, R. Brown, and P. Powers, Pittsburgh Conference of Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1968. ( 5 ) R. A. Bingham and P. Powers, 16th Annual Conference on Mass Spectrometry ASTM E-14, Pittsburgh, Pa., 1968. (6) R. A. Bingham, P. Powers, and W. A. Wolstenholme, 17th Annual Conference on Mass Spectrometry, ASTM E-14, Dallas, Texas, 1969. (7) C. A. Evans, R. J. Guidoboni, and F. D. Leipziger, Appl. Spectrosc.,24, 85 (1970). ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1203
