270
Anal. Chem. 7984, 56,278-283
normally encountered in analytical samples. Conditions for such sample interference will, however, vary significantly with sample composition, type of adsorbent, and temperature. Because of these factors, the analyst must be aware of the possibility of distortion of the analysis caused by sample interference and the significance of the formation of a monolayer. A monolayer may be formed of one or several components; however, once a monolayer is formed, the adsorbent no longer functions as an efficient trapping material. The system will act as a vapor-liquid system with the sampling capacity determined by the liquid solubility properties of the sample components in the adsorbed component or mixture. Under normal sampling conditions the sample volume is limited to less than the breakthrough volumes of any of the components, and sample interference cannot affect the results. However, if these conditions are not met or if the desorption process is not 100% efficient, the possibility exists for severe distortion of the analytical results.
Registry No. Carbon, 7440-44-0;benzene, 71-43-2;pentane, 109-66-0; acetone, 67-64-1; nitromethane, 75-52-5; propanol, 7123-8; tetrahydrofuran, 109-99-9. LITERATURE CITED (1) Bertonl, G.; Bruner, F.; Libertl, A.; Perrino, C. J. Chromatogr. 1981, 203, 263-270. (2) von Ryblnskl, W.; Findenegg, G. H. Ber. Bunsenges Phys. Chem. 1979, 83, 1127-1130.
(3) Parcher, Jon F.; Lin, Plng J. Anal. Chem. 1981, 53, 1889-1894. (4) Lin, Plng J.; Parcher, Jon F. J. Colloid Interface Sci. 1983, 91, 76-86. (5) Parcher, Jon F.; Hyver-LoCoco, Karen J . Chromatogr. Sci. 1983, 21, 304-309. (6) Di Corcia, A.; Liberti, A. "Advances in Chromatography"; Giddings, J. C., et al., Eds.; Marcel Dekker: New York, 1976; Vol. 14, pp 305-366. (7) Bruner, Fabrizio; Ciccioll, Paolo; Crescentini, Giancarlo; Pistolesl, Maria T. Anal. Chem. 1973, 45, 1851-1859. (8) Avgul, N. N.; Klselev, A. V. "Chemistry and Physics of Carbon"; Walker, Phillip L.. Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6. (9) Brunauer, S.;Emmett P. H.; Teller, E. J. Am. Chem. SOC.1938, 60, 309-319. (IO) Parcher, Jon F.; Selim, Mustafa I. Anal. Chem. 1979, 51, 2154-2156. (11) McClellan, A. L.; Harnsberger, H. F. J. Colloid Interface Sci. 1967, 23, 577-599. (12) Wang, Jack L. H.; Lu, Benjamin C.-Y. J. Appl. Chem. Biotechnol. 1971, 21, 297-299. (13) Schrelber, Loren B.; Eckert, Charles A. Ind. Eng. Chem. Process Des. Dev. 197f9 10, 572-576. (14) Thomas, Eugene R.; Newman, Bruce A.; Nicolaides, George L.; Eckert, Charles A. J. Chem. Eng. Data 1982, 27, 233-240. (15) Thomas, Eugene R.; Newman, Bruce A.; Long, Thomas C.; Wood, Douglas A.; Eckert, Charles A. J. Chem. Eng. Data 1982, 27, 399-405. (16) Parcher, Jon F.; Lin, Ping J. J. Chromatogr. 1982, 250, 21-34.
RECEIVED for review July 11,1983. Accepted October 25,1983. Acknowledgment is made to the National Science Foundation (Grant No. CHE-8207756) and to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Solvatochromic Investigation of Polarizable Polymeric Liquids' James E. Brady, Dale Bjorkman, Christian D. Herter, and Peter W. Carr* Department of Chemistry] Smith and Kolthoff Halls, University of Minnesota] Minneapolis, Minnesota 55455
The polarlty characterlstlcs of methyVphenyl OV type GLC stationary phases (OV-101, -3, -7, -11, -17, -22, -25) and low molecular weight analogues (hexamethyldlslloxane, octamethyltrlslloxane, decamethyltetraslloxane, and bls(tr1methyls1lyl)methane) are examined via the solvatochromlc comparison method. As expected, the differentlal polarity of the OV llqulds Is due In large measure lo differential polarizablllty effects and Is well correlated wlth a reactlon fleld based model. I n addltlon, a nearly constant low level of hydrogen bondlng acceptor strength Is observed. The presence of measurable hydrogen bondlng acceptance In these llqulds has Important lmpllcatlons to the Interpretation of retention of strong hydrogen bond donors on these stationary phases In GLC. Llkewlse, the lnteractlon of strong hydrogen bondlng solutes wlth the slloxane backbone of slllca gel In llquld chromatography may be a slgnlflcant, and generally neglected, effect. I t appears the detalled analysis of the orlgln of retention of strong hydrogen bond donors on these materlals may warrant reexamlnatlon.
represent solvent dipolarity-polarizability, hydrogen bond donicity, and hydrogen bond accepting strength, respectively. The relationship between the a* scale and the shift in frequency of maximum absorbance of a solvatochromic indicator is given by the equation v = vo sa* (1) where vo is nominally the indicator's frequency of maximum absorbance in cyclohexane and s is a measure of the sensitivity of a particular indicator to changes in solvent dipolar-polarizability. T o date the a* values for over 100 aprotic solvents, using a number of judiciously selected solutes from a set of 45 indicators (2), have been determined. No investigations of high molecular weight liquids have been undertaken to this point. In this work the Kamlet-Taft solvatochromic approach is applied, for the first time, to the study of a series of polymeric liquids (see Table I) of systematically varying composition. The liquids are the siloxane polymers shown below:
+
% PHENYL
Over the past 5 years Kamlet, Taft, and their co-workers have systematized the use of solvatochromism as a basis for exploring the nature and strength of interactions between a solute and a solvent (I). This had led to the development of the a*,a,and /3 linear solvation free energy scales which 'This work is dedicated to Piet Kolthoff on the anniversary of his 90th birthday. 0003-2700/84/0356-0278$01.50/0
0 1984 American Chemical Society
ov-101 OV-3 OV-7 ov-11 OV-17
0 10 20 35 50
OV-22 OV-25
65
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
-
-
-~
_ _ I -
Table I. Physical Properties of OV Type Polymeric and Low Molecular Weight Silicone Liquids mol molar density a,c % phenyl volumea, solvent ov-101
0 10
OV-3 OV-7
20 35 50 65 75
ov-11
OV-17 ov-22 OV-25 hexamethyldisiloxane (HMS) octamethyltrisiloxane (OMS) decamethyltetrasiloxane (DMS) bis( trimethylsily1)methane (BTSM) cm3/mo1, 25 "C. a Computed from data given in ref 10. light illumination with compensation, 25 i 0.1 "C. By adjustment of the dimethyl/methylphenyl monomer ratio, one can progressively increase the mole percent phenyl content up to 50% in I, and by use of the diphenyl/methylphenyl monomers, polymers containing higher phenyl content can be obtained. This series of liquids allows the polarizability contribution to a* to be scanned in a controlled manner. In a recent study (3)we have shown it possible to reproduce a* values of some 76 liquids (since expanded to 103 liquids) ranging from nondipolar, weakly polarizable fluoroalkanes to highly polarizable aromatic liquids to very polar liquids such as Me2S0 by means of the following regression equations:
31000 20000 9800 6600 3700 7100 8700 212.5 288.5 363.8 213.3 g/mL, 25 "C.
279
0.972 0.994 1.018 1.054 1.089 1.124 1.147 0.764 0.820 0.8 54 0.752
refractive indexd 1.4036 1.4423 1.4781 1.5086 1.5381 1.5654 1.5848 1.3772 1.3848 1.3895 1.417
Measured with AbbP refractometer, white
thereby test our model that polarizability contributions to R* are proportional to L(n2).
EXPERIMENTAL SECTION The indicator compounds used were 4-nitroanisole ( l ) , 1ethyl-4-nitrobenzene (4), N,N-diethyl-4-nitroaniline (6), N,Ndimethyl-4-aminobenzophenone (7), 4,4'-bis(dimethy1amino)benzophenone (8), ethyl 4-(dimethy1amino)benzoate (9), N,Ndimethyl-4-nitroaniline (13), 4-nitroaniline (14), N-methyl-4nitroaniline (17A), 4-(dimethy1amino)benzaldehyde(19), ethyl 4-aminobenzoate (20), 4-aminobenzophenone (21), 2-nitro-ptoluidine (23), 3-nitroaniline (28), N-methyl-o-nitroaniline (32), %nitroaniline (33), 2-nitro-p-anisidine (35), p-nitrosodimethylaniline (39), and 2-nitroanisole (45). Sources of the compounds were Aldrich Chemical Co. (1, 4, 7, 8, 9, 14, 19, 21, 32, 39, 45), X * = -2.54(&0.36) 6.79(*1.46)6(~) 12.41(*1.76)~(n2)- 2 3 . 1 6 ( * 7 . i 0 ) s ( ~ ) ~ ( ~ 2 ) Eastman Chemical Co. (13,17A,20,28), and J. T. Baker Chemical Co. (23, 35). Solute 6 was generously provided by M. J. Ramlet (Naval Surface Weapons Center, Silver Springs, MD). p = 0.957 (2) The numbering system above corresponds to that established by Kamlet et al. (2). For brevity we will refer to solutes, not by or name, but by number. The indicators were purified by dissolving them in a minimal amount of methylene chloride, passing the a* = -2.16(&0.24) 5.88(*1.06)6(tp) resulting solution over activated silica gel, collecting and evapo11.43(f1.14)L(n2)- 20.86(i5.10)L(n2)6(~,) rating the methylene chloride solutions, and recrystallizing the remaining solids from hot 2-propanol. Solute 6 was used as received. The purity of each solute was checked by high-pressure p = 0.965 (3) liquid c,homatography. The OV type polymeric liquids (OV-101, where L(n2)represents the optical Onsager reaction field ( 4 ) ) -3, -7, -11, -17, -22, and -25) were obtained from Anspec, Inc., and used as received. The hexamethyldisiloxane (HMS), octaO(E,) represents the Block and Walker reaction field (5),and methyltrisiloxane (OMS), and decamethyltetrasiloxane (DMS) p is the correlation coefficient. The parameters n and E are were obtained from Aldrich; bis(trimethylsily1)methane (BTSM) the solvents' refractive index and dielectric constant. The term was obtained from Petrach Systems, Inc. (Bristol, PA). These e defined below, represents the relative permittivity free of 9' liquids were purified by passage over activated silica gel. Relevant distortional polarization effects and can be obtained from physical properties are given in Table I. fundamental considerations (6) Test solutions of the indicators in the polymeric liquids were prepared by dissolving a minimal amount (typically < 0.1 mg) tp=E-n2+l (4) in 1.0 mL of liquid. Complete solution was achieved within 5 days at room temperature. The more viscous liquids (OV-22 and Prior to the above work, Kamlet and Taft approached the OV-25) required heating to 90 OC for 24 h to achieve complete issue of polarizability contributions to the a* scale in a desolution. These same liquids also required heating in order to cidedly more ad hoc fashion (1,2). They now accept our view transfer them to a cuvette. Solutions made up from HMS, OMS, DMS, and BTMS were stored under desiccation (phosphorus that polarizability contributions to a* cannot be represented pentoxide) p t i l used to reduce the potential of solvent hydrolysis. by a simple contribution which is the same value for all alkanes All spectra were obtained on a GCA-McPherson EU-700 and a different value for all aromatics. spectrophotometer (0.4 nm band-pass) using 1-mm quartz cuIn addition to providing a direct experimental approach to vettes. Refractive indexes were measured with an Abbe refracassessing the importance of solvent polarizability contributions tometer (Bausch and Lomb) thermostated to 25 i 0.1 "C by using to the r* scale, the OV liquids studied in this work are of white light illumination with compensator. considerable practical analytical importance due to their very RESULTS A N D D I S C U S S I O N common use as stationary phases in gas-liquid chromatography. The goal of this work was to examine the O V series The properties of the solvatochromic indicator dyes used of liquids in terms of both solvatochromism and chromatomay be found in Kamlet et al. (2). This paper gives the slopes graphic data so as to better understand these important ($1 and intercepts (vo) obtained in correlations against the materials and to be better able to interpret their effect on defining solvents. For purposes of this discussion it is imchromatographic separations. In addition, these liquids allow portant to note that solute dyes 1, 3, 4, 6, 7, 8, 9, 13, 19, 32, us to systematically vary the polarizability of a liquid and 39, and 45 are unable to donate hydrogen bonds (protons) and
+
+
+
+
280
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984
__
l__--l---______
Table 11. Apparent n* Indexes for OV Liquid Phases solvent
n*
ov-101 OV-3 OV-7
45
0.06 0.21 0.30 0.44 0.52 0.53 0.56 0.09
ov-11
OV-17 ov-22 OV-25 squ a1an e
Solvent 21 is excluded. puted from eq 3. a
fl* I
0.06 0.22 0.34 0.50 0.57 0.60 0.6 7
n*,
n*
0.08 0.25 0.35 0.51 0.61 0.61 0.64 0.18
0.09 0.27 0.39 0.55 0.62 0.62 0.67
32
std devb
n*ava
n*21
0.20 0.07 0.40 0.24 0.50 0.35 0.68 0.50 0.76 0.58 0.78 0.59 0.82 0.64 0.11 0.11 0.13 0.12' Standard deviation of the n* values from the mean.
?i
0.02 0.03 0.04 0.05 0.05 0.04 0.05 0.04'
0.13 0.16 0.15 0.18 0.18 0.19 . 0.18 0.01
Solute 21 is not excluded.
All OV liquids. based on eq 3 with a
E
std dev
std dev
intercept
slope
of intercept
of slope
-1.96 -2.34 -1.81 -1.85 0.12 2.02 1.95 4.43 4.32
10.51 12.32 9.92 10.3 2 0.097 -1.271 -1.240 -12.98 -12.75
0.17 0.07 0.50
0.77 0.33 2.00 2.00 0.023 0.112 0.094 2.07
1
Excluding OV-22 and OV-25. = 2.3 and n = 1.3848.
Com-
_-
--
Table 111. Results of Linear Least-Squares Analysis correlated data
0.07 0.20 0.30 0.41 0.50 0.58 0.63
0.50 0.01
0.05 0.04 0.47 0.41
corr coeff 0.987 0.999 0.887 -0.981 -0.986 -0.942 -0.953
1.81
Estimated results based on eq 2 with
therefore can be used to determine the a* values of hydrogen bond acceptor (HBA in the Kamlet-Taft notation) solvents. We will refer to these indicators as class I solutes. In contrast, solutes 14,17,20,21,23,28,33, and 35 are capable of donating hydrogen bonds, though to different degrees. These will be termed class I1 solutes. The results of the solvatochromic investigation of the OV liquids are summarized in Table 11. In this study we chose to carry out a complete matrix of tests with solutes 1, 7, 32, and 45 from class I and solute 21 from class 11. Application of a two-way block design analysis of variance (ANOVA) (7) to these data indicates the existence of significant differences between solutes. The F ratio between solutes is 91.5; this is statistically significant at an extremely high level of confidence. Upon exclusion of solute 21 the mean variance between solutes drops by nearly a factor of 6 while the estimate of the random error scarcely changes. Similarly, a t test between the average A* value of any solvent obtained with all solutes excluding 21 and the A* value obtained with solute 21 indicates a significant difference at greater than the 99% confidence level. Solute 32 cannot be separated from the others on the basis of a t test for any solvent other than OV-101. Since the A* scale is not defined with complete precision, we conservatively believe that only solute 21 should be viewed as showing systematic differences with respect to the average of the other solutes. The magnitude of the deviation is indicated in Figure 1,a plot of the average a* values and that discerned by solute 21 vs. the mole percent of phenyl groups in the polymer. We will return to the issue of hydrogen bonding interactions later. As shown in Figure 1there is a clear-cut relationship between the average R* value of the methylphenyl silicon oils and the mole percent phenyl groups. Recently we indicated (3) correlations between the A* values of a wide variety of solvents and the Block and Walker reaction field function were much improved when one considers A* to be a weighted measure of a solvent's dipolarity and polarizability as indicated by eq 2 and 3. This constituted a considerable improvement over the original Kamlet-Taft formulation which distinguishes only three classes of solvents in terms of their polarizability
__
E
=
2.3.
Estimated results
b
0.90
21
0.80
0.70 0.60
* t=
0.50 0.40
0.30
0.20 0.10
b/
0.OOl
0
I
1
20
I
' 40
'
I
60
I
1
80
' 100
% PHENYL Flgure 1. Plot of a* vs. mol % phenyl for OV liquids.
contribution to a*. As shown in Figure 1,even though there is little difference in dipolarity of a methyl and a phenyl group, the A* value of the series of solvents increases with the relative content of phenyl groups. Figure 2 indicates the change in a* is not due to an enhanced polarity of the siloxane backbone of the polymer but rather to the increased polarizability of the medium since the A* value is extremely well correlated with the optical Onsager reaction field function, which is used to represent polarizability contributions to a*. When the data for OV-22 and OV-25 are excluded from the least-squares fit, the correlation improves considerably although the slope is not seriously altered (see Table 111). We believe the distinct behavior of OV-22 and OV-25 arises from the use of diphenyl monomer units in these two liquids. An experiment analogous to that indicated by Figure 2 could be carried out by mixing low molecular weight alkane and aromatic solvents; however, as pointed out repeatedly by Kamlet and Taft (8) and others (9),mixed solvent behavior is much more complex due to the propensity of the strong
0.50
rn
0.40
-
0.30
-
0.60 0.70
*
I=
.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY
I 1
$ 1
,/
20 1
-
V
I
1 a
I
//
0 n
W
-000
0 20
021
0 22
0 23
0 24
8
v
0 25
(n2-1)/(2n2+1)
’
Flgure 2. Plot of no vs. the optical Onsager reaction field (n2- 1)/(2n2 4- 1). For results see Table 111.
solvent to “localize” about the solute in the cybotactic region. In essence, the dielectric environment local to the solute would be much different than that implied by bulk values of n or t. The use of polymeric liquids of controlled composition avoids this complication by imposing a significant barrier to the independent localization of specific substituents in the vicinity of the probe solute. The intercept and slope of the line shown in Figure 2 are in agreement with the previously developed generalized model of n* based on eq 2 and 3. If we approximate the dipolarity contribution of the siloxane backbone as being equal to that given by octamethyltrisiloxane (t = 2.3; O(t) = 0.131; O ( t J = 0.053), the intercept and slope of the plot in Figure 2 are predicted to be -1.8 f 0.5 and 10.0 f 2, respectively (see Table 111). The very significant dependence of n* cn solvent polarizability and the accuracy of eq 2 and 3 to represent these dependencies can be most simply tested in the following fashion. If we assume that the dielectric constant of octamethyltrisiloxaneprovides a good estimate of the local dipolar environment of the polymeric liquids, then eq 3 leads to the following equation for the difference in the P* value for any OV liquid and that of OMS:
“*ov - “*OMS = A(L(n20v)- L ( n 2 0 ~ s ) ) where A is a lumped parameter given below
(5)
A = 11.43 - 20.868(to~s)= 10.32 (6) As shown in Table IV the T* value of OMS is virtually zero. Thus, eq 5 and 6 may be used to compute n*ov given only its refractive index. The results are shown in the last column of Table 11. The overall average discrepancy is less than O.O3n* units which is approximately the error of measurement. The coefficients used in eq 6 were obtained from our previously reported “all” solvents general correlation and were not obtained from the present data. HMS, OMS, DMS, and BTSM were not included in that data set. This constituted a more stringent test of the model. These results clearly reinforce the accuracy of our generalized model of n* and the importance of including a “cross term” in both types of reaction field functions. In order to verify that the unique behavior of solute 21 was due to hydrogen bonding phenomena, an additional series of solutes were run with OV-101 (permethylsilicone oil), HMS, OMS, DMS, BTSM, and squalane as test solvents. The results are summarized in Tables I1 and IV. In the case of squalane, a saturated hydrocarbon, the difference between the average
4 1
I
tI
i
I
I
i
8 I
t - M b
9 0 0
I
0
0
0
282
ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984 !
I 2.00
1.80 h
1.80
1 0 Y
1.40
v
b
0" Q
1.20
3
0l o80 o0 0 0
012
I a00
I
L
mo
0.20
am
OAO
aw
0.60
Gvg
Flgure 3. Plot of a'*,
- asavvs.
For analysis see Table 111.
a* and a* obtained with solute 21 is only 0.01. No hydrogen bonding effects are indicated, nor would they be expected. A t test indicates that the a* values obtained with class I1 solutes are significantly more positive than those obtained with the class I solutes for OV-101, HMS, OMS, and DMS. Further, there is no distinguishable difference between the class I and class I1 a* values for BTSM, the nonoxygenated analogue of HMS. This establishes the inherent equivalence of a* values determined for NHB solvents, using either class I or class I1 solvents. It is clear that the hydrogen bonding effect is very much stronger with OV-101 (both effects exist at greater than 99% confidence intervals). The hydrogen bonding characteristics of the OV liquids can be further characterized. Close inspection of the data in Table I1 indicate that the discrepancy between the a* value obtained with solute 21 and the average of all other solutes tends to increase as a*ovincreases. A least-squares analysis of the data shown in Figure 3 indicates that there is a real effect (correlation coefficient = 0.89). As the polymer becomes more highly substituted with phenyl groups, Le., as a* increases, the polymer becomes a stronger hydrogen bond acceptor. This is in contrast t Q other findings that aromatic ethers are marginally weaker hydrogep bond acceptors than are alkyl ethers (1). The results obtained for the low molecular weight siloxane liquids, HMS, OMS, and DMS, indicate the hydrogen bond strength is very dependent on the number of siloxane monomer units in the molecule. We infer from Table IV that the dependence of hydrogen bond strength on chain length is very weak for solvents higher in the series than DMS. The occurrence of hydrogen bonding interactions between strong donor species and siloxane polymers (and, by extension, the silica gel matrix employed almost exclusively in highpressure liquid chromatography) has obvious implications to a detailed mechanistic interpretation of retention in these systems. There is a great deal of thermodynamic information available on the partitioning of small solutes from the gas phase into these polymeric liquids since such data are very useful in examining the gas-liquid chromatographic properties of the solvents. Recently Parcher and co-workers (IO)carried out a careful series of measurements in which the free energy of transfer for the process was measured solute, gas + solute, liquid (7) for the OV phases used in this work.
010
020
030
040
050
080
070
Anal. Chem. 1984, 5 6 , 283-288
(12) Langer, S.H.; Sheehan, R. J.; Huang, J.-C. J. Phys. Chem. 1882, 86, 4605-4618. (13) Karger, B. L.; Snyder, L. R.; Eon, C. J. Chromatogr. 1978, 125, 71-a~. (14) Liptay, W. "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1974;Vol. 1, pp 129-229.
(2) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. J. Am. Chem. SOC. 1877, 99,8027-6038. (3) Brady, J. E.; Carr, P. W. J. Phys. Chem. 1982, 86, 3053-3057. (4) Onsager, L. J. A m . Chem. SOC.1938, 58, 1486-1493. (5) Block, H.; Walker, S. M. Chem. Phys. Lett. 1873, 19, 363-364. (6) Reitz, J. R.; Milford, F. J.; Christy, R. W. "Foundatlons of Electromagnetic Theory", 3rd ed.; Addison-Wesley: Reading, MA, 1979; pp
86-87. (7) Box, G. E. P.; Hunter, W. G.; Hunter, J. S."Statistics for Experiments"; Why-Interscience: New York, 1978;Chapters 6-7. (6) Kamlet, M. J.; Kayser, E. G.; Jones, M. E.; Abboud, J.-L. M.; Eastes, J. W.; Taft, R. W. J. Phys. Chem. 1978, 82, 2477-2483. (9) Nitsche, K A . ; Suppan, P. Chimia 1882, 36, 346-348. (10) Parcher, J. F.; Hansbrough, J. R.; Koury, A. M. J. Chromatogr. Sci. 1870, 16, 183-189. (11) Conder, J. R.; Young, C. L. "Physlcochemical Measurement by Gas Chromatography"; Wiley: New York, 1979;Chapter 5.
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RECEIVED for review June 13,1983. Accepted October 3,1983. J. E. Brady was supported by an A.C.S. Analytical Divisional Full Year Fellowship sponsored by the Upjohn Company. This work was supported in part by grants from the National Science Foundation (CHE-8205187)and the 3M Co. (St.Paul, MN).
Analytical Performance of a Low-Gas-Flow Torch Optimized for Inductively Coupled Plasma Atomic Emission Spectrometry Akbar Montaser,* G . R. Huse, R. A. Wax, and Shi-Kit Chan Department of Chemistry, George Washington University, Washington, D.C. 20052
D. W. Golightly, J. S. Kane, and A. F. Dorrzapf, Jr.
US.Geological Survey, 957 National Center, Reston, Virginia 22092
An Inductively coupled Ar plasma (ICP), generated in a lowflow torch, was lnvestlgated by the simplex optimization technique for simultaneous, multieiement, atomlc emission spectrometry (AES). The variables studied included forward power, observation height, gas flow (outer, intermedlate, and nebulizer carrler) and sample uptake rate. When the ICP was operated at 720-W forward power wllh a total gas flow of 5 L/min, the signal-to-background ratios ( S I B ) of spectral llnes from 20 elements were either comparable or inferior, by a factor ranglng from 1.5 to 2, to the results obtained from a conventional Ar ICP. Matrlx effect studles on the Ca-PO, system revealed that the plasma generated in the low-flow torch was as free of vaporizatlon-atomization interferences as the conventional ICP, but easily Ionizable elements produced a greater level of suppresslon or enhancement effects which could be reduced at higher forward powers. Electron number densltles, as determlned via the series limit line merglng technlque, were lower In the plasma sustained In the low-flow torch as compared with the conventional ICP.
A number of promising plasma sources are currently being used in analytical atomic spectrometry. They are inductively coupled plasmas (ICP) ( I ) , direct current plasmas (DCP) (1, 2), and microwave-induced plasmas (MIP) (I,3-5). Among these sources, argon-supported ICPs are excellent vaporization-atomization-excitation-ionization sources which are commonly employed for analytical atomic emission spectrometry (AES). Although the Ar ICP-AES method exhibits superior analytical performance for elemental analysis of a variety of materials ( I ) , it has disadvantages of requiring a relatively high rf power, a lot of laboratory space, and high argon gas flows. With reference to the rate of gas consumption, it is important to note that the conventional ICP torch requires 15-22 L/min of argon. If the ICP is run for 40 h per week, the estimated gas cost approaches $10 000 to $12 000
per year. The relatively high operating costs and the initial cost of an ICP instrument thus can be considered as one of the impediments to the acceptance of ICP-based methods. Furthermore, because of the cited limitations, the commercially available ICP-AES instruments presently are not applicable to analysis which has to be conducted in a mobile laboratory or a ship. T o reduce the argon gas flow and the input power requirements of the ICP, a number of investigators have explored the use of water-cooled torches (6-9) or torches cooled externally by compressed air (9,10) or have reduced the diameter of the gas introduction nozzle, the annular spacing between the intermediate and the outer tube, or the actual torch size (11-1 7). Although the resulting plasmas apeared stable, they suffered from the disadvantages of exhibiting inferior detecting powers and enhanced interferences, or they required a modification of the load coil and the impedance matching network (7-1 7). Evidently, what would be desirable is an ICP torch of the size (18 mm i.d.) commonly utilized in analytical laboratories, but with internal dimensions designed to allow operation at reduced input power and gas flow levels, while a t the same time preserving the excellent analytical performance of the conventional ICP torches. Recently, i t has been possible (18-20) to operate an 18 mm i.d. torch at a total gas flow and a forward power of 5 L/min and 450 W, respectively. However, the analytical performance of this torch for the elemental analysis of a variety of samples has not yet been critically documented. In the present study, the simplex technique (21-32) is utilized to optimize the plasma sustained in a low-flow torch. By use of a suitable objective function (32) optimization is conducted simultaneously for many elements while the gas flow levels (outer, intermediate, and nebulizer aerosol carrier gas flows), forward power, observation height, and sample uptake rate are changed simultaneously. After the optimum conditions are established, the analytical capabilities of the low-flow torch are compared to those of a conventional torch for atomic emission spectrometry. This evaluation includes
0003-2700/84/0356-0283$01.50/00 1984 American Chemical Society
