950
Anal. Chem. 1988, 60, 950-958
Table I. Active/Inactive Enantiomer Ratio Data for Misoprostol Samples
59122-50-8; (11R,16R)-misoprostol, 59122-48-4; misoprostol, 59122-46-2.
LITERATURE CITED
area ratio"
areas x mean (llR,16S)/ sample/inj llS,16R llR,16S llS,16R llR,16S (llS,16R) control/l chem/2 /3
7.391 8.012 8.706
/4
6.589
degraded/l
9.218 8.573 10.275
chem/2 /3
7.536 7.664 7.471 6.202
7.674
7.219
0.94 f 0.08
8.673 8.179 9.094 7.696
9.330
8.410
0.90 f 0.07
14 9.256 The reported result is the average ratio value. Confidence intervals are calculated at the 90% level. 0.90, respectively. The data indicate that the relative levels of the active and inactive enantiomers in the control and degraded misoprostol samples do not change. Registry No. (11R,16S)-Misoprostol, 59122-49-5; (11S,16R)-misoprosto1,59122-51-9; (11S,16S)-misoprostol,
Armstrong, D. Anal. Chem. 1987, 59, 91A. Dappen, R.; Arm, H.; Meyer, V. J . Chromatogr. 1986, 373, 1. Lochmuller, C.;Souter, R. J . Chromatogr. 1975, 773, 283. Wainer, I. Chromatogr. Forum 1986, Nov-Dec, 55. Davankov, V.; Kurganov, A.; Bockiov, A. Advances in Chromatography; Giddings, J., Gruska, E., Cazes, J., Brown, P., Eds.; Marcel Dekker: New York, 1983; Voi. 22, p 71. (6) Armstrong, D. J . Lip. Cbromatogr. 1984, 7(Supp. 2), 353. (7) Hermansson, J. J . Chromatogr. 1983, 269,71. (8) Hermansson, J. J . Chromatogr. 1984, 298, 67. (9) Schill, G.; Wainer, I.; Barkan, J. J . Cbromatogr. 1986, 365, 73. (IO) Bishop, R.; Hermansson, I.; Jaderland, B.; Lindgren, G.; Pernow. C. Am. Lab. (Fairfield, Conn.) 1986, March, 138. (1) (2) (3) (4) (5)
Daryl A. Roston* Ranmali Wijayaratne Searle Research and Development 4901 Searle Parkway Skokie, Illinois 60077 RECEIVED for review September 16, 1987. Accepted January 11, 1988.
Gas Phase Retention Volume Behavior of Organic Compounds on the Sorbent Poly(oxy-m-terphenyl-2',5'-ylene) Sir: There has been considerable interest in the gas phase retention volume behavior of organic compounds on the sorbent Tenax-GC (poly(oxy-rn-terpheny1-2',5'-ylene)) (1-10). This has been due to the need to select the compound-dependent volumes of gas that can be sampled with little breakthrough at a given temperature with a particular amount of sorbent. Ambient and workplace air, stack-gas effluents, and gas streams in purge and trap (P&T)concentrators for water analysis have all been sampled with Tenax-GC. In addition to a compound's retention volume, knowledge of the chromatographic efficiency (i.e., number of theoretical plates) for that compound on the trap is also needed to predict the actual breakthrough curve (2,5,11-13). However, regardless of what the chromatographic efficiency is, a significant fraction of the influent concentration will always be breaking through when the sample volume exceeds the retention volume. Although a fair amount of retention volume data have been generated for Tenax-GC, additional analysis of that data is needed to help predict the behaviors of compounds of interest that have not as yet been studied. The correlation of retention volumes at 20 "C with boiling point temperatures was examined by Gallant et al. (4) and Brown and Purnell (5). This paper reviews (1)the basis for expecting physical constants like boiling point and vapor pressure to correlate with retention volume values and (2) a variety of such correlations for Tenax-GC. Only retention volume data from simple single-component systems will be considered. For example, although the presence of water vapor is known to have some effects on retention behavior ( I ) , only retention data obtained in water-free systems will be considered. THEORY Retention Volumes. The net retention volume VN (L) corrected for both dead volume and compressibility in an isothermal chromatographic system at temperature T (K) is
determined by the temperature-dependent equilibrium constant @(T) (L/g) for partitioning between the gas and stationary phases. When VN is normalized by the weight w (g) of stationary phase in the system, the result equals (I(14),and P ( T ) = c,/cg =
v,/w
(1)
where c, (mol/g) and c, (mol/L) are the stationary phase and gas phase concentrations, respectively. If t ' is the adjusted retention time, T, (K) the ambient temperature at the column outlet, j the gas compressibility factor, F the flow rate at T,, and V, the specific retention volume at 273.16 K, then (14) P ( T ) = V&T= t ' ( T / T , ) . P / w
(2)
V , = P ( T ) (273.16/T)
(3)
@(T)is noted above as being equal to V,p This is to emphasize that it is a type of specific retention volume (Le., the one where the volume is that occupied at T). It can be shown (15) that
d log Vg/d(l/T) = d log ( V g , T / T ) / d U / n= AHJ2.303R (4) where R = 0.001 99 kcal/mol. For gas/solid chromatography (GSC), AH, (kcal/mol) is the enthalpy of desorption from the surface with the subsequent formation of gaseous sorbate in the typical gaseous standard state of fugacity = 1atm. A plot of log V, vs 1/T has a slope of AH8/2.303R. It is the normalization by T inherent in V, that provides this desired slope. Plotting log Vg,Tor log V, vs 1 / T as some have done (3-6), on the other hand, yields a slope of (A", - RT)/2.303R (15). The value of R T is not a very strong function of T in the range T = 293-373. Therefore, when A",is approximately constant over the temperature range of interest, then both AH8/2.303R and (A", - RT)/2.303R will also be approximately constant. The quantity (AH8- R7') is the enthalpy of desorption for the
0003-2700/88/0360-0950$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
a.
All Compounds
44
1
-
: i r _ r b. Most
951
where a is a fractional efficiency (often taken to be 11,and to is a molecular vibration time s). The value of R is 0.001 99 kcal/(mol deg) in the numerator, and 8.314 X lo7 (g cm2)/(s2mol deg) in the denominator. When cg is low enough that a linear Langmuir isotherm is followed (Le., 1 >> b ~ / 7 6 0 )then , by eq 6, 8, and 9, for NB= 6X mol/cm2
Compounds
log Vg,T= 4.6
+ log [ c Y ~ ~ ~ ( T / M+ )AH8/0.0046T ~/~] (10)
-13
0
6
j
C. Alcohols
1
2
3
2
I
d.
Amines
5:
1 3
I
I
4-
The value of log MI2will vary only weakly from compound to compound. If atOis also approximately constant, a plot of log Vg,Tvs AH, will then be linear. However, AHBis often measured using eq 4, and so it will generally not be known for a compound unless Vg,Tis also already known near the T of interest. Thus, eq 10 will not be very useful in predicting unknown Vg,Tvalues based on measured AHBvalues. A more useful form for bL is needed. Correlation of log Vg,Twith log p O , pThe development of the Brunauer, Emmett, and Teller (BET) isotherm equation leads to (17, 18)
(11)
log po.293
log po.293
Figure 1. log Vg,2g3 vs log Po.293 for data of Brown and Purnell (5): (a) all data; (b) selected data: (c) alcohols only: (d) amines only.
formation of gaseous sorbate in a standard state of 1mol/L. It is also the change in internal energy (AI?,)for desorption with the production of the 1 atm standard state (3). Langmuir Sorption. When a gas/solid equilibrium is governed by a Langmuir sorption process (i.e., one that is site limited), then c, = ON,A(i04 cm2/m2)
where AHv is the enthalpy of vaporization of the pure liquid compound at T, pO,T(Torr) is the saturation vapor pressure of the liquid compound at T , and c is the BET isotherm parameter. The quantity (AH, - AHv) is termed the net enthalpy of desorption. It is one measure of how different vaporization from the surface is from the vaporization of the species of interest from its own, pure liquid. The other measure is the factor (albz/a2bl) which is a function of the net entropy of desorption, which increases as the entropy of the sorbed-state increases. Brunauer et al. (18)assumed that in many cases (albz/azbl) N 1. In the linear range of the isotherm, by eq 6,8, and 11, we then have
(5)
log
where 8 is the fraction of occupied sites, N , (mol/cm2)is the surface concentration of sorption sites, and A (m2/g) is the specific surface area of the sorbent. For an ideal gas, cg = pf760RT where p has units of Torr, and R = 0.082 (L atm)/(mol deg). Therefore
5.8
Vg,T= ON,A (lo4 cm2/m2) 760RT/p
(6)
For nonspecific, monolayer sorption where a sorbing molecule needs only the requisite surface space to allow it to deposit, N , depends only on the size of the molecule, and so
N,
N
[d/MNo1/2]2/3
(7)
where d (g/cm3) and M (g/mol) are the compound's density and molecular weight, and No = Avogadro's number. For a compound with d = 0.9, M N 80 g/mol, then N , N 6 X mol/cmz. For gas/solid Langmuir sorption, the isotherm equation for 8 is (16) b~p/760 O=
1
+ b~p/760
(8)
where bL (atm-') is a compound- and temperature-dependent constant. A kinetic analysis (16) leads to ~ i t ~ ( 1 . 0 1X3 lo6 g/(cm s2 atm)) exp[AH,/RT] bL =
N,(ZTMRT)~/~
(9)
vg,T
If N , log
=
+ log N,AT + (AH, - AHv)/0.0046T - log PO,T N
6
X
= -0.96
vg,293
(12)
mol/cm2 and T = 293 K, then
+ log A + 0.74(AH, - AHv)- log ~ 0 , 2 9 3(13)
If a given compound is a solid at the temperature of interest, the value of pO,293 used should be that of the subcooled liquid (IO),and AHvis the enthalpy of vaporization of the subcooled liquid. When a series of compounds exhibit similar values of (AHs - AHv) on a given solid, then a t a given T , values of log Vg,T for the series will tend to correlate with the corresponding log pO,Tvalues with a slope of -1.0. It may be noted that the situation is completely analogous in gas/liquid chromatography (GLC). In that case, log Vg,Twill correlate with -log pO,T (19-22) when the activity coefficients of the compounds in the liquid stationary phase (which depend upon the corresponding net enthalpy and entropy values) remain approximately constant (ref 14, eq 3.13 and 3.55). The value of A for Tenax-GC has been measured as 19 m2/g (23). However, for this work it may be more appropriate to use 6.4 m2/g since that is the value that can be deduced using isotherm data for benzene on Tenax-GC (7) together with eq 5 and 7 . (The smaller surface area as measured with benzene could be due to its exclusion from very small pores open to the small gases typically used in BET area determinations.) When (AHs- AHv)= 0, the enthalpy of the sorbed state is equal to that of the pure liquid state. For "liquidlike"
-
992
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Table I. Specific Retention Volume Values on Tenax-GC at 293 K ( Vp,291, L/g) together with Other Pertinent Values) Thermodynamic Data (References Given Pertain to Vg,293 compound
ref
vg,293
1 2 4 5
3 5 3 dichloromethane 5 3 chloroform 5 3 carbon tetrachloride 5 5 chloroethane 5 1,l-dichloroethane 4 1,2-dichloroethane 5 3 l , l , 1-trichloroethane 5 5 1,1,2-trichloroethane 1,1,1,2-tetrachloroethane 5 1,1,2,2-tetrachloroethane 5 5 vinyl chloride 5 1,l-dichloroethene 5 1,2-dichloroethene 3 trichloroethylene
-0.90 -1.11 -0.22 0.60 0.37 1.28 0.59 1.79 -0.11 1.14 1.37 1.74 0.83 1.59 2.54 2.89 3.23 -0.51 0.33 1.03 1.02
phenol o-cresol m-cresol
4 4 4
3.39 4.00 4.07
-0.35 -0.52 -0.92
182 191 202
methyl acetate ethyl acetate propyl acetate butyl acetate
5 5 5 5
0.73 1.55 2.28 2.93
2.17 1.81 1.37 0.95
57 71 102 125
acrolein acetaldehyde acetone methyl ethyl ketone
5 5 5 2 4 5
0.68 0.09 0.73 1.61 1.34 1.52
2.34 2.88 2.21 1.88 1.88 1.88
methanol ethanol
5 1 4 5
1-propanol
1
2-propanol 1-butanol
4 5 5 4
-1.40 0.06 -0.04 0.27 0.76" 0.76 0.93 0.63 1.64
1.96 1.61 1.61 1.61 1.17 1.17 1.17 1.49 0.75
4 5 4
0.51 0.89 1.24
1.07 1.07 0.36
5 5 5 4
-0.41 0.43 0.93 1.43 2.29 2.87
3.29 2.87 2.34 1.86 1.54* 0.81
n-heptane n-octane n-decane n-dodecane benzene
chloromethane
acetic acid 1-propionic acid methylamine ethylamine n-propylamine n-butylamine n-pentylamine n-hexylamine
4
4
2.75 2.03 2.03 1.52 1.02 1.02 0.11 -0.69 1.76 1.76 1.76 1.76
Hydrocarbons 36 benzene 69 to1u en e 69 98 xylenes 125 p-xylene 125 ethylbenzene 176 n-propylbenzene 219 cumene 80 trimethylbenzene 80 styrene 80 a-methylstyrene 80 butadiene
0.66 1.41 1.52 2.25 2.28 2.89 3.49 5.34 1.67 1.92 1.78 1.79
n-pentane n-hexane
5 4 5 5 4 5 4 4
log
Halogenated Aliphatics and Aromatics -24 trichloroethvlene 3.54 -24 3.54 tetrachloroethylene 2.45 40 bromomet hane 2.45 40 trichlorofluoromethane 2.14 62 2.14 62 77 dichlorodifluoromethane 1.90 77 chlorotrifluoromethane 1.90 allylchloride 12 2.95 fluorobenzene 57 2.20 chlorobenzene 84 1.78 84 1.78 74 1.98 bromobeniene 74 1.98 o-dichlorobenzene 114 1.25 m-dichlorobenzene 130 1.10 p-dichlorobenzene 146 0.68 -14 3.53 2.71 32 1,2,4-trichlorobenzene 55 2.30 87 1.76 Phenols p-cresol rn-chlorophenol 2,4,6-trichlorophenol Esters isopropyl acetate methyl acrylate ethyl acrylate
Aldehydes and Ketones 53 methyl isobutyl ketone 21 3-methyl-2-butanone 56 cyclohexanone 80 2-heptanone 80 4-heptanone 80 acetophenone 65 78 78 78
Alcohols 1-butanol 2-methyl-2-propanol 2-methylpropanol
97
97 97 82 118
1-methylpropanol 1-octanol allyl alcohol
Acids and Anhydrides 118 1-butanoic acid 118 1-pentanoic acid 141 acetic anhydride Amines benzylamine 17 di-n-butylamine 48 tri-n-butylamine 78 pyridine 104 aniline 120 -6
7 4 5 5 4 4 4 5 5 5 5 5
1.82 a.90 2.59 3.49 2.58 2.92 3.18 3.66 4.14 3.49 4.09 0.09
1.76 1.32 1.32 0.76 0.81 0.87 0.41 0.52 0.23 0.74 0.34 3.20
80 111 111 141 137 136 159 152 170 145 167 -4
4 5 5 5 3 5 3 3 5 4 4 5 6 4 10 10 4 6 10 6
1.95 1.76 2.69 0.03 0.55 -0.51 -1.27 -2.15 1.00 1.95 3.37 2.42 3.38 3.92 3.99 4.10 4.24 3.97 4.34 5.20
1.76 1.76 1.14 3.06 2.77 2.77 3.10 4.33 2.48 1.75 0.96 0.96 0.96 0.50 0.03 0.23 0.16 0.16 0.16 -0.49
87 87 121 4 24 24 -28 -81 45 85 131 131 131 156 181 173 174 174 174 213
4 6 6
4.15 5.04 6.23
-0.96 -0.66 -1.62
202 214 246
5 5 5
1.79 1:81 2.38
1.65 1.81 1.44
90 81 100
5 4 4 4 4 4
2.43 1.81 3.13 3.74 3.51 4.09
0.71 1.33 0.53 0.00 -0.08 -0.46
118 94 156 152 144 202
5 2 4 4 5 4 5 5 5
1.71 -0.13 -0.15 1.46 1.45 1.27 1.33 4.14 0.97
0.75 1.43 1.43 0.96 0.96 -1.03 1.26
118 82 82 108 108 99 99 180 96
4 4 5
2.02 2.74 0.89
-0.09 -0.70 0.56
164 187 140
4 4 4 5 5
3.20 3:28 2.69 1.89 3.33
-0.24 0.76c -0.15 1.15 -0.21
185 160 215 116 184
1.11 1.11
953
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
Table I (Continued) compound
ref
log
2 5 5 5
acetonitrile acrylonitrile epichlorohydrin
log PO,Ze3
vg,293
0.60 0.59 0.84 2.14
bp,
ref
Miscellaneous 82 ethylene oxide 82 ethyl mercaptan 78 nitrobenzene 117
1.84 1.84 1.93 1.07
Value a t 298 K. *Value a t 299 K.
compound
OC
log
5 5 5
vg,293
log P0,293
bp, "C
3.08 2.59 -0.66
14 35 211
-0.34 0.84 5.14
Value for diisobutylamine.
Table 11. Correlation Equation Papameters for Gas Phase Retention Volume on Tenax-GC as Functions of p0193(Torr), (K),and AH,(kcal/mol)
compound type
ref
figure
mixed mixed alcohols alcohols amines mixed halocarbons
5 4 5 4 5
lb 3b
log v@,293 vs 1% P0,293 slope intercept -1.44 -1.24 -1.42 -2.34 -1.00 -1.14
IC
3c Id 5a 5a
6 3
4.30 3.92 2.73 3.57 3.13 4.39 2.94
-1.16
r2
figure
log slope
0.96 0.87 0.99 0.90 0.99 0.97 0.87
2b 4b 2c 4c 2d 5b 5b
0.0258 0.0240 0.0369 0.0487 0.0183 0.0269 0.0179
Tb
intercept
1.2
-7.43 -6.82 -12.59 -17.18 -5.09 -7.41 -5.48
0.97 0.89 0.99 0.92 0.98 0.96 0.97
compound type
ref
figure
slope
A", intercept
r2
all
2, 4-7, 10, 31
6
0.414
-4.27
0.91
1%
Boiling Point $23
vg,293 v8
273
323
373
(K) 423 473
(K)
Boiling Point 523
v,,Ze3 vs
Tb
One form of the integrated Clausius-Clapeyron equation is
3 -523
(24)
log PO,T = -218.5AHq/T
+B
(15)
where B is compound-dependent and is assumed to remain constant over the temperature range of interest. (It may be noted that 218.5 = 1000/2.303R.) When pO,T = 760 Torr, then T = Tb, and -218.5AHq = (log 760 - B)Tb
(16)
On combination of eq 12, 15, and 16 1%
vg,T
5.8 d.
Amines
", - AHq + + log N,AT + A0.00462'
(B - log 760)Tb T
-B (17)
and log flg,Tmay be expected to correlate positively with Tb when (A€&- AHv) and B are roughly constant from compound to compound. For m a n y non-hydrogen-bondingcompounds, by Trouton's rule, E N 7.7 (ref 14,eq 3.30). If N , = 6 X mol/cm2, T = 293 K, and B = 7.7, then
/
43
=
log
Vg,293
= -8.7
+ log A + 0.74(AH8- A",)+ 0.016Tb (18)
-io
-2
I
,
,
,
d
#
!
#
sb
I
~
~
~
150
100
Boiling Point
I
260
('C)
/
,
! ~
- 2 ' , m ;
250 - L O
d
I
,
,~,
I
1
8
I !
>
#
160
Boiling Point
I
, a , ,
,~
lL0
,
260
,~ I
,
(OCZ
Flgure 2. log Vg,293vs T , for data of Brown and Purnell (5): (a) all data: (b) selected data; (c) alcohols only; (d) amlnes only.
sorption on Tenax-GC then, eq 13 reduces to log
vg,293
=
- log P0,293
,
For liquidlike sorption and if A = 6.4 m2/g, then
~
~
~
~
240
(14)
Correlation of log Vg,Twith T,,. Conditions may also be found for when log V{,T will correlate with Tb, the boiling point temperature in kelurns a t a total system pressure of 1 atm.
log
Vg,293
= -7.9
+ 0.016Tb
(19)
For constant (A", - A",),the slope of a plot Qf log Vg,293 vs Tb wdl be -0.016, independent of the sorbent. The same slope is predicted for the analogous plot in GLC (ref 14, eq 3.30). Although different classes of compounds do exhibit minor differences in slopes on a given liquid stationary phase, slopes near 0.013 in GLC are very common (25-27). When eq 15-19 are used, it is obviously important to remember that both T and Tb are in kelvins.
054
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988 ' T y i o m p o u n d s
,
2j
5-
.
b.' Most
Compounds!
1
-3
43
a.
Ail Compounds
Compounds
*
!
.. . k l
- -1
- 2 m - F - T - 4 log Po293
T+-2--
log P o 2 9 3
Flgurr 3. log V,,,, vs log po,283for data of Gallant et al. ( 4 ) : (a) all data; (b) selected data; (c) alcohols only; (d) amines only.
ANALYSIS OF RETENTION DATA FOR TENAX-GC Retention Data of Brown and Purnell(5). Brown and Purnell (5) obtained retention data a t 293 K (20 "C) by plotting log V , , T values obtained a t higher temperatures vs 1 / T and then extrapolating to 1/(293 K). They studied hydrocarbons, halocarbons, esters, aldehydes ketones, alcohols, amines, nitriles, and a few other compounds. Table I includes a summary of their data, and Figure 1gives plots of their data in a log v , , 2 9 3 vs log pO,293 format. The Po,293 values were obtained based on available literature data (24). The solid lines in Figure 1b-d are the best fit lines. The fit parameters are given in Table 11. Figure la, which represents the entire dataset, is characterized by a clear trend of decreasing v,,293 with increasing ~ 0 , 2 9 3 . The scatter in Figure l a is to be expected since, as noted above, different compound classes will not necessarily follow the same correlation line. Figure l b was therefore prepared by excluding the points for all of the hydrogen bonding compounds as well as certain other compounds. The excluded compounds were acetic acid, dimethylhydrazine, and epichlorohydrin, and the amines, alcohols, anhydrides, and nitriles. The resulting fit in Figure 1b is very good (r2 = 0.96). It should be noted that the aldehydes and ketones were included in the Figure l b dataset, and so polarity alone does not lead to a deviation from the type of behavior exhibited in Figure lb. The data for the alcohols and amines are plotted in parts c and d of Figure 1,respectively. For these compound classes, excellent correlations are obtained (r2 = 0.99 for both). In the case of Figure IC,the data point for methanol was not included for the fitting. For a given value of log pO,293, the alcohols exhibit log v , , 2 9 3 values that are uniformly smaller than for the Figure l b compounds. This is as expected since the alcohols are comparatively hydrophilic, and Tenax-GC
Boiling Point
('C)
Boiling Point
("C)
Flgure 4. log V , , vs T , for dataset of Gallant et al. (4): (a)all data; (b) selected data; (c) alcohols only: (d) amines only.
on the other hand is hydrophobic (28). This result may be viewed in terms of eq 12 by noting that it probably indicates smaller values of (AHs- AHv) for the alcohols than for .the compounds in Figure l b , though contributions from nonzero net entropies of desorption are also possible. In contrast to the alcohols, only the least volatile amines give log V,,B3values that are lower than those of the compounds in Figure lb; the behaviors of the more volatile amines are similar to the Figure l b compounds. The slope of the Figure l b correlation line is -1.44. While it is moderately close to -1.0 as predicted by eq 12, it is also significantly different from -1.0. Thus, as volatility increases, these compounds tend to be less well retained than expected from a correlation slope of -1.0. This could be due to a weak trend of decreasing (AHs- AHv)with increasing pO,293. Such trends in sorption energetics have also been proposed (29) to explain deviations between the observed slopes in log KO,vs log KO,correlations and the ideal slopes that can be predicted from linear free energy relationship considerations analogous to those discussed above. ( K , is the soil organic matter/water partitioning constant, and KO,is the octanol/water partitioning constant.) A trend in the net entropies of vaporization could also be playing a role in Figure lb. In addition to considering the slope of the Figure l b correlation, it is interesting to examine its vertical location relative to eq 14 as a reference line. Equation 14 is therefore plotted as a dashed line in Figure 1. It is clear that the sorption on Tenax-GC is not liquidlike for any of the compounds studied. Figure 2 shows plots of the data in a log V,,B3vs T b format. The solid lines in Figure 2b-d are the best fit lines. The fit parameters are given in Table 11. Figure 2a represents the entire dataset and so is similar to Figure 6 of Brown and Purnell (5). Figure 2a is characterized by a clear trend of increasing V,, with increasing Tb As in Figure la, the scatter is due to the range of compounds present in the full dataset.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988 Boiling Point ( K )
l o g Po 293
Boiling Point Y C )
log V,,2s3vs log po,283 and vs T , for miscellaneous data: circles, with best fit line (6);diamonds (2);star (7); squares with best fit line (3); and asterisks (70). Dashed line in 5a Is best fit from Figure l b . Dashed line in Figure 5b is best fit from Figure 2b. Flgure 5.
This type of variability has also been noticed by Krost et al. (8). Figure 2b was therefore prepared for the same compounds included in Figure lb. The resulting fit is very good (1.2 = 0.97). As in Figure l b , polarity alone does not mean that a given compound will behave in an atypical fashion. When the data for the alcohols and amines are plotted in parts c and d of Figure 2, respectively, excellent correlations are obtained (r2 = 0.99 and 0.98, respectively). For a given value of Tb, the alcohols exhibit log vg,zg3 values that are for the most part smaller than for the Figure 2b compounds. Again, this is probably due to their hydrophilic character. In a manner similar to that observed for Figure 1, however, the more volatile amines in Figure 2d plot in a range that is quite consistent with the correlation line in Figure 2b. The slope of the Figure 2b correlation line is 0.026. This is within a factor of 1.6 of the predicted value of 0.016. The factor is 1.6 is very similar to the factor of 1.4 deviation between the observed and predicted slopes for Figure lb. As tabulated in the literature (24),the distribution of B values for the Figure 2b compounds is 7.9 f 0.4, and so the observed slope of 0.026 cannot be ascribed to anomalous B values. Rather, it may be due to a weak trend in the (A",- AHv) values as discussed for Figure lb. We can also examine the vertical location of the Figure 2 data relative to what would be expected for liquidlike sorption. Equation 19 is therefore plotted as a dashed line in Figure 2. As in Figure 1,it is clear that the sorption on Tenax-GC is not liquidlike for any of the compounds studied. Retention Data of Gallant et al. (4). The compounds studied by Gallant et al. ( 4 ) included a range of n-alkanes, aromatics, halogenated hydrocarbons, ketones, amines, aliphatic alcohols, phenols, and aliphatic acids. Figure 3a is a log Vg,zg, vs log ~ 0 , 2 9 3plot for the entire dataset. The p0,293 values were obtained based on available literature data (24). For the compounds in Figure 3 (and the figures discussed below) and Table I that are solids at T = 293 K, the parameters for the liquids given in (ref 24) were used to extrapolate to 293 K, thereby yielding the values for the corresponding subcooled liquids. Figure 3b presents the data for all compounds minus the alcohols, aliphatic acids, and amines. Parts c and d of Figure 3 present the data for the alcohols and the amines, respectively. The vapor pressure value used for di-n-butylamine was that of diisobutylamine. All of the general comparative statements made in the discussion of Figure lb-d relative to Figure l a also apply to Figure 3b-d relative to Figure 3a. The slope of the Figure 3a plot (-1.24) is similar to the slope of the Figure l a plot (-1.44). In contrast to Figure Id, however, the Figure 3d
955
amines did not give a straight line. In order of increasing log pow, the Figure 3d amines are benzylamine, tri-n-butylamine, di-n-butylamine, n-hexylamine, n-pentylamine, and n-butylamine. If benzylamine and tri-n-butylamine are excluded, the data for the remaining amines do appear fairly linear. In the case of the alcohols in Figure 3c, it may be noted that the slope of the correlation line is different from that in Figure IC. Nevertheless, the least volatile compounds in Figure 3c do plot very near their counterparts in Figure IC. Figure 4 presents log Vg,293 vs T b plots for the same datasets plotted in Figure 3. The general comments made in regard to the Figure 3 plots also apply to the Figure 4 plots. It may be noted that the slope of the Figure 4b correlation line (0.024) is very similar to the slope of the Figure 2b correlation line (0.026). Gallant et al. (4) give log Vg,m3 vs boiling point correlation equations for each of the individual groups of compoun& they studied. Although their equations employ temperature as expressed in degrees Celcius, the slopes they obtained are the same as if kelvins are used (only the intercepts differ); for the nonhalogenated aliphatic hydrocarbons, the slope was found to be exactly as found for Figure 2b (0.026). The fit parameters for the correlation lines in Figures 3 and 4 are given in Table 11. Retention Data of Janak et al. ( I ) , Butler and Burke (2), Vidal-Madjar et al. (3), Eiceman and Karasek ( 6 ) , Vejrosta et al. (7), and Zaranski and Bidleman (IO). Figure 5a is a log vg,2g3 vs log po,293plot containing the data discussed in ref 2,3,6, 7, and 10. Best fit lines are included for two of the datasets (ref 3 and 6). The Figure l b best fit line is also included for comparison. The data of Eiceman and Karasek ( 6 ) ,the benzene and methyl ethyl ketone data points of Butler and Burke ( 2 ) , the benzene data point of Vejrosta et al. (3,and the dichlorobenzene data of Zaranski and Bidleman (10) all fall close to the Figure I b best fit line. The benzene data point obtained by Janak et al. (1)is similar to that of Vejrosta et al. (7) (see Table I) and was not platted. It may be noted that the chlorobenzene data point of Eiceman and Karasek (6)falls closer to the Figure l b best fit line than the point obtained by Brown and Pumell(5), suggesting that the former might be more reliable. This is further substantiated by the excellent agreement between the Eiceman and Karasek (6) and Gallant et al. ( 4 ) values for chlorobenzene given in Table I. The reason why the Vidal-Madjar et al. (3) data set deviates from the Figure 1b best fit line is unclear, particularly since several compounds are common between the two data sets. Figure 5b is a log vg,2g3 vs T b plot for the same data sets discussed in Figure 5a. The best fit line for Figure 2b is also included for comparison. All comments made in regard to Figure 5a apply to Figure 5b as well. Indeed, it may be observed that parts a and b of Figure 5 are nearly mirror images of one another. The same type of mirror image character is present in Figures 1 and 2 and also in Figures 3 and 4. Retention Data of Krost et al. ( 8 ) . As with most other studies of retention on Tenax-GC, Krost et al. (8) measured retention volumes a t temperatures higher than the temperature of interest, then extrapolated to that temperature. However, they extrapolated using plots of -log V g vs , ~T,rather than vs 1/T since some experimental data they obtained suggested that plotting vs T was more appropriate (30). The error that will result from their method will increase with (1) increasing difference between the T range over which the V,,T values are measured and the T of interest and (2) increasing AH,. The magnitude of the error can be predicted. Assuming a constant AHB,integration of the equation d log Vg,T/d(l/7') = (A",- RT)/2.303R (20) leads to
958
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1 , 1988
Table 111. Values of AH, (kcal/mol) for Tenax-GC and the Corresponding Tabulated (24) Values of AHv (kcal/mol) compound n-hexane n-octane n-decane n-dodecane benzene
toluene p-xylene ethylbenzene n-propylbenzene cumene 1,2-dichloroethane trichloroethylene fluorobenzene chlorobenzene
a 14.7 15.7 19.2 25.1 14.1 16.6 15.4 15.8 17.7 14.8 16.6 16.5 21.9
Hydrocarbons 4 4 4 4 4 5 2, 30 7 4 4 4 4 5
7.5 9.2 10.9 11.9 8.1 8.1 8.1 8.1 8.6 9.8 9.3 10.4 10.3
1,2,4-trichlorobenzene phenol m-chlorophenol o-cresol m-cresol p-cresol 2,4,6-trichlorophenol
17.8 20.9 19.5 19.5 19.8 22.9
4 6 4 4 4 6
acetone methyl ethyl ketone
13.6 14.7 14.2 15.3 17.6 20.5 19.8 19.3
5 30 4 4
5 4 4 5 4
2-methylpropanol
9.1 9.9 11.7 14.5 14.5 13.4 10.5 4.3 14.3
acetic acid propionic acid 1-butanoic acid 1-pentanoic acid
11.2 12.6 12.6 16.6
4
n-butylamine n-pentylamine n-hexylamine benzylamine di-n-butylamine tri-n-butylamine aniline
13.4 15.8 17.0 15.8 17.7 17.5 17.7
4 4 4 4 4 5
nitrobenzene acetonitrile
24.0 11.9
5 30
O.74(AH8 - AHv)
V,,B3 + log P0,ZSS + 0*15
log
5.4 4.8 6.1 9.8 4.4 6.3 5.4 5.7 6.7 3.7 5.4 4.5 8.6
3.6 3.5 3.8 4.8 3.7 3.7 3.8 3.9 4.4 3.5 3.7 3.7 4.3
4.1 5.3 5.6 7.5 6.4 7.4 7.6, s 6.1, s 7.1, s
3.3 3.9 3.9 4.5 4.5 4.6 4.6, s 4.3, s 4.9, s
11.9, s 12.0, s 12.5, s 13.5 13.6, s 14.1, s
4.4, s 6.6, s 5.2, s 4.4 4.6, s 6.5, s
3.2, s 4.5, s 3.6, s 3.3 3.3, s 4.8, s
7.6 8.1 8.1 11.1 10.0 12.5 13.4 11.7
4.4 6.6 4.5 3.1 5.6 5.9 4.7 5.6
3.1 3.6 3.4 3.3 3.8 3.9 3.6 3.8
9.4 9.7 10.4 10.4 11.0 10.7 10.4 10.4 10.9
-0.2 0.1 1.0 3.0 2.6 2.0 0.1 -4.5 2.5
0.7 1.7 2.1 2.3 2.5 2.5 1.4 1.5 2.6
9.5 12.5 11.9 13.4
1.3 0.1 0.5 2.4
1.7 1.8 2.1 2.2
NAb NA NA 11.7 NA NA 11.3
NA NA NA 3.0 NA NA 4.7
3.4 4.0 3.8 3.1 4.2 2.7 3.3
12.2 8.2
8.7 2.7
4.6 2.6
Halogenated Aliphatics and Aromatics 4 7.9 4 8.3 8.0 4 9.1 4 9.1 6 4 10.2 4 10.6, sa 10.6, s 6 11.4, s 6
13.4 15.5 15.6 19.2 17.8 20.2 20.9 18.9 21.0
bromobenzene p-dichlorobenzene
A",
ref
Phenols
Ketones
3-methyl-2-butanone cyclohexanone 2-heptanone 4-heptanone acetophenone
4 4 4
4 Alcohols
methanol ethanol 1-propanol 1-butanol 1-methylpropanol 2-methyl-2-propanol
4
4 30 4 Acids 4
4 4
Amines 4
Miscellaneous
Solid at 293 K.
Not available.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 9, MAY 1, 1988
(21) where T z is the temperature a t which the value of log Vg,T is desired and T1is the midpoint of the range for which data are available. To examine how Krost et al. (8) carried out their extrapolations, we place the preceding differential equation in the form
d log V,,T/dT = -(AH, - RT)/2.303RT2
(22)
If the right-hand side of eq 22 is assumed constant (as Krost et al. (8) in effect assumed), integration leads to log
“vg,Tz”
log
vg,T1
-k
[(AH, - RT1)/2.303RT12] (TI - Tz)
(23)
Thus, the error caused by using eq 23 rather than eq 21 is given by
’ (1/2.30i) In
-
( T 2 / T l )(24)
The error, which is always an underestimation of Vg,T2,goes to zero as Tl Tz. For T z close to T1,In (T,/T,) ( T , T l ) / T land the second and third terms very nearly cancel one another out. For AH, = 15 kcal/mol (e.g., a compound like benzene), T I = 373 K and T2 = 293 K, then log (“Vg,~2”/Vg,TJ = -0.50, and Vg,T2is underestimated by a factor of -3. For AH, = 20 kcal/mol (e.g., a compound like p-dichlorobenzene) and Tl = 423, Vg,T2is underestimated by a factor of 24. The log “Vg,294n values of Krost et al. (8)may be calculated from the data they give as “breakthrough volumes” (actually, elution volumes) using the conversion factor (3.6 mL of Tenax-GC)/(g of Tenax-GC) that may be deduced from their cited retention volume values of 0.9 L/(g of Tenax-GC) and 2 L/(7.96 mL of Tenax-GC) for vinyl chloride at 283 K. Some of the compounds studied by Krost et al. (8) were also investigated by researchers mentioned above. When the retention volume values for the compounds are compared, the Krost et al. (8) “Vg,294” value for benzene is less by a factor of -3 than the retention volume values reported by others.
957
For rn-dichlorobenzene, the Krost et al. (8) “Vg,294”value is less by a factor of -20 than the corresponding value of Zaranski and Bidleman (IO). It would be useful if the original data of Krost et al. (6) could be reextrapolated by using eq 20. Thermodynamics of Desorption. As discussed above, the slopes of plots of log V, vs 1 / T or log Vg,Tvs 1/T can be used to determine A“,. The entropy change ASs of the desorption can be determined from the y intercept. The specific values of these parameters depend upon the standard states chosen for both the sorbed and desorbed states (31,32). The reason for examining AH, values here is to permit an evaluation of the magnitude and constancy of (AH, - AHv) as it appears in equations like eq 12 and 17. Table I11 contains a summary of the AH, values available from published figures and tables giving the slopes of plots of log Vg,Tvs 1 / T ( 4 - 4 3 1 ) . The corresponding values (24) of AHvare also given. For reasons discussed above and also by Kiselev and Yashin (1.9, the value of RT for the center of the temperature range of the data has been added to the slopes to obtain AH,. The compounds in Table I11 that are solids a t 293 K have been marked. While not especially constant, many of the non-hydrogenbonding compounds exhibit similar values of 0.74 (AH, - AH,). Also given in Table I11 are the corresponding values of (log vg,zg3 + 0.15 + log po,B3) (see eq 14). This quantity provides a measure of the non-liquid-like character of the sorbed state of a given compound as it is set by the net enthalpy and entropy of vaporization. The extent to which the term (log Vg,293 + 0.15 + log p0,293) does not match O.74(AHs - AH,) provides a measure of the importance of the net entropy of vaporization; in most cases it is rather important. Brunauer et al. (16) have also noticed that when albz/azbl is assumed equal to 1.0 (net entropy of vaporization = 0), that the values of the BET isotherm parameter c obtained from fitting sorption data very often imply values of (AH, - AHv)that are only half of the actual values. The fact that (log Vg,m3+ 0.15 log ~ 0 , 2 9 3 )is more constant than 0.74(AH, - AHv) indicates the importance of compensatory net entropies of vaporization in helping keep the observed relationships between log vg,z93 and log po,293 linear. Exactly analogous comments can be made regarding the observed relationships between log Vg,m3and Tb. As regards the possible trend of decreasing (AH, - AHv)with increasing p0,293 that was suggested above as an explanation for observed slopes, it may be noted that although the limited data in Table I11 are rather scattered, they could be consistent with such a trend. Figure 6 is a plot of log Vg,293 vs AH, as based on the Table I11 data. When the 2-methyl-2-propanol (tert-butyl alcohol) data point of Butler (31)is excluded, then log Vg,zg3correlates with AH, with r2 = 0.91. Although the point for 2-methyl2-propanol as obtained from Butler (31) appears to be an outlier in this plot, the 2-methyl-2-propanol point obtained from Gallant et al. (4) is consistent with the best-fit line. Overall, it is very interesting to note that even methanol, propanol, and acetonitrile follow the correlation very well. The somewhat greater scatter a t high values of AHs may be due to errors incurred in presuming that these AH, values are the same at 293 K as they are at the temperatures at which they can be easily measured. As indicated in eq 10, the slope of the correlation at T = 293 K is predicted to be -0.74. This is a factor of 1.8 larger than the observed slope of 0.41.
+
CONCLUSIONS It is clear that useful relationships can be developed between retention volumes on sorbents like Tenax-GC and physical property data such as vapor pressure and boiling point. Many different types of compounds (including hydrocarbons, hal-
958
Anal. Chem. 1988. 60. 958-960
ogenated hydrocarbons, esters, aldehydes, ketones, and others) behave in a similar manner within the context of such relationships. Although compound classes such as alcohols and amines demonstrate different behaviors, within each class the behavior is nevertheless predictable. The availability of these relationships should be beneficial to researchers concerned with predicting retention volume values for compounds whose behavior on Tenax-GC has not as yet been studied.
Registry No. n-Pentane, 109-66-0; n-hexane, 110-54-3;nheptane, 142-82-5;n-octane, 111-65-9;n-decane, 124-18-5;n-dodecane, 112-40-3; benzene, 71-43-2; toluene, 108-88-3; xylene, 1330-20-7; p-xylene, 106-42-3; ethylbenzene, 100-41-4; nprnpylbenzene, 103-65-1; cumene, 98-82-8; trimethylbenzene, 25551-13-7;styrene, 100-42-5;a-methylstyrene, 98-83-9;butadiene, 106-99-0; chloromethane, 74-87-3; dichloromethane, 75-09-2; chloroform, 67-66-3; carbon tetrachloride, 56-23-5;chloroethane, 75-00-3;1,l-dichloroethane,75-34-3;1,2-dichloroethane,107-06-2; l,l,l-trichloroethane, 71-55-6; 1,1,2-trichloroethane, 79-00-5; 1,1,1,2-tetrachloroethane,630-20-6; 1,1,2,2-tetrachloroethane, 79-34-5;vinyl chloride, 75-01-4; 1,l-dichloroethene,75-35-4; 1,2dichloroethene, 540-59-0; trichloroethylene, 79-01-6; tetratrichloroethylene, 127-18-4; bromomethane, 74-83-9; trichlorofluoromethane, 75-69-4;dichlorodifluoromethane, 75-71-8; chlorotrifluoromethane, 75-72-9;aUyl chloride, 107-05-1;fluorobenzene, 462-06-6; chlorobenzene, 108-90-7;bromobenzene, 108-86-1;odichlorobenzene, 95-50-1; m-dichlorobenzene, 541-73-1; p-dichlorobenzene, 106-46-7;1,2,4-trichlorobenzene, 120-82-1;phenol, 108-95-2;o-cresol, 95-48-7;n-cresol, 108-39-4;p-cresol, 106-44-5; rn-chlorophenol, 108-43-0;2,4,64richlorophenol, 88-06-2;methyl acetate, 79-20-9;ethyl acetate, 141-78-6;propyl acetate, 109-60-4; butyl acetate, 123-86-4; isopropyl acetate, 108-21-4; methyl acrylate, 96-33-3; ethyl acrylate, 140-88-5; acrolein, 107-02-8; acetaldehyde, 75-07-0; acetone, 67-64-1; methyl ethyl ketone, 78-93-3;methyl isobutyl ketone, 108-10-1;3-methyl-2-butanone, 563-80-4; cyclohexanone, 108-94-1; 2-heptanone, 110-43-0; 4heptanone, 123-19-3;acetophenone, 98-86-2; methanol, 67-56-1; ethanol, 64-17-5; 1-propanol,71-23-8;2-propanol, 67-63-0; l-bu2-methyl-1-propanol, tanol, 71-36-3;2-methyl-2-propanol,75-65-0; 78-83-1; 1-methyl-1-propanol,78-92-2;1-octanol, 111-87-5;allyl alcohol, 107-18-6;acetic acid, 64-19-7; 1-propionic acid, 79-09-4; 1-butanoic acid, 107-92-6;1-pentanoic acid, 109-52-4;acetic anhydride, 108-24-7;methylamine, 74-89-5; ethylamine, 75-04-7; n-propylamine, 107-10-8;n-butylamine, 109-73-9;n-pentylamine, 110-58-7;n-hexylamine, 111-26-2;benzylamine, 100-46-9;di-nbutylamine, 111-92-2;tri-n-butylamine, 102-82-9;pyridine, 11086-1;aniline, 62-53-3;acetonitrile, 75-05-8;acrylonitrile,107-13-1; epichlorohydrin, 106-89-8; ethylene oxide, 75-21-8; ethyl mercaptan, 75-08-1; nitrobenzene, 98-95-3; Tenax-GC, 24938-68-9. LITERATURE CITED (1) Janak, J.; Ruzickova, J.; Novak, J. J. Chromatogr. 1074, 9 9 , 689.
Butler, L. D.; Burke, M. F. J. Chromatogr. Sci. 1076, 14, 117. VMal-Madjar, C.; Gonnord, M.-F.; Benchah, F.; Guiochon, G. J. Chromatogr. Sci. 1078, 16, 190. Gallant, R. F.;King, J. W.; Levins, P. L.; Plecewicz, J. F. Characterization of Sorbent Reslns for Use in Envlronmentai Sampling; U.S. EPA60017-78-054; U.S.Environmental Protection Agency: Washington, DC, March 1978. Brown, R. H.; Purneii, C. J. J . Chromatogr. 1970, 178, 79. Eiceman, G. A.; Karasek, F. W. J. Chromatogr. 1080, 200, 115. Vejrosta, J.; Roth, M.; Novak, J. J. Chromatogr. 1081, 277, 167. Krost, K. J.; Peilizzari, E. D.; Walburn, S.G.; Hubbard, S . A. Anal. Chem. 1082, 5 4 , 810. van der Straeten, D.; van Langenhove, H.; Schamp, N. J. Chromatogr. 1085, 331, 207. Zaranski, M. T.; Bidleman, T. F. J. Chromatogr. 1087, 409, 235. Cropper, F. R.; Kaminsky, S. Anal. Chem. 1063, 35, 735. Raymond, A,; Guiochon, G. J. Chromatogr. Sci. 1075, 13, 173. Senum, G. I. Environ. Sci. Technol. 1081, 15, 1073. Littlewood, A. B. Gas Chromatography;Academic: New York, 1970. Kiselev, A. V.; Yashln, Y. I. Gas-Adsorption Chromatography; Plenum: New York, 1969. Adamson. A. W. Physical Chemistry of Surfaces: Wiiey: New York, 1982. Brunauer, S.:Copeland, L. E.; Kantro, D. L. I n The Solid-Gas Interface; Flood, E. A., Ed.; Marcel Dekker: New York, 1967; Vol. 1. Brunauer, S.;Emmett, P. H.; Teller, E. J . Am. Chem. SOC.1038, 6 0 , 309. Hoare, M. R.; Purnell, J. H. Trans. Faraday SOC. 1055, 52, 222. Rose, A.; Schrodt, V. N. J. Chem. Eng. Data 1083, 8 , 9. Westcott, J. W.; Bldleman, T. F. J. Chromatogr. 1081, 210, 331. Conder, J. R.; Young, C. L. Physicochemical Measurement by Gas Chromatography: Wiley-Interscience: New York, 1979. Sakcdynskii, K.; Panlna, L.; Klinskaya, N. Chromatographia 1074, 7 , 339. Handbook of Chemistry and Physics, 51st ed.; Weast, R. C., Ed.; Chemical Rubber Co.: Cleveland, OH, 1970; p D-146. Tenney. H. M. Anal. Chem. 1058, 30, 2. Desty, D. H.; Whyman, B. H. F. Anal. Chem. 1057, 2 9 , 320. Brazhnikov, V.; Sakodynski, K. J. Chromatogr. 1068, 3 8 , 244. Daemen, J. M. H.; Dankelman, W.; Hendrlks, M. E. J. Chromatogr. Sci. 1075, 13, 79. Karickhoff, S . W. J. Hydraul. Eng. 1084, 110, 707. Pellizzari, E. D., personal communication, 1988. Butler, L. D. Ph.D. Thesis, Unlverslty of Arizona, 1978. Sawyer, D. T.; Brookman, D. J. Anal. Chem. 1068, 4 0 , 1847.
James F. Pankow Department of Environmental Science and Engineering Oregon Graduate Center 19600 N.W. Von Neumann Drive Beaverton, Oregon 97006
RECEIVED for review August 7,1987. Accepted December 22, 1987. This work was funded in part with federal support from the United States Geological Survey (USGS) under Grant 14-08-0001-A0410 and with the support of the Northwest Environmental Research Center (NWERC). The contents do not necessarily reflect the views or policies of USGS, nor does the mention of trade names or commercial products constitute endorsement for use.
TECHNICAL NOTES Supersonic Jet Atomlc Spectroscopy with Flame Atomization Lance B. Koutny, William B. Whitten,* Thomas G. Nolan, and J. Michael Ramsey Oak Ridge National Laboratory, Analytical Chemistry Division, Oak Ridge, Tennessee 37831 -6142 We have recently shown that high-resolution laser spectra of flame-atomized samples can be obtained with a low-pressure sampling technique (1). We used a form of saturation spectroscopy to obtain Doppler-free spectra. Collision broadening was reduced to a negligible value by reducing the sample pressure to as low as 0.1 Torr. There were, however, sufficient velocity-changing collisions within the residual gas to produce
strong background pedestals. Also, additional spectral lines due to crossover resonances were present, with background pedestals as well. Since these resonances occur midway between primary resonances ( I ) , they produce no additional spectral information but instead are an unnecessary hindrance to quantitative high-resolution spectroscopy. The purpose of this note is to report an improved apparatus in which the
0003-2700/88/0360-0958$01.50/00 1988 American Chemical Society
