where, again, the 72"'s are given by Equations 38-41 with Z, = 8 and Z b = 10, and
+
- _ 19.792(4.181 Z,,) [ M , f 2.857(4.302 -t- Z , )I2}
'+f1
(")
where cr(lnRab) is estimated to be 0.001. The almost linear plot of C M ~ / us. M ~M I , as generated from Equation 57, is shown in Figure 5. Note that a relative error of 15% is reached at about M I = 5,800. (3) From Retention Index of Benzene. One finds that the retention index (I,) of benzene fits the following empirical function of stationary phase molecular weight: 14580 I , = 677.8 - ____ (Mi+ 93) from which one can derive that G\T
=
+
( M I 93)2 14580 r'
(59)
where we estimate u1, to be 0.3 unit. Illustrated in Figure 5 is the dependence of c r ~ l / M 1on M I (also nearly linear for M I > 1000) as deduced from Equation 59. This alternative method for determining M I seems to give the best precision (a relative error of 15% for M I = 7,150). However, it also requires utilizing several calibration solvents to obtain the working equation (Equation 58). It would appear that, if one is to seriously consider using GLC for the stated purpose, this last alternative is
the best of the three. For one thing, it involves only relative retention parameters. More important, it can be more readily adapted to studying other polymeric series. As mentioned before, compared to n-alkane solvents, other polymers should produce a more marked chain length dependence of I,, thus reducing the relative error in the measurement of MI.
CONCLUSION Equations derived through refinement of Prigogine's theory of chain molecule mixtures have been successfully employed in interpreting and predicting the solute activity coefficient and its dependence, and that of the retention index, on the stationary phase molecular weight. The importance of the so-called structural contribution (heretofore neglected by GLC workers) to these retention quantities has been demonstrated. These theoretical developments are currently being applied to the problem of selectivity in gas and liquid chromatography and to the analysis of Vgovalues of a variety of monofunctional alkyl solutes in monofunctional n-alkyl solvents, where the solvent chain length is held approximately constant. ACKNOWLEDGMENT Helpful discussion with G. M. Janini and J . W. King are gratefully acknowledged. Received for review September 5, 1973. Accepted January 10, 1974. This research was supported by a grant from the National Science Foundation.
Role of Nitric Oxide in Positive Reactant Ions in Plasma Chromatography Francis W. Karasek and Donald W. Denney Department of Chemistry. University of Waterloo, Waterloo, Ontario
Ions of the form (H20)nH+ and (H20)n,NOC have been observed among the positive reactant ions using a coupled plasma chromatograph/quadrupole mass spectrometer, but only limited work on identification of these ions in the mobility spectra has been reported. Addition of nitric oxide to the nitrogen carrier gas in the PC instrument results in a large increase in relative abundance of the reactant ion attributed to (H20)NO+ in the mobility spectrum, confirming its mobility. An increased abundance of the (H20)NO+ ion in the reactant ion group affects the product ion spectra of different classes of organic compounds, and its greater reactivity leads to increased sensitivity of the plasma chromatograph.
Functioning at atmospheric pressure, the plasma chromatograph first creates both positive and negative ions in a carrier gas using a nickel-63 beta source. The reactant ions undergo ion-molecule reactions with trace molecules injected into the carrier gas stream. The resultant product ions are separated in a coupled ion-drift spectrometer to
give positive and negative mobility spectra characteristic of the organic molecules involved and the reactant ions generated. Both the technique of plasma chromatography (PC) and instrumentation have been described previously (1-4).
The type, reactivity, and relative concentrations of the reactant ions generated are of basic importance in the PC technique. Although a number of limited studies of their identity have been made, further work to understand and explore their function is needed. Using a combined plasma chromatograph/quadrupole mass spectrometer, various positive ions have been identified as the reactant species, using either air or nitrogen as a carrier gas. These are (HzO)H+, (HzO)zH+, (Hz0)3H+, NO+, (Hz0)NOf and (HzO)zNO+ (5-7). Their mobility spectra are the same whether air or nitrogen is used as the carrier gas. (1) F. W. Karasek, Res./Deve/op., 21 (12), 25 (1970). (2) F. W. Karasek, W. D. Kilpatrick, and M . J. Cohen, Anal. Chem., 43, 1441 (1971). (3) F. W. Karasek, Inf. J. Environ. Anal. Chem., 2, 157 (1972). (4) F. W. Karasek, 0. S. iatone, and D. M. Kane, Anal. Chem., 45, 1210 (1973). A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
NO. 6,
M A Y 1974
633
I
1
i
i
4
4-4
I
4
7
0
DIifT l i M l -MlllISiCONDS
Figure 1. Mobility spectrum of positlve reactant Ions using an air carrier and a nitrogen drift gas T h e reduced mobility values ( K O ) are calculated by K O = d / t E X P/760 X 273/T cm2/volt-sec where d = drift distance ( 6 cm) € = fleld strength (250 V/cm), t = drift time (sec), T = degrees K , p = pressure
(Torr)
Although these species have been identified, only limited work has been done to determine exactly where these ions occur in their mobility spectra and their relative effect on the ion products formed with organic compounds. The mobility spectra of the positive reactant ions obtained using either air or nitrogen that has all contaminants removed and contains only a trace (10-50 ppm) of water shows three ion peaks at characteristic reduced mobilities ( K O )The . three ion peaks are found in the positive reactant ion mobility spectra a t all temperatures from 25 to 180 "C and their reduced mobilities exhibit a slight temperature dependence. Their relative peak intensities are a function primarily of temperature and water concentration. These positive ions are observed to have different reactivities with different compound classes. Generally, the most reactive ion is the one associated with (H20)NO+, shown in Figure 1 a t a KOvalue of 2.49 cm2/ V-sec. Upon injection of a compound, this ion. of the three present, usually reacts first and returns to its original equilibrium concentration last as depletion of the injected compound occurs in the PC instrument. This study was undertaken to determine the effect of adding nitric oxide to a nitrogen carrier gas on the relative abundances of the positive reactant ions, and its subsequent effect on the product ion mobility spectra formed with organic compounds. These experimental observations confirm the mobility of the reactant ion peak associated with (H20)NO+ and indicate that an increase of this reactant ion changes the relative abundance and intensities of the product ion mobility spectra of different classes of compounds.
EXPERIMENTAL Instrumentation. The plasma chromatograph used in this study was the BETA-VI Model (Franklin GNO Corporation, P.O. Box 3206, West Palm Beach. Fla., 33402). Details of the instrumentation have been previously described (8, 9). Unless otherwise indicated in figure captions, operating conditions of the plasma chromatograph are: temperature, 136 "C; drift gas flow. 410 ml/ Wernlund, Franklin GNO Corporation, P.O. Box 3206, West Palm Beach. Fla.. 33402, personal communication, March 1973. G. W. Griffin, I . Dzidic, D. I . Carroll, R. N. Stillwell, and E. C. Horning,Ana/.Chem., 45, 1204 (1973). V. Mohnen, Atmospheric Sciences Research Center Publication Number 204. State University of New York at Albany, Albany, N . Y . , June 1972. F. W. Karasek and 0. S. Tatone, Anal. Chem., 44,1758 (1972) F. W. Karasek and D. M. Kane, Anal. Chem., 45,576 (1973). R. F.
ANALYTICAL CHEMISTRY,
VOL. 46, NO. 6 , M A Y 1974
min; carrier gas flow, 100 ml/min; electric field, 250 V/cm; injection and gating pulses, 0.1 msec; spectral scan time, 2 min; pressure, 727 to 740 Torr. All mobility values were obtained using a nitrogen drift gas. Samples of organic compounds were introduced to the plasma chromatograph uia a coupled gas chromatograph with provisions for valving a portion of an eluting gas chromatographic peak into the plasma chromatograph inlet (IO); approximately gram samples were admitted in this manner. Addition of nitric oxide to the carrier gas inlet was accomplished by a single injection with a 1-ml syringe. Reagents. The nitrogen used is Linde High Purity (99.996%) and the air is Linde Bone Dry grade. Both gases used for carrier and drift sections of the PC tube are passed through individual stainless steel traps of 2.25-liter capacity packed with 4-8 mesh Linde Molecular Sieve 1 3 X . The nitric oxide (98.5%) was obtained from, Matheson of Canada. Ltd., Whitby, Ont., the n-octane from PolyScience Corp.. Evanston, Ill., and the 2-chloronitrobenzene from Fisher Scientific Co., 1,td.. Don Mills, Ontario.
RESULTS AND DISCIJSSION Origin of Reactant Ions. There is ample evidence to expect the formation of both (H20),H+ and (HzO),NO' as stable species from the interaction between high energy beta particles and nitrogen containing only traces of oxygen and water when one considers the subsequent series of ion-molecule reactions that can occur. The following ionmolecule reactions have been reported by Good e t al. (11): N,
-
+
+
N2+
e
( 1)
+ N, h',' + H,O H20+ + 2N2 H,O+ + H,O (H,O)H+ + OH (H,o)H+ + H,O + s, (H?o)?H+ + N?f
+
2N,
N4+
--
(2) ( 3)
(4) N?
is)
Through reactions of this type and similar studies, the existence of the hydrated proton ions is well established (5-7). In studying a series of ion-molecule reactions, Thynne and Harrison (12) present sufficient data to calculate that the charge exchange reaction between Nzf and OH would be exothermic by 78 kcal/mol:
+
+
N?+ OH -3 OH' N2 ( 6) They also found the following reaction to be exothermic by 35 kcal/mol with a rate constant of he = 4.72 X cm3/sec. OH'
+
H,O
-
H,Of
+0
(7) The formation, of a stable OH+ from ionization and dissociatibn of water by electron impact was also observed by Morrison and Traeger (13). Different authors have postulated a mechanism for the formation of NO+ in ion-molecule reactions. Ferguson and coworkers (14) measured the rate constants for the following reactions a t 300 "K:
K2
+
N,'
NJC
0' 0
+
0:
NO'
+
N
(8)
NO'
+
K
(9)
NO+
+
KO
(10)
--+
(10) F. W. Karasek, 0. S. Tatone and D. W . Denney, J. Chromafogr., 87, 137 (1973). (11) A. I . Good, D. A. Durden, and P. Kebarle. J. Chem. Phys., 52, 212 (1970), (12) J. C. J. Thynne and A G. Harrison, Trans. Faraday Soc., 62, 2468 (1966). (13) J. D. Morrison and J. C. Traeger, Inf. J. Mass Speclrom. /on Phys.. 11, 77 (1973). ( 1 4 ) E E Ferguson. F C . Fehsenfeld,P . D . Golan. and A L Schme'ltek o p f , J. Geophys. Res.. 70. 4323 (1965)
I
mie-
Figure 2. Normalized mass spectrum of positive reactant ions and product ions of dimethylsulfoxide using a coupled plasma chromatograph/mass spectrometer unit. Data were obtained at 26 "C using an air carrier gas (5)
.!
N O added
1
4
a
I
ORIF1 l l M E
-
2iP
I
6
1
Q
0,
-+
-t
1.56
MlLllSiCONDS
5
4
6
DRlFl 1 l M E
-
I
4
MILLISECONDS
Figure 4. Positive mobility spectra showing the effect of nitric oxide added to the nitrogen carrier gas on the product ion mobility spectrum of normal octane. Scans are taken at 4-minute intervals with zero level raised for each scan
X:
N 1 CARRIER
lk
a
4
which they determined required no activation energy. Dunkin et al. (17) studied the reaction of N3+ with 0 2 at 200 "K and proposed the following reactions:
in20i3u+
lH201NO+
1
4
Figure 3. Alteration of relative abundance of the positive reactant ions formed with a nitrogen carrier upon addition of nitric oxide to t h e carrier gas
Of these reactions, it was concluded that reactions 10 and 11 were slow ( k l o cm3/sec and k l l cm3/ see, respectively), and as such were not a significant source of K O + . They concluded that reaction 9 was the important production mechanism for daytime NO+ ion density in the atmosphere around 120-140 km. This work further reinforced t h r results of A . Galli et al. (15), who studied ion-molecule reactions leading to NO+ formation and obtained reaction rate constants of similar magnitude. Earlier work by Talrose et al. (16) concludes that reaction 8 plays the major part in establishing the ion balance of the upper atmosphere. They also studied the reactions of the activated ions:
+ 0 + N2 + r\' + N2
NO' 02
NOL+
+
(16a) (16b)
N,
(16c) All three reactions appear to be roughly comparable in magnitude and have an overall rate constant of k 1 6 = 1 X cm3/sec. Of the three, reaction 16c does not appear to give a stable ion under plasma chromatographic conditions based on mass spectra observed for the reactant ions ( 5 , 6). It appears that there are several mechanistic routes for the formation of NO+ and its hydrated species in the plasma chromatograph. The same sets of positive reactant ions and their ion product species formed with organic compounds are observed whether an air or nitrogen carrier gas is used to create reactant ions. Since nitrogen and water are common to both gases, this suggests that formation of NO+ proceeds primarily from a series of reaction steps such as 1, 4, 6, 7, and 9 rather than those involving oxygen itself. A number of studies have been made on the formation of hydrated NO+ species. Puckett and Teague (18) in studies of water-nitric oxide mixtures observed that the NO+ ion is the precursor to a chain of reactions which result in NO+.nNO ( n = 1, 2), NO+.nH20 ( n = 1, 2, 3), and H30+.nH20 ( n = 2, 3, 4) being formed. With increasing water concentration, the NO+ (KO), species is lost in a reaction producing (H20)NO+. Reactions postulated consider NO as the third body:
+ HLO + NO (H?O)NO+ + H,O + NO NO+
---
(H,O)NO+
IH,O),NO+
+ +
NO (1;) NO (18)
Griffin and coworkers proposed similar reactions, but considered Nz as the third body (6);
+ (H20)NO+ + YO'
H?O + N2 H,O
+
N,
(H,O)NO+
+
(H20)2NO+
+
N2 (19) N L (20)
The hydration of NO+ was also observed by McAdanis (15) A . Galli. A . Giardini-Guidoni, and G. G. Volpi, J. Chem. Phys., 39, 518 (1963). (16) V L. Talrose. M. i . Markin, and I . K. Larkin, Disc. Faraday SOC., 33-34,257 (1962)
(17) D. B. Dunkin, F. C. Fehsenfeld, A. L. Schmeltekopf. and E. E. Fergus0n.J. Chem. Phys., 54,3817 (1971). (18) L. J. Puckett and M. W. Teague, J. Chem. Phys., 54,2564 (1971).
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1974
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DRIFT llME - M I l L I S i C O N D S
Figure 5. Positive mobility spectra showing the effect of nitric oxide added to the nitrogen carrier gas on the product ion mobility spectrum of 2-chloronitrobenzene. Scans are at 4-minute intervals with zero level raised for each scan
214
2
io
20
do
50
do
710
MOLECULAR WEIGH1
and Bone using a photoionization mass spectrometer to study the reaction of NO+ with water (19). The mechanisms shown in reactions 1-20 can account for formation of the major positive ,reactant ions observed in the plasma chromatograph when using either nitrogen or air with traces of water as a carrier gas. These reactions and the work of Puckett and Teague (18) on NO-HzO mixtures suggest that the addition of nitric oxide should affect the relative amounts of positive reactant ions formed. Role of Nitric Oxide. The positive reactant ion mobility spectrum along with the calculated KO values obtained using air as a carrier gas is shown in Figure 1. When using nitrogen as a carrier gas, the mobility spectrum is identical to that of air, as can be seen by comparison of Figures 1 and 3. Ions present in the positive reactant ion mobility spectra have been identified using a coupled plasma chromatograph/mass spectrometer system. Figure 2 shows the normalized mass spectrum obtained with this system for the positive reactant ions when using air as a carrier gas ( 5 ) . Also shown here are the protonated product ions produced by the injection of a trace of dimethylsulfoxide. T o determine which ion peak or peaks are associated with nitric oxide, it was introduced into a nitrogen carrier gas stream and changes in the positive reactant ion mobility spectrum were observed. The effect shown in Figure 3 indicates that the ion peak associated with (H20)NO+ increases in abundance with increased nitric oxide concentration, while the previously most abundant ion peak associated with (H20)3H+ decreases. These data indicate that the increased ion peak is related to nitric oxide concentration and tends to confirm its assignment as (HzO)NO+. By whatever routes discernible in the reactions listed in Equations 1-20, this stable reactant ion is formed; addition of nitric oxide either shifts their equilibria, or introduces new reaction paths, to result in increased formation of the (HzO)NO+ion. The effect of adding nitric oxide to the carrier gas on the positive mobility spectrum of an organic compound was investigated using a normal alkane and an aromatic compound. In Figure 4, the first scan shows the positive mobility spectrum normally obtained for n-octane with a nitrogen carrier gas. Immediately after this spectral scan was completed, nitric oxide was added to the nitrogen (19) M. J. McAUarns and L. I. Bone, Proceedings of the Nineteenih Annual Conference on Mass Spectrometry and Allied Topics, Atlanta, Ga.. May 1971, No. M 3.
636
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 6, M A Y 1974
Figure 6. Plot of reduced mobility ( K O ) vs. molecular weight for the positive reactant ions carrier gas and the next two mobility spectra were obtained. The latter spectra indicate that the addition of nitric oxide increases the intensity of the ion peak attributed to the (M - 1)+ion (10) even though the original sample concentration has been depleted by the flowing carrier gas during the scan periods. These spectra also reveal that the addition of nitric oxide promotes fragmentation of the normal alkane, as evidenced by the increased number of ion peaks in the spectra. Figure 5 indicates that a different type of effect occurs with an aromatic compound. With 2-~hloronitrobenzene, no new ion peaks occur with nitric oxide addition, but the intensities are altered. In this case, the intensity of the molecular ion peak (20) decreases relative to the ion peak attributed to (M NO)T, which shows an increased sensitivity even at the depleted sample concentration present during its scan. These results and assignment of ion peaks are in agreement with those obtained by Einolf and Munson in studies of high pressure charge exchange mass spectrometry (21). Using a nitric oxide reactant gas in the chemical ionization (CI) technique, they found that aromatic compounds gave very simple CI mass spectra, showing primarily two ions, a large (M + N O ) + and a smaller M + ion.
+
CONCLUSIONS The identity of the reactant ions formed with a nitrogen or air carrier in the plasma chromatograph as (H20)NOf and (H20),H+ is supported by both theoretical and experimental evidence. Observation of changing abundance of these ions in their mobility spectrum with addition of nitric oxide confirms mobility assignments previously made. The mobility plot shown in Figure 6 for the above ions reveals a correlation of molecular weight with mobility similar to that reported by Griffin (6). The addition of nitric oxide to the nitrogen carrier gas produces a reactant ion of increased reactivity and increases the sensitivity of the plasma chromatograph. It also changes the product ion mobility spectrum differently for compounds of different classes. Its use could prove a (20) F. W . Karasek and D. M . Kane, Ana!. Chem.. 46, 780 (1974) (21) N. Einolf and B. Munson, Int. J. Mass Spectrom. / o n Phys.. 9, 141 (1972).
valuable technique for detection and identification of compounds with plasma chromatography. Received for review August 6, 1973. Accepted November
12, 1973. The research for this paper was supported by the Defence Research Board of Canada, Grant No. 9530-116 and the National Research Council of Canada, Grant Lie. A5433.
Applications of Fourier Transform Techniques to Steric-Exclusion Chromatography T. A. M a l d a c k e r , l J. E. Davis,2 and L. B. Rogers Department of Chemistry, Purdue University. West Lafayette, Ind. 47907
Unlike earlier investigators who used synthetic functions to enhance the resolution of overlapped peaks or to obtain an improved estimate of the molecular-weight distribution, we have used the experimental nonretained peak. While the latter approach is not ideal, it does provide a mathematically simple correction for some sources of peak broadening which are either ignored or treated more empirically when using a synthetic function. Two other aspects have also been examined. The first, signalto-noise enhancement, has been obtained using a combination of different mathematical functions for diminishing or eliminating the high-frequency components while minimizing peak distortion. If component peaks of the sample are narrow, approaching the peak widths of the noise, the improvement will, of course, be relatively less. The second aspect, data reduction, appears to be promising for the economical description of complex mixtures as shown by the results of simulation studies.
The experimental peak width obtained in steric-exclusion chromatography is a function of several factors. These include contributions from instrumental artifacts, such as those associated with the detector and recorder (1-3), as well as dead volume and connections between different diameters of tubing. In addition, physicochemical factors such as diffusion and sorption will contribute to band broadening. Finally, some peak broadening may be due to the presence of a molecular-weight distribution of very similar species which are only partially resolved (4). The diffusion processes have been further divided into those that occur in the stationary phase (pores) and in the mobile phase. The latter contains contribution from solute diffusion in the presence and absence of permeation (5). The variance (u2) of the experimental elution profile can be expressed as the sum of the variances of the peak P r e s e n t address, P h a r m a c y a n d A n a l y t i c a l Research, S a n d o z W a n d e r , I n c . , R o u t e 10, East H a n o v e r , N.J. 07936 Present address, B a r n e s H o s p i t a l , C l i n i c a l C h e m i s t r y , St. L o u i s , Mo. 63110
*
A . Savitsky and M . J . E Golay, Anal. Chem.. 36, 1627 (1964). I . G. McWilliams and H. C. Bolton, Anal. Chem. 41, 1755 (1969). / b i d , p 1762 J G . Hendr1ckson.J. Polym. Sci.. PartA-2. 6 , 1903 (1968). 5 ) F . W. Billrneyer. Jr.. G. W. Johnson, and R . N. Kelley. J . Chromafogr.. 34, 316 (1968).
1) 2) 3) 4)
broadening contributions ( 4 ) . Experimenrally, optimization of conditions (6) and special techniques, such as reverse flow (4, 7), have been used to minimize individual variances. Mathematically, several approaches have also been taken to extract the “true” elution profile from the experimental one. That aspect has been of particular interest to polymer chemists because the peak shape contains important information about the molecular-weight distribution. Tung has mathematically estimated the true molecular-weight distribution from the instrumental signal (8). Others have obtained a formal solution to his equation (9) and have successfully applied it to real data. Both approaches assume a Gaussian peak shape, but that basis is not entirely adequate when non-Gaussian elements are present (IO). Fourier transform techniques have been applied to the problem of signal correction. Resolution enhancement in spectrometry has been demonstrated by Horlick ( 1 1 ) . As in chromatography, the experimental signal is the result of convolution (multiplication in the Fourier plane) of the true signal with degrading instrumental functions (12). Though several Fourier techniques have been used, we have taken the approach of effectively subtracting out (deconvoluting) undesirable variances in the time-plane signal by dividing the Fourier transform of the uncorrected signal by the Fourier transform of a function that contains the undesirable elements of the variance. Although Kirmse and Westerburg (13) have taken a similar approach to resolution enhancement, their Fourier correction function was essentially the transform of a synthetic Gaussian. Rosen and Provder have used the Fourier transform method with a nonlinear calibration curve to correct for peak broadening in gel permeation chromatography (14).
In the present study, the transform of an actiial chrcmatographic peak has been the correcting function. Tbis (6) /bid p 322 ( 7 ) L H Tung J C Moore and G W Knight J Ao,J P n l , ( v S c IO 1261 (1966) (8) L. H. Tung, J. Appl Polym. Sci.. 10, 375 (19661 (9) P. E. Pierce and J . E. Armonas. J Poiym. S o . P a r t C 21, 23 (1967). (10) J, H. Duerksen and A. E. Harnielec, J . Poiyi-i Scr P a r : C. 21, 83 (1967). (11) G. Horlick, Anal. Chem., 44, 943 (1972). (12) G. Horlick.AnaJ Chem.. 43 ( S i , 61A (19711. (13) D. W . Kirmse and A W. Westerberg. Anal C h e m 4 3 , i ; E P (1971). (14) E. M. Rosen and T. Provder, Separ. Sci.. 5 , 485 (1970)
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