ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
705
Comparison of the Negative Reactant Ions Formed in the Plasma Chromatograph by Nitrogen, Air, and Sulfur Hexafluoride as the Drift Gas with Air as the Carrier Gas Timothy W. Carr International Business Machines Corp., Dept. 350, Bldg. 3 10, Hopewell Junction, New York 12533
The negative reactant ions formed in the plasma chromatograph with zero grade air are examined with nitrogen, air, and SFB as the drift gas. The 02-,and 0,- ions are observed In the mobility spectra of air carrier with both air and SFBas the drift gas but not when N, is used as the drift. The negative ion mass spectra as well as the total ion and individual mass-identified mobility spectra are shown for air with each of the three drift gases.
Plasma chromatography (1-8) and the coupled technique of plasma chromatography/mass spectroscopy (9, 10) have been investigated in recent years as methods t o detect picogram quantities of both organic and inorganic compounds. T h e basis of the technique is the formation of both positive a n d negative ions through a series of ion-neutral molecule reactions occurring at atmospheric pressure. The phenomenon observed in the ionization process is similar t o that observed in chemical ionization mass spectrometry (11, 12). The ions produced from the carrier gas are referred to as reactant ions, since they are used to react with the trace amount of sample molecules. T h e identity of t h e reactant ions formed in the PC techniques is of particular importance in understanding the formation of product ions. Carroll, Horning, e t al. (13)have identified the positive reactant ions formed using nitrogen for both t h e carrier and drift gases as being NH4+,NO', and H(H,O),+. Recently Kim, Karasek, e t al. have also studied t h e positive reactant ions (30). T h e identity of the negative reactant ions formed with zero grade air as the carrier and nitrogen as the drift gas was the subject of a recent report by Carr (14). Most applications of the plasma chromatographic technique have involved the use of nitrogen as the drift gas. T h e purpose of this investigation is t o compare the reactant ions produced with zero grade air as the carrier gas and nitrogen, zero grade air, and sulfur hexafluoride as the drift gas.
EXPERIMENTAL The Alpha-I1 plasma chromatograph/mass spectrometer manufactured by Franklin GNO Corporation which was used in this investigation has been described previously (14, 15). This instrument consists of a Beta-VI1 plasma chromatograph coupled to a specifically modified Extranuclear Laboratories Spectr-El quadrupole mass spectrometer. The operating parameters of the plasma chromatograph used in this study are summarized in Table I. A schematic of the combined PC-MS system is shown in Figure 1. This instrument has the capability of operating in several modes so that ion mobility spectra, mass spectra, total ion mobility spectra, and mass identified mobility spectra can be obtained. The ionic mobility may be measured in either a one-grid or two-grid pulsing procedure with the Beta-VI1 plasma chromatograph mode of operation. Both grids of the drift tube can be held open, allowing all the ions produced in the ionization source to continually drift down the tube and into the quadrupole mass spectrometer, which results in atmospheric pressure ionization 0003-2700/79/035 1-0705$01.OO/O
Table I. Operating Parameters of the Alpha TI Plasma Chromatograph-Mass Spectrometer drift gas flow rate carrier gas flow rate applied voltage gate width repetition rate temperature
500 cm3/min 100 cm3/min
+ 2800V 0.2 ms 2 7 . 0 ms 210 "C
mass spectra. Total ion mobility spectra can be obtained by operating the plasma chromatograph in the normal one-grid pulsing mode and the quadrupole mass spectrometer in the total ion mode, which enables the channeltron electron multiplier detector to measure the ionic distribution as a function of time. By adjusting the mass analyzer to respond only to a single m / e value and operating the plasma chromatograph in the normal one-grid pulsing mode, the distribution of an individual ion as a function of time can be obtained. The arrival time of the individual ions can then be compared with the arrival time of the ions measured in the total ion mode to produce the mass-identified mobility spectra. A Nicolet Model SD-721A integrating ADC mounted in a Nicolet Model 1074, 4096-channel signal averager was used to digitize the accumulated plasmagrams. Usually 512 scans of 27-ms duration were collected and stored on magnetic tape with a Nicolet Model NIC-28A magnetic tape coupler and Kennedy Model 9700 tape deck. As the data were being accumulated in memory, the information was displayed on a Tektronix Model D10 oscilloscope. The data stored on the magnetic tape were entered into an IBM System/370 computer and were subsequently analyzed using VSAPL under the operating system VM/370. The position and intensity of the peaks in the mobility spectra along with a plot of the mobility spectra were then displayed on a graphics terminal from which hard copies could be obtained. The ultrahigh purity nitrogen and the zero grade air were obtained from MG Scientific Gases of Somerville, N.J.; the sulfur hexafluoride was obtained from Matheson Gas Products, Inc.
RESULTS AND DISCUSSION T h e interpretation of plasma chromatographic data is dependent upon an understanding of ion-mobility theory and the nature of ion-neutral molecule interaction a t atmospheric pressure in weak electric fields. Extensive studies have been made in the field of ionic mobility (16, 17); most studies of ion-neutral molecule reactions have been conducted at considerably lower pressures. T h e drift time, tD, of an ion in the plasma chromatograph can be expressed as 1 P 273 L tD=----
KO 760 T E
where KOis the reduced mobility of the ion, P is the pressure measured in Torr, T is the absolute temperature, E is the electric field, and L is the length of t h e drift tube. This equation shows t h a t an increase in pressure or a decrease in temperature will result in a n increase in the drift time of a n ion. This is due to the increase of ion-neutral molecule collisions as a consequence of t h e increase in t h e molecular density in the drift tube. 0 1979 American Chemical Society
I
L
A
GAS E X I T
SAMP
---+
Q U A D RODS ION MOLECULE REACTOR
E l I O N SOURCE MULTIPLIER ANODE GRID
MULTIPLIER CATHO PC DETECTOR
SCAN GRID
Figure 1. Schematic of the Alpha-I1 Plasma Chromatograph-Mass Spectrometer
I
co,
O2-
C N-
I
I
CI-
I t
d
26
I
L
3 2 35 37
NO;
42
-
J
L 46
52
n
60
(b)
Figure 2. Negative ion mobility spectrum of the reactant ions produced with (a) zero grade air as the carrier gas and nitrogen as the drift gas and (b) the associated negative ion mass spectrum
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
707
MASS 60
MASS 52
-
A
-
rA
MASS 37
MASS 35
I
MASS 26
TOTAL IONS
-
-
Figure 3. Total ion and Individual mass-identified ion mobility spectra of the negative reactant ions produced with air as the carrier gas and nitrogen as drift
T h e general theory of ion mobility in weak electric fields, based upon the kinetic theory of gases, can be expressed as
where K is the ionic mobility, N is the density of uncharged drift gas in molecules per cm3, e is the charge on the ion, m is the mass of the ion, M is the mass of the neutral drift gas molecule, k is the Boltzmann constant, T is the absolute temperature, and Ro is the average collisional cross section. From Equation 2 it can be seen that as the mass of the ion becomes much larger than the mass of the drift gas molecules, the mass dependent term of the mobility equation becomes insignificant. Therefore, the mobility of a large ion is controlled by the average collisional cross section of the ion with the neutral drift molecules. The evaluation of the average collisional cross section is dependent upon several factors, the most important of which are ionic size and the nature of the interaction forces between the ions and neutral drift gas molecules (18,19). Therefore, it is desirable from the point of view of increasing the resolution of heavy ions in the mobility spectra to use a heavy drift gas. Xenon and sulfur hexafluoride are possible candidates, but xenon is economically unfavorable. Initially it was thought that sulfur hexafluoride would be unsuitable as a drift gas because of its high electron capture cross section. However, the results of this investigation showed t h a t these fears were unwarranted. The identity and formation of the negative reactant ions produced with air as the carrier and nitrogen as the drift gas have been discussed previously (14). For comparison purposes, the negative ion mobility spectrum and the total ion mobility, as well as the individual mass-identified mobility spectra, are shown in Figures 2 and 3, respectively. The most unexpected feature of these spectra is the absence of a mobility peak molecular ion under the experimental associated with 02-. conditions used in this investigation. At 483 K the coefficient of thermal detachment of an ion is greater than the coefficient of electron from the 02-
thermal attachment (20-22). However, when the negative ion mobility spectra of the reactant ions produced from zero grade air carrier gas are compared with nitrogen, air, and sulfur hexafluoride as the drift gas, a definite mobility peak is observed for 02-with air and SF6as the drift gas as shown in Figures 4 and 5 . The reason for this is the third-body efficiency of O2 and SF6 upon the stabilizing effect in the three-body electron attachment to O 2 (20).
e-
+ O2 + M
5 0~ + M
(3
The role of the third body in this reaction is to absorb the excess energy from the 02-ion after the electron attachment and to aid in the conservation of the total angular momentum of the system. In addition to the conservation of the orbital angular momentum, the spin angular momentum must also be conserved in the attachment process. Therefore, because O2 and SF, have a greater stabilizing effect for thermal attachment than does NP,the forward reaction rate of reaction 3 is greater than the reverse reaction rate in SF6and air. For N2 as the drift gas the reverse reaction rate is greater. Nitrogen is roughly 100 times less effective than oxygen as a third body (23). The negative ion mobility spectrum produced with air as both carrier and drift gas is shown in Figure 4a, the associated negative ion mass spectrum in Figure 4b. The same ionic species are observed in the negative ion mass spectra obtained from zero air carrier with both nitrogen and air as the drift. However, the intensities of the ions are significantly different between the two drift gases. With air as the drift gas, the O2 molecular ion with an m / e value of 32 is observed to be the most intense ion. In addition to the ions observed with nitrogen as the drift, an ion with an m / e value of 64 identified as 04is found using air as the drift. The negative ion mobility spectra of air carrier with air drift is strikingly different from the spectra of air carrier with nitrogen drift, which is readily seen from a comparison of Figures 2 and 4. The negative ion mobility spectrum with nitrogen drift contains five peaks which have been identified
708
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
3.31
2.76 2.55 (a)
26
37 32 3 5
42
46
52
60
64
76
(bl
Negative ion mobility spectrum of the reactant ions produced with (a)air as both carrier and drift gases and (b) the associated negative ion mass spectra
Figure 4.
as CN-, C1-, CNO-, O(H20)2-,and COS- ( 1 4 ) . The negative ion mobility spectrum obtained with air, on the other hand, contains three main peaks. The total ion mobility spectra and the individual mass-identified mobility spectra of the negative ions formed with air as the drift gas are shown in Figure 6. The distinct mobility peaks observed for the CN- and C1 ions using nitrogen as drift are not observed when air is used as the drift gas. These peaks are obscured by the large mobility peaks comprised mainly of 02-and 0,. The 02-and 04-ions are observed under the same mobility peak due to the reversible reaction. 02-
+0 2 +M
04-
+M
(4)
T h e mobility peak observed is equal to the mobility of the 02-ion times the fraction of the current carried by the 02ion plus the mobility of the 04-ion times the fraction of the current carried by the 04-ion, which can be expressed as:
KT = Xo,-Ko,-
+ Xo,-Ko,-
(5)
where KT is the mobility of the peak observed in the spectrum, X0,-and X0,-are the fractions of the current carried by the
respective ions and KO,- and KO,- are the mobilities of the specific ions. T h e mobilities of the NOz-, CNO-, O(HzO)z-, and COS- ions are observed to be similar, whether air or nitrogen is the drift gas. These results are in agreement with the results obtained by Cohen and Wernlund (24). The negative ion mobility spectrum associated with air carrier and SF, as the drift gas is shown in Figure 5a; the negative ion mass spectrum is shown in Figure 5b. As with air as the drift gas, the 02-molecular ion is the most intense ion in the negative mode mass spectra. Several ions in the mass spectra were also found with either nitrogen or air as drift, such as CN- ( m / e 26), C1- ( m / e35, 371, CNO- ( m / e42), NOz- ( m / e 46), O(H20)z-( m / e 5 2 ) , COB-( m / e 60). As with the air drift gas, the 04-with a n m / e value of 64 was also observed. A negative of m / e 50 is observed when the drift gas is SF,, but not when the drift gas is air or nitrogen; this ion is most likely 02(HzO)-. The thermodynamics of the hydration of 02-in the gas phase to yield O2-(HZ0),with n values up to 3 have been studied by Arshadi and Kebarle (25); they determined that reaction 5 is 18.4 kcal/mole exothermic. 02-
+ H20 Z OZ(Hz0)-
(6)
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6,MAY 1979
709
O(H
02(H20)-
co,
CNO-
26
32 3537
4244 465052
6 0 64
76
96
(bl Figure 5. Negative ion mobility spectrum of the reactant ions produced with (a) air as the carrier and SF, as the drift and (b) associated negative ion mass spectrum between 10 and 1 10 amu
The bimolecular rate constant for the neutral interchange reaction of 04-and water a t 300 K was measured by Adams, Bohme, Ferguson, et al. (26) to be 1.0 x cm3 molecule-' s-' for reaction 7.
04-+ H20 5 0 2 ( H 2 0 ) -+ O2
gas is that the zero grade air is considerably drier than the SF6. The negative ion mobility spectrum with SF6drift is similar to that with air as the drift. Once again the CN- and C1mobility peaks are obscured by the 02-, 04-, and Oz(H20)- ion contribution. The total ion mobilitv swctra and the individual mass identified mobility spectra are shown in Figure 7. The ion of mass 42 (CNO-), 60 (COS-),and 52 (O(H20)2-)have been discussed previously. From Figure 7 it can readily be seen that the ions with m f e 32, 02-, and the ions with mle 50, Oz(H20)-, are under the same mobility peak. This suggests that reaction 6 is occurring. It should be noted that this peak, with SF, carrier, is of lower mobility than the corresponding peak with air as the drift gas. There are two possible exI
Reaction 6 is probably the predominant mechanism for the formation of the Oz(H20)- ion because of the greater concentration of 02-ions compared with 04-ions. However, the rate constant measured for reaction 7 indicates that almost every encounter between water and 04-leads to the interchange reaction; therefore, this reaction cannot be neglected in a discussion of the formation mechanism. The reason the 02(H20)-is observed with the SF6 and not with the air drift
_
710
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
MASS 3 2
MASS 5 0
MASS 52 MASS 60
MASS 4 2
Total ions
Flgure 6. Total ion and selected individual mass-identifiedion mobility spectra of the negative reactant ions produced with air as both the carrier and drift gas
MASS 3 2
MASS 60
MASS 46 MASS 5 2 MASS 4 2
Total ions
Flgure 7. Total ion and selected individual mass-identified ion mobility spectra of the negative reactant ions produced with air as the carrier gas and SF, as the drift gas
planations for this. The 02(H20)ion may have a lower mobility than either the 02-or 04-ion, so that its contribution lowers the overall mobility of the peak. Also, SF6may increase the average collisional cross section, which would lower the mobility. Both of these mechanisms probably contribute to the observed result. An additional peak of reduced mobility 2.08 cm2 V-' s-l is observed in the mobility spectrum of Figure 5 , and can be assigned to the SFG- ion based upon mass spectral data. This mobility is considerably higher than the value of 0.57 cm2 V-' s-' obtained by McDaniel (27) and later by Morrison, Edelson, and McAfee (28). When the drift gas was changed from SF6 back to nitrogen, the mobility of the SF6-was found to change
drastically. The residual SF, left in the plasma chromatograph formed the SF6-and SF6-ions, and the peak in the mobility spectrum drifted toward a higher mobility after starting from a mobility approximating the value found by McDaniel. This phenomenon is presently under investigation in an attempt to elucidate the mechanism involved. The change in mobility when changing from SF6to N2as the drift is not surprising. This can be accounted for by the decreasing concentration of SF6in the mixed drift gas during the change-over, with the associated change in mobility from lower t o higher value. However, the high mobility of SF6- in the SF6drift gas is not entirely clear. One explanation may be that charge transfer is taking place in the drift region of the PC tube which gives
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Table 11. Negative Reactant Ion Mobilities from N,. Air. and SF, Drift Gas
ion
CNCI 0 2 -
CNONO,O,(H,O)O(H,O),-
c0,0 4 -
SF,'
ionic weight, amu 26 35,37 32 42 46 50 52 60
64 146
mobility cm2 V-' N2
3.37 3.01 2.75 2.76 2.69 2.57
SF,
3.37 3.01 3.31 2.76 2.76
3.37 3.01 3.17 2.77 2.77 3.17 2.70 2.58 3.17 2.08
2.55 3.31
routines used in this study. The author is also indebted to Carl G. Majtenyi for the maintenance of the plasma chromatography-mass spectrometer instrument.
s-l
Air
rise t o the apparent increase in mobility. Also the intensity of the SF6-ions is lower with the SF6drift gas. This may be due to the binary ionic recombination in the gas phase (29). T h e mobility of the negative reactant ions observed for air carrier gas with nitrogen, air, and sulfur hexafluoride as the drift gas are listed in Table 11. From this investigation, it is apparent that the negative ion mobility spectrum is dependent upon the nature of the drift as well as the carrier gas. For example, the examination of chlorinated compounds with plasma chromatography using air as the drift gas would have been complicated by the large mobility peak associated with t h e 02ion. From this standpoint, the use of nitrogen as the drift gas was rather fortuitous. Also the negative ion mobility spectra associated with sulfur hexafluoride was a pleasant surprise. Because of its greater molecular weight and molecular size and because of its insulating characteristics, sulfur hexafluoride may prove to be of great value as a drift gas. Several other gases are now under investigation for use as carrier and drift gases in an effort to enhance the selectivity and sensitivity of the PC/MS technique.
711
LITERATURE CITED (1) (2) (3) (4) (5) (6) 17) i8j (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
(25) (26) (27) (28) (29) (30)
H. E. Rivercomb and E. A. Mason, Anal. Chem., 47, 970 (1975). F. W. Karasek, Anal. Chem., 46, 710A (1974). R. A. Keller and M. M. Metro, Sep. Purif. Methods, 3, 207 (1974). S . A. Benezra, J. Chromatogr. Sci., 14, 122 (1976). T. W. Carr, J. Chromatogr. Sci., 14, 85 (1977). R. A. Keller and M. M. Metro, J. Chromatogr. Sci., 12, 673 (1973). F. W. Karasek and D. A. Kane. Anal. Chem.. 46. 780 119741. .. . ~ F. W. Karasek and S. H. Kkn,-Anai, Chem., 47,1168 (1975j. F. W. Karasek, M. J. Cohen, and D. I.Carroll, Anal. Chem., 43, 390 (1971). G. W. Griffin, I.Dzidic, D. I. Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem., 45, 1204 (1973). F. W. Karasek. D. W. Denney, and E. H. DeDecker, Anal. Chem.. 46, 970 (1974). R. A. Keller, Am. Lab., 1975 (7) 35. D. I.Carroll, I.Dzidic, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, 1956 (1975). T. W. Carr, Anal. Chem., 49, 828 (1977). F. W. Karasek, S . H. Kim, and H. H. Hill, Anal. Chem., 46, 1133 (1978). E. W. McDaniel and E. A. Mason, "The Mobility and Diffusion of Ions in Gases", John Wiley and Sons, New York, 1973. E. A. Mason and H. W. Scharp, Ann. Phys., 4, 233 (1958). P. L. Patterson, J. Chem. Phys., 56, 3943 (1972). S . N. Lin, G. W. Griffin, E. C. Horning, and E. W. Wentworth, J. Chem. Phys., 60, 4994 (1974). A. V . Phelps and J. L. Pack, Phys. Rev. Lett., 6, 111 (1961). J. A. Ratcliffe and K. Weckes, "The Ionosphere in Physics of the Upper Atmosphere", Academic Press, New York, 1960. E. W. McDaniel, "Collision Phenomena in Ionized Gases", John Wiley and Sons, New York, 1964. L. M. Chanin, A. V. Phelps, and M. A. Biondi, Phys. Rev. Lett., 2, 344 119591. M. J. Cohen and R. F. Wernlund, "Multiple Chemical Analysis with the Ion Molecule Reactor Mobility Spectrometer", presented at the 28th PittsburghConference on Anawical Chemistry and Applied Spectroscopy, Cleveland, Ohio, Feb. 28-Mar. 4, 1977. M. Arshadi and P. Kebarle, J. Phys. Chem., 74, 1483 (1970). N. G. Adams, D. K. Bohme, D. B. Dunkin, F. C. Fehsenfeld, and E. C. Ferguson, J. Chem. Phys., 52, 3133 (1970). E. W. McDaniel and M. R. C. McDowell, Phys. Rev., 114, 1028 (1959). J. A. Morrison, D. Edelson, and K. B. McAfee, Fourteenth Annual Gaseous Electronics Conference, Schenectady. N.Y., October 31, 1961. M. J. Church and D. Smith, Int. J. Mass Spectrom. I o n Phys., 23, 137 (1977). S. H.Kim, K. R. Betty, and F. W. Karasek, Anal. Chem.. 50, 2006 (1978). ~
~
~
ACKNOWLEDGMENT T h e author expresses his appreciation to Gerald W. Peterson for programming the data acquisition and data analysis
RECEIVEDfor review December 6, 1978. Accepted February 9, 1979.
