2006
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
(3) H. W. Hoyer and S. Nevins, in "Analytical Calorimetry Proceedings Sympossium", Vol. 3,R. S. Porter and J. F. Johnson, Ed., Plenum, New York, 1974,p 465. (4) Y. Baba, S. Tanka, and A. Kagemoto, Polym. J . , 7,641 (1975). (5) J. W. Donovan and C. J. Mapes, J. Sci. Food Agric., 27, 197 (1976). (6) A. Finch, and D. A. Ledward, Biochim, Biophys. Acta, 278, 433 (1972). (7) Ann-Christin KochSchmidtand K. Mosbach, Bicchemisby, 16, 2101 (1977). (8) B. Cassel, T. Ohnishi, in Ref. 3, p 147. (9) S. Mabrey and J. M. Sturtevant, Proc. Natl. Acad. Sci. USA, 73,3862 (1976),Biochemistry. (10) D. L. Melchior, F. J. Scavitto. M. T. Walsh, and J. M. Steim, Thermochim. Acta, 18. 43 (1977). (1 1) B. Cassel, Hifachi Sci. Instrum. News, 73(16),no. 5. (12) L. F. Whiting and P. W. Carr. in "Analytical Colorimetry Proceedings Symposium", Vol. 4,R. S. Porter and J. F. Johnson, Ed., Plenum, New York, 1977,p 67. (13) H. M. Heuvel and K. C. J. B. Lind, Anal. Chem., 42, 1044 (1970). (14) M. J. Richardson and N. G. Savill, Thermochim. Acta., 12, 213 (1975). (15) C. M. Guttman and J. H. Flynn, Anal. Chem., 45, 408 (1973). (16) M. J. Richardson and P. Burrington, J . Thermal Anal., 6, 345 (1974). (17) B. A. Finlayson in "Mathematics in Science and Engineering", Richard Bellman, Ed., Vol. 87, Academic Press, New York, 1972. (18) L. F. Whiting and P. W. Carr, J . Electroanal. Chem., 81, 1 (1977). (19) R. Melling, F. W. Wilburn, and R. M. McIntosh, Anal. Chem., 41, 1275 (1969). (20) E. S. Freeman and B. Carrol, J. Phys. Chem., 62, 394 (1958). (21) J. H. Flynn and L. A. Wall, J. Res. Natl. Bur. Stand., Sect. A , 70(6), 487 (1966). (22) A. W. Coats and J. P. Redfern, Nature (London), 201, 66 (1964). (23) B. N. N. Achar, G. W. Brindley, and J. H. Sharp, Proc. I n t . Clay Conf., Jerusalem, 1, 67 (1966). (24) T. Ozawa, J. Thermal Anal., 2 , 301 (1970). (25) C. Spink and I. Wadso, "Methods of Biochemical Analysis", Vol. 23,David Glick, Ed., John Wiley and Sons, New York, 1976,p 66. (26) J. H. Flynn in "Thermal Analysis", Proc. Int. Conf., H. G. Wiedermann, Ed., Birkhauser, Switzerland, 1972,p 127. (27)J. H. Flynn, in Ref. 3. p 17. (28) J. V . Villadsen and W. E. Stewart, Chem. Eng. Sci., 22, 1483 (1967).
Table 111. Range of Dimensionless Parameters Studied 0 = D,/ZL2
D,/ZL2 p = AHD,C,IKT, y = AE/RT, r,* = r,/ZT, =
qsarnp
4ref
4, 4, R rS
rS
t t*
*
T TO T* V X
3.'
Z a
8 Y
4 P
8
largest value
smallest value
5.2 x 10-2 5.2 x 3.7 x 10-3 88.5 7.3 x
6.1 x 1 0 - 3 9 6.1 x 3.3 x 10-4 5.53 7.0 x
-
h e a t flux from sample h e a t flux from reference h e a t flux for ideal reaction calculated ( s i m u l a t e d ) h e a t flux allowing for temperature a n d concentration gradients in sample a n d reference gas constant t e m p e r a t u r e scan rate dimensionless t e m p e r a t u r e scan rate time dimensionless t i m e temperature initial t e m p e r a t u r e dimensionless t e m p e r a t u r e sample volume linear distance in sample dimensionless linear distance in sample Arrhenius pre-exponential factor fraction reacted of substrate thermicity factor dimensionless activation energy dimensionless t h e r m a l diffusion coefficient density of sample a n d reference dimensionless mass diffusion coefficient
RECEIVED for review September 7 , 1977. Accepted September 5,1978. The authors acknowledge support from an NSF Grant CHE 75-19412 and NIH Grant GM17913 and an ACS Analytical Summer Fellowship (L. F.
LITERATURE CITED (1) P. D. Garn, Grit. Rev. Anal. Chem., 3 (l),65 (1972). (2) H. J. Borchardt and F. Daniels, J . Am. Chem. SOC.,79, 41 (1957).
w.).
Mobility Behavior and Composition of Hydrated Positive Reactant Ions in Plasma Chromatography with Nitrogen Carrier Gas S. H. Kim, K. R. Betty, and F. W. Karasek" The Guelph- Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G7
years as a technique for ultratrace analysis of gas phase samples of organic compounds, for study of ion molecule reactions at atmospheric pressure. and for the study of electron capturing mechanisms. The instrumentation comprises a 63Ni electron capture detector, usually used in gas chromatography, from which ions are extracted and separated in an ion-drift spectrometer. Ions formed in a nitrogen carrier gas react with trace organic compounds introduced to produce characteristic product ions. A review and its references summarize the characteristics and scope of the method ( I ) . T h e reactant ions and their characteristics play an important role in the analytical aspects of plasma chromatography. T h e positive reactant ionic species of PC usually exhibit three distinct ion mobility peaks. The identity of these ions and the reaction mechanism leading to their formation when the carrier gas reacts with 63Ni electrons have been reported by various authors using ion mobility data alone or
Identity and temperature dependence of the ions associated with the three positive reactant ion mobility peaks observed in plasma chromatography (PC) with N2 carrier gas is established by plasma chromatography/mass spectrometry (PC/MS). Equilibrium clusters of the general form (H20),NH,+, (HzO),NOf, and (HzO),H+ are observed, as well as some clusters containing N2. The three ion mobility peaks can be expressed as hydration exchange reaction clusters: H20 (H20),Hf ( n = 2, 3, 4); (H20),_,NO+ (HzO),_lHt HzO $ (H,O),NO+ ( n = 1, 2, 3, ...); and (Hz0),_,NH4+ H,O e (H20),NH4+ ( n = 1, 2, 3, ...). Calculated mobilities using the Langevin and Mason-Schamp equations are in agreement with experimental data with the Mason-Schamp equation giving the best fit.
+
+
+
Plasma chromatography (PC) has been developing in recent 0003-2700/78/0350-2006$01.00/0
c
1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO 14, DECEMBER 1978
2007
,
Figure 1. Schematic diagram of Phemto-Chem MMS-160 combined PC/MS. The Phemto-Chem MMS-160 consists of a Phemto-Chem 100 ion mobility spectrometer coupled with a specially modified Extranuclear Laboratories quadrupole mass spectrometer
PC instrumentation involving an interfaced mass spectrometer (PC/MS) (2-6). Recently. work reported by Carroll (7) and Karasek (8)using PC/R;IS instrumentation leads to the conclusion t h a t the three peaks in t h e P C reactant ion spectrum in order of decreasing mobility are equilibrium mixtures comprised primarily of (H20),NH,+ ( n = 0,1. 2, ...); (H20)"NO+( n = 0,1, 2 , ...). and (H,O),H+ ( n = 2! 3. 4 T h e value of n depends upon the temperature and partial pressure of H 2 0 in t h e system. It has been observed that the reduced mobility (KO)of these equilibrium mixture ion mobility peaks increases with a temperature increase, contrary to t h e temperature independent mobility behavior predicted theoretically for a single ion. It is the purpose of this study to present further evidence for t h e composition of these cluster ions in a given mobility peak a n d t o compare t h e experimentally observed KOvalues for these reactant ion mobility peaks as a function of temperature with calculated mobilities using t h e Langevin polarization limit equation (9, 1 0 ) and t h e Mason-Schamp equation (11, 12). EXPERIMENTAL Total ion mobility spectra, mass spectra, and mass identified ion mobility spectral data were obtained using Phemto-Chem MMS-160 Ion Mobility Spectrometer-Mass Spectrometer as shown in Figure 1 (PCP Inc., 2155 Indian Rd., West Palm Beach, Fla. 33409). The instrument consists of a plasma chromatograph coupled to the quadrupole mass spectrometer. The ion mobility spectrometer used in the Phemto-Chem MMS-160 model has a total drift length of 11.61cm. which is comprised of a 5-cm section between two grids and another 5 + x cm between the last grid and the ion collector of the mass spectrometer. The distance I arises from drift length contributed by the sample orifice system of PC/MS interfaced instrument, which by calculation was found to be 1.61 cm. The mass spectral data were obtained by holding both gates (GI, G2)open (Figure l), allowing all the ions formed in the plasma chromatograph to continuously drift down through the tube and into the quadrupole mass spectrometer. The mass spectrometer was then scanned to produce a mass spectrum of the ions present. Total ion mobility spectra were obtained by operating the G, grid of the plasma chromatograph in the normal gating fashion with the Gz grid of the plasma chromatograph continuously open. and monitoring the drift times of the ions with the total ion monitor of the mass spectrometer. Finally, mass identified ion mobilities were obtained by operating the PC with the same grid gating procedure as the above but tuning the mass spectrometer to respond only to those ions having a specific m / e . To obtain accurate ion drift data, two sets of the same ion mobility spectra were recorded by the operation of the single gating mode for G I or G1, respectively. The drift time difference between the two peaks recorded by the operation of the two different modes corresponds to the drift time required for traverse of the exact distance of 5.0 cm of drift length. All the data obtained with the Phemto-Chem MMS-160 instrument were taken by signal averaging a given number of 20-ms scans in a Nicolet signal averaging
Figure 2. Total ion mobility spectra of positive reactant ions in PC taken using the Phemto-Chem MMS-160 (PCIMS). Trace a, gate 1 pulsed, gate 2 open; trace b, gate 2 pulsed, gate 1 open. The time difference between trace B and b is the ion drift time for a drift tube length of 5 cm computer (FT 1072: Nicolet Instrument Inc., Madison, \Vis. 53711). All the spectral data used in this study were subjected to multiple 3-point curve smoothing operations. The operating parameters, unless indicated otherwise in the figure captions, were as follows: N2 drift gas flow rate, 500 m l j m i n , N2 carrier gas flow rate, 100 mL/min; gate width, 0.2 ms; electric field gradient, 214-247 V/cm; typical pressure, 762-764 Torr. Carrier and drift gas inlet and sample injection port temperatures are shown in the figure captions. For samples involving NH,,the sampling technique used with the PC/MS instrument was injection by microsyringe of the head space vapor from a ",OH solution. The ammonium hydroxide solution was obtained from J. T. Baker Chemical Company. Both carrier and drift gas were N,,Linde high purity (99.996%). All of the reduced mobility data of the reactant ionic species throughout the temperature range investigated in this study were obtained using the Beta VI plasma chromatograph ( I ) . To ensure accurate reduced mobility data, the instrument was baked at 240 "C for 3 days with a high flow rate of carrier gas. The temperature was allowed to stabilize for 1 h after reaching the temperature set before sample introduction. Three or four different measurements were made for each temperature point a t 5-10 "C intervals and the average of those measurements was adopted. The temperature for an observation was reached both by cooling down from high temperature and by heating up from a lower to high temperature. Experimental reduced mobilities were calculated using the conventional equation:
KO=
d tE
273 T
- x -x
P 760
-
(KO= cm'/V.s)
(I)
where d = drift length (cm),t = drift time (s),E = electric field gradient (V/cm), T = temperature (K), and P = pressure (Torr). The operating conditions for the Beta \'I PC were as follows: N2 carrier gas, 65 m l j m i n , N2 drift gas, 330 m l j m i n ; electric field gradient, 179 to 214 Vjcm; typical pressure 726-734 Torr. RESULTS AND DISCUSSION T h e ion mobility spectrum of reactant ions obtained using the PC/R;IS as shown in Figure 2 appears essentially the same as that obtained with the Beta VI PC instrument alone. In the P C / M S instrument, ions were injected into the drift tube by the normal gating operation with pulsed and G2 opened for trace a a n d G2 pulsed a n d G I opened for trace b. T h e difference in drift time between tracing a a n d b equals t h a t for the 5 c m drift length between grids GI and GS. T h e ions which had drifted through t h e drift tube of PC were introduced into the mass analyzer through a sample aperture and scanned to be recorded. T o observe the role of the NH3 gas in forming ions of mobility peak I, 0.5 FL of t h e head space of t h e solution of
2008
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
7
B
I O
,
I2 DRIF'
I4 TIME
I6 msr:
Figure 4. (a) Total ion mobility spectra of positive reactant ions under the same conditions as in Figure 3a. (b), (c), and (d) are specific ion mobility spectra at m l e 30, 48, and 58, respectively
i N z b NH:
I
5
7
9 II 13 D R I F T TIME
15
17 m sec
m/e-l02
19
Figure 3. (a) PC/MS total ion mobility spectra of positive reactant ions at 155 OC following injection of 0.5 WL of the head space vapor of 0.03% aqueous ammonium hydroxide solution. Gate 1 pulsed and gate 2 open (drift length 11.61 cm). (b) Specific ion mobility spectra at m / e 18 under the same conditions. (c), (d), (e), (f), and (9) are at m / e 36. 46, 64, 74, and 102, respectively
0.03% of aqueous ammonium hydroxide was injected into P C / M S instrument a t 155 "C. As shown in Figure 3, the intensity of peak I was increased with injection of NH, vapor. Traces b to i show the individual ions having the same drift time are associated with each other as one clustered ion mobility peak with various masses as shown. The identity of the ions appear t o be: m / e 18 = NH4+, m / e 36 = (H20)NH4+,m / e 46 = (N2)NH4+,m / e 64 = (H20)(N2)NH4+. m / e 74 = (NJ2NH4+,and m / e 102 = (N2)3NH4+ respectively. Figure 4 shows mass identified single ion mobility spectra associated with peak I1 of the reactant ions. These ions appear t o be m / e 30 = NO', m / e 48 = (H20)KO+, and mle 58 = (N*)NO+. Figure 5 shows the same relationship for peak I11 of the reactant ions as seen in Figures 3 and 4. Trace a shows the total ion mobility spectrum, and the individual ions can be identified as m / e 37 = ( H 2 0 ) z H +m , / e 55 = (H20)3H+,m / e 65 = ( H 2 0 ) , ( N 2 ) H + ,m / e 73 = ( H z 0 l 4 H + , m / e 83 = (H20),(N2)H+,and m / e 93 = (H20)2(N2)2H+, respectively. A generalized expression for these water and N2 cluster ions can be given as (H20)n(N2),H+(8). The observation of cluster ions involving N2 is not noted in all P C / M S experiments. Some investigators believe there is a strong possibility that cluster ions involving N2 are formed during the adiabatic expansion which occurs after the sample aperture between the PC tube and the quadrupole mass spectrometer ( 1 3 ) . P C / M S data show a considerable percentage of all three reactant ions undergoing a clustering reaction with N2 presumably via hydrogen bonding between N and H. The relative % intensity ratios observed are: (H20),H+:[ ( H 2 0 )(N2)]= 80:18:2. All of the resultant clustering H+:[(HzO)2(N,)2]H+ is dependent upon the water vapor pressure and temperature and relative concentration ratio of NH, to HzO or NO to H20.
. 5
'
7
'
9
1.1 ' 13 DRIFT TIME '
.
.
15
17
.
.
.
19
m sec
Figure 5. (a) Total ion mobility spectra of positive reactant ions under the same conditions as in Figure 3a. (b), (c), (d), and (e) are specific ion mobility spectra at m / e 37, 55, 65, 73, 83, and 93, respectively
A plot of reduced mobilities of peaks I, 11, and I11 vs. temperature is shown in Figure 6. Using these data, the water vapor pressure was calculated using the equilibrium constant values obtained by Kebarle (14) from the relationship:
where I , is the intensity of the (H,O),H+ ion, Zn-l is the intensity of the (H20),_1HCion, and P His ~the~partial pressure of water, respectively. Since a value of 10-20 ppm of H 2 0 in the N2 carrier gas was given by the manufacturer, and the gas
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
; 3.0
2009
-
2.9: 22.8-
2.72.6: 2.5-
EV 2.4-I
a
2.3-
= 2.22.1, 20
. , , , , , , , , , , , , , , , , ,
30
70
50
90 110 130 TEMPERATURE
"C
170
150
190
210 MA
Figure 6. Reduced mobility vs. temperature for the reactant ions (H,O),H+,
(H,O),NO+,
6.31
32 j
IO
30
502
537
39s
437
377
371
372
and (H,O),NH,+
1 9 4 , .
20
s s O F fn.o)n n*
5S8
506
40
50
60
70
80
,,e90
om"
Figure 7. Mass spectrum of reactant ions observed by PC/MS with both PC gates open and signal averaging 2048 scans each 0.15 ms
was passed through a molecular sieve 13X trap prior to use, Torr (-1 ppm) via = our calculated value of PHzO Equation 2 appears to be a reasonable one to use in our calculations (7). A mass spectrum of the reactant ions obtained with both P C gates open is shown in Figure 7 . ( H 2 0 ) n H +Ions. T h e relative percent fraction of the protonated water cluster ion I,/ I,-1, at equilibrium with PHzO of Torr throughout the temperature range (30-210 "C) measurable in P C can be calculated using the K,-l,, values a t various temperatures. T o obtain the percent relative intensity of the (H,O),H+ clusters, we sum the ionized fractions to 100% as follows: J
( J = 5 or 6 maximum in this system) and plotted the total ionization vs. temperature. The upper plot of Figure 8 shows the fraction of Mobility Peak I11 due to (H,O),H+ vs. temperature for n = 2 , 3, 4. To indicate how the equilibrium is affected by changing the water pressure by one order of magnitude, plots are given for two different water vapor Torr, pressures. T h e closed circles represent a PHlOof and the closed squares represent a P H ?of~ Torr. At 200 "C, this peak is composed almost entirely of (H20)*H+for both partial pressures. Similar calculations have been done for the Torr, and are other two reactant ion peaks a t a PHzOof shown in the top portions of Figures 9 and 10. These theoretical calculations permit calculation of the composite mass associated with an ion mobility peak at any desired temperature for each equilibrium mixture of (H20),H+, (H20),NO+, and (H20),NH4+. The composite masses of these ions a t various temperatures are given in the
20
,
40
.
,
.
.
,
,
I
80 100 TEMPERATURE
60
.
,
.
120
140 * C
I
,
160
.
,
150
.
.
I
200
Figure 8. (Top): Distribution of protonated water cluster ion as total ionization percent vs. temperature at fixed water vapor pressure. Closed Torr and closed squares are for PHpO= circles are for PH20= I O 4 Torr. (Bottom): Reduced mobility (KO)vs. temperature for (H,O),H+. See text for discussion
upper scale of the abscissa in the lower plots of Figures 8, 9, and 10. Since we have calculated the mass of the mixtures of clustered ions in a reactant ion mobility peak, we can use this value t o calculate their respective rnobilities at any temperature using the theoretical equation. These can be compared with the experimentally observed mobilities of the three reactant ions as temperature increases. The curves in the lower portion of Figure 8 are the reduced mobilities ( K Oof ) the (H20),H+mixture calculated using the Langevin equation (polarization limit) (9) and using the Mason-Schamp equation ( 1 1 ) compared to the mobility curve experimentally measured by PC. T h e theoretical calculation steps undertaken for the ion mobility using the Langevin equation, the Mason-Schamp equation, and the modification of the data of the single ion mobility of reference 17 were as follows: The Langevin Equation used is:
where p is the density of the gas (g/cm3), M is the mass of the neutral gas, m is the mass of the ion. and D is the dielectric constant of the neutral gas. The parameter A is a function of X which is defined by the equation =
grp ( a d 4 (D l)e2
(5)
~
where P is the pressure of the gas and uI2 is the distance between the centers of the molecules and the ion at the moment of contact for a collision. The A values for individual ions can be found according to the A value calculated by
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2010
1
In,@'
lH.OlnN0
* C
TEUPERATURE
H4SS
-
34
5+3
5tS
470
OF
426
1HpO)nNO'
354
W7
y 2
303
391
304
1
.33
>3 2.
I 2
dvr 2'
40
, , , v r , , , 80 100 120 TEMPERATURE
60
22:
L Of_, 20 140
. C
160
180
200
220
Figure 9. (Top): Distribution of (H,O),NO+ as total ionization vs. temperature at fixed PHpO= Torr. (Bottom): Reduced mobility ( K O )vs. temperature for (H,O),NOf. See text for discussion
Equation 5. A tabulation of A vs. X has been compiled by Hasse (9). In this study, we have obtained the radii of the ions by using the average radius in the x,y,and z axes. Lin has used a molecular frame model for the ion molecule (15). The radius of the N2molecule was calculated to be 1.58 A from the constant b of the Van der Waals equation. T h e Mason-Schamp equation (10) used is:
\there e is electronic charge (esu), N is gas density (g/cm'). m is the mass of ion, M is the mass of the neutral gas. k is the Boltzman constant, 1 is a correction term for the higher order approximation, and is 0 for the first order when m >> M , and r, is a measure of the closest approach of the ion and neutral molecule during binary collision. T o reduce the ratio of the observed mobility to that calculated by the Langevin equation (K/Kpo1)where this ratio falls below 1.0,we adopted a 7 value of 0.65, where 7 is the parameter for the relative strength of the lo4 term in the 12-6-4 ion-molecule interaction potential equation (12). Patterson (16) has reported the relation between r,/uI2 and 7 (see Figure 2 of reference 16). Using this relationship, we calculate rmvalues for every ion, since we obtain u12 for every ion with the measured radius and the Van der Waals radius of N2. Finally Q ( l , l l * values against T* as a dimensionless temperature have been tabulated for values of 7 u p to 0.50 ( I / ) . Extension of the table beyond 7 = 0.50 is accomplished by interpolation between these tables and the results for a 12-6-4 potential equation. T" is defined by
kT T* = t
and is the ratio of thermal energy to the potential well depth
do
'
6'0'
k
r
160
120 '
'
TEUPERATURE
-----.---.-I 140 160
I80
200
220
* C
Figure 10. (Top): Distribution of (H20),NH4+ as total ionization percent vs. temperature at fixed PHZO = Torr. (Bottom): Reduced mobility ( K O )vs. temperature for (H,O),NH,'. See text for discussion
of the ion and molecule, where t is related to the molecular size parameter r , by the equation t =
e2fi
3rm4(1-
(8)
where ci is the polarizability of N2 in Other ion mobility data available for comparison are the experimentally measured ion mobility data reported by Young ( 1 7 ) . He measured the mobility of single ions of (H,O),H+ ( n = 1, 2: 3, 4, 5) in a n O2 gas stream when PH20= 1.2% in the absence of rapid interconversion of ions under various ratios of E / P o ,where E is the electric field gradient and Po is the temperature reduced pressure. T h e KO value for (H20)H+under PC conditions (0.33 V/cm Torr) was obtained by extrapolation of the curve of M o ( K O vs. ) E / P Oin Figure 4 of reference 17 over the different E / P Oratios employed in this work. It was concluded that the ratio of mobilities relative to (H,O)H'. of (H,O),H+, (H20I3H+,(H20)4Ht,and (HLO)5Ht were 0.87'7, 0.825, 0.796, and 0.778, respectively. These will differ in nitrogen carrier gas because of the polarization attraction ( I O ) and the permanent quadrupole moment (12). Using the Langevin polarization limit in Equation 4 the KO value in an N, gas stream can be calculated by (7):
where Do,is the dielectric constant of O2 (= 1.000523), D ~ J ~ is the dielectric constant of N2 (= 1.000380) (241, and KO (0,) is the mobility for a single (H20),H+ obtained by Young et al. (17). These results are tabulated in Table I. values and the relative fraction of water Using these cluster ions as shown in Figure 8 (top),the ion mobilities over the temperature range shown were calculated and plotted in the bottom figure of Figure 8. As one can see, the mobilities
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Table I . Theoretical Reduced Mobility Values for Water Cluster Ions with Oxygen and Nitrogen Drift Gas Kn(o,) cm2/V.s
ion (H,O)" (H,O),H+ ( H,O ) 1H+ (H,O )$+ (H,O),H+
Ko(N,)
2.02
2.08
1.98
2.41 2.16 2.04
calculated by the Langevin equation appear to be 18-2270 higher than the mobilities observed by PC throughout the temperature range studied. The mobility curve calculated by the Mason-Schamp equation shows excellent agreement with the observed mobility curve, especially under 100 "C, where the two curves coincide. Above 100 "C,the difference becomes somewhat larger but is still only 4% lower than the observed mobility a t 210 "C. The curve of the experimental mobilities based on the data of Young (17)appears 2-4'70 lower in the 30-50 "C range and i-lO% lower a t higher temperatures than the mobility data observed by PC. In Figures 8 , 9 , and 10, the components of the equilibrium mixture (top figure) appear to be almost entirely (H20)*H+,NO+, NH4+,respectively, a t 200-210 "C, indicating that the mobility values will approach a constant asymptotically. Those values correspond to the mobilities revealed for (H20)2H+,NO', and NH4+, respectively, in Figures 8, 9, and 10. In Figure 8, the mobility of (H20),H+ is KO= 3.05 by the Langevin equation, 2.42 by observation using PC, 2.34 by the Mason-Schamp equation and 2.16 using data from reference 17. (H20),NO+Ions. Even though the origin of NO gas in dry nitrogen carrier gas is not clearly known, several different experimental results (18-20) have demonstrated and established the mechanism for formation of NO+ or (H,O),NO+ depending on the temperature and PHBo.Since 20 ppm of O2 can exist in the N 2 gas used, the most probable reactions leading to the formation of NO+ under PC ion source conditions would be the sequences:
-
+ N2 (10) o*++ h - 2 NO + NO+ (11) S O + N2+ z NO+ + N, (12) NO+ + HZO + N2 2 (H,O)NO+ + S 2 (13) (H,O)NO+ + H 2 0 + N2 z (H,O),NO+ + N, (14) (H,O),-,NO+ + H 2 0 + N2 t (H,O),NO+ + N 2 (15a) (H2O),_lNO+ + XHZO + YN, (H20),(N2),NOt + ( X 1)HzO + ( y n)N2 (15b) -
reaction rate constant Kf for reaction 13 was reported to be 1.8 X cm6 molecule-2s-1 (20). The half life for the forward reaction is:
cm2/V.s
2.67 2.34 2.20 2.13
N2+ + 02
2011
02f
-
N2+ is formed as the fundamental precursor ion in the ion source by t h e 63Ni@ ray. Ferguson reported that Equation 10 occurs as a charge transfer reaction in the ionosphere in daylight with a rate constant 1.0 X cm3 s-' (20). This reaction could easily occur in the PC ion source. Equation 11 is responsible for the formation of NO+ in the atmosphere a t an altitude of 120-140 km where sufficient atomic state oxygen is available (20). Under these conditions, it has a rate constant of cm3 s-'. However, in N2carrier gas under P C conditions, the reaction of Equation 11 is also possible. If NO is formed via some other reaction mechanism in PC, then the NO+ could be formed by Equation 12 (18). By either Equation 11 or 12, once NO+ is formed, further hydration via Equations 13, 14, and 15a occurs with a rate constant of 10-2'-10~'1cm3 s-' as a three-body reaction (18). The clustering reaction with N2 can occur as reaction l5b. T h e forward
where [ H 2 0 ] and [N2] are the concentrations of water and nitrogen in molecules ~ m - The ~ . rate constant for the reverse reaction can be determined from the calculated equilibrium constant KO,'for Equation 13 using the van't Hoff plot in Figure 5 of reference 18 using the relationship:
Kf
K, = KO,, This KO,'value has units of Torr-' which should be converted to the corresponding units for Equation 13. T h e converted K O ,value appears as 5.05 x lo-'' cm3 molecule and t h e calculated K , is 3.56 X lo-'' cm3 molecule-' s-'. Therefore the half life for the reierse reaction is:
'
t12=--
1
KJNd
- 2.96
X
s
Considering the equilibrium time of 5.03 X lo4 s and the ion drift time 6.0 X s, lo3interchange reactions can take place and resolution by P C is not possible. Based on these considerations, it appears that an equilibrium mixture of (HzO),NO+ ions is responsible for Peak 11. We calculated the and K1.>values for Equations 13 and 14 using extrapolated data from the van't Hoff plot in Figure 5 of reference 18 to get and Kl,2values throughout the temperature range of PC. Once we obtain the Kn-',, values for Equation 15a (generalized form) ( n = 1, 2, etc.), then the relative intensity ratios can be obtained using Equation 2 and PHZo Torr) in the temperature range desired. Figure 9 presents data similar to Figure 8 except for (H20),NO+. In the same way! trace a shows the calculated mobilities by the Langevin equation, trace b shows observed PC mobilities, and trace c those calculated by the MasonSchamp equation. Trace a appears 15207~higher than trace b! while trace c is in good agreement with trace b around 100 "C but 3-6% lower below 60 "C and 5-107~ lower in the 140-220 "C range. (H20),,NH4+Ions. Peak I shows the greatest mobility and the lowest intensity of the three distinctive reactant ion peaks in PC. Various authors (2-6,21) have assigned this ion to be H 2 0 + ,(H2012H+:or ( H 2 0 ) H + . Recently Carroll designated this peak as hydrated KH4+ (7). The origin of NHj+ in N2 gas has also not been clearly established. However, the possibility of NH,, contamination in N,gas is very high because of biochemical activity in the environment (22). The role of NH, in the formation of Peak I F a s earlier shown in Figure 3. The observed ion-molecule clusters with H 2 0 are in good agreement with the results reported by Kebarle (23)and Payzant (24). T h e clustering of H 2 0 with NH, is dependent on the relative concentration ratio of [ KHd]:[HZO]. Considering the trace amount of NH:, present, the direct ionization of NH3 by a lj' ray can be neglected. Therefore, under normal PC conditions, the formation of h",+ is given by:
-
+ (H,O),H+ NH4+ + n H 2 0 (19) NH4++ H 2 0 + N2 s (H20)NH4++ K2 (20) (H20),-'NH,+ + H,O + K2 s (H,O),NH,+ + N2 (Xla) (H,0),-1NH4++ x H 2 0 + yN2 z (H20)n(N2)nlh-H4++ (X 1)H,O + (>' ~ z ) N (21b) , NH,
-
-
Fehsenfeld (22) reported the forward rate constant K f for
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ANALYTICAL CHEMISTRY, VOL 50, NO. 14, DECEMBER 1978
Equations 19 and 20 as 2.0 x lo-' em3 s-'. Using this K f and the value from the van't Hoff plot of Figure 2 of reference 23, K, and the half life can be obtained. The calculated half ) Equation 20, is 8.04 life of the forward reaction, t l l z ( K ffor X s. T h e rate constant for the reverse reaction K,, a t 23 "C, is 1.11 X cm3 5-l. giving a half life for the reverse reaction of Equation 20, tl,z(Kr)r of 9.45 X s. From these arguments, we conclude that the first peak should be assigned as a mixture of NH4+and (H,O),NH,+ species from Equation 21a, as well as some clusters with nitrogen (from Equation 21bj. We have frequently observed that the NH4+peak increases in intensity a t temperatures above 200 "C. The adsorption of NH, on stainless steel surfaces a t lower temperatures, followed by desorption a t higher temperatures and protonation via Equation 19 could be responsible for this observation. The clusters with HzO become unstable a t higher temperature. Again, using Equation 2 and KO,],K1,2 values for (H20),NH4+ obtained from the van't Hoff plot of Figure 2 in reference 24, we have calculated the relative intensity of (HzO)nNH4+series ( n = 0, 1 , 2 ) shown in Figure 10. Similarly, the corresponding masses of (H20),NH4+ species over the temperature range of 3@220 "C are shown in Figure 10. Trace a in t h e lower portion of Figure 10 shows the reduced mobilities calculated by the Langevin equation, trace b shows those observed by PC and trace c those calculated by the Mason-Schamp equation. The mobilities from the Langevin equation are higher than the observed values. The results of the Mason-Schamp equation are in good agreement under 100 "C, but above 100 "C the deviation from the observed reduced mobilities increases. Carefully observed reduced mobilities for the positive reactant ions (H20),H+, (H20),NO+, and (H,0),NH4+ in the plasma chromatography show a temperature dependence. This dependence is due to changes in the amount of ionmolecule clustering, yielding a different equilibrium mixture of clustered ions in each mobility peak a t each temperature. The changes in KOwhich differ slightly from the linear increase reported previously (21) can be calculated from the various equilibrium constants. Observation of these clustered ions by P C / M S suggests that Nzis clustered to (H20j,H+, (H20),NO+, and (H20),NH4+ possibly by hydrogen bonding to form (H,O),(N,),H+,
(HzO),NO+, and (H20),(N2),NH4+, respectively. Although no investigations were performed on the location of these clustering reactions, they could take place either while the ions are in transit in the drift tube, or during adiabatic expansion through the P C / M S sample orfice.
ACKNOWLEDGMENT T h e mass identified mobility spectra were obtained by courtesy of P C P Inc., West Palm Beach, Fla. The authors thank M. J. Cohen, C. Wernlund and R. F. Wernlund a t PCP for direct assistance in obtaining data. The authors also thank S.Rokushika in the Department of Chemistry, University of Kyoto, and D. I. Carroll and S.N. Lin of Baylor College, Houston, Texas, for helpful advice in treating the data.
LITERATURE CITED (1) F. W. Karasek, Anal. Chem., 46, 710A (1974). (2) M. J. Cohen and F. W. Karasek, J . Chromatogr. Sci., 8, 330 (1970). (3) F. W. Karasek, M. J. Cohen, and D. I.Carroll, J . Chromatogr. Sci., 9, 390 (1971). (4) S. P. Cram and S. N. Cheder, J . Chromatogr. Sci., 11, 391 (1973). ( 5 ) E. C. Horning, M. G. Horning, D. I . Carroll, I . Dzidic, and R . N. Stillwell, Anal. Chem-,, 45, 936 (1973). (6) G. W. Griffin, I. Dzidic, D. I.Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem.. 45. 1204 119731. (7) D. I . Carroll, I. Dzidic, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, 1956 (1975). (8) F . W. Karasek, S. H. Kim, and H. H. Hiil, Jr., Anal. Chem.,48, 1133 (1976). (9) H. R. Hasse, Phil. Mag., 1, 139 (1926). (10) E. W. McDaniel, "Collision Phenomena in Ionized Gases", John Wiley & Sons, New York, N.Y., 1964, pp 427-441. (11) E. A. Mason and H. W. Schamp, Jr., Ann. Phys., 4, 233 (1958). (12) H. E. Revercomb and E. A. Mason, Anal. Chem., 47, 970 (1975). (13) D. I.Carroll, Institute for Lipid Research, Baylor College of Medicine, Houston, Texas 77025, Personal Communication, September 1976. (14) P. Kebarle, J. K. Searles, A. Zolla, J. Scarborough, and M. Arshadi, J . Am. Chem. Soc., 89, 6393 (1967). (15) S. N.Lin, G. W. Griffin, E. C. Horning, and W. E. Wentworth, J , Chem. Phys., 60, 4994 (1974). (16) P. L. Patterson, J . Chem. Phys., 56, 3943 (1972). (17) C.E. Young and W. E. Falconer, J , Chem. Phys., 5 7 , 918 (1972). (18) M. A. French, L. P. Hills, and P. Kebarle, Can. J . Chem., 51, 456 (1973). (19) 2. J. Puckett and M. W. Teague, J . Chem. Phys., 5 4 , 2564 (1971). (20) E. E. Ferguson, F. C. Fehsenfeid, P. D. Goldan, and A. J. Schmeltekopt, J . Geophys. Res., 70, 4323 (1965). (21) E. C. Horning, D. I . Carroll, I . Dzidic, P. Haegele, M. G. Horning and R . N. Stillwell, J . Chromatogr. Sci., 12, 725 (1974). (22) F. C. Fehsenfeld and E. Ferguson, J . Chem. Phys., 5 9 , 6272 (1973). (23) P. Kebarle, Adv. Chem. Ser., 72, 24-27 (1968). (24) J. D. Payzant, A. J. Cunningham, and P. Kebarle, Can. J . Chem., 5 1 , 3442 (1973). I
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RECEIVED for review June 5 , 1978. Accepted August 29, 1978.
