(IO)R. S . Braman, L. L. Justen, and C. C. Foreback, Anal. Chem., 44,2195 (1972). (11) J. M. Ondov et al., Anal. Chem., 47, 1102 (1975). (12) U S . Natl. Bur. Stds. Certificate accompanying standard.
RECEIVED for review November 1,1976. Accepted February 19,1977. This research was performed at Oak Ridge National
Laboratory, which is operated for the U.S. Energy Research and Development Administration by Union Carbide Nuclear Corp. under contract No. W-7405-eng-26. The work was supported in part by the U.S. Environmental Protection Agency (IAG-DS-0713)and by the Ecological Sciences Division of this Laboratory.
Negative Ions in Plasma Chromatography-Mass Spectrometry Timothy W. Carr International Business Machines Corporation, F06/ 052, P. 0 . Box 390, Poughkeepsie, New York 12602
The negative reactant lons produced from zero grade air as the carrier gas are identified as CN-, Ci-, CNO-, O(H,O),-, and COS-. The mechanisms leading to formation of the reactant ions are discussed. The response of the plasma chromatograph to the negative ions produced from several chlorinated compounds is examined using ultra-high purity nitrogen and zero grade air as the carrier gas. The negative Ions were found to be of greater intensity using zero grade air than with nitrogen as the carrier.
Plasma chromatography ( I ,2) and atmospheric pressure ionization mass spectroscopy (3,4)have been studied in recent years as methods to detect picogram quantities of organic compounds. The basis of both techniques is the formation of both positive and negative ions through a series of ionneutral molecule reactions occurring at atmospheric pressure. The ions produced from the carrier gas which are used to react with the trace sample molecule are referred to as reactant ions. The identity of the reactant ions formed in both techniques is of particular importance in understanding the formation of product ions. A recent report by Carroll, Horning, e t al. (5), discusses the identity of the positive reactant ions formed using nitrogen as the carrier gas. The negative reactant ions formed using zero air as the carrier gas was the subject of a report by Spangler and Collins (6). The purpose of this study is to reconsider the assignment of the negative reactant ions of zero air and to compare the formation of the chloride ion using nitrogen and zero air as the carrier gas. EXPERIMENTAL The instrument used in this study was an Alpha I1 Plasma Chromatograph-Mass Spectrometer manufactured by Franklin GNO Corporation. This instrument consists of a Beta VI1 plasma chromatograph coupled to a specially modified quadrupole mass spectrometer. The mass spectrometer used is a modified Extranuclear Laboratories spectr-EL quadrupole. The combined PLC-MS instrument has been described in detail elsewhere (7-9). The Alpha I1 PLC-MS may be operated in several different modes. It may be operated solely as a plasma chromatograph capable of measuring the ionic mobility by either a one-grid or two-grid pulsing procedure. The Alpha I1 may also serve as an atmospheric pressure ionization mass spectrometer when both grids of the PLC are in the open position. In a third mode of operation,the channeltron electron muliplier detector of the mass spectrometer may be used to measure the mobility of the ions generated with the PLC in the one-gridpulsing mode and the quadrupolemass spectrometer in the total ion mode. The mass analyzer may then be adjusted to respond only to a single ionic mass. The arrival time of the individual ions could then be compared with the arrival time of the ions measured in the total ion mode. These mobility spectra will be referred to 828
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
Table I. Operation Parameters of the Plasma Chromatograph Drift gas Carrier gas Voltage Gate width Rep rate Temperature
500 cm3/min 100 cm3/min k2800 V 0.2 ms
N, N, or zero air
27.0 ms 210 "C
as total ion mobility spectra and single ions mass mobility spectra, respectively. The operating parameters of the plasma chromatograph used in this study are summarized in Table I. 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-283A magnetic tape coupler and Kennedy Model 9700 tape deck. The data stored on the magnetic tape could be analyzed by reading the tape back through the signal analyzer and displaying the dath on a Tektronix Model D10 oscilloscope. Hard copies of the data could be obtained by recording the data from the signal averager memory on a Hewlett-Packard Model 7035B x-y recorder. The chlorinated compounds used in this study were chlorobenzene, hexachlorobenzene, and 1-chlorobutane. Each was obtained from Chem Service Company's mini stockroom kit. Each of the compounds was used without additional purification. Samples of the aromatic compounds were diluted to the apg/pL chloropropriate concentrations in benzene; 14 X benzene, and 17 X lo-'' g/pL hexachlorobenzene. The 1chlorobutane was diluted to 17.7 X lo-' g/pL in n-hexane. The benzene and n-hexane were obtained from Fisher Scientific Company. The samples were measured by the syringe injection of 1 pL of the solutions directly into the sample tube.
RESULTS AND DISCUSSION Identification of Negative Reactant Ions. The positive reactant ions were observed to be the same for both nitrogen and zero air used as the carrier. The data obtained in this study are in agreement with the results obtained by Carroll, Horning, e t al. (5), in their study of the positive reactant ion formed in a nitrogen carrier gas system. Three peaks are observed in the positive mode mobility spectrum, with the corresponding ionic weights of 18, 30, and 37 as the predominant species. (See Figure 1.) Spangler and Collins (6) tentatively identified the negative reactant ions formed in zero air carrier gas as OF, OH-(H,O),, and O;(H,O), on the basis of mobility data alone. The mass spectral data obtained in this study do not agree with their assignments. A typical mobility spectrum observed for the negative reactant ions of zero air carrier gas is shown in Figure 2. The
CNO-
b
I
b
18
30
37
Figure 1. Positive mode spectra for zero air carrier gas: (a) mobility spectrum, (b) mass spectrum
Flgure 2. Negative mode spectra for zero air carrier gas: (a)mobility spectrum, (b) mass spectrum
Table 11. Reactant Ion Identification and Mobility
Peak
Ion
Mol wt
Drift time, ms
1 2 3 4 5
CNc1CNOO-(H,O),
26 35, 37 42 52 60
6.58 7.38 8.08 8.26 8.62
c0,-
-a This work.
i:,
Mobility
cmz a
b
3.37 3.01 2.75 2.69 2.57
3.28 2.94 2.70 2.59 2.53
s-l
MASS 1 2
A
0;
+ e+ CN
-
-+
Spangler and Collins
CNCN- t 0,
IMASS 01
MASS 26
reduced mobility of the ions observed agrees with that of Spangler and Collins as shown in Table 11. A total ion mass mobility spectra of these negative reactant ions is shown in Figure 3a; the individual ionic mass mobility spectra are shown in Figure 3b-h. Peak I of the mobility spectrum is observed to contain ions of mass 26 and is CN-. Ions of mass 35 and 37 are found under peak 11, which is identified as C1-. Peak I11 has ions of mass 42 and is probably CNO-; ions of mass 52 are contained in peak IV; peak V contains ions of mass 60, which is probably COB-. The CN(B22+- X2Z+) violet band system is one of the oldest known and most persistent electronic transitions of a diatomic molecule (10-1 1). The most extensively investigated sources of CN chemiluminescence are the reactions of active nitrogen with simple or halogenated hydrocarbons (12-16). Also Dondes, Harteck, and Kunz (17)observed the emission bands of the CN(B2Z)system when mixtures of nitrogen and carbon monoxide were irradiated with cr particles from a polonium-210 source. The Matheson zero grade air contains approximately 2 ppm hydrocarbon reported as methane. It is possible that the CN- ion observed may arise from contamination on the walls of the ionization source. The CNion can be formed via two mechanisms: electron capture (1) and charge exchange (2). CN
M A S 11
(1) (2)
The charge exchange reaction with the 02ion acting as the donor species is energetically feasible, since the electron affinity of oxygen is 0.42 eV while the electron affinity of the CN molecule is 3.17 eV. The charge exchange reaction
TOTAL IONS
Figure 3. Total ion and individual ion mass mobility spectra of the ions observed in the negative mode spectra of zero air: (a) total ions, (b) mass 26, (c)mass 32,(d) mass 35,(e)mass 37, (f) mass 42, (9) mass 52, and (h) mass 60
probably predominates due to the higher concentration of 0; ions compared with thermal electron under these experimental conditions. Spangler and Collins identified the first peak in the negative ion mobility spectrum as 0;. This is clearly not the case, since only ions of mass 26 are observed under the fiist mobility peak. This illustrates the errors which can be made using only the mobility data as a means of identifying ions generated in the plasma chromatograph. The second peak contains the ions of mass 35 and 37 in the ratio of approximately 3:1, which corresponds to the natural isotopic distribution of chlorine. The intensity of the second peak is observed to increase significantly as samples of chlorinated compounds are introduced into the sample tube. In the individual mass plasmagram mode of operation, the ions of mass 37 are observed to reach the mass analyzer detector 0.2 ms after the ions of mass 35. However, in the mobility spectra of the plasma chromatograph, the 3sCl- and 37Cl-are ilot resolved, and only a single peak is observed. Ions of mass 42 are observed under the third mobility peak, which is most likely the cyanato or isocyanato (OCN-) ion. Using flash photolysis, Morrow and McGrath (18)observed the reaction of molecular oxygen with CN to form NCO via reaction 3 to be 25 kcal/mol exothermic.
+
~ ~ ( 2 2 : )
0z(3~ -+ i N)C O ( ~ ~ ,+)
o ( 3 ~ )
(3)
This reaction would also result in the conservation of total ANALYTICAL CHEMISTRY, VOL. 49, NO. 6,MAY 1977
829
spin. Boden and Thrush (19) studied the kinetics of reaction 3 by electronic absorption spectroscopy and found it to have a rate constant of 4.4 x 10" cm3 mol-' s-', The CNO- ion could be formed via several mechanisms involving electron capture, charge transfer, and attachment reactions. CNO + e- --t CNO' (4) CNO + 0 ; CNO- t 0, (5) CN + 0,' -+ CNO- + 0 (6) CN- t 0, CNO- t 0 (7 1 -+
-+
There is no convenient means of determining the concentrations of the un-ionized species in the reaction region. Therefore, the extent to which the above reaction contributes to the formation of the CNO- ion cannot be readily ascertained. Probably reaction 6 is the predominant mechanism leading to the formation of CNO- because of the greater concentration of oxygen ions compared with thermal electrons under these experimental conditions. Peak IV of the negative mode mobility spectrum contains ions of mass 52 which may be O(H2O)T which is being produced in the source from oxygen and water, or it may arise from contamination in the source such as CH2F2. Henderson, Fite, and Brackman (20)showed that 0- would be formed as a result of electron collisions with molecular oxygen if the electrons possessed a t least 3.6 eV energy. 'The 0- ion may then undergo clustering reactions with water to form O-(H20)2. The absence of an ion with mass 34 which would correspond to the O(H20)-ion cast some doubt on the identification of the mass 52 ion as O(H,O)z-. If this ion is being formed from contamination such as CH2F2,then the probable mechanism involved is a charge transfer reaction. CH,F, t 0 ;
+
CH,F;
+ 0,
(8)
As suggested by Spangler and Collins, one would expect that 0; would contribute more to the reactant ion current than 0-.An ion of mass 32 is in fact observed in the negative ion mass spectrum of zero air obtained under the conditions of atmospheric pressure ionization. However, the 0 2 - ion, which is most likely the ionic species of mass 32, was found as not having a distinct mobility under the experimental conditions used in this investigation. The ion current of mass 32 was observed to have a uniform distribution as a function of time. This suggests that charge transfer is occurring between 02and molecular oxygen in the drift region of the plasma chromatograph. Another contributing factor is that, since thermal electrons are not excluded from entering the drift region when operating in the negative mode, the molecular oxygen present in the nitrogen drift gas may be involved in an electron capture reaction. The fifth peak in the negative mode mobility spectrum of zero air carrier gas was found to contain ions of mass 60 and is the COB-ion. Karasek, Cohen, and Carroll have also reported the presence of the CO, ion in mass spectral data taken on the negative reactant ion observed in plasma chromatography (21). In a study of negative ion detachment reactions using conventional drift tube mass spectrometry, Fletcher and Moruzzi also observed the COB- ion (22). The negative mode spectra associated with ultra-high purity nitrogen as the carrier contains no detectable ionic species in both the PLC and mass spectral modes of operation. The absence of ions in the negative mode and the presence of only three peaks in the positive PLC spectrum is characteristic of a clean system when nitrogen is used as the carrier gas and can be used as a cleanliness test. Comparison of Air with Nitrogen as Carrier Gas upon t h e Formation of Negative Ions. The intensity of the negative ions formed as a result of the introduction of sub830
*
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
L.,-.I-__^
D BICKOROUND y_xcc___c__
6 m ~ c
25 r n m
Figure 4. Negative mode rnoblllty spectra of hexachiorobenzene using nitrogen as carrier: (a) background of nitrogen carrier, (b) after injection of samples
Table 111. Ratio of Intensity of Ions in Air to Nitrogen Ion C1- (chlorobenzene)
Cl- (1-chlorobutane) C1- (hexachlorobenzene) C,Cl,O- (hexachlorobenzene)
Ratio air/N,
Molecules injected
3.35 5.72 3.08 2.84
7 x 10'O 1 X 10l4 6 X lo9 6 X lo9
nanogram amounts of several chlorinated compounds has been measured using both zero grade air and ultra-high purity nitrogen as the carrier gas. The response of the plasma chromatograph to the syringe injection of samples was observed to follow a simple exponential dilution relationship of the form PH = RCoe-Ft
(9)
where PH is the peak height measured, R is the response factor of the instrument to the particular sample, Co is the initial concentration of the sample, F is an experimental constant determined by the carrier flow rate and volume of the system, and t is time in seconds. The response factor, R , is dependent upon the chemical nature of the sample, carrier gas, and drift gas used. In this study, the response of the plasma chromatograph to several chlorinated compounds is compared using nitrogen and air as the carrier gas. The predominant species observed in the negative mode when 14 pg of chlorobenzene are introduced into the sample tube is the chloride, C1-, ion for both nitrogen and zero air. However, the intensity of the chloride ion was observed to be greater by a factor of approximately 3 in air than in nitrogen. A similar result was obtained for 17.7 ng of 1-chlorobutane. The chloride ion was the predominant species found in the negative mode and was approximately 5 times as intense in air than in nitrogen. The introduction of 17 pg of hexachlorobenzene into the PLC/MS resulted in the formation of two large negative product ions (Figure 4). One ion was the chloride ion; the other was determined to be a pentachlorophenoxide ion. The phenoxide ion was identified from its amu values and its isotopic distribution in the negative mode mass spectrum. Both the pentachlorophenoxide ion and the chloride ion were found to be more intense in air than in nitrogen. The ratio of the intensities of the ions measured in air to nitrogen is shown in Table 111. As suggested by Dzidic et al. (23),when zero air is used as the carrier gas, the ratio of oxygen ions to thermal electrons is on the order of lo6 to lo'. Therefore, the oxygen ions will contribute more to the ionization of trace sample molecules than thermal electrons, provided that the electron affinity
requirement is met in the charge transfer process. The mechanism for the formation of the chloride ion with zero air as the carrier for the compounds studied is probably a charge transfer reaction followed by dissociation. R-C1 + 0; R-CI- + 0 , (10) R-C1- R + C1(11) The pentachlorophenoxide ion was observed with both zero grade air and ultra-high purity nitrogen as the carrier gas. Matheson’s ultra-high purity grade nitrogen contains approximately 1 ppm of oxygen, which is several orders of magnitude greater in abundance than the hexachlorobenzene introduced into the PLC/MS. An increase in the oxygen concentration in the carrier gas by a factor of lo6 in changing from ultra-high purity nitrogen to zero grade air increases the phenoxide ion intensity only by a factor of 3. The molecular ion of hexachlorobenzene was not observed with air or with nitrogen as the carrier. These results indicate that the mechanism responsible for the production of the phenoxide ion is probably R - C1 + 0 ; (R-0)+ OC1 (12) -+
+
+
These results are in total agreement with Dzidic, Carroll, Stillwell, and Horning’s study of the phenoxide ion formation from chlorinated aromatic compounds, using atmospheric pressure ionization (API) mass spectrometry (23). Karasek has studied the negative mode spectra produced in the plasma chromatograph from several classes of chlorinated compounds (24-27). He believes that the mechanism involved in the formation of the chloride ion with nitrogen as the carrier is a dissociative electron capture. These experimental results indicate that reactions 10 and 11 play a significant role in. the formation of the chloride ion with nitrogen as well as with air as the carrier gas. Oxygen present a t a concentration of 1 ppm is in excess of the amount of sample introduced; increasing the oxygen concentration by IO5increases the intensity of the chloride ion only by a factor of 3 to 5, which is similar to the results obtained for the pentachlorophenoxide ion. The response of the plasma chromatograph to the negative ions produced from the chlorinated compounds used in this
investigation indicated that zero grade air is more suitable as a carrier gas than ultra-high purity nitrogen. The change from ultra-high purity nitrogen to zero grade air will have no effect upon the response of the plasma chromatograph in the positive mode, as indicated by the lack of change of the positive mode reactant ions. 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 PLC technique.
LITERATURE CITED F. W. Karasek, Anal. Chem., 48, 710A (1974). R. A. Keller and M. M. Metro, Sep. Purif. Methods, 3, 207 (1974). D. I.Carroll, I.Dzidlc, R. N. Stlllwell, M. 0. Hornlng, and E. C. Hornlng, Anal. Chem., 46, 706 (1974). M. McKeown and M. Siegel, Am. Lab., Nov. 1975. D. I.Carroll, I. Dzidic, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, 1956 (1975). G. E. Spangler and C. I. Collins, Anal. Chern., 47, 393 (1975). F. W. Karasek, ResJDev., 21, 34 (1970). G. W. Grlffln, I.Dzidlc, D. I. Carroll, R. N. Stlllwell, and E. C. Hornlng. Anal. Chem., 45, 1204 (1973). F. W. Karasek, S. H. Kim, and H. H. Hill, Anal. Chem., 48, 1133 (1976). W. Jevons, Proc. R . SOC., London, Ser. A , 112, 407 (1926). F. A. Jenkins, Phys. Rev., 31, 539 (1928). D. W. Setsen and N. A. Thrush, Nature (London), 200, 874 (1963). D. W. Setsen and N. A. Thrush, Proc. R. Soc., London, Ser. A , 288, 256, 275 (1956). N. H. Kiess and H. P. Brodln. Symposium on Combustion, 7th, London, Oxford, 1958 (1959), p 207. R. L. Brown and H. P. Brodin, J. Chem. Phys., 41, 2053 (1964). T. Iwai, M. I. Savadotti, and H. P.Brodln, J. Chsm. phys., 47, 3861 (1967). S. Dondes, P. Harteck, and C. Kunz, Radiaf. Res.. 27(2), 174 (1966). T. Morrow and W. D. McGrath, Trans. Faraday Soc., 82 (3), 642 (1966). J. C. Boden and N. A. Thrush, Proc. R . SOC., London, Ser. A , 305, 107 (1968). W. R. Henderson, W. L. Fite, and R. T. Brackman, Phys. Rev., 183, 157 (1969). F. W. Karasek, M. J. Cohen, and D. I. Carroll, J. Chromafogr. Sci., 8, 390 (1971). J. Fletcher and J. L. Moruzzi, Int. J . Mass Spectrom. Ion Phys., 18, 57 (1975). I. Dzidic. D. I. Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, t308 (1975). F. W. Karasek, Anal. Chem., 43, 1982 (1971). F. W. Karasek and 0. S. Tatone, Anal. Chem., 44, 1791 (1972). F. W. Karasek, 0. S. Tatone, and D. W. Denney, J. Chromafogr., 87, 137 (1973). F. W. Karasek and D. M. Kane, Anal. Chem., 48, 780 (1974).
RECEIVED for review December 17, 1976. Accepted February 17, 1977.
ANALYTICAL CHEMISTRY, VOL. 49, NO. 6, MAY 1977
831
