Anal. Chem. 1987, 59, 2289-2293
2289
Ultra-High-Resolution Daughter Ion Tandem Mass Spectra Using a Differentially Pumped Dual Cell Fourier Transform Mass Spectrometer Marcus B. Wise
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
An experlment sequence has been developed for the Nlcoiet FTMS-2000 dtfferentlaily pumped, dual cell Fourier transform mass spectrometer (FTMS) that enables ultrahigh-resolution daughter ion MS/MS spectra to be easliy acquired wlthout requiring pulsed valves for collision gas Introduction. Full advantage Is taken of the differential pumptng capabtllty of the spectrometer by forming and dlssociatlng Ions in the hlghpressure “source” cell, then pulsing the daughter ions aeross the conductance limit into the lower pressure “analyzer” cell where they are detected. Under these low-pressure conditlons it Is posslbie to obtain ultra-high-resolution spectra and exact mass measurements of the daughter Ions. By the use of this approach, resoiutlon In excess of 200000 (fwhm) has been demonstrated for Isobaric daughter ions at m / z 105. Both positive and negallve Ion collision induced dissociation (CID) spectra have been successfully generated.
Collision induced dissociation (CID) mass spectrometry/ mass spectrometry (MS/MS) is a versatile technique of increasing importance in the field of analytical chemistry (1-4). In particular, the ability to select and fragment ions of a single mass in a complex spectrum can often provide a means of identifying constituents in a mixture without the need for prior chromatographic separation or extensive sample preparation (5, 6). Although MS/MS is typically performed by using multiple analyzer mass spectrometers such as triple quadrupole (7) and hybrid (8) instruments, single analyzer spectrometers including ion cyclotron resonance (9)) ion traps (IO), and Fourier transform mass spectrometers (FTMS) (11,12) have also been used successfully. With the exception of FTMS and some hybrid instruments, most MS/MS instruments are only capable of producing lowto medium-resolution daughter ion spectra. Although medium-resolution CID spectra are often sufficient for identification purposes, there are situations when high-resolution measurement of daughter ions is necessary. For instance, complex mixtures often contain numerous components that have isobaric parent ions which may fragment under CID conditions to form daughter ions of the same nominal mass. High-mass resolution and exact mass measurement capability are therefore features that are especially desirable for the interpretation of such CID spectra, enabling isobaric daughter ions to be resolved and empirical molecular formulas to be determined. This makes FTMS a potentially useful choice for such MS/MS applications. Additional capabilities of FTMS that further increase its usefulness for MS/MS include rapid data acquisition, signal averaging, and the ability to study ion/molecule reactions of daughter ions. Finally, since collisional dissociation events in FTMS are separated in time instead of space, it is possible to perform multiple stages of MS/MS (often referred to as MS/MS”) simply by utilizing more sophisticated experiment sequences (13) rather than increasing the mechanical complexity of the instrument.
Low-energy collision-induced dissociation with an FTMS was first demonstrated in 1982 (12, 14). Since then, the technique has been applied to a variety of analytical problems (15-17) and has also been used as a convenient means of generating precursor ions for sophisticated ion/molecule reaction studies (18). FTMS has been used successfully in some instances to dissociate molecules in the presence of a low pressure of collision gas allowing daughter ion resolution of up to 9200 (fwhm) at m / z 866 (15) to be obtained. However, because CID in an FTMS cell is a multiple collision process, high dissociation efficiencies often require the use of collision gas pressures in excess of 1 X lo4 Torr. This, in turn, produces a rapid decay of the ion image current signal and results in a loss of mass resolution. To circumvent this problem, pulsed valves have been used to admit a momentary high pressure of collision gas into the FTMS cell during the CID event, with the gas pumped away prior to ion detection. This configuration allows ions to be detected at much lower pressures, substantially increasing mass resolution. For example, with this arrangement, resolution of 20 000 (fwhm) has been demonstrated for isobaric ions at m / z 105 by using an FTMS equipped with a 0.9-T magnet (19). The difficulty with this approach arises from the long delay (typically several hundred milliseconds) between parent ion dissociation and daughter ion detection while the collision gas is pumped away. During this time, undesirable reactions may occur between the daughter ions and neutral molecules in the cell, resulting in nonrepresentative CID spectra. Additionally, the long pump down time between scans significantly increases the time required for signal averaging a large number of spectra. An alternative approach has been to couple a quadrupole mass spectrometer with an FTMS, wherein parent selection is achieved with the quadrupole and daughter ion analysis is performed with the FTMS (20). A mass resolution of 140000 (fwhm) at m / z 78 has been demonstrated with this system. Recently, a differentially pumped, dual cell Fourier transform mass spectrometer was commercially introduced (21). This instrument has two independent trapped-ion cells separated by a vacuum conductance limit which is capable of supporting a pressure differential between the cells of approximately 3 orders of magnitude. The conductance limit also serves as a common trapping plate for each cell. With this dual cell design it is possible to separate experiment events not only in time but also in space if desired. Ions may be moved between the two cells by momentarily grounding the voltage applied to the conductance limit plate and allowing the ion populations in the two cells to equilibrate. This allows ions that are formed under high-pressure conditions (lo4 Torr) in the source cell to be detected under low-pressure (lo4 Torr) conditions in the analyzer cell favorable for high resolution and exact mass measurements (21). Due to the small diameter (2 mm) of the conductance limit orifice, efficient ion partitioning between the source and the analyzer requires that the ions be located near the center of the cell and that the cyclotron radius be less than that of the conductance limit. Because of this, it was believed that dual
0003-2700/87/0359-2289$01.50/00 1987 American Chemical Soclety
2290
ANALYTICAL CHEMISTRY, VOL. 59,
NO. 18, SEPTEMBER 15, 1987
t N A L Y Z E < I O N QUENCH P A R E Y T :ON
EXCITE
~~
,
2:
LO
65
80
TIME
00
'20
40
160
RECE:LE
IBP
230
(rsec
Figure 1. FTMS pulse sequence used for acquisition of ultra-highresolution daughter ion CID spectra.
cell FTMS could not be used for the transfer of CID product ions between cells. We have shown, however, that it is indeed possible to collisionally dissociate ions with high efficiency in the source by using a static pressure of collision gas and to obtain ultra-high-resolution spectra of the daughter ions by partitioning the ion population between the cells for detection in the analyzer under low-pressure conditions. Furthermore, without the need for pulsed valves and long pump down times, data acquisition for signal averaging of spectra is faster.
EXPERIMENTAL SECTION All experiments were performed with a Nicolet Instruments, Inc., FTMS-2000 Fourier transform mass spectrometer. This instrument is equipped with a 3.1-T superconducting magnet and two 17/8-in.cubic trapped ion cells separated by a 2-mm-diameter conductance limit orifice. Each cell is individually pumped by a lo00 L/s oil diffusion pump capable of reaching base pressures of 3 X lo4 Torr. Reagent gases were added to the spectrometer via 1-L batch inlets equipped with molecular leaks. Samples and reagents were obtained commercially and used as received. Liquid samples were subjected to several freezepump-thaw cycles in order to remove dissolved gases prior to introduction into the spectrometer. Sample pressures in the Torr as measured with a FTMS cell were typically 2 X Bayard-Alpert ionization gauge. Argon (99.9%) was used as the collision gas for all CID experiments and was maintained in the spectrometer at a,constant total pressure (sample plus argon) of approximately 5 X lo4 Torr. At these source pressures, the analyzer pressure was typically 1 X lo-* Torr or lower. When ionization energies in excess of 15 V were used, it was necessary to continually eject Ar' ions ( m l z 40) from the cell during the ionization event in order to avoid excessive space charge effects and loss of dynamic range for the signals of interest. This ejection was accomplished by exciting argon ions at their cyclotron frequency through the use of the "frequency during the beam" (FDB) command supported by the Nicolet FTMS software. The experiment sequence described in this paper is available from the author on an 8-in. floppy disk. Source code and program documentation are also available.
RESULTS AND DISCUSSION The experiment sequence used for the acquisition of ultra-high-resolution daughter ion CID spectra is represented in Figure 1. The first event in the sequence is an ion quench pulse, which is applied to both the source and analyzer cells to remove spurious ions. This is followed by the ionization event in which the electron beam is turned on for a brief period
of time (typically 10 ms). After formation, ions are trapped in the source region for a period of time (delay 1)and allowed to undergo ion/molecule reactions with reagent gases. The ion trapping time may be varied from 0 to 500 s enabling secondary and higher order reaction products to be formed. Because of the high-pressure present in the source region and the resulting loss of ion trapping efficiency, it is necessary to use a trapping potential of 2-3 V, which is somewhat higher than normal. The CID parent ion is isolated by ejecting all ions at masses higher and lower than that of the parent ion. Two swept frequency events (ejection sweeps 1,2) are applied to the FTMS cell for this purpose. The length of time required for each of these events is determined by the frequency sweep rate and the range of ion masses to be ejected. Immediately following the ejection sweeps, a quench pulse is applied to the analyzer cell (without affecting the source cell) to ensure that any ions detected in the analyzer cell originated in the source region. Excitation of the parent ion is accomplished by irradiating it with a fixed-frequency sine wave equal to its cyclotron frequency. Typically, a constant excitation voltage is applied to the source cell and the translational energy of the ion is varied by altering the length of time that the ion is irradiated a t its cyclotron frequency. For most experiments, the output of the irradiating oscillator (38 V, peak to peak) is attenuated by 25 dB and the irradiation time ranges from 100 to 1000 ps. For an ion of mass 100, this corresponds to a translational energy range (laboratory frame of reference) of 5 eV to 2.8 keV at a magnetic field strength of 3.1 T. After excitation, the parent ions are allowed to undergo multiple collisions with argon atoms for a period of 50-1Qo ms, resulting in collisional activation and fragmentation by way of accessible low-energy paths. This collision period is also believed to provide some collisional cooling of the daughter ions, which may aid in transfer of the ions into the analyzer cell, as will be discussed later. After the ion collision period, the trapping voltage is reduced to a level of 0.75 V and the conductance limit is grounded for 100-300 ps, allowing ions in the source cell to migrate into the analyzer cell. The partitioning of ions between the two cells is primarily a function of their oscillatory motion parallel to the magnetic field (21). Since the frequency of these oscillations is mass dependent, an optimum transfer time must be selected for each experiment in order to efficiently move the ions of interest into the analyzer (22, 23). Lower mass ions oscillate at higher frequencies and move into the analyzer cell more rapidly than heavier ions, leading to discrimination aginst higher masses when short transfer times (100 ps) are used. Once the ions have been partitioned between the two cells, they may be detected immediately or trapped for an additional period of time (delay 2)) so that ion/molecule reactions of the CID daughter ions may be allowed to occur. Ions present in the analyzer region are detected either by application of a broad-band excitation signal for medium-resolution spectra or by narrow-band heterodyne detection of a limited mass range for high-resolution spectra. The reduced trapping voltage (0.75 V) is also used during ion detection to minimize the effects of the trapping fields on the accuracy of the mass measurements. An example of medium resolution CID spectra obtained by using the previously described experiment sequence is shown in Figure 2. The parent ion, m/z 120, was generated by 10-eV electron impact on 1,3,5-trimethylbenzene. The low ionization potential was used to help minimize ion fragmentation during the beam event and to prevent the formation of Arf ions in the cell. Other conditions used to obtain these spectra include
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 CONDUCTANCE L I M I T CLOSED
ANALYZER RECEIVE
I
-5b
-
6b
7b
8b
9b
MASS I N A . M
U.
160
110
li0
0 e V C I D OF MASS 120 FROM 1. 3, 5-TRIMETHYLBENZENE
125 e V C I D OF MASS 120 F R O M 1. 3. 5-TRIMETHYLBENZENE
G
50
..
60
70
80 90 MASS I N A . M. U.
100
110
120
5 0 0 e V C I D OF MASS 120 FROM 1, 3. 5-TRIMETHYLBENZENE
0
Flgure 2. Medium-resolution CID spectra of mlz 120 formed by electron impact on 1,3,5-trhhylbenzene: (a) analyzer detection with conductance limit electronically closed: (b) parent ion at mlz 120 detected in analyzer with no excitation; (c) CID spectrum of parent ion excited to 125 eV (laboratory frame of reference); (d) CID spectrum of parent ion excited to 500 eV (laboratory frame of reference).
a detection band width of 2 MHz, 64K data points, and 25 averaged scans. In order to be certain that all ions in the analyzer cell were properly quenched prior to partitioning of the ions between the cells, an experiment was performed in which the conductance limit potential was maintained a t the trapping potential (0.75 V) during the partitioning event. Under these conditions, ions formed in the source were prevented from entering the analyzer. As shown in Figure 2A, no ions were observed in the analyzer indicating that quenching of possible interfering ions was efficient. A spectrum of the parent ion with no excitation is shown in Figure 2B. All other ions below m/z 110 were ejected from the source cell by the swept frequency events. Excitation of the m / z 120 ion to 125 eV (laboratory frame of reference) resulted in a decrease in the measured parent ion intensity and the appearance of fragment ions at m / z 77,79,91,103,
2291
and 105 (Figure 2C). The measured mass resolution of the m / z 105 ion was 3 200 (fwhm). Further excitation at 500 eV led to nearly complete loss of the parent ion peak at m / z 120 and increased daughter ion abundances with the m Jz 77 ion as the most abundant fragment ion as shown in Figure 2D. Because of the large pressure differential between the source and analyzer cells, the rate of decay for the image current in the two cells is significantly different, leading to an apparent decrease in ion abundance for the faster decaying signal. As a result, it is difficult to accurately measure the ion partition efficiency between the cells under CID conditions by simply comparing the apparent ion abundances in the analyzer vs. the source. However, the efficiency for analyzer detection of daughter ions and excited parent ions (mlz 120) was determined relative to the analyzer detection of the parent ion with no translational excitation. These efficiencies were calculated as the combined total ion current of daughter ions and undissociated parent ion relative to the ion current of parent ion with no excitation. This measurement reflects not only the CID efficiency of the parent ion, but also is an indication of how efficiently the daughter ions are transferred into the analyzer. The range of energies studied was 0-500 eV and the observed average efficiency was 30%, indicating efficient CID daughter ion production, as well as very good ion partitioning efficiency. A slight decrease in the apparent efficiency was observed at higher excitation energies (19% at 500 eV), which was likely due to the formation of a greater percentage of lower mass ions, which may not have been partitioned as efficiently into the analyzer as the higher mass ions at the transfer time selected for these experiments (23). Increased cyclotron radii and further removal of the ions from the center of the cell may have also contributed to decreased ion partitioning efficiencies at higher CID energies. An experiment was also performed in order to determine the maximum radius to which a parent ion could be excited in the source cell without resulting in complete loss of the daughter ion signal in the analyzer cell. For this experiment, a daughter ion at mlz 39 was monitored for the dissociation of the m / z 164 molecular ion of eugenol (2-methoxy-4-allylphenol). A collision gas pressure of 3 x lo4 Torr was used and the ion dissociation time in the source was 100 ms. No daughter ion signal was observed at mlz 39 until the parent ion was excited to a translational energy of 89 eV (laboratory frame of reference). This corresponded to a orbital radius of 5.6 mm for the mJz 164 parent ion, which was nearly 6 times the radius of the conductance limit orifice. The maximum signal intensity for the daughter ion at mlz 39 was found to occur at a parent ion excitation energy of 175 eV (8-mm parent ion radius). At higher translational energies, the intensity of the daughter ion decreased considerably, although reasonably good signal to noise was maintained up to 800 eV excitation for the parent ion (17-mm parent ion radius). Beyond 800 eV parent ion excitation, a discernable signal for the daugher ion was observed up to a maximum parent ion excitation energy of 1400 eV. This corresponds to a maximum parent ion radius of 22 mm, which is close to the maximum radius that can be achieved by an ion in a 17/s-in (24-mm radius) cell. To demonstrate the ultra-high-resolution capability of the experiment sequence, an equimolar mixture of 1,3,5-trimethylbenzene and acetophenone was introduced into the FTMS through the batch inlet at a pressure of 4 X Torr. Argon was added to the cell for a total pressure of 6 X lo4 Torr in the source and 1X lods Torr in the analyzer. Parent ions at a nominal mass of 120 amu were generated by 10-eV electron impact and all ions of lower mass were ejected from the cell by using the swept frequency pulses. At an excitation energy of 125 eV, both acetophenone and 1,3,5-trimethyl-
2292
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
C I D OF ACETOPHENONE AND 1 . 3, 5-TRIMETHYLBENZENE
125 e V
RES=21 1K
..
C I D OF EUGENOL m/z
1 6 4 I N CLOVE O I L
2 0 6 eV
0-
:*ng*
1
i/z
105.0694
C,H50*
6b
sb
io
1 do 1 MASS I N A . M . U .
1i
o
160
1
0
nil 105,031:
C I D OF PURE EUGENOL m/z
164
206 eV
105 02
-0
MASS IN A M
U
Flgwe 3. Ultra-hiih-resolution (21 1 000 fwhm) CID spectrum of m l z 105 daughter ions resulting from dissociatlon of m l z 120 formed by electron impact on a mixture of acetophenone and 1,3,5-trimethyl-
u
::
-
< - 1 w .'
benzene.
- W'L
60 46 F R O M CID OF 2 . 4 - D N T
M/Z
'8'
I
70 e V ELECTRON IMPACT SPECTRUM
of
PURE EUGENOL
I
u .
MASS I N A . M . U.
45 b9a
45 992
MASS :h
45 9 9 4
A M U
45 996
45 99e
Flgure 4. High-resolution (320000 fwhm) CID spectrum of m l z 46 daughter ion (NO,-) resulting from dissociation of the m lz 181 parent ion of 2,4dinitrotoluene.
benzene undergo loss of CH, under CID conditions to form daughter ions at m / z 105.0335 (C7H60+)and m / z 105.0699 (C8H9+),respectively. These ions were partitioned into the analyzer cell after a 100-ms collision period in the source. The ions were observed with a resolution of 211 OOO (fwhm),using heterodyne detection and signal averaging 100 scans, as shown in Figure 3. Because resolution is inversely proportional to mass in the FTMS, the ability to achieve resolution of this magnitude for lower mass ions is important if medium- to high-resolution is to be obtained for higher mass ions. For example, the demonstrated resolution of 200000 at mlz 105 corresponds to a theoretical resolution of only 20 000 at mlz 1000. This CID technique has been found to work well for both positive and negative ions, as shown by the negative ion CID spectrum of 2,4-dinitrotoluene in Figure 4. This spectrum waa obtained by CID of the mlz 181ion formed by dissociative electron capture. At a parent ion excitation energy of 92 eV (laboratory frame of reference), the primary daughter ion observed was m / z 46 correspondingto NO,. The peak shown in Figure 4 represents a single scan with a measured resolution in excess of 320000 (fwhm) obtained by using heterodyne detection. Based on the results of the experiment with 1,3,5-trimethylbenzene and acetophenone, slightly higher resolution would have been predicted for the mlz 46 ion;
Flgure 5. Medium-resolution spectra of eugenol: (a)CID spectrum of m l z 164 isolated from an electron impact spectrum of clove oil; (b) CID spectrum of m l r 164 from pure eugenol; (c)electron impact mass spectrum of pure eugenol. however, space charge effects from electrons trapped in the cell along with the negative ions may have been responsible for a loss of resolution. The addition of an electron ejection oscillator to the FTMS should eliminate this problem. In order to examine the applicability of this CID technique to mixture analysis and to investigate the severity of mass discrimination resulting from ion transfer through the conductance limit, this technique was used to qualitatively determine eugenol in clove oil. The m / z 164 molecular ion of eugenol was isolated from a low-energy electron impact spectrum of clove bud oil, which is a mixture of eugenol, eugenol acetate, and several terpenes (24). The parent ion was excited to translational energies ranging from 10-400 eV and allowed to undergo collisions with argon for 75 ms in the source prior to transfer into the analyzer region. Ion partitioning times of between 100 and 300 ks were examined and it was found that shorter times enhanced the population of the lighter ions in the analyzer and longer times enhanced the population of the heavier ions. An intermediate time of 120 ps was found to be optimal for this sample with comparable relative ion intensities observed in both the source and analyzer over the entire mass range of 50-200 amu. An example CID spectrum of eugenol from the clove bud oil sample is shown in Figure 5A. A total of 100 scans (64K data points each) were averaged and a parent ion excitation energy of 206 eV was used. With a detection band width of 1 MHz, the resolution at m / z 1 2 1 was 3000 (fwhm). For comparison, Figure 5B shows a CID spectrum of pure eugenol
2293
Anal. Chem. 1987,5 9 , 2293-2302
that was obtained by using identical experimental parameters. The similarity of these spectra indicates that the m/z 164 ion present in the clove oil sample was indeed the molecular ion of eugenol. Both of these spectra are remarkably similar to the 70-eV electron impact spectrum of pure eugenol as shown in Figure 5C, with many of the same characteristic fragment ions observed over the entire mass range. It appears that two factors are particularly important in the success of these experiments. First, the use of elevated trapping potentials probably enables a larger population of ions to be trapped in the cell at the high pressures used in the source. Therefore, even if a low-efficiency ion transfer occurs, a sufficient number of ions for detection may still migrate into the analyzer. Second, by using a longer collision time in combination with the high pressure of collision gas, there is also an increased probability of collisionally cooling translationally excited ions that otherwise might not pass through the small orifice of the conductance limit. Finally, it is also possible that the sudden reduction in trapping voltage at the time of ion transfer may perturb theions in a way that increases transfer efficiency.
CONCLUSION The results of this work indicate that both parent ions and daughter ions generated under static high-pressure CID conditions in the source cell of the FTMS, can be partitioned between the source and low-pressure analyzer in abundances that are easily detectable. By the use of the technique described in this paper, it is possible to routinely obtain highresolution daughter ion spectra without the use of pulsed valves or serious loss of CID efficiency. Finally, the reproducibility of results over a period of several months has been excellent, further adding to the analytical utility of this high-resolution CID method.
LITERATURE CITED Tandem Mass SepctroTetry; McLafferty, F. W., Ed.; Wiley: New York, 1983. Sack, T. A.; Lapp, R. L.; Gross, M. L.; Kimble, B. J. Int. J . Mass Spectrom. Ion Processes 1984, 61, 191-213. Cooks, R. G. Spectros.: Int. J . 1984, 3, 129-131. Johnson, J. V.; Yost, R. A. Anal. Chem. 1985, 5 7 , 75SA-788A. McLafferty, F. W.; Bockoff, F. M. Anal. Chem. 1978, 50, 69-76. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81A-92A. Yost, R. A.; Enke, C. G. Anal. Cbem. 1979, 5 1 , 1251A-1264A. Glish, Gary L.; McLuckey, Scott A. Anal. Chem. 1986, 5 8 , 1889-1892. Wise, M. B. Ph.D. Thesis, Purdue University, 1984. McLuckey, Scott A.; Glish, Gary L.; Kelley, Paul E. Anal. Cbem. 1987, 59, 1670-1674. Johlman, Carolyn L.; White, Robert L.; Wilkins, Charles L. Mass Spectram. Rev. 1983, 2 , 389-415. Cody, R. B.; Burnier, R. C.; Freiser, B. S. Anal. Chem. 1982, 5 4 , 96-101. Cody, R. B.; Burnier, R. C.; Cassady, C. J.; Freiser, B. S. Anal. Chem. 1982, 5 4 , 2225-2228. Cody, R. 8.; Freiser, B. S.Anal. Cbem. 1982, 5 4 , 1433-1435. White, R. L.; Wilkins, C. L. Anal. Cbem. 1982, 5 4 , 2211-2215. Cody, Robert B.; Amster, I.Jonathan; McLafferty, Fred W. Proc. Natl. Acad. Sci. U . S . A . 1985, 8 2 , 6367-6370. Hunt, Donald F.; Shabanowitz, Jeffrey; Yates, John R.; McIver, R. T., Jr.; Hunter, R. L.; Syka, John E. P.; Amy, Jon Anal. Chem. 1985, 5 7 , 2733-2735. Hettich, R. L.; Freiser, B. S. J . Am. Chem. SOC. 1985, 107, 6222. Carlin, T. J.; Freiser. B. S. Anal. Chern. 1983, 55, 574-578. McIver, R. T.; Hunter, R. L.; Bowers, W. D. Int. J . Mass Spectrom. Ion Processes 1965, 64 67. Cody, Robert 8.; Kinsinger, James A.; Ghaderi, Sahba Anal. Chim. Acta 1985, 178, 43-66. Cody, R. B., Nicolet Instruments, Inc., private communication, 1986. Giancaspro, Carlo; Verndun, R. Francis Anal. Chem. 1986, 58, 2099-2101. Masada, Yoshiro Analysis of Essential Oils by Chromatography and Mass Spectrometry; Wiley: New York, 1976. ~
RECEIVED for review December 16, 1986. Accepted June 1, 1987. Research sponsored by the Office of Health and Environmental Research, U.S.Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
Characterization of Replacement-Ion Chromatography Employing Cation Replacement and Flame-Spectrometric Detection Leonard J. Galante and Gary M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 A relatively new detection method for ion chromatography (IC)called replacement-Ion chromatography (RIC)Is critically evaluated. The capabilities and limitations of cationreplacement RIC for cation and anion separatlon and determination were explored by uslng a conventlonal dual-column IC system followed by a continuously regenerating Nafion cation-exchange fiber (the replacement column). This third cdumn converts all lonk solutes into their lithium salts, whlch are detected by an acetylene-air flame-emission spectrometer. Although poor figures of merit were observed for cation chromatography, the method is analytically useful for anion chromatography. Anlon detection llmits are 3-50 ng and depend upon the replacement-Ion background produced by the eluent. This background llmitatlon of RIC and the infiuence of varlous system parameters are dlscussed in detail. The universal calibration capabiltly of RIC Is also evaluated.
Ion chromatography (IC) is becoming an increasingly popular technique for separating, identifying, and detecting
solvated ions. At present, the conductivity detector is the most widely used detector in IC because of its simplicity and generality. Two conductometric-based IC detection methods exist. Suppressed IC was introduced in the mid-1970s by Small and co-workers (1). The technique employs a second column called the “suppressor” to minimize the conductivity of eluent (background)ions and enhance the detectability of solute ions. Later Gjerde et al. developed nonsuppressed IC (2-4), in which low-capacity analytical columns and dilute, low-conductivity eluents are employed. The intrinsic advantages and disadvantages of the two methods have been critically compared in the literature ( 5 , 6 ) . Detection limits for suppressed IC are in the range of 0.1-1.0 pg/mL and about an order of magnitude higher for nonsuppressed IC because of its higher background conductivity. Also, separator columns for nonsuppressed IC necessarily have limited sample capacity. These two effects account for the much larger linear dynamic range observed in suppressed IC.
0003-2700/87/0359-2293$01.50/00 1987 American Chemical Society
