Anal. Chem. 1991, 63,2061-2064
by -714 < 4 < 714, the locations of the other quadrants following therefrom. During excitation, a time-dependent voltage difference, V(t),is applied equally across the excitation electrodes located a t -714 < I#J < 714 and 3714 < I#J < 5714. Through first order in the field coordinates, odd-order terms are zero, and the electric field during excitation, E@),is spatially uniform in the x direction, as in the case of the tetragonal trap ( x = p cos 4, p = ( x 2 y 2 ) + ) :
+
(11.8) where
Here again, eq 1.8 and eq 11.8 are identical except that a in eq 1.8 becomes d in eq 11.8. Thus, the results of this paper are easily generalizableto the cylindrical ion trap, configured as described above, with the help of eqs 11.3-11.9 and the transformation a d. In particular, substituting d for a in eqs. 1-18 wherever a appears completes the corresponding derivations for this type of cylindrical trap. For cylindrical ion traps configured differently than that described above, a and y are unchanged, whereas must be determined from the expansion of the solution to Laplace’s equation with the appropriate boundary conditions. The interested reader is advised to consult ref 29 for details of that procedure.
-
LITERATURE CITED (1) Marshal, A. 0.; Grosshens, P. B. Anal. CY”. 1881, 63, 215A-229A. (2) Comlsarow, M. B.; Marshall, A. G. (2”. Phys. Lett. 1874, 2 5 , 282-283. (3) Marshall, A. 0.;Verdun, F. R. Fourier Transfwms in Optlcal. NMR, and Mass Specfromefry: A User’s Handbook; Elsevier: Amsterdam, 1990. (4) McIver, R. T., Jr.; Hunter, R. L.; Baykut, G. R. Anal. Chem. 1868, 6 1 , 491-493. (5) McIver, R. T., Jr.; Baykut. G.; Hunter, R. L. Int, J. Mass Spectrom. Ion Processes 1889, 8 9 , 343-358.
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Ledford, E. B., Jr.; Rempel, D. L. Gross, M. L. Anal. Chem. 1884, 5 6 , 2744-2748. Rempel, D. L.; Ledford, E. B., Jr.; Gross, M. L. Anal. Chem. 1887, 5 9 , 2527-2532. Comlsarow, M. B.; Marshall, A. 0. Chem. Phys. Lett. 1874, 2 6 , 489-490. Marshall, A. 0.; Roe, D. C. J. Chem. Phys. 1860, 73, 1581-1590. Wang, M.; Marshall, A. 0. Int. J. Mass Spectrom. Ion Processes 1880. 100, 323-348. Marshall, A. G.;Wang, T.4. L.; Ricca, T. L. J . Am. Chem. SOC. 1865, 107, 7893-7897. Chen, L.; Wang, T.-C. L.; Ricca, T. L.; Marshall, A. G. Anal. Chem. 1967. 5 9 , 449-454. Marshall, A. G.; Wang, T.4. L.; Chen, L.; Ricca, T. L. Am. Chem. SOC. Symp. Ser. 1887, No. 359, 21-33. Hearn, B. A.; Watson, C. H.; Baykut, 0.;Eyler, J. R. Int. J. Mass Spectrom. Ion Processes 1880, 9 5 , 299-316. Guan, S . J. Chem. Phys. 1888, 91, 775-777. Guan, S . Proceedings of the 39th American Society of Mass Spectrometry Conference, Nashville, TN, May, 199 1; Abstract TOB9:30. Grosshans. P. B.; Marshall, A. G. Inf. J. Mass Spectrom. Ion Processes 1880. 100, 347-379. van der Hart, W. J.; van der Guchte, W. J. Int. J. Mass Spectrom. Ion Processes 1888. 82, 17-31. Dunbar, R. C. (Department of Chemistry, Case Western Reserve University) Private communication of unpublished results, 1989. Rempel, D. L.; Huang, S. K.; Gross, M. L. Int. J . Mass Spectrom. Ion Processes 1886. 70. 163-184. Randier, P.; Bodenhausen, G.; Rapin, J.; Hourlet, R.; Giiumann, T. Chem. Phys. Lett. 1867, 138, 195-200. Pfgndler, P.; Bodenhausen, 0.; Rapln, J.; Walser, M.-E.; Giiumann, T. J. Am. Chem. SOC.1866, 110, 5625-5628. Benslmon, M.; Zhao, G.; Giiumann, T. Chem. Phys. Lett. 1868, 157, 97- 100. Boyce, W. E.; DiPrlma, R. C. Elementary Differentiel Equations and Boundary Value Problems, 4th ed.; John Wiley and Sons: New York, 1986; pp 279-322. Grosshans, P. 8.; Shields, P. J.; Marshall, A. G. J . Chem. Phys. 1881, 9 4 , 5341-5352. Hop, C. E. C. A.; McMahon, T. B.; Willet, G. D. Int. J. Mass Spectrom. Ion Processes 1880, 101, 191-208. Sharp, T. E.; Eyler, J. R.; Li, E. Int. J. Mass Spectrom. Ion Processes 1872, 9 , 421-439. Byrne, J.; Farago, P. S. R o c . Phys. SOC. London 1865, 8 6 , 801. Kofel, P.; Allemann, M.; Kellerhals, Hp.; Wanczek, K.-P. Int. J. Mass Spectrom. Ion Processes 1866, 74, 1-12.
RECEIVED for review March 8,1991. Accepted June 14,1991. This work was supported by grants (to A.G.M.) from the USA National Science Foundation (CHE-9021058)and The Ohio State University.
CORRESPONDENCE Pulsed Flame-A
Novel Concept for Molecular Detection
Sir: Flame-based detectors for gas chromatographs (GC) and air impurities are extensively used and constitute the major selective molecular detectors today (1). Most notable are the (a) flame ionization detector (FID), which is the most commonly used GC detector and which is selective for organic molecules, (b) flame photometer detector (FPD), which is mainly used for the selective detection of sulfur- and phosphorus-containing molecules, (c) thermionic ionization detector (TID), which is sometimescalled the nitrogen and phosphorus detector (NPD), (d) flame infrared (IR) emission detector (FIRED), which is based on hot-flame IR emission, and (e) atomic absorption (AA) detection method for trace-metal analysis. All these and other detectors are based on the continuous existence of a flame and combustion of gases, usually hydrogen and oxygen (either without or with air). The detected mol0003-2700/91/0363-2061$02.50/0
ecule, when introduced to the W e , may indicate ita presence by the formation of charge carriers (positive ions, negative ions, or electrons) or excited species that emit electromagnetic radiation (ultraviolet (UV), visible or infrared (IR)). Alternatively, the combustion can create new species that are amenable to detection in spectroscopic techniques such as fluorescence excitation or atomic and molecular absorption spectroscopy in the UV, visible, or IR spectral ranges. Obviously, it is desirable to further improve these detectors in terms of their sensitivity, selectivity, gas consumption (hydrogen), and simplicity (price). We describe here a new approach (2) for the design of flame detectors that is based on a pulsed-flame operation instead of the usual continuous mode. This pulsed-flame detector possesses the potential of improving flame-based detectors in all the above-mentionedproperties as well as showing some 0 1991 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991 PULSED F L A M E PHOTOMETRIC EMISSION
3 TIME ( m s e c )
Flgure 1. Schematic diagram of a pulsebflame photometer detector (PFPD): (1) hydrogen source; (2) air (or oxygen) inlet: (3) gas mixing and pulsed-flame volume; (4) pulsed-flame chamber that can be separately heated to avoid molecular adsorption on the wails: (5) heated wire pulsed-flame igniter, (6) igniter power supply; (7) electrical feedthroughs and light protective shield; (8) light detection system including (8a) window, (8b) light optics, (8c) light filter, and (8d) photomultiplier: (9) pulsed signal processor. unique and useful new features.
PULSED-FLAME DETECTOR (PFD) The design of a pulsed-flame detector is similar to that of a conventional continuously operated flame-based detector with three main modifications: (1)The combustible gases and sample molecules are accumulated in a small combustion chamber prior to the ignition. (2) The total gas mixture flow rate is reduced to allow the flame to propagate back to the gas sources. (3) The Ni/Cr hot wire igniter is continuously heated (or another pulsed periodic ignition method is used). Figure 1 presents a schematic diagram of a pulsed-flame photometer detector (PFPD), which was successfully operated in our laboratory. The hydrogen gas is fed (1)and mixed with the air or oxygen (2) in the enclosed volume (3). The combustible gas mixture flows in the heated structure (4) to the continuously heated Ni/Cr (or Pt) flame igniter (5), which is powered by the power supply (6). The ignition wire and detector structure are light-shielded (7) to minimize stray-light detection. The ignited gas mixture forms a flame near the igniter. This flame propagates to the gas sources where it is self-terminated, since neither of the gases in volumes 1 and 2 is combustible by itself. The pulsed-flame light emission is viewed by the quartz light guide (8b) through the sapphire window (8a) and is detected by a monochromator (Jarrell-Ash 0.2 m) or by an appropriate filter (8c)-photomultiplier (8d) detection system. The pulsed signal is processed (9) by a gated amplifier and plotted versus time. A pulsed-flame ionization detector (PFID) is similar in its design but the igniter is placed in the center of the flame chamber and is electrically biased (usually negatively, to avoid alkali positive ion emission from the igniter wire), while the flame chamber serves as an electron current collector. The PFPD was mounted on a GC (Carlo-Erba Vega 6000). with a homemade mount. This mount includes a separate entrance for controlled vaporization rate of liquids in a small “melting-point” glass tube. In this way the working parameters of the detector could be conveniently optimized without the GC that served only for the gas supply. Typical operation conditions are hydrogen flow rate of 5-15 cm3/min, air flow rate 1-4 cm3/min, pulse rate of 3-10 Hz (for pulsed-flame chamber volume of 0.05 cm3),and pulse duration of 1-0.5 ms (depending on the viewing window) with air and 0.5-0.25 ms with pure oxygen. At a given total gas flow rate the repetition
Flgure 2. Pulsed-flame photometric emission through a monochromator, versus time: (A) emission from a pulsed flame of H,/ak at 312 nm (OH’ emission)or from cyclohexane seeded In the flame at 432 nm (CH’ emission); (B) emission from dimethyl phosphate in cyclohexane seeded In H,/air flame at 526 nm (HPO’ emlsion);(C) emission from a 0.1 % solution of dimethyl sulfoxide in cyclohexane seeded In H,/air flame at 400 nm (S2* emission). A hydrogen-rich flame was used throughout. rate was stable within 1% for several hours. In Figure 2 we show characteristic temporal light emission behaviors. The time dependence shown was obtained with an averaging digital oscilloscope (Le-Croy 9400). The ion pulses collected on the heated wire igniter served for triggering, or alternatively internal triggering could be used. As shown, the light emission is pulsed and the pulses are short (- 1 ms). Trace A shows the light emission as viewed through the monochromator at 312 nm and which belongs to OH* emission. Identical results are obtained with hydrocarbon emission at 432 nm (CH* emission). Trace B was obtained by viewing the phosphorus HPO* emission a t 526 nm from the combustion of 0.1% dimethyl methylphosphonate (DMP) in cyclohexane. The main observation is that the pulsed-light emission from phosphorus can be separated in time from any hydrocarbon background emission. Trace C shows the emission of a few hundred parts per million (ppm) of a sulfur compound dimethyl sulfoxide (DMSO) in cyclohexane at 400 nm (0.1% DMSO in cyclohexane results in about 200 ppm gas-phase DMSO). The emission of S2*has a structure in time and is broader and further delayed than that emerging from phosphorus compounds. Presently, the origin of this time delay is not clear but a possible conjecture is that C02or H 2 0 formation (which involves with OH* and CH* emissions) is fast and irreversible and thus is completed within the time scale of flame propagation. After the flame front was extinguished we can assume that, in the hydrogen-rich mixture, all the oxygen was combusted. However, reactions with atomic (or molecular) hydrogen can further proceed and thus the formation of S2*or HPO* according to the several possibilities mentioned in the literature (3) can be delayed. In fact the light emission time dependence can be viewed as the distance dependence in conventional flames with premixed gases. This situation is also similar to the dual-flame approach of Patterson (4,5), where the sample molecules are partially combusted in the first flame and the second flame, which is in analogy with the delayed combustion, serves for the chemiluminescence reactions. In Figure 3 we show the pulsed-flame photometric emission spectra of cyclohexane solutions with DMSO or DMMP. The emission spectra were measured with a box-car integrator as a gated amplifier (Ortek Brookdeal 9415), while the mono-
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'FPD EMISSION OF C S H , ~SOLUTIONS
n
A
FID
I METWANOL
2 TOLUENE
3 DMSO 4 DMP 5 DECANE
6.PULSED FPD SULFUR MODE
60 pg/sec
-
C PULSED FPD
PHOSPHOROUS
MODE
J
\
d . k, 600 500 I
400 300 WAVELENGTH (nm)
2
Flgue 3. PulsedRame photometric emtssbn of cyclohexane solutions viewed through a 0.2-m scanning monochromator. The upper trace was obtained with a gated amplifier having a short undelayed gate. Dimethyl sulfoxk%-cycbhexane dilute sdutlon (0.1%) was used. The observed emission Is of OH' and CH' (the peak at 624 nm is the secondorder 312-nm OH' emission). The mkldle trace was obtained with a delayed gate from the combustion of dimethyl methyl-
phosphonate-cyclohexane dHute solution. The HPO' emission spectrum Is observed ahd Is isolated from the early time OH', CH' emission. The lower trace was obtained with a delayed gate from the combustion of dimethyl sulfoxide-cyclohexane dilute solution (as in the upper trace). The observed S2* emission spectrum is unperturbed by any hydrocarbon-related emission.
chromator was scanned and the cyclohexane solution was continuously vaporized at a rate of 1pg/s. The upper trace A was obtained while the gate was positioned on the "small" early time emission shown in Figure 2C. The hydrocarbon emission spectrum is clearly observed together with the OH* emission a t 312 nm and its second order appearance through the monochromator at 624 nm. Note that no sulfur emission is detected. However, while the same exact solution and pulsed-flame conditions were used, but with a delayed gate to match the sulfur emission shown in Figure 2C, the sulfur emission is clearly observed in Figure 3 trace C. It is clearly observed that this spectrum is completely free from any interference from the OH* or CH* emission shown in trace A. Similar behavior is shown in trace B for the phosphorus (HPO*) emission from DMP-cyclohexane solutions. Figure 3 demonstrates the ability to control the detection selectivity, which can be much higher in comparison with that of the conventional FPD. Thus, under these conditions PFPD may turn into a specific detection method. In Figure 4 we demonstrate this enhanced selectivity in the GC detection of a methanol solution of 1% toluene (2),0.1% decane (51, lo-' DMSO (3), and 10 ppm dimethyl methylphosphonate (DMP) (4). A capillary column (J&W DB-10.25 mm id., 30 m long) was used at 140 "C and a 2%" head-one photomultiplier (Hamamatsu R269) was used with color glass filters (Schott BG12 and WG 360 for sulfur and VG9 for phosphorus). Samples measuring 1 p L were injected in traces A and C, and 0.7 p L was injected in trace B. The injected amount was
lo pg/sec
0
I
/I
2 3 TIME(min)
4
Flgure 4. Selective GC detection of sulfur and phosphorus: methanol solution of 1% toluene (2), 0.1 % decane (5), lo-' DMSO (3), and 10 PPM DMP (4). The upper trace was obtained by a conventional FID, while the lower traces were obtained by our pulsed FPD. A 1-pL solution was injected in traces A and C, and 0.7 pL, in trace B. The injected amount was further split into a 1OO:l ratio.
further split by a 1:lOO ratio. Thus about 0.7 ng of DMSO and 0.1 ng of DMP were injected into the column and reached the detector a t a rate of 60 pg of S/s and 10 pg of P/s. The upper trace A represents the Conventional FID detection of this mixture, while traces B and C represent the PFPD detection in the sulfur- and phosphorus-selective modes, respectively. The total lack of any foreign peaks including that of the solvent implies selectivity against carbon-containing molecules better than 10'. This value is calculated from the relative peak heights in traces B and C, which are divided by their molar fraction in the solution. We also note that the sulfur-phosphorus selectivity is very good, partially due to their different delayed emission time and partially due to differential optimal gas mixture conditions, which is more hydrogen rich for sulfur (in addition to differences in the emission wavelengths). In a separate set of measurements we have found that the minimum detected level (MDL) is better than g of S/s and of P/s. The MDL achieved depends on the details of the internal structure of the pulsed-flame chamber, and further progress is anticipated in our current and future studies of PFPD.
ADVANTAGES AND APPLICATIONS The pulsed signal and added dimension of its time dependence results in the following emerging possible advantages. (a) Higher Sensitivity. The pulsed signal results in a reduced contribution of amplifer noise. This can be combined with the reduction of chemical noise by its separation in time to enhance the detection sensitivity. (b) Improved Selectivity, The possible separation in time of the signal from unwanted chemical background results in
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a substantial improvement of the chemical or functional group selectivity. (c) Additional Molecular Information. The added time dimension provides new and complementary functional group or atomic information that broadens the scope of selectivity of existing flame-based detectors as well as allowing for new pulsed-flame detection schemes. For example, PFPD can be used as a sensitive halogen-selective detector (6) or nitrogen-selective detector, as was recently found in our laboratory. Similarly, pulsed FIDs can potentially be used as a halogenselective detector due to their expected delayed action as flame retardants. (d) Lower Hydrogen Consumption. Since there is no need to hold or keep a continuous flame, the hydrogen flow can be arbitrarily reduced. Our pulsed FID typically works with 2 mL/min hydrogen flow. This flow can be further reduced by using H, as the carrier gas or at a lower repetition rate or by reducing the flame chamber volume. This hydrogen fuel saving is of special importance in field portable detectors. (e) Detection of Unseparated Mixtures and Solutions. The added dimension of time dependence allows the detection of unseparated mixtures, as demonstrated in Figures 2 and 3. The ratio between the (wavelength dependent) early hydrocarbon peak and delayed sulfur peak in time permits its quantification since the hydrocarbon peak can serve as an internal standard. The road to flame photometric detection of HPLC is now open. We believe that pulsed flame is a new technology that can be applied to all the flame-based detectors such as FID, FPD, TID, FIRED, and AA. At this early stage, the limited information gathered from the operation of our PFPD is insufficient to assess all the possibilities. However, it seems that one of its most promising directions might be the detection of SFC (7)and HPLC (8), where the added selectivity is a highly desirable feature. The spectroscopic detection of flame-generatedspecies can also greatly benefit from the time separation of flame interferences. In addition to the usual GC detection, the reduced H2 consumption is of large importance in mobile detectors. We note that pulsed flames offer unique possibilities in combustion and flame ignition research. Laser ignition-detection schemes (9) with unequaled time resolution of up to 10 ps are estimated to be possible with
radical vibrational energy distribution information. Finally, we also note that the pulse operation mode can simplify the size and cost of flame-based detectors by reducing the size of the hydrogen bottle or generator and by replacing the complex and delicate two photomultipliers and their highvoltage power supply (in a differential FPD (IO))with a single silicon photodiode.
ACKNOWLEDGMENT The contribution of Eran Yavin to the early experiments of PFPD is greatly appreciated. This work was inspired by several stimulating discussions with Brad E. Farch and laser-pulsed flame ignition experiments performed at the US. Army BRL at Aberdeen Proving Ground and also with J. B. Morris and R. J. Locke. Registry No. S,1104-34-9;P,1123-14-0. LITERATURE CITED (1) Dressler, M. Selective Gas Chmarupaphlc Derectws: Elsevier: Amsterdam, 1986. (2) Amirav, A. Pulsed Flame Detector Method and Apparatus. Israel Patent Application No. 95617, Sept 1990. US., European, and Japan Patent application submission Aug 1991. (3) (a) Farwell, S. 0.; Gage, D. R.; Kagel, R. A. J . Chromatogr. Scl. 1981, 79, 358. (b) Farwell, S. 0.; Barinaga, C. J. J . Chrmtogr. Sei. 1988, 24, 483. (4) Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978, 50, 339. (5) Patterson, P. L. Anal. Chem. 1978, 50, 345. (6) Bowman, M. C.; Beroza, M. J . Cbrmtogr. Sei. lB69, 7, 484. (7) Richter, B. E.; Bornhop. D. J.; Swanson, J. T.; Wangsgaard, J. Q.; Andersen, M. R. J . Chrometcgr. Sci. 1989, 27, 303. (8) McGuffin, V. L.; Novotny, M. Anal. Chem. 1981, 53, 946. (9) (a) Morris, J. B.; Forch, B. E.; Mirlolek, A. W. Appl. Spectrosc. 1990, 44, 1040. (b) Forch, 8. E.; Mlrlolek, A. W. Combust. Fkme 1991, 85, 254. (10) Am, W. A.; Miller, 6.; Sun, X. Y. Anal. Chem. 1990, 62, 2453.
Eitan Atar Sergey Cheskis Aviv Amirav* School of Chemistry Sackler Faculty of Exact Sciences Tel Aviv University Ramat Aviv 69978 Tel Aviv, Israel RECEIVED for review January 14,1991. Accepted May 17,1991.
Preforming Ions in Solution via Charge-Transfer Complexation for Analysis by Electrospray Ionization Mass Spectrometry Sir: Electrospray (ES) ionization is rapidly developing as a method to produce gas-phase ions from analyte species in solution for subsequent analysis by mass spectrometry. The combination of ES ionization with mass spectrometry (MS), first demonstrated by Fenn and co-workers ( 1 , 2 ) ,has proven useful in the analysis of involatile, polar, and thermally labile compounds, especially high molecular weight biopolymers. Electrospray also serves to interface the mass spectrometer with a variety of liquid phase separation methods, including HPLC and CZE (see refs 3-1 for recent ESMS reviews). Electrospray ionization can be viewed as an ionization process involving two steps. First, highly charged droplets of a solution containing the analyte are dispersed a t atmospheric pressure. This usually is accomplished by application of a high potential difference (typically 3-5 kV) between a 0003-2700/91/0363-2064$02.50/0
capillary needle, through which the analyte solution is flowing at a low rate (typically 1-10 pL/min), and the atmospheric sampling aperture of the mass spectrometer, which are typically separated by 0.5-2.0 cm. This dispersal is followed by droplet evaporation and finally ion evaporation or desorption to yield gas-phase ions that can be sampled and analyzed by the mass spectrometer. While the detailed mechanism for ion evaporation or ion desorption is currently at issue (3-9,it has become clear that best ESMS results, in terms of both sensitivity and detection limits, are achieved for compounds that are already ions in solution. This is in direct analogy to FAB and SIMS where best performance is observed for preformed ions (8). Species that are ionic in solution and have been analyzed by ESMS include, for example, metal salts (I, 9-11) and organic salts (e.g., alkylphosphoniumsalts (12),and 0 1991 American Chemical Society
