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Anal. Chem. 1982, 54, 1644-1646
anthroquinone were 1ng, 10 ng, and 6 ng, respectively. The previous data give a general idea of the relative standard deviation and limits of detection for the compounds in Table I. It should be mentioned that the relative standard deviation and limit of detection data were obtained from 1% PAA-NaBr mixtures and not from 0.5% PAA-NaC1 mixtures as in Table I. Carbazole and 2-aminoanthroquinone gave relatively weak signals on both 1%PAA-NaBr and 0.5% PAA-NaC1, but 5,6-benzoquinoline gave strong RTP signals on both mixtures. The 0.5% PAA-NaC1 mixture induced a RTP signal three times greater from 5,6-benzoquinoline compared to the signal obtained from the 1% PAA-NaBr mixture. Thus, the PAANaCl mixture was used to obtain the RTP data in Table I.
Hurtublse, R. J. Talanta 1981, 2 8 , 145-148. Hurtubise, R. J.; Datterio, R. A. Am. Lab. (Fairfield, Conn.) 1981, 13 (1 l),58-62. Parker, R. T.; Freedlander, R. S.:DUnlaD, R. 6. Anal. Chim. Acta 1880, 779, 189-205. Parker, R. T.; Freedlander, R. S.; Duniap, R. 6. Anal. Chim. Acta
---. --.
Ig80. 120. 1-17.
Hurtubise, R. J. “Soild Surface Luminescence Analysis: Theory, Instrumentation, Applications”; Marcel Dekker: New York, 1981;Chapters 3,5,and 7. Later, D. W.; Lee, M. L.; Wllson, B. W. Anal. Chem. 1982, 54.
117-1 23. Whttehurst, D. D.; Mlchell. T. 0.; Farcasiu, M. “Coal Liquefaction: The Chemlstry and Technology of Thermal Process”; Academic Press: New York, 1980. Ford, C. D.; Hurtublse, R. J. Anal. Chem. 1979, 5 7 , 659-663. Von Wandruszka, R. M. A,; Hurtubise, R. J. Anal. Chem. 1978, 48,
1784-1788.
LITERATURE CITED Ford, C. D.; Hurtublse, R. J. Anal. Chem. 1980, 5 2 , 656-662. Paynter, R. A.; Wellons, S. L.; Winefordner, J. D. Anal. Chem. 1974, 46, 736-738. Parker, R. T.; Freedlander, R. S.; Schulman, E. M.; Dunlap, R. 8. Anal. Chem. 1979, 5 7 , 1921-1926. Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 5 7 , 1915-1921. Meyers, M. L.; Seybold, P. G. Anal. Chem. 1979, 5 7 , 1809-1612. Schulman, E. M.; Parker, R . T. J . Phys. Chem. 1977, 87,
1932-1939. de Lima. C. G.; Nlcola, E. M. Anal. Chem. 1978, 50, 1658-1665. Bower, E. L.; Winefordner, J. D. Anal. Chlm. Acta 1878, 702, 1-13. Cline Love, L. J.: Skrllec. M.; Habarta, J. G. Anal. Chem. 1080, 5 2 ,
754-759.
S. M. Ramasamy R. J. Hurtubise* Department of Chemistry The University of Wyoming Laramie, Wyoming 82071
RECEIVED for review April 9, 1982. Accepted May 26,1982. This work was supported by the Department of Energy (Office of Basic Energy Sciences) under Contract No. DE-AC0280ER10624.
AIDS FOR ANALYTICAL CHEMISTS Magnetic Enhancement of an Ionization Source for Glow Discharge Mass Spectrometry B. L. Bentz and W. W. Harrison* Deparfment of Chemistty, Universiw of Virginia, Charlottesville, Virginia 2290 I
Magnetic field coupling to a gas discharge finds practical employment in several types of present-day low-pressure discharge assemblies, for example, in the duoplasmatron ion source and in commercial sputter atomization units serving to deposit coatings or films of sputtered material on selected substrates. To date, gas discharge ionization sources for glow discharge mass spectrometry (GDMS) (I), an analytical technique developed by researchers for inorganic solids analysis (2-5) and thin film process control (6),and which recently has seen the appearance of a commercially available ionization unit (7), have been without magnetic field superposition. This report describes the testing of a cylindrical diode ionization source with added magnet, designed for GDMS use, and presents new data showing that accommodation of a magnetic field affords enhancements in mass analyzed ion signals arising from the sputtered neutral atom fraction. The negative glow region of a weakly ionized dc glow discharge is rich in electrons with varying energies (8). In general, the effect of a magnetic field on a glow discharge is via the electrons; at the fields normally used, the ions are insignificantly influenced. The lifetime of an electron in the discharge can be increased by using a magnetic field to redirect electron motion in a manner to increase the net electron path length (9). In so doing, the probability of ion formation by electron collision can be enhanced owing to a more efficient use of the available electron supply. An externally applied magnetic field may, in principle, be superimposed on a glow discharge either parellel to the electric field or in a perpendicular manner. 0003-2700/82/0354-1844$01.25/0
Both of these arrangements are effective in causing enhanced electron activity. Another important advantage of a transversally applied magnetic field, apart from providing increased ionization, is that the discharge can be operated a t lower source pressures for a given discharge current (IO) due to its effect on electrons in the cathode dark space. This is advantageous for a plasma ion sampling system, such as our quadrupole-based solids mass spectrometer described previously (II), because lower source pressures reduce the pumping load on the vacuum system and also lessen redeposition of sputtered cathode particles in the source.
EXPERIMENTAL SECTION We studied a simple source geometry using two conducting, concentric cylindersas electrodes, a design referred to as a coaxial cathode ion source (CCIS) (12) as is evident when the electrodes are viewed head-on (see Figure 1). The geometry of this source enabled convenientmodification to accommodate a magnetic field superimposed on the discharge. A variable field, solenoidal electromagnet was formed by winding 22 gauge insulated copper wire around the cylindricalstainless steel anode (1.52 cm i.d., 2.07 cm o.d., 6.7 cm long) with sufficientturns (14 turns/cm) and layers (20) to generate a calculated (13) static magnetic field strength of 325 G at the axis and center of the source for a magnet current of 1A, supplied by a Trygon Electronics (Westbury, NY) 40-V, 10-A dc power supply. The magnetic field was roughly mapped in the source interior by use of a Hall probe. Good calibration agreement (within 1-2%) was found at the source axis between the calculated field strength and the measured value. Use of the electromagnet,rather than a permanent ring magnet, allows the field strength to be varied such that conditions might be estab0 1982 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
Table I. Effect of Magnetic Field on Extracted Ion Signals from the Coaxial Cathode Ion Source rn /e, ion current (nA) conditions 14 18 20 28 29 40 magnetic field off 0.60 1.02 1.40 0.21 3.81 41.5 comer coaxial cathode, 30 mA discharge current, 616V discharge voltage, P = 1.30 tlorr A magnetic field on 7.25 8.90 5.30 4.70 31.0 340 30 mA discharge current, 512 V discharge voltage, magnet current: 700 mA (-225 G ) 8.2 magnet on/magnet off 12.1 8.7 3.8 22.3 8.1 NEGATIVE GLOW
1645
63 0.88
0.15
7.80
3.85
8.9
J
80
25.6
?
NDRICAL DE
Y-ELECTRO TRAPPED IN ORBIT
CATHODE DARK SPACE
M A G N E T IC FIELD UPWARDS
Flgure 1. Representation of electron transpot? processes in a coaxial cathcde glow discharge iori source wlth a superimposed axial magnetic field (adapted from ref 18).
900 400 600 000
'0
1000
Magnet Current, mA
Figure 2. Effeci of magnet current on discharge current: Ar gas, 450 V discharge voltage.
lished which allow selective ion enhancements.
RESULTS The applied magnetic field was roughly parallel to the source axis, and owing to the cylindrical geometry of the CCIS, the magnetic field was approximately tiransverse (perpendicular) to the dischargle electric field. Hence, the modified source now contained crossed electric and magnetic fields, the effects of which are shown schematically IUI Figure 1. Addition of the magnetic field changes the electron transport processes. Those electrons which enter the negative glow region are believed to become trapped on cycloidal-like paths which orbit but do not reach the cathode (14). Ultimately, the electron exchanges energy in repeated ionizing and excitation collisions with gas particles, and (diffuses,in a gyro motion, to the anode. The voltage-current characteristics off the modified source were examined with an interest in learning more about the ionization processes in the discharge. Figure 2 shows that at a fixed Ar source pressure, the CCIS discharge current increases with magnet current (directly related to the magnetic field strength), at least for magnet currents above -600 mA (correspondingto a field of -200 G). At a pressure of 1.0 torr and with a 1-A magnet current, the discharge current increased by a factor of -5 relative to field off conditions. In this study, the Kepco supply powering the CCIS was voltage programmed, maintaining the discharge voltage ,at 450 V. The results show that magnetic fields of approximately 200 G and greater cause a measurable increase in discharge ionization. It was observed also that at EI fixed discharge voltage and magnetic field strength, the CCIS discharge current increased with pressure as shown in Figure 3. This is due to more collision targets a t higher source pressures. Application of the magnetic field caused a visually discernible reduction in the volume (thickness) of the cathode dark space and negative glow regions; increasingly higher magnet currents causeld a greater contraction of these regions. From basic glow discharge theory, the product of cathode dark
lq
Magnet
on
?
40
I
1
104
i
/
.e0 1.00 120 1.40 1.60 Source Pressure, torr Figure 3. Effect of magnetic field on discharge current as a function of source pressure: Ar gas, 450 V discharge voltage, 800 mA magnet current.
space thickness times'pressure equals a constant value (15). Apparently, the magnetic field produces a discharge environment which simulates an increase in gas pressure. The lowest pressure at which a discharge could acceptably be maintained in the CCIS without the use of the magnetic field was -0.9 torr for an argon discharge at 30 mA. Use of 300-450 G magnetic fields lowered the pressure limit by a factor of 4-5 (Le., to -0.2 torr), working at a constant discharge current. The role of the magnetic field is to cause electrons to remain in the discharge region for longer periods of time, which increases the probability of ion formation per charged particle,
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Anal. Chem. 1082, 5 4 , 1646-1647
thus increasing the ionization efficiency of the source. Upon using the mass spectrometer to monitor extracted ion signals from the modified source, ion signal enhancements of 6-9 times were observed for the sputtered sample neutral atoms subsequently post-ionized in the discharge. Table I presents typical data for mass scans with and without the magnetic field on. In this example, a polycrystalline copper sample was subjected to ion bombardment in an argon discharge. The ion signals from background contaminant species (N+,HzO+) and ions arising from the discharge sputter gas (Arz+)also increased with application of the magnetic field. The Cu+:Ar+ ion ratio maintained approximately the same value when the magnetic field was applied as when the source was operated without the magnet. For elements whose nominal mass positions are not masked by interferences, enhanced GDMS trace detection sensitivites are expected owing to the increase in ion yield. The analytical value of this magnetic coupling has yet to be fully evaluated for mass spectrometry. The dc electromagnet employed,while easy to construct, may not represent the optimum configuration of the magnetic field in the source. This gives expectation of developing more efficient source models which have different magnetic field distributions which could improve the ionization interactions. It has been reported that in the glow discharge, the concentrations of ionic species (both atomic and molecular) vary spatially throughout the body of the discharge (17,181. A redesigned electromagnet with properly defined and controlled magnetic field geometry might allow the observation of selective ion enhancements as the position of discharge regions changes with magnetic field
application.
LITERATURE CITED (I) Coburn, J. W.; Harrison, W. W. Appi. Specfroso. Rev. 1981, 17,
95-130. (2) Coburn, J. W.; Taglauer, E.; Kay, E. J . Appl. Phys. 1974, 45, 1779-1786. (3) Coburn, J. W.; Kay, E. Appl. Phys. Left. 1971, 19, 350-352. (4) Bruhn, C. G.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1978, 50, 373-375. (5) Bruhn, C. Q.; Bentz, B. L.; Harrison, W. W. Anal. Chem. 1979, 51, 673-678. (6) Hofmann, D.; Wechsung, R. Proc. ISPC-4 1979, 2, 622-627. (7) “Plasma Discharge Source”; Vacuum Generators Analytical Ltd.: Cheshire, England, Product Details Bulletln, April 1981. (8) Wiilett, C. S. ”Introductlon to Gas Lasers: Population Inversion Mechanisms”; Pergamon Press: Oxford, 1974; Chapter 3. (9) Maisel, L. In “Handbook of Thin Flim Technology”; -. McGraw-Hi11: New York, 1970; Chapter 4. ( I O ) Kay, E. J. Appi. Phys. 1963, 3 4 , 760-768. (11) Bentz, B. L.; Bruhn, C. G.; Harrlson, W. W. Inf. J . Spechom. Ion PhW. 1978. 28. 409-425. (12) Harrlson, W.W.’; Bentz,B. L. Anal. Chem. 1979, 51, 1853-1855. (13) Kip, A. F. “Fundamentais of Electrlclty and Magnetism”; McGraw-HIII, New York, 1969; Chapter 8, (14) Chapln, J. S. ResJDev. 1974 (Jan), 37-40. (15) Coblne, J. D. “Qaseous Conductors, Theory and Engineerlng Appllcations”; Dover: New York, 1941. (16) Thornton, J. A. J . Vac. Sci. Technd. 1978, 15, 171-177. (17) Howarka, F.; Llndlnger, W.; Pahl, M. I n f . J . Mass Specfrom. Ion Phy6. t973, 12, 67-77. (18) Knewstubb, P. F.; Tlckner, A. W. J . Chem. Phys. 1982, 36, 684-693.
RECEIVED for review July 1,1981. Resubmitted February 8, 1982. Accepted April 14, 1982. Support is gratefully acknowledged from the Department of Energy, Division of Chemical Sciences, and the National Institutes of Health.
Septumless Injection Port for Capillary Gas Chromatography Joachim Greter” and Goran Sttihle Department of Clinical Chemistry, University of Gothenburg, 5 4 13 45 Gothenburg, Sweden
In capillary gas chromatography, problems arising from the injection port septum are quite common and very disturbing in routine on-column injection work. The main problems encountered are bleed from the septum at elevated injector block temperatures and particles torn from the septum into the column by the injection needle. These particles are deleterious to column performance and in the worst case may clog the column. For some injection ports with a septum, devices have been designed to overcome problems from septum bleeding, but as far as we know none of these devices safely prevent septum particles from coming into the column, if on-column injection is to be applied. Two years ago the only commercially available septumless on-column injector for capillary columns (1) did not fit our budget and, like other septumless injection devices known to us, did not seem to be suitable for a trouble-free continuous use with an autosampler. We therefore designed, built, and tested the injection port shown in Figure 1which can be made in 2 days at a material cost of less than $30 by any skilled technician. The injection port was easily fitted to different gas chromatographs (Carlo Erba, Hewlett-Packard, Varian) in our laboratory, the only difference being the thread cut into the block (Figure 1F). To prevent carrier gas and sample from leaking out during the injection, we fitted the syringe with a piece of high-temperature septum (Figure 1B) which is tightly pressed against 0003-2700/82/0354-1646$0 1.25/0
the injector block (Figure IC)at least 20 s before and after the injection. A representative chromatogram obtained with this injection port is shown in Figure 2 (chromatogramB). The comparison with the chromatogram obtained using an injection port septum under otherwise identical conditions (chromatogram A) reveals the absence of septum bleeding but no significant differences in peak shapes, heights, and separation. The high-temperature septum was obtained from Varian (Part No. 69-000179) and had been conditioned 8 h at 350 “C. Flash heater block and FID detector temperatures were 350 OC (3770 gas chromatograph with manually switchable constant flow and constant pressure regulators, Varian, Palo Alto, CA), and the temperature in the injection port measured a t the rotating stainless steel disk (Figure 1E) was 120 “C. The needle was guided about 4 cm into a 15 cm long, 0.7 mm i.d. glass-lined stainless steel tubing (SGE, Melbourn, Australia) to which capillary columns of different internal diameters can be attached by the reversed cone principle without any dead volume (not shown in Figure 1). Since the sample is not really injected into the column but into a glass-lined tube attached to the column, we call this type of injection ‘‘quasi on column”. The oven temperature was programmed from 40 “C to 300 OC a t a rate of 10 “C/min. The final temperature was held for 5 min. The carrier gas was hydrogen a t a flow rate of 20 mL/min during injection (constant pressure regulated), and 2 mL/min during the temperature program (constant flow 0 1982 American Chemical Society
