Anal. Chem. 1988, 60, 1990-1994
1990
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Figure 1. Schematic diagram of apparatus for demonstrating super-
conductivity. If the material is a powder, a superconductor may be detected by sprinkling the powder into a container of liquid nitrogen with a magnet in or beneath it (8). The superconducting material avoids the region of high magnetic flux, leaving a bare spot. According to the authors, 2% of superconducting powder admixed with inert copper oxide can be detected in this way. This method requires the use of only simple, inexpensive equipment that, except for the liquid nitrogen, is likely to be available in most laboratories. It will be useful for the detection of superconductivity in known compounds.
EXPERIMENTAL SECTION Apparatus. The apparatus (Figure 1) consisted of a clean, dry wide-mouth Dewar flask filled to within 3 cm of the top with liquid nitrogen. (Caution:contact with bare skin can cause injury due to frostbite). An aluminum cup was fabricated by forming a disk of aluminum foil (Alcoa heavy duty) against the bottom of a round-bottom flask. The diameter of the disk and roundbottom flask were the same, and both were somewhat smaller than the inner diameter of the Dewar flask. A Teflon-coated Alnico V magnetic stirring bar 38 mm long by 7.9 mm in diameter (Dynalab Corp., Rochester, NY) was inserted into a piece of plastic tubing to serve as a magnetic probe. Procedure. The piece or powder to be tested for superconductivity was placed in the cup and floated on the surface of the liquid nitrogen. To ensure that the sample was thoroughly cooled, liquid nitrogen was added a few drops at a time to the cup, until a small amount remained. When the magnet was brought to within 1 cm of the sample, the presence of superconductivitywas demonstrated by repulsion between the sample and the magnet. RESULTS AND DISCUSSION This procedure was used to detect the presence of superconductivity in samples of Y-BaXu oxide, prepared according
to a published method (9). The prepared material was found by X-ray diffraction to be primarily composed of the superconducting phase (IO). Bulk and powdered samples ranging down to 0.1 g gave a positive response. When the superconducting powder was mixed with powder of the same chemical composition that was not superconducting, the repulsive effect was observed down to 5% (w/w). Larger A1 cups were used to provide the necessary buoyancy with large samples. Repulsion was not observed with an empty AI cup or with a nonconducting material. When the experiment is properly conducted, even relatively weak repulsive effects can be observed because of the ease of movement of the floating Al cup. Some precautions are necessary to avoid extraneous effects that may either mask the repulsive motions or be mistaken for them. Obviously, air currents can cause motion. Furthermore, bubbles rising within the liquid nitrogen cause currents at the surface. The liquid bubbled vigorously when first placed in the Dewar flask, but the bubbling ceased, resulting in a quiet surface. After some use, when the liquid became contaminated with ice crystals, continuous streams of fine bubbles made it necessary to empty, clean, and refill the flask. The design of the AI cup was important in avoiding another interfering effect, sticking of the cup to the wall of the Dewar flask due to a film of liquid drawn up by capillary attraction. The flaring sides of the cup prevented a close approach and minimized this problem. In addition to experimental factors that can mask the presence of superconductivity, if the physical dimensions of the superconducting particles are too small relative to the London penetration depth, the Meissner effect will be suppressed (11). The suppression is more pronounced in a ceramic superconductor because of the low density of conduction electrons. If this apparatus is used as a quick check for superconductivity in unknowns, it must be remembered that false negative responses are possible. Therefore, a negative result does not necessarily prove the complete absence of superconductivity.
LITERATURE CITED (1) Daigani, R. Chem. Eng. News 1987, 65(19), 7-16. (2) Fisher, G.; Schober, M. Am. Cefam. SOC. Bull. 1987, 66, 1087-1092. (3) Hor, P. H.; et al. Phys. Rev. Lett. 1987, 58, 1891-1894. (4) Murphy, D. W.; et al. Phys. Rev. Lett. 1987, 58, 1888-1890. (5) Wang, H. H.;et ai. Inorg. Chem. 1987. 26. 1190-1192. (6) Juergens, F. H.: et al. J . Chem. Educ. 1987, 64(10), 851-853. (7) Kittel, Charles. Introductlon To Sol& State Physics, 3rd ed.; Wiley: New York, 1966; p 341. (8) Baker, R.; Thompson, J. C. J . Chem. Educ. lg87, 64(10), 853. (9) Cava, R. J.; et at. Phys. Rev. Lett. 1987. 58, 1676-1679. (10) Wong-Ng, W.; et al. A&. Ceram. Mater. 1987, 2, No. 38, Special Issue, 565-576. (11) Kittei, Charles. Introduction To SolM State Physics, 3rd eo'.;Wiley: New York, 1966: p 354.
RECEIVED for review October 6,1987. Accepted April 26,1988.
Electron-Ionization-Like Mass Spectra by Capillary Supercritical Fluid Chromatography/Charge Exchange Mass Spectrometry Edgar D. Lee, Shih-Hsien Hsu, and Jack D. Henion* Equine Drug Testing and Toxicology, New York State College of Veterinary Medicine, Cornell University, 925 Warren Drive, Ithaca, New York 14850 Coupling supercritical fluid chromatography with mass spectrometry (SFC/MS) has been suggested as a method for
solving various analytical problems that are not amenable to gas chromatography/mass spectrometry (GC/MS) or liquid
0003-2700/88/0360-1990$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
chromatography/mass spectrometry (LC/MS) ( I ) . For example, supercritical fluid chromatography is able to separate more polar, less volatile, and more thermally labile compounds than gas chromatography (2,3) with much higher chromatographic efficiencies than have been demonstrated by highpressure liquid chromatography (4). Most data presented from SFC/MS experiments have been obtained by using conventional chemical ionization (CI) methods ( I , 5-8) with the exception of a few examples of electron ionization (EI) (9-13). This is primarily due to the decrease in sensitivity achieved for E1 SFC/MS compared to that of E1 GC/MS for similar compounds. The decrease in E1 sensitivity may result from increased ion source pressure during the course of the SFC pressure program or another possibility is the formation of dry analyte particles emerging from the SFC restrictor due (14) to the rapid cooling of the expanding supercritical fluid. If the ionization chamber is opened up as is the case in most E1 sources to allow improved pumping in this region, the residence time of the analyte particles within the ionization region should be decreased and the analyte may not have enough time to volatilize prior to ionization. Unfortunately, the reasons for reduced E1 SFC/MS sensitivity are not yet well understood. The use of CI circumvents this problem by “tightening” the ionization chamber and increasing the pressure within the ion source. This higher pressure condition in the ionization chamber increases the analyte residence time in the ion source and improves heat conduction allowing vaporization of the analyte particles so conventional CI can take place using methane, ammonia, or other suitable reagent gases. SFC/MS under CI conditions has comparable sensitivity to GC/MS but lacks the structural information available from electron ionization ( I , 5-8, 13). The use of charge exchange (CE) to obtain library-searchable EI-like mass spectra has been reported in the literature (15). By use of CE to obtain “pseudo” E1 data, the mass spectrometer could be used in its CI configuration and still obtain meaningful structural information. Two of the most common supercritical fluids used for SFC, carbon dioxide and nitrous oxide, have ionization potentials of 13.6 and 12.9 eV, respectively ( I @ , which is above the ionization potential of most organic compounds. Because of the low flow rates from SFC capillary columns, the use of additional “make-up” reagent gases can be used, such as nitrogen, helium, and argon. Thus the use of charge exchange ionization is a practical way to obtain EI-like mass spectra by SFC/MS. In this report we describe capillary SFC/MS results obtained by using this approach on a modified commercial benchtop GC/MS system.
EXPERIMENTAL SECTION The chromatographic system consisted of a Lee Scientific Model 502 pump with a Model 602 controller (Salt Lake City, UT), a Hewlett-Packard5890 gas chromatograph oven (Avondale, PA), and a Valco Model AC14W injector (Houston,TX) equipped with a 0.2-pL sample loop and a SFC splitter (Scientific Glass Engineering, Austin, TX). The supercritical carbon dioxide (Scott Specialty Gases, Plumstead, PA) was delivered to a 50 pm i.d. X 10 m or a 25 pm i.d. X 5 m SB-methyl or SB-octyl capillary column (Lee Scientific, Inc.), which was housed in the chromatographic oven. The restriction at the column exit was the integral type reported by Guthrie (17). It was adjusted to allow 1-3 mL/min of gas flow at 1atm with a head pressure on the column of 200 atm. The restrictor was then fed through the vacuum seal to the entrance of the CI ion source. The chromatographic pressure ramps and temperatures of each separation are given in the Figures. The mass spectrometer used was a modified Hewlett-Packard 5970 mass selective detector (MSD). Modifications included alteration of the vacuum manifold to incorporate a 330 L/s Model TMP 330 turbomolecular pump (Balzers, Hudson, NH), use of a modified Hewlett-Packard 5995 chemical ionization source, and
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placement of the vacuum housing through the wall of the GC oven to preclude the need for an extra heated transfer line region. The Hewlett-Packard chemical ionization source (Figure 1) was modified to accommodate higher gas flow by enlarging the ion exit orifice (A) and the electron entrance orifice (B) to 400 pm. The standard 5995 CI source “spindle” (C) was shortened and fitted with a copper insert (D) to facilitate heat transfer to the exit of the capillary restrictor (E). Additional makeup gas (L) was added to the CI ion source coaxially to the capillary restrictor. The additional gas was used to pressurize the ion source for optimum CE SFC/MS conditions. Helium was chosen as the CI makeup gas because it is more easily pumped than COz by the mass spectrometer vacuum system and ita higher ionization potential (16)induces more extensive fragmentation (15). The ion source was pressurized so that the analyzer ion gauge tube read between 2 X and 4 X Torr (uncorrected) at mid-range
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Flgure 3. Charge exchange supercritical fluid chromatography/mass spectrometry total ion current profile for the separation of five acid drugs, 10-60 ng per component: (A) ibuprofen, (B) flufenamic acid, (C) naproxen, (D) phenylbutazone, (E) unknown impurity, (F) indomethacin. Conditions were as follows: 50 pm i.d. X 10 m SEmethyl capillary column; supercrttical carbon dioxide at 150 OC. of the chromatographic pressure or density ramp used for the separation being performed. The source heater (F)was used to heat the restrictor area and was maintained at 250 “C. The
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electron energy of the MSD circuit was modified to allow variation from 0 to -250 V instead of the fixed -70 V output typically used for E1 conditions. The electron energy was varied with continuous coaxial introduction of PFTBA under CE operating conditions and the ion abundances at m / z 131,219, and 502 were optimized at -230 V. The typical mass range scanned in this work was 90-450 daltons at one scan per two seconds. The scan was begun at m / z 90 to preclude scanning through the dimer of carbon dioxide at m/z 88. After conventional tuning using perfluorotributylamine (PFTBA), the overall performance of the SFC/MS instrument was evaluated by injecting a dilute dichloromethane solution of poly(propy1ene glycol), average molecular weight 425. If a satisfactory total ion chromatogram was obtained, the system was used for further experiments. The conventional E1 GC/MS determination of boldenone was accomplished on a HewlettPackard 5840/5985B GC/MS system equipped with a 0.32 mm i.d. X 25 m SE-54 fused-silica capillary column (Hewlett-Packard, Avondale, PA). The GC oven temperature was programed from 150 to 260 OC a t 15 deg/min after a 1-min isothermal period. The carrier gas was helium maintained at a linear velocity of 30 m/s. The determination of boldenone under true E1 SFC/MS conditions was accomplished on the same GC/MS system with modifications for SFC operations. The experimental details of this work are reported elsewhere (18). Equine urine samples were obtained from experimental horses following therapeutic administration of ibuprofen. Ibuprofen (2
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