drive system for rapid mixing. The entire operating cycle is actuated by pulses from a digital sequencing system or, if desired, by manual switches. The flow system is vertical to minimize problems with air bubbles. The flow system has a mixing time and a flow dead time which are comparable to those of instruments which are commercially available. One further advantage of this system is the ease of sample handling which, in the case of the commercial instruments, can be quite tedious. In addition, the stopped-flow spectrophotometer has been shown to be useful for kinetic analyses based on reactions of widely varying rates.
ACKNOWLEDGMENT
The authors acknowledge the excellent workmanship and cooperation of Charles Hacker and Russell Geyer of the Michigan State University Chemistry Department Machine Shop in the construction of the stopped-flow system. RECEIVED for review June 10, 1971. Accepted September 27, 1971. Presented at the 160th National Meeting, ACS, Chicago, Ill., September 1970. Financial support of the National Science Foundation (Grant No. GP-18123) is gratefully acknowledged.
Modification of an AEI MS12 Mass Spectrometer for Chemical Ionization A. M. Hogg Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada
An AEI MS12 medium resolution mass spectrometer has been modified to permit chemical ionization studies at source pressures in the region of 0.8 Torr. A new source pumping system gives a pumping speed of 125 liters/sec at the source and a new ionization chamber of similar overall dimensions to those of the original source, but gas-tight except for a 0.013-in. electron entrance hole and a 0.002-in. x 0.200-in. ion exit slit, is used. Both sample and reactant gas are admitted through a specially constructed direct probe which utilizes the existing insertion lock. Sensitivity is high in the chemical ionization mode and the source may also be used in the electron impact mode with sensitivity similar to that of the original source. The original source can be reinstalled at any time to return the instrument to its normal specifications.
CHEMICAL IONIZATION is the name given by Field and Munson ( I , 2) to a technique employing ion-molecule reactions between ions of a reagent gas at high concentration with molecules of a sample gas to produce a mass spectrum which differs considerably from that produced by direct ionization of the sample gas by electron impact. The analytical value of this technique has been amply demonstrated over the last few years by the original developers and other workers (3-19) (1) M. S . B. Munson and F. H. Field, J. Amer. Chem. SOC.,88, 2621 (1966). (2) F. H. Field in “Advances in Mass Spectrometry,” Vol. 4, E. Kendrick, Ed., Institute of Petroleum, London, 1968. (3) M. S . B. Munson and F. H. Field, J . Amer. Chem. SOC.,88, 4337 (1966). (4) F. H. Field, M. S. B Munson, and D. A . Becker, Adoan. Chem. Ser., 58, 167 (1966). (5) M. S . B. Munson and F. H. Field, J . Amer. Chem. SOC.,89, 1047 (1967). (6) F. H. Field and M: S . B. Munson, ibid., p 4272. (7) F. H. Field, ibid., p 5328. (8) F. H. Field, Peter Hamlet, and W. F. Libby, ihid., p 6035. (9) F. H. Field, ihid., 90, 5649 (1968). (10) Zbid., 91, 2827 (1969). (11) Zbid., p 6334. (12) D. P. Weeks and F. H. Field, ibid., 92, 1600 (1970). (13) F. H. Field, ibid., p 2672. 39, 388 (1967). (14) E. Gelpi and J. Oro, ANAL. CHEM., (15) H. M. Fales, G. W. A . Milne, and M. L. Vestal, J. Amer. Chem. Soc., 91, 3682 (1969). (16) H. M. Fales, H. Lloyd, and G. W. A. Milne, ihid., 92, 1590 (1970). (17) H. Ziffer, H. M. Fales, G. W. A. Milne, and F. H. Field, ihid., p 1597.
and the modification of mass spectrometers for chemical ionization has been described ( I , 20, 21). Work in this laboratory on ion-molecule reactions was begun several years ago (22,237 and directed toward an understanding of the kinetics of the systems and determination of basic thermodynamic quantities and so the instruments developed for this work (22, 24) have been designed with sources operating over wide ranges of pressure (0.05-200 Torr) and temperature (20-630°C) coupled to analyzers of relatively low resolving power. The requirements for a chemical ionization mass spectrometer for the study of large organic molecules are much less exacting, with the exception of the need for adequate resolving power and the provision of a simple means for rapid introduction of samples of low volatility in a routine fashion. Accordingly a medium resolution analytical mass spectrometer, an AEI MS12, has been modified so that chemical ionization can be applied to routine analytical problems. EXPERIMENTAL
Design Criteria. The MS12 in this laboratory is used principally for MSIGC work where maximum sensitivity is required and, for this purpose, it is fitted with a helium separator and a direct 4 atm ml/min leak both of which contact the source block in operation but are readily retractable for source removal. It was felt that a compromise dual EIjCI source could not be expected to perform as well as the original E1 source without the added complexity of dual filaments and a two stage ionization chamber. While such a design may be attempted at a later date, it seemed wiser to concentrate on optimum CI performance in the initial source design. Consequently, the following minimum goals were set: Provision should be made for operating the source in the E1 mode but not at the expense of degraded CI operation.
(18) H. M. Fales, G. W. A. Milne and T. Axenrod, ANAL. CHEM., 42, 1432 (1970). (19) G. W. A. Milne, T. Axenrod, and H. M. Fales, J. Amer. Chem. SOC.,92,5170 (1970). (20) J. H. Futrell and L. H. Wojcik, Reo. Sci. Instruni., 42, 244 (1971). (21) J. Michnowicz and B. Munson, Org. Mass Spectrom., V4, 481 (1970). (22) P. Kebarle and A. M. Hogg, J. Chem. Phys., 42,668 (1965). (23) A. M. Hogg and P. Kebarle, ibid., 43,449 (1965). (24) P. Kebarle, Advan. Chem. Ser., 72, 24 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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Figure 1. Photograph of modified MSlZ showing pumping system and chemical ionization sample introduction probe As most of the work in this laboratory involves low volatility solids which require direct probe introduction, this should be the principal means of admitting samples in the CI mode. Provision for handling gases and volatile liquids should be made but this is of minor importance. The CI source should be of comparable reliability t o the existing E1 source and the instrument resolution should not be significantly degraded with R.P. = 1000 easily obtainable with wide slits. The safety of the operator should be ensured by having only grounded metal surfaces exposed. It should be a simple matter t o reinstall the original source to obtain optimum E1 performance and the inlet systems used with it, insertion lock, helium separator, direct G C leak, and calibrating gas inlet, should remain in place when the CI sourceisinuse. Pumping System. A high capacity source pumping system (Figure 1) has been in use on the MS12 for over a year, to provide greater flexibility in MS/GC work where it is advantageous if large flow rates of helium into the source can be tolerated and it has adequate capacity for chemical ionization work. The system consists of an Edwards E 0 4 pump with 600 liters/sec capacity, an Edwards NTM4 liquid nitrogen trap, and an Edwards QSB4 butterfly valve. The manufacturer's quoted overall pumping speed for air of this system is 150 litersisec. A new source housing of the same dimensions as the original has been constructed but in place of the existing small pumping port (2-in. diameter) an opening approximately 3 inches X 5 inches has been cut and to it is welded a fabricated rectangular duct which terminates in a 7-in. diameter circular flange situated 4 inches from the center line of the source. An elbow made from 6-in. 0.d. stainless pipe with suitable flanges connects the source housing pumping flange to the top of the butterfly valve so that the center line of the pumping tower is 9 inches from the center line of the source. The combined conductance of the duct and elbow for air is calculated to be 700 literslsec leading t o a pumping speed for air at the source of 125 liters/sec. The original source nude ion gauge is mounted in a T-piece attached to the elbow and a 22-mm window in the end flange of the T-piece permits a view of the source through the rectangular port in the source housing. A small lamp can also be brought close to the window to provide better illumination of the source as viewed through another window in the source flange. These windows are also valuable for detecting the onset of glow discharge as the source pressure is raised above the safe working level, but below the point where audible arcs occur. Gold gaskets are used in the source and ion gauge housings 228
Shield Trap
k,
,)-Repeller
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and viton O-rings in the -oumving . -tower. Aviezon BW oil is used in the pump. The diffusion DumD.is rouahed through the existing '/An. line by the original 60 liters/& rotary pump which it shares with the analyzer diffusion pump. Tests made with a short 1-in. fore pump line and an additional 150 literlmin rotary pump showed no improvement in pressures, as read on the source and analyzer ion gauges, when the source was operated with a throughput of 8 atmospheric ml/min of methane, a flow in excess of that required for chemical ionization operation. Two other minor changes were made to accommodate the new pumping system. The source "neck" was rotated through 180' to allow clearance for the source slit adjusting mechanism which now protrudes from the side opposite the pump. This has the added advantage of bringing the source isolation valve into a position below the source housing and readily accessible from the front of the instrument. The insertion lock was modified to accept the current concentric AEI direct insertion probes and rotated 30' from its original position to provide clearance between its pumping lines and the new pumping tower. Ion Source. A new ion source, Figure 2, has been constructed with outside dimensions identical to those of the original 4-port AEI source. Internally, however, it is significantly different. The ionization chamber is a circular cavity of about '/An. diameter and the repeller and trap are mounted on a tightly fitting base plate with their electrical connections made via a dual miniature feedthrougb (part of a Gow-Mac thermal conductivity cell filament assembly). The top of the chamber is integral with the source block and has a 0.200-in. diameter hole in it across which are welded two pieces of stainless steel razor blade t o form a 0.002-in. X 0.200-in. ion exit slit.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
15K 5W
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Figure 4. Schematic diagram of modifications to MS12 source siipply chassis
-45
Lusec “Met rosi1” Leak
-Quartz to Pyrex Graded Seal
1
Kovar to Quartz Graded Seal Heater and Thermocouple
,Figure 3. Chemical ionization sample introduction probe
The cut-out for mounting the filament is deeper than on the normal source and has a 0.013-in. diameter hole in it to permit entrance of the electrons. The filament assembly is made from a standard AEI tungsten filament with the addition of an emission collector plate and a further mica insulator. The emission collector is simply an electrically isolated plate with a 0.040411. hole in it mounted between the filament and the electron entrance aperture. The “Trap” lead from the source supply chassis can be connected either to this plate or to the trap by moving connectors on the source flange outside the vacuum. This permits operation in either trap or emission stabilized modes. The electrode which is not in use is electrically connected to the source block. Four ports are drilled in the block, however only one, opposite the insertion lock, penetrates into the ionization chamber. The other three are blind holes terminating about 0.050 inch short of the chamber. They can be drilled out easily at a later date if required.
A probe guide made from Rulon is attached to the completed port and held in place by a screw in the same manner as that used for the original stainless steel guide. Two stainless steel encapsulated heaters (Hottwatt I/&. diam X 1 inch long, 35 ohms) are fitted in wells in the source block parallel to the electron beam. Wells for two more heaters of the same type are provided but are not used at present. A new source flange has also been constructed which differs from the standard AEI flange in two ways. A 22-mm diameter window replaces the feedthrough almost directly above the probe guide, permitting visual monitoring of the probe mating with the guide and the remaining 10 feedthroughs are all of a dual concentric type which allows two leads insulated from each other to a maximum of a few hundred volts and from ground for 8 kV to be connected at each position, thus giving a total of 20 feedthroughs into the vacuum. Not all are required at present but the spares may be used in future modifications. The overall source maintains the same clearance between the block and the collimating magnets and between the beam centering plates and the source adjustable slit cover plate as the standard source. Probe. Both sample and reactant gas are introduced uia a specially constructed probe (See Figure 3). The shaft is constructed from 0.478-in. diameter stainless tube which fits the existing MS12 insertion lock and a Kovar to quartz graded seal is mounted on the end. A ring seal between the outer tube and an inner quartz tube is ground to a 45” taper to permit a good seal to be made with the 43” taper of the probe guide on the source. The use of quartz rather than borosilicate glass is necessitated by the poor electrical insulating properties of the latter at elevated temperatures. A slight constriction 2 cm from the tip supports the sample container which is made from melting point capillary. Inside the stainless steel handle, a quartz to borosilicate glass graded seal is made in the inner tube and a 45 lusec “Metrosil” (sintered silicon carbide) leak is sealed inside the borosilicate glass. The inner tube terminates in a specially designed seal using a rubber O-ring which connects it to a short length of 3/is-in.brass pipe which extends from the end of the handle. A tubular heater is constructed from a thin-walled stainless steel former with a Cr/Al thermocouple attached and over which a glass fiber sleeve is placed and a nichrome heating wire wound. The heater is inserted into the probe and heats the region from the tip to a point just past the constriction in the inner quartz tube. A cable and plug connect the probe to a simple power supply which consists of a 115 V/12.5 V filament transformer whose primary is controlled by a variable autotransformer.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
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To MELeod Gauge
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Figure 5. Diagram of reactant gas pressure regulation system
An adjustable locking collar on the probe shaft can be set to touch the top of the insertion lock housing when the probe tip is snugly mated with the probe guide (as seen through the viewing port) allowing repeated sample introductions without danger of damaging the probe tip or the source quartz support plates. Source Supplies. The rectifier tube, VI, in the MS12 source supply chassis has been replaced by silicon diodes and the 5-V winding of T1 which is now freed is used to supply power to a floating 420 V, 2 mA power supply (Figure 4). A string of six 68 V (nominal) Zener diodes is placed across the power supply output and by means of an auxiliary rotary switch marked “eV Coarse” on the source supply control panel, any voltage from 0-420 V in 68 V steps can be inserted in series with the existing 80 V adjustable electron accelerating voltage. A further position on the eV meter switch gives 500 V f.s.d. The power supply is mounted inside the source supply chassis. Reactant Gas Regulation. The 45-lusec leak in the probe was chosen so that reactant gas supplied at one atmosphere pressure to its input side would result in an ion chamber pressure of about 0.8 Torr and a flow of about 4 atm ml/min through the source pumping system, A cylinder of compressed reactant gas (normally methane) is fitted with a pressure regulator, a one-liter neoprene inhalator bag and a coarse needle valve (Hoke No. 3232M4B) as shown in Figure 5 and mounted in a support attached to the tube unit frame close to the ion source. A short piece of 3/32-in.i.d. rubber hose connects the needle valve to the brass input pipe on the probe handle. The inhalator bag is inflated and the regulator closed. The needle valve is normally operated fully open but it can be closed partially to reduce the ion source pressure below 0.8 Torr, always bearing in mind that if the pressure in the inlet side of the leak drops much below 1/10 atm, there will be a danger of sparking. The inhalator bag need only be refilled every three to four hours when using the maximum flow rate. Somewhat surprisingly, after a few days operation, no appreciable water or other impurity is evolved from the large internal rubber surface of the inlet system; however when some other gas (e.g., isobutane) is used, a new inhalator bag and rubber hose must be used. The extra expense of having each compressed gas cylinder fitted with an inhalator bag, needle valve, and hose is negligible. Source Pressure Measurement. A special pressure measuring probe, Figure 6, has been constructed by means of which the pressure inside the ion chamber can be measured with a rotating McLeod Gauge while reactant gas is flowing but with the high voltage switched off. The needle valve (Vacoa MV25) is opened in stages and the source pressure calibrated cs. the readings on the source pumping ion gauge and the thermocouple gauge on the source rotary pump. The calibration must be repeated for each reactant gas used because of the different responses of the ion and thermocouple gauges to different gases. The total flow of gas through the system is measured by means of a soap bubble flow meter on the exhaust of the source rotary pump. For a one-atmosphere differential 230
Figure 6. Diagram of ion chamber pressure measuring probe
across the leak, this total flow does not vary appreciably with the nature of the reactant gas. The only possible variable in this calibration procedure is the tightness of the seals made between the probe tip, the Rulon guide and the source block. This may deteriorate with age if the guide is not replaced at intervals but so far the original guide is still in use and the calibration has notchanged. RESULTS AND DISCUSSION
Probe samples can be handled in the CI source as quickly as in the normal system and no special skills are required in its operation. Routine operating conditions are: 0.8 Torr methane reactant, 100 pA total emission current, 320 V electron energy, repeller at +9 V relative to the source block, 8 KV ion accelerating voltage, 1.2 KV electron multiplier voltage, and 0.004-in. source slit width. Total ion monitor current under these conditions is off scale and estimated by extrapolation to be 52,000 monitor units (7.8 X A). Pure methane gives a m/e = 17 base peak with an intensity of 30,000 LRP collector units (2.1 X A positive ion current). When Lproline is admitted on the probe, an intensity at m/e = 116 (M 1)+ of greater than 10,000 LRP collector units (7 X A positive ion current) is easily obtained. While the sensitivity under these conditions is adequate even for samples where the (M 1)+ ion is much less abundant than those of amino acids, a further gain of about X3 can be obtained by using 500 pA emission and multiplier gain can be increased by a factor at least X100. Resolution is comparable to that obtained under E1 with the same slit widths. The indicated pressures during operation (gauges calibrated for air) are 4 x 10-4 Torr on the source ion gauge, 1 X Torr on the analyzer ion gauge, and 0.14 Torr on the tube unit rotary pump thermocouple gauge. No arcing has been experienced at any time with 0.8 Torr methane but when isobutane is used, it is necessary to lower the source pressure to about 0.6 Torr before intermittant sparks can be eliminated. Addition of small partial pressures of secondary reactant gas to a methane stream at 0.8 Torr does not lead to arcing. In the E1 mode, the electron beam can be stabilized either by trap or emission current. However when using trap stabilization a 10-pA trap current requires 450 pA total emission and at 70 eV electron energy, the beam is not very stable.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 2, FEBRUARY 1972
+
+
Best stability and sensitivity for air is obtained at 300 V electron energy and 0 to f l volts relative to the source block on the repeller. Sensitivity for air using these conditions and 10 pA trap current is almost identical to that of the original E1 source using 100 pA trap current. Samples may be admitted either by the normal magnetically coupled direct probe or by a variable leak gas probe. The procedure for the admission of samples in the CI mode differs from that used with E1 in that the total ion monitor cannot be used as a guide to the partial pressure of sample in the source. The MS12 is equipped with a circuit which permits rapid repetitive magnet scans over a selected mass range coupled to an oscilloscope display. It is convenient to use this while the probe tip is being warmed and to record scans when ions begin to appear at masses higher than those of the reactant gas. Normally several scans are recorded at increasing probe temperature in order to obtain sufficient sample partial pressure for adequate sensitivity but below the point where sample ionisample molecule interactions become pronounced. Some preliminary studies have been made using gaseous and volatile liquid samples which are injected via a septum in a small glass T-piece inserted in the rubber hose connecting the probe to the reactant gas supply. The results are quite satisfactory and this means is also used for adding secondary reactants (e.g. methanol) to the reactant gas stream. A
more sophisticated gas introduction system could be easily devised if the need for it is demonstrated. CONCLUSIONS
A satisfactory system for chemical ionization studies has been devised which can in principle be applied to most commercial mass spectrometers. It differs from other published conversions of commercial instruments (20, 21) mainly in the ease with which sample temperature can be controlled independently of the source temperature, the simplicity of the reactant gas pressure regulation, and the means of obtaining adequate high voltage insulation. Also, with the exception of the high capacity pumping system, the instrument can be returned to its original configuration by merely exchanging sources, a procedure which takes no more than 15 minutes. This permits any standard accessories provided by the instrument manufacturer to be used. The pumping system reduces the time required to reach an acceptable background after a source change by a factor of about 10 and would be a valuable modification for virtually all commercial mass spectrometers even when the use of chemical ionization is not considered.
RECEIVED for review July 19,1971. Accepted September 27, 1971.
Interfering Ions in the Elemental Analysis of Biological Samples by Mass Spectrometry N. L. Gregory Clinical Research Centre, Harrow, Middlesex HA1 3UJ, England In a detailed analysis of a dc arc source mass spectrum of blood, in which a resolution of 19,000 was attained, most spectrum lines were shown to be multiplets. By precise mass measurement, many element lines were identified in the presence of interfering lines at the same mass number, and atomic compositions were assigned to interfering ions. The nature and extent of the interference was calculated from microdensitometer records and the interfering lines together with minimum resolving powers required for given separations have been tabulated. Singly charged ions were much more abundant than multiply charged ions for all elements. Many compound ions appeared, and doubly charged compound ions were present.
INA NUMBER of papers dealing with the possible use of spark source mass spectrometry in the analysis of biological samples (1-3), it has been concluded that analyses can be performed for many elements lying within a range of concentrations from parts per thousand to parts per hundred million. The writers recognize that the method must frequently be regarded as giving upper limits of concentration of certain elements rather than absolute measurements because inter(1) R. M. Jones, W. F. Kuhn, and C. Varsel, ANAL.CHEM., 40, 10 (1968). (2) C. A. Evans and G. H. Morrison, ibid.,p 869. (3) G. H. Morrison, “Trace Substances in Environmental Health11,” D. D. Hemphill, Ed., University of Missouri, Columbia, Mo., 1969, p 307.
ference can occur. This happens when a spectrum line which is used in analysis for the element in question incorporates an unresolved contribution due to another ion. The interfering ion may be an isotope of another element having the same nominal mass, e.g., 48Ca may interfere with 48Ti or 40A with 40Ca; an isotope of another element having twice (or another integral multiple of) the same nominal mass, e.g., doubly ionized S4Fe may interfere with 27Al; or a compound ion which may be due to other elements within the sample, e.g., 32S2interfering with e4Zn; or derived from the matrix of the electrode in which the sample is embedded, e.g., ‘ZCz interfering with Z4Mg; or due to residual gas in the spectrometer, e.g., l4Nz and CO may both interfere with measurements of 28Si. The extent to which one ion interferes with another of a given mass difference depends on the resolution of the spectrometer. In most of the spark source mass spectrometry of biological samples so far reported, a radio-frequency spark has been used, giving a resolving power of up to 3000. At this resolution hydrocarbon ions can sometimes be resolved from single element lines at the same nominal mass, but in general one line only is seen at each unit mass. It is usual to guard against interference by using lines due to ions which bear multiple charges and which appear at nonintegral mass, by confirming that isotope ratios are correct, by calibration with mixtures of known composition, and by comparison with other methods. In spite of these precau-
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