Emitter Current Programmer for Field Desorption Mass Spectrometry J. W. Maine, B. Soltmann, J. F. Holland, N. D. Young, J.
N. Gerber,’ and C. C. Sweeley”
Department of Biochemistry, Michigan State University, East Lansing, Mich. 48824
The practice of field desorptlon mass spectrometry has shown that continuous control of the emitter temperature is essential both for the production of quality FD spectra and for obtaining reproducible results from successlve analyses of the same sample. In this study, an attempt to obtain temperature regulation by means of emitter current control has been made. A solid state device has been developed that regulates the current through the emitter wire to within 0.1 mA at any level between 0 and 80 mA. The unit also provides linear programming from an initial to a final current at any of four selectable rates. The resulting current regulation has provided adequate temperature regulation to enable, with ease, the determination of the optimum anode temperature (current) for obtaining the best FD spectra for each of the various compounds analyzed. Reproducibility between different emitters is sufficient to allow separations of sorbed mixtures based on selection of successive optimum temperatures by pre-determined current adjustments. Repetitive scanning of the mass spectrometer under computer control during current programming results in the production of reconstructed mass chromatograms plotted, in essence, on a thermal axis. This device has been called an emitter current programmer (ECP) and in studies, to date, has been essential for the production of reproducible FD spectra, and when used in the programmed mode shows great promise as a new approach to resolving solid phase mixtures by differential desorption processes.
Field desorption mass spectrometry (FD-MS) has shown great promise as a method of analysis for many types of nonvolatile chemical compounds (1-10). In this technique, a sample is deposited on an emitter anode of activated tungsten wire and placed in a high electric field in the mass spectrometer. Ions are produced by desorption of molecules from the solid phase matrix by a quantum mechanical tunneling effect ( 1 1 ) . As an aid in the production of ions, thermal energy is applied to the sorbed sample by passing a small current through the emitter wire ( 1 2 ) . Several experimental difficulties have been encountered by users of this technique (13, 1 4 ) . A major problem has been the generation of reproducible mass spectra from successive analyses of the sample. Experience has revealed that the standard method of manual control of the emitter current, by means of a ten-turn helipot, is ineffective in reproducing the emitter heating parameter. This shortcoming has been overcome with a solid state emitter current programmer (ECP), which provides a mechanism of precisely controlling the magnitude and rate of change of current in the emitter wire. The details of the fabrication of the ECP are provided in this note, along with preliminary studies that illustrate the impact this device can have on FD-MS. Of particular note are the capability of providing optimum temperature regulation for specific ion desorpPresent address, Institute for Basic Research in Mental Retardation, Staten Island, New York 10314.
tion studies, and the capability of producing programmed thermal gradients which allow separation and analysis of multiple components in complex mixtures by mass chromatography or selected ion monitoring techniques.
EXPERIMENTAL Instrumentation. Mass spectrometry studies were performed on a Varian MAT CH5-DF with a combination EI/FI/FD ion source. This instrument is interfaced to a DEC PDP-11/40 computer using programs similar to those described previously (15, 16). Outputting from the computer was accomplished by a Tektronix Model 4010 graphic computer terminal with a Tektronix Model 4610 hard copy unit. Sample Preparation. FD emitters are comprised of 10-micron tungsten wires activated in a benzonitrile environment in a device commercially available from Varian MAT. Uridine and adenosine were purchased from Sigma Chemical Co. (St. Louis, Mo.) and were weighed into screw-capped test tubes with Teflon liners. Doubly distilled water was added to give 10 mg/ml stock solutions. Emitter wires were coated with samples from these solutions using a dipping technique (I.?), or by use of a novel two-wire drop suspension method developed in our laboratory. In the latter method, one or more drops from a syringe are suspended between two parallel emitter wires placed in close proximity. The volume of fluid applied in this manner changes from a free-falling spherical to an oblate form. When the wires are carefully separated, the entire volume of sample solution can be collected on one of the emitter wires because of the surface tension of the solvent. This technique increases the volume of sample solution on the emitter wire as compared to the conventional dipping method. In the situation where the surface tension of the solvent is very low, multiple drops are evaporated on the suspended wires, before separation, resulting in preparation of two loaded emitter wires. Fabrication of ECP. Several factors concerning the operation of the ECP dictated the primary design of the device. They were: 1) setting of the initial and final current, 2) selection of a timebased rate of change from initial to final current, 3) current regulation to better than 0.1 mA, 4) real-time digital display of the emitter current, and 5) provision for remote computer operation. As shown in Figure 1,circuits of standard digital design employing available commercial components were used to fabricate the ECP. Logic components of the integrated circuits are all of the standard “7400” series T T L and the analog-to-digital converter is an Analog Devices, No. 7520. The thumbwheel switches used to set the starting and ending current are Digitran, type B-293-002, with true binary output code. The panel meter is a Weston Model 1230 dc milliammeter (0-100 mA). A detailed circuit diagram will be furnished upon individual request. The critical electrical consideration was the transfer of the control signals from the low voltage section to the regulating section, which lies a t 3 kV above ground. This was accomplished by high resistance optical couplers, GE H-15A1, which transmit information while ensuring electrical isolation. The actual control function is attained by an analog regulator circuit inserted in series with the CH5-DF emitter current supply and the emitter wire itself. Circuit sensing and feedback circuits permit closed loop regulation using a DAC output voltage comparison and level control.
RESULTS A N D CONCLUSIONS The emitter current programmer regulates the current to within 0.1 mA a t any level between 0 and 80 mA. For temperature programming, the operator may choose any initial and final current in this range and select one of four programming rates of 0.1, 0.5, 1.67, and 16.67 mA/s to provide a linear current “sweep.” These specifications allow the opANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
427
low voltage :15V + 5 V
I
high voltoge 3KV
FD
-
ECP - Mass Chromatography
:8
1 .L7
I
Adenosine M i l
parollel to serial
'
'
I
2i. E C c m P 2631
I
-
v)
d Q
I i I
starting current
DAC 1
I I
data system preset stort O stop manual
I 1 I
clock q
=
J
p
j
control
Flgure 3. Reconstructed mass chromatograms of uridine and adenosine plotted on a time (temperature) axis FMiTTER CURRENT REG U L AT0 R
Figure 1. Block diagram of the emitter current programmer (ECP). Essential components are identified in the text
~~
Table I. Reproducibility of Emitter Current Programmer WCP)
F D - E C P -TIC
Figure 2. Total ion intensity for the desorption of uridine as a function of time (temperature) during a linear current program (0-65 mA)
erator a wide range of experimental current conditions for use in FD-MS analyses. The best anode temperature (BAT) for a specific compound is defined as the temperature (current) where the maximum number of M+ or M I + ions are created and the best field desorption mass spectrum is generated (17). A typical experiment, as shown in Figure 2, consists of monitoring the total ion current while sweeping the emitter current. Without ECP,this is a difficult experiment to perform and nearly impossible to reproduce, but with the device excellent reproducibility is attained with ease. The results in Figure 2 show the determination of the BAT for uridine. Similar curves have been obtained with other classes of compounds using the linear current sweep of the
+
428
Current, m A
1 2 3 4 5
19.2 t 0.4 21.3 i 0.4 20.3 21.0 20.2
The mass spectrometer was scanned repetitively a t 6-s intervals while the ECP generated a linear current sweep. The data were collected by the computer and reconstructed mass chromatograms (18) were assembled. Differentiating ions were chosen to indicate the degree of separation of uridine and adenosine. In contrast to the problem of choosing characteristic ions in electron impact mass spectrometry, with FD mass spectrometry the predominant production of molecular ion (M)+ and (MH)+ makes the choice an easy one in most instances. In Figure 3, for example, mle 244 and 245 are the molecular and (MH)+ ions for uridine and those a t mle 267 and 268 are molecular and (MH)+ ions for adenosine. The areas under the peaks of these ions were approximately proportional to the concentrations of the compounds deposited on the emitter wire, indicating that it may be possible to use ratios of these areas to those of internal standards for quantitative analyses as has been done previously by GC-MStechniques (15,18). In the applications studied to date, ECP has been essential to the production of reproducible FD spectra. Temperature gradient FD analysis produced by emitter current programming has shown promise as a new approach that should be particularly effective in resolving solid phase mixtures by differential desorption processes.
LITERATURE CITED (1)H.-R. Schulten and H. D. Beckey. Angew. Chem., int. Ed. Engl., 14, 403 (1975). (2)H.-R. Schulten and D. E. Games, Biomed. Mass Spectrorn., 1, 120 (1974). (3)K. L. Rinehart, Jr., J. Carter Cook, Jr.. K. H. Maurer, and V. Rapp, J. Antibiot., 27, l(1974). (4)D. A. Brent, P. deMiranda, and H.-R. Schulten, J. Pharm. Sci., 63, 1370 (1974). (5)G.W. Wood and P.-Y. Lau, Biomed. Mass Spectrom., 1, 154 (1974).
ECP. Reproducibility of the BAT with repeated dipping of the emitter wire into an aqueous sample of uridine was within 0.4 mA (Table I) indicating an effective temperature regulation commensurate to this current variation. Table I also shows a series of experiments where the BAT for uridine was determined with different emitter wires, all of which were activated under the same conditions. A greater variation was observed in this case than with repeated sampling on the same wire, indicating that while the BAT of a chemical compound is not an absolute parameter, it is a reasonably precise estimation of sample temperature. Figure 3 illustrates the potential value of ECP for qualitative and quantitative analyses of mixtures by FD-MS.
Wire N o ,
No. of measurements
(6) D. E. Games, M. P. Games, A. H. Jackson, A. H. Olavesen, M. Rossiter, and P. J. Winterburn. Tetrahedron Lett., 27, 2377 (1974). (7)J. R. Hass, M. C. Sammons, M. M. Bursey, B. J. Kukuch. and R. P. Buck, Org. Mass Spectrorn., 9, 952 (1972). (8)D. A. Brent, D. J. Rouse, M. C. Sammons, and M. M. Bursey, Tetrahedron Lett., 42, 4127 (1973). (9)M. C. Sammons, M. M. Bursey, and D. A. Brent, Biorned. Mass Spectrom., 1, 169 (1974). (10)M. C. Sammons, M. M. Bursey, and C. K. White, Anal. Chem., 47, 1165 (1975). (1 1) Gomer, "Field Emission and Field Ionization". Harvard University Press,
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Cambridge, Mass.. 1961. (12) H. D. Beckey, lnt. J. Mass Spectrom. /on. Phys., 2, 500 (1969). (13) Field Desorption Users Workshop, Biochemistry Department, Michigan State University, East Lansing, Mich., March 1974. (14) Field Desorption Users Workshop, Chemistry Department, University of Illinois, Urbana, Ill., December 1974. (15) R. A. Laine, N. D. Young, J. N. Gerber, and C. C. Sweeley, Biomed. Mass Spectrom., 1, 10 (1974). (16) C. C. Sweeley, B. D. Ray, W. I. Wood, J. F. Holland, and M. I. Krichevsky, Anal. Chem., 42, 1505 (1970). (17) H. U. Winkler and H. D. Beckey, Org. Mass Spectrom., 6, 655 (1972).
(18) R. A. Hites and K. Biemann, Anal. Chem., 42, 855 (1970).
RECEIVEDfor review July 21, 1975. Accepted November 10, 1975, This work was supported in part by a postdoctora1 fellowship (to JNG) from the Smith, Kline and French Laboratories and a research grant (RR-00480) from the Biotechnology Resources Branch, National Institutes of Health.
Multielement Charged Particle Activation Analysis with X-ray Counting J. R. McGinley and
E. A.
Schweikert’
Center for Trace Characterization, Deparfment of Chemistry, Texas A&M University, College Station, Texas 77843
Energy dispersive x-ray counting of radioactive species produced by charged particle activation was studied as a nondestructive multielement trace analysis method for elements of Z 1 26. Experimentally determined thick target yields and interference-free detection limits are presented for proton and deuteron activation products of 37 elements. These detection limits range from 1 X pg for Mo to 3.4 pg for Rh for a 6 pA-hr irradiation with 20 MeV protons. The application of this technique is illustrated with data on the simultaneous determination of up to 26 elements in NBS standard glass samples at the 500-, 50- and 1-ppm levels using 20-MeV proton activation.
The aim of this study was to expand the scope of charged particle activation analysis by examining the possibilities of complementing y-ray spectrometry with nondispersive delayed x-ray counting. The use of x-ray counting was first reported in neutron activation analysis by Shenberg et al. ( I ) and Pillay e t al. (2). More recently, Mantel et al. extended this work by surveying a large number of x-ray emitters resulting from (n,y) reactions (3, 4).With the exception of a note in conjunction with this project (j),no work has been reported, so far, on the analytical possibilities offered by charged particle activation followed by x-ray counting. An interesting observation in this connection is that most of the radionuclides of medium and high 2 elements produced by charged particle reactions decay principally by internal conversion or electron capture and are thus predominantly x-ray emitters. The advantages of nuclear activation followed by nondispersive x-ray spectrometry have been discussed elsewhere ( 5 ) .The main features are: the direct relationship between the x-ray energy and the atomic number of the pertinent element; the relatively simple structure of x-ray spectra (in comparison with y-ray spectra); the availability of radioactive decay rates as an additional criterion of identification. These advantages must be weighed against possible limitations arising from self-absorption and enhancement effects common to all x-ray techniques. An additional limitation proper to nondispersive x-ray counting of radioactive samples arises when p activity is present, which results in increased background. This study focused on the feasibility of nondispersive x-ray counting following proton and deuteron bombard-
ment as a means of multielement, nondestructive trace analysis of medium and high 2 elements. Topics examined include: evaluation of activation reactions and detectors; interference- free detection limits for proton and deuteron activation products of 37 elements; application of the technique illustrated with data on multielement determination in NBS SRM Glass samples a t the 500-, 50-, and 1-ppm levels using 20-MeV proton activation.
EXPERIMENTAL Irradiation. All irradiations were carried out a t the Texas A & M University 88-inch sector focused variable energy cyclotron. Nominal irradiation energies were 20 MeV for both proton and deuteron bombardment. In order to preserve sample integrity, the maximum beam intensities used were 3 NA per cmy. Samples. Thick target yields and interference-free detection limits were determined by irradiating pellets of approximately 2-mm thickness and containing known amounts of the element of interest. When necessary, reagent grade graphite was used as a binding material. Three standard reference glass samples from the National Bureau of Standards (SRM 610, 612, and 614) were analyzed using this method. These samples consisted of 72% SiO2, 12% CaO, 14% NalO, 2% A1203, and 61 trace elements nominally doped a t the 500-, 50-, and 1-ppm levels respectively. Counting. Two detectors were used in this work: a Si(Li) of 28.3-mm2 surface area, 3-mm active depth, 174-eV FWHM at 5.9 keV, and a thin Ge(Li) of 1984-mm2surface area, 7.25-mm active depth, 238-eV FWHM at 5.9 keV. Output signals were processed through a Canberra spectroscopy amplifier and fed into a 1024 channel analyzer. Quantitation. Quantitation of SRM 610 was done by comparing the peak area of the x ray from the nuclide of interest with the same peak area from a standard. The standards consisted of 70% SiO2, 10% CaO, 12% NaO, 2% A1203, and contained six of the elements of interest in known concentrations. A sufficient number of these standards were prepared to include all of the elements sought. Copper monitor foils (25 km thick) were used to normalize sample and standard irradiation conditions. After the concentrations of the elements of interest had been determined in SRM 610, all other samples were compared to this material for quantitation.
RESULTS AND DISCUSSION Activation Reactions. A comprehensive survey was made in order to determine the reactions of interest by proton and deuteron activation. The criteria for selection were as follows: Simple, low threshold, nuclear reactions; target nuclides having high natural abundance; product nuclides decaying primarily by electron capture or, in the case of isomeric transitions, having a high internal conversion to y-ray emission yield. ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
429
