Anal. Chern. 1983, 55, 1437-1440 1437

(11) Pawel, R. E.; Pemsler, J. P.; Evans, C. A,, Jr. J . .€/ectrot:hem. SOC.

(12) Hodgkin, N. M. In "Metallographic Specimen Preparation"; McCall, J.

320-322.

2029-2034. 1972, 119, 24-29.

L.. Mueller. W. M.. Eds.: Plenum Press: New York. 1974 DO 297-306.

(15) Aberth, W.; Straub, K. M.; Burlingame, A. L. Anal. Cbem. l982!, 54,

I r r ~ - . - . _ .

(13) Taylor, L. C. E. IniiTkes./Dev. 1961, 23, 124-i28. (14) Maiioney, J. F.; Perel, J.; Forrester, A. T. Appl. fhys. Lett. 1981, 38, RECEIVED for review January 21, 1983. Accepted April 4,1983.

Lineair Mass Reflectron with a Laser Photoionization Source for Time-of-Flight Mass Spectrometry

D. M. L,ubman,*l W. E. Bell,* and M. N. Kronlck' Quanta-Ray, Inc., 1250 Charleston Road, Mountaln View, California 94043

Time-of-flight devices have many features which make them the mass spectrometer of choice in photoionization experi- ments. Most significantly, the time-of-flight mass spectrom- eter (TOFMS) can display the full mass spectrum of ions created by a single laser pulse. For laser photoionization sources with low repetition rates this is especially important because of the low duty cycle. The TOFMS in theory can provide unlimited mass range in contrast to the quadrupole mass spectrometer in which the transmission falls off rapidly as the mass increases. The TOF device is also mechanically and electrically simple compared to other types of mass spectrometers and is thus less susceptible to failure since there are no movable parts or scanning fields. Perhaps the most significant drawback of present linear TOF devices based on the design of Wiley and McLaren ( I ) is the limited resolution achievable. Typically the resolution of such devices built in-house has been on the order of 250 for a 1.5-m device using a thermal beam. The main limitation on the resolution is the initial energy spread of the ions created, which causes a spread in time of the ions arriving at the detector. The initial energy spread must be minimized or compensated for in the field free drift region if the width of the ion packets is to be minimized and the resolution increased.

Recently, Mamyrin and Shmikk have reported a new TOFMS with substantially increased resolution (2,3) which has found use in several laser-based applications ( 4 4 ) . This device, named the mass reflectron, can achieve a resolution of as high as 3000 in the width of the peak at half-height with a drift length of 1.6 m. The device compensates for the difference in the time-of-flight of the ions of different energies by means of a system of electrostatic fields and an ion reflector which results in focusing of the ion packets in space and time at the detector. The ion reflector which is the essential feature of the reflectron, allows ions with greater velocities to penetrate a greater distance into the reflector region than ions with slower velocities. Thus, ions with greater velocities will travel a greater distance and spend more time in the reflector region than ions with slower velocities. Of course, in the field free drift region the faster ions will spend less time. The reflector scheme therefore compensates for the initial spread in ion velocities. If the total flight time in the drift and reflector region can be made almost the same for every ion of the same mle ratio, then the ion packet widths will be short, as is necessary for the attainment of high resolution.

The original devices built by Mamyrin and collaborators used a V-shaped trajectory. The problems with this design were the difficulty of focusing the ion beam in the angles of emission from the source and the increased diameter of the

Present address:

Present address: Jerome Instruments Corm. Jerome. .AZ.

Department of Chemistry, University of Michigan, Ann Arbor, MI 48109.

Present address: Applied Biosystems, 85O'Lincoln Centre Dr., Foster City, CA 94404.

analyzer chamber (2) . As a result, this group then designed a linear mass reflectron which circumvented these problems (3). For an ion drifft length of 0.6 m they were able to obtain a resolution of 1200. This design allows the construction of a mass spectrometer of small size with high resolution.

The literature on the practical details of building reflectrons is rather sketchy. In this paper we present the design o f a linear mass reflectron based on the article of Mamyrin and Shmikk and present results obtained by using UV laser ra- diation as the ionizing source for gas-phase molecules. In addition, we discuss the utility of using UV laser radiation as the ionizing source in TOF mass spectrometry.

EXPERIMENTAL SECTION The reflectron consisted of three mechanical parts: ( I ) the

reflectron focusing plate structure and reflector, (2) the vacuum chamber, and (3) the drift tube with the detector housing. The reflectron focusing plate structure was constructed as an iade- pendent unit on a 6 in. stainless steel conflat flange which could be mounted on the vacuum chamber (Figure 1). The structure was supported by four 1/4 in. diameter, 6'/4 in. long rods screwed into the flange. The threads on the rods were slotted in order to allow the threads to be pumped out. The plate spacings (see Figure 2) were the same as those used by Mamyrin and Shmikk (3) where dl = 0.25 cm, d2 = d3 = d5 = 0.5 cm, d4 = 10 cm, L1 = 2 cm, and L, = 56 cm. The voltages on the plates were originally intended to be the same as provided in this reference. The plates were 13/4 in. stainless steel disks slotted for the four support rods with a 3/s in. aperture in the center. The focusing plates used a f i e Ni mesh in order to keep the ions traveling in straight paths. The mesh used was1 Ni (Buckbee-Mears, St. Paul, MN) LOO line@. with a translmission of 82%. Etched stainless steel mesh can also be used since it is less likely to warp, especially if heated, as compared to copper or nickel. However, a comparable stainless steel mesh will have significantly lower transmission. The fine mesh is clamped between two plates to keep it as flat as possible. It can also be spot-welded to one plate, but it is rather difficult to keep the grid perfectly flat by using this method. A warped grid will affect the trajectory of the ions and eventually ithe resolution. The spacers between the plates are made of Maror, a machinable ceramic (Corning Glass Works, Corning, NY). Each spacer has an extended lip to space the plates from the support rod to prevent shorting. The whole assembly is fastened to the rods by four nuts.

In order to prevent complete loss of ions between the reflector and plate P3, guard rings were used to maintain a homogeneous field. Plates were spaced -3/ls in. apart so that 19 plates were used. Obviously a greater number of guard rings produces a more homogeneous field, but creates greater difficulty in assembly. A chain of 10-M Q resistors was used to create the appropriate potential on each guard ring as the voltage drops from -1000 V on the reflector to the -500 V on PB. The guard rings were also spaced by Macor spacers which included a lip to prevent shorting to the support rods. The voltage for the reflector and focusing plates was brought through vacuum feedthroughs and the electrical leads were soldered to the plate with low-temperature silver solder. The leads were insulated with glass sleeving and stray fields were shielded with stainless steel tubing. The reflectfor

0003-2700/83/0355-1497$01.~0/0 0 1983 American Chemical Society

1438 a ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983

metal shielding

elecfrical connection

Refleotor 1020V

lonlzatlon Region

6 O.D. Conilal Flange

- 1 P, P, p* P,

390V 655V V r l l O V

Flgure 1. Drawn-out view of reflectron structure on 6 in. 0.d. conflat flange.

I L, = Sbcm

P, P. ' I.lP,P.P. 'di = 0,sm

L 2cm d i 02,cm d. - mom d, = 05cm

d.-ossm

Flgure 2. Voltages and plate spacings used in Mamyrin deslgn upon which this reflectron was based. The voltages actually used are different from those presented above and are given in Figure 1.

is a stainless steel disk machined onto a stainless steel plate. The reflector disk is 3/4 in. in diameter and is polished to a smooth flat surface. Surface fiiish is important because any imperfections may cause a distortion in the field which will affect the trajectory of the ions.

The ions must pass through the accelerating region a second time since they are reflected back on their original trajectory. The voltage on PI must be dropped to be approximately equal to that of P2 at some appropriate delay time after the laser pulse. A schematic of the electrical circuit used to provide such a drop is shown in Figure 3. One power supply generates a voltage from an adjustable voltage divider network which is used to provide a voltage to each plate. Each voltage is independently adjustable. With the circuit shown in Figure 3, which can drop the voltage up to 300 V for several milliseconds, the voltage on P1 is dropped

-3 ps following the creation of the ions. An adjustable delay is provided so that the correct delay can be experimentally deter- mined in order to allow ions to pass through the acceleration region.

The reflectron structure which is mounted on a 6 in. conflat flange mates onto a six-way cross. The port on the opposite side is connected to the flight tube which is constructed of 1.5 in. 0.d. stainless steel tubing. The total drift distance of the chamber and tube is 56 cm. The detector a t the end of the drift region is a Varian microchannel plate (MCP), Model 8911ZS. This device is capable of a gain IO7. The risetime is subnanosecond and the typical output into 50 s2 is in volts of signal. A grid is placed in front of the microchannel plate in order to provide a field-free region.

The orthogonal set of ports are mated to 6 in. conflat flanges with 23/4 in. conflat adapters on one side for 2ll2 in. Suprasil windows which are sealed by Viton gaskets under vacuum. Quartz windows are necessary to allow UV radiation to pass through the interaction region, In order to maximize the resolution, the laser beam is condensed from 6 mm to 1 mm by using a telescope consisting of a quartz positive and negative lens with a focal length ratio of 6:1.

The laser source is a Quanta-Ray Nd:YAG laser which produces light at 1.06 pm. The primary beam can be frequency quadrupled in KD*P to product light a t 266 nm to be used as the pho- toionization source, The primary beam can also be frequency doubled to 532 nm or tripled to 355 nm and used to pump a dye laser which in turn can be frequency doubled to produce tunable

R., IOMh

+ POOV,

t IOV,

Trigger

Flgure 3. Electronic circuit used to drop pulse on plate PI. The delay time ( T I ) is given by T , = ( R 2 R 3 ) C r / 2 and the time the pulse is dropped ( T 2 ) is given by T , = R,C2/2 which in the above flgure is equal to 4.1 ms.

ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983 1439

signal but of course decrease the resolution. The signal can actually be best maximized by using a rectangular slit of illumination. If the size of the laser beam is increased in the direction parallel to the plane of the detector, the signal will increase without loss of resolution. However, if the beam increases in the direction perpendicular to the plane of the detector, there will be a decrease in the resolution.

A problem encountered with the reflectron is that the transmission will be lower than for a linear TOF because of the increased number of grids the ions must pass through. In a linear TOF the ions will pass through two grids in the accelerating region. If for Ni mesh (100 lines/in.) the transmission is 82%, then the total transmission will be no greater than 67%. If in the reflectron the ions pass through 10 grids, then the tr,msmission is - 14%. The total efficiency of the device can be calculated as follows: A 1.5-V signal peak, 20 ns fwhm into 50 SI, is equivalent to 3 X lo9 electrons/pullse. If the detector provides a gain of lo7)(, then 300 ions are produced per pulse. A pressure of 2 x torr of the com- pound under study is equivalent to 6 X lo9 molecules/cm3 which for 0.1 cm diameter laser beam X 0.95 cm long reflectron plate aperture is equivalent to 6 X lo7 molecules in the in- teraction region. If the ionization occurs with 10% efficiency, then the total efficiency of our device is -0.005% and the efficiency exclusive of the grids is -0.04%.

Although the reflectron has the disadvantage of lower transmission than a linear TOF and is mechanically more difficult to build, it has the great advantage of superior res- olution. In addition, lower voltages are required for the re- flectron. The highest voltage used for the reflectron was 1CW V on the reflector as compared to more typically -5000 V used on the first accelerating plate of our linear TOF. The reflectron also is physically much smaller than a comparable linear TOF. The reflectron used in this study provided a resolution of -630 at mol wt 93 with a drift length of 0.6 m as compared to a resolution of only 185 for a linear TOF with a drift length of l.!j m. In order to obtain a comparable resolution to a reflectron, a linear TOF would have to be unreasonably long.

In this study laser multiphoton ionization was used as the ionization source. The process used is resonant two-photon ionization where the iflist photon is absorbed by a real resonant state and the second photon takes the molecule above its ionization limit. The scientific virtues of laser photoionization have been discussed elsewhere (8-23). In terms of the TOF device a laser source provides an easily focusable source of photons for ionization. Focusing a low-energy electron beam to 1 mm or less is difficult because of space charge effects. The laser provides u 5-11s pulse so that the ion packets will not be limited by the pulse width of the ionization source. Producing an electron beam pulse of 20 ns is not a trivial task. The laser is an ionization source which is external to and independent of the reflectron device and the vacuum system. An electron gun necessary to produce a pulsed electron beam is nontrivial to build and must be mounted within the vacuum system. The electron gun has the potential to become con- taminated, and if the filament burns out, the vacuum system must be opened to tlhe atmosphere to replace it. Of course, these problems do not arise when laser produced photons are the source of ionization.

The use of laser multiphoton ionization when used in conjunction with a high-resolution reflectron device has many potential applications for analytical chemistry. One such application is in laser desorption or laser microprobe exper- iments (4 ,5) where the wide energy spread in the ions created from the surface can be minimized. Another possibility is as an aid in gas-phase analysis of organic compounds. Laser RZPI can provide soft ionization, i.e., production of moleculm

Ionization Signal I VOI t s I

al

0 4

Ionization Signal ivoltsl I O 8

-200 0 +zoo i I O : E l 2

nsec

1440 Anal. Chem. 1983, 55, 1440-1442

ions, with high efficiency. Thus, one can easily separate and distinguish the I3C isotope peak at M+ + 1 even for fairly large organic molecules. The ratio of (M+ + 1)/M+ should provide the number of carbons in the compound. Thus, even if other elements are present in an organic compound, the number of carbons can be calculated. In addition, efficient and selective ionization spectroscopy can be used for isotope discrimination (24) and a high-resolution reflectron can add further to that preselected discrimination.

A microchannel plate was used as the detector. The great advantage of this device is the high gain (lo'), speed (sub- nanosecond risetime), and large active detector area (1 in. in this study). A number of other electron multipliers are available and can be used. A channeltron electron multiplier should not be used because of its cone-shaped detector surface. The cone shape will cause different flight times depending on which point on the surface the ion arrives, thus decreasing the resolution. Activated Cu-Be devices can be used if fast enough; however, the gain is degraded when exposed to water vapor in the air. Some of the Cu-Be devices are only mar- ginally fast enough.

The simplicity of the TOF device makes it inexpensive to build. I t is mechanically much simpler than a quadrupole or double focusing field mass spectrometer and, thus, can be built in any lab. The cost of building the reflectron in parts and machining time was on the order of $2200, not including the detector or power supply. The vacuum feedthroughs which are commercially ~ $ 1 0 0 each can be replaced by crude in- house feedthroughs to reduce the cost. Eight feedthroughs were used so that the saving is significant. The cost of the electron multiplier can vary from $400 up to $5000, depending on the type and quality of the multiplier used.

ACKNOWLEDGMENT We wish to thank R. J. Rorden and D. Billings for design

and construction of the pulsed electronics circuit.

LITERATURE CITED (1) Why, W. C.; McLaren, I. H. Rev. Scl. Instrum. 1955. 26, 1150, (2) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov.

fhys.-JETP (Engl. Transl.) 1973, 37, 45. (3) Mamyrin, 5. A.; Shmikk, D. V. Sov. Phys.-JETf (Engl. Transl.)

1979, 49, 762. (4) Kaufmann, R.; Hillenkamp, F.; Wechsung, R. Med. frog. Tecbnol.

3979, 6, 109. (5) Denoyer, E.; Van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal.

Chem. 5982, 54, 26A. (6) Boesi, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . fhys, Chem.

1982, 86, 4857. (7) Lubman, D. M.; Kronick. M. N. Anal. Chem. 1982, 54, 660. (8) Dietz, T. G.; Duncan, M. A.; Liverman, M. G.; Smalley, R. E. Chem.

Phys. Lett. 1980, 70, 246. (9) Frueholz, R.; Wessel, J.; Wheatley, E. Anal. Chem. 1980, 52, 281.

(10) Lubman, D. M.; Naaman, R.; Zare, R. N. J. Chem. fhys. 1980, 72, 3034.

(11) Seaver, M.; Hudgens, J. W.; DeCorpo, J. J. I n t . J . Mass Spectrom. Ion. fhys. 1980, 34, 159.

(12) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 70, 2574. (13) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 77, 1359. (14) Llchtln, D. A.; DattaGhosh, S.; Newton, K, R.; Bernsteln, R. B. Chem.

fhys. Lett. 1980, 75, 214. (15) Boesl, U.; Neusser, H. J.; Schlag, E. W. J . Chem. fhys. 1980, 72,

4327. (16) cooper, C. D.; Williamson, A. D.; Miller, J. C.; Compton, R. N. J .

Chem. Phys. 1880, 73, 1527. (17) Fisanick, G. J.; Eichelberger, T. S., IV; Heath, B. A,; Robin, M. B. J .

Chem. Pbys. 1980, 72, 5571. (18) Fisanick, G. J.; Elchelberger, T. S., IV J . Chem. fhys. 1981, 74,

6692. (19) Reilly, J. P.; Kompa, K. L. J . Chem. fhys. 1980, 73, 5468. (20) Antonov, V. S.; Knyazev, I. N.; Letokhov, V. S.; Matuik, V. M.; Mosh-

ev, V. G.; Potopov, V. K. Opt. Lett. 1978, 3 , 37. (21) Antonov, V. S.; Letokhov, V. S. Appl. Phys. 1981, 24, 89. (22) Kiimcak, C.; Wessei, J. Anal. Chsm. 1980, 52, 1283. (23) Parker, D. H.; El-Sayed, M. A. Chem. Pbys. 1979, 42, 379. (24) Lubman, D. M.; Zare, R. N. Anal. Chem. 1982, 54, 2117.

RECEIVED for review October 25, 1982. Accepted March 7, 1983. This work received partial financial support from the U.S. Army Research Office, Contract No. DAAG-29-81-C-0023.

Generation of Formaldehyde in Test Atmospheres with Low Concentrations of Hydrogen and Carbon Monoxide

Roland E. Muller and Ulrlch Schurath" Institut fur Physikalische Chemie der Universitat Bonn, Wegelerstrasse 12, 0-5380 Bonn I, West Germany

Considerable progress has recently been made in the analysis of very low formaldehyde concentrations in tropos- pheric air (1-3). Formaldehyde is also an indoor air pollutant, due to outgassing of plastic materials and adhesives, which is suspected to be carcinogenic (4). Formaldehyde test gases for calibration purposes cannot be prepared from the pure monomer which has a strong tendency to polymerize. Geisling et al. (5) describe a dynamic procedure for the generation of formaldehyde in a test atmosphere by the depolymerization of trioxane on a carborundum catalyst at 160 "C. The trimer is continuously evaporated into a carrier gas from a thermo- stated diffusion cell. We needed about 100 ppm formaldehyde in air for a continuous actinometer, designed to monitor the photolysis frequency of formaldehyde in daylight. In order to measure the very low photochemical CO and H2 yields with a specific detector (6), the background concentrations of these gases in the unexposed test gas had to be a t most in the low parts-per-billion range. We have tested carborundum catalysts under conditions suitable for the depolymerization of trioxane and found that they produce 2180 ppb CO and 240 ppb H2 by the thermal decomposition of 90 ppm formaldehyde in a test gas. We have therefore developed a permeation source

which eliminates the problem of thermal decomposition of formaldehyde.

EXPERIMENTAL SECTIQN The aldehyde source consists of a gastight permeation cell,

either a stainless steel cylinder of 60 mL volume, or a sealed 300-mL Pyrex flask to further reduce heterogeneous decompo- sition, which is loosely packed with paraformaldehyde and quartz wool (Merck "Paraformaldehyd reinst", Erg B 6), as shown in Figure 1. A Teflon tube (3.2 mm o.d., 2 mm id.) is tightly sealed into the cell. Aldehyde vapor in thermal equilibrium with pa- raformaldehyde diffuses through the Teflon tube into the carrier gas which flows through the loop. Active tube lengths up to 266 cm were used. Constant flow rates are maintained by means of ASM flow controllers.

The permeation cell is contained in a thermostated brass box (temperature control better than h0.1 "C) , which is thermally isolated with mineral wool. The box temperature is digitally displayed and can be varied continuously over the ranges 0-60 " C or 80-140 "C. For the lower temperature range, the isolated box is cooled externally.

Long path UV absorption is used to measure the aldehyde concentration in the test gas, as shown schematically in Figure 1. The absorption cell is a Pyrex tube of 248 cm length, 2.0 cm

0003-2700/83/0355-1440$01.50/0 0 1983 American Chemical Soclety