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Anal. Chem. 1985, 57, 1880-1883
Real-Time Measurement of Sodium Chloride in Individual Aerosol Particles by Mass Spectrometry M. P. Sinha* J e t Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109
S. K. Friedlander Department of Chemical Engineering, University of California, Los Angeles, California 90024
The method of particle analysis by mass spectrometry has been applied to the quantitative measurement of sodium chloride in individual particles on a real-time basis. Particks of known masses are individually introduced, in the form of a beam, into a miniature Knudsen cell oven (1600 K). The oven is fabricated from rhenium metal sheet (0.018 mm thick) and Is situated in the ion source of a quadrupole mass spectrometer. A particle once lnslde the oven Is trapped and completely volatilized; this overcomes the problem of partial volatilization due to particles bouncing from the fliament surface. Individual particles are thermally volatilized and ionized inside the rhenium oven and produce discrete sodium ion pulses whose intensities are measured with the quadrupole mass spectrometer. An ion pulse width of several miiiisecpnds (4-12 ms) is found for particles in the mass range 1.3 X IO-‘’ to 5.4 X IO-’’ g. The sodium ion intensity is found to be proportional to the (particle The intensity distribution for monodisperse aerosol particles possesses a geometric standard deviation of 1.09 showing that the method can be used for the determination of the mass distribution function with good resolution in a polydisperse aerosol.
The chemical analysis of aerosol particles is generally made on a collection of particles. The sample is collected on a filter paper or on an impactor stage and analyzed by various physical and chemical methods (1,2). These methods usually determine the elemental composition of the accumulated material. In some analytical methods such as chromatography and mass spectrometry ( 3 , 4 ) ,the molecdar composition of the aerosol material is also obtained. Analysis of bulk samples provides information on the chemical composition averaged over the deposited particulate material and over the collection time. A significant amount of time elapses between the sample collection, its preparation, and the analytical procedure. In some cases, artifacts (5)occur in the sampling and analysis processes. In recent years, the composition of individual particles has attracted attention for aerosol analysis. Single particles contain much information about the health and environmental effects of aerosols. For example, some particles may be rich in lead, polynuclear aromatic hydrocarbons, and other toxic compounds depending on their origin and history. A related example is the detection of biological particles in air. Chemical analysis of single particles will also provide a more conclusive identification of their emission sources. High spatial resolution microprobe methods (6) such as electron microscopy with energy-dispersive X-ray analysis, secondary ion mass spectrometry, and laser microprobe mass spectrometry (7) have been used. In all these methods, the particles must be collected on a suitable substrate before analysis. We have recently developed in our laboratory a method for the contin0003-2700/85/0357-1880$01.50/0
uous, real-time analysis of individual aerosol particles. In this method, known as particle analysis by mass spectrometry (PAMS), the aerosol particles are introduced with high efficiency directly into the ion source of a mass spectrometer (MS) and their mass spectra are obtained on a single particle basis (8-10) * The direct admission of aerosols into the ion source was first reported by Davis (11,12). Myers and Fite (13) also used this technique for aerosol analysis. However, these investigators did not make a systematic study of the efficiency of the transmission of the particles into the ion source or of the relationship between particle mass and ion intensity. Recently, Stoffels and Lagergren (14)have made studies of a direct-inlet mass spectrometer (DIMS) for real-time analysis of aerosol particles. A rhenium filament was used for both the vaporization and ionization of particles. The ion yield was found to be proportional to the square of the particle diameter (0.8-1.7 pm diameter). A decrease of several orders of magnitude in the ion yield was found for larger particles. They attribute this to particle-bounce from the flat filament surface. In our earlier studies on PAMS for the mass spectrometric analysis of aerosol particles, we used a heated rhenium filament for the volatilization of the particle and the resulting plume of vapor was ionized by electron impaction. Electron-impact ionization was preferred for its general applicability. This method of ionization has also been employed by Allen and Gould (15).Surface ionization although efficient is limited to chemical species of low ionization potentials (C7.0 eV). The rhenium filament was shaped into a V-groove in the PAMS technique to facilitiate complete volatilization of particles. Results on various aerosol particles composed of ammonium sulfate, adipic acid, glutaric acid, and dioctyl phthalate (0.8-2.0 pm diameter) showed a linear dependence (8) of MS signal with particle volume indicating a complete vaporization of the particles in the V-type filament. Since this observation is based on the measurements of particles in a rather narrow size range and also on particles composed of volatile chemical species, the results need to be extended to other particle sizes and to aerosols composed of different chemical species, The extent of particle volatilization may depend on several parameters such as melting and boiling points, mechanical properties, and particle size and shape (16,
17). This paper describes the results of experiments with sodium chloride aerosol particles which have a much higher boiling point than those we studied earlier and have a crystalline structure. Sodium chloride aerosols are frequently generated by nebulization for the delivery of therapeutic or diagnostic agents to human or animal subjects (18,19).These aerosols are generated from saline solutions in which small concentrations of the active agents are also present. The mass distribution of the active compounds in particles can be calculated from the sodium chloride content of the particles and from the knowledge of their concentration in the solution 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985 AEROSOL GENERATOR/
PARTICLE
*
QUADRUPOLE MASS SPECTROMETER
INTEGRATOR/ PULSE SHAPER I
KNUDSEN CELL
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through the capillary is about 50 atm cm3 mi&. The particles which are much heavier than the carrier gas molecules remain preferentiallyconcentrated along the axis and are thus efficiently sampled through the skimmer. A detailed study of the physical characteristics of the particle beam has been made by Estes et al. (20) and Sinha et al. (8, 9). The beam generator has a transmission efficiency of about 80% in the size range of 0.3-5.0 pm diameter Particles. The other important property of a particle beam is its divergence. Estes et al. have also measured the beam divergence as a function of particle size. The transmission efficiency of the beam generator and the beam divergence are particularly important when the beam is generated from a polydisperse aerosol sample and only a part of the cross section of the beam is analyzed. The particle beams are highly directed, forward peaked, and provide a well-controlled method for the introduction of aerosol particles directly into the ion source of a mass spectrometer. The particle beam enters the mass spectrometer chamber through the skimmer. The MS chamber is differentially pumped and an operational pressure of lo6 torr is maintained with an oil diffusion pump. Liquid-nitrogen-cooled traps are provided at the top of the pumps to stop back streaming of pump oil. The particles impinge on a hot rhenium filament situated in the ion source. The linear relationship between the MS signal and particle mass did not hold when the original V-groove filament was used for the volatilization and ionization of sodium chloride particles. This probably resulted from the refractory nature of sodium chloride particles compared to the organic particles used in earlier PAMS studies. Sodium chloride particles may be bouncing off the filament surface before complete volatilization. For quantitative measurement of chemical species, the complete volatilization of particles is important. This is particularly true for multicomponent, and heterogeneous particles. To overcome the particle bounce problem, the V-filament was replaced by a miniature Knudsen cell oven (-3 mm diameter, 5 mm in length). The lower part of the oven was pressed to form an angle of -30' (Figure 1). It was fabricated of zone-refinedrhenium sheet (0.018 mm thick) and was resistively heated to 1600 K during operation. An opening of -1.5 mm was made in the oven for the introduction of the particle beam. The oven was situated between the repeller and the extraction plates of the ion optics. The new ion source was mounted on the quadrupole mass analyzer (Uthe Technology, Inc., Model lOOC). The purpose of filament design was to trap the particles inside the oven for completevolatilization. The vapor molecules are themally dissociated and a fraction of the resulting sodium atoms is ionized on the surface before effusing out of the oven. The oven was situated at a distance of 6 cm downstream from the nozzle. At this distance, the beam cross section diameter was more than 1.5 mm. The particles hitting the outer surface of the oven may bounce off it with the attendant incomplete volatilization. In order to eliminate this problem, a collimator (1mm opening), made of stainless steel, was placed just above the oven entrance. Under these conditions,Na+ ions are produced inside the oven only. The collimator, however, reduces the number of particles analyzed in a given time. It also introduces a bias in favor of the particle with smaller divergence for the analysis of a polydisperse aerosol sample. Particles hitting the collimator are reflected without any significant volatilization or ionization and are thus not analyzed. The discrimination can be accounted for by knowing the beam divergence for the various size particles. Each particle inside the oven produces a burst of Na+ ions which are monitored by the quadrupole mass analyzer. The MS is manually set at 23 u and the intensity is measured by integrating the area under the ion pulse from the electron multiplier of the MS. The duration of integration is adjusted to cover the pulse width. Immediately following the signal integration, the background is integrated for the same length of time and subtracted from the accumulated signal. The peak voltage on the integrator is then suitably shaped and fed into a pulse height analyzer (Canberra Industries, Inc., Model 8100) for the measurement of the intensity distributions from a large number of particles N
I
Figure 1. Schematic diagram of the partlcle analyzer.
used for aerosol generation. For an efficient administration of these aerosols, their mass distribution should be optimized with respect to their retention. Sodium chloride is also an important constituent of marine aerosols, and the PAMS system should be adaptable t o field measurements. A calibration curve for the MS signal as a function of the mass of the sodium chloride particles bas prepared using monodisperse aerosols as described in this paper. The salt particles in these studies were both volatilized and ionized thermally because of the high ionization efficiency for this technique for alkali metal compounds.
EXPERIMENTAL SECTION Particle analysis by mass spectrometry is a combinationof the particle beam technique with mass spectrometry. Figure 1shows a schematicof the PAMS system. Ita details have been described previously (8, 9). Briefly, a particle beam generator is used to introduce particles directly into the ion source. It consists of a capillary nozzle (100 pm diameter, 5 mm length) followed by a set of two skimmers (360 pm and 500 pm orifices). The regions between the nozzle and the first skimmer and between the two skimmers are differentially pumped. The second skimmer was removed in the present work in order to minimize the distance between the nozzle and the ion source of the quadrupole mass spectrometer. Monodisperse aerosols of different sizes were produced from aqueous solutions of sodium chloride using a vibrating-orificegenerator. This is well suited to the generation of monodisperse aerosol particles of known masses. In this method a jet of liquid is produced by forcing the solution through an orifice in a plate. The plate is mounted on a piezoelectric material which serves as a transducer for vibrating the plate at a known frequency. The liquid jet breaks into droplets of uniform diameter which are introduced into a stream of nitrogen gas. The aerosol is then neutralized by passing it through a neutralizer containing 8SKr and dried by passing through a thermal drier before its introduction into the beam tube. These droplets produce the solid particles after drying. The particle mass, m is calculated from the concentration of the salt solution and the operating conditions of the generator using the equation m = (QC/f) where Q is the solution feed rate, C is the salt concentration (mass/volume),and f is the vibration frequency. The quantities Q and f can be easily measured. Monodispere particles of different masses were generated from solutions of various concentrations,and different size orifices were used to control Q; the vibration frequency, f , was optimized for each value of Q. A beam of particles is produced by the expansion of the aerosol through the capillary nozzle into a vacuum. The volumetric flow
(
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1000).
RESULTS AND DISCUSSION Characteristics of the M S Signal. The ion pulse width, 7,was found to vary from 4 to 12 ms for the different mass
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Fwre 2. Pulse height distributions of (a)optical pulses scattered by monodisperse sodium chlorae aerosol particles of calculated d!ameter, 1.8 pm. Pulses were mmitaed on an optical paale counter. The ordlnate represents the number of particles and the abscism represents the channel numbr of the PHA proportional to the peak pulse intensity. (b) Mass spectral sodium ion signal intensities for the ths
Rgue 3. (a)Bimodal optical pulse height distribution. The two peaks correspond to the monomer (0.83p m diameter) and the dimer (1.04 pm diameter) particles produced in the vibraling-arificeaerosol generator. (b) Mass spectral intensity distribution of Na' ions of the same aerosol particles.
above input aerosol.
particles. The increased width of several milliseconds in the present studies, as contrasted with a typical pulse width of 200 ps when the volatilization was made with a V-type rehenium filament, may be attributed to the longer residence of the particle inside the oven and to the slow effusion rate of the ion from the oven. This puts a limitation on the number of particles that can he measured per unit time in order to minimize the coincidence error. A coincidence error in the m a a ~measurement occurs when the time interval hetween two successive particles entering the oven is less than 7. The counts per second a t 5% coincidence should he between 12 and 4 over the range of particle mass studied (21). Figure 2 shows the results of the intensity measurement of sodium ions. The measurement was made with a monodisperse aerosol generated from a vibrating orifice aerosol generator. The calculated diameter of the particles is 1.8 pm based on bulk density of the salt. Figure 2a is the optical size distribution of the input aerosol measured with an optical particle counter (Climet Corp., Model 226) and the pulse height analyzer (PHA), and Figure 2h shows the MS intensity distribution of the sodium ion pulses from the above aerml. The ion intensity distribution is sufficiently narrow for the monodisperse aerosol and shows the reproducibility of the volatilization and ionization process. The peak of the distribution measures the average intensity of the particle mass peak. The signal intensity (proportional to the channel number of PHA) distribution of Figure 2b when plotted against the cumulative percent of particle counts on a probability-graph paper yields a straight line. A linear plot shows a lognormal distribution of the MS signal. The geometric standard deviation, ug,of the distribution is 1.09.indicating that the mass spectrometric measurements can he used for the mass distribution of polydisperse aerosols with good resolution. For example, Figure 3 shows the resolution for an aerosol composed of particles of two sizes. Figure 3a is the optical size distribution of aerosol generated from a vihrating-orifice aerosol generator. The first peak corresponds to the calculated diameter of 0.83 pm. The second peak corre-
0. I 0.1
I 1 .o
, , , I , , , ,I
, , , I ,,
10
1w
PARTICLE M*sS (x 10-12g)
Flgure 4.
Variation of Na+ ion intensity with particle mas.
s p n d s to the dimer particle formed by the coagulation of two monomer particles. The mass spectral measurements for this aerosol sample are shown in Figure 3b. The resolution of the two sizes is clearly seen. Mass Calibration. For the calibration of the MS signal with respect to mass, monodisperse particles of known masses were used. Figure 4 is the log-log plot of the ion intensity, I, as a function of particle mass. A straight line with a slope of 0.86 is obtained showing that I m0.%in the mass range 1.3 X to 5.4 X 1 0 . ' g. We believe that the particles are completely volatilized in the oven. The departure of the exponent from unity may be due to the complex effusion profile of the sodium ion vapor from the oven and through the collimator situated above the oven. The oven was biased a t a voltage intermediate between those of the repeller and the extractor. The power law relationship between the maas and the MS signal makes the recalihration of the PAMS (r
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9. AUGUST 1985
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Thermal volatilization followed hy electron impact ionization haa been previously used in the PAMS measurements. The present PAMS system with the quadrupole mass spectrometer is limited to the measurement of a single mass peak from an individual particle. This results from the slow mass scan rate and from the brevity of the ion pulse width. A nonscanningmaas spectrograph with a sensitive ion detector is needed for the complete mass spectral measurements from a single particle composed of more than one chemical species. Such a mass spectrograph (Matttauch-Herzog type), with an ultrasensitive detector (ZZ),known as electrooptical ion detector, has been constructed. Work on measurement of particulate material with this second generation PAMS instrument is in progress in our laboratory.
Flgure 5. Electron photomicrograph of soclum chloride particles. collected on a Nucleopore filter of 0.6 r m pore diameter.
system convenient for any minor changes in the MS perf o m m chacteriatica. This can be aecomphhed by making a single measurement with particles of known maas within the masa range over which the power law holds. The calibration curve is being applied to the measurement of particle mass distribution for a polydisperse sodium chloride aerosol. Mass determination can be used for the particle size measurements provided information about their shape and density is known. For liquid aerosols, the droplets are spherical and the hulk liquid density can he used. However, for aerosols composed of a crystalline solid, the density is uncertain and the shape is not well determined. Only the mass equivalent spherical diameter can he calculated for such solids for an assumed value of the density. Figure 5 shows an electron photomicrograph of sodium chloride particles. The particle appears to be composed of cubic crystals and there is a strong indication of void spaces within the particles. Consequently, the normal hulk density of sodium chloride cannot he used for accurate particle size calculations from the mass measurements. Similarly, it is difficult to infer the mass distribution from optical size measurements using a counter calibrated with polystyrene latex particles. The oven design for the volatilizing unit overcame the particlehounce problem experienced by other investigators (14). The quantitative nature of the results, together with the fact that masa spectrometric measurements posseas small standard deviation (Figure 4) for monodisperse aerosol particles, makes PAMS a suitable method for the mass determination of chemical species in individual particles. For particles composed of chemical species that cannot he efficiently surface ionized, volatilization of the particle inside the oven can be combined with electron-impact ionization.
ACKNOWLEDGMENT The authors wish to express their thanks to James Hill for his contribution in the construction of the experimental apparatus and to James Stoffels for his valuable discussions. LITERATURE CITED (1) FOX. D. L.: JeMles. H. E. Anal. Chem. 1983. 55. 233R. (2) Fox. D. L.: JeWes, H. E. Anal. Wem. 1981. 53,1R and reterenc~6
...
herd" 0.~....
(3) Knights, R. L. A&. EnvKon. Scl. Techno. 1980. 9 . 237. (4) Macho, E. S.: Kumar. K. S.: Hoffman. M. K. NBS Spec. mi.1979, No. 519, 101. (5) Clemem, R. E.: Karasek. F. W. Int. J. EnvuWr. Anal. Own. 1979. 7 , 109, (8) Heinrich. K. F. J. i n "Environmental pollutants": Tcdbara, T. Y.. Coleman. J. R., Dahneks. B. E., Fe!dman, I., E&.: Plenum Press: New Ywk. 1978: p 241. (7) Kaufmann. R.: Wlser. P. NBS Spec. MI. 1880, No. 533, 199. (8) Sinha, M. P.: GiHin. C. E.: Norris. D. D.: Estes. T: V i b r . v.: Friedlan. der. S. K. J . CoiioM Interface Sci. 1992, 87, 140. (9) Sinha. M. P.: Mae. R. M.: Vilker, V. L.: Friedlander. S. K. Int. J . Mass Spectom. Ion PIoCesJes 1984, 57, 125. (10) Sinha. M. P.; Plae. R. M.: Friedlander. S. K.; Vilkern. V. L. J . A@. Envirm. h4bmbiol. 1985. 49. (No. 8). (11) Davis. W. D. J . Vac. Sci. Technol. 197%. 10. 278. (12) Davis. W. D. Envlron. Sci. Technol. 1977. 11. 587. (13) Myers. R. L.; Fits W. L. Environ. Sci. Techno. 1975. 9 . 334. (14) Slotfels, J. J.; Lagergren. C. R. Int. J . M a s Spemom. Ion phys. 1981. 40, 243. (15) Allen, J.: h ! d . R. K. Rev. Sei. I n s m . 1981, 52. 804. (18) Dahneke. B. J . C M Intertsce Scl. 1971. 37, 342. (17) Dahneke. B. J . CoUM Inhnface Scl. 1972. 40, 1. (18) Hayes. M. H.: Taplln. G. V.; Chopra. S. K.: Knox. D. E.: Elam. D. Radioicgy 1979, 131. 256. (19) Muir, D. J . &I. FhysioI. i987. 23.210. (20) Esles, T. J.: Vllker. V. L.: Friedlander. S. K. J . C o w Interface sci. 1983. 93. M~ ~~~~, (21) Thornson, 0 . H.; Pipani. J. F. J . phys. E 1971. 4 , 359. (22) Boenger. H.: Gmin. c. E.: Nmis. D. D. in 'wunichannei images Detectors": Talml, Y., Ed.: American Chemical Society: Washington. D C 1979 Symp. Ser. NO. 102. p 291.
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for review March 8,1985. Accepted April 25,1985. This work was performed a t the Jet Propulsion Laboratory, California Institute of Technology, and was supported by the National Science Foundation under Grant CPE 83-18686 through National Aeronautics and Space Administration.
