STANDARD OPERATING PROCEDURE (SOP) FOR THE FIELD OPERATION OF THE AERODYNE AEROSOL MASS SPECTROMETER (AMS)

Transcription

1 1 of 9 STANDARD OPERATING PROCEDURE (SOP) FOR THE FIELD OPERATION OF THE AERODYNE AEROSOL MASS SPECTROMETER (AMS) Prepared by : Frank Drewnick, John Jayne Reviewed by: Volker A. Mohnen (PMTACS-NY QA Officer) Approved by: Kenneth L. Demerjian (PMTACS-NY Project Director) Atmospheric Sciences Research Center, State University of New York at Albany; Aerodyne Research, Inc.

2 2 of 9 TABLE OF CONTENTS 1. SCOPE AND APPLICATION 2. SUMMARY OF METHOD 3. HEALTH & SAFETY WARNINGS 4. CAUTIONS 5. INTERFERENCES 6. APPARATUS 7. INSTRUMENT CALIBRATION 8. SAMPLE COLLECTION 9. DATA MANAGEMENT 10. TROUBLESHOOTING 11. QUALITY CONTROL 12. REFERENCES 1. SCOPE AND APPLICATION 1.1 The Aerodyne Aerosol Mass Spectrometer (AMS) has been recently developed to provide real-time size and size resolved composition information for ambient aerosol. The development of this instrument directly responds to the need for such instrumentation to help better characterize and understand the PM 2.5 air quality issues. The AMS also has many uses as a laboratory tool to study aerosol chemical and physical properties. When used for ambient aerosol measurement the instrument provides real-time size resolved mass spectrometric analysis of volatile and semivolatile components, i.e., water, ammonium nitrate, ammonium sulfate or organic substances. The instrument cannot be used for the analysis of involatile substances as elemental carbon, carbonates or silicates. The AMS falls into the category of new technology instrumentation and as such standard operating procedures have not yet been developed. There are, however, calibration procedures which must be followed to ensure quantitative operation. These procedures are described below

3 3 of 9 2. SUMMARY OF METHOD 2.1 Ambient air is sampled through an aerodynamic particle focusing lens into a differentially pumped vacuum chamber. Particles in the range of 50 nm to 2000 nm (50 % cut off) are focused by this lens to a fine particle beam (~1 mm diameter). On the small particle size end collection efficiency at the detector is limited by reduced focusing and on the large size end it is limited by impaction at the lens inlet. Particle transmission within this range is quantitative. The particle beam passes through a skimmer and an aperture and impacts a resistively heated surface, where volatile and semivolatile substances are flash vaporized. 2.2 The vapor molecules are electron impact ionized, extracted and focused into a quadrupole mass spectrometer (QMS) where they are mass filtered. The ions that pass through the QMS are detected by an electron multiplier. Efficient separation of the vaporization and ionization processes allow the ion signals to be quantified. The signals are recorded, processed and stored by the data acquisition computer. The ion signals are proportional to the mass of volatile or semi-volatile material in /on the particles. The exact relationship must be established through calibration. The data system provides a real-time display of the acquired data. 3. HEALTH AND SAFETY WARNINGS 3.1 For normal operation, general good laboratory practices are sufficient. 3.2 When disconnecting cables from the instrument for maintenance or repairs make sure that no high voltages are applied (esp.multiplier and ion gauge). 4. CAUTIONS Precautions with this instrument primarily deal with standard vacuum and mass spectrometric techniques. 4.1 For instrument shut down for maintenance or repairs, the turbo molecular pumps should be allowed to spin-down before venting the instrument for at least 10 minutes to avoid damage by heavy gas loads. This helps prolong the life of the turbo pumps.

4 4 of To reduce danger of oxidation of the electron emitting filaments and the particle vaporization heater, both should be turned off at least 15 minutes before venting the system. 5. INTERFERENCES 5.1 Since the detection process is based on electron impact ionization, fragmentation of the molecular species from this ionization process can result in multiple species being detected at the same mass-to-charge quadrupole setting. This is particularly true for organic molecules. Monitoring ion fragments at multiple quadrupole settings can help correctly identify the parent molecules. In addition, since the ionization/detection process is a commonly used method (i.e., GC-MS) standard mass spectral reference libraries can be used to help identify mass spectra. 5.2 Non-isokinetic sampling of the ambient air will cause false concentration values. Subisokinetic sampling will over-represent large particles, super-isokinetic sampling will under-represent large particles. To avoid these interferences in correct determination of the mass concentration use an isokinetic inlet for the extraction of the sample flow. 5.3 Since the aerodynamic inlet of the AMS has limited transmission for large particles above 500 nm and limited focusing properties for small particles below 60 nm the mass concentration, calculated from these particle size ranges will always be underestimated. A correction of these interferences due to limited transmission of the inlet is only possible when the size distribution of the particles of a certain chemical composition is known. 5.4 Different species show different collection, evaporation and ionization efficiencies which will lead to incorrect mass concentrations. To take these different measuring efficiencies into account, establish appropriate correction factors for the different species. This correction of the raw data may be done later.

5 5 of 9 6. APPARATUS 6.1 The Aerodyne Aerosol Mass Spectrometer is a custom built vacuum chamber that couples an aerodynamic particle inlet system with a particle time-of-flight tube and a detection chamber. The inlet system provides efficient and quantitative delivery of particles to the detection chamber. Particle aerodynamic diameter is determined via a time-of-flight measurement following quadrupole mass spectrometric detection. Particle detection and aerosol mass measurement relies on efficient flash vaporization and hence quantitative measurement is limited to volatile and semi-volatile aerosol species. 6.2 The AMS vacuum chamber/quadrupole system occupies a volume of approximately 2 x 2 x 3 feet and is typically mounted on a bechtop. The data acquisition computer, the quadrupole (and other) control electronics, are housed in a standard half rack ~32 tall. The complete system, vacuum chamber and electronics weights ~120 kg and uses ~700 Watts of power. 6.3 The Aerosol Mass Spectrometer consists of a custom built aluminum chamber which is pumped with four Varian V70LP and one Varian V250 turbo pumps. The pressure is measured with two Varian ConvecTorr gauges and a Varian MBA-100 ion gauge. The aerodynamic inlet, chopper and resistively heated vaporizer are custom made. Ions are separated with a Balzers QMG 422 quadrupole mass spectrometer and detected with an ETP electron multiplier AF140. The multiplier signal is amplified by an Advanced Research Instruments PMT-5 preamplifier. 6.4 The mass spec data are recorded by National Instruments data acquisition boards PCI-6110E and PCI-6024E, via two National Instruments data acquisition interface boards BNC Data are processed, displayed and stored by a fast PC with sufficient storage capacity. A CD writer is used for long time data storage. The turbo pumps, chopper and vaporizer are controlled by custom made electronics. The pressure gauges are controlled by a Varian sentorr gauge controller. The mass spectrometer is controlled by a Balzers QMS 422 QMS control box.

6 6 of 9 7. INSTRUMENT CALIBRATION 7.1 Particle Aerodynamic Diameter. The AMS determines particle aerodynamic diameter by a particle time-of-flight or velocity measurement. As a result of the gas expansion into vacuum, particles of different diameters are accelerated to different terminal velocities, larger heavier particles are accelerated to slower velocities and smaller lighter particles reach faster velocities. A mechanical beam chopper modulates the flight of the particles to the detector. Particle detection is synchronized to the phase of the chopper providing a measure of velocity. Typically the AMS size measurement is calibrated using NIST traceable standard polystyrene latex spheres (PSLs). These standard size particles can be introduced to the sampling inlet by atomizing an aqueous solution of PSLs (the vaporized PSL is detected by the quadrupole at a mass-to-charge ratio 104 amu, the styrene molecule). Alternatively, the AMS size (and particle mass) calibration can also be performed using particles of different sizes and compositions generated with a differential mobility particle analyzer/sizer (DMA). For time-of-flight calibration sample and analyze monodisperse aerosol in the timeof-flight mode. Localize the maximum of the flight-time distribution and note the corresponding flight time together with the nominal size of the particles. Conduct this procedure for at least 10 different particle sizes. Since size selection with a DMA leads to particles with a given mobility size, the particle sizes have to be converted to aerodynamic sizes by multiplication of the mobility diameters with particle density and shape factor. Fit the expression vg v = * b 1+ ( D / D ) aero to the datapoints of v (= l/t TOF ) and D aero. Type the fitted coefficients v g, D * and b in the appropriate parameter tab to calibrate the particle size - flight time conversion. Since the flight time particle diameter correlation only depends on the geometry of the aerodynamic lens and the length of the flight path this calibration needs to be done only after changes in these parts. 7.2 Mass concentration calibration: The DMA allows the mass spectrometric ion signals to be most easily quantified since known compositions and sizes (ie, known aerosol mass) can be readily generated. The calibration of the mass measurement is performed by determining the number of ions detected of a particular species for single particles of a given size. The ability to detect individual particles makes this approach practical. In principle, the AMS can be calibrated with many different species, however, in practice current calibration procedures involve sampling mixtures of NH 4 NO 3, (NH 4 ) 2 SO 4 and one or two organic aerosols to generate response curves. These calibration procedures are typically performed once a day.

7 7 of An important aspect of the mass calibration procedure involves calibration of the electron multiplier since the gain of this device gradually decreases with time. The data collection software has built in functions to automatically verify and correct for the gain of the electron multiplier. In addition, the data collection software routinely monitors the absolute ion intensity of N 2 and O 2 (from the air) which should remain constant since the gas flow into the instrument is constant (see below). Any changes in these signals can be used to correct for systematic decreases in the multiplier gain. 7.3 Inlet flow calibration: The sample flow rate into the instrument is regulated by a critical aperture on 100 µm diameter. The flow rate is monitored and recorded constantly by measuring the differential pressure across a laminar flow element. The laminar flow meter can be calibrated using a Gilibrator or a soap film bubble meter. The instrument samples ~0.1 LPM. 8. SAMPLE COLLECTION 8.1 Collect aerosol samples from ambient air via a 10 lpm, 2,5 µm cut cyclone of type URG EN. Control the airflow through the cyclone with a critical. Use a diaphragm pump for sampling flow. Chose the tubing diameter to minimize loss due to diffusion, settling and impaction. 8.2 Sample the AMS inlet flow (90 cm 3 /min) from the cyclone flow with an isokinetic sampling probe (custom made). 8.3 Use the AMS in the alternating mode, alternating between ToF mode and MS mode. Adequate sampling times in each mode before switching to the other mode are between 5 seconds and 1 minute. 8.4 In the ToF mode sample masses 32 (air beam, for signal normalization), 30, 46 (nitrate), 48 and 64 amu (sulfate). Additional masses may be sampled. Before selecting additional masses, check the mass spectrum for their abundance. 8.5 In the MS mode scan mass range from 1 to 300 amu. 8.6 If necessary correct mass calibration to optimal signal intensity. Check mass spectrometer tuning. Check multiplier gain. Set multiplier gain to default value (2*10 6 ). Check single ion threshold in ToF mode. For detailed description of these procedures, see AMS manual.

8 8 of Auto-save the ToF- and MS-data after adequate averaging time. Averaging times should be between 5 seconds and 15 minutes, depending on aerosol concentration and question of research. Auto-save the main logfile at the same time intervals. Additional logfiles may be saved if deserved. 8.8 Note all deviations from the normal performance of the instrument in the bound AMS record book. Note all extraordinary external conditions that could lead to changes in the measured data in the AMS record book. 8.9 While aerosol collection continuously check inlet flow rate, turbo pump performance and temperature, chopper performance (including chopper servo), filament emission current and vacuum chamber pressure. 9. DATA MANAGEMENT The AMS is operated in two modes and automatically alternates between the two at preset intervals. In one mode, the quadrupole is scanned over its full range (0-300 amu) and full mass spectrometric information is measured but without information on aerosol size (the particle beam chopper is moved out of the beam). In the other mode the quadrupole is step scanned over pre-selected masses and the beam chopper modulated the flight of the particles. At each quadrupole setting, particle time-offlight is measured and hence size distributions are recorded for each setting. The data acquisition system can be programmed to log data at preset interval. The instrument is designed to sample around-the-clock. Approximately 50 MB of data are generated in a 24 hr period with 10 min sampling. Data analysis programs are currently being developed to analyze, process and interpret the data. 10. TROUBLESHOOTING This instrument is designed to run unattended with daily or twice daily performance checks by a trained operator. Since this is a research-grade instrument problems that might arise will require specialized diagnostics from the trained operator. There are no formal guidelines for trouble shooting. Some routine checks of instrument performance include the following: inlet flow rate, vacuum pressure, electron multiplier gain, ionization efficiency and mass spectrum calibration.

9 9 of QUALITY CONTROL 11.1 To consider the decay of the multiplier gain the actual gain of the multiplier has to be verified with a gain check as described in the AMS manual. Make this gain check at least once a day. Note the change in gain in the AMS record book Check the tuning of the mass spectrometer and verify ionization efficiency. Note the changes in the AMS record book Check the tuning of the chopper position at least once a week. The procedure is described in detail in the AMS manual. Note changes in the AMS record book Make a copy of the raw data on an independent data storage device at least once a day. Make a note of the copied data in the AMS record book Document all irregularities of the measurements and the instrument performance in the AMS record book. Mark data, taken with low inlet flow (below 1 cm 3 /s) or under high vacuum pressure (above 5*10-7 torr). 12. REFERENCES J. T. Jayne et. al.: Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles; Aerosol Sci. Technol. 33 (2000) AMS Manual Balzers QMG 422 Operating Manual, Balzers document no. BG BE (9801)