Thermal Desorption Chemical Ionization Mass Spectrometer

Overview

gif animation of ion trap in action

Thermal Desorption Chemical Ionization Mass Spectrometry (TDCIMS) is an instrument that is capable of measuring the chemical composition of particles as small as 4 nm. It accomplishes this with a sensitivity that makes it possible to measure the molecular composition of nanoparticles at ambient concentrations in the atmosphere. The instrument builds on our experience in developing highly sensitive selected ion chemical ionization mass spectrometers for the detection of atmospheric gas-phase compounds at concentrations below 105 molecules/cm3 [Mauldin et al., 1999]. In our experimental setup, described in detail below, aerosols are charged and then collected by electrostatic deposition on a metal filament. Then the filament is slid into a chemical ionization region where it is resistively heated to evaporate the aerosol. The desorbed molecules are ionized at atmospheric pressure by proton transfer with protonated water clusters or oxygen anions. Ions are then transferred to a triple quadrupole mass spectrometer for mass analysis.

The instrument, is made of an electrostatic precipitator for collecting charged particles, an evaporation-ionization chamber, and a triple quadrupole mass spectrometer equipped with a collision induced dissociation (CID) chamber. The CID chamber is used to strip clusters down to their core ion before they are mass, or tandem mass, analyzed. The tandem mass spectrometry capability is useful for analyzing particles containing mixtures of organic compounds. The system is equipped with a unipolar charger and a nanometer aerosol differential mobility analyzer (nano-DMA) [Chen et al., 1998] at the inlet to charge and select particles of a given size, and with a condensation particle counter at the outlet, which is used to quantify the collected aerosol mass.

Particle Collection

The electrostatic precipitator consists of a 6 cm inside diameter, 30 cm long coaxial collection tube, with a central ceramic rod holding a metal filament made of nichrome (90% Ni/10% Cr) (figure 2). A flow of air (1-35 sl/m) containing charged ultrafine particles is continuously drawn in through the outer annulus. The inner tube (3.5 cm OD) extends 25 cm into the collection tube, and is flushed with 3 sl/min of ultra-pure nitrogen that protects the collection filament from contamination by gaseous compounds present in the particle-laden air flowing in the outer annulus. The combined flow is then drawn radially out through a hole pattern downstream, insuring flow symmetry at the outlet. In the collection position, the metal filament protrudes 0.8 cm from the end of the inner tube. The exact position is set within 0.02 cm by a computer controlled linear actuator that also rapidly slides the filament from the collection position into the evaporation-ionization chamber. Figure 2 shows the precipitator in collection mode. Here, an electric field is maintained between the outermost stainless steel cylinder and the filament to move charged particles from the sample airflow, through the nitrogen sheath gas and onto the filament. The collection potential on the filament is thus usually set to ~4000 V, balancing high collection efficiency with the potential for corona emission around the filament at excessive voltages. Particles are collected for a time period that corresponds to the accumulation of a few picograms or more on the wire, usually varying from one to ten minutes. After collection, the wire is slid into the evaporation-ionization chamber for analysis. This process is described next.

Sample Desorption and Ionization

This step occurs within the evaporation-ionization chamber, shown above. The evaporation-ionization chamber is a cylindrical chamber made of 4 stainless steel rings separated by electrical insulators. The central ring is coated with a radioactive material (231Am) emitting a particles that ionize the buffer gas mixture to form nitrogen and oxygen ions that subsequently react to form H3O+, O2- and CO3- as the primary stable ions. These ions will then react with the compounds evaporated from the aerosol. The ion source assembly is encapsulated in a heating ribbon and heated to ~70°C to avoid condensation of the evaporated material back onto the walls. Three nitrogen flows are fed into the chamber. The first one (0.03 sl/min) flushes the ionization chamber pushing evaporated compounds slowly towards the vacuum system aperture. This flow is kept small, as is the volume of the evaporation-ionization region (2.5 ml), in order to minimize dilution of the evaporated material, and to increase its residence time in the ionization region. As the vacuum system pumps a total flow of 0.13 sl/min, 0.1 sl/min are added to this first flow in front of the ionization region. The third flow (0.6 sl/min) is directed towards the collection region, isolating the ion source from the electrostatic precipitator and the sample air.

The collection filament can be resistively heated by pulsing a known current through it. Filament temperature vs. current was measured in a separate test by spot welding a thermocouple to the tip. Controlling the current leads to a known temperature with an accuracy of ±5°C. When in evaporation mode, the filament is located in the center of the ionization chamber. The filament and the rings that comprise the chamber are electrically biased to force ions to travel axially from the ion source to the vacuum entrance aperture. These and all mass spectrometer lenses are computer controlled, allowing automated switching from positive to negative ion measurement. In the example shown here, positive ion mode is used to detect molecules M and M´ produced by the evaporation of the collected aerosol, which readily reacts with H3O+ water clusters to create MH+ and MH´+. We can use this method to detect the following compounds in aerosols:

  • sulfate
  • nitrate
  • organic acids
  • oxygenated and unsaturated organics

Calibration

The figures above shows typical ion peaks for ammonium sulfate aerosol. In the positive ion mode, a clear signal can be seen for NH4+ (18 amu), when the filament is heated in the ionization chamber (upper two plots above). The peak of evaporated ammonium persists for less than 5 s, and reaches a maximum of 1990 counts per second above a 320 counts per second background corresponding to a signal to noise ratio of 6.2 (calculated as the peak height divided by the standard deviation of the background). The collected aerosol mass for the ammonium peak is 3.2 pg, corresponding to 14 nm particles collected at a concentration of 2750 cm-3 for 60 s. Tests were also performed in negative ion mode to establish instrument response for sulfate in ammonium sulfate aerosols. A peak is observed for bisulfate (the stable form of sulfate in the gas phase) when the filament is heated in the ionization chamber (lower two plots above). The peak for bisulfate shows the result for 10 nm particles being collected at a concentration of 4300 cm-3 for 120 s, resulting a collected aerosol mass of 1.8 pg. In the case of bisulfate ion, the bottom figure shows two peaks arising from sample desorption and ionization. The presence of two peaks suggests two separate mechanisms for the formation of bisulfate from ammonium sulfate aerosol. The signal to noise ratio for the bisulfate trace in the bottom figure is 60.

A series of tests were made to compare ion peak integrated area to the collected aerosol mass. For these experiments, ammonium sulfate aerosols were generated and size-classified with the nano-DMA into 14 and 10 nm aerosols for measurements of ammonium and sulfate, respectively. Collection times were varied from 15 s to 5 min to vary the collected aerosol mass. Plots of integrated ion peak area versus collected aerosol mass are shown in figure 3. In the plot, uncertainties associated with generating ammonium sulfate particles of known diameter were primarily responsible for the error in the total aerosol mass. The uncertainty in the measurement of the integrated peak area arises from the manual integration scheme that we employed for this study. The integrated peak areas shown in figure 3 are the net areas obtained by subtracting the background peak areas from that of the collected aerosols. The data thus represent the chemical compounds present only in the aerosol phase. For both ammonium and sulfate, the trends are linear. In the case of bisulfate (figure 3b), a slight departure from linearity can be seen for collected aerosol mass below 1 pg. Although the cause of this departure from linearity is not known, since it occurs near the detection limit we can state a effective detection limit (1 pg) that corresponds to the lowest mass for which fully linear calibration is observed.

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ACOM | Atmospheric Chemistry Observations & Modeling