Field ToF-AMS Operation

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Revision as of 07:31, 7 May 2009 by Jose (talk | contribs) (→‎Inlet and Dryer Set up)
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The purpose of this page is to serve as a repository of best practices on how to calibrate, operate, and document the calibrations/operation of a ToF-AMS during a field study. Much of it will also be relevant for lab studies.

Once the data have been acquired, please refer to the ToF-AMS Field Data Analysis Guide to guide the (very complex) data analysis phase.

This page was created by the Jimenez Group at CU-Boulder and Peter DeCarlo from PSI, with contributions from multiple other AMS Users.

Inlet and Dryer Set up

General References on Particle Transmission thru Inlets

For a guide on particle transmission on inlets, see the following references:

For calculation of losses thru inlets:

Other useful inlet-related info:

Dryer Set Up

It is highly recommended that air is dried before AMS analsysis in order to reduce uncertainties in the bounce-related collection efficiency (Eb) and possible losses in particle transmission (E_L) if the particles grow too large at times of high RH. For more information see the CE section of the Field Data Analysis Guide.

There are 2 basic types of dryers that can be used. You need 2 dryers if you use diffusion dryers (like the TSI one with silica gel) so that you can regenerate one while the other is being used. However they often have to be replaced daily (depending on ambient RH and size) and doing that automatically is very complicated. (Although possible, IFT Leipzig has built some industrial-quality systems to do this). An easier alternative is to use a nafion dryer. We recommend dryers with metal casing, as those with plastic casing were observed to lead to losses of charged particles by P. McMurry's group (Jose can provide a report on this upon request). For the small flows that are typically needed, the MD series dryers from Perma Pure work well. What you need to do is to plumb the aerosol flow through the "straight" path, and then put a critical orifice on one end of the "outside" path and a pump on the other end. You want to have some flow on the outside path, but at a much reduced pressure (~1/10 of 1 atm). What drives the drying is the absolute humidity, so you are dropping that by x10, and that will drive the gas-phase H2O on the aerosol flow through the nafion membrane and to the outside path. A typical Gast pump/compressor will do. Very important, be very careful with the fittings on the dryer, as if rotated strongly one can break the internal connections of the nafion tube, and then you have a large leak and end up sampling room air.

If is worth convincing yourself that you don't have a leak by putting a filter before the dryer and sampling with a CPC, and similarly (w/o the filter) convincing yourself that particle number losses thru the dryer are not important.

Tuning the Ion Source and ToF-MS

Note: these are some notes on tuning put together by P. DeCarlo and J.L. Jimenez, and circulated via email to Tofwerk and some core users. The responses that we got via email are inserted in the original list. There appear to be different schools of thought on how to tune, so these are given here only as guidelines.

  • ORDER OF TUNING
    • Tune ion source first for signal, then tune reflectron voltages for resolution, then tune Pulsers for detailed peak shape
  • ION SOURCE:
    • The tuning appears to depend on the emission current (maybe really filament current?) in a way that we don't understand. So it is important to tune at the emission current that will be used during measurements (typically 2 mA)
    • Don't change ion chamber from design values for that ToF by more than 10% (design values = 49V for cToF, 35V for vToF, and 8.5 for wToF, from Katrin's email on 3/1/2006)
    • In general we don't tune the ion chamber. Rather we tune the heater bias, which will typically be a few volts lower than the ion chamber value.
      • Response from Achim T: Tuning the ion chamber makes a lot of sense. It affects the ion signal a lot by a better extraction of the ions into the Pulser region. In the moment we have no evidence that changes in a certain range (not up to 80 V) will lead to a loss of small ions. The post acceleration affects this loss in a similar way like the ion chamber voltage. By increasing it the ions get a higher velocity perpendicular to the ion extraction. This has the opposite effect to increasing the ion chamber voltage.
    • The difference between the ion chamber and the filament should be 70 V (this is the electron energy)
      • Response from Katrin F: This is somewhat flexible, since depending on where you set your extraction lens voltage you may have a range of potentials in the ion source (due to field penetration). The problem is that what you see on the tof detector will be a folding of the Energy acceptance of the TOF (which is on maximum at the 49eV for the C etc) And the energy distribution of the ions transferred in the TOF extraction. Depending on your LV settings you will get ions of different energies into the TOF extraction. To be sure that you have preferentially 70eV ions one would need to compare to a reference spectrum, and tune the LV until you reproduce the reference spectrum. In principle you should be fine if you use the same voltages relative to the ion source as with the quad settings (what voltages did you use there to get the 70eV reference spectra?) for filament, ion chamber, and extraction lens.
    • The ion energy into the ToF orthogonal extractor is determined by the ion chamber. Changing the ion energy will change the m/z-dependence of the extraction efficiency. This should not be changed during a campaign.
    • Deflector flange needs to walk with the deflector voltage, small changes between the two are okay, and fractional volts can help a lot
    • Electron Pusher is not connected and is disabled in recent TPS software versions.
  • HIGH VOLTAGE:
    • Tune reflectron grid and backplane together. This is the best way to increase resolution.
      • Response from Katrin F: Just tuning RG is sufficient.
    • Lens: this was added to tune off V-mode ions in W-mode. It should be high (~2800 V) in W mode and low (~1000) in V-mode
      • Response from Achim T: Tuning the lens voltage helps a lot by reducing the vmode ions in W mode and by increasing the signal. The quoted numbers of 1000 V for Vmode and 2800 V for Wmade are typical but they vary a lot. There are instruments which have the best vmode performance with a lense voltage of 2300 V.
    • MCP and post-acceleration are NOT tuned, these are set for amplification depending on other considerations (saturation, signal-to-noise, etc.)
    • Hardmirror doesn't have much effect on signal or resolution, no need to tune much
      • Response from Katrin F: Yes, hardmirror should NOT be tuned
      • Response from Achim T: The hardmirror has an effect on the peak shape and signal height. But you may have to vary it by some hundred Volts. In a well tuned instrument it has really no effect.
  • PULSERS
    • The three should be tuned together
      • Response from Katrin F: I would prefer in the future that the pulser is left at the settings I ship it. You can get the same effects by just tuning the LV.
    • These should NOT be set higher than 800V
    • Tuning these 3 together is the best way to tune out leading and tailing edges
    • U+Low is not connected and doesn't do anything

Standard Field Calibration procedures

Data Acquisition and Instrument Control Software (DAQ): Make sure you are doing data acquisition with the latest version of Acquisition software whether a ToF or a Quad. (upgrade before field campaign, and check for any problems)

Saving Frequency: at least 5 min data on fixed time grid (on the hour).

m/z Range: at least scan m/z 0-300 (Q). One should go higher with the ToF instruments around m/z 10-500 is encouraged.

m/z's for Q-AMS PToF Mode: For particle TOF data the following masses should be used at a minimum: 16, 28, 30, 35, 43, 44, 46, 48, 55, 57, 60, 64

JMS Mode: For Q-AMS Jump mass spec (JMS) mode can be used, but it is not necessary for ground based monitoring unless mass concentrations are very low (remote clean sites). Note that the m/z's used in JMS mode are hardwired to be the same as those in PToF mode.

Vaporizer temperature: 600C is standard.

Dryer: If you have the ability to dry the aerosol inlet, do that. It simplifies the application of the componsition-dependent CE (Eb), see the Field Data Analysis Wiki for details. A nafion dryer from Perma-Pure (MD-110 with Steel casing, AVOID plastic casings) is recommended. The drying counterflow can be provided by ambient air whose pressure is reduced with a critical orifice, so no dry air supply is needed. Diffusion dryers filled with silica gel can also be used but require constant monitoring of the indicator and frequent changes which is more prone to leaks. Depending on the humidity and the sample flow rate they may be spent and no longer dry after only one day of sampling. If you cannot dry, at the very least measure of the RH at the AMS inlet is needed if the inlet line is not dried. This measurement is best done in the line to the pump for the AMS bypass flow, to avoid perturbing the flow going into the AMS which could lead to particle losses.

Calibration Frequency Description
Inline HEPA Filter At least every 3 days, best to do every day at the site.

Also do this every time the instrument has been vented/pumped down.

30 minute (if every 3 days) or 10 min (every day) HEPA filtered, particle free air from inlet (NOT air from the room which will have increased CO2).

Do not filter at the same time every day. See suggested schedule below.

HEPA should be put on inlet outside (if possible) and this will also allow a check for leaks in the inlet system, using a CPC or the AMS itself (although the CPC is much more sensitive to small leaks).

Note that there is an Inlet delay, so one should reinitialize (F5) AMS acquisition after the inlet residence time has elapsed so that the data files are not mixed filter/ambient data

IE calibration Every Week (minimum)

2x a week better, and anytime the instrument has been pumped down, or there has been a problem. Specifically it is CRITICAL to do at least one new IE calibration if a filament and/or MCP/electron multiplier are changed.

Standard Ammonium Nitrate IE calibration.
Baseline / SI calibration Before every IE calibration See Instructions for Bitwise below
Size Calibration Do at beginning and end of Campaign, and if there is a large instrument problem (e.g. loss of a pump etc.) Standard AMS size calibration

Note that size calibration changes if lens pressure changes. So if you change the orifice and the lens P changes more than 3%, you should do another size cal. (changes in P are amplified into larger changes in size)

Flow Calibration Do at beginning and end of Campaign, or if you change or clean the cirtical orifice, or if there is a problem or change on the baratron P gage used to quantify AMS flow Standard AMS flow calibration

Please note whether the flowmeter you are using reports volumetric or mass flow. For volumetric flow, note whether it reports it under ambient T & P conditions, or under some standard conditions (e.g STP of 1 atm and 273 K, but people use many other definitions).

Clock Synchonization Computer clocks can drift by up to 1 min per day (occasionally worse). Either set up automatically or keep track of instrument clock in relation to other clocks.

Suggested Filter Times (perform the filter blank during the time interval indicated):

Day of week Week 1 Week 2 Week 3 Week 4 Week 5
Monday 800-900 1000-1100 1200-1300 1400-1500 1600-1700
Tuesday 1100-1200 1300-1400 1500-1600 1700-1800 900-1000
Wednesday 1200-1300 1400-1500 1600-1700 800-900 1000-1100
Thursday 1500-1600 1700-1800 900-1000 1100-1200 1300-1400
Friday 1600-1700 800-900 1000-1100 1200-1300 1400-1500
Saturday 900-1000 1100-1200 1300-1400 1500-1600 1700-1800
Sunday 1200-1300 1400-1500 1600-1700 800-900 1000-1100

Calibrations

  • BitWise

Instructions for use of BitWise

  • Lens Alignment
  • IE Calibration
  • Size Calibration
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