Aircraft PTR-MS

The PTR-MS (Proton Transfer Reaction Mass Spectrometer) is an instrument that works on the principle of proton transfer from H₃O⁺ to any organic molecule with a higher proton affinity.

H₃O⁺ + VOC → VOCH⁺ + H₂O

The instrument is made up of three main components: (i) an ion source, where water vapour is ionised by hollow cathode discharge; (ii) a drift tube, where the sample is introduced and proton transfer occurs; and (iii) a detection system, consisting of a quadrupole mass analyser and electron multiplier. The instrument operates under a vacuum maintained by three turbomolecular pumps and is controlled via an integral lap-top computer.

Air is sampled continuously into the instrument so no sample pre-treatment or pre-concentration is required. Blanks (zeros) are performed by diverting the sample flow through a heated catalytic converter at 350°C.

The quadrupole mass analyser separates the protonated molecules according to their mass so that the monitored masses are sampled sequentially by the electron multiplier detector. Typical cycle times range from 1 – 20 seconds depending upon the number of compounds monitored. The quadrupole has a low mass resolution and essentially detects integer masses. For this reason, there are sometimes isobaric interferences that need to be taken into account. For example, the quadrupole cannot distinguish between isomers such as the isoprene oxidation products MVK and MACR and reports only the sum of the 2 isomers. Similarly, monoterpenes are reported as a sum of all monoterpenes. Table 1 (below) lists the most commonly monitored compounds.

References

Murphy, J.G., D.E. Oram and C.E. Reeves, Measurements of volatile organic compounds over West Africa, Atmos. Chem. Phys., 10, 5281-5294, 2010.

Lindinger, W., Hansel, A., and Jordan, A.: On-line monitoring of volatile organic compounds at pptv levels by means of proton-transfer-reaction mass spectrometry (PTR-MS) – Medical applications, food control, and environmental research, Int. J. Mass. Spectrom., 173, 191–241, 1998.

Table 1: Commonly monitored compounds using PTRMS

The instrument is calibrated in the laboratory before and after each detachment. It is also calibrated on the ground several times during a detachment. Ideally, this is carried out post-flight, but can also be done on non-flying days. The procedure involves the dilution of a calibrated gas standard (500 ppb VOC in nitrogen) in VOC-free air using a series of mass flow controllers.

Minor servicing (pump upkeep, ion source cleaning, detector replacement, etc) is carried out during non-campaign periods.

Consumables

• The instrument requires VOC-free air (e.g. BOC BTCA-178) for pre-flight flushing of the inlet and for the dilution of gas standards. Normally two 10 litre cylinders (V size) are sufficient for a 4-week detachment.
• A new electron multiplier detector (£1500) is normally installed before each campaign.
• A small contribution (typically £1K-£2K) towards longer-term maintenance costs (pump servicing, replacement gas standards, etc) may also be requested.

Costs

• Instrument Insurance
  • This system must be insured by the user for £200K and covers loss, theft or damage to the instrument: damage is that over and above general wear and tear. The system has been designed to be rugged and autonomous. Even so, the end-user must respect the fact that the system is a precision optical instrument that must be treated with great care.
  • The user is responsible for the instrument from the time it leaves the AMF to the time it is returned and signed off as in an acceptable operating condition by the IS: this will be done as soon as is possible on its return.
• Public Liability Insurance
  • NCAS is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
• Shipping Expenses
  • The user is liable for all costs arising from the shipping of the instrument both to and from a deployment.
• IS T&S
  • The user is responsible for coving the travel and subsistence expenses of the IS while attending the instrument.

Shipping

The instrument would normally be installed on the FAAM aircraft.

If the instrument requires shipping it is supplied in a purpose built heat-treated wooden crate (120 x 80 x 125 cm; 230 kg).

Additional items for shipping (may vary depending on length of campaign period):

• 1 x Zarges crate containing instrument spares, calibration rig, tools, etc (80x60x60 cm; ~70kg)

• 1 x calibrated gas standard in wooden crate (148x30x28 cm; 50 kg)

• 2 x 10-litre gas cylinders containing BTCA-178 VOC-free air (95cm x 15cm OD; 20kg per cylinder)

The instrument is normally located on the aircraft.

For aircraft detachments lab space for the calibration equipment is required. The equipment consists of a gas standard cylinder and mass flow controllers. Power (mains, 230V) is required at all times for the flow controllers.

For use in ground-based studies, the instrument has a footprint of 110 x 62 cm and so requires floor space of approximately 2-3 m2 to include room for the calibrated gas standards and associated equipment. The instrument works best in a temperature controlled (air conditioned) room.

Power requirements: 230V/50 Hz, max power approximately 1.3 kW

Manual handling

• The calibration procedure involves bringing pressurised gas cylinders onto the aircraft for short periods of time. Be careful when lifting and manoeuvring these cylinders and do not drop onto the floor of the aircraft. Ensure the cylinders are secured during use.

Electric safety

• Low risk of electrical shock.
• Do not remove any panels or the ion source insulation housing whilst the equipment is switched on.
• Do not attempt any electrical work during flights. Repairs or modifications should only be carried out by trained personnel.

Attended Operation

• The instrument will normally be operated by the IS who will follow risk-assessed procedures. The instrument can be left unattended whilst in operation

Thermal

• Do not touch the catalytic converter when switched on.

Noise

• The instrument contains 3 turbopumps. If these become excessively noisy, or the operator notices a distinct change in pitch, then the instrument should be switched off immediately. Any failure of the rotor will be contained within the pump.

Weight:

170 kg (instrument only)

230 kg (instrument and packing crate)

Footprint: FAAM aircraft rack – 110 x 62 x 110 cm (L x W x H)

Power requirements: 230 V / 50 Hz; 1300 VA (max)

The instrument is capable of producing near real-time data for a selected range of VOCs including methanol, acetone, acetaldehyde, acetic acid, acetonitrile, benzene, toluene, isoprene, the sum of methyl vinyl ketone and methacrolein (MVK+MACR), methyl ethyl ketone (MEK) and the sum of all monoterpenes.

The PTRMS is a scanning instrument meaning that it measures a set of preselected masses (compounds) sequentially with a typical repeat (cycle) time of 1-20 seconds. The actual sampling rate depends on the number of compounds analysed and the type of flying being undertaken.

Most compounds are calibrated to internationally recognised calibration scales.

Field Data

• The instrument produces a range of out files and all are text format.
• The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.

Archive Data

• Data is provided in NetCDF files following the AMOF data standard
• Files contain no more than 24hr of data.
• Instrument name is
  • ncas-ptrms-1
• The data product(s) associated with this instrument:
  • voc-concentration
• Example data file
  • How to use example data
  • ncas-ptrms-1_ral_29001225_voc-concentration_v0.1.nc