Mass spectrometry

Mass spectrometric methods PTR-MS is a suitable technique when the flux is low and flux ratio varies a lot since this method has a fast response time and high sensitivity (Taipale et al.,2008). The major application of PTR-MS is atmospheric sciences. This technique is compact and robust so it can be used for very different measurement approaches like at measurement stations (for example SMEAR- stations in Finland), ships, air-crafts and vehicles (Blake et al., 2009). Ionization happens trough the proton transfer reaction:

 

H3O+ + R → H2O + RH+

 

Proton transfer from H3O+ is a soft ionization method which limits fragmentation and improves quality of the mass spectrum and identification accuracy of the different compounds. An advantage of H3O+ ions is that the proton transfer processes are mainly non-dissociative which means that only one type of secondary ions species will form from individual neutral reagent (Lindinger et al., 1998).

 

PTR-MS is a sensitive technique for the low fluxes and small VOC samples and this technique will analyze trace gases from the air samples with direct inlet (Warneke et al., 2003). This technique is does not require sample preparation which means that samples can be taken straight from the chamber headspace into drift tube. For this reason, an instrument is suitable for automatic chamber measurements. The response time of this instrument is very fast and it can be used to collect online-data fast directly from the experiment site. The concentration quantification can be executed without standard gases. You can combine this technique to the quadruple mass filters or a time of flight analyzers. PTR-MS is quantitative and fast technique which measures one compound within 0,1 s. The instrument measures mass range 33-137 Th and it is based on the calibration standards. The detection limit is low which is crucial for this study since emissions rates of VOCs are very low and closure time should be as sort as possible so that we can avoid changes of temperature and relative humidity in the headspace during the closure time since this will affect immediately for the flux rate by biasing the results. For this study, the major advantages of this technique is the wide range of VOCs which it will detect separately but very light hydrocarbons are under the detection limit for this technique. PTR-MS can also detect oxidation products of VOCs.

 

The fast response time of this technique means that it can be used for the ecosystem scale monitoring with EC (eddy covariance) and DEC (disjunct eddy covariance). This can be used to measure VOC gradient in the boreal forest. This technique gives results as nominal mass which means that the identification ability is low. PTR-Ms is not a suitable method for gas analyses since it cannot separate compounds with same molecular mass (Lindinger et al., 1998) and for this reason I need another mass spectrometric technique, GC-MS (gas-chromatography mass-spectrometry).

 

The GC is an effective instrument when we wish to separate compounds into various components but it is unreliable for identification of specific species. When GC is used first to separate compounds and MS after that for identification we can detect masses of different elements more precisely. The disadvantages is a very long analyzing time which limits opportunities to use this technique. It takes one hour to analyze an adsorbent sample and when samples are analyzed with calibration samples the analyzing time is pretty long. This technique is very important in this study since the knowledge of the VOC sources and released compounds is very limited.

Disadvantages of this technique is that GC will not separate all the species completely which means that MS receives an unclear signal which transfer to the mass spectrum which is used to the identify the peaks of different species based on the calibration samples.

 

The advantages of the PTR-TOF-MS instruments is that it measures very low VOCs concentrations (a few pptv) with high mass resolution (over 6000 molecules min-1).This would be useful since the fluxes are very small and changes in the flux rate can be difficult to detect. A disadvantage of PTR-TOF is that when ions are produces in the drift tube they will be pulsed into time-of flight region (Jordan et al., 2009). The traditional PTR-MS detects a constant current if the analyzed ions which means that the large ion loses are smaller.