28 PROTON TRANSFER REACTION MASS SPECTROMETRY ( PTR - MS ) Yujie Wang , Chengyin Shen , Jianquan Li , Haihe Jiang , and Yannan Chu 28.1 INTRODUCTION is measured. If a chromatographic separation method is not used prior to MS, then the resulting mass spectra Proton transfer reaction mass spectrometry ( PTR - MS ) from EI may be so complicated that identifi cation and was fi rst developed at the Institute of Ion Physics of quantifi cation of the compounds can be very diffi cult. In Innsbruck University in the 1990s. Nowadays, PTR - MS PTR - MS instrument, the hollow cathode discharge is is a well - developed and commercially available tech- served as a typical ion source [1] , although plane elec- nique for the on - line monitoring of trace volatile organic trode direct current discharge [2] and radioactive ion- compound s ( VOC s) down to parts per trillion by volume ization sources [3] recently have been reported. All of (ppt) level. PTR - MS has some advantages such as rapid the ion sources are used to generate clean and intense + response, soft chemical ionization ( CI ), absolute quan- primary reagent ions like H3 O . Water vapor is a regular tifi cation, and high sensitivity. In general, a standard gas in the hollow cathode discharge where H2 O mole- PTR - MS instrument consists of external ion source, cule can be ionized according to the following ways [4] : drift tube, and mass analysis detection system. Figure 28.1 illustrates the basic composition of the PTR - MS + ee+→++HO22 H O 2, (28.1) instrument constructed in our laboratory using a quad- +→+++ rupole mass spectrometer as the detection system. eeHO2 H OH 2, (28.2) + ee+→++HO22 O H 2, (28.3) 28.1.1 Ion Source + ee+→HO22 HO +2. (28.4) Perhaps the most remarkable feature of PTR - MS is the special CI mode through well - controlled proton trans- The above ions are injected into a short source drift fer reaction, in which the neutral molecule M may be region and further react with H2 O ultimately leading to + converted to a nearly unique protonated molecular ion the formation of H3 O via ion – molecule reactions: MH+ . This ionization mode is completely different from ++ traditional mass spectrometry (MS) where electron HHOHOH22+→ 2 + 2 (28.5a) + impact ( EI ) with energy of 70 eV is often used to ionize →+HO3 H (28.5b) chemicals like VOCs. Although the EI source has been ++ HHOHOH+→ + (28.6) widely used with the commercial MS instruments most 22 ++ coupled with a variety of chromatography techniques, OHOHOO+→22 + (28.7) these MS platforms have a major defi ciency: in the ++ OH+→ H23 O H O + O (28.8a) course of ionization, the molecule will be dissociated to + →+HO OH (28.8b) many fragment ions. This extensive fragmentation may 2 ++ result in complex mass spectra especially when a mixture HO223+→ HO HO + OH (28.9) Mass Spectrometry Handbook, First Edition. Edited by Mike S. Lee. © 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc. 605 606 PROTON TRANSFER REACTION MASS SPECTROMETRY (PTR-MS) HC SD quadrupole mass filter SEM Water in Water out Water IC sample inlet turbo pump turbo pump ion drift tube ion detection system source FIGURE 28.1 Schematic diagram of the PTR - MS instrument that contains a hollow cathode (HC), a source drift (SD) region, an intermediate chamber (IC), and a secondary electron multiplier (SEM). Unfortunately, the water vapor in the source drift 28.1.2 Drift Tube region inevitably can form a few of cluster ions + The drift tube consists of a number of metal rings that H O (H O) via the three - body combination process 3 2 n are equally separated from each other by insulated rings. Between the adjacent metal rings, a series of resis- + + HO3212() HOnn− ++→ HO A HO 32 () HO +≥ A (),n 1 tors is connected. A high voltage power supplier pro- duces a voltage gradient and establishes a homogeneous (28.10) electric fi eld along the axis of the ion reaction drift tube. The primary H O + ions are extracted into the ion where A is a third body. In addition, small amounts of 3 + + reaction region and can react with analyte M present in NO and O2 ions occurred due to sample air diffusion the sample air, which through the inlet is added to the into the source region from the downstream drift tube. upstream of the ion reaction drift tube. According to Thus, an inlet of venturi type has been employed on the values of proton affi nity ( PA ) (see Table 28.1 ), the some PTR - MS systems to prevent air from entering the + + reagent ion H3 O does not react with the main compo- source drift region [5,6] . At last, the H3 O ions produced > nents in air like N2 , O2 , and CO2 . In contrast, the reagent in the ion source can have the purity up to 99.5%. Thus, ion can undergo proton transfer reaction with M as long unlike the selected ion fl ow tube mass spectrometry as the PA of M exceeds that of H O [6] : (SIFT - MS) technique [7] , the mass fi lter for the primary 2 + ionic selection is not needed and the H 3 O ions can be MHO+→+++ MHHO. (28.11) directly injected into the drift tube. In some PTR - MS, 32 + 6∼ 7 the ion intensity of H3 O is available at 10 10 counts Thus, the ambient air can be directly introduced to per second on a mass spectrometer installed in the achieve an on - line measurement in the PTR - MS opera- vacuum chamber at the end of the drift tube. Eventually, tion. Due to the presence of electric fi eld, in the reaction the limitation of detection of PTR - MS can reach low region, the ion energy is closely related to the reduced - ppt level. fi eld E/N , where E is the electric fi eld and N is the + Instead of H3 O , other primary reagent ions, such as number density of gas in the drift tube. In a typical + + + NH4 , NO , and O2 , have been investigated in PTR - MS PTR - MS measurement, E/N is required to set to an instrument [8 – 10] . Because the ion chemistry for these appropriate value normally in the range of 120 ∼ 160 Td ions is not only proton transfer reaction, the technique (1 Td = 1 0 − 17 Vcm 2 /molecule), which may restrain the + is sometimes called CI reaction MS. However, the formation of the water cluster ions H3 O (H2 O)n potential benefi ts of using these alternative reagents are (n = 1 – 3) to avoid the ligand switch reaction with + usually minimal, and to our knowledge, H3 O is still the analyte M [6] : dominant reagent ion employed in PTR - MS research +++→ + [1,6,11,12] . HO32() HOnn M HO 3212 () HO− M HO . (28.12) INTRODUCTION 607 TABLE 28.1 Proton Affi nities of Some Compounds mass spectrometer ( TOF - MS ) [14 – 16] , ion trap mass Compound Molecular Molecular Proton spectrometer (IT - MS) [17] , and linear ion trap mass Formula Weight Affi nity [13] spectrometer (LIT - MS) [18] . These MS techniques have (kJ/mol) been used to distinguish isomeric/isobaric compounds as discussed in the later section. Helium He 4 177.8 Neon Ne 20 198.8 Argon Ar 40 369.2 28.1.4 Absolute Quantifi cation Oxygen O2 32 421 Normally, PTR - MS can determine the absolute concen- Nitrogen N2 28 493.8 Carbon dioxide CO2 44 540.5 trations of trace VOCs according to well - established Methane CH4 16 543.5 ion – molecular reaction kinetics. If trace analyte M + + Carbon monoxide CO 28 594 reacts with H3 O , then the H3 O signal does not decline Ethane C2 H6 30 596.3 signifi cantly and can be deemed to be a constant. Thus, + Ethylene C2 H4 28 680.5 the density of product ions [MH ] at the end of the drift Water H2O 18 691 tube is given in Equation 28.13 [6] : Hydrogen sulfi de H2 S 34 705 Hydrogen cyanide HCN 27 712.9 ++−=−kt[]M Formic acid HCOOH 46 742 [][](),MH H30 O 1 e (28.13) Benzene C6 H6 78 750.4 + Propene C3 H6 42 751.6 where [H3 O ]0 is the density of reagent ions at the end Methanol CH3 OH 32 754.3 of the drift tube in the absence of analyte M, k is the Acetaldehyde CH3 COH 44 768.5 rate constant of reaction 28.11, and t is the average reac- Ethanol C2 H5 OH 46 776.4 tion time the ions spend in the drift tube. In the trace Acetonitrile CH3 CN 41 779.2 analysis case k [M] t << 1, Equation 28.13 can be further Acetic acid CH3 COOH 60 783.7 deduced to the following form Toluene C7 H8 92 784 Propanal CH3 CH2 COH 58 786 + O - xylene C H 106 796 []MH 1 8 10 []M = . Acetone CH COCH 58 812 + (28.14) 3 3 []HO30kt Isoprene CH2 C(CH3 ) 68 826.4 CHCH2 Ammonia NH3 17 853.6 Equation 28.14 is often used in a conventional Aniline C6 H7 N 93 882.5 PTR - MS measurement. However, when the concentra- tion of analyte M is rather high, the intensity change of + reagent ions H3 O is not neglectable. In this case, the relation k [M] t << 1 is not tenable; therefore, the regular Equation 28.14 is no longer suitable for concentration However, a higher reduced - fi eld E/N can cause the determination. For a more reliable measurement, the collision - induced dissociation (CID) of the protonated following Equation 28.15 , deduced from Equation 28.13 , products, thereby complicating the identifi cation of can be used to determine the concentration of analyte detected analytes.