Simultaneous Measurement of Nitric Oxide (NO) and NitrogenDioxide (NO2) in Simulated Automobile Exhaust Using MediumPressure Ionization–Mass Spectrometry

LUKE CHANDLER SHORT,* RUDIGER FREY, and THORSTEN BENTERBergische Universitat Wuppertal, Fachbereich C—Mathematik und Naturwissenschaften, Gaubstr. 20, 42097 Wuppertal, Germany (L.C.S.,T.B.); and Bruker Daltonik, GmbH, Fahrenheitstr. 4, 28359 Bremen, Germany (R.F.)

In previous papers we have demonstrated two different, two-color

resonance-enhanced multiphoton ionization (REMPI) schemes for the

simultaneous measurement of trace amounts (ppbV to pptV) of nitrogen

monoxide (NO) and nitrogen dioxide (NO2). The goal of this study is to

provide a laser ionization–mass spectrometric scheme capable of

measuring ppmV to ppthV concentrations of NO and NO2 within vehicle

exhaust containing up to ppthV of aromatic hydrocarbons and a time

frame of seconds. Two ionization schemes are used here to measure NO

and NO2 in simulated automobile exhaust with three different sources.

REMPI Scheme 1 uses broad-bandwidth light and an effusive source to

measure NO (limit of detection (LOD) 300 ppmV), NO2 (LOD 100 ppmV),

and aromatic hydrocarbons (via photoionization) along with fragments

(via electron impact). REMPI Scheme 2 uses narrow-bandwidth light and

a medium pressure laser ionization (MPLI) source to measure NO (LOD

60 ppmV), NO2 (LOD 3 ppmV), and fragments (via electron impact). The

LOD is determined using 10-second sampling times. A newly developed

delayed-ion extraction technique for MPLI is then applied to REMPI

Scheme 2, dramatically reducing the electron impact signal, so that only

NO and NO2 are observed. We conclude that Scheme 2 with delayed-

electron extraction is best suited for measuring in situ NO and NO2 within

engine exhaust.

Index Headings: Nitric monoxide; Nitrogen dioxide; Exhaust analysis;

Medium pressure laser ionization; MPLI; Resonance-enhanced multi-

photon ionization; REMPI.

INTRODUCTION

Nitrogen oxides are significant in their contribution toatmospheric chemistry,1 as well as being medically relevant,providing an indirect measure of oxidative stress andinflammation in the lungs.2,3 Anthropogenic sources ofnitrogen monoxide (NO) are high temperature combustionprocesses, such as automobile engines.4 Natural sources of NOare lightning, biomass burning, nitrification/denitrification, anddecomposition of nitrite in soils.1 A significant source ofnitrogen dioxide (NO2) is the atmospheric oxidation cycleinvolving NO.1,5 Roadside measurements indicate that dieselexhaust could also prove to be an underestimated direct sourceof NO2 in urban areas.6 Mixing ratios for both NO and NO2

have been measured at the low pptV in remote atmospheres7,8

and up to hundreds of ppbV in urban areas.1,9 Thesimultaneous measurement of both NO and NO2 is importantin characterizing the highly dynamic combustion properties ofengines, as several factors can influence the NOx (¼NOþNO2)balance, including speed, load, and fuel used by theengine.10–12 The typical amount of NO emitted from a dieselengine is 0.34 g km�1, with 13 to 39% NO2 of total NOx.13

Heavy-duty diesel trucks equipped with a particle trap convertNO to NO2, oxidizing soot, but also raise the %NO2 from 30 to70%.13 Gasoline engines produce ;1/10th less total NOx

emissions than diesel engines, with a range of 4.3% (passengercar) to 17.6% (light-duty truck) NO2/NOx.14 The measuredconcentration of NO in the exhaust at the tail pipe can rangefrom several hundreds to tens-of-thousands ppmV NO,depending on where the sample is taken and the conditionsof the engine.

Analytical Techniques for NOx Measurement. Theimportance of accurate, in situ measurements of nitrogenoxides is reflected by the large number of analytical techniquespreviously developed. These techniques include infrared andultraviolet/visible absorption,15 laser magnetic resonancespectroscopy (LMRS),16 fluorescence,17 chemilumines-cence,15,18–20 and ionization spectroscopy.21 A challenge tosome of these detection techniques is the limited capability ofabsolute determination and quantification of NO in thepresence of other nitrogen oxides, while others do not haveappropriate detection limits with time-resolved sampling.

Spectroscopic techniques used to measure NO rely upon theabsorption of light at a particular wavelength,15 as described bythe Beer–Lambert law. At room temperature, molecules withadjacent absorption bands often obscure molecular identity andquantification. For sensitive gas-phase detection, long absorp-tion paths are necessary, requiring considerable space. Laser-induced fluorescence (LIF) is a technique that quantifies NO inair by collecting fluorescent emission after suitable laserexcitation of NO, e.g., excitation to the A2R state.22 TheLMRS16 technique measures light absorption by a paramagneticmolecule as a strong magnetic field is varied. As the magneticfield is switched on, degenerate states split (Zeeman effect),resulting in a change in the absorption of NO at a particularwavelength. This effect and the resulting absorption bands aresignificantly simplified for molecules with a single unpairedelectron, i.e., a radical species, so that molecules such as NOare ideally suited for this technique. Although LMRS has anappropriate detection limit, often below ppbV, a significantdrawback is that it can only measure paramagnetic molecules.Also, the setup requires a bulky magnet to producea sufficiently strong magnetic field.

One powerful chemical technique to measure NO ischemiluminescence.19,20 Sampled NO is titrated with ozone,resulting in electronically excited nitrogen dioxide (NO2*) andoxygen. This NO2* subsequently relaxes to produce a photon(590 to 2800 nm) that can be efficiently measured usinga photomultiplier tube. Sampled NO2 can be measured by firstconverting NO2 into NO, such as with luminal (5-amino-2,3-dihydro-1,4-phthalazinedione) in an alkaline solution,19 per-miting the indirect, non-simultaneous measurement of NO2.

Received 22 June 2005; accepted 18 November 2005.* Author to whom correspondence should be sent. E-mail: lcshort@uci.

edu or lcshort@uni-wuppertal.de.

208 Volume 60, Number 2, 2006 APPLIED SPECTROSCOPY0003-7028/06/6002-0208$2.00/0

� 2006 Society for Applied Spectroscopy

Although this technique has both good temporal resolution andsensitivity (pptV), it can only be used to measure nitrogenoxides and ozone and cannot directly distinguish the twooxides of nitrogen.

Two techniques availible for real-time measurement of NOin exhaust are either non-photoionization or photoionizationspectrometry. An example of a non-photoionization techniqueis selected ion flow tube mass spectrometry, which has beensuccessfully used to measure NOx in exhaust.23 This techniquerelies on the reaction of NO with O2

þ, resulting in NOþ.However, this method has difficulty quantitatively measuringNO2 due to competing reactions. Photoionization techniquesinclude photoelectron spectroscopy of NO24 and NO2.25–27

Multiphoton ionization (MPI) techniques are more selective,particularly if molecule-specific resonances are used, termedresonance-enhanced MPI (REMPI). Trace detection of NOwith REMPI-MS has been demonstrated with one-color(1þ1),28 (2þ1),29 (3þ1), and (2þ2),30 and two-color (1þ10)schemes.31 The species NO2 has been measured using two-color REMPI via (1þ10).32 A single-color photoionizationscheme has been developed to measure NO and NO2 withinengine exhaust using 230 nm light.33 NOþ resulting fromphotoionization of NO2 has a large initial kinetic energydistribution, leading to a broader peak than NOþ formed fromphotoionization of NO. However, this scheme cannot providean absolute determination of either NO or NO2 in the presenceof both oxides.

Our group has previously demonstrated two schemes usingtwo-color REMPI to simultaneously measure low pptVconcentrations of NO and NO2 using either broad-bandwidthlight (Eqs. 1 to 3c)21 or narrow-bandwidth light (Eqs. 4a to 5c).31

Analysis of atmospheric nitrogen oxides using REMPI-MShas several advantages over other techniques, because it cansimultaneously measure NO and NO2, has a good sensitivity of;pptV with a jet-REMPI-MS system or ;ppmV for either aneffusive source or a medium pressure laser ionization (MPLI)-REMPI-MS system, can measure other relevent gases such asaromatic hydrocarbons, and can be made into a portable system.

Two methods using photoionization to simultaneouslymeasure NO and NO2 are explored here: Scheme 1 usesa broad-bandwidth light system for the MPI detection of NOand REMPI of NO2 and aromatic hydrocarbons, with a BrukerTurboTOF-MS for ion detection. Scheme 2 uses a narrow-bandwidth laser system for the REMPI of NO and NO2, witha home-built, compact time-of-flight mass spectrometer (TOF-MS) for ion detection. Laser intensities used here for bothschemes are significantly lower compared to previous studiesdue to the concentration range of NO (ppmV to ppthV) andmatrix of aromatic hydrocarbons (ppmV to ppthV) studied,minimizing the broadening and saturation of the measuredsignal in simulated exhaust.

Previous mass spectrometric techniques often rely upon anion lens system that is susceptable to the generation ofphotoelectrons during laser ionization. These low-energyelectrons are believed to be accelerated towards the chargedfront plate (repeller) of the MPLI source, resulting in the non-selective ionization and fragmentation of species within thesampled gas. Even with a scheme that does not directlyphotoionize aromatic hydrocarbons, ionization via photoelec-trons can result in the signals N2

þ and C3H4þ, the latter being

a fragmentation peak of aromatic hydrocarbons. A recentlydeveloped delayed-ion extraction technique for MPLI-MS is

applied here, substantially reducing the detection of such non-selective, photoelectron signals. A more detailed review of thistechnique will be provided in a forthcoming publication.

THEORY

The goal of a MPI scheme is the selective production of ionsfrom a target molecule within a sample matrix. If a uniqueenough ionization pathway is used, the background signal fromother ionized species is minimal. With an intermediateexcitation step, as with REMPI, ionization selectivity andefficiency is enhanced. Further selectivity is provided by thecoupling of photoionization with a mass-resolving technique,e.g., TOF-MS. Two photoionization schemes are presentedhere that can be used to simultaneously measure the chemicalspecies NO and NO2.

Broad-Bandwidth Resonance-Enhanced MultiphotonIonization of NO, NO2, and Aromatic Hydrocarbons(Scheme 1). For Scheme 1, a one-color MPI scheme is usedto ionize NO (Eq. 1). Light at 266 nm is generated as thefourth-harmonic from a Nd:YAG laser:

NO2ðX 2PÞ!hvð266nmÞNOþðNOþþe�Þ ð1Þ

The same wavelength of light is used to ionize aromatichydrocarbons, AHþ, contained within the mixture, via one-color REMPI (Eqs. 2a through 2d). Significant fragmentation,Fþ, of aromatic hydrocarbons has been observed in thiswavelength range with laser fluxes .MW cm�2,34,35 a result ofthe large density of metastable states, AH*, accessible from theabsorption of .2 photons (Eqs. 2c and 2d).

AHðS01A1gÞ!hvð226nmÞ

AHðS11B2uÞ ð2aÞ

AHðS11B2uÞ!hvð226nmÞ

AH� ð2bÞ

AH�!hvð226nmÞAHþ þ e� ð2cÞ

!.1hvð266nmÞFþ þ e� ð2dÞ

The chemical species NO2 is detected using a two-colorREMPI route previously published.21 The third-harmonic froma Nd:YAG laser (355 nm) is used to pump an opticalparametric oscillator (OPO), producing tunable, broad-band-width light at 508 nm. This light is absorbed by NO2,populating the A (2B1) state. The fourth-harmonic froma Nd:YAG laser (266 nm) is then used to further excite NO2

A (2B1) to the 3pr Rydberg state, followed by a third 266 nmphoton, resulting in ionization:

NO2ð~X 2A1Þ!hvð508nmÞNO2ð~A 2B1Þ ð3aÞ

NO2ð~A 2B1Þ!hvð266nmÞNO2ð3pr 2Rþu Þ ð3bÞ

NO2ð3pr 2Rþu Þ!hvð266nmÞNOþ2 þ e� ð3cÞ

APPLIED SPECTROSCOPY 209

The 508 nm photon is followed ;10 ns later by the absorptionof a 266 nm photon, ensuring the population of the A (2B1)state of NO2 before 266 nm is absorbed. Using two colors fromone laser, routes for NO and NO2 are presented in Fig. 1.

Selectivity for NO2 is obtained via the 3pr Rydberg state,not the lower A (2B1) state. Since light from a broad-bandwidthlaser excites several rotational states, the initial rotationaldistribution of states is not resolved. This REMPI pathway istherefore not sensitive to the initial temperature of themolecule, so that substantial adiabatic cooling resulting fromjet expansion, e.g., jet-REMPI, is not necessary. A simpler,effusive source is thus used with Scheme 1.

Narrow-Bandwidth Resonance-Enhanced MultiphotonIonization of NO and NO2 (Scheme 2). A two-color, REMPIscheme is used to ionize NO (Eqs. 4a and 4b).31 Light isgenerated from a XeCl*-pumped dye laser using Coumarin 120dye (k ¼ 423 to 462 nm), then the second-harmonic isgenerated using a beta barium borate (BBO) crystal (k¼ 212 to231 nm). Light at 308 nm is generated from a separate, time-independent XeCl* laser.

NOðX 2PÞ!hvð215nmÞNOðA 2RþÞ ð4aÞ

NOðA 2RþÞ!hvð308nmÞNOþ þ e� ð4bÞ

Although absorption of two photons at 215 nm by NO ispossible, the absorption cross-section for the second photon isgreater for a photon at 308 nm than at 215 nm. This is becausethe total absorbed energy of (215 nmþ 308 nm) correlates wellto the ionization potential of NO, resulting in a high absorptioncross-section.31

For Scheme 2, NO2 is detected using a two-color REMPIscheme demonstrated previously (Eqs. 5a to 5c).31 Light at 430nm is generated from a XeCl*-pumped dye laser withCoumarin 120 dye. This light is absorbed by NO2, populatingthe A (2B1) state. Absorption of 308 nm from the second XeCl*laser leads to the population of the 3pr Rydberg state. Finally,absorption of either 430 nm or 308 nm results in ionization:

NO2ð~X 2A1Þ!hvð430nmÞNO2ð~A 2B1Þ ð5aÞ

NO2ð~A 2B1Þ!hvð308nmÞNO2ð3pr 2Rþu Þ ð5bÞ

NO2ð3pr 2Rþu Þ!hvð308nm or 430nmÞNOþ2 þ e� ð5cÞ

Using these three colors from two lasers (430 nm, 215 nm, and308 nm), efficient routes for NO and NO2 are presented in Fig. 2.

Scheme 2 REMPI is more sensitive to NO and NO2, becauseboth NO and NO2 are ionized via resonance enhancedpathways; however, aromatic hydrocarbons are not efficientlyionized, because the absorption cross-section for these speciesat these wavelengths is relatively low.

One consideration with narrow-bandwidth REMPI is that itis best suited with a jet-cooled source of molecules. At lowermolecular temperature, the energy distribution of the narrow-bandwidth light correlates well with the narrow distribution ofrotational states within the molecule. At a higher moleculartemperature and thus a larger state distribution, narrow-bandwidth light excites a smaller fraction of states, decreasingthe ionization efficiency.

Photo-Fragmentation of NO2. Careful selection of thetunable radiation must be made to avoid the dissociative regionof NO2 between 420 nm , k , 180 nm. McKeachie et al.21

discuss the MPI dissociation route, resulting in a high-vibrational-state radical NO and ground-state atomic oxygen,

NO2ð~X 2A1Þ!hv

NOðX 2PÞ þ Oð3PÞ ð6Þ

This NO molecule will then add to the background NO signal,from which it is nearly indistinguishable. Dissociation of NO2

is avoided by selecting an energy that reaches the A (2B1) state,but does not access this dissociation region. For both REMPIschemes discussed in this paper, excitation energy of NO2 at508 nm (Scheme 1) or 430 nm (Scheme 2) is below the onsetof this dissociative region. This ensures a high ionizationefficiency of NO2

þ and also avoids photodissociation of NO2.

MATERIALS AND METHODS

Mass Spectrometer Used with Scheme 1. Experimentsdescribed here with broad-bandwidth light (Scheme 1) are done

FIG. 1. (Left) Scheme for one-color (1þ1) non-resonant MPI of NO. (Middle) Scheme for two-color REMPI of NO2. (Right) Absorption cross-section for thedissociative channel for NO2 is from ;2 to 5 eV,21 shown here with the quantum efficiency for this channel (solid line, NO2!NOþO).

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with a Bruker TurboTOF-MS linear time-of-flight massanalyzer. The mass spectrometer consists of a 0.8 m flight-tube, dual channel plate detector, and high-speed dataacquisition electronics. Either an effusive source or an MPLIsource is substituted for the original inlet stage. The applicationand benefits of using an MPLI source are describedelsewhere.36,37

Mass Spectrometer Used with Scheme 2. The massspectrometer used with the narrow-bandwidth light system(Scheme 2) is a home-built, mobile TOF instrument equippedwith a single-stage reflectron. The 50 3 20 3 20 (L 3 W 3 H)cm3 body is constructed as one piece from aluminum to bedimensionally compatible with a Pfeiffer TC-600 three-portturbo drag pump backed by a Pfeiffer Duo-10 roughing pump(Pfeiffer Vacuum; Asslar, Germany). The two, high-vacuumports of the pump are directly connected to the differentialpumping chamber and analyzer region of the main housing. Inthe present experiments, both chambers are separated by a 1mm i.d. orifice, which could be easily replaced to matchvarious operating pressure conditions. The mass spectrometerhousing is equipped with vacuum ports for pressure monitoringas well as with three fused silica laser windows and variouselectrical feed-throughs. A compact, 40-electrode gridlessreflectron is set 28 off-axis to reflect the ions coaxially toa center-holed, cascaded multi-channel plate assembly (Hama-matsu, F2223-21SH; Herrsching, Germany) mounted directly

onto the second Einzel lens plate. This system is designed and

constructed based on extensive ion trajectory calculationsdescribed in detail elsewhere.36,38 Data acquisition is accom-

plished with a 100 ps time-to-digital converter (Ortec, 9353;Meerbusch, Germany), a pulse delay generator (National

Instruments, BNC-2121; Austin, TX), and data acquisition

electronics (National Instruments, DAQ-6602; Austin, TX), allmounted into a Pentium PC running LabView in a Windows

XP programming environment.

Sources for the Time-of-Flight–Mass Spectrometry. Twotypes of sources are tested with the TurboTOF-MS and REMPI

Scheme 1. First, an MPLI source (Fig. 3, left) is used at an

operating pressure of ,10�5 mbar, monitored by a coldcathode gauge (PKR 251 Compact Full Range, Pfeiffer

Vacuum; Asslar, Germany). The jet-cooled temperature ofthe gas in the ionization volume resulting from an MPLI

source is ;100 K.38 Second, a 0.3 mm i.d. effusive source

(Fig. 3, middle) is used at an operating pressure of ,10�4

mbar and a gas temperature of ;250 K. The calculated

pressure, Pm, and temperature, Tm, of sampled gas within theionization volume using an effusive source are based on the

adiabatic expansion equations:39

Pm ¼ P02

jþ 1

� � jj�1

ð7Þ

FIG. 2. (Left) Scheme for one-color (1þ1) REMPI of NO. (Right) Scheme for the two-color (1þ10þ1(0)) REMPI of NO2. The ;430 nm/;215 nm light from thedye laser arrives concomitant with the 308 nm light from the XeCl* laser.

FIG. 3. (Left) MPLI inlet for the Turbo-TOF-MS with Scheme 1. (Middle) Effusive inlet with Scheme 1. (Right) MPLI inlet for the home-built TOF-MS andScheme 2. Arrows¼ gas flow, R¼ Repeller electrode, and dotted circle ¼ ionization region.

APPLIED SPECTROSCOPY 211

Tm ¼ T02

jþ 1

� �ð8Þ

where P0 and T0 are the initial pressure and temperature of thegas, respectively, and j is the adiabatic expansion coefficient,with j(N2) ¼ 1.400 and j(He) ¼ 1.660.

An important feature of the MPLI source is that the sample isa short, high-density pulse of molecules, while the effusivesource ‘‘leaks’’ a constant stream of gas into the ionizationchamber. The pulse of gas from the MPLI source has a typicallocal pressure of up to 10 mbar, or a sample density of 1017

molecules cm�3.36 In comparison, the effusive source hasa calculated sample density, q0, of 1013 molecules cm�3,calculated assuming an adiabatic expansion, with the ideal gascoeffecient, R,

q0 ¼p0

RT0ð9Þ

As higher sample density generally results in a lower detectionlimit, the MPLI source has potentially a 3-to-4-fold lowerdetection limit than an effusive source.

A third type of MPLI source is used with the home-builtTOF-MS system and REMPI Scheme 2 (see Fig. 3, right). Thissource is similar to the MPLI source used with the TurboTOF-MS, except that the back-pressure on the membrane of the inlet

is kept at ;0.8 atm. Also, the extraction electrode mounteddirectly on the repeller plate is removed.

Broad-Bandwidth Laser System (Scheme 1). The broad-bandwidth light for MPI and REMPI is generated by a home-built tuneable light source consisting of a Nd:YAG lasercoupled to an OPO (Fig. 4). This light source is based on thedesign by Boon-Engering et al., with some modification.40 Thethird-harmonic (355 nm) light pumps the OPO, witha wavelength range of 490 to 600 nm, dependent upon theangle between the nonlinear BBO crystal and the pump beam.Power output for the OPO is constant at 0.3 mJ over thewavelength range used, with a fluence of ;10 MW cm�2. Thefourth-harmonic (266 nm) light is used directly for excitation/photoionization. The power output of the 266 nm light is 5 mJ,with a fluence of ;100 MW cm�2.

We used this broad-bandwidth light for the MPI of NO andthe REMPI of NO2 and aromatic hydrocarbons (Scheme 1).Two ion sources for the mass spectrometer are tested with thisscheme: The more sensitive but complex MPLI source and theless sensitive but simplified effusive source. The entire lasersystem is very compact, sitting on top of the TurboTOF massspectrometer.

Narrow-Bandwidth Laser System (Scheme 2). Thenarrow-bandwidth light for REMPI using Scheme 2 isgenerated from two lasers (Fig. 5). The first laser is a COMPexXeCl* excimer laser (Lambda Physik, Gottingen, Germany).The resulting beam has 10 mJ at 308 nm, with a fluence of 10MW cm�2. A second COMPex XeCl* pumps a ScanMate dyelaser (Lambda Physik, Gottingen, Germany) with Coumarin120 dye, resulting in narrow-bandwidth, tunable light from 420to 460 nm with a peak power of 10 mJ and a typical fluence of;100 MW cm�2. A 20 cm lens is used to adjust the fluence,diameter, and position of this beam.

The narrow-bandwidth light produced for REMPI (Scheme2) is used to measure NO and NO2 efficiently. A more portable,broad-bandwidth laser could be used with this scheme, andwould better match a MPLI source, but was unavailable for useat the time.

Delayed-Ion Extraction Applied to Medium PressureLaser Ionization–Mass Spectrometry. Following the pulse oflight used for photoionization, scattered or stray light can reachmetal surfaces on the inside of the ionization chamber. Fromextensive calculations, it was determined that the most likelysource of these photoelectrons is the extraction electrodeopposite the repeller electrode.36,38 The resulting electrons arethen accelerated towards the positively charged repellerelectrode (see Fig. 3), leading to non-selective, electron-impact

FIG. 4. Schematic diagram of the Nd:YAG and OPO. Key: KDP 1, second-harmonic generator; KDP 2, third-harmonic generator; KDP 3, fourth-harmonic generator; BS 1, beam splitter (355 nm reflect); BS 2, beam splitter(visible light reflect); BS 3, beam splitter (visible reflect); BD 1 and BD 2, beamdumps; M1, M2, M3, and M4, mirrors; OPO, optical parametric oscillator.Ovals between BS1 and BS3 represent two collimating lenses. No additionalcomponents are used.

FIG. 5. Schematic diagram of the narrow-bandwidth light source. A pulse generator (BNC-2121) is used to synchronize the two laser systems and MS. Fixed light isproduced by the COMPex XeCl* laser (k1¼ 308 nm). Tunable light is produced by a XeCl*-pumped ScanMate dye laser (k2¼ 430 to 464 nm) followed by thesecond-harmonic generation (SHG) using a BBO crystal (k2/2 ¼ 215 to 232 nm).

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ionization of molecules within the jet. One technique to reducethe generation of photoelectrons has been previously in-troduced36 and is briefly described here.

Typically, the extraction electrode is maintained at groundpotential, pulling the photogenerated cations towards the ionoptics. Unfortunately, following the first pulse of light,scattered or stray light results in the generation of electronsfrom the extraction electrode, as described by the workfunction:

KEe� ¼ hv� U ð10Þ

where KEe� is the kinetic energy of the photoelectron generated,hv is the energy of light incident upon the metal, and U is theelectron–metal binding energy, i.e., work function, of the metal.For the experiments presented here, the repeller electrode on theMPLI source and the extraction electrode are composed of Cr–Ni–Steel (Mf. Nr. 1.4541) and the main body is composed ofaluminum. For photoemission electrons to be generated fromthe metal surfaces, light that reaches the extraction electrodemust have a wavelength ,303 nm and light that reaches themain body must have a wavelength ,276 nm.41 As mentioned,the main source of photoelectrons that intersect the sample gasjet originate from the extraction electrode opposite the repellerelectrode. The most energetic light used with either Scheme 1(266 nm) or Scheme 2 (215 nm) is sufficient to generatephotoelectrons from the extraction electrode.

To minimize this effect on the MPLI-MS system usingScheme 2, the extraction electrode is maintained at a voltageslightly less than that of the repeller electrode up to the time ofionization. Photoelectrons are subsequently reabsorbed by theextraction electrode during the initial pulse of light. The cationsgenerated within the gas jet are only slightly acceleratedtowards the extraction electrode at this time. Approximately250 ns after photoionization, the extraction electrode is thenpulsed to ground, drawing the photogenerated cations towardsthe ion optics. A fast, high voltage transistor switch (BehlkeElectronics, GmbH, Kronberg, Germany) is used in positive-pulse mode to provide the necessary high-voltage pulses to theextraction electrode.

Chemicals Used with the Bruker TurboTOF and Scheme1. Nitrogen (N2) is used as the carrier and diluting gas(99.995% purity, Messer, Sulzbach, Germany). A bottle of NOand NO2 is listed by the manufacturer as 97.0% NO, 1.4%NO2, 0.9% nitrous oxide, and 0.02% nitrous acid (Messer,Sulzbach, Germany). Mass flow controllers dilute the samplegas with a flow of N2. A mixture of 118 ppmV (10:1:5) BTX(benzene:toluene:o-xylene; balance N2, provided by Messer) isused to simulate diesel engine exhaust.

Chemicals Used with the Home-Built Time-of-Flight–Mass Spectrometer and Scheme 2. Helium is used as thediluting gas (99.999% purity, Messer, Sulzbach, Germany).Sampled NO is prepared by diluting pure NO in N2 to 30 bar,until ;1% is obtained. This mixture has been analyzed bya 520 Fourier transform infrared (FT-IR) Spectrometer(Nicolet, Kwun Tong, Hong Kong) to contain 2200 ppmVNO and 32.9 ppmV NO2 in N2. A mixture of 900 ppmV BTXis made by vaporizing benzene, toluene, and o-xylene inspecific concentrations and then diluting with He to 30 bar.This mixture has been analyzed by FT-IR to contain (2:1:1)BTX. Mass flow controllers are used to mix the sample gasesor dilute with helium. For the experiments with Scheme 2 and

delayed-ion extraction, a mixture of 1% NO and 0.01% NO2 ismade by diluting pure NO in He to 100 bar.

RESULTS AND DISCUSSION

NO and NO2 Detection via Resonance-Enhanced Mul-tiphoton Ionization–Mass Spectrometry with MediumPressure Laser Ionization Source (Scheme 1). Previouslyour group demonstrated the analytical detection of NO andNO2 in a gas mixture with two-color REMPI (Scheme 1) andan MPLI source with a TurboTOF-MS.21 Integration over 20seconds provided a detection limit of 10 pptV NO and 20 pptVNO2. Taking a sample of NO and NO2 in nitrogen, threeexperiments are performed to demonstrate two-color REMPIand record a REMPI spectrum of NO2 using the MPLI source(Fig. 6).

From Fig. 6, 266 nm light results in one-color MPI of NO,while use of 508 nm light alone yields no measurable ions.However, if 508 nm light first populates the A(2B1) state ofNO2, and then ionization with the absorption of two photons of266 nm light, NO2

þ is detected. Both NO and NO2 aremeasured independently of each other using this two-colorscheme. The origin of the 3pr Rydberg state of NO2 is at;55 840 cm�1, which coincides with the m/z ¼ 46 signalcentered at k1(548 nm) þ k2(266 nm). This compares to theassignment by McKeachie et al. of 55 630 cm�1, k1(549 nm)þk2(266 nm), via (1þ10þ10) REMPI21 and to the assignment by

FIG. 6. (Left top) Demonstration that the NOþ signal is not dependant onOPO tunable light, but results from (10þ10) MPI of NO at 266 nm. (Leftbottom) NO2

þ signal resulting from (1þ10þ1(0)) REMPI of NO2. (Right) Massspectrum taken from a mixture of NO and NO2 using (a) ;508 nm, (b) 266 nm,and (c) ;508 nm with 266 nm. MPLI source is used for sample introduction.

FIG. 7. Wavelength scan of OPO light (60.2 nm)þ266 nm, while monitoring[NO2

þ] with TurboTOF via (1þ10þ10) REMPI with: (top) effusive source and(bottom) MPLI source. Vibronic band assignments for NO2 (3pr 2Rþu) arebased on Haber.42

APPLIED SPECTROSCOPY 213

Haber et al. of 55 650 cm�1, k1 ¼ k2 ¼ 359 nm, via (2þ1)REMPI,42 both using the jet region to generate ions.

NO and NO2 Detection via Resonance-Enhanced Mul-tiphoton Ionization–Mass Spectrometry with an EffusiveVersus Medium Pressure Laser Ionization Source (Scheme1). A direct comparison is made using REMPI-MS (Scheme 1)between the more complex MPLI source and the simplereffusive source (Fig. 7). The wavelength of the OPO wascalibrated at the beginning of the experiments, using theeffusive source, but thereafter recalibration was not convenient.Vibrational assignments are thus made with the effusive sourcedata. Although the signal intensity of the MPLI source isgreater than the effusive source, the distribution of the signalsrepresenting the rotational states do not appear to be influencedby the molecular temperature of the sampled jet. For example,the v1 vibronic band has a ratio of 1:1 for the two firstvibrational states with both the effusive source (;250 K) andthe MPLI source (;100 K). Therefore, for the two-colorREMPI path (Scheme 1), the only significant advantage tousing the more complicated MPLI source versus the simplereffusive source is the gain in signal intensity, a result ofa higher sample density in the source.

Analysis of NO and NO2 in BTX with an Effusive Source

(Scheme 1). NO and NO2 are sampled in simulated engineexhaust composed of 118 ppmV BTX (10 benzene:1 toluene:5o-xylene), 5400 ppmV NO, 780 ppmV NO2, and a balance ofN2 (Fig. 8). This mixture is similar to that used by Salasc andSkoglundh to measure synthetic exhaust over different metalcatalysts.43 Our first experiments with sampling this gasmixture with the MPLI source showed that although it is moresensitive to NOx, the source is too sensitive for direct samplingof typical exhaust containing hundreds of ppmV aromatichydrocarbons. This is because the ion density resulting fromREMPI of aromatic hydrocarbons with 266 nm light results insignal broadening and saturation of the detector. As theeffusive detector results in a much lower overall sample densitythan with the MPLI source, it is used here in conjunction withScheme 1 REMPI.

Using the effusive source to measure a mixture of NO, NO2,and BTX, we can identify the fragments originating from the 6-carbon BTX parent molecules, NO, NO2, and some impuritiespresent in the BTX bottle (C . 6). Both NO and NO2 arevisible when the concentration of NO and NO2 are severalpercent of the sampled gas, as is the case for diesel engines(Fig. 8, left and right (spectrum a)). In this scenario, only theNOþ peak is saturated. When the concentration of NO and NO2

FIG. 8. Scheme 1 REMPI with an effusive source. (Left and right (a)) 40% NO and 3% NO2 mixture with 10 ppmV BTX, (right (b)) 10:10:118 ppmV mixture ofNO:NO2:BTX. Mass spectrum is the sum of 100 samples over 10 seconds.

FIG. 9. (Left top) NOþ signal from (1þ1) REMPI using an MPLI source and 1% NO. (Left bottom) NO2þ signal from (1þ10þ1(0)) REMPI using 1% NO2

generated. (Right) Mass spectrum taken at 431.6 nm with a 1:1 mixture of NO:NO2, demonstrating the enhanced sensitivity of this REMPI scheme to NO2 versusNO.

214 Volume 60, Number 2, 2006

is limited to 10 ppmV, indicative of exhaust from a gasolineengine, then the NO2

þ signal is not easily observed. For thescenario with low NO/NO2 concentration (Fig. 8, right(spectrum b)) a limit of detection (LOD, 5r noise and S/N �2) for NO is 300 ppmV and for NO2 is 100 ppmV.

NO and NO2 Detection via Resonance-Enhanced Mul-tiphoton Ionization–Mass Spectrometry with a MediumPressure Laser Ionization Source (Scheme 2). Previously wehave demonstrated analytical application of NO and NO2 ina gas mixture with two-color-REMPI (Scheme 2) and an MPLIsource connected to a TurboTOF-MS.21 Using Scheme 2 andthe homebuilt TOF-MS system, the wavelength-dependantphotoionization of NO and NO2 is found (Fig. 9). Dependingupon the experiment, the wavelength can be selected to eitherfavor ionization of NO or NO2.

Analysis of NO and NO2 in BTX with a MediumPressure Laser Ionization Source (Scheme 2). A mixture ofNO (220 ppmV), NO2 (3 ppmV), and BTX (90 ppmV) is made

using mass flow controllers, balanced in N2. For ionizationa tunable wavelength of 430.940 nm (SHG¼ 215.47 nm) with308 nm is used, resulting in (1þ10þ1) REMPI (Fig. 10). Theconcentration of BTX is low to minimize fragmentation signalsresulting from electron impact. Detection of NO and NO2

within the mixture of gas containing air and BTX is greatlysimplified when Scheme 2 is used, since the two colors of lightdo not yield significant quantities of ions from aromatichydrocarbons. However, light at 215 nm and 308 nm results inthe electron impact signal of aromatic hydrocarbons, C3H4

þ,and also presumably NOþ from both NO and NO2. Using thismixture and sampling 1000 shots over 100 seconds, the LOD at2r for NO is 20 ppmV (S/N¼ 280) and for NO2 is 1 ppmV (S/N¼ 50). For 100 shots sampled over 10 seconds, the LOD forNO is 60 ppmV (S/N¼ 90) and for NO2 is 3 ppmV (S/N¼ 20).

Delayed-Ion Extraction with Medium Pressure LaserIonization–Mass Spectrometry and Scheme 2. A mixture ofNO (200 ppmV), NO2 (20 ppmV), and BTX (900 ppmV) ismade using mass flow controllers, with no additional dilution.These concentrations reflect the typical values observed ingasoline engine exhaust. For ionization, a tunable wavelengthof 430.940 nm (SHG ¼ 215.47 nm) with 308 nm is used,resulting in (1þ10þ1) REMPI (Fig. 11).

When the extraction electrode is not pulsed to a high positivevoltage during photoionization, photoelectrons are generatedon the extraction electrode (0 V) and accelerate towards thereflector (600 V). With a greater concentration of BTX (900ppmV) in Fig. 11 as compared to Fig. 10 (450 ppmV), a muchmore congested mass spectrum is observed, with electron-impact fragmentation signals at Heþ, CH3

þ, C2H2þ, and C3H4

þ.Due to electron impact, the NOþ signal is likely generated fromboth NO and NO2.

When the extraction electrode voltage is maintained at 500 Vup to the end of the ionization event, the photoelectronsproduced on the extraction electrode are captured, resulting inminimal electron-impact signals. Fragmentation of NO2,leading to generation of NOþ, is demonstrated by comparingthe relative magnitude of NOþ signals in scenario [1] versus

FIG. 10. Measurement of (1100:17:450 ppmV) mixture of NO:NO2:BTXusing the MPLI source and Scheme 2. Laser light (;430/215 nm) is notfocused to minimize fragmentation. (Top) Only dye laser is used, (middle)only XeCl* laser is used, and (bottom) both dye and XeCl* lasers are used.

FIG. 11. Measurement of (200:20:900 ppmV) mixture of NO:NO2:BTX using the MPLI source and Technique 2. Laser light (;430/215 nm) is focused to increaseintensity of signals. [1] Extraction electrode is maintained at ground potential during the ionization event, resulting in electron impact signals, and [2] extractionelectrode is maintained at ;500 V until after ionization event. The star symbol indicates the onset of laser ionization.

APPLIED SPECTROSCOPY 215

scenario [2]. The concentration of NO in the gas phase is ten-fold greater than NO2; however, due to the wavelengthselected, the photoionization efficiency of NO2 is greater thanNO. Thus, the saturated signal at 30 amu (NOþ) for scenario[1] is most likely a result of the non-selective electron-impactof NO and charge-transfer reactions.

CONCLUSION

In comparing the two schemes with each other, we see thatthere is a clear advantage to using Scheme 2 versus Scheme 1to measure NO and NO2 in complex mixtures of aromatichydrocarbons. However, for the measurement of NO and NO2

in exhaust from diesel engines, both schemes have adequateLODs. Use of a delayed-ion extraction technique with MPLI-MS and Scheme 2 dramatically simplifies the mass spectrawhile measuring NO and NO2 in simulated exhaust mixtures.This demonstrates that there is significant interference fromelectron-impact signals generated from standard photoioniza-tion MPLI-MS systems. In a forthcoming paper, we willdiscuss the generation of these photoelectrons within the MPLIsource and then further detail how delayed-ion extraction canbe adapted to MPLI-MS.

ACKNOWLEDGMENTS

This work is supported by the NSF Atmospheric Program, grant no.0083436, and in part by the BMBF, Germany, within projects 01 LO 950/6 and07 AK 302/0. LCS acknowledges support through a fellowship of the FulbrightFoundation. We would also like to thank Dr. W.E. van der Veer for his designand construction of the OPO light source, Dr. R. Kurtenbach and AnneHeinrich for the preparation of the NOx and BTX standards, and Dr. I Barnesfor his FT-IR measurements of BTX, NO, and NO2.

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