Anal. Chem. 1995, 65, 1090-1702
1090
Ion Mobility Spectrometry of Hydrazine, Monomethylhydrazine, and Ammonia in Air with 5-Nonanone Reagent Gas Gary A. Eiceman,' Michael R. Salazar, and Michael R. Rodriguez Department of Chemistry, New Mexico State University, Las Cruces, New Mexico 88003-0001
Thomas F. Limero, Steve W. Beck, and John H. Cross KRUG Life Sciences, Inc., Houston, Texas 77058
Rebbeca Young Toxic Vapor Detection Laboratory, National Aeronautical and Space Administration, John F. Kennedy Space Center, Florida 32899
John T. James Biomedical Operations and Research Branch, National Aeronautical and Space Administration, Johnson Space Center, Houston, Texas 77058
Hydrazine (HZ) and monomethylhydrazine (MMH) in air were monitored continuously using a hand-held ion mobility spectrometer equipped with membrane inlet, ssNi ion source, acetone reagent gas, and ambient temperature drift tube. Response characteristics included detection limit, 6 ppb;linear range, 10-600 ppb;saturated response, >2 ppmj and stable response after 15-30 min. Ammonia interfered in hydrazines detection through a product ion with the same drift time as that for MMH and HZ. Acetone reagent gas was replaced with 5-nonanone to alter drift times of product ions and separate ammonia from MMH and HZ. Patterns in mobility spectra, ion identifications from mass spectra, and fragmentation cross-sections from collisional-induced dissociations suggest that drift times are governed by ioncluster equilibria in the drift regionof the mobility spectrometer. Practical aspects including calibration,stability, and reproducibility are reported from the use of a hand-heldmobility spectrometer on the space shuttle Atlantisduring mission STS37.
INTRODUCTION Hydrazine ( N ~ Hor I HZ) and monomethylhydrazine (N2H3CH3 or MMH) are hypergolic fuels, i.e., substances that combust upon contact with an oxidant without an external ignition source. This characteristic had led to extensive use of these fuels in propulsion systems for spacecraft, rockets, and satellites. However, the toxic properties of HZ and MMH necessitate the protection of personnel working with or near these substances with adequate ambient air monitoring. The threshold limit for inhalation exposure specified by the American Conference of Governmental Industrial Hygienists is 100 ppb,' and a revised value of 10 ppb has been suggested. Risk of nasal injury from such exposures has prompted the (1) American Conference on Governmental Industrial Hygienists. 1%&199I ThresholdLimitValuesforChemicalSubstancesandphysical Agents and BiologicalExposureIndices;ACGIH Cincinnati,OH, 1990. 0003-2700/93/0365- 1686$04.00/0
National Aeronautical and Space Administration to establish 2 ppb as a maximum allowable concentration in spacecraft, and further MMH toxicity studies were recommended. Thus, hand-held field monitors are needed to provide quick response at the 1-10 ppb level and continuous, interference-free operation. Desirable features for such monitors also include stable calibration, rugged construction, and consumable-free operation. Some chemical propertiea of hydrazines facilitate specificity in detection and include oxidation potentials for Coulometric titrations2 or chemical reactivity toward ketones for derivatization gas chromatography.3 These methods are both reliable with low detection limits and not well suited to field use. Portable hydrazine monitors based upon potentiometry4t6and colorimetric-based paper tapes6 are presently used for air monitoring of MMH and HZ but exhibit limitations in detection limits or unacceptable calibration drift. No commercially available technology embodies the desired features of rugged design, speed of response, stability, specificity, and convenience for monitoring hydrazines at low ppb vapor levels. An exploratory study7 using ion mobility spectrometry (IMS) was motivated by the strong proton affinities of hydrazines since the first step in IMS detection, Le., analyte ionization, occurs through gas-phase proton transfer reactions. In IMS, vapors are ionized in air and characterized in a weak electric field at atmospheric pressure.8 Ionization depends on the charge exchange of protons or electrons between a reservoir of charge and neutral vapors. This ionization is competitive and imparts a primary level of specificity to detection. The mobilities of gas-phase ions at ambient pressure are inversely related to the ratio of collisional (2) Miller, E. L.; Johnson, R. P.; J o h n , H. Determination of Propellant Hydrazines in Aqueous Solutions by Constant Current Coulometric Titrations; Instruction No. 320-31-145, Revision D; NASA-WS'IF Laa Cruces, NM, Mar 2,1988. (3) Holtzclaw,J. R.; Fhe, S. L.; Wyatt, J. R.; Rounbehler, D. P.; Fine, D. H. Anal. Chem. 1984,56, 2952-2956. (4) Delgado, R.; Johnson, B.; Johnson, H. Calibration of Interscan Compact Portable Analyzers Model 4186 & 4164; Instruction No. 32031-069; NASA-WSTF Las CNCW,NM, Aug 24,1989. (5) Young, R.; Mattaon, C. Preliminary Evaluation of Hypergolic Fuel Vapor Detectors; Report KSC-DL-3395;1990. (6) Young, R.; Beers,M. Advanced Hydrazine Vapor Detector Syetem Technology Evaluation;Report KSC-DL-3406; Chapter 3, in prese. (7) Leaeure, C. S.; Eiceman, G. A. Anal. Chem. 1986,57,1890-1894. (8) Eiceman, G. A. Crit. Reu. Anal. Chem. 1991,22 (1,2), 471-490. 0 1993 American Cbmical Soclety
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1, 1993
cross-section (i.e., size)tocharge. This feature of IMS provides a second dimension of selectivity and makes IMS more than a general vapor sensor. A significant development in IMS occurred when military establishments in the U.S. and U.K. pioneered small, light-weight, battery-operated, hand-held ion mobility spectrometers for combat use.8 Unfortunately, intrinsic features of these designs were not wholly compatible with hydrazine sensing due to the high chemical reactivity of hydrazines. For example, ambient temperature drift tube, membrane inlet! alternate reagent gas chemistry,'O and recirculated gas flow11 provided surfaces where vapors can adsorb or read. In preliminary trials, these limitations were eclipsed by a severe interference from ammonia which exhibited product ions with the same drift times as ions from the hydrazines. This arose seemingly from poor ion separations associated with ion clusters between ammonia and the reagent gas (acetone) and not from preferential ionization.12 The main objective of this present investigation was the development of reagent gas chemistry to allow separation of MMH and HZ from ammonia in an ambient temperature drift tube, and this was attained using 5-nonanone. A second goal was a thorough exploration of the basis for the resolution of peaks using mobility spectra, ion identifications with ion mobility spectrometry-tandem mass spectrometry, and collisioninduced dissociation studies of relevant clusters with tandem mass spectrometry. EXPERIMENTAL SECTION
Instrumentation. Ion Mobility Spectrometers. Two handheld IMS instruments, employed in the early stages of this project, were unmodified Advanced Vapor Monitors (AVMs) from Graseby Ionics, Ltd. (Watford, Herts, U.K.). These devices had the same electronics, pneumatics, and drift tube as military chemical agent monitors.1s An AVM with acetone reagent gas was used at Kennedy Space Center (KSC), FL, in May 1989 for exploratory studies including calibration and interference tests. In a second AVM, the original permeation source was replaced with a diffusion source containing 5-nonanone. The source was made from polytetrafluoroethylene (PTFE) tubing (6 mm 0.d. X 30 mm long) and was packed with ca. 100mg of montmorillonite treatedwith 10-200rL of 5-nonanone;a hole (0.070-mmdiameter) was made in the tubing using a syringe needle. In each AVM, recirculatedflows are conditioned with ketone vapor before return to both the ion source and the drift regions of the ion mobility spectrometer. Operating parameters for both AVMs were established by the manufacturer and included lOmCi WNi sources; 12-mm-long drift regions; drift gas flow, 200 mL/min; field strengths, 244 V/cm; inlet sample flow, 0.5 L/min; shutter pulse width, 180 rs; shutter repetition rate, 40 Hz; and drift tube temperature, ambient. In the AVM, a methylsilicone membrane (75-150r m thick) isolatesthe inlet flowfrom the ionizationregion. The inlet nozzle is roughly 40 OC, and the membrane temperature is intended for 100 OC. Under the operating conditions of sample flow, the membrane temperature has not been verified. Signals were processed using digital signal averaging with an interface board and software (Advanced Signal Processor or ASP, Graseby Ionics, Ltd.) installed on IBM-AT-compatible computers. Parameters selected with the ASP software included number of scans per spectrum, 10-1000, and number of samples per spectrum, 512. The number of scans per spectrum was 64 for studies with 5-nonanone as the reagent gas. (9)Spangler, G.E.;Canico, J. P. Int. J. Mass Spectrom. Zon Phys. 1983,52,267-287. (10)Proctor, J.; Todd, J. F. J. Anal. Chem. 1984,56,1794. (11)Bradshaw, R. F. D.; Brokenshire, J. L. Trace Vapor Detector; United States Patent 4,317,995;Mar 2, 1982. (12)Young,R. AReportonTeetResultaonIon MobilitySpectrometry Detection of Hydrazines Conducted at Toxic Vapor Detection Lab; KSC, Internal Report, June 26,1989. (13)Eiceman, G.A.; Snyder, A. P.; Blyth, D. A. Int. J. Environ. Anal. Chem. 1990,38,415-422.
1897
An enhanced-ion mobility spectrometer (E-IMS), also from Graseby Ionics, Ltd., was used to determine the effect of 5-nonanone vapor levels on drift times of product ions and to measure the effect of nozzle temperature on response times. The effect of temperature on reduced mobility was also studied through control of the drift tube temperature from 30 to 70 OC at 5-nonanonevapor levels from 0 to 1ppm. Vapor concentrations for MMH, HZ, and NH, were each approximately 100 ppb, and the nozzle temperature was 60 "C. The E-IMS was operated with air flows of 200 mL/min in both drift and reaction regions. In nozzle temperature studies, the drift tube temperature was 70 "C, and the nozzle was set to temperatures from 25 to 60 OC. A third AVM was configured at Graseby Ionics, Ltd., with 5-nonanone ionization chemistry and was flight hardware for the space shuttle. This instument was launched on-board Atlantis (STS-37, April 1991) and underwent pre- and postflight calibrations at KSC withindependently verified vapor levels of MMH and HZ. Zon Mobility Spectrometer-Tandem Mass Spectrometer and Atmospheric Pressure Chemical Ionization (APCI) Tandem Mass Spectrometer. A TAGA 6000 tandem mass spectrometer (MS/MS) from Sciex, Inc. (Toronto, Ontario, CANADA), was used in collisional-induceddissociation (CID) studies to determine relative cross-sections of ions created from HZ, MMH, and ammonia with 5-nonanone. The APCI-MS/MS was operated under nominal condition^^^ with an exception that the dry nitrogen plenum gas was replaced with bottled air conditioned with 5-nonanone vapors. This arrangement provided the same ion clusters observed in the IMS/tandem MS configuration. In CID studies, an ion was selected in the first quadrupole, the ion was injected into an argon gas curtain in a second quadrupole, and the fragments were analyzed in the third quadrupole. Conditions and data reduction for these experiments have been reported.16 The tandem MS was also configured with an ion mobility spectrometer in place of the conventional corona discharge source. The ion mobility spectrometer was equipped with an WNiion source and shutter identical to those in the hand-held ion mobility spectrometer. The drift region was 4 cm long with an electric field of 230 V/cm. The end of the drift tube was an insulator and was placed against the plate used for corona discharges; the last conducting ring in the drift region was placed at 1400 V and was 1 cm from this plate (at 650 V+). The ion mobility spectrometer was at ambient temperature. The ion shutter was operated fully open to provide improved ion yield to the mass spectrometer. The IMS/tandem MS was used only for identification of ions created in the IMS under conditions similar to those in the hand-held units. Vapor Generators. The generation of MMH and HZ vapors of known concentrations was aggravated by the chemical reactivity of these hydrazines toward surfaces of most kindsale Two Kin Tek Model 360 vapor generators produced vapors from permeation sources. The vapor generator at NMSU was used to generate approximate vapor levels, and vapors from the thermostated chamber were delivered to the AVM through a 50-cm length of 6-mm 0.d. PTFE tubing. The AVM inlet nozzle was shielded from cross drafts in the fume hood with a Teflon sleeve (30 cm diameter X 10 cm long). The vapor generator at KSC was equipped on the vapor delivery tube with a calibrated Interscan Model 4000 detector to confirm vapor levels. This reference detector was calibrated versus Coulometric titrations.2 An Omega DP2000 temperature-humidity sensor and Miller-Nelson Model HCS-301 control system for flow, humidity, and temperature were also incorporated into this generator. A thermally insulated glass vessel allowed hydrazine vapors and dilution air to mix before delivery to the AVM and reference detector. Supporting Instrumentation. A Model 5890A gas chromatograph from Hewlett-Packard Co. (Avondale, PA) was equipped with 15- m DB-1 fused silica capillary column, flame ionization (14)Snyder, A.P.;Harden, C. S. Org. Mass Spectrom. 1990,25,301. (15)van Houte, J. J.; de Koster, C. G.; van Thuijl, J. Int. J. Mass Spectrom. Ion Processes 1992,115,173. (16)Taffe, P.A.; Rose-Pehrsson, S. L.; Wyatt, J. R. Material Compatibility with Threshold Limit Value Levels of Monomethyl Hydrazine; Naval Research Laboratory (NRL) Memorandum Report 6291;Oct 26, 1988,pp 7-17.
18B8
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1, 1993
detector, and splitless injector and was used to verify 5-nonanone levels in the drift gas of the E-IMS. Conditions for determining trapped 5-nonanonevapors were as follows: column temperature, isothermal at 150 "C; and injector and detector temperatures, 200 "C. Vapors of 5-nonanone were trapped in 1mL of acetone using a constant flow sampler (Model 224-43XI3, SKC, Inc., Eighty Four, PA) and microimpinger (SKC, Inc.) at the vent tube of the E-IMS. The recorder for the KSC studies was a BBC, Goerz Metrawatt Model SI130 recorder and that at NMSU was a HP Model 15002recorder with Model 443 input amplifier. Chemicals, Gases, and Reagents. Permeation tubes of Hz, MMH, and ammonia were obtained from Kin-Tek, Inc. (Texas City, TX). Liquid hydrazine, monomethylhydrazine, and 5-nonanone were obtained from Aldrich Chemical Co. (Milwaukee, WI). Air for the vapor generators was delivered from bottled breathing air at NMSU and from an oilless compressor with a Balston purification train at KSC. Vapors of 5-nonanone were trapped in distilled-in-glass-gradeacetone (Burdick and Jackson, Inc., Muskegon, MI). Procedures. Generation of Vapors. Vapors of MMH, HZ, and ammonia were used in every study and were generated with permeation tubes using a protocol common to both NMSU and KSC. The vapor generator was placed at aspecified temperature and gas flow rate and allowed to equilibrate for periods from 30 min for flow changes or 1-12 h for temperature changes. These times allowed passivating or conditioning of surfaces permitting a continuous stream of vapors at steady concentrations. Vapor levels at NMSU were calculated from calibrated permeation rates with appropriate dilution flows and were regarded as approximations only (i.e., not confirmed vapor levels). All vapor levels reported in this work for calibrations were confirmed at KSC using Coulometric measurements directly or indirectly.2 Exposure of AVM to Vapors. A single exposure began with a background measurement of clean air prior to presentation of the mobility spectrometer to the vapor stream. The exposure to vapors, once started, was maintained until the response became stable; then the AVM was exposed again to clean air until the signal was restored to or near the original baseline conditions. Intensity of the product ion was taken from the signal-averaging board and recorded on a strip chart recorder. The scale expansion factor in the signalprocessing software and hardware was usually adjusted for maximum response without saturating the recorder. Interference Tests. Interference studies were conducted by exposing an AVM with acetone reagent gas to 2-propanol,acetone, ammonia, and a halocarbon mixture (equal parts of 1,1,2trifluorotrichloroethane and methylene chloride). The potential interferent was metered into a T-union of the vapor steam with a syringe pump and diluted with an air flow of 1 L/min. Concentrations of interferences were approximately 10-100 ppm except for ammonia which was a known level of 90 ppm. Alternate Reagent Gas Chemistry. Ionization chemistry with reagent gases other than acetone occurred through replacing the permeation source from the sieve packs of the AVM. The sieves packs were vacuum baked for 10 h typically at 40 "C to remove residual acetone. Other permeation or diffusion sources were placed in a receptacle of the sieve pack of the hand-held mobility spectrometers, and the instruments were assembled without further changes. Nonamne Reagent Gas Concentrations. Vapors of 5-nonanone in the drift tube of the E-IMS were determined by trapping exhaust flow in an ice cooled microimpinger charged with ca. 2 mL of acetone. The microimpinger was attached to the SKC, Inc., constant flow sampler with a flow of 200 mL/min; flow was sampled for 5 min. The final volume of sample was adjusted to 2 mL by evaporation with a nitrogen stream or addition of fresh solvent in calibrated conical vials. Samples were analyzed with a gas chromatograph that had been calibrated previously using standard solutions of 5-nonanone in acetone. Concentrations of 5-nonanone in the drift gas were calculated from the mass of 5-nonanone trapped and the volume of drift gas sampled with the impinger. Nozzle Temperature Study. The temperature of the inlet nozzle on the E-IMS was controlled manually and measured using a thermocouple inserted ca. 2 cm into the nozzle. The instrument was exposed to an estimated 150 ppb of MMH at nozzle temperature of 22 "C,and the product ion intensity was monitored
continuously. This was repeated with measured temperatures of 24,26,29,33,39,43, and 60 "C. Surface temperatures of the nozzle were not corrected for cooling effects of the air flow rate (0.5 L/min). Presentation of Spectra and Calculation of Reduced Mobilities. Drift times or mobilities of ions in ion mobility spectrometers are influenced by temperature and pressure and in cma/ can be compared as normalized or reduced mobilities (KO V.s) using KO = K(273/7') (P/760) where K is experimental mobility, Tis drift tube temperature in Kelvin, and Pis ambient pressure in Torr. In the discussion below,actual mobility spectra are presented for direct assessment of results. However,findings are shown from studies at KSC and NMSU, and drift times were affected by elevations (0-10 m and ca. 1300 m, respectively). Consequently, direct comparisons of the results are complicated by the experimental sequence spanning 26 months and the specific locations of individual studies.
RESULTS A N D DISCUSSION IMSof Hydrazine Vapors with Acetone Reagent Gas. Mobility spectra for HZ and MMH with acetone (Ac) reagent gas chemistry exhibited a single intense ion with drift time of 8.77 ms for MMH and 9.08 ms for HZ. The product ion drift times corresponded to reduced mobilities (KO) of 1.61 and 1.56 cm2/V.s for MMH and HZ, respectively, and were longer than that for the reactant ion, a dimer ion (Ac)zH+," with drift time of 7.65 ms. These mobilities differed from earlier reports of larger KOvalues or smaller ions7 for MMH and HZ product ions and suggested ion cluster in the handheld mobility spectrometer of the type MMH*Ac*H+. Product ions for vapors of MMH and HZ at low ppb levels appeared within 1-3 s of the start of an exposure with a hand-held mobility spectrometer, and a stable, maximum response occurred after 15-30 min of exposure. This gradual drift was attributed to surface passivating from reactive hydrazine or vapor condensation on the inlet surfaces or membrane and closely paralleled trends observed from the passage of hydrazine vapors through tubes of comparable materials.16 Intensity of the reactant ion was restored in varying degrees following an exposure event and return to clean air source. The time for this was governed by the level of exposure and varied from 15 min for a low ppb exposures to over 30 min for exposures over 2 ppm (not shown). Results from repetitive exposures to MMH vapors from 119 to 1090 ppb separated by exposures to clean air are shown in Figure 1. A measurable memory effect was evident in the voltage polygon for the clean air exposures and constituted 50-100 mV on a 160-mV base line offset. As expected, the effect was most noticeable after elevated (1090ppb) levels. The effects seemed reversible or minor providing exposures a t low (10-100 ppb) levels. The reproducibility of response for ppb levels of MMH and HZ is given in Table I as peak heights (in mV) corrected for background exposures to purified air. Standard deviations from replicate exposures were 0.5-16 mV, which corresponded to relative standard deviations of 2-16 ?6 The response curve for MMH is shown in Figure 2 and exhibited a working range from 10 to 2000 ppb and a linear range from 50 to 450 ppb with saturated response above 2 ppm. Saturation in ion sources of ion mobility spectrometers is governed by the dynamic but limited reservoir of reactant ions's and had little significance in this work with attention directed toward the 10-100 ppb range. The working range in Figure 2 is larger than previous IMS studies7 and was attributed to inefficiency in yield through the membrane inlet and to surface passivating of the nozzle. Detection limits were estimated as 6 ppb for an AVM with acetone reagent gas using criteria proposed by Long and Winefordnerlg from lo00 scans requiring ca. 15 of signal averaging.
.
(17) Preston, J. M.; Rajadhyax, L. Anal. Chem. 1988,60, 31.
ANALYTICAL CHEMISTRY. VOL. 85. NO. 13. J U Y 1. 1993 10999 3M,
Vonage Polygonlor Clean Air E x p u r e !
/ 300
i
S
._ E
/
v
&2oo a,
I Y
m
a,
a
IW
II
Exposures
Flgmo 1. Signalfwproductionasabarplothomrepetltlveexposues
to MMH vapors. Between exposures of ca. 20 mln to vapor levels (shaded bars). tha AVM we8 supplied puMed ak fw 20 mln and ti-m backgrwnd signal determined (unshaded bars). Both vapor sources and purltied air contained 45% rektbe humldny. Concentrations of exposure are marked. The trend in background signal levels is shown In the upper frame. ACEXNE
WLLXSS%
MrmR
Table 1. Reproducibility of Peak Heights for Production Ions in A V M with Acetone Reagent Ion Chemistry' vapor concn of MMH (in ppb)< avpeakheightb 6 13 25 50 119 227 452 1090
X
11
SD
14
0.5
%RSD
n
12
2
4 3
37 4 10
2
3
57 3 4 3
100 16 16 5
204 13 7 4
376 11 3 3
vapor mncn of HZ (in pphP av peak heightb
110
210
421
X (mV) SD (mV)
148 3 2 4
220 16
208
%RSD n
7 5
24 12 4
a These values were drawn from the experiment shown in Fgurr 3. Peak heighta were subtractad from bane line rasponse to a clean air exposure. Concentrations obtained from Inbraean 4wO freshly cnlibrated against vapor levels confirmed by coulometric analysis.2
Mobility spectra for some anticipated interferences with acetone reagent gas are shown in Figure 3 (solid line) and did not elicit much response a t elevated levels (50+ ppm), except for ammonia. Proton affinities for 2-propanol and the halocarbon mixture are low compared to acetone," and product ions were not expected. However, exposure to these vapors caused a slight broadening in the reactant ion peak, possiblefromclusterreactionsinthedriftregion. The product ion for 450 ppb of MMH was observed in the presence of each interference (dotted line spectra in Figure 3) with alight quantitative variation in peak height. However, the product ion for ammonia was coincident with that for MMH and HZ. This constituted an unacceptable condition due to the prevalence of ammonia in ambient air and enclosed atmospheres, and the possihility of false positives from ammonia with hand-held mobility spectrometers was unwelcome. (18) Siegel, M.W. Atmaaphsric P m m Ionhtian Chemistry; In P@ma Chromofomph>;Can. T.W., Ed.; Plenum Publiabem: New ~~
5
10
15
5
10
15
Drii Time (ms) Flgue 3. Moblllty specrra fmm AVM wllh acetone reagent gas fw potentla1Interferema(solld lima spectra) associsfed with grourd-based activnies of shale and orrboard space shut%. Concentraths of Interferences were ca. 50-100 ppm. Spectra of MMH at 450 ppb In the presence of 50-100 ppm levels of thase impurftles are shown wlth dotted lines and offset. The product ion for MMH 18 shown wlth a vertical dashed line. and the reactant ion peak (RIP) Is (Ac)rH+.All scales furespMlseanddrltttlmeareti-msame. Spectra were collected at KSC. Ammonia is a comparatively small ion, and a fast reduced mobility a t 150 "C with water-based ionization chemistry was previously reported' as 2.80 cm2/V.s, in contrast to 2.43 cm2lV.s for MMH and 2.48 cm2/V.s for HZ. The drift time of ammonia with acetone reagent gas was longer than the reactant ion peak (AchH+ at 25 "C, suggesting a cluster ion such as (A@.",+. The predominant ions of ammonia with acetone ionization chemistry were determined with AF'CI/ tandem MS as the monomer cluster (n= 1). rnlr 76,and the dimer cluster (n = 2), rnlz 134. The coincidence in drift times for MMH or HZ and ammonia could originate from either of two causes: (1) similarities in collisional cross-sections for cluster ions of stable structure or (2) differences between ion-cluster equilibria for ions and molecules traversing the drift region to yield KO values that are weight sums of equilibrium constituenta.18 These possibilities are explored in the next section.
ANALYTICAL CHEMISTRY, VOL. 05, NO. 13, JULY 1, 1993
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Table 11. Effect of Humidity on Peak Heights for MMH Product Ions at Vapor Level of 452 ppb relative humidity (%) peak height (mV) SD (mV) % RSD n 15 40 50 71
196 218 215 215
3 0 5 1
2 0 2 0.4
3 4 4 4
MMH 93 ppb
E's00
.-
1200
' \ ,& Id i:JuL , 1
, ,
600
10
15
10
15
Drift Time (ms) Figure 4. Mobility spectra for MMH and hydrazine from AVM with Snonanone reagent gas chemlstry. Low and medium vapor levels are shown, and product ions for respective vapors are shown with dotted vertical line. A dashed line shows where the product ion for ammonia was observed. The Ion near 15 ms in the MMH was an impurity, possibly hydrazlne. The reactant ion peak (RIP) is principally (5-Non)*.H+.
Effects or interferences from humidity, a potential difficulty in coastal environments such as KSC,were minor as shown in Table 11. IMS of Hydrazine Vapors with 5-Nonanone Reagent Gas. Mobility spectra for MMH and HZ with 5-nonanone (5-Non) reagent gas chemistry are shown in Figure 4 for medium (50-100 ppb) and elevated (300 ppb) vapor levels. The drift times for the principal peak in each mobility spectrum was 13.3 ms for MMH, 15.4 ms for HZ, and 16.3 for ammonia. The mobility spectra did not exhibit monomer peak and dimer peak patterns commonly seen in IMS, and only a single peak was observed throughout the working range. The reactant ion peak was characterized by IMS/MS and was comprised of two major ions which were the monomer ion, 5-Non.H+ (m/z143) and the dimer ion (5-Non)z.H+ (mlz 285) in nearly equal abundance. These ions were subjected to MS/MS analysis, and structures were confirmed. The product ions for each vapor showed three ions including protonated base, MH+, a single ketone-ion cluster, 5-Non.MH+,and a ketone ion cluster with two ketone neutrals, (5-Non)rMH+. The masses from IMS/MS characterization in each case were m/z 47,189, and 331 for MMH; m/z 33,175, and 317 for HZ; and m/z 18,160, and 302 for ammonia. The coexistence of three ions (base and two cluster ions) is evident in Figure 5 (top frame) for HZ, and the mass spectrum shows relative abundance of product ions as base < (5-Non)z.MH+ < 5-Non*MH+. The trend in relative abundance of product ions was also seen for the other vapors. Ions were individually characterized by MS/MS analysis where logical neutral losses (5-Non) confirmed the assignment of (5-Non)z.MH+as shown in Figure 5 (bottom frame). In the IMS/MS characterization of MMH, significant levels of hydrazine were observed supporting the interpretation that the peak at 15.1 ms was a hydrazine impurity. The ions were also examined using CID experiments to measure relative stabilities of ions, and results are shown in
I 10
50
I 1 100
150
I
200
I 250
I II] 300
Mass (m/z) Mass spectra of product Ions for hydrazlne wlth S-nonanone reagent gas (top frame) and mass spectrum from CID fragmentatlon of Ion m/z317 (bottomframe). The presence of the M-H+, 5-Non-MH+, and (S-Non)pMH+Is evldent in the top frame with resldualof the reactant ions at 143 (monomer)and 285 (dimer). In the bottom frame, results from CID show loss of one and two 5-nonanones and the remaining M.H+ Ion for hydrazine. Figure 5.
Table 111. Results from Collision-InducedDissociation (MS/MS) Studies of Ions from MMH, HZ, and Ammonia with 6-Nonanone Reagent Gas ion mass (rnlz) slope0 PA (kcaUmol) 5-Non.NH,+ (5-Non)rNHdt 5-Non*HZ.H+ (5-Non)rHZ.H+ 5-Non.MMH.H+ (5-Non)rMMH.H+
160 302 175 317 189 331
34 183 108 142 134 187
204.0 204.7 214.1
a Slope from plot of In (Z/ZBu) veraua collision gas thickness which serves as relative fragmentation cross-section in units of 10-16 cml.
Table 111. The slopes for plots of In (ion intensity/total ion intensity) versus collision gas thickness provide a measure of relative stability with large slopes for unstable ions. Thus, 5-Non.MH+ ions increased in stability in the trend MMH < HZ < ammonia. Ions of (5-Non)l.MH+ were less stable than 5-Non.MH+ ions and stability increased MMH < ammonia < HZ. The cause for these differences may be found in the structure of the ion as governed by proton affinities. Hiraoka21 found that as proton affiity increased, bond energies of proton-held dimer cations decreased. Hiraoka also found22 that increases in proton affinities caused decreases in exothermicity of solvation of protic hydrogen. Ion Equilibria and Drift Times. Results from the IMS, IMS/MS, and MS/MS studies showed that for a given peak in the mobility spectrum there may exist three separate (21) Hiraoka, K.; Takimoto, H.; Yamabe, S. J.Phys. Chem. 1986,90,
5910.
(22) Hiraoka, K.Can.J. Chem. 1987,65,1258.
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Table IV. E l l e a on Drift Times by bNonanone Vapor Lsveeh in IMS Drift Tube
drift timea (nu)of bnonaue mncn (ppb) 9.4
MMn
nz "a
RIP volofbnonanone in diffwion murce 01L)
37b
6.88 9.04 7.08 11.85 5.57 11.78 6.04 10.80 5 20
48* 9.14 11.95 11.85 10.94
80
58
62
86
9.94 9.57 11.94 12.12 12.48 12.84 12.11 12.41 12.94 10.94 11.01 11.07 100 140 160
The drift tube temperature was ambient (25 *C). b Indieah an internlatad value. e RIP is the reactant ion waL.
ngU.7. R e s p o ~ e oAVMwlth5mnam1eagentgasvaausthe f lw expowe to MMH at several m c e n l r a h s . COncentraHoM are cakulated and are shown In the upper r!ght caner of each tram Tabls V. Effect of Tamrratnrs in Inlet of
setting on
temp ("0 measd in nomle
1 2 3 4 7 8 10 11
22 22 24 26 39 43 56
E-IMS
, 0
,
I o ' A ' "en ' en 13m Concentration of S N o n m (ppb) Flpm8. Drfft times of the reamnt ion. and loM f a MMH. hydrazine. In the drm tube. Note and ammnla fa vapa leveb of 5-ncna~)tm thechangelntheaxrambgt8cale. hconcmfratlmattheemme dght data polnt b 1.2 ppm
equilibria for product ions of each vapor as shown in eqs 1-3. formation of unclustered protonated base
M + C$I,,O.H+
t M*Ht
+ C$IlsO.H
(1)
formation of cluster ion with a single ketone
M + C&ilsO.Ht
=M*C$IlSO.H+
(2)
formation of cluster ion with two ketones
M + (C$IlSO),-H+
M*(C&ils0)2*H+
(3)
The mechanism and structures of intermediates or producta have not been determined. Moreover, additional equilibria could be written for reactions between the reactant ion dimer (bNon)ZH+ to give uncluster protonated base and a cluster with a single ketone. Based on appearance of the mobility spectra and the findings of Preston and Rajadhyax," these equilibria coexist within the ion s w a m traversing the drift region of the ion mobility spectrometer. According to their work, peak shape in the mobility spectrum is not noticeably affected providing the equilibria are fast compared to the speed of ion drift. Consequently,the observed peak mobility is a weighted average of mobilities of individual ions in the ion swarm. As expected for an equilibrium, the relative ion abundance8 should be governed hy both temperature and concentration of the 5-nonanonewhich should exist throughout the entire drift tube of the hand-held mobility spec-
a The
60
full scale b i n )
timetofull d e (rnin)
20 20 14 11 9 3 4 3
36 34 28 21 20 10 13 9
timeto9096
temwraturs was m e a s d in the inlet n d e .
trometers since the ketone was delivered to both source and drift gas flows. The vapor level of the ketone d r a m a t i d y affected drift times as shown in Table IV and Figure 6;major changes occurred in the region from 0 to 9.4 ppb and a t levels greater than 80ppb. All product ions appeared with H+(HzO) source chemistry near 6 ma drift time. A small addition of 5-nonanone caused an immediate shift to nearly 12 ma for HZ and ammonia; MMH underwent a lesser change than this to ca. 9 ms. A further shift occurred a t concentrations of bnonanone greater than 1 ppm, providing the resolution between MMH and ammonia and partial resolution between HZ and ammonia. The trend toward longer drift times with increased levels of 5-nonanoneisconsistentwiththe equilibria written in eqs 1-3. Temperature also affected reduced mobilities (not shown) with a trend toward larger mobilities a t higher temperatures. This was consistent with a shift toward desolvated ions with greater mobilities as expected with the additionalenergyfromtemperature todeclusterthe (5-Non)yMH+ and bNonMH+ ions. The procedures and calculations necessary to measure enthalpies of cluster reactions in I M S 7were considered for use here; however, the procedures were deemed not directly applicable to multiple equilibria per eqs 1-3. Analytical Aspects of Detecting Hydrazines Using Hand-Held Mobility Spectrometers. The speed of response of an AVM when exposed to MMH and HZ vapors with 5-nonanone reagent gas is shown in Figure 7 and was virtually the same as that for an AVM with acetone reagent gas. This was consistent with and attributed to passivating of themembrane and inlet nozzle surfaces which were identical in both AVMs. Times of 15-30 min to stable and maximum response are unacceptably long and could be due to either simple condensation or to material reactivity." The role of condensation was examined by measuring the time to the
1702
ANALYTICAL CHEMISTRY, VOL. 65, NO. 13, JULY 1, 1993
Table VI. Differences in Calibration of AVM with 5-Nonanone Reagent Gas before and after Use on Atlantis for STS-37 peak height (mVp concn (ppb)
MMH 0 9 42 94 304 617
preflight
postflight
difference in percent relative
0 26 63 118 438 805
0 18 64 120 440 772
0 -31 1.6 1.7 0.46 -4.1 av 7.8
0 6.3 14.8 33.3 61.5
0 4.6 13.7 39.2 60.8
0 -27 -7.4 17 -1.1 av 13.1
hydrazine
0 40 96 296 601
Peak heights were corrected for background offset.
level response at elevated temperatures of nozzle and membrane in an E-IMS. At low temperatures from ambient to 40 OC, the time to either 90% full-scale response or to full-scale response were 40 min or longer (Table V). At elevated temperatures of 60"C,the response times were 5 minor lower, providing some evidence that surface adsorption was responsible for the hysteresis seen in Figure 1. A solution to
this behavior will involve careful combinations of materials, design, dimensions, and manufacturing technologies. The calibrations of an AVM with 5-nonanone reagent gas are given in Table VI before and after the launch of Atlantis on mission STS-37. Relative standard deviations were between 2 and 10% in most instances, which is comparable or has lese variation over that seen earlier with the acetonebased AVM in hourly reproducibility trials. The response toward MMH and HZ with 5-nonanone reagent gas was comparable to that in Figure 2 (acetone reagent gas), presumably from the same poor yield across the membrane in the inlet. In summary, the AVM exhibited suitable long term stability and show no obvious failure through the extremes of a shuttle mission, including the launch.
ACKNOWLEDGMENT We wish to acknowledge the laboratory assistance of Zachary Laney and funding by KRUG International, Inc., to NMSU through Contract 50,003. The assistance of John Brokenshire of Graseby Ionics, LM.(Analytical Division), is gratefully recognized as are the comments on the manuscript by A. Peter Snyder and David Blyth. Useful discussion and tandem mass spectrometry work of Robert Ewing and Zeev Karpas are greatfully appreciated.
RECEIVED for review October 26, 1992. Accepted March 9, 1993.
