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Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level
International
ELSEVIER
Journal
of Mass Spectrometry
and Ion Processes
149/150 (1995) 609-619
Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level* A. Hansela, A. Jordana,
R. Holzingera,
P. Prazellera,
W. Vogelb, W. Lindingera,*
aInstitut ftirIonenphysik, Leopold-Franzens Universitiit Innsbruck, Technikerstr. 25. A 6020 Innsbruck, Austria bUniversitiitsklinik fiir Innere Medizin. Leopold-Franzens Universitiir Innsbruck, A 6020 Innsbruck, Austria Received
6 July 1995; accepted
7 July 1995
Abstract A system for trace gas analysis using proton transfer reaction mass spectrometry (PTR-MS) has been developed which allows for on-line measurements of components with concentrations as low as 1 ppb. The method is based on reactions of HsO+ ions, which perform non-dissociative proton transfer to most of the common organic trace constituents but do not react with any of the components present in clean air. Examples of analysis of breath taken from smokers and non-smokers as well as from patients suffering from cirrhosis of the liver, and of air in buildings as well as of ambient air taken at a road crossing demonstrate the wide range of applicability of this method. An enhanced level of acetonitrile in the breath is a most suitable indicator that a person is a smoker. Enhanced levels of propanol strongly indicate that a person has a severe liver deficiency. Keywords:
Breath gas analysis;
Proton
transfer
reaction
1. Introduction
On-line gas analysis based on conventional mass spectrometry using electron impact ionization suffers from the strong fragmentation of molecular ionic species [l]. Especially when a mixture of organic components is to be analyzed, the complexity of breakup patterns puts severe constraints on the quantitative analysis of the concentrations of these components. Recent attempts to use thermal
mass spectrometry
(PTR-MS);
Trace components
charge transfer processes to ionize the neutrals to be analyzed showed some improvement in that the breakup of the molecular components could be significantly reduced [2]. Even better results were obtained by using proton transfer reactions for the ionization of trace constituents [3]. When HsO+ is used as the proton donor, most of the typical organic trace components MiHi (where M is a combination of C, 0, N and S atoms) in air are ionized by proton transfer processes: HsO+ + MiHi * MiH,‘+r + Hz0
* Dedicated
to Professor his 60th birthday. * Corresponding author.
David Smith FRS on the occasion
0168-1176/95/$09.50 0 1995 Elsevier SSDI 0168-l 176(95)04294-6
Science
of
B.V. All rights
in air
(1)
These reactions are invariably fast, whenever they are exoergic, with rate coefficients, k, reserved
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A. Hansel et al./International
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close to the collisional limiting values, k0 E 10e9 cm3 s-’ (k, is the Langevin rate coefficient, kL in the case of non-polar neutrals [4] or the capture rate coefficient k, in the case of polar neutral reactants [5]). Hz0 has a proton affinity of 7.22 eV [6], and common organic molecules have proton affinities in the range from 7 to 9 eV, thus making most of the relevant proton transfer reactions involving H30f ions slightly exoergic, but keeping the exoergicity low enough, so that breakup of the neutrals to be detected only seldom occurs. Proton transfer reaction mass spectrometry (PTR-MS) [3] was demonstrated to be applicable for trace gas analysis by using a conventional selected-ion flow drift tube (SIFDT) [7]. H30f ions originating from a high presure electron impact ion source were preselected by means of a quadrupole mass filter and injected into the drift zone of the Innsbruck SIFDT, which was operated with helium buffer gas. The gas mixture to be analyzed was added through a reactant gas inlet, from which point on H30’ ions were allowed to perform reactive collisions with the respective neutral reactants. The product ions of the reaction were then detected at a downstream ion detection system. From the measured primary and product ion count rates, the densities of the reactive neutral components were calculated in much the same way as rate coefficients are usually determined using small flows of neutral reactants in SIFDT systems [7], with the difference that now the rate coefficients are known and the densities are to be calculated [3]. H30+ is a most suitable proton donor, when trace components in air are to be investigated, because H30f does not react with any of the common constituents in air. They all have proton affinities lower than HZ0 (see Table l), and therefore cannot cause significant loss of H30+. The resonant proton transfer between H30+ and H20 does not alter the density of H30+ ions either, thus
und Ion Processes
149/150 (1995) 609-619
it is not a loss term for H30+. There is, however, some loss of H30f due to association reactions, especially the one with HZ0 to form H30+ - HZ0 due to the ternary reaction
PI H30+ + HZ0 + M %
HsO+ .H*O+M (2)
where M is any neutral partner performing collisional stabilization of an excited complex (H30+. H20)* formed in a binary collision between H30+ and H20. Keeping the drift section at elevated E/N (E being the electric field strength and N the buffer gas number density), drastically reduces this loss process, as k3 has a strongly negative energy dependence [8] and also because of collisional breakup of H30f - HZ0 at elevated E/N. PTR-MS using the Innsbruck SIFDT apparatus has been successfully applied so Table Proton various
1 affinities of common components in clean air and of organic components (from Ref. [6])
Component
He Ne Ar 02 N2 co2 CH4 N2O
co H2O C4H10 H2S
HCN C6H6 C3H6
HCOOH CH30H CH,CN CH3COH C2HSOH CH3COCH3 NH,
Proton
affinities
(kcal mol-‘)
(ev)
42.5 48.1 88.6 100.9 118.2 130.9 132 136.5 141.9 166.5 163.3 170.2 171.4 181.9 179.8 178.8 181.9 188.0 186.6 188.3 196.7 204.0
1.8 2.1 3.8 4.4 5.1 5.1 5.7 5.9 6.2 7.2 7.1 7.4 7.4 7.8 7.8 7.8 7.9 8.1 8.1 8.2 8.5 8.8
A. Hansel et aLlInternational Journal of Mass Spectrometry and Ion Processes 149jl50 (1995) 609-619
far to perform on-line measurements of trace components, such as ethanol, methanol and acetone, in human breath [3,9]. The limits for the quantitative analysis of these components were mole fractions of about 0.1 ppm. In this work we report on a PTR-MS system, the sensitivity of which is increased by two orders of magnitude, so that trace constituents in air of the order of 1 ppb are measurable in on-line experiments. This allows for the investigation of concentrations of components such as benzene, acetonitrile, isoprene, and dimethyl sulfide and others in human breath, also acetaldehyde, formaldehyde, methanol, ethanol and others in buildings as well as in ambient air.
611
they may undergo a reaction A+ + R + products
(3)
and specifically in the case of reactant ions HsO+, these perform a proton transfer reaction (if energetically allowed) HsO+ + R A
RH+ + Hz0
(4)
Therefore, by analogy with the detailed description in Refs. [3] and [7], at the end of the reaction section, bordering an ion detection system (Fig. l), the density of product ions [RH+] is given by [RH+] = [H,OflO(l - e-k(R1f)M [HsO’],,[R]kt (5)
2. Experimental The main goal of building this new system was to achieve a better sensitivity than is possible with the convential SIFDT. Let us therefore make a general remark: in most swarm type experiments [7] and especially in drift experiments (such as a SIFDT), primary (reactant) ions A+ travel through a buffer gas B (see Fig. 1) to which the reactant gas R is added in small amounts, so that the density [B] is much larger than the density [RI. On the way through the reaction region (Z = 0 to Z = L) the ions perform many non-reactive collisions with buffer gas atoms or molecules; however, once they collide with a reactant gas particle, DC electric field metal rings -__--1__--_I ?? . . mass ?? ?? . . spectrometer . . o d ion &Lo. 0 ’ . 0. Ion detection source . . 0 . o system . . 0 ?? I I---------i T-inert buffer gas B reactant neutral R N - 1016cm-3 N - 101~ cm-3
reactant ion *+
, z=o
Fig. 1. Schematic
I
z=L representation
of a drift experiment.
where [H30’10 is the density of H30f ions in absence of reactant neutrals in the buffer gas, and k is the reaction rate constant for the proton transfer reaction (4). Reaction rate constants are reported for many proton transfer processes in the literature [lo], or can be calculated as mentioned above. The typical uncertainties are f20%. t is the average time or “reaction time” the ions spend in the reaction region. Where [R] denotes small densities of trace constituents, then [RH+] < [HsO’] M [H30+10 = constant. The ion detection system measures count rates i(H30’) and i(RH+), which are proportional to the respective densities of these ions. To reach a high sensitivity thus requires generating a high ion count rate i(RH+) per unit density [R] in the gas to be analyzed. This obviously can be achieved by keeping the density [H30’] high and by not diluting the gas to be analyzed in an additional buffer gas like helium, but by using the air itself (which contains the trace constituents to be analyzed) as the buffer gas. This can be done when HsO+ ions are used as the ionic reactant species because these ions do not react with the components of air, as discussed above. With these considerations in mind, a
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A. Hansel et al./Iniernational Journal of’A4a.v Spectrometry and Ion Processes I49/150 (1995) 609-619
system was built, a schematic drawing of which is shown in Fig. 2. The required high density of primary ions HsO+ is provided by means of a hollowcathode ion source, from which ions are extracted into a short “source drift region” filled with water vapor. After passing this small drift section, the ions (H30’) reach a large reaction region which is in the form of a drift section of 22 cm length and 5 cm inner diameter, filled with the air (pressure M 0.2 Torr) containing trace constituents to be analyzed. No further buffer gas is needed and therefore the original mole fraction of R in air is retained in the reaction region. On the way from the Venturi type inlet [7] to the downstream end of the drift section, HsO+ ions undergo non-reactive collisions with any of the common components in air (see Table I), but a small fraction (typically in the order of a percent) react with trace constituents. Before going into more detail about the reactions occurring in the reaction region, let us ask why there are only H30+ ions entering this region. Hollow-cathode discharges, which have been investigated in detail in this laboratory [l l-l 31, have proven to be intensive
pump +
pump 1‘
Hz0 -vapor
[air to be analyzed]
inlets
ion source
Fig. 2. Schematic
representation
sources of secondary and tertiary ions. Within the axial region of a cylindrical cathode, there is the so-called negative glow, a nearly fieldfree cylindrical region the potential of which is close to anode potential. Between this negative glow and the cathode is the “cathode fall”, a potential drop of typically a few hundred volts, where ions are produced by highly energetic (a few hundred electronvolts) electrons colliding with neutrals. These primary ions undergo many collisions with the neutral gas in the hollow cathode, thereby having ample chance to undergo reactions to form secondary and tertiary products, till they finally diffuse toward the edge of the field-free negative glow, after which they traverse the cathode fall region, gaining enough energy to release secondary electrons upon their impact on the metallic cathode wall. These electrons in turn are attracted by the cathode fall and gain a few hundred electronvolts kinetic energy, which enables them to make several ionizing collisions within the negative glow region. Thus a plasma of relatively high density (lO1o-lO” cmP3) is retained. In both axial directions the negative glow is limited by anode plates, one of which has a small hole
high vacuum
drift tube
of the present
PTR-MS
ion detection system
system: HC, hollow cathode;
SD, drift region; VI, Venturi
type inlet.
A. Hansel et al./International Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 609-619
(diameter 1.0 mm) through which ions leave toward the adjacent “source drift section”. When HZ0 is used as a discharge gas in the hollow cathode, most of the ions emerging from the negative glow are already in the form of H30f. As Table 2 shows, all the ions produced by electron impact from H20, like O+, H+, Hl, OH+ and H20+ react with HZ0 with collisional rate constants and finally end up as HsO+ ions. The small fraction of primary ions produced by electron impact ionization within or outside the anode hole are allowed to undergo reactive collisions with HZ0 in the “source drift region”, so they also end up largely as H30 + ions. This hollow-cathode ion source provides H30f ions with a purity of about 99.5% or better. This situation has two advantages. High concentrations and therefore count rates of primary H30+ ions are obtained in the ion detection system (typical count rates are several lo5 counts SC’) and no quadrupole system needs to be installed to preselct the reactant ions HsO+ before entering the reaction region of the system. The only significant impurity ions observed are 0: ions, which are produced within the “source drift region” due to the charge transfer from H2Of ions to O2 diffusing from the reaction region toward the ion source system, or by direct electron impact ionization of OZ. As 0; does not react with HZ0 [lo] it is not converted into another ionic Table 2 Reactions of ions, which arc produced by electron H20, with Hz0 (data are taken from Ref. [IO])
impact
upon
Reaction
Rate constant (cm3 SC’)
0+ H+
2.6 8.2 3.4 3.7 1.3 1.8 1.8
+HzO+HzO++O +H20+H20++H +H20+H30++H H; + H20+ + OH+ + Hz0 + H30+ + + H20+ + HzO+ + H,O --* H30+ +
H2 0 OH OH
x x x x x x x
10m9 10-9 10-9 10-9 10-9 10m9 lo-’
k
613
form, once it is produced in an HZ0 environment. In contrast, Nz or N+ is rapidly converted into H30t in successive reactions with H20. Let us return to the reactions of H30+ in the reaction region. The last important quantity we have to obtain in order to calculate the density [R] according to Eq. (5) is the reaction time t, the time the HsO+ ions take to traverse the length of the drift tube. t can be measured directly, either by pulsing the ions at the entrance and at the exit of the drift tube and monitoring the arrival time spectrum, or by calculating it from mobility values, CL, of HsO+ in air, reported in the literature [14]. The drift velocity ud of the ions, which is obtained from the relation vd = pE, was always kept high enough so that it was large compared to the flow velocity of the air (to be analyzed) through the drift tube. The mean relative kinetic energy ICE,, between HsO+ and neutral reactants was calculated when necessary according to standard procedures as described by McFarland et al
[151. Usually, throughout the measurements, E/N and KEcm were kept at values of 120 Td and about 0.25 eV respectively, which is a good compromise between avoiding on the one hand formation of too much HsO+ .H20, and higher clusters HsO+ 9 (H20)n, n being 2,3,. . .) the reaction of which with the neutrals would obscure the data due to switching reactions [ 161 H30+
- (I&O),
x R+H20
+ R + H30+.
n=
1,2,3...
(H20)n-1 (6)
and on the other hand breaking up of product ions due to collisions with neutrals in the drift section [17,18]. Under these circumstances typical count rates i(RH+) of 1 s-r per ppb are reached. For many masses the background is of the order of 0.3-0.6 counts s-l, so that
614
A. Hansel et al./Intemational Journal
concentrations good accuracy.
qf Mass Spectrornetry and Ion Processes 149/150 (1995) 609-619
of 1 ppb are measurable
3. Distinguishing the same mass
with
between ionic components of
In special cases a variation of E/N is quite useful for the identification of ions. Fig. 3 shows mass scans at different E/N values. The strong increase of mass 55 with decreasing E/N easily allows it to be identified as H30+(H20)2, being produced in association processes at low E/N, whereas mass 59 does not increase strongly, being protonated acetone, which is produced in the binary proton transfer from HsO+ to acetone. A second way to help identify ionic components is the observation of isotopic abundances. Mass 69, which is observed in the analysis of human breath may be protonated isoprene (CsH,‘) or the mixed water-
methanol cluster ion CH30H - (H20)2H+. At E/N = 120 Td, six cycles of count rates for masses 69 and 70 were obtained as shown in Table 3, yielding an average isotopic mass ratio of M69/M70 = 94.1%/5.9% which is quite close to the calculated one (94.4%/ 5.4%), assuming that the molecule in question contains five carbon atoms, and is thus isoprene. At lower E/N the mass ratio M69/ M70 changed toward 97.4512.55 at E/N = 18.5 Td, indicating that a substantial amount of CH30H(H20)2Hf was obscuring the measurement. In this specific case neglecting this contribution would result in an error in the obtained density of isoprene by a factor of 15. Finally, different ionic constituents having the same mass or even the same ionic composition but different isomeric structures may be identified due to their different mobilities or drift velocities in the buffer gas. Pulsing of the H30f ions at the entrance to the reaction
Et03 1
Et01 k 54
Fig. 3. Mass scans from M54 to M60.5 taken at different values of E/N. 8 denotes E/N = 26.4 Td; 7, 35.2 Td; 6, 45.6 Td; 5, 59.2 Td; 4,75.2 Td; 3,92.8 Td; 2, 107.2 Td and 1, 120 Td. The much stronger increase of M55 as compared to the increase of M59 with decrea .sing IS p ro d wed in a ternary reaction, while M59, protonated acetone, is a binary product. E/N indicates that M55 (HjO+(H20),)
A. Hansel et al./International Journal of Mass Spectrometry and Ion Processes 1491150 (1995) 609-619 Table 3 Count rates (counts s-r) for masses 69 and 70 and corresponding isotopic abundances (in percent) obtained in five count cycles
fragment
615
ions:
H30+ + C3H70H
+ (C3H70H)H+
+ H20 30%
Cycle
Count
1 2 3 4 5 Average
l-5
rates
Isotopic abundances
--+ C3HT + products
M69
M70
M69
M70
86.11 88.88 83.38 84.98 86.58
5.46 5.74 5.41 5.36 5.21
90.04 93.93 93.90 94.07 94.32
5.96 6.07 6.10 5.93 5.68
85.98
5.44
94.06
5.94
region and detection of the arrival time spectrum of the product ions at the end of the drift section leads to results as shown in Fig. 4. The data show significantly differing spectra for protonated dimethyl ether, ethanol and formic acid, all appearing at mass 47. These results can be used as references to compare the arrival time spectrum of an unknown ionic species of mass 47 in order to help indentify it. There are still cases where proton transfer from H30+ ions to neutrals result in fragmentation of the product ions. At thermal energies the reaction of HsO+ with iso-propanol leads to both protonated propanol ions and C3HT
0 100
150
200
250
300
350
400
450
500
time (in units of 10%) Fig. 4. Arrival time spectra of the primary ion H,O+ and of product ions (protonated dimethyl ether, ethanol and formic acid respectively), all appearing at mass 47.
(7) 70%
(8) As Fig. 5 shows, the fraction of C3Hf product ions reaches 100% at mean relative kinetic energies, KE,,, approximately 1.5 eV. With this in mind and remembering that performs switching processes HjO+ - Hz0 with n-propanol, propanol could be unambiguously detected in human breath and its concentration was measured in many test persons as will be shown below.
4. Results 4.1. Trace components
in human breath
4.1 .I. Persons suffering from liver cirrhosis It is common knowledge that acetone is enhanced in the blood, urine and breath of persons suffering from diabetes [19], and monitoring of acetone has become a useful tool for the diagnosis of this illness. In the
E,, WI Fig. 5. Dependent of the relative abundances of the product ions (C,H,OH)H’ and CsHT resulting from proton transfer from HsO+ to propanol on the mean relative kinetic energy of the reactants.
616
A. Hansel et al./International
Journal of Mass Spectrometry
100 200 300 400 concentration of isoprene [ppb]
Fig. 6. Concentrations of isoprene in the breath of healthy persons (divided into smokers and non-smokers) and in the breath of patients suffering from cirrhosis of the liver.
same way it would be highly desirable to find other components which act as an indicator for specific illnesses. It is especially difficult to know about the actual state of the human liver by using only non-intrusive methods. We have compared the concentrations of methanol, ethanol, isoprene, butanone and propanol in the breath of healthy persons and patients suffering from liver cirrhosis respectively. There was no significant difference in the contents of methanol and ethanol in the two groups of persons and in view of
and Ion Processes 149/150 (1995) 609-619
existing detailed reports on methanol and ethanol in the blood and breath of humans [9,19 and references cited therein], there is no need for further discussion of these data. We therefore will concentrate on the other components mentioned above. Results on the contents of isoprene in the breath of healthy persons, divided into smokers and nonsmokers, are shown in Fig. 6. The average concentration of isoprene in smokers and non-smokers is about 250 ppb and the concentrations in persons suffering from liver cirrhosis seem to be slightly lower, but not significantly so. The present values agree satisfactorily with average values of 150 ppb obtained by Mendis et al. [20] by means of gas chromatography. Fig. 7 presents concentrations of butanone in the same sets of test persons as for Fig. 6. While healthy persons (smokers and non-smokers) show values typically from 5 to 25 ppb, the concentrations in several liver patients reach values up to more than 40 and 50 ppb. Again the overlap between the concentrations in healthy persons and liver patients is still too big to allow for a significant differentiation between the two groups. In contrast, propanol shows quite significantly enhanced densities in the breath of cirrhosis patients compared with healthy
6 4 m 5
6
2 0
4
$2 20 =
6 4 2 0
concentration
of butanone
[ppb]
Fig. 7. Concentrations of butanone in the breath of healthy persons (divided into smokers and non-smokers) and in the breath of patients suffering from cirrhosis of the liver.
concentration Fig. 8. Concentrations non-smokers.
of propanol
of propanol
[ppb]
in the breath of smokers and
617
A. Hansel et al./Iniernational Journal of Mass Speciromerry and Ion Processes 149/150 (1995) 609-619
35 30
??healthy persons
D
25 I--r r-l 20 2
nonsmoker
15
5: 10 $ a5
.n n,~.~.,....,..~.,..;; u-, I...
0
500
1000
1500
2000
2500
3000
6000
xw
0
$
a
E I?
6
m
smoker, smoking lor 2 cigaretts per day
%i 0
5
10
15
20
25
30
concentration of acetonitrile [ppb]
concentration of propanol [ppb] Fig. 9. Concentrations of propanol in the breath of 41 healthy persons (same as in Fig. 8) and of 15 patients suffering from cirrhosis of the liver.
Fig. 11. Concentrations of acetonitrile in the breath of nonsmokers (same as in Fig. 10) and of smokers, smoking l-2 cigarettes per day.
subjects. While all measured concentrations of 41 healthy persons (Fig. g), both smokers and non-smokers, were below 300 ppb, only two out of 15 patients suffering from liver cirrhosis had concentrations below 300 ppb and 12 had concentrations between 300 and 3000 ppb, one even more than 6000 ppb (Fig. 9). These data indicate that PTR-MS might be used for screening purposes in order to detect persons suffering from severe liver deficiencies.
acetonitrile in the breath of smokers and nonsmokers [21] using this new PTR-MS system and could show that acetonitrile is a most suitable indicator of whether a given subject is a smoker or not. The concentration of acetonitrile in the breath of smokers is typically 30-100 ppb while that for non-smokers is only about 5 to 10 ppb (Fig. 10). Even if someone smokes only 1 or 2 cigarettes per day, the acetonitrile concentration is enhanced significantly over that for non-smokers (Fig. 11). Wester et al. [22] have reported on
4.1.2. Acetonitrile in the breath of smokers and non-smokers Recently we have reported on the analysis of
B smokes 5 cigarettes
time [hours] 50
100
150
200
concentration of acetonitrile [ppb] Fig. 10. Concentrations of acetonitrile in the breath of smokers and non-smokers (taken from Ref. [21]), smoking more than 10 cigarettes per day.
Fig. 12. Concentrations of acetonitrile in the breath of two smokers, who stopped smoking at time 0. Person B started smoking again after about 100 h, while person A (one of the authors, A.J.) continued to abstain from smoking till concentrations of acetonitrile as low as for non-smokers were reached after about 7 days.
618
A. Hansel et al./International
Journal of Mass Spectrometry
enhanced levels of benzene in the breath of smokers; however after someone has smoked a cigarette, the benzene concentration in the breath decreases to normal values within about I h [21]. In contrast, the concentration of acetonitrile takes about one week to decrease to values typical for non-smokers (see Fig. 12); thus even if someone does not smoke a cigarette for several days, this person can still be identified as being a smoker. 4.2. Formaldehyde, methanol, ethanol, propanol, acetaldehyde, acetone and toluene in the air of typical homes Air was taken from five rooms in four different homes in Innsbruck and mass spectra were taken by passing this air through the PTR-MS system. All rooms are in buildings which were constructed more than 20 years ago. The furniture in three of these rooms is at least 10 years old. Two rooms have been equipped with new furniture and carpets only two months before the measurements were done. The sampling of air was done by means of a pre-evacuated glass vessel of about 5 1 volume. The windows of the rooms under investigation were kept closed for about 2-3 h before the sampling was done. The above-mentioned components were identified unambiguously by PTR-MS, but also many other components were observed, the identification of which has not yet been done. Table 4 shows the measured concentrations
Table 4 Concentrations
(in ppb) of various
components
in the air sampled
and Ion Processes 149/150 (1995) 609-619
of the components which were unambiguously identified. The concentrations of formaldehyde and ethanol are quite similar in the old and newly furnished rooms. In contrast, all other components are strongly enhanced (by at least an order of magnitude) in the two newly furnished rooms, reaching concentrations as high as about 5000 ppb for methanol, about 100 ppb for toluene, about 650 ppb for acetone and about 400 ppb for propanol. It is quite interesting to note that four of the five rooms investigated show concentrations of formaldehyde higher than 24 ppb, which is the maximum concentration allowed by Austrian law for humans to be permanently exposed to. 4.3. Trace components crossing
in the air taken at a road
In the same way as air was sampled in various rooms, we also took air from a road crossing, which is typically frequented by 4060 cars per minute. The air temperature at the time of sampling was 20°C and there was moderate wind of about 3 m SK’. The following concentrations were measured (all in units of ppb): formaldeyde, 48; acetaldehyde 40; methanol, 14; ethanol, 70; butanol, 15; benzene, 28; pentanol, 9; toluene, 53; xylene, 13. Many other com60; diethylbenzene, ponents still have to be identified, the sum of which certainly exceeds the sum concentration of the above components.
from five rooms
Room
Formaldehyde
Acetaldehyde
Methanol
Ethanol
Propanol
Acetone
Toluene
1 (new) 2 (new) 3 (old) 4 (old) 5 (old)
27 61 40 42 4
195 150 51 33 20
1360 5342 59 98 53
612 398 501 642 501
399 269 36 38 20
596 657 39 69 40
72 97 5 6 3
Room
1 and 2 were newly furnished
2 months
before the measurements
were done; Rooms
3-5 had old furniture.
A. Hansel et al./International
Journal of Mass Spectrometry
Conclusion
It was the aim of this work to improve the sensitivity of the gas analyzing method PTRMS (which previously had a sensitivity suitable for measuring trace components on the ppm level or slightly below) by building a system operating with a hollow-cathode ion source, which allows us to produce a high density of reactant ions H30+, and injecting them into a drift reaction region without mass spectrometric preselection. Furthermore, the air to be analyzed itself acts as the buffer gas; therefore no dilution by any buffer gas is needed, so that the trace components to be analyzed are present at their original mole fraction. In this way the rates of reaction of HsO+ with the trace components under investigation are large enough to yield product ion count rates of 1 count s-l per ppb of typical organic components and therefore on-line measurements of trace constituents at the ppb level can be done simultaneously for a large variety of components. Thus the goal of the present work is fully reached in that the sensitivity of PTR-MS was increased by more than two orders of magnitude.
Acknowledgment
This project (P10360) was rejected by the Austrian Science Foundation (Fonds zur Forderung der Wissenschaftlichen Forschung).
and Ion Processes
1491150 (1995) 609-619
619
References PI T.D.
Mark, in L.G. Christophorou (Ed.), ElectronMolecule Interations and their Applications, Vol. 1, Academic Press, New York, 1984, pp. 251-334. PI W. Lindinger, J. Hirber and H. Paretzke, Int. J. Mass Spectrom. Ion Processes, 129 (1993) 79. [31 A. Lagg, J. Taucher, A. Hansel and W. Lindinger, Int. J. Mass Spectrom. Ion Processes, 134 (1994) 55. and D.P. Stevenson, J. Chem. Phys., 29 141 G. Gioumousis (1993) 294. 151 T. Su and W.J. Chesnavish, J. Chem. Phys., 76 (1982) 5182. WI S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (Suppl. 1) 1988. 171 W. Lindinger, in J.H. Futrell (Ed.), Gaseous Ion Chemistry and Mass Spcctrometry, Wiley, New York, 1986, Chapter 7. PI V.M. Bierbaum, M.F. Golde and F. Kaufman, J. Chem. Phys., 65 (1976) 2715. t91 J. Taucher, A. Lagg, A. Hansel, W. Vogel and W. Lindinger, Alcoholism: Clin. Exp. Res., 19 (1995) in press. t101 Y. Ikezoe, S. Matsuoka and A. Viggiano, Gas Phase IonMolecule Reaction Rate Constants through 1986, Maruzen Company, Ltd., Tokyo, 1987. Pll M. Pahl, W. Lindinger and F. Howorka, Z. Naturforsch., Teil A, 27 (1972) 678. P21 W. Lindinger, Phys. Rev. A, 7 (1973) 328. W. Lindinger and M. Pahl, Int. J. Mass u31 F. Howorka, Spectrom. Ion Phys., 12 (1973) 67. 1141 H. Ellis, R. Pai, E. McDaniel, E. Mason and L. Viehland, At. Data Nucl. Data Tables, 17 (1976) 77. D.L. Albritton, F.C. Fehsenfeld, E.E. Cl51 M. McFarland, Ferguson and A.L. Schmeltekopf, J. Chem. Phys., 59 (1973) 6610, 6620. U61 C. Prazmarer, A. Hansel, A. Jordan, H. Kraus and W. Lindinger, Int. J. Mass Spectrom. Ion Processes, 129 (1993) 121. 1171 A. Viggiano, F. Dale and J.F. Paulson, J. Chem. Phys., 88 (1988) 2469. [181 Y.K. Lau, S. Ikuta and P. Kabarle, J. Am. Chem. Sot., 104 (1992) 1462. 1191 W. Wirth and C. Gloxhuber, Toxikologie, Georg Thieme Verlag, Stuttgart, 1994. PO1 S. Mendis, P. Sabatka and D. Euler, Clin. Chem., 40 (1994) 8. Pll A. Jordan, A. Hansel, R. Holzinger and W. Lindinger, Int. J. Mass Spectrom. Ion Processes, in press. P21 R.C. Wester, HI. Maibach, L.D. Gruenke and J.C. Craig, J. Toxicol. Environ. Health, 14 (1986) 567.
ELSEVIER
Journal
of Mass Spectrometry
and Ion Processes
149/150 (1995) 609-619
Proton transfer reaction mass spectrometry: on-line trace gas analysis at the ppb level* A. Hansela, A. Jordana,
R. Holzingera,
P. Prazellera,
W. Vogelb, W. Lindingera,*
aInstitut ftirIonenphysik, Leopold-Franzens Universitiit Innsbruck, Technikerstr. 25. A 6020 Innsbruck, Austria bUniversitiitsklinik fiir Innere Medizin. Leopold-Franzens Universitiir Innsbruck, A 6020 Innsbruck, Austria Received
6 July 1995; accepted
7 July 1995
Abstract A system for trace gas analysis using proton transfer reaction mass spectrometry (PTR-MS) has been developed which allows for on-line measurements of components with concentrations as low as 1 ppb. The method is based on reactions of HsO+ ions, which perform non-dissociative proton transfer to most of the common organic trace constituents but do not react with any of the components present in clean air. Examples of analysis of breath taken from smokers and non-smokers as well as from patients suffering from cirrhosis of the liver, and of air in buildings as well as of ambient air taken at a road crossing demonstrate the wide range of applicability of this method. An enhanced level of acetonitrile in the breath is a most suitable indicator that a person is a smoker. Enhanced levels of propanol strongly indicate that a person has a severe liver deficiency. Keywords:
Breath gas analysis;
Proton
transfer
reaction
1. Introduction
On-line gas analysis based on conventional mass spectrometry using electron impact ionization suffers from the strong fragmentation of molecular ionic species [l]. Especially when a mixture of organic components is to be analyzed, the complexity of breakup patterns puts severe constraints on the quantitative analysis of the concentrations of these components. Recent attempts to use thermal
mass spectrometry
(PTR-MS);
Trace components
charge transfer processes to ionize the neutrals to be analyzed showed some improvement in that the breakup of the molecular components could be significantly reduced [2]. Even better results were obtained by using proton transfer reactions for the ionization of trace constituents [3]. When HsO+ is used as the proton donor, most of the typical organic trace components MiHi (where M is a combination of C, 0, N and S atoms) in air are ionized by proton transfer processes: HsO+ + MiHi * MiH,‘+r + Hz0
* Dedicated
to Professor his 60th birthday. * Corresponding author.
David Smith FRS on the occasion
0168-1176/95/$09.50 0 1995 Elsevier SSDI 0168-l 176(95)04294-6
Science
of
B.V. All rights
in air
(1)
These reactions are invariably fast, whenever they are exoergic, with rate coefficients, k, reserved
610
A. Hansel et al./International
Journal of Mass Spectrometry
close to the collisional limiting values, k0 E 10e9 cm3 s-’ (k, is the Langevin rate coefficient, kL in the case of non-polar neutrals [4] or the capture rate coefficient k, in the case of polar neutral reactants [5]). Hz0 has a proton affinity of 7.22 eV [6], and common organic molecules have proton affinities in the range from 7 to 9 eV, thus making most of the relevant proton transfer reactions involving H30f ions slightly exoergic, but keeping the exoergicity low enough, so that breakup of the neutrals to be detected only seldom occurs. Proton transfer reaction mass spectrometry (PTR-MS) [3] was demonstrated to be applicable for trace gas analysis by using a conventional selected-ion flow drift tube (SIFDT) [7]. H30f ions originating from a high presure electron impact ion source were preselected by means of a quadrupole mass filter and injected into the drift zone of the Innsbruck SIFDT, which was operated with helium buffer gas. The gas mixture to be analyzed was added through a reactant gas inlet, from which point on H30’ ions were allowed to perform reactive collisions with the respective neutral reactants. The product ions of the reaction were then detected at a downstream ion detection system. From the measured primary and product ion count rates, the densities of the reactive neutral components were calculated in much the same way as rate coefficients are usually determined using small flows of neutral reactants in SIFDT systems [7], with the difference that now the rate coefficients are known and the densities are to be calculated [3]. H30+ is a most suitable proton donor, when trace components in air are to be investigated, because H30f does not react with any of the common constituents in air. They all have proton affinities lower than HZ0 (see Table l), and therefore cannot cause significant loss of H30+. The resonant proton transfer between H30+ and H20 does not alter the density of H30+ ions either, thus
und Ion Processes
149/150 (1995) 609-619
it is not a loss term for H30+. There is, however, some loss of H30f due to association reactions, especially the one with HZ0 to form H30+ - HZ0 due to the ternary reaction
PI H30+ + HZ0 + M %
HsO+ .H*O+M (2)
where M is any neutral partner performing collisional stabilization of an excited complex (H30+. H20)* formed in a binary collision between H30+ and H20. Keeping the drift section at elevated E/N (E being the electric field strength and N the buffer gas number density), drastically reduces this loss process, as k3 has a strongly negative energy dependence [8] and also because of collisional breakup of H30f - HZ0 at elevated E/N. PTR-MS using the Innsbruck SIFDT apparatus has been successfully applied so Table Proton various
1 affinities of common components in clean air and of organic components (from Ref. [6])
Component
He Ne Ar 02 N2 co2 CH4 N2O
co H2O C4H10 H2S
HCN C6H6 C3H6
HCOOH CH30H CH,CN CH3COH C2HSOH CH3COCH3 NH,
Proton
affinities
(kcal mol-‘)
(ev)
42.5 48.1 88.6 100.9 118.2 130.9 132 136.5 141.9 166.5 163.3 170.2 171.4 181.9 179.8 178.8 181.9 188.0 186.6 188.3 196.7 204.0
1.8 2.1 3.8 4.4 5.1 5.1 5.7 5.9 6.2 7.2 7.1 7.4 7.4 7.8 7.8 7.8 7.9 8.1 8.1 8.2 8.5 8.8
A. Hansel et aLlInternational Journal of Mass Spectrometry and Ion Processes 149jl50 (1995) 609-619
far to perform on-line measurements of trace components, such as ethanol, methanol and acetone, in human breath [3,9]. The limits for the quantitative analysis of these components were mole fractions of about 0.1 ppm. In this work we report on a PTR-MS system, the sensitivity of which is increased by two orders of magnitude, so that trace constituents in air of the order of 1 ppb are measurable in on-line experiments. This allows for the investigation of concentrations of components such as benzene, acetonitrile, isoprene, and dimethyl sulfide and others in human breath, also acetaldehyde, formaldehyde, methanol, ethanol and others in buildings as well as in ambient air.
611
they may undergo a reaction A+ + R + products
(3)
and specifically in the case of reactant ions HsO+, these perform a proton transfer reaction (if energetically allowed) HsO+ + R A
RH+ + Hz0
(4)
Therefore, by analogy with the detailed description in Refs. [3] and [7], at the end of the reaction section, bordering an ion detection system (Fig. l), the density of product ions [RH+] is given by [RH+] = [H,OflO(l - e-k(R1f)M [HsO’],,[R]kt (5)
2. Experimental The main goal of building this new system was to achieve a better sensitivity than is possible with the convential SIFDT. Let us therefore make a general remark: in most swarm type experiments [7] and especially in drift experiments (such as a SIFDT), primary (reactant) ions A+ travel through a buffer gas B (see Fig. 1) to which the reactant gas R is added in small amounts, so that the density [B] is much larger than the density [RI. On the way through the reaction region (Z = 0 to Z = L) the ions perform many non-reactive collisions with buffer gas atoms or molecules; however, once they collide with a reactant gas particle, DC electric field metal rings -__--1__--_I ?? . . mass ?? ?? . . spectrometer . . o d ion &Lo. 0 ’ . 0. Ion detection source . . 0 . o system . . 0 ?? I I---------i T-inert buffer gas B reactant neutral R N - 1016cm-3 N - 101~ cm-3
reactant ion *+
, z=o
Fig. 1. Schematic
I
z=L representation
of a drift experiment.
where [H30’10 is the density of H30f ions in absence of reactant neutrals in the buffer gas, and k is the reaction rate constant for the proton transfer reaction (4). Reaction rate constants are reported for many proton transfer processes in the literature [lo], or can be calculated as mentioned above. The typical uncertainties are f20%. t is the average time or “reaction time” the ions spend in the reaction region. Where [R] denotes small densities of trace constituents, then [RH+] < [HsO’] M [H30+10 = constant. The ion detection system measures count rates i(H30’) and i(RH+), which are proportional to the respective densities of these ions. To reach a high sensitivity thus requires generating a high ion count rate i(RH+) per unit density [R] in the gas to be analyzed. This obviously can be achieved by keeping the density [H30’] high and by not diluting the gas to be analyzed in an additional buffer gas like helium, but by using the air itself (which contains the trace constituents to be analyzed) as the buffer gas. This can be done when HsO+ ions are used as the ionic reactant species because these ions do not react with the components of air, as discussed above. With these considerations in mind, a
612
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system was built, a schematic drawing of which is shown in Fig. 2. The required high density of primary ions HsO+ is provided by means of a hollowcathode ion source, from which ions are extracted into a short “source drift region” filled with water vapor. After passing this small drift section, the ions (H30’) reach a large reaction region which is in the form of a drift section of 22 cm length and 5 cm inner diameter, filled with the air (pressure M 0.2 Torr) containing trace constituents to be analyzed. No further buffer gas is needed and therefore the original mole fraction of R in air is retained in the reaction region. On the way from the Venturi type inlet [7] to the downstream end of the drift section, HsO+ ions undergo non-reactive collisions with any of the common components in air (see Table I), but a small fraction (typically in the order of a percent) react with trace constituents. Before going into more detail about the reactions occurring in the reaction region, let us ask why there are only H30+ ions entering this region. Hollow-cathode discharges, which have been investigated in detail in this laboratory [l l-l 31, have proven to be intensive
pump +
pump 1‘
Hz0 -vapor
[air to be analyzed]
inlets
ion source
Fig. 2. Schematic
representation
sources of secondary and tertiary ions. Within the axial region of a cylindrical cathode, there is the so-called negative glow, a nearly fieldfree cylindrical region the potential of which is close to anode potential. Between this negative glow and the cathode is the “cathode fall”, a potential drop of typically a few hundred volts, where ions are produced by highly energetic (a few hundred electronvolts) electrons colliding with neutrals. These primary ions undergo many collisions with the neutral gas in the hollow cathode, thereby having ample chance to undergo reactions to form secondary and tertiary products, till they finally diffuse toward the edge of the field-free negative glow, after which they traverse the cathode fall region, gaining enough energy to release secondary electrons upon their impact on the metallic cathode wall. These electrons in turn are attracted by the cathode fall and gain a few hundred electronvolts kinetic energy, which enables them to make several ionizing collisions within the negative glow region. Thus a plasma of relatively high density (lO1o-lO” cmP3) is retained. In both axial directions the negative glow is limited by anode plates, one of which has a small hole
high vacuum
drift tube
of the present
PTR-MS
ion detection system
system: HC, hollow cathode;
SD, drift region; VI, Venturi
type inlet.
A. Hansel et al./International Journal of Mass Spectrometry and Ion Processes 149/150 (1995) 609-619
(diameter 1.0 mm) through which ions leave toward the adjacent “source drift section”. When HZ0 is used as a discharge gas in the hollow cathode, most of the ions emerging from the negative glow are already in the form of H30f. As Table 2 shows, all the ions produced by electron impact from H20, like O+, H+, Hl, OH+ and H20+ react with HZ0 with collisional rate constants and finally end up as HsO+ ions. The small fraction of primary ions produced by electron impact ionization within or outside the anode hole are allowed to undergo reactive collisions with HZ0 in the “source drift region”, so they also end up largely as H30 + ions. This hollow-cathode ion source provides H30f ions with a purity of about 99.5% or better. This situation has two advantages. High concentrations and therefore count rates of primary H30+ ions are obtained in the ion detection system (typical count rates are several lo5 counts SC’) and no quadrupole system needs to be installed to preselct the reactant ions HsO+ before entering the reaction region of the system. The only significant impurity ions observed are 0: ions, which are produced within the “source drift region” due to the charge transfer from H2Of ions to O2 diffusing from the reaction region toward the ion source system, or by direct electron impact ionization of OZ. As 0; does not react with HZ0 [lo] it is not converted into another ionic Table 2 Reactions of ions, which arc produced by electron H20, with Hz0 (data are taken from Ref. [IO])
impact
upon
Reaction
Rate constant (cm3 SC’)
0+ H+
2.6 8.2 3.4 3.7 1.3 1.8 1.8
+HzO+HzO++O +H20+H20++H +H20+H30++H H; + H20+ + OH+ + Hz0 + H30+ + + H20+ + HzO+ + H,O --* H30+ +
H2 0 OH OH
x x x x x x x
10m9 10-9 10-9 10-9 10-9 10m9 lo-’
k
613
form, once it is produced in an HZ0 environment. In contrast, Nz or N+ is rapidly converted into H30t in successive reactions with H20. Let us return to the reactions of H30+ in the reaction region. The last important quantity we have to obtain in order to calculate the density [R] according to Eq. (5) is the reaction time t, the time the HsO+ ions take to traverse the length of the drift tube. t can be measured directly, either by pulsing the ions at the entrance and at the exit of the drift tube and monitoring the arrival time spectrum, or by calculating it from mobility values, CL, of HsO+ in air, reported in the literature [14]. The drift velocity ud of the ions, which is obtained from the relation vd = pE, was always kept high enough so that it was large compared to the flow velocity of the air (to be analyzed) through the drift tube. The mean relative kinetic energy ICE,, between HsO+ and neutral reactants was calculated when necessary according to standard procedures as described by McFarland et al
[151. Usually, throughout the measurements, E/N and KEcm were kept at values of 120 Td and about 0.25 eV respectively, which is a good compromise between avoiding on the one hand formation of too much HsO+ .H20, and higher clusters HsO+ 9 (H20)n, n being 2,3,. . .) the reaction of which with the neutrals would obscure the data due to switching reactions [ 161 H30+
- (I&O),
x R+H20
+ R + H30+.
n=
1,2,3...
(H20)n-1 (6)
and on the other hand breaking up of product ions due to collisions with neutrals in the drift section [17,18]. Under these circumstances typical count rates i(RH+) of 1 s-r per ppb are reached. For many masses the background is of the order of 0.3-0.6 counts s-l, so that
614
A. Hansel et al./Intemational Journal
concentrations good accuracy.
qf Mass Spectrornetry and Ion Processes 149/150 (1995) 609-619
of 1 ppb are measurable
3. Distinguishing the same mass
with
between ionic components of
In special cases a variation of E/N is quite useful for the identification of ions. Fig. 3 shows mass scans at different E/N values. The strong increase of mass 55 with decreasing E/N easily allows it to be identified as H30+(H20)2, being produced in association processes at low E/N, whereas mass 59 does not increase strongly, being protonated acetone, which is produced in the binary proton transfer from HsO+ to acetone. A second way to help identify ionic components is the observation of isotopic abundances. Mass 69, which is observed in the analysis of human breath may be protonated isoprene (CsH,‘) or the mixed water-
methanol cluster ion CH30H - (H20)2H+. At E/N = 120 Td, six cycles of count rates for masses 69 and 70 were obtained as shown in Table 3, yielding an average isotopic mass ratio of M69/M70 = 94.1%/5.9% which is quite close to the calculated one (94.4%/ 5.4%), assuming that the molecule in question contains five carbon atoms, and is thus isoprene. At lower E/N the mass ratio M69/ M70 changed toward 97.4512.55 at E/N = 18.5 Td, indicating that a substantial amount of CH30H(H20)2Hf was obscuring the measurement. In this specific case neglecting this contribution would result in an error in the obtained density of isoprene by a factor of 15. Finally, different ionic constituents having the same mass or even the same ionic composition but different isomeric structures may be identified due to their different mobilities or drift velocities in the buffer gas. Pulsing of the H30f ions at the entrance to the reaction
Et03 1
Et01 k 54
Fig. 3. Mass scans from M54 to M60.5 taken at different values of E/N. 8 denotes E/N = 26.4 Td; 7, 35.2 Td; 6, 45.6 Td; 5, 59.2 Td; 4,75.2 Td; 3,92.8 Td; 2, 107.2 Td and 1, 120 Td. The much stronger increase of M55 as compared to the increase of M59 with decrea .sing IS p ro d wed in a ternary reaction, while M59, protonated acetone, is a binary product. E/N indicates that M55 (HjO+(H20),)
A. Hansel et al./International Journal of Mass Spectrometry and Ion Processes 1491150 (1995) 609-619 Table 3 Count rates (counts s-r) for masses 69 and 70 and corresponding isotopic abundances (in percent) obtained in five count cycles
fragment
615
ions:
H30+ + C3H70H
+ (C3H70H)H+
+ H20 30%
Cycle
Count
1 2 3 4 5 Average
l-5
rates
Isotopic abundances
--+ C3HT + products
M69
M70
M69
M70
86.11 88.88 83.38 84.98 86.58
5.46 5.74 5.41 5.36 5.21
90.04 93.93 93.90 94.07 94.32
5.96 6.07 6.10 5.93 5.68
85.98
5.44
94.06
5.94
region and detection of the arrival time spectrum of the product ions at the end of the drift section leads to results as shown in Fig. 4. The data show significantly differing spectra for protonated dimethyl ether, ethanol and formic acid, all appearing at mass 47. These results can be used as references to compare the arrival time spectrum of an unknown ionic species of mass 47 in order to help indentify it. There are still cases where proton transfer from H30+ ions to neutrals result in fragmentation of the product ions. At thermal energies the reaction of HsO+ with iso-propanol leads to both protonated propanol ions and C3HT
0 100
150
200
250
300
350
400
450
500
time (in units of 10%) Fig. 4. Arrival time spectra of the primary ion H,O+ and of product ions (protonated dimethyl ether, ethanol and formic acid respectively), all appearing at mass 47.
(7) 70%
(8) As Fig. 5 shows, the fraction of C3Hf product ions reaches 100% at mean relative kinetic energies, KE,,, approximately 1.5 eV. With this in mind and remembering that performs switching processes HjO+ - Hz0 with n-propanol, propanol could be unambiguously detected in human breath and its concentration was measured in many test persons as will be shown below.
4. Results 4.1. Trace components
in human breath
4.1 .I. Persons suffering from liver cirrhosis It is common knowledge that acetone is enhanced in the blood, urine and breath of persons suffering from diabetes [19], and monitoring of acetone has become a useful tool for the diagnosis of this illness. In the
E,, WI Fig. 5. Dependent of the relative abundances of the product ions (C,H,OH)H’ and CsHT resulting from proton transfer from HsO+ to propanol on the mean relative kinetic energy of the reactants.
616
A. Hansel et al./International
Journal of Mass Spectrometry
100 200 300 400 concentration of isoprene [ppb]
Fig. 6. Concentrations of isoprene in the breath of healthy persons (divided into smokers and non-smokers) and in the breath of patients suffering from cirrhosis of the liver.
same way it would be highly desirable to find other components which act as an indicator for specific illnesses. It is especially difficult to know about the actual state of the human liver by using only non-intrusive methods. We have compared the concentrations of methanol, ethanol, isoprene, butanone and propanol in the breath of healthy persons and patients suffering from liver cirrhosis respectively. There was no significant difference in the contents of methanol and ethanol in the two groups of persons and in view of
and Ion Processes 149/150 (1995) 609-619
existing detailed reports on methanol and ethanol in the blood and breath of humans [9,19 and references cited therein], there is no need for further discussion of these data. We therefore will concentrate on the other components mentioned above. Results on the contents of isoprene in the breath of healthy persons, divided into smokers and nonsmokers, are shown in Fig. 6. The average concentration of isoprene in smokers and non-smokers is about 250 ppb and the concentrations in persons suffering from liver cirrhosis seem to be slightly lower, but not significantly so. The present values agree satisfactorily with average values of 150 ppb obtained by Mendis et al. [20] by means of gas chromatography. Fig. 7 presents concentrations of butanone in the same sets of test persons as for Fig. 6. While healthy persons (smokers and non-smokers) show values typically from 5 to 25 ppb, the concentrations in several liver patients reach values up to more than 40 and 50 ppb. Again the overlap between the concentrations in healthy persons and liver patients is still too big to allow for a significant differentiation between the two groups. In contrast, propanol shows quite significantly enhanced densities in the breath of cirrhosis patients compared with healthy
6 4 m 5
6
2 0
4
$2 20 =
6 4 2 0
concentration
of butanone
[ppb]
Fig. 7. Concentrations of butanone in the breath of healthy persons (divided into smokers and non-smokers) and in the breath of patients suffering from cirrhosis of the liver.
concentration Fig. 8. Concentrations non-smokers.
of propanol
of propanol
[ppb]
in the breath of smokers and
617
A. Hansel et al./Iniernational Journal of Mass Speciromerry and Ion Processes 149/150 (1995) 609-619
35 30
??healthy persons
D
25 I--r r-l 20 2
nonsmoker
15
5: 10 $ a5
.n n,~.~.,....,..~.,..;; u-, I...
0
500
1000
1500
2000
2500
3000
6000
xw
0
$
a
E I?
6
m
smoker, smoking lor 2 cigaretts per day
%i 0
5
10
15
20
25
30
concentration of acetonitrile [ppb]
concentration of propanol [ppb] Fig. 9. Concentrations of propanol in the breath of 41 healthy persons (same as in Fig. 8) and of 15 patients suffering from cirrhosis of the liver.
Fig. 11. Concentrations of acetonitrile in the breath of nonsmokers (same as in Fig. 10) and of smokers, smoking l-2 cigarettes per day.
subjects. While all measured concentrations of 41 healthy persons (Fig. g), both smokers and non-smokers, were below 300 ppb, only two out of 15 patients suffering from liver cirrhosis had concentrations below 300 ppb and 12 had concentrations between 300 and 3000 ppb, one even more than 6000 ppb (Fig. 9). These data indicate that PTR-MS might be used for screening purposes in order to detect persons suffering from severe liver deficiencies.
acetonitrile in the breath of smokers and nonsmokers [21] using this new PTR-MS system and could show that acetonitrile is a most suitable indicator of whether a given subject is a smoker or not. The concentration of acetonitrile in the breath of smokers is typically 30-100 ppb while that for non-smokers is only about 5 to 10 ppb (Fig. 10). Even if someone smokes only 1 or 2 cigarettes per day, the acetonitrile concentration is enhanced significantly over that for non-smokers (Fig. 11). Wester et al. [22] have reported on
4.1.2. Acetonitrile in the breath of smokers and non-smokers Recently we have reported on the analysis of
B smokes 5 cigarettes
time [hours] 50
100
150
200
concentration of acetonitrile [ppb] Fig. 10. Concentrations of acetonitrile in the breath of smokers and non-smokers (taken from Ref. [21]), smoking more than 10 cigarettes per day.
Fig. 12. Concentrations of acetonitrile in the breath of two smokers, who stopped smoking at time 0. Person B started smoking again after about 100 h, while person A (one of the authors, A.J.) continued to abstain from smoking till concentrations of acetonitrile as low as for non-smokers were reached after about 7 days.
618
A. Hansel et al./International
Journal of Mass Spectrometry
enhanced levels of benzene in the breath of smokers; however after someone has smoked a cigarette, the benzene concentration in the breath decreases to normal values within about I h [21]. In contrast, the concentration of acetonitrile takes about one week to decrease to values typical for non-smokers (see Fig. 12); thus even if someone does not smoke a cigarette for several days, this person can still be identified as being a smoker. 4.2. Formaldehyde, methanol, ethanol, propanol, acetaldehyde, acetone and toluene in the air of typical homes Air was taken from five rooms in four different homes in Innsbruck and mass spectra were taken by passing this air through the PTR-MS system. All rooms are in buildings which were constructed more than 20 years ago. The furniture in three of these rooms is at least 10 years old. Two rooms have been equipped with new furniture and carpets only two months before the measurements were done. The sampling of air was done by means of a pre-evacuated glass vessel of about 5 1 volume. The windows of the rooms under investigation were kept closed for about 2-3 h before the sampling was done. The above-mentioned components were identified unambiguously by PTR-MS, but also many other components were observed, the identification of which has not yet been done. Table 4 shows the measured concentrations
Table 4 Concentrations
(in ppb) of various
components
in the air sampled
and Ion Processes 149/150 (1995) 609-619
of the components which were unambiguously identified. The concentrations of formaldehyde and ethanol are quite similar in the old and newly furnished rooms. In contrast, all other components are strongly enhanced (by at least an order of magnitude) in the two newly furnished rooms, reaching concentrations as high as about 5000 ppb for methanol, about 100 ppb for toluene, about 650 ppb for acetone and about 400 ppb for propanol. It is quite interesting to note that four of the five rooms investigated show concentrations of formaldehyde higher than 24 ppb, which is the maximum concentration allowed by Austrian law for humans to be permanently exposed to. 4.3. Trace components crossing
in the air taken at a road
In the same way as air was sampled in various rooms, we also took air from a road crossing, which is typically frequented by 4060 cars per minute. The air temperature at the time of sampling was 20°C and there was moderate wind of about 3 m SK’. The following concentrations were measured (all in units of ppb): formaldeyde, 48; acetaldehyde 40; methanol, 14; ethanol, 70; butanol, 15; benzene, 28; pentanol, 9; toluene, 53; xylene, 13. Many other com60; diethylbenzene, ponents still have to be identified, the sum of which certainly exceeds the sum concentration of the above components.
from five rooms
Room
Formaldehyde
Acetaldehyde
Methanol
Ethanol
Propanol
Acetone
Toluene
1 (new) 2 (new) 3 (old) 4 (old) 5 (old)
27 61 40 42 4
195 150 51 33 20
1360 5342 59 98 53
612 398 501 642 501
399 269 36 38 20
596 657 39 69 40
72 97 5 6 3
Room
1 and 2 were newly furnished
2 months
before the measurements
were done; Rooms
3-5 had old furniture.
A. Hansel et al./International
Journal of Mass Spectrometry
Conclusion
It was the aim of this work to improve the sensitivity of the gas analyzing method PTRMS (which previously had a sensitivity suitable for measuring trace components on the ppm level or slightly below) by building a system operating with a hollow-cathode ion source, which allows us to produce a high density of reactant ions H30+, and injecting them into a drift reaction region without mass spectrometric preselection. Furthermore, the air to be analyzed itself acts as the buffer gas; therefore no dilution by any buffer gas is needed, so that the trace components to be analyzed are present at their original mole fraction. In this way the rates of reaction of HsO+ with the trace components under investigation are large enough to yield product ion count rates of 1 count s-l per ppb of typical organic components and therefore on-line measurements of trace constituents at the ppb level can be done simultaneously for a large variety of components. Thus the goal of the present work is fully reached in that the sensitivity of PTR-MS was increased by more than two orders of magnitude.
Acknowledgment
This project (P10360) was rejected by the Austrian Science Foundation (Fonds zur Forderung der Wissenschaftlichen Forschung).
and Ion Processes
1491150 (1995) 609-619
619
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