Amu. occxp. Hjt., VoL 40. No. 6, pp. 615-626, 1996
Britiih Occmntionml Hygiene Society
Crown copyright p 1996 Pub&hed by B x v k r Science Ltd
AlTrifhu reserved. Printed in G r o t Britain
0003-4S7S/96 115.00+0 00
PH: S0003^«878(96)00028-2
THE SELECTED ION FLOW TUBE (SIFT)—A NOVEL
TECHNIQUE FOR BIOLOGICAL MONITORING
P. Spanel,*f P. Rolfe,* B. RajanJ and D. Smith*
'Department of Biomedical Engineering and Medical Physics, University of Keele, Thornburrow Drive,
Hartshill, Stoke-on-Trent ST4 7QB, U.K.; and JHealth and Safety Executive, Stanley Precinct, Bootle
L20 3QZ, U.K.
{Received 28 March 1996)
Abstract—We describe the use of our selected ion flow tube (SIFT) technique for the rapid
detection and quantification of trace gases in atmospheric air, with special reference to the analysis
of human breath. It is based on the chemical ionization of the breath trace gases to the exclusion of
the major breath gases, using 'soft' proton transfer from H 3 O + ions. Breath samples can either be
introduced into the SIFT from bags or by direct breathing into the apparatus, the advantage of the
latter approach being that surface active gases such as ammonia and many organic vapours which
adsorb onto bag surfaces can be more accurately quantified. We present examples of the analysis
of laboratory air, the breath of a non-smoker and of a smoker taken from bag samples, and
illustrate the rapid time response of the technique by showing the time profile of acetone on breath
during direct breathing into the apparatus. The current partial pressure sensitivity of our SIFT
method is within the range 30ppb to in excess of lOOppm, but with further development the device
could be made more sensitive, 1 ppb being well within reach. A transportable SIFT device is under
development which will have applications in environmental, medical and biological research,
health and safety monitoring, and in clinical diagnosis. Crown copyright © 1996 Published by
Elsevier Science Ltd.
INTRODUCTION
Modern industry uses a wide range of volatile substances which are hazardous to
health. During use, individuals may be exposed to these substances via inhalation,
skin absorption or ingestion. If the exposure is not prevented or controlled to an
acceptable level, the health of the exposed individual may be adversely affected. In
the U.K., the requirements to prevent and control the health risks from substances
hazardous to health are laid down in the Control of Substances Hazardous to
Health Regulations (COSHH, 1994). Health risk management procedures and
efforts to comply with the COSHH may entail exposure monitoring.
Biological monitoring is a way of assessing the absorption of hazardous
substances to which an individual may be exposed at the workplace (HSE, 1992). It
involves the measurement of a substance and/or its metabolite in body fluids,
usually blood, urine or exhaled breath. It is particularly useful because it can reflect
absorption via all routes.
The use of exhaled breath analysis for workplace air contaminants was brought
into prominence when exhaled air indices were incorporated into the biological
exposure indices (BEI) of the threshold limit values (ACGIH, 1987). The technique
fAuthor to whom correspondence should be addressed.
615
616
P. Spanil et al.
is not, however, a new one. It was first advocated as a tool for biological monitoring
in the workplace in the early 1960s (Stewart et al., 1961). There are a number of
reviews on the theory and policy of exhaled breath analysis (Fiserova-Bergerova,
1983; Astrand, 1975; Thomas, 1991). It is well known that significant amounts of
volatile organic compounds absorbed by the body appear in the exhaled breath,
sometimes with their metabolites. If these trace gases can be reliably detected and
quantified, it is theoretically possible to calculate total exposure during the period
monitored.
The detection and quantification of trace gases in exhaled breath are performed
using a variety of techniques, including gas chromatography (Monster et al., 1993),
gas chromatography/mass spectrometry (GCMS) (Phillips and Greenberg, 1992)
and conventional mass spectrometry using semi-permeable membrane barriers
(Campbell et al., 1985) to prevent interference from the major components of the
atmosphere. However, it is a common practice to concentrate the trace gases in
exhaled breath on a pre-trap (Money and Gray, 1989) to increase the potential for
detection and to achieve acceptable accuracy and precision. The pre-concentration
procedure also helps in the selective removal of the copious amount of water in
exhaled breath. These procedures are often laborious, costly per sample and time
consuming.
The conventional transportable mass spectrometers have a number of
limitations. In this technique, the gas to be analysed is introduced into an electron
impact ion source of the instrument through a controlled leak (so that the pressure
in the analyser can be maintained around 10~5 torr). The electron impact results in a
complicated mass spectrum because each molecular component of the mixture
produces several peaks due to fragmentation or cracking of molecules. When several
trace gases are present the resulting mass spectrum requires careful interpretation
and quantification. Therefore, most of the exhaled breath analyses by direct air
sampling mass spectrometry technique have been limited to components present at
percentage or fractional percentage levels.
Clearly, a more desirable exhaled breath analysis procedure would be one in
which the breathflowsdirectly from the subject into the analytical equipment whose
features overcome some or all the problems mentioned above. The novel SIFT
technique described in this paper provides a new method by which to perform
exhaled breath analysis.
INSTRUMENTATION
Background of the SIFT technique
The SIFT technique was conceived and developed by Smith and co-workers to
study reactions in the gas phase. It soon became a standard method for the study of
ion-neutral reactions at or near to thermal interaction energies (Smith and Adams,
1987), including interstellar gas phase reactions (Smith, 1992; Smith and Spanel,
1992). In the course of this research, Smith and SpanSl realised its potential for the
detection and analysis of trace gases (Span61 and Smith, 1995).
SIFT for biological monitoring
617
The principle of the SIFT
The principle of the SIFT technique is simple and a line diagram of the current
apparatus, indicating the essential features, is shown in Fig. 1. Positive ions are
created in the microwave discharge ion source (low and high pressure electron
impact sources were also previously commonly used). A current of ions of a given
mass-to-charge ratio (H3O + , for example) is extracted from the mixture of ion
species created in the source using a quadrupole mass filter. This current of selected
ions is then injected into a fast-flowing inert carrier gas stream (usually pure helium
at a pressure of about 1 torr) via a Venturi-type orifice, diameter typically 1-2 mm.
As long as the ions are injected at sufficiently low energy, they do not fragment on
collision with helium atoms and they are carried along theflowtube (approximately
1 m long) as a thermalized ion swarm with a MaxwelMan velocity distribution and a
temperature equal to that of the carrier gas (normally room temperature). The
thermalized ions are sampled from the flowing swarm via a pinhole (approximately
0.3 mm diameter) located at the downstream end of the flow-tube, into a
differentially-pumped quadrupole mass spectrometer. After mass analysis the ions
are detected by a channeltron/pulse counting system. The resulting mass spectra are
then recorded by an on-line computer for analysis and storage.
The SIFT can be used to determine the rate coefficient (that is, the efficiency) for
the reaction of the primary ions in the swarm with a known reactant gas and also the
product ions of the reaction. To carry out such a study, first the reactant gas is
introduced into the carrier gas via a mass flow meter in controlled and measured
amounts through the entry port (see Fig. 1). Then, the decay rate of the primary
(injected) ion current and the growth of the product ion currents as a function of the
reactant gas flow rate (the reactant gas number density) in the flow-tube are
observed using the mass spectrometer/channeltron system. The rate coefficient for
the reaction is then calculated according to a well established procedure which is
described in detail elsewhere (Smith and Adams, 1987).
Helium
Reactant gas
Ion injection c a r r i e r «** < or a i r
orifice
Microwave
resonator
H3O+
Ion source
gas
(Ar/H 2 O)
Roots pump
o r b r e a t h sam
Ple>
Channeltron
ion detector
Carrier gas flow
Ion sampling orifice
Injection quadrupole
mass filter
Injection diffusion pump
Detection quadrupole
mass spectrometer
10 cm
Detection diffusion pump
Fig. 1. A schematic diagram of the selected ion flow tube (SIFT) apparatus as configured for trace gas
analysis, showing the H j O + ion source (microwave resonator discharge) and injection quadrupole mass
filter, the flow tube with the gas sample (air or breath) inlet, and the downstream quadrupole mass
spectrometer and detection system which is used to detect and quantify the ions produced by chemical
ionization of the trace gases in the gas sample.
618
P. Spanil et al.
Trace gas analysis
For trace gas analysis using the SIFT the above procedure is in a sense reversed.
If the rate coefficient for the reaction of a particular primary ion with a trace gas is
known (if not it can be measured using the above procedure), then the trace gas
concentration in a sample of air or breath introduced into the SIFT can be
determined. Thus, exhaled breath is introduced into the SIFT via the sample inlet
port (see Fig. 1) where the trace gases react with the injected primary ions, and the
product ions are detected and quantified by the detection/analyser system.
The primary ions have to be chosen intelligently; most importantly, they must
not react at a significant rate with the major components of the breath sample (N2,
O2, H2O, Ar and CO2) but they must react efficiently with the trace gases to be
detected producing identifiable product ions. These stringent requirements are best
fulfilled by H3O + , O2+ and NO + ions (Spanel et al., 1995; Spanel and Smith, 1995,
1996), all of which have been successfully used during the feasibility testing of the
SIFT technique for trace gas analysis.
The best way to describe this technique of quantitative analysis more clearly is by
way of an example. For this, we chose the reaction of H3O"1" primary ions with
acetone (CH3COCH3) molecules, a common trace gas in exhaled breath. When the
primary ion, H 3 O + , reacts with an acetone molecule a proton is transferred from the
H3O + to the acetone molecules thus:
H 3 O + + CH3COCH3->H2O + CH3COCH3. H +
(1)
An important point to emphasise here is that such proton transfer reactions
almost always result in a single ionic product, unlike the multiple fragmentation that
results following electron impact ionization. This allows even complex multicomponent mixtures to be quantitatively analysed. It is well known that when the
proton affinity of the acceptor molecule, (acetone in this example), exceeds that of
the donor molecule (water in this example) then proton transfer proceeds with unit
efficiency, that is, the proton transfers in every collision between the protonated
donor molecule (H 3 O + ) and the proton acceptor molecule (CH3COCH3). Then the
rate coefficient, k, for the reaction is the collisional rate coefficient, which is readily
calculated. So, when a sample of exhaled breath containing acetone is introduced
into the SIFT, reaction (1) proceeds resulting in some loss of the H 3 O + ions and the
production of the protonated acetone ions, CH3COCH3.H+. It is simple to show
(Smith and Adams, 1987) that the count rate (/) of the H 3 O + at the downstream
detector in the presence of acetone is related to the count rate (/0) in the absence of
acetone, by:
/=/ o exp-A:[A]/ = /oexp-fc[y4]—
(2)
where / is the time spent by the ions in the flow tube, / is the length of the reaction
region of the flow tube, e is an end correction to / (about 1 cm) associated with the
inlet port, and v/ is the ion flow velocity which is readily determined (Smith and
Adams, 1987). [A] is the required number density of the acetone molecules in the
carrier gas. The relationship between [A] in cm~3 and the flow rates of the helium
SIFT for biological monitoring
619
carrier gas, <DC, and the acetone, <DA, the carrier gas pressure, pg (in torr) and the
absolute temperature, Tg, is given by:
[A] = 3.54 x l0i6pg
(3)
where 3.54xlO16 is the conversion coefficient between gas pressure (torr) and
number density (cm"3) of molecules at 273 K.
The count rate of the product ion CH3COCH3. H + is not of primary concern
for the determination of the rate coefficient for the reaction (1), but it is the essence
of the SIFT technique for trace gas analysis. When the flow rate of the reactant gas
(acetone) is very small, the fractional decay of the primary ion (H 3 O + ) count rate is
negligible but the much smaller count rate of the product ions (CH3COCH3.H+)
can still be determined accurately. When fc[A]/<d it is easy to show [following
Equation (2)] that the product ion count rate, Ip, is related to the primary ion count
rate, /, as:
y = k[A}t
(4)
Thus the count rate of the product ion is directly proportional to the number
density of the trace gas (acetone) when it is small enough only to reduce slightly the
count rate of the primary ions. This is the important factor in the application of the
SIFT to trace gas analysis because it allows the concentrations of the trace gases in a
multicomponent mixture such as breath that react with H 3 O + to be readily
calculated from the observed count rates of each product ion species. Thus our SIFT
technique determines absolute partial pressures of trace gases in air (or breath) at
atmospheric pressure using only the measured flow rates of the carrier gas and the
sampled air, the pressure in the flow tube and the ratio of the product ion and
precursor ion count rates, and it does not require calibration with prepared standard
mixtures. Nevertheless, we have indeed carried out cross validation with known
concentrations of different mixtures of organic vapours prepared by the well
established dynamic dilution technique (MDHS, 1990). The results of these detailed
studies will be reported elsewhere (Smith et al., in preparation). The current
apparatus, initially designed to study gas phase reactions, is able to quantify the
partial pressures of several trace gases simultaneously, rapidly and accurately down
to about 30ppb and to in excess of lOOppm. With further development of this
technique, notably of the primary ion source, we anticipate that the partial pressures
of trace gases down to 1 ppb will be measurable. It is important to note that the
stainless steel tube through which the moist breath sample flows into the SIFT is
heated to a temperature above 100°C to inhibit the condensation of water and the
trace gases onto the tube surface. This is particularly important when following the
time profiles of the trace gases on the breath as described later.
The flow rate of exhaled breath or air into the SIFT is a first order parameter in
determining the sensitivity of the method. It can either be varied over a wide range
and accurately measured and stabilized using a mass flow controller or be
maintained at a fixed level using a calibrated capillary leak directly from the
atmosphere. The partial pressure of the sampled exhaled breath or air in the carrier
620
P. Spanil el al.
gas can be up to a few per cent of the carrier gas pressure without seriously
disturbing the flow (without causing turbulence).
Another major feature of the SIFT technique is its short time response (currently
20 ms) to changes in the trace gas concentrations in the sampled air or breath.
Therefore, it is possible to observe real time fluctuations in concentrations in a
breath sample and to quantify trace gas concentration in a single exhalation of
breath as we show later.
SOME ILLUSTRATIVE RESULTS
In order to illustrate the applicability of the SIFT technique in exhaled breath
analysis, we present some selected examples of trace gas analyses obtained using
H 3 O + precursor ions. We also illustrate the rapid time response of the SIFT using
an example the time profile of breath acetone obtained from single exhalations of
breath directly into the SIFT. All the data presented here were obtained at room
temperature.
Helium carrier gas
First, it is important to establish the levels of detectable impurities in the SIFT
carrier gas (helium in these experiments). This is simply achieved by injecting the
H 3 O + ions and recording the mass spectrum using the downstream mass
spectrometer. A typical result is shown in Fig. 2(a), where the mass spectrometer
count rates are shown on a log scale to illustrate the count rate range and the
detection sensitivity. The scan covers the ion mass range from 10 to HOu and was
taken for a period of 120 s, a time interval of 1.2 s per mass unit. As can be seen in
Fig. 2(a), the other major ions on the spectrum are O^ (from the photoionization of
impurity O2 which is present in the helium carrier gas at a level of about 1 ppm) and
H 3 O + .(H2O) cluster ions formed in three-body reactions between the H 3 O + ions
and the trace of water in the carrier gas (also at a partial pressure of about 1 ppm)
(see SpanSl and Smith, 1995). The ion signals with intensities of about 3 counts s~'
indicate the presence of impurities (apparently residual methanol and ethanol from
previous experiments) at a level of about 106 molecules cm~3. This amounts to
partial pressures of about 50 parts per trillion in the carrier gas which has number
density of 2 x 1016cm~3.
Laboratory air
Figure 2(b) shows the mass spectrum obtained after the introduction of
laboratory air into the helium at a flow rate of 1 torr I s " 1 , that is, about 1% of the
helium flow rate which we used in all the measurements that are reported below. As
can be seen, the major additional ions on the spectrum, representing a few per cent
of the total ion count rate, are the water cluster ions H 3 O + .(H 2 O)2 and
H 3 O + . (H2O)3 formed by the sequential three-body reactions of the H 3 O + .H 2 O
ions with the water in the atmospheric air sample. The other ions on the spectrum
indicate the trace gases present in the laboratory atmospheric air, several of which
are immediately recognisable, including gethanolo ethanol anO ammoniO, at paptial
pressures sn the 3 to lOOOppb raOge, much lower revels t an the acceptabOe
exposure stanOards (s(e HSE, 1996). TOe value of the elFT tecenique to
SIFT for biological monitoring
621
(a)
19/
H3O
10'
H3O.H2O
10'
32
10'
33
20
47
30
40
60
50
70
80
90
80
90 95
(b)
10'
37
Ammonia
10 3
Methanol
(780 ppb)
55
Ethanol
(80 ppb) 73
102
57
10'
65
20
30
40
50
60
70
u
Fig. 2. (a) The ion spectrum observed in the SIFT downstream following the injection of H3O + into
cylinder helium at a pressure of 0.5 torr. Note that the signal levels vary over four to five orders of
magnitude. The O2" ions result from the photoionization of trace Oj impurity in the helium by ultraviolet
light from the microwave discharge that is the ion source. The H3O + . H2O ions are formed in a threebody reaction between the H 3 O + ions and water impurity in the helium. Note the appearance of the
HiDO + ion at 20 u and the H 3 O + isotopic ion at 21 u. The other ions seen on this spectrum are the result
of reactions of the H 3 O + ions with impurities in the helium (see text), (b) The spectrum obtained
following the introduction of a small flow (1 torr per 1. s~') of laboratory air into the helium. Note the
appearance of the water cluster ions H3O + . ( F ^ O ) ] ^ and their associated isotopic variants (open bars).
Note also the ions at 18 u from ammonia, 33, 51 and 69 u from methanol, and 47, 65 and 83 u from
ethanol in the laboratory air. The concentration of these trace gases are as given in parts per billion of the
air sample. There are also recognisable traces of acetaldehyde, acetone and acetic acid in the air at the
» 30 ppb leveL (The laboratory is next door to a chemistry laboratory.)
622
P. Spanil et at.
environmental monitoring is immediately clear, not least for the accurate
quantification of common industrial solvents in the environment, and for the
detection of gases and vapours which could be harmful even at very low
concentrations such as benzene (HSE, 1996).
Normal human breath
As a third example, a spectral scan obtained using a breath sample from a
healthy, non-smoking individual contained in a clean sampling bag (previously
purged with clean, dry air) is shown in Fig. 3(a). Some of the peaks seen on this
breath sample are at greater intensities on the breath than in the laboratory air
sample, notably the peaks at masses 18 and 69 u, and the peaks at 59 and 77 u. The
peak at 18 u is protonated ammonia, NH4". The 69 u peak indicates the presence of
isoprene (detected after protonation as CH2C(CH3)CHCH2.H+) which overlaps in
mass the dihydrate of the protonated ethanol ion (SpanSl and Smith, 1995).
Fortunately, the simultaneous presence of the protonated methanol ion (33 u) and
its monohydrate (51 u) allows the separate quantification of both isoprene and
methanol. The other two peaks correspond to acetone (detected as CH3COCH3. H +
and its monohydrate CH3COCH3.H"1". H2O). These data demonstrate the fact that
protonated non-polar and weakly polar gases, including isoprene, do not form
hydrates (and so there is only one peak on the spectrum for isoprene) whilst the
polar gases do form hydrated ions (and so there are two peaks on the spectra for
acetone separated by 18u) (see Spanel and Smith, 1995 for details). Other gases
present on this breath sample are ethanol (giving ions at 47 u, the monohydrate at
65 u, and the dihydrate at 83 u) and acetaldehyde (similarly giving ions at 45, 63 and
81 u). The partial pressures of the gases on the breath can be obtained following
Equations (3) and (4) by first summing all the product ion count rates on the
spectrum associated with each individual neutral trace gas species, and then
rationing this sum to the sum of each precursor H3O + . (H2O)n ion, weighted by the
corresponding reaction rate coefficients. The partial pressure of acetone on the
breath of healthy individuals is typically 0.3 to 2ppm, and that of isoprene is usually
within the range 0.1 to lppm. Isoprene is known to be the most abundant
hydrocarbon on breath (Gelmont et al., 1981).
Breath of a smoker
As a preliminary study of the influence of smoking on breath composition we
present SIFT spectra obtained for the breath of a smoker. Figure 3b shows the
spectrum obtained from the sample of the smoker's breath 30 min after smoking the
cigarette. It can be seen immediately that more ionic species are present than are
detected on the sample of non-smoker's breath [Fig. 3(a)]. The peak at 28 u is more
obvious on the breath of the smoker and is indicative that HCN (known to be
present in cigarette smoke) is retained on the smoker's breath. An ion appears at
43 u which we attribute to the presence of propene on the smoker's breath. The peak
at 79 u could be indicative of protonated benzene or protonated fumaronitrile
(CNCH=CHCNH+), but the most obvious additional ion occurs at 42 u which must
be protonated acetonitrile, CH3CNH + . Protonated acetonitrile is known from our
previous studies readily to form hydrates, and so it is no surprise to see its
monohydrate and dihydrate appear on this spectrum at masses 60 and 78 u
SIFT for biological monitoring
623
Isoprene
Breath of smoker
19
10'
73
55
Propene
(40 ppb)
Benzene?
(50 ppb)
32
Acetonitrile
(120 ppb)
Hydrogen
cyanide
18
(50 ppb)
21
36
28
10'
30
87
20
30
40
50
60
70
80
Fig. 3. (a) The ipectrum of the breath of a non-smoker clearly showing the ions derived from acetone (59
and 77 u), isoprene (69 u), ethanol (47, 65 and 83 u), methanol (33, 51 and 69 u) and traces of acetaldehyde
related peaks (45, 63 and 81 u). Note that the ion at 69 n is due to protonated isoprene and the dihydrate
of protonated methanol (separated according to the procedure described in our recent paper (Smith and
Spanel, 1996). The concentrations of the major impurity species in the air are as indicated in parts per
billion, (b) The spectrum of the breath of a smoker. The most obvious additional ions (compared to the
non-smoker's breath) are at 42, 60 and 78 u which are CH 3 CN.H + and its first and second hydrates, and
43 u (protonated propene, C3H7"). The mass 79 u could be protonated benzene, but there are other
possibilities for this ion (e.g. protonated fumaronitrile). Mass 28 u (protonated HCN) is also more
obvious on this spectrum, but it is also present on the laboratory air spectrum (a).
P. Spanil et at.
624
respectively. It is also clear that the ions due to ethanol are more intense on this
smoker's breath than on the non-smoker's breath; this subject had probably been
drinking as well as smoking! The acetone and isoprene concentrations are not so
different from those for the non-smoker's breath, and are certainly within the likely
natural variations of these compounds on breath. However, it is clear that HCN,
CH3CN and CH2=CHCH3 persist on the breath of smokers, and so they are good
indicators that an individual has been smoking. Gearly, an enormous amount of
interesting and valuable research can be done in following up these very preliminary
studies.
Time profile of breath acetone
To demonstrate the rapid time response and the 'real time' analysis feature of the
SIFT technique, we show in Fig. 4 the partial pressure of acetone on an individuals
breath as he breaths directly into the SIFT. These data are obtained by rapidly
switching the downstream quadrupole mass spectrometer between the precursor
ions H 3 O + .(H 2 0)o, 1,2,3 ions and the acetone derived ions at 59 and 77 u. The
calculation of the partial pressure of the acetone is carried out rapidly by the on-line
computer. It is worth noting that in Fig. 4 the initial rapid rise in the amount of
acetone exhaled during the first 2 s as the upper airways are emptied and then the
slower further increase as the alveolar breath is exhaled. The fluctuations on each
profile are real indicators of the acetone flow fluctuations, because the time constant
of the system is only about 20 ms.
1.6
Acetone
1.4
1.2
1.0
i.0.8
Q.
0.6
0.4
02
10
15
20
25
30
35
40
45
Time [s]
Fig. 4. Time profiles of the partial pressure of breath acetone (in parts per million, ppm, of the breath gas)
from a subject breathing with a cycle of about once per 10 s directly into the SIFT apparatus. These
profiles are calculated using the known rate coefficients for the ion-molecule reactions involved from the
count ratei of the acetone-derived product ions (at 59 and 77 u) and the count rates of the primary
(precursor) ions (19, 37, 55 and 73 u). Time dependencies of these count rates were obtained by rapid
switching of the SIFT mass spectrometer.
SIFT for biological monitoring
625
DISCUSSION AND CONCLUSIONS
We have shown that the SIFT technique offers a number of advantages for
exhaled breath analysis. The technique does not suffer from the presence of water
vapour in breath. 'Soft' ionization of trace gases usually produces only one product
ion for each molecular component in the exhaled breath, and the technique can
provide absolute partial pressures of trace gases in air and on breath currently
within the range 30ppb to in excess of lOOppm without the need for pre-calibration
of the instrument. Sampling of breath can either be carried out by taking bag
samples or by direct breathing into the apparatus. The latter procedure turns out to
be especially valuable for the detection of gases (vapours) that are surface active and
readily adsorb onto bag surfaces. It is also a very rapid analytical procedure.
The applicability of the SIFT technique for exhaled breath analysis has now been
established using our strictly laboratory based apparatus. Currently, we are using
this apparatus to carry out breath analyses of patients with various clinical
conditions (for example, renal failure, pre-eclampsia and gut colonization by
Helicobacter pylori) in collaboration with clinical specialists, and obtaining some
remarkable results (§pan£l et ai, 1996). The current SIFT is very large and requires
high carrier gas flow rates maintained by a high volume displacement Roots vacuum
pump. However, our calculations have shown that a smaller transportable
instrument can be built without any loss of selectivity or sensitivity. At present,
we are in the early stages of building such a transportable SIFT apparatus.
Acknowledgements—We have benefitted from useful discussions with John Cocker, John Thompson and
Simon Davies.
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