Introduction
With increasing evidence that anthropogenic chlorinated and brominated
hydrocarbons can be transported into the stratosphere and release
chlorine and bromine atoms that can deplete ozone in catalytic
cycles , the production and
use of such species were regulated under the Montreal Protocol in
1987. Most of these fully halogenated compounds are declining in the
atmosphere . However, many partially halogenated
compounds are still increasing in the atmosphere , as
are some newly detected fully halogenated species
. Also, many fluorocarbons which do not destroy
stratospheric ozone and are thus not regulated under the protocol show
increasing trends in the atmosphere . Although these fluorocarbons do not destroy ozone,
many of them are strong greenhouse gases with long atmospheric lifetimes,
resulting in increased radiative forcing of the troposphere. Therefore, the
need persists for continuous measurements to identify new compounds in the
atmosphere and monitor and document their atmospheric trends. The mass
spectrometric instrument commonly used for halocarbon analysis is the
quadrupole mass spectrometer (QP MS) . Besides the QP MS, the use of high mass resolving and extremely
sensitive sector field MS has also been reported .
Time-of-flight mass spectrometry (TOF MS) has only been applied sporadically for measurements of atmospheric
trace gases and in particular not with focus on halocarbons.
The main advantage of coupling a TOF MS to a gas chromatograph (GC) over
using the QP MS is the intrinsic full mass range acquisition and the better
mass resolution and accuracy. The identification of unknown peaks is
significantly facilitated by these advantages and the use of more narrow mass
intervals is expected to reduce interferences and background noise. In
addition, much higher data acquisition rates are possible using TOF MS, which
is an advantage for fast chromatography. A TOF MS instrument can measure more
than 10000 mass spectra per second. They are added up and averaged over a
certain time period to yield the desired time resolution. The possibility of
operating the TOF MS at high data rates is also of high interest for fast
chromatography and narrow peaks, for which the operating frequency of quadrupole
instruments (especially when measuring several ions) can be a limiting
factor. The maximum time resolution for the TOF MS used in this study is
50 Hz. An increase in the data frequency will lead to decreased
signal-to-noise levels. The data frequency must therefore be optimised to
provide a sufficient number of data points per chromatographic peak while
keeping the signal-to-noise level as high as possible. In contrast, a QP MS
is a mass filter and will only measure one mass at a time. It needs to scan
many individual masses sequentially to register a full mass spectrum. To
achieve high sensitivity, QP MS are therefore often operated in single ion
monitoring (SIM) mode in which the instrument is tuned to only one or a few
selected ion masses and all other ions do not pass the quadrupole mass
filter. Regardless of these limitations of the QP MS, it is widely used in
analytical chemistry due to its stability, ease of operation, high degree of
linearity, good reproducibility as well as sensitivity. Especially for
atmospheric monitoring the advantage of obtaining the full mass information
from the TOF instrument might allow retrospective quantifications of species
which were not target at the time of the measurement. For this purpose the
TOF MS must be well characterised (in particular with respect to linearity)
and the calibration gas used during the measurements must contain measurable
amounts of the retrospective substances and be traceable to an absolute
scale.
In this paper, a comparison of a state-of-the-art QP MS and a TOF MS is
presented, with both mass spectrometers being coupled to the same gas
chromatographic system. The instrumental setup is described in Sect. .
The GC QP MS system was characterised and used before for studies by
and showed consistent results in the
international comparison IHALACE (International Halocarbons in Air Comparison
Experiment) with the NOAA (National Oceanic and Atmospheric Administration)
network . We discuss the use of TOF MS in atmospheric trace
gas measurements, in particular for the detection and quantification of
halocarbons, focusing on four substances: CFC-11, CFC-12, Halon-1211 and
Iodomethane. These four substances cover the boiling point and typical
concentration range of a total of 35 substances analysed. The six key
parameters for atmospheric trace gas measurements discussed in this paper are
(1) mass resolution and (2) mass accuracy of the detectors, (3) stability of
the mass axis and instrument sensitivity, (4) detector sensitivity
represented by the limits of detection (LOD), (5) reproducibility of the
measurement procedure and (6) the linearity of the detectors for varying
amounts of analyte. The underlying experiments are described in Sect.
and their results are discussed in Sect. . Section summarises
the results of this work.
Schematic of the cooling head. The aluminium cylinder which contains
the sample loop is placed on top of the Stirling cooler's cold end. Electric
connectors are located at each end of the sample loop for resistive heating.
Instrumental
Preconcentration unit
Atmospheric mixing ratios (mole fractions) of halocarbons are very low, i.e.
in the parts per trillion (ppt) to parts per quadrillion range (ppq). To
achieve signals clearly distinguished from noise in GC MS analysis, a sample
preconcentration procedure is required. In this work, the method of sample
preconcentration on adsorptive material followed by thermodesorption prior to
gas chromatographic separation was used. Figure shows
a schematic of the preconcentration unit; an explanation follows.
A similar setup was described by . A 1/16 inch
stainless steel tube (sample loop, ID = 1 mm, length = 15 cm)
packed with HayeSep D (10 mg) adsorption material was cooled to a
temperature of -80 ∘C for sample preconcentration. The
sample flow during preconcentration was adjusted to 50 mL min-1
controlled by a needle valve. For cooling, a Stirling cooler was used (Global
Cooling, Inc., model M150). The sample loop was placed inside a cooled
aluminium cylinder (cooling head) and was thermally and electrically isolated
with two layers of glass silk and one layer of Teflon shrinking hose. The
cooling head was thermally isolated towards ambient air with two layers of
Aeroflex-HF material. All sample components which were not trapped on the
adsorption material were collected in a 2 L stainless steel flask
equipped with a pressure sensor. The pressure difference between beginning
and end of the preconcentration phase was recorded to calculate the
preconcentration volume. After the preconcentration phase, the sample loop
was heated resistively to +180 ∘C in a few seconds for
instantaneous injection of the trapped analyte fraction onto the GC column.
Desorption temperature was maintained for 4 min to clean the sample loop
from all remaining compounds. All tubing (stainless steel) used for sample
transfer between sample flask and preconcentration unit as well as
preconcentration unit and GC was heated to 80 ∘C to avoid loss
of analytes to the tubing wall.
Gas chromatograph
An Agilent Technologies 7890A GC with a Gas Pro PLOT column (0.32 mm
inner diameter) was used for separation of analytes according to their
boiling points. The column had a total length of 30 m, divided inside
the GC oven into 7.5 m pre-column (backwards flushable) and
22.5 m main column. Purified helium 5.0 (Alphagaz 1, Air Liquide,
Inc.) was used as carrier gas. The GC was operated with constant carrier gas
pressure on both pre- and main column. The temperature program of the GC
consisted of five phases. (1) For the first 2 min, the temperature was
kept at 50 ∘C. (2) Then the oven was heated at a rate of
15 ∘Cmin-1 up to 95 ∘C, (3) from thereon
at
10 ∘Cmin-1 up to 135 ∘C and (4) then at
a rate of 22 ∘Cmin-1 up to 200 ∘C. (5) The
final temperature of 200 ∘C was kept for 2.95 min. The resulting runtime was 17.95 min. The
pre-column was flushed backwards with carrier gas after 12.6 min to avoid
contamination with high-boiling substances. The gas chromatographic column
was connected to the QP MS and the TOF MS using a Valco three-port union and
two fused silica transfer lines. The transfer line to the QP MS had a total
length of 0.70 m with an inner diameter of 0.1 mm, and the
transfer line to the TOF MS had a total length of 2.10 m with an
inner diameter of 0.15 mm. Based on the length, temperatures and
inner diameters of the transfer lines, a split ratio of 63 : 37 (TOF MS : QP MS)
was calculated. Using the ratios of the peak areas of the quadrupole when
receiving the entire sample (TOF transfer line plugged) to those obtained in
the split mode, a spilt ratio of 66 : 34 was calculated. We have adapted this
latter value as it is based on actual measurements rather than calculations.
All parts of the transfer lines outside the GC oven were heated to
200 ∘C.
Mass spectrometer
The two mass spectrometers in comparison were (1) an Agilent Technologies
5975C QP MS and (2) a Markes International (former ALMSCO) Bench TOF-dx E-24
MS. Both MS were operated in electron ionisation mode with an ionisation
energy of 70 eV and ioniser temperatures of 230 ∘C.
The QP MS was operated in SIM and SCAN mode (see Table for more
information). As the GC was operated in constant pressure mode, i.e. the head
pressure of the columns were kept constant, the carrier gas flow into the two
MS therefore varied according to the temperature ramp during each gas
chromatographic run. Pressures inside the ion flight tubes of the MS
therefore also varied; the TOF MS had a pressure range from
1.8 × 10-6 to 1.6 × 10-6 hPa and the QP
MS had a pressure range from 2.1 × 10-5 to
1.8 × 10-5 hPa. The Bench TOF-dx uses a direct ion
extraction technique with an acceleration voltage of 5 kV. In
contrast to many other TOF instruments, the ions are accelerated directly from
the ion source into the drift tube instead of extracting them from the ion
source and then accelerating them orthogonally to the extraction direction
(orthogonal extraction). The direct extraction method in combination with the
high acceleration energy orients the instrument towards a high sensitivity,
especially for heavier ions (five technologies GmbH, G. Horner and
P. Schanen, personal communication, 2014). The TOF MS was set up to detect
mass ranges from 45 to 500 m / z; higher and lower m / z were
discarded. The reason to discard ions with m / z ratio below 45 was to
eliminate a large part of the CO2 which is trapped by our
preconcentration method and can lead to saturation of the detector.
A schematic of the Bench TOF-dx is given in Fig. . The spectra
extraction rate was adjusted to 4 Hz to get a data acquisition rate
comparable to that of the QP MS.
Scheme for the direct ion extraction of the Bench TOF-dx direct
extraction (five technologies GmbH, G. Horner and P. Schanen, personal
communication, 2014). The red dotted line represents a typical ion path.
Experimental
All characterisation experiments were conducted using a high-pressure
air sample (50 L Aluminium flask, 70 bar) filled in
2007 at Jungfraujoch, Switzerland. Prior to preconcentration, the air
sample was dried using a heated (70 ∘C)
Mg(ClO4)2 water trap. Halocarbon mixing ratios were
assigned to this reference gas by calibration against an AGAGE
(Advanced Global Atmospheric Gas Experiment) gas standard
(H-218). Table shows reference gas mixing ratios of specific
substances discussed in this paper.
Mixing ratios in ppt in the reference gas used in this work for the
discussed substances.
Substance
Formula
MR [ppt]
Scale
CFC-12
CCl2F2
544.42
SIO-05
CFC-11
CCl3F
250.79
Halon 1211
CBrClF2
4.41
Iodomethane
CH3I
0.88
NOAA-Dec09
Dwell time settings for given substance fragments in QP MS modes
with a data frequency of ≈3 Hz. SCAN mode (1): QP scanned
from 50 to 500 u with 1.66 scans per second and a dwell time of
3.7 ms. Optimised (opti.) SIM mode (2): settings used for
measurements on which LOD calculation was based, with 310 ms dwell time
per ion and a scan rate of 3 scans per second. Operational SIM mode (3):
default settings, used for reproducibility and linearity experiments with 3
scans per second.
Substance
Fragment
m/z
QP SCAN mode
Optimised (opti.) SIM mode
Operational (oper.) SIM mode
[u]
dwell time [ms]
dwell time [ms]
for LOD calculation (1)
for LOD calculation (2)
for LOD calculation (3)
1.66 scans per second
3 scans per second
3 scans per second
CFC-12
CCl35F2+
85
50 to 500 u
50
CFC-11
CCl235F+
101
310 ms dwell time
70
Halon 1211
CCl35F2+
85
3.7 ms dwell time
100
Iodomethane
CH3I+
142
70
Measurement procedure and data evaluation
To ensure measurement quality, both MS were tuned in regular intervals
(autotune by operating software) at least every 2 months but especially
before sample measurements and/or characterisation experiments. Autotune
options of both mass spectrometers were used without further manual
adjustments. To increase the sensitivity and linearity of the TOF MS, its
detector voltage was increased by 30 V, as described in Sect.
. Additionally, a zero measurement (evacuated sample loop), a blank
measurement (preconcentration of purified Helium 5.0) and two calibration gas
measurements were conducted to condition the system before every measurement
series. At the end of every measurement series, another blank measurement was
added. Every measurement series itself consisted of a calibration measurement
followed by two sample measurements (same sample). This sequence of three
measurements was repeated n times depending on the type of experiment and
then terminated by a calibration measurement. For characterisation
experiments both calibration and sample measurements were taken from the same
gas cylinder (reference gas, see description above) but treated differently
in data evaluation, e.g. as a calibration or sample measurement.
Chromatographic peaks were integrated with a custom designed software
written in the programming language IDL. The peak integration is based not on
a standard baseline integration method commonly used in chromatographic
applications but on a peak fitting algorithm. For the results shown here
Gaussian fits were used for peak integration. This software was also used for
data processing by and described there. Noise calculation
was performed on baseline sections of the ion mass traces of interest. The
noise level was determined as the 3-fold standard deviation of the residuals
between data points and a second degree polynomial fit through these data
points. This approach accounts for a drifting non-linear baseline. Otherwise,
a non-linear baseline would cause an overestimation of the noise level. The
integrated detector signal was divided by the preconcentration volume to get
the detector response per sample volume. To account for detector drift during
measurement series, the calibration measurements bracketing the sample pairs
were interpolated linearly. Thereby, interpolated calibration points are
generated for each sample measurement. The response for each sample was then
derived by calculating the quotient between sample and corresponding
interpolated calibration point. Experiments were conducted to analyse six key
parameters (Sect. to ) important for measurements of
halogenated trace gases in the atmosphere: mass resolution, mass accuracy,
limits of detection, stability of the mass axis and instrument sensitivity,
measurement precision and reproducibility as well as detector linearity.
Mass resolution
The mass resolution (R) is defined as follows:
R=mΔm,
with Δm being the full width at half maximum (FWHM) of the
exact mass m of the ion signal.
The mass resolution determines whether two neighbouring mass peaks can be
separated from each other. It is considered an instrument property,
i.e. influenced only by internal factors like instrument geometry, ion
optics, etc. The mass resolution of the TOF MS was calculated with its
operating software ProtoTOF in a mass calibration tune. The QP MS was
operated with MS Chemstation (Agilent Technologies, Inc.) which only
processes unit mass resolution, independent of mass range.
Mass accuracy
The mass accuracy (δa) defined as
δa[ppm]=m-mmmm×10-6
and quantifies the deviation between a measured ion mass mm and the
according expected exact mass m of each fragment. Like mass resolution, it
is considered an instrument property. In this work, so called 1 amu
centroid mass spectra are used to calculate mass accuracy. The exact mass
is thus taken as the maximum intensity of the mass spectrum within
a certain window (±0.5 u) around the nominal mass. Mass accuracy
was calculated for four different ion masses of four different substances:
HFC-134a (CF3+, 68.995 u), CFC-12 (CF235Cl+,
84.866 u), CFC-11 (CF35Cl2+, 100.936 u,) and
methyl iodide (CH3I+, 141.928 u), which cover most of the mass
range of the substance peaks in our chromatogram. Individual values for the
mass accuracy were taken at the maximum of each chromatographic peak. Data
from reproducibility experiments (see Sect. ) as well as regular
sample measurements were analysed to gain information about mass accuracy for
the four exemplary ion masses. Only measurements taken under well-equilibrated conditions were used for this analysis. As the first two
measurements of a measurement day often show enhanced variability they were
excluded from the analysis of the mass accuracy.
Stability of the mass axis and instrument sensitivity
To evaluate the stability of the two mass spectrometers with respect to
sensitivity and accuracy of the mass axis, a reproducibility experiment was
used. The relative difference between the minimum and maximum detector
response of the day and the 1σ standard deviation of all measurements
over this day were taken as measures of the drift. For drift in mass accuracy
over the day, the mean value and the 1σ standard deviation are given
for the main masses for the following four compounds: HFC-134a
(CF3+, 68.995 u), CFC-12 (CF235Cl+,
84.866 u), CFC-11 (CF35Cl2+, 100.936 u,) and
Iodomethane (CH3I+, 141.928 u). To evaluate the stability of
the mass accuracy over a longer time period, the mass accuracy was calculated
on measurement days with different time differences since the last mass
calibration tune.
Three exemplary halocarbon/hydrocarbon fragment pairs with equal
unit mass but differing exact mass. The qualitative separating resolution
(qual. Rsep) with nσ = 2 and the quantitative separating
resolution (quan. Rsep) with nσ=8.
Exact mass
Δm
Qual.
Quant.
Fragment
m [u]
[u]
Rsep
Rsep
(nσ=2)
(nσ=8)
CClF2+
84.966
0.136
>600
>2500
C6H13+
85.102
CF3+
68.995
0.075
>900
>3700
C5H9+
69.070
C2H335Cl37Cl+
98.958
0.159
>600
>2500
C7H15+
99.117
The difference of the minimal (Min) and maximal (Max) values in %
in one reproducibility experiment for the relative response is shown with a
1σ relative standard deviation (RSD) over all measurements (20) on
this day. In the comment line the trend of the calibration gas over the day
is given.
Mass
Substance
Max-Min
RSD
Comment
spectrometer
[ % ]
[ % ]
TOF MS
CFC-12
4
1.41
linear
QP MS
CFC-12
4
1.28
linear
TOF MS
CFC-11
5
1.32
linear
QP MS
CFC-11
5
1.38
linear
TOF MS
Halon-1211
7
1.97
linear
QP MS
Halon-1211
1
0.63
linear
TOF MS
Iodomethane
10
3.73
scatter
QP MS
Iodomethane
5
1.92
scatter
Limits of detection
The lowest amount of a substance that can reliably be proven is considered to
be its LOD and serves as a measure for the sensitivity of the analytical
system. Based on the assumption that a molecule fragment (f) can be
detected when its detector signal height (Hfi) is equal to or higher
than 3 times the signal noise (Nfi) on the adjacent baseline
(signal-to-noise level (S/N)>3), a limit of detection for
a fragment (fi) from an analyte substance (Si) with a mass
(mSi) in the injected sample can be calculated as
LODSi=3⋅Nfi⋅mSiHfi.
For comparison with the QP MS, the LOD of both instruments were
calculated from calibration gas measurements by linear downscaling. Possible detector non-linearities were omitted in this
case. The LOD error was considered to be the standard deviation of 10
calculated limits of detection. Different settings of the QP MS (SCAN
mode (1), optimised (opti.) SIM mode (2) and operational (oper.) SIM
mode (3)) were applied. In the SCAN mode (1), the quadrupole MS
scanned from 50 to 500 u (comparable to the mass range of the
TOF MS) with a dwell time of ≈3.7 msion-1 and
a scan rate of 1.66 scans per second. In the optimised SIM mode (2),
the quadrupole MS measured only one ion with a dwell time of
310 ms with ≈3 scans per second. In the operational
SIM mode (3) the quadrupole MS measured several masses (up to six) in
one scan with individual dwell times given in Table and
≈3 scans per second.
The limit of detection (LOD) in ppq and pg of the
substances CFC-12, CFC-11, Halon-1211 and Iodomethane in 1 L of air
sample per detector. The dwell times and settings for the QP MS are given in
Table . The given errors are 1σ standard
deviation.
LOD TOF
LOD TOF
LOD QP
LOD QP
LOD QP
LOD QP
LOD QP
LOD QP
Substance
[ppq]
[pg]
[ppq]
[pg]
[ppq]
[pg]
[ppq]
[pg]
SCAN (1)
SCAN (1)
opti. SIM (2)
opti. SIM (2)
oper. SIM (3)
oper. SIM (3)
CFC-12
25±2
0.12±0.02
241±19
1.18±0.09
21±3
0.10±0.01
48±6
0.23±0.30
CFC-11
31±2
0.17±0.02
370±19
2.05±0.29
36±1
0.20±0.01
64±9
0.35±0.05
Halon-1211
27±2
0.182±0.004
276±53
1.84±0.13
36.0±0.3
0.240±0.002
43±5
0.29±0.02
Iodomethane
12.00±0.01
0.069±0.001
Not a Number
Not a Number
16±1
0.090±0.003
42±2
0.24±0.05
The LOD in pg and ppq were calculated for 0.28 L
sample volume with respect to the split ratio (see Sect. )
and then extrapolated to 1 L of ambient air.
Schematic display of two different mass resolutions (blue and black
curves). Two signals on masses 84.966 and 85.102 u with equal
intensities demonstrate the mass separation with R=600 (blue curve) and
R=3700 (black curve). Assuming Gaussian peak shapes for the signals, R=3700 separates both peak by 8σ (quantitative separation) and R=600
separates them by only 2σ (qualitative separation).
Reproducibility and measurement precision
The measurement precision describes the repeatability of a measurement. We
determine the precision from the reproducibility (i.e. the standard
deviation) of the measurements. The mean reproducibility is derived from
dedicated multiple experiments designed to assess measurement precision
(reproducibility experiment). Reproducibility was analysed over five
measurement series, conducted on 5 different days, to give the mean
measurement precision. Every experiment followed the procedure described in
Sect. , with a total of 19 evaluated measurements of the same
ambient air sample. A subset of the samples was treated as standard, the
other part as unknown samples (two samples bracketed by two standards). Every
individual measurement of these five series was conducted with
a preconcentration volume of 0.28 L of the reference gas. Two
additional reproducibility experiments were conducted with a higher
preconcentration volume of 1 L to assess the possible dependence of
the reproducibility on the preconcentrated sample volume. For each sample
pair, the standard deviation of the relative response was calculated, summed up
over all pairs and divided by the number of pairs to form the sample pair
measurement reproducibility of that measurement series. The described
procedure was applied to all analysed substances and reproducibility
experiments. The mean value of measurement reproducibilities is considered to
be the measurement precision of the system for the respective substance and
volume.
The reproducibility (REP) for the QP MS and the TOF MS as a mean
value of five measurement series with 20 measurements each and
a preconcentration volume of 0.28 L. The given errors are 1σ
standard deviation over five reproducibility experiments.
Substance
Formula
REP QP [ % ]
REP TOF [ % ]
CFC-12
CCl2F2
0.56±0.31
0.56±0.18
CFC-11
CCl3F
0.45±0.26
0.54±0.23
Halon-1211
CBrClF2
1.56±0.52
0.94±0.39
Iodomethane
CH3I
3.96±0.72
3.44±1.61
Detector linearity
Detector linearity was analysed in two linearity experiments by
varying the default preconcentration volume of 0.28 L by
factors of 0.33, 0.66, 1.25 and 2 (sample positions in the measurement
sequence, see Sect. ). As calibration measurements, the
default preconcentration volume was used. For comparison, detector
responses were calculated as the ratio of the area of
a chromatographic peak (A) to the preconcentration volume (V). All
detector responses were normalised to 1 (relative detector response)
by dividing them by the mean A/V of the calibration measurements. An
ideally linear detector would show a relative response of 1 for any
preconcentration volume used. The errors for the linearity
measurements were derived as the 3-fold standard deviation given
from reproducibility experiments.
Results and discussion
Mass resolution
If mass resolution is sufficiently high, it is possible to separate mass
peaks of equal unit mass but differing exact mass. This separation
drastically enhances the possibility to identify specific molecule fragments
and to reduce cross-sensitivity. For halocarbon analysis, it is interesting
to separate halogenated molecule fragments with exact masses typically below
unit mass from other fragments with exact masses typically at or slightly
above unit mass (e.g. hydrocarbon fragments). It could then be possible to
reduce background noise generated by interfering ion signals or even
compensate co-elution of non-target species from the GC column. For
quantitative analysis the separation of adjacent mass signals implicates
a possible loss of signal area when both mass peaks are not fully separated.
The imposed error, i.e. the peak area lost due to separation, should not
decrease measurement precision and should therefore be lower than the
targeted measurement precision, in our case 0.1 %.
The reproducibility (REP) for the QP MS and the TOF MS as a mean
value of two measurement series with 20 measurements each and a
preconcentration volume of 1.00 L. The given errors are 1σ standard
deviation over two reproducibility experiments.
Substance
Formula
REP QP [ % ]
REP TOF [ % ]
CFC-12
CCl2F2
0.22±0.10
0.23±0.09
CFC-11
CCl3F
0.14±0.03
0.16±0.00
Halon-1211
CBrClF2
0.60±0.05
0.55±0.21
Iodomethane
CH3I
1.31±0.23
0.99±0.30
For this purpose, the definition of a qualitative and a quantitative
separating resolution RSep is introduced (see Fig.
for an illustration). Assuming a Gaussian peak shape (normal distribution) of
the ion signal on the mass axis, a separation of two neighbouring signals
m1 and m2 (with m2>m1) by 8σ (SD, 4σ per peak) is
considered a quantitative separation (less than 0.01 % loss of peak area)
while a separation by less than 8σ is considered to be only
a qualitative separation. Further assuming that 1σ is approximately
1/2 FWHM (or 1/2 Δm respectively) and that Δm1 is not
significantly different from Δm2, one can estimate RSep
(at m1 or m2) for a known (m2-m1) difference:
Rsep=m1Δm1=m12⋅(m2-m1)nσ.
For a value of nσ = 8, Eq. () gives the quantitative
separating resolution, while for a value of nσ = 2 it gives a qualitative
separating resolution. Table shows some examples for qualitative and
quantitative separating resolutions required for separation of halogenated
mass fragments from hydrocarbon molecule fragments with slightly different
masses.
To separate e.g. the CClF2+ ion signal from the C6H13+
ion signal qualitatively, a resolution of 600 is necessary. For
a quantitative separation, the mass resolution has to be R=3700 according
to the definition of 8σ separation (see above). For the Bench
TOF-dx, the calculated mass resolution was R=1000 at mass
218.985 u for the fragment C4F9+ in a mass calibration tune
by the software ProtoTOF. This allows a qualitative separation of two
neighbouring mass peaks like the ones listed in Table , e.g. the
separation of mass 84.966 u to mass 85.102 u. An example of a
mass spectrum centred around 85 u is shown in Fig. for
a chromatogram of a typical ambient air sample at a retention time of 11.35
minutes. Two mass peaks, one centred at 84.943 u
(CH35Cl37Cl+), a fragment of the Trichloromethane
(CHCl3) molecule and one with a mass slightly above unit mass, can be
clearly distinguished. The higher mass is the result of an unidentified
hydrocarbon peak eluting shortly before the Trichloromethane peak.
The resulting chromatogram centred at 11.3 minutes is shown in Fig.
. Three different mass ranges were extracted from the raw
data, the nominal mass range from 84.5 u to 85.5 u, the lower
mass range from 84.7 u to 85.0 u and the higher mass range
from 85.0 u to 85.3 u. When extracting the information
centred around the unit mass range, a double peak is observed. An extraction
of the lower mass range of the 85 u signal yields a much lower signal
in the earlier eluting peak yet the signal cannot be reduced to baseline
level. An extraction of the higher mass range of the signal gives a larger
signal for the earlier eluting peak, but again the signal does not drop to
baseline level.
This shows that the mass resolution of the Bench TOF-dx is sufficient to
qualitatively show that two different fragments are present but that the
resolution does not allow the separatation of these fragments in a way sufficient for
quantifications. For a quantitative separation as defined above, the mass
resolution of the Bench TOF-dx is not sufficient without further data
processing steps like a peak deconvolution.
So-called 0.01 u mass spectrum of the substance
Trichloromethane. Two mass peaks are shown. The higher one by mass
84.9 u is identified as the molecule fragment
(CH35Cl37Cl+) and the other one by mass 85.1 u is an
unidentified hydrocarbon peak.
Mass accuracy
While sufficient mass resolution is necessary for an unambiguous separation
of two mass peaks, mass accuracy is in addition needed for chemical
identification of the detected ion. The better the mass accuracy, the lower
the number of possible fragments that might be the source of the mass signal.
The mass accuracy for the Bench TOF-dx was found to be in a range of 50 to
170 ppm for a mass range from 69 u to 142 u. Mass
accuracies for the analysed target masses were determined as follows:
(100±60) ppm for mass 68.995 u, (80±50) ppm for 84.966 u,
(120±50) ppm for 100.936 u and (130±40) ppm for 141.928 u. A correlation between the displayed
masses is observed: when the accuracy of one mass is decreased, the others are,
too. There is no correlation given by the proximity of target masses to
tuning compound (PFTBA, e.g. 68.995 u) masses. A suspected reason
for the instability of the mass axis is the instrument temperature and
resulting changes in material elongation. This is, however, speculation. At
a mass resolution of R=1000 at ion mass 85 u and an accuracy of
100 ppm, the mass difference between measured and exact mass would be
10 % of the FWHM of this mass peak (or 5 % at 50 ppm). The
stability and absolute accuracy in the determination of the exact mass is
thus not a significant additional limitation in the ability of the Bench
TOF-dx to separate different ions (see Sect. ).
Stability of the mass axis and instrument sensitivity
A reproducibility experiment was used to evaluate the stability of two
detectors over a measurement series (typically 10 h). For that purpose, the
minimum and maximum value of the detector response relative to all recorded
responses and the 1-fold relative standard deviation of all recorded
responses were used (see Table ).
For the substances CFC-11 and CFC-12 the drift of the sensitivity of the TOF
MS and QP MS are on the same level. For the low concentrated substances, the
drift of the TOF MS is higher than that of the QP MS.
A chromatogram of an unidentified hydrocarbon peak (smaller one)
eluting slightly earlier than the higher Trichloromethane peak. The nominal
mass 85 u (black) shows a double peak. By choosing the lower mass
range (84.7 u to 85.0 u; red) a lower signal for the
unidentified hydrocarbon peak is observed, and by choosing the higher mass
range (85.0 u to 85.3 u, blue) a lower signal for the
Trichloromethane peak is observed.
For evaluating the stability of the mass axis, the drift over a day was
calculated as mean accuracy and standard deviation (1σ). The stability
over a long time period was observed over different days away from a mass
accuracy tune. As shown in Sect. the mass accuracy of the Bench
TOF-dx was observed to be on the order of 50–170 ppm. Within this
uncertainty no drift of the mass axis with time could be observed for periods
of up to 19 days after the mass axis calibration. The stability and absolute
accuracy in the determination of the exact mass is thus not a significant
additional limitation in the ability of the Bench TOF-dx to separate
different ions (see Sect. ).
Limits of detection
For halocarbon measurement, sensitivity is an important issue as
atmospheric concentrations can be below 1 pgL-1 of
ambient air, especially for newly released anthropogenic
species. Table shows the calculated LOD for the QP and the
TOF MS for the four selected species with different measurement
settings of the quadrupole MS detector.
For the QP MS, the signal-to-noise level of a certain m / z depends on
the concentration and dwell time. The dwell time represents the time interval
in which the quadrupole mass filter is tuned to the specific mass-to-charge
ratio m / z before switching to the next mass setting. Lower dwell
times will decrease sensitivity but allow for more different mass filter
settings per scan, resulting in more different m / z monitored per
time. Higher dwell times increase the detector sensitivity towards specified
m / z ratios but reduce the number of m / z monitored per time.
For this work, data based on three different instrument settings were used for
LOD calculation (see Table ). The SCAN mode of the QP MS was
chosen for a direct comparison with the TOF MS (scan range from 45 u
to 500 u) and is shown in Table (1). Higher and lower
m / z ratios were discarded. Reducing the scan range will result in
better detection limits for the QP MS and theoretically also for the TOF MS
as long as no significant amounts of ions heavier than the chosen upper scan
limit are produced in the ion source. Remaining ions in the TOF MS flight
tube from a preceding extraction would result in unambiguous detector
signals. The optimised SIM mode monitors only one m / z of the
respective substance, Table (2). In measurements of ambient air,
several m / z are usually monitored simultaneously (operational SIM
mode (3)). The dwell times are optimised for the different substances. For
substances with high concentration shorter dwell times are chosen, while the
dwell time is increased for substances with low concentrations in order to
increase the sensitivity. Only one ion is measured for most species in order
to reach optimum sensitivity. As a consequence, limits of detection are
higher in such measurements as in the optimised SIM mode. Respective LOD for
the discussed dwell time settings are shown in Table .
In comparison to the QP MS, the TOF MS is up to 12 times more
sensitive than the QP MS in the SCAN mode. In the optimised SIM mode
with increased dwell times (2) for specific ion masses, limits of detection in quadrupole MS and time-of-flight MS are similar. During
routine measurements (operational SIM mode (3)), the limits of detection of the TOF MS were up to a factor of 3 lower than those of
the QP MS.
Linearity graphs of CFC-11 (CFCl2+ fragment) based on two
different linearity experiments (red and black plots in each graph). Primary
x axis (lower): mass on column in ng. Secondary x axis (upper):
preconcentration volume variation in % versus a default preconcentration
volume of 0.28 L (dashed line). Y axis: deviation from the
normalised relative detector response versus the detector response of the
default preconcentration volume). For every preconcentration volume, the
relative response should be one in case of a linear detector behaviour
(dashed line). The error bars show the three-fold measurement precision, on
the left-hand side for the QP MS and on the right-hand side for the TOF MS.
The second linearity experiment (black) of the TOF MS was conducted with an
decreased detector voltage (-2274.8 V instead of
-2244.8 V).
Reproducibility
A high measurement precision is required as it is of great importance to
detect very small variability of halocarbons in the atmosphere, e.g. to
characterise trends of highly persistent substances . Table shows exemplary reproducibilities
for both instruments based on a preconcentration volume of 0.28 L.
The reproducibility is rather similar for both MS, with values below 1 %
for the species with high ambient air concentrations and therefore high
signal-to-noise levels (CFC-12 and CFC-11). For the species with lower
concentration and lower signal-to-noise levels the reproducibility of the TOF
seems to be slightly but not significantly better (see Table ).
Same figure as Fig. for the substance CFC-12
(CF2Cl+ fragment).
The reproducibilities shown in Table are based on measurements with
a relatively small sample volume. Larger preconcentration volumes should
result in better reproducibilities as signal-to-noise levels are increased
and error sources during sample preparation should become smaller relative to
the sample volume. Therefore, two reproducibility experiments with a larger
preconcentration volume of 1 L were performed. The results are shown
in Table .
The increase of the preconcentration volume to 1 L yields
a significant improvement of the measurement precision. The high signal-to-noise species CFC-12 and CFC-11 now show reproducibilities below 0.3 %
for the QP and for the TOF. For the low signal-to-noise species Halon-1211
and CH3I the reproducibilities are improved by a factor of up to 4
for the TOF MS and by a factor of up to 3 for the QP MS, with the TOF
instrument showing better reproducibilities. As for the TOF MS, the detector
itself was found to be a limitation to higher preconcentration volumes as it
showed saturation effects for some analysed ions already at 0.5 L
preconcentrated sample. For example, CFC-12 had to be evaluated on mass 87 u
(relative abundance: 32.6 %) and CFC-11 on mass 103 u (relative
abundance: 65.7 %) as both main quantifier ion masses (85
and 101 u) showed saturation in the respective retention time
windows. This saturation reflects the limited dynamic range of the analog to
digital converter (memory of 8 bits) used in the Bench TOF-dx.
Linearity
For the calculation of the mixing ratio of a measured substance, its
detector signal has to be correlated with the signal of the same
substance in a calibration measurement with known mixing ratio. If the
detector behaves linearly, this correlation is linear and the
calculation of the mixing ratio is straight forward. As mixing ratios
in different air samples might vary to a great extent (e.g. diurnal
variations of short-lived substances) , a linear detector simplifies data evaluation to a great
extent. Furthermore, retrospective analysis of substances that were not
identified at the time of measurement is possible without an unknown error
due to detector non-linearity. Figures and show linearity
plots for the QP MS for the CFC-11 and CFC-12 based on two linearity
experiments. The QP MS showed a linear behaviour within the measurement
errors (3-fold measurement reproducibility for the respective substance).
This linearity test includes possible effects of the preconcentration unit
(quantitative adsorption and desorption) as well as the determination of the
preconcentration volume, the GC and data processing (signal integration).
Figures and illustrate results from the two linearity
experiments for the TOF MS. For CFC-11 (Fig. ) a deviation from
linearity for small preconcentration volumes of nearly 10 % is observed,
while detector behaviour is close to the ideal value for high
preconcentration volumes. The red curve was derived based on the standard
detector voltage of -2244.8 V. An decrease of the detector voltage
by -30 V brought slight improvements but did not solve the issue.
Figure shows a linearity plot for the substance CFC-12. For CFC-12
the detector is considered to be linear within the error bars. Both detectors
compared in this work depend on the same sample preparation and separation
steps before detection. As measurement reproducibilities of QP MS and TOF MS
were not significantly different, the direct comparison is possible without
limitations. The examples displayed for the QP MS and the TOF MS are two of
35 substances measured and analysed. The QP MS showed linear behaviour for
all substances within the uncertainty range. The non-linearity of the TOF-MS
was highest for the low preconcentration volume (33 %, 0.09 L)
with deviations of -10 to +20 % compared to a standard preconcentration
volume of 100 % (0.28 L). For most substances the instrument
showed a similar behaviour as observed for CFC-11 (decreased sensitivity for
low amounts of analyte) while some species showed the opposite behaviour
(increased sensitivity with decreasing amount of analyte). Reasons for this
conflicting behaviour are still subject to further investigations.
Proportionality of detector signal against the amount of analyte in the
sample over the given concentration range was thus found for the QP MS but
only for some species in the TOF MS. If the detector does not behave
linearly, the relationship between the integrated peak area and the
atmospheric concentration has to be approximated by a fit function. In order
to generate this fit function, additional measurements with varying
preconcentration volumes are necessary before each measurement series. This
procedure was found to be necessary for the TOF MS. It lengthens measurement
series, implies an additional error source and requires additional time for
data processing.
Conclusions
A Markes International Bench TOF-dx was compared to an Agilent Technologies
5975 QP MS with respect to the measurement of halogenated trace gases in the
atmosphere. Both detectors ran in parallel (66:34 split) after cryogenic
preconcentration and gas chromatographic separation of the air sample. The
comparison included the mass resolution, mass accuracy, the limit of
detection, the measurement precision (reproducibility) and the detector
linearity. The TOF MS showed a resolution of 1000 and a Δm of 0.071
at mass 219.995 u with a mass accuracy of 50 to 170 ppm.
Therefore it is able to qualitatively separate ion signals at different exact
mass but equal unit mass (for example the mass 84.966 u from the mass
85.106 u by a Δm of 0.136). This qualitative mass separation
of the TOF MS could be sufficient for improved substance identification and
is an advantage over the QP MS. The QP MS does not allow for separation of
exact masses as the mass resolution of QP MS instruments is generally too low
(R≈200) for that purpose. The analysis of detection limits showed
that the TOF MS is generally more sensitive than the QP MS (despite using
selected ion monitoring mode). The LOD of the QP in the SCAN mode are up to
a factor of 12 higher than the LOD of the TOF MS. LOD of the TOF MS are lower
by factors of up to 3 (Table ) in comparison to the QP MS with
operational SIM mode settings used for routine measurements. In the SIM mode
with only one quantifier (optimised SIM mode) the TOF MS is similar to the QP
MS. In that respect, the TOF MS with its very high sensitivity and full mass
range information provides a considerable advantage compared to a QP MS. The
reproducibility of both instruments was found to be on an equal level with
slightly better reproducibilities of the QP MS at high signal-to-noise levels
and slightly better reproducibilities of the TOF MS for low-concentrated
species. Regarding detector linearity, the Bench TOF-dx in its current
configuration could not compete with the QP MS. A high degree of linearity
is,
however, necessary for high accuracy measurements in trace gas analysis. The
encountered non-linearities necessitate a correction which adds an error
source, especially when there is a large concentration difference between
sample and calibration measurement. It furthermore complicates measurements
as well as data evaluation. For other applications where concentration
variability is significantly higher than the non-linearity of the detector,
the observed detector non-linearities might not be of such high relevance.
In conclusion, the TOF MS does show advantages with respect to mass resolution and
sensitivity without losing the full mass spectra information. Persisting
non-linearities are a big disadvantage but might be conquered in the future
by developments in detector electronics. With reduced non-linearities, TOF MS
could well be the technology of the future for the analysis of halogenated
trace gases in the atmosphere, despite the significantly higher costs of the
TOF MS in comparison to QP MS instruments. These conclusions are only valid
for the Markes International Bench TOF-dx E-24 MS and atmospheric trace gas
measurements and might turn out differently for another field of research or
another TOF MS.