pittcon 2011 mass spectrometry for hydrogen application: novel high dynamic mass range (hdmr) magnet
TRANSCRIPT
ITT | OI Analytical, 2148 Pelham Parkway, Bldg 400, Pelham, AL 35124Omar Hadjar, Gottfried Kibelka, Scott Kassan, Chad Cameron, Ken Kuhn
Experimental Results
IntroductionMany industries rely on the tight control of chemical processes to profitably produce a high quality product. High sample throughput and a dependable analyzer enable real-time analyses from receiving inspection to post-production verification. Spatially dispersive, non-scanning mass spectrometers (MS) continuously provide full mass spectra, thus
enabling rapid analytical techniques with enhanced sensitivity.
Miniaturizing a sector-field MS and joining it with a detector array that combines high spatial resolution and linear response results in a transportable instrument offering speed and precision, the IonCam . Ion beams shaped and focused by the ion optical elements of the double-focusing MS are detected by an IonCCD in the focal plane of the
instrument. This allows the observation of many chemical processes in real time. Sequences of mass spectrometric frames up to 360 frames/s (< 3 ms temporal resolution) will greatly help understanding the dynamics of various processes.
Experimental Setup
References
Simulation: Design upgrade
The ions are
finally m/z dispersed
and energy focused on to a focal plane.
The m/z peak width is proportional to the
slit width, the magnetic radius and inversely proportional
to the electrostatic radius. The mass range of the analyzer, better
defined as Mmax/Mmin, is given by the analyzer geometry. The first parameter
is the blind plane length (lbp), defined as the distance between the ion entrance
on the magnet and the first pixel of the detector array. The second parameter is the focal plane
length (lfp) which is defined by the array detector length: Mmax/Mmin = (1 + lfp / lbp)2
1170 1200 1230 1260 1290 1320
-10
0
10
20
30
1170 1200 1230 1260 1290 1320
-10000
0
10000
20000
30000
10-1
100
101
102
103
102
103
104
105
106
+101
with averaging
103 with
out avera
ging
100 ion/dN
Ion
CC
D r
esp
on
se a
t 1
5 m
s in
t. t
ime
(dN
)
ion beam current (pA)
50 ion/dN
1 frame:
S/N=188/107=1.8
Ion
CC
D s
ign
al
(dN
)
pixel number
200 frames:
S/N=188/10.7=18
Ion
CC
D s
ign
al
(dN
)
pixel number
The IonCCD1, 2 is designed around
the MH-MS analyzer, hence made of
a single array of pixels (one-
dimension) spanning 51 mm length,
matching the focal plane length (lfp)
and 1.5 mm pixel height matching
the magnet air gap. The pixel width is
21 m and insulating gap is 3 m
providing a pitch or spatial resolution
of 24 m. The result is a total of
2126 pixels with 88 % pixel area
ratio (PAR). The detector operates
from atmospheric pressure to high
vacuum as no high voltage is used.
The IonCCD analogue voltage signal is digitized
by a 16 bit ADC so all data are expressed in digital
numbers (dN). The IonCCD has a response of 100
ions/dN/pixel.2 The noise floor when not cooled
and frame averaged is 9 dN/pixel. In terms of ion
current density, the IonCCD has a limit of
detection (LOD) of 0.5 fA/pixel.2 The detector
provides up to 360 frames or spectra per second
providing about 3 ms temporal resolution.
Imin(A/pixel)=R*3*n*q/t
Temporal resolution: 3 ms
Integration time: t=83 s-5s
Frame rate: 360 Hz
Spatial resolution (pitch): 24 m
Array size: 51 x 1.5 mm2
Total pixels: 2126
Noise floor: n=9dN
Response: R=100 ions/dN/pixel
Single-frame dynamic range: >1000
Detector size: 105 x 67 x 30 mm3
Operation Pressure: 103 to 10-8 torr
Operation bias voltage: 0 to 3 kV
0 200 400 600 800 1000 1200 1400 1600 1800 2000
10
100
1000
10000
Ion
CC
D s
ign
al (
dN
)
IonCCD pixel number
10 20 30 40 50 280 290 300 310 320 330 340
10
100
Ion
CC
D s
ign
al (
dN
)
IonCCD pixel number
To achieve atomic hydrogen detection, the standard magnetic sector (1.04 T) used for routine mass window detection of [8,
128] u is replaced by a weaker one (0.38 T) allowing mass window detection of [1, 16] u & [4, 64] u. In the case of the 0.38 T
magnet, the first and second window are achieved by a 1000 and 250 V acceleration respectively. The mass spectrum shown
below is produced with a 1000 V acceleration and the signal is expressed in a logarithmic scale. The orange and black spectra
are taken with a He sampling bag on and off respectively. The atomic hydrogen ion, or proton, originates from the residual gas
and is visible on both spectra. The ion peak at mass 2 u in the graph bellow is produced by doubly charged He ( alpha particle)
and or molecular hydrogen ions. The zoom around mass 1 u and 2 u is shown to the right. The zoom clearly shows the base line
separated double peak structure demonstrating that both species are detected and nicely separated.
-1 0 1 2 3 4 5 6 7 8 9 10 48 49 50 51
0.0
0.5
1.0
1.5
2.0
2.5
m=
71
u,
FW
HM
= 1
19
m
m=
69
u,
FW
HM
= 1
12
m
m=
70
u,
FW
HM
= 1
12
m
m=
68
u,
FW
HM
= 1
08
m
m=
72
u,
FW
HM
= 1
21
m
m=
67
u,
FW
HM
= 1
14
m
m=
6 u
, F
WH
M=
46
m
m=
5 u
, F
WH
M=
45
m
m=
4 u
, F
WH
M=
49
m
m=
3 u
, F
WH
M=
61
m
m=
2 u
, F
WH
M=
92
m
last active pixel
Tra
nsm
issi
on
(2
4
m b
in h
isto
gra
m)
IonCCD array (mm)first active pixel
1.000 T 1.045 T
1.2 kV
100 m slit
50 mm ESA
m=
1 u
, F
WH
M=
19
7
m
High Mass Dynamic Range Magnetic Assembly
0 1 2 3 4 5 6 66 67 68 69 70 71 72 73
0.0
0.5
1.0
1.5
2.0
2.5
M=
72
.03
9 u
, M
= 0
.30
8 u
M=
71
.02
9 u
, M
= 0
.30
1 u
M=
70
.00
0 u
, M
= 0
.28
2 u
M=
68
.98
7 u
, M
= 0
.27
9 u
M=
67
.96
7 u
, M
= 0
.26
7 u
M=
66
.95
1 u
, M
= 0
.28
0 u
M=
6.0
37
u,
M=
0.0
33
u
M=
5.0
07
u,
M=
0.0
29
u
M=
3.9
86
u,
M=
0.0
28
u
M=
2.9
79
u,
M=
0.0
30
u
M=
1.9
82
u,
M=
0.0
37
u
1.045 T
last active pixel
Tra
nsm
issi
on
(2
4
m b
in h
isto
gra
m)
m (u)first active pixel
1.000 T
1.2 kV
100 m slit
50 mm ESA
M=
1.0
14
u,
M=
0.0
56
u
High Mass Dynamic Range Magnetic Assembly
3rd
order polynomial fit using 12 peaks
The mass range of the present MH-MS appears to be acceptable when it is tuned for a minimum mass of
C+, resulting in a mass window of about 200 u. However for hydrogen applications, the mass range drops
to a very narrow mass window of 16 u. One can consider scanning the energy to produce different mass
windows which then can be stitched. This would, first, defeat the whole purpose of the non-scanning
instrument. Secondly, it would decrease the duty cycle drastically. Calibration of masses must be
performed for every energy. Also, the signal must be corrected for the dependence of ion transmission on
the energy. Last but not least, more complex software is needed to orchestrate all of the steps above. To
attack this problem, one could think of an increase in the dynamic mass range providing a acceptable
detectable mass window at single acceleration. This is achieved either by increasing the array detector
length (or butting two IonCCDs) and increasing the magnet size or reducing the blind plane length (lbp).
While the first option is very costly with analyzer size increase, the second option is straight forward with
no instrument size increase, allowing the use of the same detector. This last option calls only for a slight
change of the magnet shape, referred to as High Dynamic Mass Range Magnet (HDMR). The question is
how would the peak shapes and mass calibration suffer from such a change? The simulation above shows
the scenarios where lbp is shortened from 17 to 7 mm, theoretically increasing the dynamic mass range
from 16 to 69. A dynamic mass range of 72 is simulated. This is a welcome byproduct of the reduced lbp
(lower B-field at low mass due to the lack of magnetic material). The mass calibration, in this case, would
be performed using both B values for low (blue) and high mass (green), introducing no serious calibration
issues, especially at such unit mass resolution. However this outcome needs to be proven experimentally
in the near future. The result is a full mass range as observed in the above simulated spectra.
Mass calibrated spectrum showing the mass resolution of the
instrument over the detected mass window [1, 72] u.
Conclusions
Beam profile of the dispersed m/z ions with energy and direction
focus along the 51 mm long focal plane IonCCD array detector.
• Non-scanning MH-MS instrument is used in combination with a focal plane array detector IonCCD
• The IonCCD shows a 103 dynamic range within a single frame with a 0.5 fA/pixel sensitivity
• The IonCCD acquisition speed of 360 Hz is key for fast GC and GCxGC (< 100 ms peak) applications
• Preliminary experimental data show promising results with Hydrogen to Oxygen detection
• Baseline separation between He2+ & H2+ (2 u nominal mass) is achieved at 1000 V and 0.38 T
• Simulation suggests a possible dynamic mass range increase with no loss of instrument performance
At 70 eV, the electron impact ionization cross section3 of H2 is about 1*10-16 cm2, smaller than
those of N2 and O2 which are about the same, 2.6*10-16 cm2. The tight focus of H+ and H2+ due
to the short magnetic radii compensate the latter effect. Despite the fact H+ and O+ are
detected with a lag of 3 s at 1000 V (SIMION 8.0), the simultaneous detection statement
remains correct at 99.9 %, considering the 3 ms temporal resolution of the existing camera (83
s minimum integration time). In the best case scenario, with the ultimate camera system
matching the chip’s maximum frame rate of 2 kHz (500 s temporal resolution), the
simultaneous detection will only drop to 99.4 %. The dynamic mass range increase (see next
part) will increase the above detection lag to about 5 s, which is of no consequence (99 %).
1. M. P. Sinha and M. Wadsworth. Miniature focal plane mass spectrometer with 1000-pixel modified-CCD detector array for direct ion
measurement. Rev. Sci. Instr.76 025103(2005).
2. O. Hadjar et al. IonCCDTM for direct position-sensitive charged-particle detection: from electrons and keV ions to hyperthermal biomolecular
ions. J. Am. Soc. Mass Spectrom. Online (2011) April issue cover.
3. W. Hwang and Y.-K. Kim. New model for electron-impact ionization cross sections of molecules. J. Chem. Phys. 104 2955(1996).
Monitoring Hydrogen and Gaseous Fuels
using a Double-Focusing Mass Spectrometer
Pittcon 2011, Atlanta
The gas phase sample is introduced through a fused silica capillary to the ionization chamber. The
molecules are ionized by 70 eV electron impact generated by a hot rhenium filament, not
shown below for clarity. The electron impact (EI) ion source is biased
at 1000V. The double focusing analyzer of Mattauch-Herzog geometry
(MH-MS) is operated at ground potential. The ion energy is
then defined by the EI source bias. The ions are
extracted through a 100 m object slit and
directionally focused through
a 31.8o electrostatic
analyzer.