Evaluation of the Classification Performance of the New Centrifugal Particle Mass Analyzer
Jonathan P.R. Symonds
Cambustion Ltd, J6 The Paddocks, 347 Cherry Hinton Road, Cambridge CB1 8DH, UK
[email protected]
PSL spheres have in the past been used to characterise
the prototype CPMA (Olfert et al., 2006), and both the
commercial Kanomax APM 3600 (Tajima et al., 2011)
and APM 3601 (Tajima et al., 2009) instruments. This
is an initially appealing method; PSL is a certified
standard for size (and hence if the density is known,
mass can be estimated). The monodispersity of the
PSL peak minimizes the population of doubly charged
particles (of twice the setpoint mass) which may pass
through the DMA. Note that the DMA is still required
to remove the “surfactant mode” from the PSL and to
ensure all particles are charged.
Using a mass equivalent size metric, the resolution of the
CPMA may be narrower than that of the DMA, and almost
certainly will be narrower than the PSL size distribution:
Penetration
radial
location (m)
90%
+
6.00E-01
5.00E-01
1+
4.00E-01
3.00E-01
0 .03
4400n n
mm
0.04
0.05
0.0 6
41
41nm
nm
Desired Mass
0 .07
0.0 8
0 .09
0.1
length along classifier (m)
Too Heavy
unstable transfer fn
Too Light
The CPMA, by contrast to the APM, sets up a system
of forces which balance across the entire classification
region. It achieves this by spinning the inner cylinder
slightly faster than the outer cylinder:
Static Penetration (losses by diffusion)
50%
40%
Dynamic Penetration
30%
2+
0.00E+00
60
65
70
75
80
85
0
90
50
95
Dp* (nm)
The other drawback is that the NaCl particles are
cubic, and this affects their classification in the DMA.
However, their dynamic shape factor can be used to
correct for this:
100
150
DMA Size (nm)
200
250
The next plot shows the losses only attributable to
dynamic operation of the CPMA, calculated by
correcting the total losses by the static losses.
Dynamic Particle Loss (diffusion corrected)
100%
90%
χ f , NaCl = 1.08
d meCc (d me )
χf =
d veCc (d ve )
Response (a.u.)
0.02
60%
0%
1.00E-01
0%
3399n n
mm
70%
10%
0%
0 .01
80%
20%
2.00E-01
0.052 5
0
Penetration
100%
7.00E-01
100%
100%
0.0 5
In the plot below, the blue data points show the
penetration with the CPMA not rotating and the voltage
switched off. Losses in this case represent those due to
diffusion in the 1 mm gap, and other transport losses
such as impaction. This then defines the d50, static ≈
20 nm. The pink points show the penetration with the
CPMA operational (at Rm=3), corrected for multiply
charged particles and the difference in the widths of
the transfer functions of the DMA and CPMA.
80nm NaCl
Use of Polystyrene Latex Spheres
0 .05 5
ω inner = ω outer
Penetration & Particle Loss
Penetration (%)
However, as described, the system of forces is unstable. Only particles entering exactly in the correct location between the cylinders will be correctly classified.
As the centrifugal force increases with increasing diameter, and the electric force increases with decreasing diameter, then particles entering closer to the outer cylinder will be subjected to a larger than desired
centrifugal force, and those entering closer to the inner cylinder will be subjected to a larger than desired
electric force; in either case particles of the desired
mass:charge ratio will be lost to the walls, and the
transmission efficiency of the device is reduced.
two drawbacks to using NaCl. The aerosol is not
monodisperse, so there may be multiply charged
particles exiting the DMA. These will be counted by
the first CPC, but not the second when the CPMA is
set to the peak mass, so these must be accounted for
when calculating the transmission efficiency. This can
be done by extending the CPMA scan to include peaks
from the multiply charged particles, and adding their
post-CPMA population onto the singly charged particle
population for the purposes of efficiency calculation.
Particle Loss (%)
The Centrifugal Particle Mass Analyzer (CPMA, Olfert & Collings, 2005) classifies aerosol particles by
their mass:charge ratio. Like the Aerosol Particle Mass
Analyzer (APM, Ehara et al., 1996), it uses opposing
centrifugal and electrical fields. Pre-charged particles
enter a classification region between two concentric cylinders which are rotating. The rotation causes
a centrifugal force to be applied to the particles deflecting them outwards, whilst a potential difference
applied between the cylinders deflects them inwards.
Only particles of the correct mass:charge ratio such
that the two forces balance will emerge from the classification section, otherwise they will be lost to either
of the cylinder walls.
using the drift limited model proposed by Reavell et
al. (2011). The resolution parameter, Rm is defined as
below:
Output/Input Ratio [s]
The CPMA
mDMA =
π
6
ρd ve 3
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
Mass Accuracy
50
And at Rm=5:
The data below were taken using NaCl, firstly at Rm=3,
with 1.5 lpm flow through the CPMA:
100
150
DMA Size (nm)
200
250
Penetration
100%
90%
Normalised Dp
0.01
Therefore, the CPMA at its peak will only ever allow a
certain fraction of the total aerosol to pass; this has to
be taken into account when calculating the penetration
efficiency.
0.09
0.1
length along classifier (m)
neutrally stable
For an otherwise identical analyzer, Olfert et al. (2006)
experimentally proved that this method does increase
the penetration efficiency of the analyzer at the desired
mass setpoint.
In 2011, Cambustion launched a commercial version
of the CPMA, with a 200 mm long classifier, 120 mm
in diameter and a 1 mm gap between the electrodes.
This paper characterizes the classification performance
of this version of the CPMA.
100%
90%
80%
40%
30%
20%
10%
1.41
1.61
1.81
2.01
2.21
2.41
2.61
Mp* (fg)
The test aerosols used here are Polystyrene Latex
Spheres and Sodium Chloride. The CPMA is scanned
for mass by counter-varying the speed and voltage; this
ensures a constant resolution across the entire scan,
unlike scanning the voltage alone. The appropriate
speed and voltage for the desired mass and resolution
is automatically calculated by the CPMA interface
Here one considers the use of a Sodium Chloride
aerosol instead. The advantages of this aerosol are
that only one species is present, it can be used at
smaller sizes than PSL, and the technique is reliant on
the accuracy of the DMA alone. There are, however,
Acknowledgements: The author would like to thank Dr Jason Olfert of the University of Alberta for useful discussions and the use of some of
the line drawings above, and Kingsley Reavell of Cambustion.
Presented at the American Association for Aerosol Research (AAAR) Conference, Minneapolis 2012
200
160
140
120
80
90
100
150
200
250
Dynamic Particle Loss (diffusion corrected)
70%
60%
50%
40%
30%
20%
50
Conclusions
0%
-10%
-20%
-30%
-40%
100
1000
DMA Size (nm)
And then at Rm=5:
Mass calculated from DMA size (fg)
0.01
0.1
1
10
10
Mc = 1.036 Md + 0.005
R2 = 0.998
1
60
50
40
30
0.1
200
250
The CPMA method agrees with the calculated DMA
mass to within 5% across the range of sizes tested
(20 nm to 200 nm). No appreciable loss in accuracy
is observed for the smaller particles, unlike results
reported by Tajima et al, 2009 (for the APM3601)
which show increasing disagreement with the DMA
mass as the particle size is reduced, reaching a 50%
error at 20 nm. Results for the APM3600 (Tajima et
al 2011) show good mass accuracy at smaller sizes,
so it is not known what causes this issue in the 3601.
The CPMA does use two autoswitching voltage ranges
as it was found that a single voltage supply was not
accurate across the entire range from 0.1 to 1000 V.
If the voltage and speed and classifier dimensions are
well controlled, then the accuracy of the CPMA should
exceed that of the DMA method with assumed particle
shape and density.
Some loss of particles is observed at the highest speeds
(= smallest particles & narrowest resolution), probably
due to dynamic forces in the inlet and outlet regions
of the classifier.
Bibliography
• J.S. Olfert & N. Collings. New method for particle mass classification—the
Couette centrifugal particle mass analyzer, Journal of Aerosol Science 36 (11)
pp1338–1352 (2005)
Comparison with “DMA Mass”
50%
0.01
100
150
DMA Size (nm)
• K. Ehara, C. Hagwood & K.J. Coakley. Novel method to classify aerosol
particles according to their mass-to-charge ratio — aerosol particle mass
analyzer, Journal of Aerosol Science 27 pp217–234 (1996)
DMA Size (nm)
• J.S. Olfert, K.St.J. Reavell, M.G. Rushton & N. Collings. The Experimental
Transfer Function of the Couette Centrifugal Particle Mass Analyzer, Journal of
Aerosol Science, 37 pp1840–1852 (2006)
40%
% difference in mass
The experiments in this paper use a DMA to classify
(and pre-charge, by virtue of the bipolar neutralizer)
aerosol by electrical mobility diameter, and then the
CPMA is step scanned, producing a mass versus
concentration spectrum for particles of that diameter
(e.g. Olfert et al., 2007).
70
60
50
40
10%
CPMA Mass (fg)
Tandem DMA-CPMA Experiments
There are a number of issues with using PSL for this
kind of validation work. Firstly, though a certified
standard for size, the tolerance in size quoted, when
converted to mass can become unacceptable. For
example, Thermo Scientific PSL at 50 nm has a standard
deviation of 7 nm (or 15%), which corresponds to 45%
in the mass domain. Secondly, the aerosol produced
by nebulizing PSL also contains a large concentration
of particles consisting of impurities in the water and
surfactant chemicals used in the preparation of the
PSL suspension. These particles will be of a different
density to the PSL particles of interest, and therefore
can cause artefacts when selected by the CPMA; this is
especially a problem for smaller PSL particles. Thirdly,
the success of this method depends on the accurate
overlap of the DMA transfer function with the PSL
distribution — the accuracy is as much a function of
the accuracy of the DMA as the accuracy of the PSL.
100
DMA Size (nm)
0
20%
-50%
Use of Sodium Chloride
50
0%
30%
10
As well as showing agreement in terms of mass and
transmission efficiency, this also validates the drift
based model of Reavell et al (2011) used to calculate
the speed and voltage setpoints of the CPMA from the
desired mass and resolution.
0
10%
40%
50%
0%
80%
Comparison with “DMA mass”
50%
60%
0%
1.21
DMA Size (nm)
Peak (DMA at PSL peak) = 1.86 fg
CPMA peak = 1.89 fg
Error = 1.8%
Upscan
Downscan
Model
Dynamic Penetration
30%
90%
CPMA Mass Scan, 150nm PSL, Rm= 5.13
70%
40%
10%
Particle Loss (%)
0 .08
Static Penetration (losses by diffusion)
50%
20%
0.001
200
0.07
180
0.06
160
41 n m
0.05
140
40nm
0.04
120
0.03
100
39nm
0.02
90
0.01
60%
100%
80
0
70%
0.01
70
0 .05
0%
0%
1
0.1
30
0.0525
R = 0.999
20
(CPMA)
10
10
2
% difference in mass
1/radius
radial
location (m)
ωα
100%
100%
1
MC = 1.047 Md + 0.0006
The figure below shows an upscan and a downscan of
150 nm Thermo Scientific PSL, classified with a TSI 3081
DMA, compared with the theoretical “transmission”
which takes into account the difference in the widths
of the source aerosol, and the transfer function of the
DMA and CPMA.
Output/Input Ratio [m]
0.055
0.1
CPMA Mass (fg)
The forces are therefore neutrally stable:
Mass calculated from DMA size (fg)
Penetration (%)
80%
30%
20%
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dimension of particles emitted from a light-duty diesel vehicle with a diesel
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10%
0%
• K.St.J. Reavell, J.P.R. Symonds & M.G. Rushton,. Simplified approximations to
Centrifugal Particle Mass Analyser performance, European Aerosol Conference,
Manchester (2011)
-10%
-20%
• N. Tajima, N. Fukushima, K. Ehara, H. Sakurai. Mass Range and Optimized
Operation of the Aerosol Particle Mass Analyzer, Aerosol Science and Technology
45 pp196–214 (2011)
-30%
-40%
-50%
10
100
DMA Size (nm)
1000
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Standard Operating Conditions of a miniaturized Aerosol Particle Mass Analyzer,
American Association for Aerosol Research conference (2009)