209:!131
Title of the Invention
METHOD AND APPARATUS FOR MONITORING
SUB-MICRON PARTICLES
Backgro~ n~ d of the Invention
Field of the Invention
The present invention relates to a method and apparatus for
monitoring or detecting sub-micron particles on a novel principle which
is completely different from the conventional techniques.
The method according to the present invention is advantageously
applicable to monitor and control impurity particles in fluids such as pure
water and ultrapure water used in electronics industries, biotechnology,
medical and pharmaceutical application and foods industries. The present
method can be used to evaluate the performance of separation membranes
and filtration systems.
. Related Arks
The conventional methods for monitoring or detecting particles in
2 0 fluid are classified into following four categories:
(1) Shadow system in which decremc,nt of light intensity caused by
travelling particles in fluid passing across an optical axis of parallel
ray.
(2) ' Microscope system in which fine particles in fluid are caught by a
2~5 membrane filter or the like and are observed or counted by a
electron scanning microscope.
(3) Light scattering system in which fluid is inadiated with an intensive
light such as a laser beam and the resulting scattered light is
1
CA 02091131 2004-04-16
collected by ~ 1eI-°fs so that the focused light is detc~;tb:~~:~.~,~ a
photo
Iu~t~p~ier.
~rrza~in~ systel~~ in which a fluid is irrat~xated w~ ~~. ~ ~ ~- ~~
~~°.
resulting contrast of light is detected by a photo-~~~,~~~: array and an
irr~a~o of particles in the fluid is forn~ecl by a cor~-~~~~~;
~e~~ techniques such as ultrasonic scattering technique are also
proposed.
~n the case of the shadow system ~1~, however, detection of ~°in~
particles is Iirr~ite~ to tile particle size of about 1 ~rr~ and hence this
1 0 CleteCtlon SySteIl1 Can IlOt be i~Se~ far sub-micron particles. fns the
microscope system ~~. j, n~IOre than half day is required to obtain the
result.
'the light scattering system ;.~) is the rrl~ir~ current of developrnent
In particle counters or detectors and now ultra-fine particles having the
particle size of less than ~.~ ~ ~.n~ can be detected by us~na light source
having shorter wave Length such as argon laser. In fact, ;Japanese pate nt
laid-open I~o. 4-~~,~~~ clise~oses a technique to dete~'~Iyne the pre~ase
number of fine contaminant particles each having the particle sake of
lower than ~?.0 ~ p.m cont<ained in ultrapure water. '~'hls patent proposes to
2 ~ use two detectors each receyve the scattered laght so t.l°~at a
particle counter
produces a ~5lg,na~ ~vl~el~ two detectol°s detect the scattered light
sin~ultaneous~y. '~'nis system, however, requires a high-dower laser as
well as very ser~s~tive photo-~~~uitiplier, resulting in a :ergs costl~l
system.
~tL~i more, II"t t~"lIS SyStGIT't, preCISC a~lTIrITeigt ~.7etween an AXIS of
~~Ll;Cia
'~. '~ .'5'trear~Co'I'~t't~.Tlrl~n~ p'GIr tiC~ed to ~e C'L~.tF°.Cted
a~'~''a ~.n ot-3tiC~ a ial~ a~ r~~L~':~-
~~.~E ~~'~'~~r ECi ~t:~8ui°~ ~a'Ie 1'C,ilil~l~Ity fi~ ~t:Cc~~ur~~Clii:.
.~~s.paneSC ~3at~nt ~d3v
open o. ~~,-~~:~ discloses an autorl~ated apparatus which facilitates this
~11~;I~ner~t.
1
CA 02091131 2004-04-16
Japanese patent. lays-open I~o, ~~-1_~~3~ discloses ~ variation of the
imaging s~rsten~ ~4 ~. In ~I~is patent; a l~.ser ~~eairr~pinges ve~ic~Iwv to
flow of s~~.ple limpid ~.nd the diffracted and scattered light is passed
through ~ ~ourie~°-transformation optical system or a lerxs to produce
a
~raunhofer diffraction image which is treated ire order to evaluate fine
p~.rticles in the liquid. Ire this patent, ~ diameter of a laser beam is
enlarged to obtain a parallel ra-y which is directed to the s~pie li~ui.<
~°his system re~~aires a complicated computer systean,
'f'herefore. an ob~°cx of the present invention is to provide ~, method
which permits to detect fine particles of sub-micron as contaminant in
fluida in particular, pyre water or ~zltr~:pure water by a simple and very
Summary of the Invention
One aspect of the present invention provides a method for detecting
sub-micron particles in fluid comprising converging a light beam from a
coherent light source so that the resulting focused light passes through a
stream of fluid containing particles therein in such a manner that a focus of
the focused light is located in the stream of fluid, receiving a light passed
-,, ,-, through the stream of fluid and diffracted by the particles by a photo-
detector
which is positioned at an opposite side of the coherent light source with
respect to the stream and on an optical axis of the light beam to produce
electrical signals, and counting numbers of particles in the stream by
treating
the electrical signals, wherein said photo-detector comprises a photo-diode
array .
s .... T'c..:~.~ ~~~~'-'~' f ~~ts. , ~ ~i ...~_. f~ :°.~ :
j.C.,~,.......s., . r' ....,NS,.~ ~ .~.°_~F 5.23'"s.~.
~n~c~'c~~ particles ~n fiuic~ con~t~rising a coherent light source, an optical
s~etC'.I~~ ~~~' ~,t'~n~~~'a'~fr~ltr ~ ~l~:~i~ ~~.aq'1~ ~:1$~It.r~~ ~~ ~~ ~~~~
~C3~'I~r~'n~ ~~g~?~.
~~=s:a~~~ ~_~~ ~~:C1~~~F c~ ~..~iF'~ia~<t~~~ ~ig~'~~:. t ~:~1~ ~~"P3'C~ug~1
w~'ilc~ a St~c~.~~f
X
CA 02091131 2004-04-16
~~c.=.~i,a ~~'J're~-",~.f.I~tik~i~ ~~di'r:,...1~to l~iCl~~t%iI GI't~ ~'~in~:.
IGC~~.ev. 1~1 t~Ifv n~rlgl"It~C~rtl~~.:i~i
a~ a f~c~.s of tt~~e converged light bearr~, a phota-detector whici~ is
posltionect at an opposite side of the coherent light source with respect ~o
the stream ~. and on are c~pticai axis o~ the light ~ear~r~ tc~ produce
electrical
~ig~xdls, and ar electric circuit for counting numbers of particles ir~ t~~e
streairs by treating the electrical signals, wherein said coherent light
source is
a semiconductor Laser.
The coherent light source is pre~era~iy a Laser diode and the Iphat~-
erector c~rr~prises r?referG~b(y at least one photo-diode, more preferahi~t a
photo-diode array ~°~..~.k~ged perpendicularly to the direr-tion o1'
the strewn
~r~c:4 also perpe~~dic~~$a~°ay to the optical axis. The optical system
can be a
lens. preferably, the edectl'IC C1I°cu~t includes differs _ _z~ R~ ~.,
~ - ~ ~~: ~_~~,°
multiplying signals from elements in the photo-detects- array. The cell
can be a dart of a transparent tube through which a stream c~~: iilu id
I ~ containing part~GLeS is flown.
The present invention is based on such surprising and unexpected
finding t'h~t the existence of sub-rr~icron particles in a liquid stream can
lie
detected or I~nonitored b5l utilising di~'fraction phenomenon of a
transmitted Iiglzt, which is observed when a converged coherence light is
focussed on the iic~uid strea~r~. In fact, it is rat l~nown$ to use tl~e
tr~ansrrlitted light of a converged light directly for detecting fine
particles.
lri tl~e conventional detection technique, dispersed particles are irradiated
with an illumination paru~les ray so that the resuitir~g transrr~itted light
image is p~urier-transi~ormed ~s is described in the ,lapanese patent laid
'~~.r~'c~ :'z ~ ~s_i :a j.~.'
Another aspect of the invention provides a method for measuring
sub-micron particles in a fluid comprising converging a Light beam from a
coherent Light source (1) so that the resulting focused light passes through
1
CA 02091131 2004-04-16
a stream (3) of fluid containing particles therein in such a manner that a
focus of the focused light is located in said stream (3) of fluid, receiving a
light passed through said stream (3) and diffracted by said particles by
means of a photo-detector (4) which is positioned at an opposite side of
said coherent light source ( 1 ) with respect to said stream (3 ) and
substantially on an optical axis of said light beam to produce electrical
signals, and treating said electrical signals from said photo-detector (4) to
count a number of particles in said stream by using a predetermined
calibration curve; and wherein said focus is located in a focus portion of
said stream of fluid which has a cross sectional area at least as great as a
maximum cross sectional area in portions of said stream of fluid upstream
and downstream relative to said focus portion.
The invention also provides an apparatus for measuring sub-
micron particles in a fluid comprising a coherent light source ( 1 ), an
optical system (2) for converging a light beam emitted out of said
coherent light source to produce a converged light, a cell (3) through
which a stream of fluid containing particles is flows and being located in
the neighborhood of a focus of said converged light beam, a photo-
detector (4) which is positioned at opposite side of said coherent light
source ( 1 ) with respect to said stream and substantially on an optical axis
of said light beam to produce electrical signals, and an electric circuit for
treating said electrical signals from said photo-detector (4) to count a
number of particles in said stream by using a predetermined calibration
curve; and wherein said cell has a cross sectional area at least as great as a
maximum cross sectional area in portions of said stream of fluid upstream
and downstream relative to said cell.
The detection principle of the method according to the present
invention is different from those of known methods but the theory why the
particles in fluid are detected precisely by the method according to the
4a
2091131
present invention can not be explained completely at this stage. Following
is one of probable explanations.
Now, we will refer to attached drawings.
brief Descri t~ion of the d~aw_in~s
Fig. 1 is a perspective view illustrating the principle of the detection
method according to the present invention.
Fag. 2 is an illustration for explaining a conventional technique.
Fig. 3 is an illustration similar to Fig. 2 but is for explaining the
detection method according to the present invention.
Fig. 4 is an example of a differential amplifier used in an apparatus
according to the present invention.
Fig. 5 and Fig. 6 are graphs each showing a relation between the
particle density in fluid and the number of particles detected in Example 1
and 2.
Fig. 7 and Fig. g are graphs each showing a relation between the
number of particles detected in ultrapure water after an ultrapurification
unit starts at different contamination levels
Fig. 9 shows graphs of development in contamination with bacteria
2.0 in ultrapure water left in ambient for four days.
At first, the conventional detection method is explained with
refernng to Fig. 2 in which a fine particle is irradiated with a parallel ray
and the resulting diffracted light is converged by a lens. In this case, so-
t 5 called Fraunhofer diffraction image shown at the right side of Fig. 2 is
observed. Fraunhofer diffraction is a well-known physical phenomenon
which is used in particle counters. A general term of "diffraction" is used
to define "all phenomena, that can't be explained by the linearity of light"
and can be described by Fresnel-Huygens' principle. However, according
5
20~1.~3~.
to this principle, the case when the radius of a particle becomes lower
than a wave length of light used can't be explained by the Fraunhofer
diffraction phenomenon but is described .as scattering phenomenon of
light. In fact, if the radius of a particle becomes smaller than a wave
length of light used, the diffraction can't occur any more in a parallel ray
because each particle functions like a point source and scatters light
In much detailed scientific theory, the diffraction phenomenon is
explained as a kind of scattering phenomenon and can be described by the
Mie scattering theory which is derived ~ strictly from the Maxwell's
electro-magnetic equation. Since the Mie scattering theory is complicated
and is difficult to be handled, an approximated equation is generally made
in the relation between the radius "r" of a particle and the wave length
"~," of light used (Rayleigh scattering for "r < ~.", Mie scattering far the
case when "r is nearly equal to ~,", Fraunhofer scattering for "r > ~,").
1 5 According to the conventional Fraunhofer diffraction theory of a
parallel ray, the divergent angle ~6 of diffraction caused by an
obstruction or fine particle is represented by an equation of ~8 = 1.22
~./D, in which "D" is a diameter of the particle (D = 2r). The divergent
angle O8 of diffraction increases with decrement of the diameter of the
2 0 particle and becomes to 90 ° when D is equal to 0.78 ~,. Usually,
this
value of the divergent angle dg is the detection limit, so that the detectable
minimum particle size is 0.52 itm at a wave length ~. = 0.67 um. In fact,
a diffraction image of a particle whose particle size is lower than a wave
length used is not easily obtainable in experiments.
2 5 The present inventors found surprisingly such a fact that, when a
particle is placed in the neighborhood of a focus of a converged beam
focussed by a lens, the diffraction angle becomes sa small so that such
finer particles becomes detectable, even if their,particle sizes are smaller
than the wave length used. On this finding, the present inventors
6
~
' CA 02091131 2004-04-16
completed the present invention which provides a novel method which
permits to detect sub-mica~on particles. The most irnport~~:~~: rrd~v:~:~~~~~
~;~.
the rnethocl according to the present invention in indus~:.:~,T resides in
that
sub-micron paz°ticles can be detected at high sensitivit~°
~~.::~ ~~~r_tE~ lE<g z
precision by a simple combination of a cheap laser {light source) and a
cheap photo-diode {pickup).
In the method according to the present invention, the diffracted
image can be obtained for a particle whose particle size is smaller than 0.1
~.m which is not observable in known techniques, Cf course, particles
1 0 having the particle size of bigger than ~.1 p.m also can be detectable
with
high sensitivity by the method according to the present invention.
Description of the Preferred Embodiment
Now, the present invention will be described with referring to
drawings but the present invention should not be limited to an
embodiment shown in the drawings.
An apparatus for monitoring or detecting fine particles shown in
IFig. 1 illustrating principle of the detection method according to' the
present invention comprises a laser {1) as a coherent light source, an
2 0 optical system, preferably a lens {2) for converging a light beam emitted
out of the coherent light source to produce a converged light., a cell {3)
through which a stream of fluid containing particles is flown; which is
located in the neighborhood of a focus of the converged light beam, a
photo-detector {4) which is positioned at an opposite side of the cohe&ent
2 ~ light source {1) with respect to the stream and substanti~li~- ~n an
o~ti~ai
axis of the light beam such as a photo-diode or a photo-i~ ~,~-::warray, and
an
electric circuit {not shown) for canverting the resulti~i~ $s~z ~~~t~~~sit~Fr
signals or a diffraction image detected by tl~e photo-det~~~G .° ~~~~
electrical signals: i'rc~~~ ~~hic='~ ~xurnbe~~s of ~artic les in '~._,. stream
are
t
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CA 02091131 2004-04-16
counted. III elements ~~sed in the present invention are available on
market and are very cheap.
The coherent light source may consist of a laser (~ j and a
collimator lens system (F'ig. 3). The laser (~~ can be any laser but is
preferably a laser diode or semiconductor laser of small power. In other
words, a cheap laser diode can be used advantageously in the method
according to the present invention. sensitivity increases with decrement
of the wave length of laser oscillation. Inventors confirmed that tl~e
detection principle of the present ia~vention can be applicable for a laser
1 0 diode wh~~~ n===~=:v~- _.~ ~n~lie~ t'~s.~n l m, for example 4.~ m~.
The focal distance ;~ the optical system or lens (2) for
convert, ~ ~ l~g~~ l.~e~ ~ _.: ~~.~te~.~~ined in function of the particle size
to
be detected. p'or example, a lens laving the facal distance f = 1(1 mm may
be used to detecting a fine particle whose. particle size is t~.~ ~.m.
1 S The cell (3) must be transparent at least on light-receiving face and
light exiting face but can have a very simple structure because no
consideration is reduired to stray light. The cell (3~ has not necessarily a
rectangular section shown in Fig. 1 but can have any section. 'I'he cell (~)
can be a separate piece from a tube for a stream of fluid containing
2 0 particles bvt, according to another advantage of the present invention, is
preferably a part of a transparent tube througl°F which the strewn of
fluid
containing particles is flown. The transparent tube can be made of fluoro
resin in order to resist chemicals.
n practice, a suitable adjusting mechanism is preferably used for
2 ~ positioning the optical systerr~ (2) so that the focused beam is focuses
in
the neighborhood of tl~e center of the cell (~~.
~'ot so high sensitivity is retluired in the photo-detector (4) if the
the photo-detector (4~ can detect the diffraction image hidden. in the
transrnltt~'~ ~~~~~~~ Ii: ~11?~i ~u~'~~5~. aø,"~dl~~~'"~~~~~; ~'v,l~,'~'a ~J~
~:,a:~~~: ~ al~v ~x~,~tt.~.~
CA 02091131 2004-04-16
detector (~:) ~ ~_~ ~~~~~:y~ _~ 4~n~.~le photo-diode but preferably
constitutes
of a photo t~~~:a~;~ array. .., y~hoto-diode alley is preferabav arraa~ged
perpendiculGx~y L~ .r~~- ~sa~~ ~~~n of the stream and also perpendicularly to
the optical axis.
The diffraction image or the distribution of intensity of a converged
light observed in the photo-detector (4) used in tl~e apparatus according to
the present invention is illustrated at the right side of Fig. 3.
In practice; signals from elements in the photo-detector array are
multiplied in differential amplifiers to improve the SN ratio of the p ;~to...
1 0 detector (4~ in such a manner x.~~fr: ~~~ ~;~~~,ri~ s~~~~~R c~tl' ~e~~; 3~
~_~:-~~~~~~d
when the elements in the photo-de~:ec.6.o= an-ay are irradiated uniformly or
no particle passes througi~ tire ce~~ ~ °>i. ~~Yie suita'~le e.lectF
~#~; sib ;~.~.r~°~ E e,
represents characteristics (number, size etc) of the particles is produced
when any change in intensity caused by the diffracticx~r ioa~~~ ~:<
t 5 converged light is appeared i~~ the elements in the photo-def.e ~:~.~_~-
array.
F~~ ~. iryctr~ates an example of differential amplifiers for a ~: ow..
detector array ,. f-w,~ ~ ~tia~g of four photo-diode elements. 'I'he values of
~'~;si~-~:-,~:~ces ia~ ti°~u ~ ifferential atr~plifiers of Fig. 4 are
adjusted in such a
manner that zero output signal (e) is produced when identical output
2 0 signals are produced at the output (a to d) of all photo-diode elements or
the aI1 photo-diode elements are Irradiated with a light ~f identical
intensity. Therefore, the output signal (e) of the differential amplifiers of
Fig. 4 changes when any Change in intensity is appeared in the output
signal, for example (a) of the photo-diode elements.
2 ~ In the embodin~ea~t~ ~ r x ~:;.~ ~ ~ . ~ and Fig. ~, all elements of a
Laser (I), a lens (2)., a cel=and ....,~ ,F~f-diodes (~i are
are°~nged on
straight line but they can be ar=ra~yeu u~~ ~~on-linear line by using suitable
mirrors) in known manner so as to reduce the total size or length of the
apparatus.
CA 02091131 2004-04-16
~lowa Fxamples of the ~nonitorinldetection method according to
the present invention will be shown in 1~,xaz~ples.
Example 1
S Fine particles were detected by using the principle shown in 1~ig. I
under following conditions and procedure:
~xnerxment conditions
Laser: Semiconductor laser
(wave length ~ 670 nm,
1 ~ power = Q. 5 m~W)
Focal distance of a converging lens: 10 mm
Liquid tested : r Itrapure water
Flow rate of tire liquid : IOQ mm/sec
Diameter of fine particles added: Q.208 err:
1 ~ Photo-detector: photo-dic~~~.:=:: array (32 elements)
Differential amplifiers : P'ig, q.
Experi~r~ent procedure
Three liquid samples having different particle concentrations
(dilution of 1 to .~ times) were prepared and flown at a rate of I2Q ml/min
2. ~ through the apparatus illustrated ~n Fig. 1. ~'l~e resulting change in
f:~e
number of particles detected is shown in ~'ig. .
This Example reveals such a fact that the method according to the
present invention is applicai~le to detect a particle having the particle size
off' t~.2Q~ ~.rr~.
example 2
Example 1 was repeated but the particle size of particles introduced
in ultrapure water was changed to C~.1 ~.m,
CA 02091131 2004-04-16
'~""l~e resulting change in the nu~r~~er o~ particles detectec is shown in
Fig. ~. ~~'hys ~xarnple reveals thai tire method a~:co~°d~~Fg i~t he
preue~:~i
~i~ven~IOr~ I~ app~~c~.~°~~ ~~ a ~~s~.~3'T'~ ~~r dei~:,~~,tr'zart~~~e~
s"~av~r'a~~
particle size of .l la.rn.
~xamole ~
~'he method according to the present invention was applied to
ultrapure water produced in are ac:tusl industrial water purification unit.
~ curve ''~" ira Fig. l shows a relation between the nurnher of
1 f~ particles detected (ar°bitra~°y unit] by the method
according to the present
invention and time duration after the purification unit starts.
For comparison, the sane Liquid sample was tested in two detectors
of known scattering method. ~'wo curves "~'f and p°~'' in Fige ~ show
the
results obtained ny the known scattering sysiems in which makers of the
~ detectors indicate that particles shove ~.1 yn aid ~.~ ~,m are detectable
respectively.
xa
example ~ was repeated for another ultrapure water wh~c:3~is more
purified than Fxample 3.
....~ a ~rp=» ~ .' . ~ :4. a ~~~ ..... . ..F. y ~. . more
_. _ t . s. ~ < : ~. E _ ;~' ' ' : '. _ ti'~a'
particles are detectable in the method according to the present
~ai~d.ai~F~rea 0,.v .>~C.aa~a~a t,Li earv:, s,traivsm;taLs~iilCC~ ~~.Ge~e~-
~sxz~ ds~~y.~a~3e.:~.
F ri'i j 'lg. J ~~ ~. 3~ ~s sip~ax"e~"x~ CtF~~ cfhe Is~~rc''~~'J"~~ r'' ~~:' N
"'~ ~.~ ~'
~~ ,.~a~°'~.~~,i~
rtzzhCl~ aG~:~~'C~c~I~~ ~C~ ~~~ ~ res~~~~. ~3~Lv~i~.~.~~~ _~ ~~~s~a~a~ga~~Ya
b.
prop~rtior~~ to tl~;e nun~hex o~ pal°ticles actually present gr~ ii~uid
o~° try tr~c
~~umi~er ot= particles detected ~°p~ ~~3e a:c~~K~:en L~o~-~al
sc~.i~erir~g ~~~eti:o.
CA 02091131 2004-04-16
'~erefore..the ~netl~od accor~in,~ to tire present invention can be used as a
particle counter by using a suitable calibration curve.
Example ~
T'he method according to the present invention was applied to
ultrapure water le~'t an ambient atmosphere. lvlamely, the purity o~
ultrapure water was examined daily for four days. '~'he results are sown
in Fig. 9.
In Fig. ~5 a cc.~rve "C" show a relation between time duration (da~rs~
Z ~ and the number of colonies determined by a known culture technidue in
which ultrapure water samples were cultured on medium.
'~'hree curves of °'~'~ and "~ls ~~" are overlapped with the curve
'°.q: in Fig. 9. ~i°~e result o1' ie~e method according to tlR~e
present invention
is shown by the curve ''A". while curves of "~14 82°' correspond to the
conventional scattering ~netE~od,
1 ig. ~° revels such a fact that the result obtained by tire ~netl~od
aceo~'ding to tlae present invention has a stronger resemblance to the actual
value than the con~.~entiona.l scattering nee' $~ u~ t {-~ ~~-__::; ~;
method aecordrt~g to the present inventioa~ i> ~~.~~:=~IiKin medical uses or
pl~ar~nace~.atical uses.
3 ''
l
