8.23.5
Raman Gas Senso o Hyd ogen
De ec ion ia Non-Dispe si e and
Dispe si e App oaches
Fabio Melison, Lo enzo Cocola and Luca Pole o
Special Issue
In a ed and Raman Spec al Sensing o Food and Indus ial Applica ions
Edi ed by
P o . D . Ba y K. La ine
A icle
h ps://doi.o g/10.3390/s25134190
Academic Edi o : Ba y K. La ine
Recei ed: 5 June 2025
Re ised: 26 June 2025
Accep ed: 29 June 2025
Published: 5 July 2025
Ci a ion: Melison, F.; Cocola, L.;
Pole o, L. Raman Gas Senso o
Hyd ogen De ec ion ia
Non-Dispe si e and Dispe si e
App oaches. Senso s 2025,25, 4190.
h ps://doi.o g/10.3390/s25134190
Copy igh : © 2025 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license
(h ps://c ea i ecommons.o g/
licenses/by/4.0/).
A icle
Raman Gas Senso o Hyd ogen De ec ion ia Non-Dispe si e
and Dispe si e App oaches
Fabio Melison * , Lo enzo Cocola and Luca Pole o
Na ional Resea ch Council o I aly, Ins i u e o Pho onics and Nano echnologies, CNR-IFN, Via T asea 7,
35131 Pado a, I aly; lo enzo.cocola@cn .i (L.C.); luca.pole o@cn .i (L.P.)
*Co espondence: abio.melison@cn .i
Abs ac
The cu en solici ude in hyd ogen p oduc ion and i s u iliza ion as a g eenhouse-neu al
ene gy ec o pushed deep in e es in de eloping new and eliable sys ems in ended o
i s de ec ion. Mos senso s a ailable on he ma ke o e eliable pe o mance; howe e ,
hei limi a ions, such as es ic ed dynamic ange, hys e esis, eliance on consumables,
ansduce –sample in e ac ion, and sample dispe sion in o he en i onmen , a e no easily
o e come. In his pape , a non-dispe si e Raman e ec -based sys em is p esen ed and
compa ed wi h i s dispe si e al e na i e. This app oach in insically gua an ees no sample
dispe sion o p epa a ion, as no di ec con ac is equi ed be ween he sample and he
ansduce . Mo eo e , he echnique does no su e om hys e esis and eco e ing ime
issues. The esul s, e alua ed in e ms o sample p essu es and came a in eg a ion ime,
demons a e p omising signal- o-noise a io (SNR) and limi o de ec ion (LOD) alues,
indica ing s ong po en ial o di ec ield applica ion.
Keywo ds: Raman spec oscopy; hyd ogen de ec ion; gas sensing; op ical senso ; ield-
deployable ins umen a ion; non-dispe si e Raman
1. In oduc ion
Hyd ogen is cu en ly aken in o conside a ion as a enewable ene gy ca ie . The
mo i a ions lie in he ac ha i is ligh , s o able, eac i e, has a high ene gy con en pe
uni o mass, and i can be eadily p oduced a an indus ial scale. Supplying hyd ogen o
indus ial use s is now a majo business globally. Demand o hyd ogen in i s pu e o m is
a ound 100 million ons pe yea (M H2/y ) [1].
Mo eo e , he use o ossil uels as he p ima y ene gy sou ce is unsus ainable due
o he apid deple ion o na u al ese es and he economic challenges associa ed wi h
exploi ing un apped esou ces. In addi ion, he g owing a en ion o g eenhouse gas
emissions is di ec ing he go e nmen s, and hus he indus ial sec o , owa ds employing
enewable and g eenhouse gasses ee sou ces o ene gy [2–4].
Hyd ogen is highly eac i e; on Ea h, i is always ound combined wi h o he elemen s.
In pa icula , i is mainly ex ac ed om wa e , hyd oca bons, and biomasses, and no all
he p ocesses used o p oduce hyd ogen a e g eenhouse gas- ee in he o e all balance
o inpu ene gy and ou pu emissions. Fo example, hyd ogen p oduc ion h ough he
oxida i e con e sion o hyd oca bons, a well-es ablished indus ial p ocess, gene a es no
only hyd ogen, bu also ca bon monoxide and ca bon dioxide. Howe e , when his p ocess
is applied o he by-p oduc s o o he indus ial ac i i ies, i becomes possible o e ie e
Senso s 2025,25, 4190 h ps://doi.o g/10.3390/s25134190
Senso s 2025,25, 4190 2 o 14
hyd ogen om g eenhouse gases ha a e mo e impac ul han ca bon oxides [
5
]. So, he
needs in measu ing he hyd ogen con en in gas mix u es co e i s en i e li ecycle.
F om i s p oduc ion and owa ds i s anspo a ion and u iliza ion, i is essen ial o
accomplish he ollowing: (i) Main ain compliance wi h egula o y equi emen s such as
sa e y and quali y s anda ds. (ii) Op imize he e iciency o he appa a us in ol ed in i s
con e sion o ene gy, e.g., indus ial applica ions and uel cells equi e a p ope le el o
pu i y. (iii) Gua an ee sa e y in he en i onmen s in ol ed in i s ansi , p e en ing and
quan i ying any losses in he p ocess.
Di e en echniques a e applied in hyd ogen measu emen s depending on he pa icu-
la equi emen s o be sa is ied: sensi i i y, esponse and eco e y ime, ull-scale, c oss- alk
wi h impu i ies, e c. The p ima y me hods adop ed by he indus ial sec o include he
ollowing: (i) Elec ochemical senso s: ampe ome ic, po en iomen ic, o conduc oc o-
me ic. Thei ansduce ope a ions a e based on he hyd ogen oxida ion on an elec ode.
These senso s a e widely used and hey a e cos -e ec i e, po able, and hey can each high
accu acy (<5 ppm); on he o he hand, hei eco e y ime depends on he senso empe -
a u e, hey su e om c oss-sensi i i y wi h o he molecules such as ca bon monoxide
and me hane, and hey could unde go hys e esis o e ime [
6
–
9
]. (ii) The mal conduc i i y
de ec o s (TCDs) a e gene ally used in gas ch oma og aphy o measu e he con en o
a pa icula elemen o molecule. This ype o ansduce s uses he inhe en ly di e en
he mal conduc i i y cha ac e is ic o gases o measu e concen a ions. They ep esen a
non-speci ic, obus , and ma u e echnology, commonly used as e e ence ins umen a ion.
TCDs lead o accu a e esul s wi h low LODs and wide dynamic anges; con e sely, due o
hei non-speci ici y, hey equi e some so o sample manipula ion and in gene al, hey a e
cos -impac i e o some applica ions [
10
–
12
]. (iii) Ca aly ic combus ion senso s a e widely
used in indus ial sa e y moni o ing o de ec lammable gases, including hyd ogen. Deep
in e es and esea ch a e g owing in de eloping new and inc easingly high-pe o ming
ma e ials o use as a ansduc o . Thei p inciple is based on he hyd ogen oxida ion on a
ca aly ic su ace, and he gas concen a ion is p opo ional o he empe a u e a ia ion
o he su ace i sel . This ype o senso has a limi ed dynamic ange and equi es di ec
con ac be ween he sample and he senso i sel [
12
–
15
]. (i ) Tunable diode lase abso p-
ion spec oscopy (TDLAS) has been ecen ly applied o hyd ogen de ec ion. TDLAS is
based on he pho on abso p ion by molecules a de e mined ene gy le els. The me hod is
ex ensi ely employed o eliably measu e he concen a ion o gasses such as wa e , ca bon
dioxide, me hane, e c. I has been ecen ly applied o hyd ogen de ec ion by exploi ing
i s abso p ion lines a 2121.8 nm and 4712.9 nm. Some esea ch s udies demons a ed i s
applicabili y o hyd ogen de ec ion by p o iding de ec ion limi s which comply wi h he
sa e y s anda ds and egula ions [16,17].
2. Ma e ials and Me hods
Raman spec oscopy uses ligh o p obe ib a ional (and o a ional in case o gases)
ene gy le els o molecules. Di e en ly om IR spec oscopy, which is based on he abso p-
ion o pho ons a he p ecise ene gy le els o hose o he ib a ions, Raman spec oscopy
is based on he inelas ic sca e ing o he inciden pho ons. The ou coming pho ons a e
shi ed in ene gy (loss o gain) in ela ion o he ib a ional inge p in s o he molecules
hi by he adia ion [18–22].
In gaseous samples analysis, Raman spec oscopy esul s is a non-in asi e app oach as
i only equi es a lase beam o pass h ough he sample and he op ical windows necessa y
o collec he sca e ed ligh . As in he IR spec oscopy, he di ec con ac be ween he
sample and he de ec ing sec ion o he ins umen can be easily a oided; his me hod does
no equi e any di ec in e ac ion wi h he sample. I does no equi e sample p epa a ion;
Senso s 2025,25, 4190 3 o 14
only dus emo al is necessa y o p e en pa icle-induced sca e ing o he lase beam,
which could comp omise da a acquisi ion. In addi ion, he e a e no issues ela ed o ime
eco e y, as once he sample exi s om he lase –sample in e ac ion olume, he sys em is
immedia ely eady o he subsequen analysis. Ul ima ely, because he Raman emission
o wa e does no spec ally o e lap wi h ha o hyd ogen, emo ing wa e apo is
unnecessa y. Howe e , i emains essen ial o a oid he mal condi ions ha could lead o
wa e condensa ion.
Thanks o ecen ad ancemen s in CMOS indus ial came as (such as imp o ed quan-
um e iciency, back-illumina ed senso s, and educed he mal noise), high-powe lase
echnology, and he abili y o ope a e in he isible ange, Raman spec oscopy is gain-
ing inc eased a en ion as a echnique o analyzing gaseous samples. This echnology is
poised o play an expanded ole in indus ial se ings, as p io s udies demons a ed i s
applicabili y bo h wi hin and beyond esea ch labo a o ies [23–28].
In his wo k, a Raman sys em designed o he de ec ion o hyd ogen, ni ogen,
and oxygen is p esen ed. P e ious s udies demons a ed he easibili y o accu a ely
de ec ing ace amoun s o hyd ogen by implemen ing a non-dispe si e app oach combined
wi h a mul i-pass cell o achie e op ical powe s close o 100 W. Wi h in eg a ion imes
anging om 10 min o 12 h, limi s o de ec ion (LODs) as low as 20 ppb ha e been
achie ed a a sample p essu e o 2 ba [
29
]. O he s udies ha e shown ha i is possible o
use comme cial ins umen a ion o de ec hyd ogen, eaching LODs as low as 775 ppm,
wi h acquisi ion imes anging om 1 o 15 min [
30
]. This wo k aims o highligh he
easibili y o de eloping an ins umen speci ically designed o indus ial use, ea u ing
ope a ing p essu es and in eg a ion imes compa ible wi h ypical p ocess con ol dynamics.
The sys em is concei ed o be as compac and s aigh o wa d as possible, while s ill
main aining he pe o mance le el o a scien i ic-g ade ins umen . The sys em has been
designed wi h u u e op imiza ion and in eg a ion in o indus ial p oduc ion in mind,
he eby enabling po en ial ield applica ions o Raman-based echniques.
The p esen ed sys em is cha ac e ized by a non-dispe si e app oach as he Raman
sca e ings o he h ee molecules unde analysis a e selec ed by employing di e en band-
pass il e s, which allow he p ope wa eleng hs o pass h ough he il e s hemsel es and
o be collec ed by a came a. The pe o mances achie able wi h his design a e compa ed
o a dispe si e app oach which employs a di ac ion g a ing spec ome e o analyze he
acqui ed spec um simul aneously.
In Raman spec oscopy, bo h dispe si e and non-dispe si e app oaches exhibi inhe -
en ad an ages and limi a ions ela ed o hei espec i e designs. These di e ences become
pa icula ly signi ican when conside ing he complexi y o he equi ed componen s and
he alignmen p ocedu es du ing assembly, ac o s ha a e especially c i ical in he con ex
o indus ial eplica ion o ins umen a ion.
Dispe si e sys ems ypically ely on a combina ion o objec i es, di ac ion g a ings,
en ance sli s, de ec o s, and in some cases, mi o s, which collec i ely esul in a mo e
complex op ical and mechanical design. The op imal alignmen o all hese componen s is
c ucial o ensu e e icien signal collec ion and high spec al esolu ion. Achie ing his o en
equi es p ecise ma ching in he nume ical ape u es o he op ical elemen s o minimize
signal losses, which in u n equen ly necessi a es he use o cus om, non-comme cial
componen s. Mo eo e , he assembly p ocess is ime-consuming and highly sensi i e o
mechanical ole ances. Al hough mode n dispe si e sys ems ha e become mo e compac
and obus , Raman spec oscopy o gaseous samples o en demands cus om-designed
spec ome e s wi h high op ical h oughpu and ailo ed con igu a ions speci ic o he
applica ion. This leads o a high deg ee o sys em complexi y, which poses signi ican
challenges o la ge-scale manu ac u ing and ield deploymen .
Senso s 2025,25, 4190 4 o 14
On he o he hand, non-dispe si e sys ems, which commonly employ op ical il e s,
a e gene ally cha ac e ized by a educed numbe o op ical componen s. As a esul , hese
sys ems ea u e a simpli ied s uc u e ha does no equi e di ac ion g a ings o en ance
sli s. The de ec o is ypically posi ioned di ec ly along he op ical axis o he collec ing
op ics, e ec i ely educing bo h sys em complexi y and he likelihood o misalignmen
du ing ope a ions. Consequen ly, he o e all design is easie o implemen , and he
op ical h oughpu can be eadily enhanced using only comme cially a ailable componen s.
These cha ac e is ics make he non-dispe si e con igu a ion pa icula ly appealing o
indus ial applica ions, whe e epea abili y, obus ness, and ease o assembly a e essen ial
equi emen s. Non-dispe si e sys ems a e, howe e , limi ed in e ms o spec al co e age,
as hey can only analyze he emissions selec ed by he op ical il e s. This makes hem
pa icula ly well sui ed o applica ions in which only a ew Raman peaks a e o in e es . In
such cases, he ade-o in spec al ange is ou weighed by he p ac ical bene i s o educed
sys em complexi y, imp o ed manu ac u abili y, and enhanced mechanical s abili y.
Bo h he acquisi ion sys ems he e p esen ed a e designed o eco d and analyze he
Raman S okes ea u es o he h ee molecula species. Gi en he simple wo-a om s uc u e
o hese molecules, hei Raman emissions esul in dis inc i e peak-shaped spec a. In he
case o complex-shaped spec a, wi h o e lapped emissions, he non-dispe si e app oach
could be di icul o implemen o e en impossible wi hou he use o se e al il e s and
c oss- alk cha ac e iza ions in he spec al egions sampled by he band-pass il e s.
The Raman shi s o oxygen, ni ogen, and hyd ogen a e as ollows: 1556 cm
−1
,
2331 cm
−1
, and 4161 cm
−1
, espec i ely. Hyd ogen has also o he o a ional emission peaks,
close o he pump wa eleng h, no aken in o conside a ion in his wo k [18,19,31,32].
Using a lase sou ce wi h emissions cen e ed a a wa eleng h o 532 nm, he esul ing
Raman peaks will be a wa eleng hs cen e ed a abou 580 nm, 607 nm, and 683 nm
o oxygen, ni ogen, and hyd ogen, espec i ely. The il e s selec ed o his s udy a e
comme cially a ailable (Tho labs, New on, NJ, USA), ha d-coa ed band-pass il e s wi h
cen al wa eleng hs o 580 nm, 610 nm, and 680 nm. All he il e s a e cha ac e ized
by a FWHM o 10 nm, wi h ansmi ance peaks exceeding 90% o he incoming ligh .
Addi ionally, hei op ical densi ies ou side he ansmission bandwid h a e g ea e han 5.
An explana o y g aph is epo ed in Figu e 1, in which he ele an Raman emissions
no malized o he p ope c oss sec ions a e d awn in he igu e wi h blue lines. The
Raman emission no in ol ed in his s udy, he o a ional Raman emission o hyd ogen,
is no epo ed. The ansmi ances o he band-pass il e s and he long-pass il e used
in he non-dispe si e se up a e also epo ed wi h ed lines. The ib a ional Raman
emissions o hyd ogen and ni ogen a e no pe ec ly cen e ed wi h he ansmission bands
o hei espec i e band-pass il e s. Howe e , he hyd ogen emission peak co esponds o
app oxima ely 87% ansmi ance o i s il e , while he ni ogen emission peak co esponds
o abou 93% ansmi ance o i s il e .
Possible in e e ing molecules in hyd ogen p oduc ion en i onmen s include me hane,
ca bon monoxide, and ca bon dioxide: (i) Me hane is cha ac e ized by a s ong Raman
emission (c oss-sec ion
≈
8.6) co esponding o he
ν1
symme ic C–H s e ching mode,
loca ed a ound 2917–2919 cm
−1
. Wi h he exci a ion sou ce used in his se up, he co e-
sponding Raman-shi ed wa eleng h is 629.7 nm, well ou side he ansmission bands o
he il e s implemen ed. Howe e , me hane is a mo e complex molecule han ni ogen,
oxygen, o hyd ogen, which leads o addi ional emissions, such as he
ν2
bending mode
a 1535 cm
−1
, co esponding o 579.3 nm. This emission nea ly o e laps wi h he Raman
peak o oxygen. The e o e, o applica ions equi ing simul aneous de ec ion o me hane
and oxygen, an addi ional il e cen e ed a 630 nm should be employed o collec he
me hane
ν1
signal, and a c oss-sensi i i y analysis mus be pe o med p io o deploymen .
Senso s 2025,25, 4190 5 o 14
Simila ly, me hane also exhibi s weak Raman ea u es in he 3500–4000 cm
−1
ange, which
is a ibu ed o o e ones, ho bands, and combina ion bands. These weak ea u es may
in e e e wi h he hyd ogen signal, and an analogous c oss-sensi i i y e alua ion, as de-
sc ibed o oxygen, is equi ed. (ii) Ca bon monoxide has a Raman peak a 2143 cm
−1
(c oss-sec ion
≈
0.9), co esponding o 600.5 nm. This peak does no in e e e wi h any
o he molecula signals conside ed in his s udy. (iii) Ca bon dioxide exhibi s a Fe mi
double wi h peaks a 1388 cm
−1
(
ν1
) and 1285 cm
−1
(2
ν2
), and wi h Raman c oss-sec ions
o app oxima ely 1.1 and 0.8, espec i ely. The co esponding wa eleng hs a e 574.4 nm
and 571.0 nm. The i s peak lies wi hin he ~8% ansmission ange o he oxygen il e ,
while he second alls en i ely ou side he ansmission bands o all he used il e s. In he
p esence o CO
2
, o ensu e accu a e measu emen , i is ad isable o in oduce an addi ional
il e cen e ed a 570 nm and o pe o m a O
2
-CO
2
c oss-sensi i i y calib a ion p io o
ope a ion. These obse a ions hold ue as long as comme cially a ailable il e s wi h an
FWHM
≈
10 nm a e used. Fo speci ic applica ions equi ing a highe deg ee o selec i i y,
i emains possible o employ cus om-made il e s speci ically designed o achie e pe ec
alignmen be ween he spec al peaks and passbands while simul aneously minimizing
hei FWHM as much as possible.
Figu e 1. Raman emissions and il e ansmi ances.
On he o he side, he dispe si e sys em acqui es he spec ally dispe sed Raman
emissions. To dis inguish o e lapping spec al componen s, such as hose abo e discussed,
i is necessa y o acqui e a da ase o pu e calib a ions o be used in a i ing p ocedu e.
C oss-sensi i i y is inhe en ly mi iga ed by he i ing algo i hm, which de e mines he
op imal combina ion o calib a ion spec a ha bes ma ches he acqui ed spec um [
26
,
28
].
A simpli ied schema ic o he non-dispe si e sys em is epo ed in Figu e 2. The sys em
exci es spon aneous Raman emission ia a 532 nm DPSS doubled Nd:YAG con inuous
wa e lase (a); such a sou ce p o ides an op ical powe o abou 1.5 W, and i s emission
linewid h is less han 0.1 nm, wi h a spa ial mode nea o TEM00. The collima ed adia ion
emi ed by he lase sou ce is cha ac e ized by a linea pola iza ion, wi h he elec ic ield
o ien ed in pa allel o he y-axis shown in Figu e 2. The beam is ocused in o he gas cell,
(c) hanks o a 1-inch diame e , 50 mm ocal leng h, an i- e lec ion coa ed ocusing lens
(b). The gas cell is cus om-designed, and i is ealized in b ass. I inco po a es he housing
o accommoda e he ocusing lens di ec ly in o i s body. All op ical windows p esen in
he gas cell a e o a 1-inch diame e , b oadband an i- e lec ion-coa ed, 3 mm- hick, and
Senso s 2025,25, 4190 6 o 14
moun ed in he gas cell body; wo windows a e placed in co espondence o he beam
ocus; on he ou pu side o he cell, an op ical window is placed o o m a 45
◦
angle wi h
he incoming beam. A e his window, he beam is e mina ed in o a beam dump (d).
The pa ial e lec ion is ansmi ed h ough an addi ional window and collec ed by a
pho odiode (m) used o no malize e en ual d i s in he op ical powe o he lase sou ce
du ing acquisi ions. A ligh di use (l) is placed be o e he pho odiode o homogenize he
collec ed ligh .
Figu e 2. Sys em schema ic.
The collec ing op ics is composed by wo, 1-inch-diame e Has ings ach oma ic iple s
(e, h) wi h ocal leng hs o 40 mm, leading o an e ec i e ape u e /1.66. A long-pass il e
( ) wi h a 550 nm cu -o wa eleng h and a band-pass il e (g), whose ansmi ance is
cen e ed a he wa eleng h o he Raman emission o be de ec ed, is placed be ween he wo
ach oma ic iple s, whe e he ligh is collima ed. To acqui e di e en Raman emissions,
he band-pass il e s a e moun ed on a il e s wheel s a ion. The dis ances be ween he gas
cell cen e , he wo ach oma ic iple s, and he CMOS senso o he came a a e adjus ed o
ec ea e an image o he lase beam ocused in he gas cell cen e on o he came a senso .
The came a (i) is an indus ial un-cooled CMOS 2.3 MP wi h a pixel a ea o 5.86
×
5.86
µ
m
2
,
which is based on he SONY IMX249 monoch oma ic senso . An aluminum sphe ical mi o
(n), wi h a ocal leng h o 25 mm, is placed on he opposi e side o he collec ing op ics a a
dis ance o 50 mm. Wi h his mi o , he emission sca e ed owa ds he opposi e side o
he came a is e ocused in he gas cell cen e and hen edi ec ed owa ds he collec ing
op ics. I s p esence con ibu es by almos doubling, excep o losses and non-ideali ies, he
signal eco ded by he came a senso .
The dispe si e se up used o he compa ison is cha ac e ized by he same coupling
op ics composed by he wo Has ings iple s (e, h). The coupling op ics ec ea e an image
o a lase beam ocused in he gas cell on o he ocal plane o a sli -less di ac ion g a ing
spec ome e , which p o ides abou 11 nm/mm spec al dispe sion on o he same came a
used in he non-dispe si e se up. The spec ome e consis s o wo ixed ocal leng h
objec i es and a di ac ion g a ing, adop ing he same op ical design success ully es ed
o Raman emission om na u al gas [
28
]. The en ance objec i e is a 50 mm /2, while he
ou pu objec i e is a 25 mm /1.4; hey a e bo h di ec ed owa ds he di ac ion g a ing o
o m an angle o 71
◦
. The di ac ion g a ing has a g oo e densi y o 1200 g oo es/mm and
Senso s 2025,25, 4190 7 o 14
a blaze wa eleng h o 750 nm (Edmund 43-210). The long-pass il e ( ) is posi ioned in
he inpu a m o he spec ome e , be ween he inpu objec i e and he di ac ion g a ing,
whe e he ligh en e ing h ough he sli is collima ed; he sli is adjus ed o minimize he
s ay ligh coming om he gas cell, while a oiding igne ing o he lase beam image
p ojec ed in o i (1.2 mm along he y axis). The e ec i e inpu ocal a io o he spec ome e
is /2.8, limi ed by he ape u e o he ou pu objec i e. In e ms o ligh collec ed by he
came a, he e is abou 40% in signal losses i compa ed o he non-dispe si e se up. The
spec al ange co e ed by he CMOS came a allows he simul aneous acquisi ion o he
Raman emission o oxygen, ni ogen, and hyd ogen, al hough a clea igne ing due o he
ou pu objec i e is obse ed on he lines closed o he edges o he CMOS came a. To a oid
igne ing, hyd ogen and ni ogen-oxygen ha e been acqui ed sepa a ely in consecu i e
acquisi ions by il ing he g a ing in o de o ha e he Raman lines be acqui ed in he cen al
egion o he CMOS came a.
The dimensions o he non-dispe si e se up a e 35 cm
×
10 cm
×
25 cm along he x, y,
and z axis, espec i ely. While he dimensions o he dispe si e se up a e 35 cm x 15 cm
x 30 cm along he same axis. These olumes could be sligh ly educed by e-design and
op imiza ions. Fu he mo e, hese dimensions a e compa ible wi h he manu ac u ing o
po able ins umen a ion. In a p ac ical case, he ins umen could be housed in a ame,
o sa e y and eliabili y easons, and anspo ed in o a sui case.
3. Expe imen al S uc u e
To analyze he sys em’s pe o mance, a ce i ied gas mix u e om a bo le has been
p essu ized in o he gas cell olume a h ee di e en p essu es. Consequen ly, he molecu-
la Raman emissions ha e been acqui ed wi h he wo se ups and he signals ha e been
elabo a ed as discussed, subsequen ly. The ce i ied gas bo le (RISAM GAS) composi ion
is as ollows: hyd ogen 2% / , ni ogen 77.1% / , and oxygen 20.9% / . No mix u e
manipula ion has been pe o med, only dus emo al ia a po ous il e (5 µm).
The es s ha e been pe o med a di e en absolu e p essu es and in eg a ion imes in
he anges o 1–3 ba and 2–10 s. Fo each condi ion, 100 came a ames ha e been acqui ed
o de e mine sys em pe o mance. In addi ion, 100 ames ha e been acqui ed o each
came a in eg a ion ime wi h he gas cell pumped down o 2 mba by a memb ane pump
o he de e mina ion o he backg ound le els. Fu he mo e, each image is co ec ed by
he co esponding came a da k ame and inally no malized o i s pho odiode eading o
ake in o accoun possible a ia ions in he lase in ensi y.
The mix u e in o he gas cell is injec ed ia a needle al e. A digi al absolu e manome-
e (D uck DPI 104, Gene al Elec ic, Bos on, MA, USA) measu es he e ec i e cell p essu e.
Figu e 3shows examples o (4
×
4 binned) ames acqui ed wi h bo h sys ems. In
he dispe si e sys em, he emission is di ac ed by he g a ing, and i is also possible o
app ecia e he Pe z al ield cu a u e [33].
The s anda d de ia ions o he esidual signal in images acqui ed wi h he gas cell
pumped o 2 mba we e conside ed as he sys em’s backg ound noise in he absence o gas
molecules. The p esence o a speci ic molecule is de ec ed when i s signal exceeds a leas
h ee imes his alue.
Senso s 2025,25, 4190 8 o 14
(d)
(a)
(b)
(e)
(c)
Figu e 3. Image o Raman emission inside he cell using O
2
il e (a), N
2
il e (b), and H
2
il e (c).
(d) Spec um o N2(le emission) and O2( igh emission) emissions. (e) Spec um o H2emission.
4. Discussion
As discussed in Sec ion 3, a e he a e age da k ame sub ac ion, he signal is
de ined as he a e age o he pixel alues (PV) in he ame po ion in which he Raman
emission is collec ed, minus he a e age alue in he egions, wi h he same a ea, in which
he e is no de ec ed emission. Exp ession (1) esumes he da a elabo a ion o a single
ame o he came a a e he a e age da k ame sub ac ion:
Signal = ∑
REA
PV −∑
NREA
PV!/A, (1)
whe e REA is he Raman emission a ea, NREA is a non-Raman emission a ea close o
REA wi h he same a ea A. In Figu e 4, he hyd ogen Raman emissions and he ela ed
REA and NREA a e epo ed o bo h he se ups. To de ine he REA and NREA egions
o each se up, ames we e a e aged along he lase p opaga ion di ec ion. This yields a
one-dimensional a ay whose leng h co esponds o he numbe o senso columns in he
non-dispe si e case and o he numbe o senso ows in he dispe si e case. The esul ing
signal is hen app oxima ed wi h a Gaussian i . The s a and end poin s o he REA
egions a e iden i ied as he mean
±
3
σ
o he i ed unc ion. This p ocedu e was applied
o he mos in ense signals o each molecule (10-s exposu e ime and highes p essu e);
