Key features
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Photoacoustic gas analysis module
for laboratory FTIRs
Includes sensitive photoacoustic cellwith ultra sensitive patented cantileverenhanced optical microphone
Includes a user programmable gas
exchange unit
Optical path length: 8 cm
Total internal gas volume: 30 ml
Fits all major FTIR instruments

FTIR-PAS setup

PA101 replaces the conventional long
path gas cell and the IR detector
Conventional FTIR gas measurement

PA101 photoacoustic gas analysis setup

Large gas volume
Non-linear
Limited dynamic range
Frequent backgroundmeasurement
Low gas volume
Highly linear
Wide dynamic range
Backgroundmeasurement onceDirect measurement in PAS vs.
Transmission spectroscopy

The absorption is measured directly in PAS, which makes the measurement very accurate andfree of drift. This is the key factor for the unbeatable stability and reliability of the PAS method.
In transmission spectroscopy the small decrease of the large signal is measured, and therefore,
small changes in the background signal create large errors and drifts in the measurement result.

Molecular vibrations
H2O molecule vibration modes
and resonance frequencies!
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1.10 x 1014 Hz
3652 cm-1
2.74 µm

4.80 x 1013 Hz
1595 cm-1
6.27 µm!

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1.12 x 1014 Hz
3756 cm-1
2.66 µm!

Signal is generated by infrared absorption into thedifferent molecular vibrations in the PAS.
Molecules do not only move with a high speed,
but they also rotate and vibrate.
Different molecules have different vibrationalfrequencies depending on the shape of themolecule, the forces between the atoms and themass of its atoms.
Single molecule can have several different
vibration modes.
Vibrations are able to absorb infrared radiation,
which has the same wavelengths as thevibrations.
Increased vibration leads to faster movement inthe molecular collisions and the temperature israised in the absorption.

Gas temperature and pressure
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Heating of the gas increases the temperature and thepressure in the photoacoustic cell.
Gas is a collection of molecules that are in acontinuous random motion. They are constantly
colliding to each other and also to the surrounding
walls.
One liter of gas contains 3 x 1022 molecules (NTP).
The absolute temperature of the gas is proportional tothe kinetic energy (i.e. to the square of the velocity) ofgas molecules. At 20°C temperature the averagevelocity of gas molecules is 407 m/s. At 40°C the it is
421 m/s.
The bombardment of the molecules to the sensor
element creates a pressure, which rises along with thetemperature. E.g. at 20 °C the molecules create thepressure of 1.013 x 105 N/m2 in the photoacoustic celland after heating to 40 °C the pressure is 1.08 x 105 N/m2 .

FTIR-PAS principle!

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In FTIR-PAS the sample
cell and the IR detector
are replaced with thephotoacoustic gas cell
Only the absorbed partof the radiation
produces PA signal – nogas, no signal i.e. zerobackground
Only one recordedinterferogram has to betransformed to get an
infrared spectrum of thesample gas

Cantilever enhanced optical microphone!
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Cantilever pressure sensor provides high sensitivity to smallpressure variations.
The movement of the cantilever is measured with a laser readout
interferometer.
The microphone has highly linear response over a very wide
dynamic range.
The response of the cantilever remains stabile, even if theambient temperature is varied.
Performance: Sensitivity: ~10V/Pa, noise level: minimum
detectable pressure variation in the PA101 cell: ~5 x 10-7 Pa/√Hz(RMS)Performance
Sample: CH4 1000ppm
Measurement time: 60 s
FTIR device: Thermo Antaris IGS
Resolution: 8 cm-1
HeNe laser frequency: 2.5 kHz
Pressure: 1 atm
Max signal@3017: 221
Noise(2200-2100 cm-1) : 0.081 peakto-peak
SNR: 2728
Detection limit: 0.4 ppm with 60 s
measurement time

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Measurement with 1000ppmof CH4
Measurement time over 3hours
Repeatability <0.5% (peak topeak measured fromsignal@3017cm-1)
Background repeatability<0.05% of the signal
Fluctuation of the instrument
IR source power are also
included in the measurement
results
Measurement time: 1 min

0.746

0.0025

0.744

0.002

0.742

0.0015

0.740

0.001

0.738

0.0005

0.736
19:12

0
20:24

21:36

Time

22:48

Average background signal@ 2200-2100 cm-1!

Max signal at 3017cm-1

Repeatability

Water subtraction with PA101!

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Short optical path length gives
linear response on a wide
dynamic range.
Linear response allows linear
spectral mathematics, e.g.
subtraction of two spectra.

Bruker Tensor 27 + PA101 accessory
90 ppm NO sample with high concentration of residual water
Water spectrum for subtraction was measured from ambient air

Wet methane - 100 ppm!
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Bruker Tensor 27 + PA101
accessory
100 ppm CH4 sample withhigh concentration ofresidual water
Water spectrum for
subtraction was measured
from a sample created witha gas generator

Low volume!

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The low total volume of system gives highsensitivity when only small amount of gas
is available.
The gas exchanger allows the recirculationof this gas which enables e.g. monitoring ofsystem which emits gas with low
concentration.
E.g. if the sample emits N units of gas
molecules and it is connected to system of
1 L the concentration is N/1 L. Whenconnected to PA101 with volume of 30 ml
and with e.g. 70 ml external volume, theglass and tubes, the concentration is 10 N/
1 L – 10 times higher sensitivity!

Applications! Low sample volume applications:!
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Headspace analysisAnalysis of synthesis processes
Analysis of decomposition processes
Outgassing of materials! Wet gases! Measurements requiring wide dynamicrange