Frequency modulation spectroscopy
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24 SPECTROSCOPYEUROPE www.spectroscopyeurope.com PROCESS COLUMN Frequency modulation spectroscopy (FMS) is opening up exciting new appli- cations using diode lasers in process spectroscopy. Diode lasers have found several appli- cations in process monitoring. They are low cost, have narrow line widths and are well suited for high-resolution spec- troscopy. For these reasons, most appli- cations have been in gas monitoring over medium to long pathlengths. The high specificity offered by the ability to measure individual rotational bands has been useful for both atmospheric trace gas analysis and process gas analysis, typically emissions monitoring. Lead salt lasers are available as sources for the mid- infrared region, but require cooling to liquid nitrogen temperatures, while near infrared diodes (based on III–V semicon- ductors) operate at room temperature and are commercially available. However, the relative weakness of absorption of NIR overtones has tended to limit applications to long pathlength measurements. The wavelength of diode lasers may be tuned by temperature and/or current control, with the range depending on the individual laser materials and structure. Tuning the wavelength offers the oppor- tunity both to scan the laser through an absorbance line and also to apply a high frequency modulation to shift the detec- tion bandwidth to high frequency where laser intensity noise is reduced towards the shot noise limit and subsequently the signal-to-noise ratio is increased by several orders of magnitude. The concept is analogous to the way voice, video and data information are encoded in the side bands of a radio transmission carrier wave. A diode laser is frequency modulated by superimposing a radio frequency oscil- lation, Ω, onto the diode injection current. The spectral output of a radio frequency modulated diode laser, shown at the top of Figure 1, consists of a carrier frequency ω c and side band frequencies, ω c ± Ω. When the laser frequency is then tuned through an absorption band, the amount of light absorbed, which by Beer’s Law is proportional to the gas concentration, is “written” into the side band frequencies. This is shown schematically as a decrease in the amplitude of the upper side band (ω c ± Ω) at the top of Figure 1. The absorption information is extracted using phase-sensitive detection techniques. A mixer demodulates the radio frequency signal and outputs a voltage proportional to gas concentration. The demodulated absorption feature is shown in Figure 2, note that the FMS output is the derivative of the absorbance band. The gas concen- tration is proportional to the peak-to-peak amplitude of the FM signal. So now we have a spectroscopic tech- nique which has good sensitivity for specific gases over relatively short path- lengths. Diode lasers are available at frequencies which are suitable to measure several gases which have absorbances in the NIR, including water and oxygen. (Oxygen has an electric dipole forbidden absorbance band between 760 and 766 nm with P and R branch rotational fine structure.) Frequency modulation spectroscopy John Andrews and Paul Dallin Clairet Scientific Ltd, 17Scirocco Close, Moulton Park Industrial Estate, Northampton NN3 6AP, UK. E-mail: [email protected] Current + Temperature Controller Diode Laser Scan Controller rf Oscillator Amp. Sample Detector ωc – Ω ωc+Ω ωc – Ω ωc + Ω Mixer A/D Converter Computer LO RF IF ωc ωc ωc (a) (b) Figure 1. (a) Frequency and intensity profile of a diode laser beam: unmodulated, modulated with no absorption and modulated with absorption. (b) Schematic diagram of the frequency modulation spectroscopy technique. The frequency modulated diode laser output is converted to an amplitude modulation after passing through a gas sample which absorbs at a particular wave- length. The amplitude modulation is proportional to gas concentration and can be phase sensi- tively detected.
Transcript of Frequency modulation spectroscopy
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Frequency modulation spectroscopy(FMS) is opening up exciting new appli-cations using diode lasers in processspectroscopy.
Diode lasers have found several appli-cations in process monitoring. They arelow cost, have narrow line widths and arewell suited for high-resolution spec-troscopy. For these reasons, most appli-cations have been in gas monitoring overmedium to long pathlengths. The highspecificity offered by the ability tomeasure individual rotational bands hasbeen useful for both atmospheric tracegas analysis and process gas analysis,typically emissions monitoring. Lead saltlasers are available as sources for the mid-infrared region, but require cooling toliquid nitrogen temperatures, while nearinfrared diodes (based on III–V semicon-ductors) operate at room temperatureand are commercially available. However,the relative weakness of absorption of NIRovertones has tended to limit applicationsto long pathlength measurements.
The wavelength of diode lasers may betuned by temperature and/or currentcontrol, with the range depending on theindividual laser materials and structure.Tuning the wavelength offers the oppor-tunity both to scan the laser through anabsorbance line and also to apply a highfrequency modulation to shift the detec-tion bandwidth to high frequency wherelaser intensity noise is reduced towardsthe shot noise limit and subsequently thesignal-to-noise ratio is increased byseveral orders of magnitude. The conceptis analogous to the way voice, video anddata information are encoded in the sidebands of a radio transmission carrierwave.
A diode laser is frequency modulatedby superimposing a radio frequency oscil-
lation, Ω, onto the diode injection current.The spectral output of a radio frequencymodulated diode laser, shown at the topof Figure 1, consists of a carrier frequencyωc and side band frequencies, ωc ± Ω.When the laser frequency is then tunedthrough an absorption band, the amountof light absorbed, which by Beer’s Law isproportional to the gas concentration, is“written” into the side band frequencies.This is shown schematically as a decreasein the amplitude of the upper side band(ωc ± Ω) at the top of Figure 1. Theabsorption information is extracted usingphase-sensitive detection techniques. Amixer demodulates the radio frequencysignal and outputs a voltage proportional
to gas concentration. The demodulatedabsorption feature is shown in Figure 2,note that the FMS output is the derivativeof the absorbance band. The gas concen-tration is proportional to the peak-to-peakamplitude of the FM signal.
So now we have a spectroscopic tech-nique which has good sensitivity forspecific gases over relatively short path-lengths. Diode lasers are available atfrequencies which are suitable tomeasure several gases which haveabsorbances in the NIR, including waterand oxygen. (Oxygen has an electricdipole forbidden absorbance bandbetween 760 and 766 nm with P and Rbranch rotational fine structure.)
Frequency modulationspectroscopyJohn Andrews and Paul DallinClairet Scientific Ltd, 17 Scirocco Close, Moulton Park Industrial Estate, Northampton NN3 6AP, UK. E-mail: [email protected]
Current +Temperature
Controller
DiodeLaser
ScanController
rfOscillator Amp.
Sample Detector
ωc – Ω
ωc+Ω
ωc – Ω
ωc + Ω
Mixer
A/D Converter
Computer
LO RF
IF
ωc ωcωc
(a)
(b)
Figure 1. (a) Frequency and intensity profile of a diode laser beam: unmodulated, modulated
with no absorption and modulated with absorption. (b) Schematic diagram of the frequency
modulation spectroscopy technique. The frequency modulated diode laser output is converted to
an amplitude modulation after passing through a gas sample which absorbs at a particular wave-
length. The amplitude modulation is proportional to gas concentration and can be phase sensi-
tively detected.
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Applications of FMSFMS has found several applications in thepharmaceutical industry as a headspaceanalysis tool. With the growth of biotech-nology a growing proportion of the prod-ucts from the industry are parenterals:drugs designed to be administered intra-venously or by injection. Typically theseare liquids (or freeze dried solids) pack-aged in glass or plastic vials, and aremanufactured under aseptic conditions.They are packaged under an inert nitro-gen atmosphere or under vacuum toprevent oxidative degradation and it isimportant to know if oxygen has beenexcluded from the package and whetherair can enter the vials through cracks orproblems with stoppers. The US FDA hasrecently produced a draft guidance docu-ment which states: “A container closuresystem that permits penetration of air, ormicroorganisms, is unsuitable for a ster-ile product. Any damaged or defective
units should be detected, and removed,during inspection of the final sealed prod-uct. Safeguards should be implementedto strictly preclude shipment of productthat may lack container closure integrityand lead to nonsterility. Equipment suit-ability problems or incoming container orclosure deficiencies have caused loss ofcontainer closure system integrity. Asexamples, failure to detect vials fracturedby faulty machinery, or by mishandlingof bulk finished stock, has led to drugrecalls. If damage that is not readilydetected leads to loss of containerclosure integrity, improved proceduresshould be rapidly implemented toprevent and detect such defects.”1 Thereare two ways in which FMS can helpmeet this requirement: oxygen measure-ments or pressure measurements.
As shown above, oxygen concentrationcan be measured by FMS. An instrumentcalibration constant, k, is determined
using a vial filled with a certified gasmixture of 20% oxygen and 80% nitro-gen. When the 20% oxygen referencestandard is inserted to the instrument, thesystem response (signal amplitude, S,and power, P) is measured and stored.The instrument records the measuredsignal and detected laser power, thenusing the known amount of oxygencomputes a calibration constant k,
k = (P / S) × % oxygen
where P is the detected laser power, S isthe peak-to-peak absorption signal (seeFigure 2) and % oxygen is the concen-tration in the calibration vial. Themeasured values of P and S are typicallythe result of 1000 averaged highfrequency scans, collected in less than2 s. A commercial instrument has beendeveloped for laboratory and on-line anal-ysis. Ingress of air into packages is readilydetected by monitoring oxygen content.
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Because the instrument is working in theNIR, glass and most plastic packages aresuitable candidates for measurement.
Pressure in a sealed container may bemonitored spectroscopically by measur-ing the width of a single waterabsorbance. The broadening of the widthof a gas-phase rotational absorption lineis largely due to a combination of colli-sion broadening and Doppler broadening,therefore the width of the FMS absorptionsignal is proportional to total headspacepressure. Figure 3 shows a water bandresponse for a set of four calibration stan-dards at known pressures, showing howthe FMS signal width varies with totalpressure.
The speed of FMS (sub-secondmeasurement times) lends itself to on-line measurement of parenteral contain-ers in real time. The non-invasive,non-destructive nature of the FMSmeasurement is ideal for 100% inspec-tion of containers which can containexpensive product and where the onlyalternative is destructive sampling ofrandom samples from the batch.Commercial instruments have beendeveloped, installed and validated tomeasure the pressure in thousands ofvials per day using FMS technology(Figure 5).
So, a relatively new spectroscopic tech-nique has found applications within phar-
maceutical production processes world-wide. Another Process Spectroscopysuccess story!
Further reading1. Sterile Drug Products Produced by
Aseptic Processing–Current GoodManufacturing Practice. www.fda.gov/cder/guidance/1874dft.htm
2. I. Linnerud, P. Kaspersen, T. Jaeger,“Gas monitoring in the process indus-try using diode laser spectroscopy”,Appl. Phys. B 6677,, 297–305 (1998).
3. J.A. Silver, “Frequency-modulationspectroscopy for trace species detec-tion: theory and comparison amongexperimental methods”, Appl. Optics3311,, 707–717 (1992).
4. J. Veale, Non-Destructive HeadspaceGas Analysers. Technical Note,Lighthouse Instruments Inc.,Charlottesville, VA, USA.
Figure 5. Commercial FMS spectrometer
for 100% inspection of headspace pressure
in sealed vials capable of measuring 100s of
vials per day (courtesy of Lighthouse
Instruments Inc.).
Figure 4. Calibration curve using standards of known pressure. The instrument responds linearly
to pressure over the range 0.04 atm to 1.0 atm.
Figure 2. Frequency modulation signals from oxygen absorption. The
peak-to-peak amplitude of each spectrum is proportional to oxygen
concentration (noted to the right of each scan). These spectra were
taken through 1″ diameter glass containers filled with certified gas
mixtures of oxygen in nitrogen.
Figure 3. Water band width for vials with different headspace pres-
sures.
