FTIRTalkLetterVol.16

7/31/2019 FTIRTalkLetterVol.16

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C103-E084


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Spectroscopy Business Unit, Analytical & Measuring Instruments Division, Kiyoshi Wada

Gas Cell

1. Lambert-Beer Law

The absorption intensity o a material can be defnedaccording to the Lambert-Beer law. The Lambert-Beerlaw, expressed by equation (1) o Fig. 1, establishes therelationship among incident light I0, the lighttransmitted through the sample It, the opticalpathlength d, the sample concentration C, and theabsorbance A. , reerred to as absorption coefcient, is

a constant that indicates the amount o light that canbe absorbed by a substance.

2.1 Short-Path Gas CellsLight is transmitted in a straight line rom the inlet tothe outlet o a short-path gas cell, just as in liquid cells,and is thereore also called a single-path gas cell. Fig. 2shows cells having a cylindrically-shaped structure withinwhich the sample gas is sealed. Window plates areattached at both ends o the cylinder to allowtransmission o inrared light. Although the opticalpathlength is actually the length o the gas cell, its

maximum length is limited to 15 cm since it is mountedinside the FTIR sample compartment. Shimadzu oersgas cells with optical pathlengths o 5 cm and 10 cm. Asinrared light displays little attenuation, it can bemeasured using a standard FTIR DLATGS detector.The gas cell is made o glass, and the window plate isselected rom among KBr, NaCl, KRS-5, depending onthe sample characteristics. The concentration that canbe measured depends on the substance, butconcentrations rom 10 ppm up to several percent canbe measured with a 10 cm cell. For example, in the caseo methane, a concentration range between 69 ppm to27,500 ppm can be measured, as indicated in Table 1.

2. Types o Gas Cells

There are 2 types o gas cells, those having short opticalpathlengths and those with long optical pathlengths.

2.2 Long-Path Gas Cells

Measurement o gas samples o low concentration inthe order o ppm, requires a long optical pathlengthcell. The detection limit o methane is on the order o 1ppm, which is 1/100 o what can be measured in a 10 cmgas cell. To measure methane at this detection limit, agas cell with a pathlength 100 times greater, one o 10m, would be required. But a 10 m-long single-path gascell cannot be mounted in an FTIR. Thereore, along-path gas cell permitting back-and-orth light travelbetween two mirrors is used. Since light repeatedlytravels back and orth in the cell, this type o long-pathgas cell is also reerred to as a multipath gas cell.

Assuming a constant optical pathlength, theconcentration can be calculated rom the absorbanceusing a calibration curve. Moreover, i the approximateconcentration o the sample is known, the opticalpathlength necessary or measuring a sample can becalculated rom the ratio o the absorbance to theproduct o the concentration and the molar absorptioncoefcient.The absorption spectra o gas samples can be measuredby enclosing them in a container, in the same manner asliquid samples, and irradiating them with inrared light.

Since the concentrations o liquid and solid samples arehigh due to their relatively high densities, measurementis conducted using optical pathlengths shorter than 1mm. Gas samples, on the other hand, are relatively lowin density, and are thereore measured using opticalpathlengths rom a ew centimeters to tens o meters.

Samples o various states can be analyzed by FTIR by selecting the appropriate accessory attachment. Here weintroduce the structures and method or selecting a gas cell used or measurement o gas samples.

d

I0 I t

Concentration C

Fig. 1 Diagram Illustrating Lambert-Beer Law

log(I0/I t)=log(1/T)=Cd=A(1)

Fig. 2 5 cm Gas Cell (let) and 10 cm Gas Cell (right)

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d

M1 M2Gas Cell

Ater traveling repeatedly back and orth betweenmirror M1 and mirror M2, as shown in Fig. 3, the lightexits the gas cell toward the detector. One round trip oinrared light is equivalent to twice the distance dbetween the mirrors. I d is 50 cm, the opticalpathlength using 10 round trips becomes 10 m.

Long-path gas cells can be o a fxed optical pathlengthor a variable optical pathlength. In general, the fxedoptical pathlength type o gas cell is convenient sinceno maintenance is required. Although the variableoptical pathlength gas cell is convenient ormeasurement o gases o varying concentration ranges,a laser must be used or adjustment o the M2 mirrorwhen changing the pathlength.When light conducts N round trips to travel the opticalpath, it is reected (2N - 1) times by the mirrors. With10 round trips it is reected 19 times. When using a 95% reectivity mirror, inrared light is attenuated up to38 % (0.9519 = 0.38) ater 19 times reections. For thatreason, a high-sensitivity MCT detector is used instead

o the standard FTIR DLATGS detector.In addition, installation and removal o the tubing canbe a time-consuming, burdensome chore due to thelarge size and weight o these gas cells. Thus, ratherthan installing it inside the FTIR sample compartment,it is convenient to set up the long-path gas cell alongwith a gas cell box beside the FTIR unit.

Fig. 3 Diagram o Long-Path Gas Cell

Fig. 4 Gas Cell and Gas Cell Box Positioned Beside the IRPrestige-21

Fig. 6 TG-FTIR System

Fig. 5 Glass Body and Metallic Body Gas Cells

Glass Body Gas Cell Metallic Body Gas Cell

Gas cells are available with a glass body or a metallicbody, as shown in Fig. 5. The glass type is inexpensiveand more commonly used, but is not applicable orgases that contain hydrogen uoride, which etchesglass. In such cases, a metallic gas cell with a uorineresin-coated interior surace is used. In addition, both

the glass and metallic types are available withheater-wrapped bodies to allow heating o the gas cell.Window plates consisting o KBr, CaF2, BaF2, etc. can beused. When used in combination with an MCT detector,BaF2 window plates are convenient because they areresistant to steam and are applicable to measurementover a wavenumber range comparable to that o anMCT detector.

2.3 Heated Gas Cell or TG-FTIR SystemFig. 6 shows a TG-FTIR system used or measurement othe gas generated rom the thermal analyzer. A heatedgas cell is used in the TG-FTIR system. A temperature o200 C or 250 C is possible using a gas cell pathlengtho 10 cm, and gas generated during heating o thesample in the TG passes to the gas cell. Window plates

consisting o KBr, NaCl, CaF2, BaF2, etc. can be used.

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Gas Substance Lower Limit Upper Limit

1,3 Butadiene

2-Methyl Pentane

Acetaldehyde

Acetone

Acetylene

BenzeneCCl2F2

CCl3CF3

CCl3F

CCl4

CClF2CClF2

CF4

CH3COOCH3

CH3COOH

CHCl3

CHClF2

CO

CO2

Cyclohexane

Diethyl Ether

Ethane

Ethanol

EthyleneH2O

HF

HCl

HBr

HCN

HCHO

HCOOH

HNO2

HNO3

HS

Isoprene

Methane

Methanol

NH3

n-Hexane

NO

NO2

N2O

n-Pentane

Ozone

Phosphine

Propane

SF6

SO2

Styrene

Toluene

o-Xylen

m-Xylen

p-Xylene

5.2

3.5

10.0

9.4

0.6

0.90.9

2.9

0.9

0.9

2.3

0.1

5.3

4.2

1.8

1.8

9.4

1.9

1.3

4.9

7.8

19.0

4.224.0

2.0

7.8

40.0

1.4

4.3

3.8

0.9

3.6

1420.0

5.4

6.9

5.4

2.7

5.0

18.0

2.5

2.7

6.6

10.0

39.0

4.0

0.2

5.2

7.6

6.7

4.4

7.5

7.7

2,080

1,410

4,060

3,760

230

340347

1,175

360

357

920

36

2,130

1,670

739

705

3,750

770

500

1,940

3,100

7,500

1,6809,720

800

3,120

16,100

578

1,710

1,500

363

1,450

570,000

2,180

2,750

2,160

1,090

2,010

7,040

1,000

1,090

2,640

4,070

15,800

1,611

70

2,060

3,020

2,680

1,740

3,006

3,100

Concentration (ppm)

Note) These values should be used only as a guideline because the upper and lower

limits will vary with such measurement conditions as integration requency, etc.

3. Sample Concentration Ranges Measurable

by a Gas Cell

The range o sample concentrations that can bemeasured by a 1 m pathlength gas cell is shown in Table1 Superscript Note). Since intensity is proportional to theoptical pathlength according to the Lambert-Beer law,the sample concentrations that can be measured will be10-old those listed when using a 10 cm gas cell, and 1/10those listed when using a 10 m gas cell.

Table 1 Quantitation Upper and Lower Limits or Typical Gases

(with 1 m gas cell)

4. Gas Cell Selection and Considerations

Select a gas cell based on the ollowing considerations.

Concentration o Measurement Target GasSelect the optical pathlength based on the gas

concentration. Select a short-path gas cell or asufciently high concentration, and a long-path gas cellor low concentrations.

Types o Gases in the Sample MatrixDepending on the types o gases present, overlappingo absorption peaks may occur, preventing analysis.I the matrix includes corrosive gases, select a glass gascell or a corrosion-resistant metallic gas cell. Since a glassgas cell cannot be used with a gas mixture containinghydrogen uoride, such gas mixtures will require theselection o a metallic gas cell with a uorinated resincoating.For gases that contain water vapor, select KRS-5 (5 cm /10 cm gas cell) or BaF2 window plates.

Detector and Installation MethodWhen using a short-path gas cell, it should be installedin the FTIR sample compartment, and a standardDLATGS detector can be used.A long-path gas cell can either be mounted in the FTIRsample compartment with installation o the optionalFTIR MCT detector, or it can be mounted in the MCTdetector-equipped gas cell box positioned beside theFTIR.

Heating Option

Water vapor and hydrogen halides can be adsorbed tothe interior walls o tubing and the gas cell body,preventing the output o correct concentrations as wellas the complete replacement o gases. This adsorptioncan be prevented by heating the tubing and gas cellbody.



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Measurement Method ABCs:Diuse Refectance Method

Along with the widespread use o the single-reection ATR method, measurement o samples in the solid, powder andliquid states has become quite easy. However, when measuring hard solid objects with coarse suraces, and powders, it canbe difcult to obtain a good inrared spectrum due to insufcient contact o the sample with the ATR prism. For thesesamples, the diuse reectance method is applicable.Here we introduce and describe the eatures, measurement principle and application examples o the diuse reectancemethod.

Kyoto Applications Development Center, Analytical Applications Department, Analytical & Measuring Instruments Division,

Hirokazu Taniguchi

One method o measuring inrared spectra o powderand solid samples by FTIR is the KBr (potassium bromide)pellet method. The KBr pellet method involves mixing

the sample with KBr powder, pressing it into pellet ormand then measuring the inrared transmittance spectrum.For a detailed description, reer to FTIR Talk Vol. 14.On the other hand, the diuse reectance method is, asindicated by its name, a method by which an inraredspectrum is obtained by measuring the diuse light thatis reected rom the surace o the sample. According tothe 15th Edition o the Japanese Pharmacopoeia, the KBrpellet method, the ATR method, and the diusereectance method are all specifed as applicable ormeasurement o solid samples. The diuse reectancemethod requires little time or pretreatment since pelletormation is unnecessary; thereore, it is oten used as aconfrmation test method.

In addition, while the ATR method permits extremelyeasy measurement o solids, powders and liquids, closecontact between the surace o the sample and the ATRprism is critical or obtaining a good inrared spectrum.Since it is difcult to achieve such close contact with solidsamples having a rough surace, as well as with powders,it may be difcult to obtain sufcient peak intensity.Also, cautioun is necessary when orcing a close contactbetween a hard crystalline sample and the ATR prism toavoid damaging the ATR prism.With the diuse reectance these concerns do not exist.Furthermore, this method eatures better capabilitythan the transmittance method in providing inormationregarding substances that are attached or adhering tothe sample surace.

1. Introduction

When light is irradiated onto a powder sample, as shownin Fig. 1, specular reected light rom the sample suraceis produced. In addition, diuse reected light (scattered

light) emerges rom the sample surace ater frst beingtransmitted into the interior o the sample particles andrepeatedly reected among the particles. Because o thisscattering o light, the method o generating an inraredspectrum o a powder using diuse reected light isreerred to as the diuse reectance method.

Diuse reected light, which has been repeatedlytransmitted within the powder sample, gives a spectrumsimilar to an ordinary transmission spectrum. However,light in a wavenumber range where absorption is weak

may be repeatedly reected inside the sample particlesand emerge in such a way that it is not collected ordetection. Because o this, bands in diuse reectedspectra may appear weaker than the correspondingtransmittance-acquired bands and not be proportional toconcentration. This, thereore, led to the manipulation odiuse reectance spectra by the Kubelka-Munkunction (K-M unction) to make the spectra morecomparable to transmittance spectra and suitable orquantitative analysis.

2. What Is the Diuse Refectance Method?

Specular ReectedLight

Fig. 1 Diagram o Light Dispersion with Powder Sample

Incident Light

Diuse ReectedLight



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To detector

Sample

(Sample)

(Standard powder)

Here, f(R) is the K-M unction, R is the absolutereectance, Kis the molecular absorption coefcient,and S is the scattering coefcient. However, due to thedifculty in measuring the absolute reectance R othe sample, in actual measurement, it is obtained bymeasuring the relative reectance r o a standard

powder, such as KBr or KCl (potassium chloride), with amolecular absorption coefcient Knear 0 in themeasurement region.

The light measured as diuse reectance actuallyincludes the specular reectance light at the sample

surace. Obtaining a more accurate diuse reectancespectrum requires a reduction in specular reectancelight, which necessitates use o powder consisting osmaller sized particles. By reducing the particle size tothe same level o the wavelength, the scatteringefciency is maximized while the ratio o specularreectance light is reduced. In addition to particle size,the shape and loading state are also important actors.Powder samples are not measured as in a typicalmeasurement, but diluted to an appropriateconcentration (about 1 to 10 %) with KBr or KCl standardpowder.

The eect o Kubelka-Munk transormation (K-Btransormation) is explained here using the diusereectance spectrum o caeine as an example. Fig. 4shows the diuse reectance spectrum, and Fig. 5 shows

the spectrum obtained ollowing Kubelka-Munktransormation.

A weak absorption band is relatively pronounced in thediuse reectance spectrum, but ollowing K-Mtransormation, these absorption intensities are reduced,and the relative intensity between peaks causes thespectrum to resemble a transmittance spectrum. TheY-axis in the K-M-transormed graph shows the K-Munction, which should not be conused with thepre-transormation absorbance (A).

4. Diuse Refectance Spectrum andKubelka-Munk Transormation

Fig. 2 DRS-8000 Diuse Reectance Attachment

Fig. 3 Optical System o DRS-8000 Diuse Reectance Attachment

Fig. 4 Diuse Reectance Spectrum o Caeine

Fig. 5 Caeine Diuse Reectance Spectrum Ater K-M Transormation

Fig. 2 is a photograph o the DRS-8000 diusereectance attachment, and Fig. 3 shows its opticalsystem. Inrared light is irradiated onto the sample viamirror M3 in Fig. 3, and the diuse reected lightcollected by mirror M4 is routed to the detector viamirrors M5 and M6. Two types o sample holders (2 mmdia., 4 mm dia., both 1 mm deep) are available orloading o the powder sample; or aluminum disposablesample cups (6 mm dia., 1.5 mm deep) can also be used.

3. Diuse Refectance Attachment



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A Company, ATR Method

B Company, ATR Method

A Company, Diuse Refectance Method

B Company, Diuse Refectance Method

100 % (Undiluted)

Fig. 6 Lactose Diuse Refectance Spectra at Dierence Dilution Concentrations

To obtain an inrared spectrum o a powder sample usingthe diuse refectance method or conrmationtesting, etc., it is necessary to adjust the sampleconcentration using a diluti on agent such as KBr ormost o the sam ple . The target sample concentrationat the start o the adjustment process is typically around5 % by weight, but since the molecular absorptioncoecient actually diers depending on the sample, theconcentration must be urther adjusted while checkingthe obtained refectance and absorbance. Generally,sample concentration adjustment is considered to begood i the intensity o the strongest peak in thespectrum is about 10 % using refectance (or 1 byabsorbance). However, with the diuse refectancemethod, peak saturation at 10 % refectance may occurdue to the infuence o refection rom the samplesurace, making it appropriate to lower theconcentration to obtain a slightly higher refectance. Thediuse refectance spectra o lactose in Fig. 6, obtainedusing concentrations o 1, 5, 20 and 100 % by weight,

illustrate the eect o concentration adjustment.

On the other hand, to conrm a peak attributable to alow-concentration substance in a mixture o substances,the concentration o the sample can be increasedintentionally to accentuate the target peak.

5. Adjusting the Sample Concentration

Introduced here are examples o the measurement o 2types o commercially available rockwool products.Rockwool, consisting o mineral bers produced at hightemperature, has traditionally been used as a heatinsulator and sound absorber material. Rockwool itsel isan inorganic compound, but a small amount o bondingresin is oten added to commercial rockwool products toimprove product orm. We conducted single-refectionATR and diuse refectance measurement o the resinbonded rockwool products to investigate the dierencesbetween the bonding resin substances.

6. Measurement Example

Fig. 7 and 8 show the inrared spectra obtained using theATR and diuse refectance methods. In bothmeasurements, the samples were measured in theiroriginal brous state, without pulverizing them. For thediuse refectance measurement, the bers were placedin the sample holder, and the ground surace mirrorprovided with the instrument was used as the reerence.

In the inrared spectra obtained by ATR, broad peaksattributable to inorganic compounds can be seen in theregion below 1200 cm-1, and aint peaks believed tooriginate rom the bonding resin are seen rom 4000 to

1200 cm-1. However, the intensities o these peaks remainat a very small level due to the poor contact between thesamples and the ATR prism. On the other hand, in theinrared spectra obtained using the diuse refectancemethod, the peaks rom 4000 to 1300 cm-1 are relativelypronounced, and the dierences between the 2 productsare quite apparent.

Fig. 7 Inrared Spectra o Rockwool Products by ATR Method

Fig. 8 Inrared Spectra o Rockwool Products by Diuse Refectance Method



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Use o a diuse refectance attachment equipped with aheated chamber permits measurement o the samplechemical changes and molecular state changes duringheating o the sample. Measurement is also possibleunder vacuum or in a controlled atmosphere

(replacement gas) environment. Here we introduce anexample o measurement o the thermal dehydrationprocess in silica gel using the heated chamber diuserefectance attachment.Fig. 9 shows a surace model o silica gel. The surace osilica gel contains hydroxyl groups covalently bound withsilicon (silanol groups). These silanol groups can exist notonly independently as ree, isolated silanol groups, butbonded to other silanol groups via hydrogen bond, andbound to water through adsorption.

Fig. 10 shows the inrared spectra measured duringheating o silica gel powder rom ambient temperature(30 C) to 800 C (30 C, 100 C, and thereater at 100 C

intervals). The peak at 3740 cm-1 is due to absorption byisolated, ree silanol groups, and the broad peak rom3700 to 3000 cm-1 is due to water molecules that arehydrogen bound to the silanol groups. As thetemperature is increased, the hydrogen-bound watermolecules are driven o as seen rom the elimination othe broad peak rom 3700 to 3000 cm-1.

7. Heated Chamber Diuse Refectance

Measurement

The diuse refectance method is widely used or varioustypes o measurement in addition to those introducedhere. Reer to FTIR Talk Vol. 10 or an in-depth discussion.Because absorption is weak in the inrared region,diuse refectance measurement o inrared spectra canbe measured without pretreatment procedures such as

sample dilution with KBr powder. Furthermore, powdersamples can be measured as is by placing them in a glassor plastic cup, and special accessories are available.

8. Conclusion

Fig. 9 Silica Gel Surace Model

Fig. 10 Diuse Refectance Inrared Spectra During Heating o Silica Gel

Fig. 11 Sample Heating Chamber

(Heated Vacuum Diuse Refectance System by S.T. Japan Inc.)

In addition to the silica gel discussed here,heating-associated measurement o kaolin (aluminum

silicate) and ABS resin (acrylonitrile butadiene styrenecopolymer) is addressed in Application News A398.

Due to insucient space in this issue,

the Q&A section is omitted.


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Sample Compartment-Integrated ATR Measurement Attachment Series

for Shimadzu IRAffinity-1 Fourier Transform Infrared Spectrophotometer

Introduced here are accessories that urther enhance the eectiveness o the ShimadzuIRAfnity-1 Fourier Transorm Inrared Spectrophotometer. Customers that have the IRAfnity-1

can utilize the sample compartment-integrated ATR attachment. For details, please contact your

Shimadzu representative.

Three total reflectance measurement attachmentsare available:

GladiATR 10

Single-Reflection AttenuatedTotal Reflection Attachment

MIRacle 10

Single-Reflection AttenuatedTotal Reflection Attachment

HATR 10

Horizontal Type Total AttenuatedReflection Attachment

IRAffinity-1 with MIRacle 10 attached

Integration with the sample compartment greatly acilitates operation and purge perormance.Since the space above the sample compartment can be eectively used, efcient sampling is enabled. Further,

complete enclosure o the sample compartment prevents contamination o the interior.

With the MIRacle 10, the sample mounting ace is positioned higher than the IRAfnity-1 main unit cover, thereby

allowing measurement o samples wider than the sample compartment (area width) without cutting them.

These accessories are specifc to the IRAfnity-1. Existing accessories must be used or the IRPrestige-21 or the FTIR-8000 Series.


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JQA-0376

SHIMADZU CORPORATION. International Marketing Division

3. Kanda-Nishikicho 1-chome, Chiyoda-ku, Tokyo 101-8448, Japan

Phone: 81(3)3219-5641 Fax. 81(3)3219-5710URL http://www.shimadzu.com

Founded in 1875, Shimadzu Corporation, a leader in the

development of advanced technologies, has a distinguished

history of innovation built on the foundation of contributing to

society through science and technology. We maintain a global

network of sales, service, technical support and applications

centers on six continents, and have established long-term

relationships with a host of highly trained distributors located

in over 100 countries. For information about Shimadzu, and to

contact your local office, please visit our Web site at

www.shimadzu.com

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FTIRTalkLetterVol.16