Introduction of FTIR, ATR, Fiber Optics, and ATR Probes in Spectroscopy and Associated Designs - Part II

This article describes the basic principles of FTIR, Attenuated Total Reflection-Infrared Spectroscopy (ATR-IR), Fiber Optical Spectroscopy techniques and their applications in in-line reaction monitor and process control. The design and selection of fiber coupler, fiber optical cable and fiber probe are briefly introduced.

Contents Outline

1. Introduction of FTIR

1.1. What is FTIR?

1.2. Why Mid-Infrared?

1.3. What is ATR-IR

2. Introduction of Fiber Optics and ATR Probes

2.1. Why fiber optics?

2.2. How to select the ATR fiber optic probes?

3. Design of ATR Probe System

3.1. General design of ATR probes

3.2. ATR Crystals choice

3.3. Selection of Mid-IR Optical fibers

3.4. Fiber Probe Coupler to IR spectrometers

4. Benefits and Applications of Fiber Optics and ATR Probes

4.1. Benefits of Fiber Optic probes in Spectroscopy

4.2. Typical Applications of ATR Fiber Optic probes in Spectroscopy

3. Design of ATR Probe System

3.1 General Design of ATR Probes

ATR probe consists of: ATR crystal tip at the end of immersible part (so-called shaft) and Mid-IR optical fiber bifurcated to 2 legs with SMA connectors (Figure 6).

Figure 6 ATR Probe Components

ATR crystal tip and Mid-IR optical fiber are the most important parts of the probe. Figure 7 shows the design of the two-bounce reflection ATR crystals. The heads of input and output fibers attach to the bottom of an ATR crystal of conic and pyramid shape. The IR beam introduced from the input fiber enters the ATR crystals reflects twice and transmit through output fibers to the detector. The connection between the fiber heads with the ATR crystals is carefully configured to withstand high outer pressure and produce better sensitivity.

Figure 7 ATR probe and the crystal tip configuration

The probes are also carefully configured with proper protective materials for different samples and working environments. Probes with metal shaft and robust stainless-steel protective conduit are suitable for reaction monitoring in the plant and in a lab reactor. Probes with polymer shaft and replaceable ATR tips are suitable for lab application. A overview of our ATR probes and associated designs for different requirements of temperature and pressure are listed in Table 1.

Table 1 ATR Probe Overview

Current we have 5 series of ATR fiber probes developed for different applications such as reaction monitoring in lab, pilot plant and for full automated process control:

• Fiber Optic ATR-Probes (Shaft-in-shaft design)

• Fiber Optic ATR-Probes for Harsh Environment

• Fiber Optic ATR-Probes for Lab Application

• Fiber Optic ATR-Probes

• Fiber Optic ATR Loop Probe

More product details and classification can be found on our web store.

3.2 ATR Crystal Choices

Commonly used ATR crystals are diamond, ZnSe, Si, Ge and ZrO2 with their own advantages and physical and optical properties. An overview of different ATR crystals and their properties are summarized in Table 2.

Table 2 ATR tips made by different crystals and their properties

3.3 Mid-IR Optical Fibers

There are two main types of IR-Fibers: Chalcogenide IR-fibers in the spectra range of 1.1-6.5 um and Polycrystalline IR-fibers in the spectra range of 3-18 um.

• Chalcogenide IR-Fibers

Chalcogenide As-S glass fiber transmits IR-radiation in the spectral range of 1.1 – 6.5μm. High performance CIR core/clad fiber are drawn with core diameters spanning from 8µm to 500µm. Advanced drawing process with double polymer jacket provides a superior mechanical strength and high flexibility of CIR- fibers. Low optical losses and small absorption peaks over the mentioned spectral range ensure a successful use of CIR-fiber for a wide range of applications.

Table 3 CIR Fibers Specification

Transmission range	9,000-1,550 cm-1
Core/clad material	As2S3/As-S
Core/Clad/jacket diameter (µm)	500/550/690
Protective jacket	Double polymer
Core Refractive Index	2.4
Numerical Aperture	0.3
Operating temperature (°C)	-200 < T < 90
Minimal bending radius (mm)	120

Figure 8 CIR Fiber cable structure and optical transmission

• Polycrystalline IR-fibers

Core / Clad Polycrystalline Infra-Red (PIR-) fibers transmits over a broad spectral range 3 – 18 μm. Highest performance PIR core/clad fiber are extruded with core diameters span from 240µm to 860µm. Continuously improved extrusion process provides a superior optical quality and mechanical strength of PIR- fibers. Low optical losses without absorption peaks over the mentioned spectral range ensure a successful use of PIR- fiber for a broad range of applications.

Table 4 PIR Fiber specification

Transmission Range	3100 -6000 cm-1
Core/Clap Material	AgClBr
Core/Clap Diameter (um)	900/1000
Protective Jacket	No
Core Reflective Index	2.15
Numerical Aperture	0.5
Operating Temperature (c)	-270 < T < 140
Minimum Bend Radius (mm)	150

Figure 9 PIR fiber cable structure and optical transmission

3.4 Fiber Probe Coupler

The fiber probe couplers enable coupling of fiber optics with FTIR spectrometers when installed in their sample chambers. This eliminates the need to prepare samples and makes remote analysis easy for molecular reaction monitoring in-line. This expands in-line spectroscopy capability for benchtop FTIR spectrometer, and the high-resolution benchtop spectrometer also improves the performance of the fiber optics.

Figure 10 ATR probe coupling with FTIR spectrometer

A variety of models were designed for different FTIR spectrometers and hardware:

Fiber probe coupler FPC-2M enables efficient coupling of fiber probes with iS5 – the smallest FTIR spectrometer from Thermo. FPC-2M design is based on two off-axe parabolic mirrors inside the standard accessories iD1 which can be adjusted for maximum signal from SMA-terminated probe.

Fiber probe coupler FPC-6M provides the highest efficiency coupling of any fiber probe with bench FTIR-spectrometer when it´s installed in its sample chamber. Thus FPC-6M enables inline reaction monitoring in lab, while it can be also modified for process- control with industrial FTIR and robust probes with SMA or other customized connectors
Fiber probe coupler UNIVERSAL are designed to be compatible with any FTIR spectrometer by using a customized baseplate.
Fiber probe couplers for IR detectors in LN‐cooled Dewars were designed for the coupling of IR-fiber optical cables with SMA termination to an MCT / InSb detector installed inside an LN-cooled Dewar with a side window. With the MCT detector the fiber optic probes can achieve better sensitivity.
Refocusing and Collimating Fiber Lens Objectives were designed to provide maximum coupling efficiency between the output from Mid-IR lasers and chalcogenide and polycrystalline optical fibers. The aspheric design of focusing objectives provides a focal spot smaller than the fiber core diameter, allowing for precision alignment.

More product details and classification can be found on our web store.

4. Benefits and Applications of Fiber Optics and ATR Probes

4.1 Benefits of Fiber Optic Probes in Spectroscopy

Fiber optics are not only revolutionizing the way optical instruments are designed but are also opening many new opportunities for novel measurement techniques and analytical applications. Some key advantages are as follows:

No light-tight sample chamber is required - improving operator speed and efficiency.
A wide range of standard or bespoke sampling accessories are easily fitted – enabling large, awkward, or previously impossible samples to be measured.
The light source can be located outside the main optics bench – reducing stray light and temperature effects and so improving linearity and reducing drift.
The spectrophotometer is easily adapted for other measurements like fluorescence, turbidity, radiometry, reflectance, etc.
Maintenance, calibration and service are all significantly reduced and simplified.

4.2 Typical Applications of ATR Probes in Spectroscopy

ATR probes in spectroscopy are mainly used for in-line spectroscopy. Typical applications include, but not limited to:

Reaction monitoring: In chemical reaction monitoring, Mid Infrared (MIR) spectroscopy has advantages over near infrared (NIR) analysis as it can look more easily at functional groups for liquid processes and provide simultaneous monitoring of many chemical species.
Process control for refinery production, fuels, bio-fuels, solvent, specialty chemicals, food products, and pharmaceutical products.
Cancer diagnostic by differentiation of normal and diseased tissue in organs, detection of early stages of malignancy, and monitoring abnormal cell growth and proliferation in tissue sections.
In-situ real time chemical determination in chemical & petrochemical.
Quality control and moisture analysis in food industry.

Introduction of FTIR, ATR, Fiber Optics, and ATR Probes in Spectroscopy and Associated Designs - Part II

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