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FTIR Spectrometer GAS analysis Module | SIMTRUM Photonics Store

Products / Light Analysis /Spectrometers /FT Infrared Spectrometer(900-16000nm) /FTIR Spectrometer GAS analysis Module /
FTIR Spectrometer GAS analysis Module
Overview Specifications Working Principle Q&A Related Products Order Now!
SIMRUM'S GASEX OEM STANDARD is the optimal solution for system integrators who need to measure complex gas mixtures. The complete GASEX OEM represents a highly efficient, rugged and fully integrated solution for gas spectroscopy in our product portfolio.
 
Our OEM010 module is coupled to a low volume (0.2L) heated (up to 200°C) gas cell, in which light experiences multiple reflections, resulting in an integrated optical path of 5m with more than 50% transmission (or 20cm in the short cell configuration). The cell's internal optics is rhodium and gold coated, making it extremely resistant against most chemicals including acid gases such as HF, HCl, HBr, and many other gases like CO,CO2, NO, NO2, NH3, SO2,NOx, CH4, VOC's,...
 
The GASEX OEM has been standardized to operate with a resolution of 0.5cm-1, which makes it compliant with the newest legal regulations for emission measurements. Featuring a USB 3.0 connection, the unit can also host an embedded single-board computer with Ethernet connection for standalone operation. Typical limit of detection (LoD) are below 1 ppm for most of the gases - with 20s integration time.
 

 

Features

  • 5m heated gas cell with small volume (0.2L)
  • Optimized Gas cell and FTIR Assembly
  • USB connectivity and SDK available
  • Resolution of 2 cm-1 down to 0.5 cm-1 Resolution of up to 0.5cm-1
  • MCT Detector with high detectivity TE-cooled MCT 

GASEX OEM with 5m long cell
 
GASEX OEM with 20cm long cell

Principle

The Gasex system consist of an interferometer module including also the MCT detector and an IR light source and a Multi-reflection gas cell. The light is produced by a black body radiation module and modulated by a dual corner cube interferometer.The light is then coupled into the Gas cell (where the gas to be analysed is injected) where it is relfected back and forth 32 times. Finally the light focused on the MCT detector.

 
Dimensions

FTIR
Product code GASEX-OEM-060-4TE GASEX-OEM-085-4TE GASEX-OEM-120-4TE
Beamsplitter material CaF2 ZnSe
Spectral range [cm⁻¹] 5'000 - 1'660 6'600 - 1'200 5'000 - 830
Spectral range [μm] 2.0 - 6.0 1.5 - 8.5 2.0 - 12.0
D* [cm Hz¹/²W⁻¹] >1x10E¹¹ >8x10E⁹ >4x10E⁹
Signal-to-noise ratio
(0.5cm⁻¹, 1 min scan)		 >55'000:1 >35'000:1 >35'000:1
Detector type MCT, 4 stages thermoelectrically cooled
Interferometer design Permanently aligned with dual retro-reflector
Resolution [cm⁻¹] 8, 4, 2, 1 or 0.5 cm⁻¹
Wavenumber repeatability <10ppm
Scan frequency >1.5 Hz @0.5cm⁻¹
Internal reference laser Temperature-stabilized solid-state laser @850nm
Absotrans(TM) Active suppression of Water and Carbon dioxide inside the interferometer housing
A/D converter 24 bit
Operating temperature 10°C - 40°C
Humidity 90 % (Not condensing)
Communication interface USB 2.0
Built-in light source SiC globar stabilized at 1550 K
Power requirement (FT-IR only) <35W @12VDC
Software interface Windows 7/10/11, Labview, net compatible dll,
API for controlling the instrument via our DLL
GAS CELL
Design Multi - pass, permanently aligned, directly mated to the FTIR
Path length [m] 5 or 0.2 (short cell)
Internal volume [Litres] 0.2
Zero gas loss > 3 dB (less than 50%)
Operation temperature [°C] -20 to 200 (not condensing)
Construction Aluminium with inert coating
Parabolic mirrors Aluminium body with rhodium protected gold coating
Heaters for stabilization at 180°C Electric 400W peak power, consumption in steady state: around 30W
Thermostat PID (integrated or customizable)
Operation voltage of heaters 220V, (customizable for other voltages)
Gas connectors 6 mm Swagelock (customizable)
SYSTEM (GAS CELL + FTIR)
Dimensions [mm] 380 x 180 x 120
Total weight [kg] 3.9

 

Detector choice recommendation

  • The 4-TEC 2-12 µm MCT detector can be considered the best choice for a general  application. It is covering a broad spectral range while proposing a high detectivity.
  • The 4-TEC 2-6 µm MCT detector is offered for specific applications where a narrower  spectral range is not a handicap and a higher detectivity is required (for instance fast response of a low concentration of a chemical species).

Measurement example

Below some examples of Spectra Measured with the Gasex system 0.5 cm-1 resolution:



FT-IR spectroscopy: high resolution measurements

Introduction

Fourier Transform infrared (FT-IR) spectrometers have proven to be efficient and reliable tools for a large variety of applications targeting the near infrared (NIR) and mid-infrared (MIR) regions of the electromagnetic spectrum. One of the most critical specification of FT-IR spectrometers is their spectral resolution , as this defines the scale of the features to be distinguished e.g. in the absorbance spectrum of a gas. It is thus quite natural to wish for the highest achievable resolution when purchasing an FT-IR spectrometer. Due to their operational mode, achieving high resolution measurements using FT-IR spectrometers is however not entirely straightforward. This technical note explains the main limits of high resolution FT-IR measurements.

 

Fig .1 Simplified FT-IR setup

 

A simplified FT-IR is depicted in Fig.1. It consists of a beamsplitter, a fixed mirror and a movable mirror. Light from a purely monochromatic light source is split in two beams, assumedly in equal parts (50% beamsplitter). Each beam bounces back on either mirror (fixed or movable) before being recombined and focused onto the detector. When the movable mirror is at the same distance from the beamsplitter as the fixed mirror, both beams travel the same distance and recombine in-phase, yielding constructive interference. By displacing the moving mirror by a distance ℓ, one introduces an optical path difference (OPD) between the two beams Δ given by:Δ=2ℓ.

Upon recombination at the detector, the optical intensity as a function of OPD is given by: I(0)=0.5I0[1+cos(2πν0∆)]=IDC+IAC(∆).

Where I0 is the source intensity at wavenumber ν0 (the wavenumber is the reciprocal of the wavelength). The AC part of the interference record is labelled IAC(Δ) and is called the interferogram. For a purely monochromatic source (as considered here), the interferogram is a pure cosine function.

 

Fig 2. Interferogram of a purely monochromatic light source

 

Here the wavenumber (or equivalently the wavelength) of the source as well as its intensity can be retrieved from a direct observation of the interferogram (amplitude and period of the cosine function). For a broadband source, the interferogram IAC(Δ) and spectrum I0(ν) of the source are related via a Fourier transform operation:

I0(v)=∫∆maxIAC(∆)cos(2πν0∆)d∆

Obviously, the OPD cannot be made arbitrarily large and has to reach a value Δmax that is defined by technological design. Given the nature of the relationship between the interferogram and the spectrum (Fourier transform), it turns out that Δmax also defines the achievable spectral resolution. To a first approximation, the spectral resolution Δν of an FT-IR is given by:

∆ν=(∆max)-1

and is often expressed in cm-1. So why not simply increase the maximum OPD to enhance the spatial resolution of an instrument ? While this is true, special care has to be considered when operating at high resolution. As explained hereafter, the mirror maximum displacement is limited by the divergence of the system and the dimensions of the detector.

 

 

HR measurement: effect of beam divergence

We consider the exact same setup as in Fig.1. The beam divergence is accounted for by observing the behavior of the so called "extreme ray", which makes an angle α with respect to the "central ray" discussed previously.

 

 

Fig 3. FT-IR setup with a divergent source

 

 

The two extreme rays (reflected from either the fixed or the moving mirror) hit the lens with an angle α, unlike the central ray which hits the lens at normal incidence. They are thus focused on another point on the detector. Moreover, their OPD is shorter than for the central ray:

∆ext=2ℓcos(α)

The larger the angle α, the greater the difference with the central ray OPD Δcen=2ℓ. Consider now the case where the difference in OPD between the central ray and the extreme ray is equal to one half of the source wavelength, that is:

∆cen-∆ext=2ℓ[1-cos(α)]=λ/2

In this scenario, when the central rays are in phase, then the extreme rays are out of phase (and vice versa). Consequently, the intensity over the detector surface follows the profile shown in Fig. 4.

 

Fig 4. Intensity over the detector surface due to a highly diverging beam

 

 

Since the detector yields a single value that corresponds to the average intensity received on its surface, the signal detected in this case corresponds to the average optical intensity only, and all information regarding the interference signal vanishes. Practically speaking, the interferogram will start losing contrast as the moving mirror is scanned as shown in Fig. 5.

 

 

Fig 5. Loss in interferogram contrast due to a diverging beam

 

 

This effect is naturally existing in all interferometers based instruments (such as FT-IR) and cannot be avoided. It can however be properly managed by appropriately trading-off the parameters involved in equation (6), namely :

  • Δν: the effect is more pronounced for high resolution measurements due to the larger OPD required.
  • α: the effect is more pronounced for a highly diverging beam.
  • λ: the effect is more pronounced at short wavelengths (large wavenumbers).

 

For most applications in solids and liquids, the size of the observable features is typically broader than 2cm-1, and high resolution (HR) measurements performed at 0.5cm-1 are usually not required. In addition, these would prove challenging due to the added contrast loss described in this document. HR measurements might still be a viable option for specific applications, such as e.g. laser characterization, where the highly collimated laser beam prevents the dramatic loss of the interferogram contrast.


1.What resolution should I use for my application?

In FTIR, resolution is traded-off with two other experimental metrics that are acquisition time and signal-to-noise ratio (SNR). Increasing resolution, meaning reducing the resolution parameter number, will result in longer acquisition time, and poorer SNR. In general, it is recommended to work at the "worst" possible resolution, that is the limit resolution that allows to distinguish the features of the sample or substance that you are characterizing. Liquids and solids present broader features than gases or gas mixtures and can generally be probed with standard resolution instruments (down to 2cm-1). Gas analysis or light sources characterization (typically lasers) usually benefit from a sharper resolution of 0.5cm-1.

 

2.How is the equivalent wavelength resolution calculated?

Due to its working principle, FT-IR provides uniformly sampled spectra in the form of wavenumber (ν) within a given spectral range, with the unit of cm-1. Wavenumber is simply defined as the reciprocal of wavelength (λ). The resolution of a FT-IR is a constant wavenumber (Δν), but varies with the wavelength (Δ λ) due to the inverse relationship between these two units. The conversion between wavenumbers resolution and wavelengths resolution is Δλ=λ2 · Δν, as shown in the following figure:


 

3.What is the acquisition rate of this spectrometer?

The acquisition rate varies with the resolution. At the standard resolution of (4cm-1), the scanning rate is ~5Hz, and at the high resolution of (0.5cm-1), the scanning rate is ~1Hz, that is, the scanning rate is inversely proportional to the resolution.

 

 

4.What are the differences between DLADTGS detectors and MCT detectors?

DLADTGS is based on the pyroelectric effect. When exposed to infrared radiation, its temperature will change, causing the polarization inside the crystal to change. MCT is a bandgap type photoconductive detector. DLADTGS has a wider spectral response range (up to 18-20 μm), and MCT detectors utilize thermoelectric cooling to achieve higher sensitivity, which can suppress dark current and improve their signal-to-noise ratio.

 

 

5.How much better is the performance of LN4 cooled detectors than that of MCT cooled detectors?

The SNR of the LN4 cooling system is approximately ten times that of the MCT cooling system.

 

 

6.Why is the performance of the Fourier transform spectrometer superior to that of the dispersive grating spectrometer?

The performance of a grating spectrometer mainly depends on the slit and detector. Due to the different dark noise of each pixel point, additional noise will be introduced during measurement. FTIR spectrometers are generally superior to dispersive grating spectrometers due to their higher light throughput, better signal-to-noise ratio, higher spectral resolution and wider wavelength range. These advantages make FTIR spectrometers the preferred choice in many applications, especially in places where high sensitivity and accuracy are required.

 

 

7.Can an FTIR spectrometer be used to measure pulsed laser?

Sure, but the power of the laser must be limited to avoid irreversible damage to the detector. The average power shall not exceed 25mW, and the peak power of pulses shorter than 1µs shall not exceed 100W. It is recommended to use a set of fixed or variable attenuators to adjust the optical power to avoid detector saturation. Secondly, the repetition frequency of the laser must exceed 25khz to avoid numerical artifacts (aliasing) in the measured spectrum.

 

 

8.Can the concentration of the substance being measured be obtained directly?

No. A spectrometer can only provide the measured spectrum. Users must obtain the concentration of substances in the sample through specific algorithms and calibration data.

 

 

9.Can this spectrometer be used for mineral identification?

The FT-NIR spectrometer is highly suitable for mineral identification. FT-NIR is capable of generating high-quality and high-resolution (better than 1nm) reflection spectra within the spectral range of 900-2550nm (SWIR) within a few seconds.


Related products

Model Wavelength Range Resolution Detector  
  FT-NIR spectrometer 900-2500 nm 2/4/8 cm-1(user selectable) Extended type InGaAs PIN photodiode, 2TE cooled
  FT-MIR spectrometer 200-16000 nm 0.5/2/4/8cm-1(user selectable) MCT (4-TE cooled)
MCT (LN2 cooled)
DLATGS
  Fiber coupled FT-IR spectrometer 200-16000 nm 2/4/ 8cm-1 (user selectable) InGaAs (2-TE cooled)
MCT (4-TE cooled)
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  VIS-NIR-FIB spectrometer 350-2500 nm <1.5 nm silicon array detector (3648 pixels) 16-bit ADC.extended range InGaAs photodiode, 2TE cooled, 24-bit ADC.
  VIS-NIR-DR spectrometer 360-2500 nm 5 nm Extended range InGaAs detector

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Compare Model Drawings & Specs Availability Reference Price
(USD)
LA-AR01-GAS-060-4TE
Spectral Range ：2-6 μm Detector Peak D* ：>1x10¹¹ cm Hz¹/²W⁻¹ Signal - to - noise ratio ：>55'000:1 Resolution ：8, 4, 2, 1 or 0.5 cm⁻¹ 		4-6 Weeks $47192.00
LA-AR01-GAS-085-4TE
Spectral Range ：1.5 - 8.5 μm Detector Peak D* ：>8x10⁹cm Hz¹/²W⁻¹ Signal - to - noise ratio ：>35'000:1 Resolution ：8, 4, 2, 1 or 0.5 cm⁻¹ 		4-6 Weeks $47192.00
LA-AR01-GAS-120-4TE
Spectral Range ：2-12 μm Detector Peak D* ：>4x10⁹ cm Hz¹/²W⁻¹ Signal - to - noise ratio ：>35'000:1 Resolution ：8, 4, 2, 1 or 0.5 cm⁻¹ 		4-6 Weeks $47192.00

LA-AR01-GAS-120-4TE - Parameter

LA-AR01-GAS-085-4TE - Parameter

LA-AR01-GAS-060-4TE - Parameter

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