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A White Paper from FOSS NIR Spectrometer Technology Comparison This whitepaper defines NIR spectrometer technology specifications, the impact of these specifications on measurement performance, the different spectrometer technologies and the pros and cons of each technology. By: Hans Villemoes Andersen, Håkan Wedelsbäck, Per Waaben Hansen P/N 1026672, Issue 1, November 2013 Dedicated Analytical Solutions Table of Contents Introduction 2 Definition of specifications 3 Impact from the specifications 6 Description the different technologies 8 Pros and cons for the different technologies 11 Conclusion 12 References 14 Introduction A number of different technologies are available for building NIR spectrometers. In this document we compare FT-NIR Interferometers, Scanning Grating Monochromators and Fixed Grating Detector Diode Array technologies. FT-IR Interferometer is at present the preferred technology for IR (λ > 2500nm). Grating is the preferred technology for UV/VIS (λ < 780nm). In-between, in the NIR region (780nm < λ < 2500nm), the advantages of both technologies overlap and they can, with a proper design, be used for designing excellent analytical instruments. The preferred spectrometer technology is always application dependent. Different applications have different requirements to be matched with the advantages and disadvantages of each technology. In this document we describe the definitions of commonly used specifications, their impact on measurement performance, the different spectrometer technologies and the pros and cons of each technology. Page 2 Definition of specifications Wavelength and Wavenumber The light is characterized by a wavelength. It is defined as the distance the light passes in vacuum during one period. The most common unit for wavelength is nm (nano meter) corresponding to 10-9 m. In the visible range from 400 to 780 nm, the wavelength is observed as the color of the light. The light is characterized by a frequency. The relationship between wavelength and frequency is given by: c= λ ∙ v Where c is the speed of light in vacuum, λ is the wavelength and v is the frequency of the light. In spectroscopy, it is common to use the units of cm-1 for frequency especially when using FTIR. The unit cm-1 is called wave number or inverse centimeter, and represents the number of wavelengths that it takes for a length of 1 cm. Since the units used for the abscissa are reciprocal of each other, praxis of plotting spectra with cm-1 abscissa with high values to the left has been used. The conversion from λ in nm to v in cm-1 and reverse is done by: v = 107 / λ and λ = 107 / v When transforming resolution between wavelength and wavenumber, this equation must be used with care. See under the resolution section. Wavelength range The wavelength range for NIR is often defined as the range from 780 – 2500 nm (12820 – 4000 cm-1) (ref 9). The 2500 nm value is well accepted, but for the short wavelengths many different values in the range from 600 to 800 nm are used. (ref 1). The spectral region with shorter wavelengths (400 – 780 nm) is called the visible range. The spectral region with longer wavelengths (2500 – 25000 nm) is called the mid infrared region. Wavelength accuracy and precision Wavelength accuracy is how well the wavelength axis of an instrument matches a reference wavelength axis. Wavelength precision is how well an instrument repeats its own wavelength axis. The wavelength accuracy and precision is measured in the same units as the wavelength axis e.g. nm. The same applies for a wavenumber scale where accuracy and precision is measured in cm-1. Resolution The resolution Δλ or Δv of a spectrometer determines the spectral features that can be separated. Resolution is also called bandwidth, band pass or instrument line shape ( ILS ). The resolution of a spectrometer is normally defined as the full width half height (FWHH or FWHM) defined by the optical system including a standard processing. The resolution may also be defined by the Rayleigh criterion which may represent a little different number. It is important to notice that the resolution is independent of the distance between the sample points. About 4 sample points are needed to represent the signal over the resolution. Higher sample density is mostly of cosmetic value. Page 3 The resolution is measured in the same units as the abscissa. Alternatively a dimensionless relative resolution R can be defined. R(λ) = Δλ / λ and R(v) = Δv / v The resolution behaves differently depending on whether it is Δλ or Δv. If Δλ is constant as a function of λ then Δv will change proportionally to v2 as function of v. Resolution of a Monochromator on different scales. Constant on a wavelength scale Resolution of a FTNIR on different scales. Constant on a wavenumber scale To convert a resolution from an abscissa of wavelength to the two wavelengths defining the bandwidth, formula 1 can be used and then subtracted to give the resolution on the new abscissa. If the relative resolution is small R << 1 then the value of R will be the same on both abscissas and R can be used to convert the resolution. Note that in all cases the transformation is only valid for one wavelength. Noise Noise is variation in the signal response for repeated measurements on the same stable sample. In a broad interpretation, noise limits the precision of most measurements. Signal to Noise Ratio (SNR) is specified as the signal at 100% transmission relative to the noise calculated as the standard deviation of the same signal for a number of measurements. A useful measure for signal to noise is absorbance units (Abs), as it relates directly the absorbencies measured in the spectrum. It must however be also be stated at which resolution, measurement time and wavelength range the signal to noise is calculated, as these factors affect the number. The number may be normalized with respect to time using the unit Abs sqrt(s). This is done by multiplicating the noise measured in absorbance units with the square root of the measurement time. Measurement time The measurement time is composed of an integration time where signal from the sample is measured, and a handling time which is the time used for handling sample and results. By changing the integration time e.g. the number of scan averages, the noise in the signal can be altered. If the noise is stationary, the noise is reduced proportionally to the square root of the integration time. Often, the number of averaged scans is used as the term for integration time, but it needs the additional information of time Page 4 pr. scan to be comparable between instruments. Many fast scans and few slow scans represent the same integration time and, as such, the same noise, so the number of scans only makes sense in a predefined context. Drift Slow changes of the instrument response are called drift. It may be interpreted as a low frequency noise. It can be measured in absorbance in a unit time ‘Abs/s’. In many cases, drift limits the signal to noise that can be obtained by increasing integration time. This is because the error caused by the drift increases while the noise is reduced as a function time. At some point, the drift exceeds the noise and signal to noise ratio starts to decline again. Stray light Stray light is when unwanted light reaches the detector and distorts the spectrum. Two different types of stray light are often observed in a system. The first type is related to the sample handling where some part of the light reaches the detector without interacting with the sample e.g. in reflection mode where light reflected from the sample cup reaches the detector and limits the dynamic range in the spectrum. In transmission the same is seen when light bypasses the sample. The second type of stray light is when wavelengths outside the selected bandwidth are included in the light measured by the spectrometer. Dynamic range and gain The difference between the highest and the lowest possible absorbance in a spectrum is called the spectral dynamic range. It can be measured as a ratio of the highest and lowest signal or in absorbance units as log10 to the ratio. The spectral dynamic range may be influenced by the stray light. In FT-NIR instruments the spectral dynamic range is sometime specified as 'photometric accuracy' and presented as transmission percentage (x %T) (ref. 11). The instrument may have variable gain which offsets the absorbance scale relative to zero absorbance. The ratio between the noise level in the most sensitive setting and the maximum tolerable signal is called the instrument dynamic range. It is measured as a ratio or in absorbance units. Instrument dynamic range is larger than the spectral dynamic range. The instrument dynamic range is also called photometric range. Photometric linearity The linearity of the photometric axis is the deviation from linearity of log10 of the signal value against log10 to the true signal attenuation. The value is calculated as the root mean square error at one wavelength. The photometric linearity is affected by stray light and limited dynamic range. In FT-NIR instruments the 'photometric accuracy' is sometimes specified as a transmission percentage (x %T) (ref. 11). This number is equivalent to spectral dynamic range as described in the previous section when it is converted to absorbance units. It assumes that deviation from linearity is caused by an addition of a constant error signal in the singlebeam spectrum. Page 5 Impact from the specifications Wavelength and Wavenumber Wavelength and wavenumber scale can equally well be applied. Traditionally wavelength scale has been mostly used in NIR, but the use of wavenumber is also common. The choice is often made from the spectrometer technology, where grating based instruments operate with wavelength scale and FT based systems operate with wavenumber scale. Wavelength range There is always a tradeoff to be made between wavelength range, resolution, measurement time and signal to noise ratio. Harmonics of the absorbance bands repeat at shorter and shorter wavelengths along with weaker signal and deeper penetration depth for each harmonic. The wavelength range must cover the absorption bands needed for the application. For specific applications a limited wavelength range covering a selected range of absorption bands with improved signal to noise is often advantageous. As an example, when measuring inhomogeneous samples like grain and meat in transmission mode, it is advantageous to use a wavelength region with deep penetration depth to reduce the sampling error. Instruments for more general use should cover the full NIR wavelength range. Wavelength accuracy Wavelength accuracy is important for model transferability and accuracy. Wavelength shifts typically introduce bias and obstruct transferability. Corrections with internal wavelength standards or stabilization of prediction models for wavelength scale errors are two well proven methods to reduce the impact of wavelength drift. (ref 7, ref 8) Wavelength accuracy is important for identification of narrow absorption bands. Resolution In most situations when performing quantitative analysis the resolution is of less importance than the other tradeoffs. It has been shown by K. H. Norris (ref 4) and K. H. Esbensen et al. (ref. 10) that it is advantageous to improve SNR by broadening the resolution. P. R. Armstrong et al (ref 3) concluded that a 10nm grating instrument and a 8cm-1 FT instrument have comparable performance for quantitative analysis of protein, amylose, ash, hardness index and moisture in wheat. O. Kolomiets and H. W. Siesler (ref 6) concluded that, for quantitative analysis of a narrow band drug active ingredient, high resolution measurement does not improve the measurement accuracy. However they claim that the high resolution can improve the wavelength calibration accuracy when the calibration is preformed with a narrow band wavelength standard. Signal to noise ratio Repeatability is strongly affected by signal to noise ratio. The noise as a function of wavelength is different for FTNIR and grating instruments and it affects the relative weight of absorption bands when prediction models are created. The noise consists of random and repetitive components. The random components can be averaged out by co-adding scans. Detector and preamplifier noise are typically Page 6 random. Repetetive noise as readout and digitizing noise cannot be reduced by averaging scans. Measurement time There is often a trade-off between measurement time and precision. With a shorter measurement time for each scan, more scans can be averaged during a fixed sample analysis time. SNR will improve with the square root of number of scans until the point where drift will start to dominate over random noise. Drift The photometric stability is important for instrument performance. The usual cause of drift is detector and/or lamp temperature shift. Also, changes in water vapor may cause changes in spectrum. It can be corrected for by measuring a photometric reference standard at a rate faster than the drift and then using the information for correction. Typically this correction is done for every sample measured. Stray light The type stray light that is related to the sample handling unit limits the maximum absorbance reading to the level caused by the stray light alone. If, for instance, 1% of the sample illumination intensity is reaching the detector without interaction with the sample, the maximum absorbance level that can be measured is 2AU. When approaching this absorbance level the photometric linearity is affected and saturates. Spectral stray light will also limit the measurement performance. The impact from this source of error is very sample and application dependant. In general, stray light is less severe if it occurs at wavelengths close to the selected wavelength. Dynamic range The requirements for instrument dynamic range are much larger for transmission measurements ( 4 - 6 AU ) than for reflectance measurements ( 2 - 3 AU ). To meet the requirements, programmable gain detector amplifiers and/or optical attenuators for the reference measurement are used. Photometric linearity In NIR spectroscopy, the useful NIR raw signal is a weak modulation superimposed on a strong background. Good photometric linearity is required to make the measurement independent of the background intensity. The demand for photometric linearity is higher for FT compared to grating instruments because the signal to background ratio is more disadvantageous and nonlinearities cause substantial distortion when the Fourier transformation is performed. Page 7 Description of the different technologies Scanning grating The key component for this design is the diffraction grating. A grating is an optical component with a periodic structure, which splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the periodic structure and the wavelength of the light. This relationship between the spacing and the angles of the incident and diffracted beams of light is known as the grating equation. mλ = d (sinα + sinβ) m:diffraction order λ: wavelength α: entrance angle vs. grating normal β: exit angle vs. grating normal d: grating period Light from source enters the monochromator through the entrance slit and is dispersed by the grating and focused at the exit slit plane. A limited wavelength region of the spectrum exits from the monochromator through the exit slit and illuminates the sample to be analyzed. A broad band detector detects the light transmitted through (or reflected by) the sample. Wavelength selection is performed by rotating the grating by means of a motor. The angle of the grating is continuously measured and converted to represent the center wavelength of the wavelength region exiting through the slit. The wavelength range is determined by the angular range of grating rotation and the dispersion of the grating. The resolution is determined by the dispersion of the grating and the width of entrance and exit slits. Page 8 Fixed grating DDA Light from source illuminates the sample to be analyzed. Light transmitted through (or reflected by) the sample enter the spectrometer through the entrance slit and is dispersed by the grating and focused at the detector array where the detection is performed. Wavelength selection is performed by reading different pixels of the detector array. The wavelength range is determined by the width of the detector array and the dispersion of the grating. The optical resolution is determined by the dispersion of the grating and the width of entrance slit. The detector must have a number of pixels required by wavelength range and resolution. FT-NIR Different manifestations exist, but the characteristics are common. A FT-NIR spectrometer is based on the principal of the Michelson interferometer. This is illustrated on the figure below. A parallel light bundle is passed through the interferometer and is split in two bundles by a beamsplitter. They are reflected back from two mirrors onto the beamsplitter and recombine. When the distances are equal they add constructively, but if the path length is different an interference pattern results. This interference pattern changes in intensity as one of the mirrors is moved. The interference pattern as function of the mirror movement is called the interferogram. The interferogram must be sampled at equal spaced path differences. Information on the path differences is obtained by using a laser as a second source in the interferometer. Deviation below 1 nm affects performance. This is a reason for the inherited sensitivity to vibrations and sound. See the next figure. The interferogram of the laser is a sine wave where one period corresponds to a path difference of one wavelength of the laser light or a physical movement of the mirror of one half laser wavelength. Page 9 The NIR interferogram is sampled at equal path differences determined from the laser interferogram and Fourier transformed to a spectrum. This raw spectrum is called the single beam spectrum. See figure below. Because the units on the abscissa for the interferogram is length, the units on the abscissa for the spectrum is length-1. This is proportional to wave number and this is the reason that cm-1 is used as the abscissa in FT-NIR systems. Interferogram Spectrum The spectral resolution gets narrower when the travel of the mirror is increased. This limits the light passed through the interferometer. This is equivalent to the slit width in a monochromator. The spectral resolution is constant on a wave number scale which implies that the relative resolution decreases with increasing wavelengths. The noise on the single beam spectrum is the same all over the spectrum. By placing a sample in the path of the light a measurement of the sample spectrum can be done. Both transmission and reflection measurements are possible. All the light of the interferometer hits the sample all the time. Page 10 Pros and cons for the different technologies Scanning grating + The best signal/noise ratio. + The best wavelength range, particularly in the visible region. + In pre-dispersive configuration the sample heating is very low. - A precise internal wavelength standard is required to achieve good wavelength precision. - For instruments with a broad wavelength range, grating anomalies create wavelength regions with rapid intensity variation. - Order sorting filters needed for instruments with a broad wavelength range. (> 1 octave) Fix grating DDA + Robust and vibration tolerant design. + Simultaneous measurement of the full spectrum making it tolerant to sample movement. - A tradeoff between wavelength range, resolution and signal to noise ratio is needed. Commonly, the wavelength range needs to be restricted to be able to meet the requirements for signal to noise ratio. - Independent detectors with individual offset and drift properties that need normalization. - Deteriorated signal / noise ratio around the upper limit of the wavelength range for each detector technology. For example, around 1050 nm with a Si detector. Or 1600-1650 nm of InGaAs. - Significant sample heating due to broad band sample illumination. - Order sorting filters needed for instruments with a broad wavelength range. (> 1 octave) FT-NIR + The best wavelength axis precision, making spectra easily transferable. + Can have high spectral resolution. High resolution is useful for identification of substances but not essential for quantitative measurements. + Resolution can be changed. - Lower signal/noise ratio than a monochromator, particularly at short wavelengths. This can be improved by reducing the resolution. - Limited wavelength range in the visible region (below 850 nm). - The most vibration sensitive technology. - Significant sample heating due to broad band sample illumination. Page 11 Conclusion FT-NIR Interferometers, Scanning Grating Monochromators and Fixed Grating Detector Diode Array technologies -the three spectrometer technologies in this comparison all have their individual advantages and disadvantages. Major distinctions between spectrometer technologies + - Scanning Grating · Signal to Noise Ratio · Wavelength range · Measures needed to improve wavelength accuracy · Lower resolution Fix Grating DDA · Robustness · Signal to Noise Ratio · Sample heating FT-NIR · Wavelength accuracy · Resolution · Vibration Sensitive · Sample heating Preferred use for the different technologies Preferred technology for: Scanning Grating · Quantitative measurements for food and agricultural products. Fix Grating DDA · Process instrumentation FT-NIR · Qualitative measurements in the laboratory (Identification) · Possibly quantitative for narrow bandwidth absorbers When choosing a spectrometer technology, the required application offers a good guide. Different applications have different requirements for the spectrometer and the preferred spectrometer technology is always application dependent. Quantitative measurements for food and agricultural products For routine analysis applications in the food and agricultural industries a scanning grating spectrometer is a proven choice. This includes benchtop compositional analysis of cereal grain, feed, milk powder, ground meat or similar as well as adulteration screening. This type of spectrometer is ideal for quantative measurements across a broad spectrum of applications with reliable, repeatable accuracy. The broad wavelength range enables the use of this technology for a very wide range of applications including those such as colour in fish food or similar requiring the visible region. When using NIR transmission for measuring inhomogeneous grain or meat samples it is an advantage to use the Short Wave NIR range (850 – 1050nm) where the light penetration is good and the premium signal to noise ratio offered by a scanning grating monochromator is essential. A further advantage is that there is little sample heating when the sample is illuminated by a monochromator. On the minus side, wavelength accuracy is not given by the design as for FT-NIR. An accurate internal wavelength standard is required to achieve good wavelength accuracy. However, like all other aspects of NIR technology, scanning grating spectrometers are constantly evolving and many of the latest generation spectrometers are using this internal wavelength standard and can demonstrate good wavelength accuracy, ensuring that this option remains the optimal choice for many applications. Page 12 Process instrumentation For measurements close to production processes, for example in a feed mill where an analyser is used close to the processing line or is mounted directly in the process for continuous in-line measurements, fixed grating DDA is the best option. A fixed grating DDA spectrometer can be used to create robust and vibration-tolerant instruments ideal for use inside production plants. The simultaneous measurement of the full spectrum also makes it tolerant to sample movement. In other words, it is the preferred technology for in-line applications. Downsides need to be considered however. In particular, a tradeoff between wavelength range, resolution and signal to noise ratio must be made. Inevitably, the wavelength range must be restricted to achieve an acceptable signal to noise ratio. Qualitative measurements in the laboratory For spectroscopic assignments of pure chemicals where narrow instrument bandwidth is needed, FT-NIR has advantages. The high resolution makes it applicable for pin-pointing substances having narrow absorption bands. The wavelength axis accuracy makes it easy to transfer spectra and the resolution can be adjusted to obtain the best tradeoff between wavelength resolution and signal to noise ratio. Disadvantages include a lower signal/noise ratio than a monochromator, particularly at short wavelengths and the omitted visible wavelength range (below 850 nm). FT NIR is also a vibration-sensitive technology and the design of an instrument must take this into account for applications in a production environment. Page 13 References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) Handbook of Vibration Spectroscopy, Edited by Chalmers and Griffiths, 2002 Handbook of Near-Infrared analysis, Edited by Burns and Ciurczak, 1992 Comparison of Dispersive and Fourier-Transform NIR Instruments for Measur- ing Grain and Flour Attributes, P.R Armstrong et al, 2006 Interactions among instrument band pass, instrument noise, sample-absorber bandwidth and calibration error, K.H. Norris 1998 Diversity in NIR instrumentation, E. Stark 2004 The influence of spectral resolution on the quantitative near infrared spectro- scopic determination of an active ingredient in a solid drug formulation, O. Kolomiets and H. W. Siesler 2004 Wavelength standard, Patent US5936727, M.W. Trygstad 1999 Method of reducing the susceptibility to interference of a measuring instrument, Patent US4944589, K.G. Nordqvist 1990 ASTM E131 - 10 Standard Terminology Relating to Molecular Spectroscopy Resolving the resolution myth, K.H. Esbensen, P. Geladi and A. Larsen, NIR news vol 23 no 7 Specification of Components, Methods, and Parameters in Fourier Transform Spectroscopy by Michelson and Related Interferometers: IUPAC Recommenda- tions 1998, J.E. Bertie, Applied Spectroscopy, vol. 55(4), 2001. FOSS Foss Allé 1 DK-3400 Hilleroed Denmark Tel.: +45 7010 3370 Fax: +45 7010 3371 [email protected] www.foss.dk Page 14