Spectrometer Optics

A monochromator is required to separate the absorption line of interest from other spectral lines emitted from the HCL and from other elements in the atomizer that are also emitting their spectra. Because the radiation source produces such narrow lines, spectral interference is not common. Therefore the monochromator does not need high resolution. 

Most commercial AAS systems have the monochromator, optics, and detector designed for the measurement of one wavelength at a time they are single-element instruments. There are a few systems available that do perform multielement determinations simultaneously, using an Echelle spectrometer (discussed in Chapter 2) and a bank of HCLs all focused on the atomizer. The limitation to this approach is not the sources or the spectrometer or the detector, but the atomizer. The atomizer can only be at one set of conditions, and those conditions will not necessarily be optimum for all of the elements being measured. There will be a tradeoff in detection limits for some of the elements. 

The spectrometer system for AAS can be configured as a single-beam system, as shown in Fig. 6.8, as a double-beam system, shown in Fig. 6.14, or as a pseudo-double-beam system, which will not be discussed. (See the reference by Beaty and Kerber for a description of this system.) Note that in AAS the sample cell is placed in front of the monochromator, unlike UV/VIS spectrometers for molecular absorption or spectrophotometry, where the sample is placed after the monochromator. 

Commercial AAS instrumentation may be purchased with fixed shts or with variable slits. Fixed mechanical slit widths are available so that the resolution and sensitivity are acceptable for most analytical purposes at lower cost than instruments with variable sht widths. Variable slit widths are desirable for maximum flexibility, especially if samples are varied and complex. Instruments that have both flame and graphite furnace atomizers often have separate sets of slits of different heights for each atomizer. The furnace slits are usually shorter to avoid having emission from the small diameter incandescent furnace reach the detector. In general, the analyst should use the widest slit widths that minimize the stray light that reaches the detector while spectrally isolating a single resonance line for the analyte from the HCL. 

The common detector for AAS is the PMT. The construction and operation of a PMT has been described in Chapter 5. While PMTs are the most common detectors, solid-state single and multichannel detectors such as PDAs (discussed in Chapter 5) and CCDs (discussed in Chapter 7) are increasingly being used in AAS spectrometers. Many small systems, particularly those dedicated to one element such as a dedicated CVAAS mercury analyzer, use solid-state detectors instead of PMTs. Multielement simultaneous AAS systems also use multichannel solid-state detectors to measure more than one wavelength at a time. 

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Analytical Methods. A Schimadzu Liquid Chromatograph was used to monitor the reaction conversion and to assign chemical and chiral purity to the final product. Structures were verified by HNMR spectra obtained on a Bruker (Model UltraShield 400 spectrometer). Optical rotations were measured on a Perkin Elmer Model 341 Polarimeter. 

Analysis of terrestrial samples demonstrated that the tuning of the ion extraction system, and consequently the amount of instrumental mass fractionation, was very sensitive to charge build-up on the sample, the position of the primary beam relative to the spectrometer optic axis, and the position of the sample relative to the extraction lens. To optimize reproducible tuning of the extraction system from sample to sample, we developed the following criteria (1) resistance of the sample (Au-coated) to ground less than 106 fi (2) alignment of the primary beam to within 10 pm using... 

This method is commonly nsed on spectral data to correct for multiplicative variations between spectra. In spectroscopy, snch variations often originate from nnintended or uncontrolled differences in sample path length (or effective path length, in the case of reflectance spectroscopy), caused by variations in sample physical properties (particle size, thickness), sample preparation, sample presentation, and perhaps even variations in spectrometer optics. Snch variations can be particularly problematic because they are confounded with mnltiplicative effects from changes in component concentrations, which often constitute the signal in qnantitative applications. It is important to note that multiplicative variations cannot be removed by derivatives, mean-centering or variable-wise scaling. 

Mixture analysis has given rise to several methods, made possible with the range of spectrometers available. Computers dedicated to the spectrometer optical platform include software that can treat a great number of data points obtained from sample spectra and standard solutions. 

Figure 6.2 General layout of the FTIR imaging spectrometer. Optical elements F and G illuminate the sampling accessory with light from the spectrometer optical elements B and E focus the light from the sampling accessory onto the FPA detector aperture C is used to control the overall light level reaching the detector, and filter B is used to block light from outside the desired spectral region.

Dougherty TP, Heilweil EJ. Dual beam subpicosecond broadband infrared spectrometer. Optics Lett 1994 19 129-131. 

Bonmarin M, Helbing J (2008) A picosecond time-resolved vibrational circular dichroism spectrometer. Optics Lett 33 2086-2088... 

K. at a frequency of approximately 9.0 Gc/sec. with a superheterodyne spectrometer. Optical measurements were made at room temperature and 77°K. with a Cary Model No. 14 spectrometer and a Jarrell-Ash F-6 spectrometer using photographic plates. 

The requirements of the spectrometer used for fhe CARS experimenf are not necessarily specific to CARS specfromefry and as such it is possible to purchase a spectrometer to suit. The typical nitrogen CARS spectral profile is abouf 100 cm or about 1.5 nm wide. Since the CARS signal can be quite weak, it is necessary to have a sensitive detector array that can be gated. It is necessary, as well, to have the source-side (laser beams) and detection-side (spectrometer) optics synchronized such that the laser beams arrive at the same time and the detector is open when the CARS signal is generated. This should all happen for every laser pulse. 

Conventional IR spectrometers follow the same principles as described for UV/vis spectrometers, but, bearing in mind that IR radiation is basically radiant heat, the detectors are usually sensitive thermocouples and the monochromator/spectrometer optics must be adapted to transmit in this spectral region. Samples are frequently prepared as thin films or solid dispersions between IR transparent optical surfaces (KBr discs, etc.). 

It is usually unwise to deal with samples whose absorbances are greater than 2.0. An absorbance of 2.0 corresponds to a percent transmittance of 1% so that 99% of the incident light has been absorbed at that wavelength. At higher absorbances, the percentage of transmitted light is so small that it becomes comparable in size with light losses in the spectrometer optics, and experimental measurements are therefore subject to considerable error. (Now try Exercise 20D.)... 

By checking the Reference Correction box the effects of the spectrometer optics can be corrected for. This requires a spectrum of a tungsten lamp for calibration. 

The function of the interferometer in a Fourier transform infrared spectrometer has been presented. An FT-IR spectrometer optical layout is now described and information is provided for each element of a typical spectrometer design. A schematic of a typical FT-IR optical design is given in Figure 6. 

Figure 9. Dual-beam spectrometer optical layout as used for GC-IR measurements. (Reproduced from reference 28, by permission of the American Chemical Society, copyright 1978).

FIGURE 3-20. Use of a collimating lens and a focusing lens in a spectrometer optical system. 

Elements such as tungsten, zirconium, uranium, and the rare earth elements have multiple spectral lines, which make line selection a difficult task. The degree of interference and sample composition is related to what is called the critical concentration ratio (CCR), which is defined as the ratio of the concentration of interferent i to that of the analyte a at which the ratio of the line intensities IJla is equal to unity. If the measured concentration ratio exceeds the CCR, the intensity of the interferent line will be higher than that of the analyte line and will be detrimental to accuracy. In some spectrometers, optical cross-talk in the region of the exit slit and detector will present itself as a direct overlap. 

Figure 2.27 An echelle spectrometer optical layout. The eohelle grating disperses the light to a second wavelength selector, called a cross disperser. The cross disperser may be a prism or a conventional grating. [ 1993-2014 PerkinElmer, Inc. All rights reserved. Printed with permission.

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