Basic Components of Spectroscopic Instrumentation
The instruments used in spectroscopy consist of several common components, including a source of energy that can be input to the sample, a means for isolat- ing a narrow range of wavelengths, a detector for measuring the signal, and a sig- nal processor to display the signal in a form convenient for the analyst. In this section we introduce the basic components used to construct spectroscopic in struments.
All forms of spectroscopy require a source of energy. In absorption and scattering spectroscopy this energy is supplied by photons. Emission and luminescence spec- troscopy use thermal, radiant (photon), or chemical energy to promote the analyte to a less stable, higher energy state.
A source of electromagnetic radiation must provide an output that is both intense and stable in the desired region of the elec- tromagnetic spectrum. Sources of electromagnetic radiation are classified as either continuum or line sources. A continuum source emits radiation over a wide range of wavelengths, with a relatively smooth variation in intensity as a function of wave- length (Figure 10.8). Line sources, on the other hand, emit radiation at a few se- lected, narrow wavelength ranges (Figure 10.9). Table 10.3 provides a list of the most common sources of electromagnetic radiation.
The most common sources of thermal energy are flames and plasmas. Flame sources use the combustion of a fuel and an oxidant such as acetylene and air, to achieve temperatures of 2000–3400 K. Plasmas, which are hot, ionized gases, provide temperatures of 6000–10,000 K.
Exothermic reactions also may serve as a source of energy. In chemiluminescence the analyte is raised to a higher-energy state by means of a chemical reaction, emitting characteristic radiation when it returns to a lower-energy state. When the chemical reaction results from a biological or enzy- matic reaction, the emission of radiation is called bioluminescence. Commercially available “light sticks” and the flash of light from a firefly are examples of chemilu- minescence and bioluminescence, respectively.
In Nessler’s original colorimetric method for ammonia, no attempt was made to narrow the wavelength range of visible light passing through the sample. If more than one component in the sample contributes to the absorption of radiation, however, then a quantitative analysis using Nessler’s original method becomes impossible. For this reason we usually try to select a single wavelength where the analyte is the only absorbing species. Unfortunately, we can- not isolate a single wavelength of radiation from a continuum source. Instead, a wavelength selector passes a narrow band of radiation (Figure 10.10) characterized by a nominal wavelength, an effective bandwidth, and a maximum throughput of radiation. The effective bandwidth is defined as the width of the radiation at half the maximum throughput.
The ideal wavelength selector has a high throughput of radiation and a nar- row effective bandwidth. A high throughput is desirable because more photons pass through the wavelength selector, giving a stronger signal with less back- ground noise. A narrow effective bandwidth provides a higher resolution, with spectral features separated by more than twice the effective bandwidth being resolved. Generally these two features of a wavelength selector are in opposition (Figure 10.11). Conditions favoring a higher throughput of radiation usually pro- vide less resolution. Decreasing the effective bandwidth improves resolution, but at the cost of a noisier signal. For a qualitative analysis, resolution is generally more important than the throughput of radiation; thus, smaller effective band- widths are desirable. In a quantitative analysis a higher throughput of radiation is usually desirable.
The simplest method for isolating a narrow band of radiation is to use an absorption or interference filter. Absorption filters work by selectively absorbing radiation from a narrow region of the electromagnetic spectrum. Interference filters use constructive and destructive interference to isolate a narrow range of wavelengths. A simple example of an absorption filter is a piece of colored glass. A purple filter, for example, removes the complementary color green from 500–560 nm. Commercially available absorption filters provide effective band- widths from 30–250 nm. The maximum throughput for the smallest effective band- passes, however, may be only 10% of the source’s emission intensity over that range of wavelengths. Interference filters are more expensive than absorption filters, but have narrower effective bandwidths, typically 10–20 nm, with maximum through- puts of at least 40%.
One limitation of an absorption or interference filter is that they do not allow for a continuous selection of wavelength. If measurements need to be made at two wavelengths, then the filter must bechanged in between measurements. A further limitation is that filters are available for only selected nominal ranges of wavelengths. An alternative approach to wave- length selection, which provides for a continuous variation of wavelength, is the monochromator.
The construction of a typical monochromator is shown in Figure 10.12. Radia- tion from the source enters the monochromator through an entrance slit. The radi- ation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with a large number of parallel grooves (see inset to Figure 10.12).
Diffraction by the grating disperses the radiation in space, where a second mirror focuses the radiation onto a planar surface containing an exit slit. In some monochromators a prism is used in place of the diffraction grating.
Radiation exits the monochromator and passes to the detector. As shown in Figure 10.12, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite effective bandwidth.
The choice of which wavelength exits the monochromator is determined by rotating the diffraction grating. A narrower exit slit provides a smaller effective bandwidth and better reso- lution, but allows a smaller throughput of radiation.
Monochromators are classified as ei- ther fixed-wavelength or scanning. In a fixed-wavelength monochromator, the wavelength is selected by manually rotating the grating. Normally, a fixed-wavelength monochromator is only used for quantita- tive analyses where measurements are made at one or two wavelengths. A scan- ning monochromator includes a drive mechanism that continuously rotates the grating, allowing successive wavelengths to exit from the monochromator. Scan- ning monochromators are used to acquire spectra and, when operated in a fixed- wavelength mode, for quantitative analysis.
An interferometer pro- vides an alternative approach for wave- length selection. Instead of filtering or dispersing the electromagnetic radiation, an interferometer simultaneously allows source radiation of all wavelengths to reach the detector (Figure 10.13). Radia- tion from the source is focused on a beam splitter that transmits half of the radiation to a fixed mirror, while reflecting the other half to a movable mirror. The radia- tion recombines at the beam splitter, where constructive and destructive inter- ference determines, for each wavelength, the intensity of light reaching the de- tector. As the moving mirror changes position, the wavelengths of light experi- encing maximum constructive interference and maximum destructive interference also changes. The signal at the detector shows intensity as a func- tion of the moving mirror’s position, expressed in units of distance or time. The result is called an interferogram, or a time domain spectrum. The time domain spectrum is converted mathematically, by a process called a Fourier transform, to the normal spectrum (also called a frequency domain spectrum) of intensity as a function of the radiation’s energy.
In comparison with a monochromator, interferometers provide two signifi- cant advantages. The first advantage, which is termed Jacquinot’s advantage, re- sults from the higher throughput of source radiation. Since an interferometer does not use slits and has fewer optical components from which radiation can be scattered and lost, the throughput of radiation reaching the detector is 80–200 times greater than that achieved with a monochromator.
The result is an improved signal-to-noise ratio. The second advantage, which is called Fellgett’s ad- vantage, reflects a savings in the time needed to obtain a spectrum. Since all fre- quencies are monitored simultaneously, an entire spectrum can be recorded in approximately 1 s, as compared to 10–15 min with a scanning monochromator.
The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of pho- tons into an easily measured electrical signal. Ideally the detector’s signal, S, should be a linear function of the electromagnetic radiation’s power, P,
S = kP + D
where k is the detector’s sensitivity, and D is the detector’s dark current, or the background electric current when all radiation from the source is blocked from the detector.
Two general classes of transducers are used for optical spectroscopy, several examples of which are listed in Table 10.4. Phototubes and photomultipliers contain a photosensitive surface that absorbs radiation in the ultraviolet, visible, and near infrared (IR), producing an electric current propor- tional to the number of photons reaching the transducer. Other photon detec- tors use a semiconductor as the photosensitive surface. When the semiconductor absorbs photons, valence electrons move to the semiconductor’s conduction band, producing a measurable current. One advantage of the Si photodiode is that it is easily miniaturized. Groups of photodiodes may be gathered together in a linear array containing from 64 to 4096 individual photodiodes. With a width of 25 μm per diode, for example, a linear array of 2048 photodiodes requires only 51.2 mm of linear space. By placing a photodiode array along the mono- chromator’s focal plane, it is possible to monitor simultaneously an entire range of wavelengths.
Infrared radiation generally does not have sufficient en- ergy to produce a measurable current when using a photon transducer. A thermal transducer, therefore, is used for infrared spectroscopy. The absorption of in- frared photons by a thermal transducer increases its temperature, changing one or more of its characteristic properties.
The pneumatic transducer, for example, consists of a small tube filled with xenon gas equipped with an IR-transparent window at one end, and a flexible membrane at the other end. A blackened sur- face in the tube absorbs photons, increasing the temperature and, therefore, the pressure of the gas. The greater pressure in the tube causes the flexible mem- brane to move in and out, and this displacement is monitored to produce an electrical signal.
The electrical signal generated by the transducer is sent to a signal processor where it is displayed in a more convenient form for the analyst. Examples of signal proces- sors include analog or digital meters, recorders, and computers equipped with digi- tal acquisition boards. The signal processor also may be used to calibrate the detec- tor’s response, to amplify the signal from the detector, to remove noise by filtering, or to mathematically transform the signal.