Echelle Grating Spectrometers in Analytical Spectrometry - American

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Peter N. Keliher and Charles C. Wohlers 1

Instrumentation

Chemistry Department, Villanova University, Villanova, Pa. 19085

Echelle Grating Spectrometers in Analytical Spectrometry Almost since its inception nearly 100 years ago, one of the major problems facing analytical spectrometry has been the limitation of spectrometer resolution and dispersion on analytical results. This problem was quite important to spectroscopists decades ago when they tried to perform such analyses as boron in steel and were severely limited by spectral interference, or when they were required to do such things as determine single isotopes of uranium. Analyses like these required very high dispersion and resolution; thus, these qualities often came to be the most i m p o r t a n t factors when designing spectrometers. For this reason, gratings replaced prisms, and grating spectrometers themselves began to be m a d e with ever longer focal lengths for higher linear dispersion and with more finely ruled gratings for higher resolution. These solutions began to become exhausted in the 1940's when focal lengths grew so long t h a t light losses became too great even for such bright sources as the DC arc, and it became too difficult to rule gratings with finer a n d finer groove spacings a n d still keep efficiency reasonably high and ghost intensities reasonably low. In the late 1940's, Harrison and his group a t M I T decided to take a completely different approach in designing a high resolution and dispersion spectrometer, a n d in so doing developed the échelle grating (1, 2). T h e y did this by applying the same basic formulas for diffraction gratings t h a t others had, b u t instead came u p with a different conclusion. T h e basic formula for resolution, assuming t h a t the angle between the incident a n d diffracted rays is small, is λ 2 Nd- sin β — = = mN Δλ λ where Ν is the n u m b e r of grooves in t h e grating, d is the groove spacing, β

1 Present address, Jarrell-Ash Division, 590 Lincoln Street, Waltham, Mass. 02154.

Figure 1. Echelle grating: W, grating width; d, groove spacing; r, angle of reflec­ tion; /, angle of incidence

is the angle between the diffracted ray a n d t h e grating normal, a n d m is t h e order in which t h e grating is used. In­ stead of using high groove densities (i.e., large n u m b e r of grooves) to achieve high resolution, the échelle grating increases t h e blaze angle (and the order) to achieve very high resolution. T h e equation for dispersion, again assuming t h a t the angle between the incident and diffracted rays is small, is dl d\

2 f - tan β _ mf λ d · cos β

where / is t h e focal length of the spec­ trometer. Here, the échelle grating spectrometer gives high dispersion again by using a high blaze angle and high orders instead of long focal lengths. T h e échelle grating is shown in Figure 1. T h e échelle grating appears

quite similar to a normal blazed plane grating except t h a t t h e "short side" of the grooves is used, i.e., échelle gratings are designed to be used a t blaze angles greater t h a n 45°. In this way, a spectrometer using an échelle grating gives high dispersion without a very long focal length and high resolution without extremely fine groove spacings. T h e effect of this high blaze angle and use of high orders is shown in Figures 2 and 3 which compare the blaze of conventional and échelle spectrometers. Some advantages of the échelle grating are indicated in Table I which compares two "typical" conventional and échelle spectrometers of 0.5-m focal length. As may be seen, t h e échelle spectrometer has better t h a n one order of magnitude higher resolution and dispersion with no sacrifice of optical speed. Table I also emphasizes an import a n t property of échelle gratings, t h e

ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976 · 333 A

Table I. Comparison of Typical Conventional and Echelle Spectrometers

Figure 2. Typical spectrometer blazed at 3000 À; represents grating blaze function

Figure 3. Echelle spectrometer blazed at all wavelengths; represents grating blaze function. Stacked orders fall within optimum blaze region

use of multiple high orders. These orders must be separated from each other in some way, which is not generally necessary in conventional spectrometers. This is accomplished by placing an auxiliary dispersing element, usually a prism or low dispersion grating in the spectrometer so that it disperses wavelengths at right angles to the échelle and thus effectively separates the orders. Typical échelle spectra are shown in Figures 4 and 5. In the photograph of the 150-W xenon lamp continuum (Figure 4), one can see the different orders appearing as horizontal lines, with the lowest order (longest wavelength) appearing at the bottom and the highest order (shortest wavelength) appearing at the top. Here, the prism-cross dispersion element is separating the wavelengths in the vertical direction, while the échelle grating separates them horizontally. The free spectral range (FSR— wavelength range best covered by one order) varies from 1.8 nm at 200 nm to 11.1 nm at 500 nm for this grating. This type of two-dimensional pattern can be used in either a spectrographic mode or a photoelectric mode. When an échelle spectrometer is used as a monochromator in the photoelectric

Focal length Groove density Angle of diffraction Width, W Order (m) at 300 nm Resolution at 300 nm Linear dispersion at 300 nm Reciprocal linear dispersion at 300 nm /"-Number

mode, two wavelength dials are required, one to change orders and the other to select the wavelength within an order. Otherwise, the operation and appearance of an échelle monochromator are the same as any conventional monochromator. Early models of the only commercially available échelle monochromator (Spectrametrics, Inc., Andover, Mass.) used arbitrary coordinates for the vertical (order) and horizontal (wavelength) dials. Wavelengths were obtained by using "Wavelength-Arbitrary Coordinate" tables provided by the manufacturer (3). For example, the Cu 324.7-nm line in the 79th order was "dialed in" by adjusting the vertical counter to 50148 and the horizontal counter to 50587. Newer systems manufactured by Spectrametrics have a more convenient direct wavelength control. As with normal monochromators, some "peaking" may be necessary after the wavelength has been set. Sometimes a spectral line will appear in more than one order, but there will be a most desired order with respect to relative signal intensity. Several examples of this are given in Table II. The two-dimensional pattern of wavelengths found with an échelle monochromator can, however, be a disadvantage in some cases. There is a limitation on slit height (the maximum slit height is generally 1 mm or less) so that interference between orders is avoided. This can affect the luminosity (or light throughput) when photoelectric readout is used. The luminosity of a monochromator (for photoelectric readout) is given by ^ RxtshWH Φ=

-cos β

ψ

where β λ is the brightness of the source directed toward the entrance slit in the wavelength interval selected by the monochromator, t is the effi­ ciency of the optics, s is the slit width, h is the slit height, W is the grating width, and H is the grating height. This equation assumes that the spec­ tral line width is smaller than the spectral slit width used, and that the

334 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976

Conventional

Echelle

0.5 M 1200 grooves/mm

0.5 M 79 grooves/mm

10° 22'

63° 26'

52 mm

128 mm 75

1 62,400 0.61 m m / n m

16 Â/mm f/9.8

763,000 6.65 m m / n m 1.5 Â / m m //8.8

slit width is sufficiently larger than the critical slit width so that diffrac­ tion effects are negligible. Therefore, if one wants to compare the luminosity of échelle and conventional spectrometers having similar focal lengths, important factors to consider are the angle of diffraction β, the slit height h, and the grating effi­ ciency. If the échelle grating is physically somewhat larger than a conventional grating, as is often the case, this will make up for any loss of luminosity due to the higher angle of diffraction. This leaves the slit height as the principal limitation on luminosity in an échelle spectrometer when the unit is used in the photoelectric mode. However, balanced against this is the favorable grating efficiency of the échelle as discussed earlier. For a given spectral resolution, a high resolution échelle spectrometer can have much wider slits than a medium resolution conventional spectrometer, thus increasing the relative luminosity of the échelle instrument. Historical Background

As mentioned previously, the échelle grating was first developed by Harrison and coworkers in 1949 (1, 2) to provide a high resolution and dispersion spectrometer in a small, high luminosity package. Although there was some early analytical interest (4, 5), this did not continue, probably due to the difficulties of ruling high angle of diffraction, ghost-free, gratings and also due to the difficulties of using photographic detection with short slits. This first problem was overcome in time by Harrison and his group at MIT, primarily through the development of the interferometrically controlled ruling of gratings (6-8), which was designed specifically for ruling échelle gratings. In spite of these improvements, échelle gratings found little use in analytical spectrometry and instead found their first widespread use in astronomy (9-12), as is often the case with new developments in spectrometry. In 1969 a commercial échelle spec-

Τ

Decreasing Wavelength (Increasing Orders)

4

Increasing Wavelength Within Order

Figure 4. Echelle spectrogram of 150-W xenon arc lamp. Note distinct orders

trometer was introduced by Spectrametrics. This system was referred to as a "SpectraSpan" and was described in some detail by Elliott (13) and later by Matz (14). By the simple adjust­ ment of a mirror, the SpectraSpan could be immediately converted from the spectrograph^ to the photoelec­ tric (monochromator) mode. Since 1970 a number of reports have ap­ peared concerning the application of the Spectrametrics system in analyti­ cal spectrometry (15-22). Applications

There are four major areas in which échelle spectrometers might be useful in analytical spectrometry: providing lower detection limits and greater selectivity in emission spectrometry through the decrease in spectral background; use of a continuum source in atomic absorption spectrometry; measurement of spectral line profiles; and multielement analysis systems. These areas are considered in detail below. Emission Spectrometry

The various factors influencing detection limits in emission spectrometry have been covered theoretically by Winefordner and Vickers (23) and also by Laqua et al. (24), who specifically considered the effect of monochromator resolution. The latter workers concluded that, for best detection limits, one should use a very high resolution monochromator and have a slit width equal to 1.5 times the critical slit width, assuming that source noise is the limiting factor. In emission spectrometry the two major sources of noise which limit the sensitivity are from, the source (flame flicker, etc.) and from the photomultiplier tube (PMT) detector (dark current). Since P M T dark current noise will remain constant, it is important to decrease the source noise from the background by decreasing the spectral

Figure 5. 1,000 ppm iron solution aspirated into premixed ni­ trous oxide-acetylene flame. Note sodium impurity in solu­ tion; 589.0-589.6-nm doublet, lower right of photograph

Table I I . Relative Intensities of Spectral Lines in Different Orders0 Rel

Line, nm Cadmium, 228.8

Mercury, 253.7

Mercury, 434.8 Sodium, 489.0

Order intensity 110 111 112 113 114 99 100 101 102 57 58 59 42 43 44

11 28 100 39 5 12 34 100 40 13 100 82 51 100 27

0 See refs. 13—1 7 and 21 for description of instrumental system used.

bandpass until P M T noise predominates or until the spectral line width is reached. Therefore, it is an advantage to have a high resolution, high luminosity monochromator so that one can easily isolate a spectral line profile from its background without loss of light throughput (compared to a medium resolution system) and consequent decrease of the source noise relative to the P M T noise. Since it is relatively easy to design . an échelle monochromator to have ten times the resolution and dispersion of conventional medium resolution monochromators of the type used in atomic absorption, significant improvements in detection limits should be possible, particularly when high background "atom reservoirs" such as the nitrous oxide-acetylene flame or various "plasma"-type systems are used. Also, spectral interferences, such as CaOH band emission on barium 553.5-nm atomic emission, or samarium 492.41 nm on neodymium 492.45-nm atomic emission, are elimi-

336 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976

nated when a high resolution échelle monochromator is used (17). This important practical property of an échelle spectrometer (increase in specificity) should be of great importance in emission spectrometry as high temperature and high background plasmas (DC, microwave, or RF) become more and more commonly used. Atomic Absorption

Another application of échelle gratings is in the area of atomic absorption using a continuum source (AAC). This use of a continuum source (xenon lamp) in atomic absorption would be an advantage in that one would need only one source for all elements, instead of a different spectral source (hollow cathode lamps or electrodeless discharge lamps) for each element. This would, of course, save considerable time and expense when a number of different elements are to be analyzed. Also, it would make background correction and multielement analysis in atomic absorption considerably easier. It is difficult to use a conventional medium resolution monochromator in AAC since it cannot completely isolate the spectral line from the continuum background and thus will have a very low signal-to-background ratio. However, with an échelle monochromator, the spectral line can be isolated and sensitive results quite similar to those using spectral sources may be obtained. This has been shown by the authors (19, 22) for two versions of a modified commercially available échelle monochromator; some typical results are shown in Table III. Here, sensitivities (defined as the concentration necessary to give 1% absorption of the incident light, i.e., an absorbance of 0.0044) are compared for the AAC system with values from hollow cathode lamp (HCL) sources both from published tables (25) and also from experimental values obtained on the

Table III. Sensitivities Using Continuum and Line Sources Sensitivity, A A L a

Element and line, nm

Ref. 25

Silver, 328.1 A l u m i n u m , 309.2 Calcium, 422.7 Chromium, 357.9 Copper, 324.7 Iron, 248.3 Potassium, 766.5 Magnesium, 285.2 Sodium, 589.0 Lead, 283.3 Vanadium, 318.5 C

0.06 1.0 0.08 0.1 0.09 0.12 0.04 0.007 0.015 0.5 1.7

Ref s. 19, 22>> 0.2 2.7 0.15 0.35 0.065 0.15 0.035 0.016 0.045 1.1 2.7

Refs. 19, 22b 0.15 4.1 0.20 0.30 0.20 0.3 0.06 0.020 0.025 1.5 13.0

" Sensitivity is defined as the concentration necessary to give 1 % absorption of the inci­ dent light (0.0044 absorbance). A l l values are in p p m . Air—acetylene flame used for all elements except aluminum and vanadium where nitrous oxide—acetylene flame used. ο Villanova results. c Multiplet, only one line of which is used in A A C .

The authors were able to demonstrate, however, the dependence of atomic absorption response on HCL line width (20). A particular aluminum HCL went through a maximum in the line width as the current was increased, and this exactly corresponded to the same current, 15 mA, where a minimum in the atomic absorption response was observed. Scans of spectral line profiles have also been made (17) showing the onset of self-absorption and self-reversal in lines emitted by microwave-powered electrodeless discharge lamps. Further progress in the determination of spectral line profiles using an échelle monochromator would probably require an instrument of higher resolution than that used by the authors. Multielement Analysis

same échelle system using wide slits. It was found possible to determine all elements having resonance lines at wavelengths greater than 225 nm, which includes the great majority of elements normally determined by atomic absorption. The sensitivities shown in Table III for AAC are somewhat worse than those for AAL (atomic absorption using a spectral line source) for most elements, but not to any great extent. The AAC results could be improved by using an échelle monochromator specifically designed for AAC. Detection limits, which compared less favorably for AAC than did sensitivities because of higher source noise, could be improved by using a higher luminosity monochromator and/or a more intense and stable lamp; this would also extend the lower wavelength limit below 225 nm. Very recently, Zander and coworkers (26) have described an echelle-AAC system with built in wavelength modulation. Wavelength modulation for AAC has the significant advantage of suppressing most source noise (27). The result is improved signal-to-noise ratios and detection limits. Detection limits in the echelleA AC-wavelength modulation mode are three times better than in normal echelle-AAC. Working curves are linear over about three orders of magnitude of concentration. Spectral Line Profiles The scanning of spectral line profiles does not have a direct application to analytical work, but knowledge of spectral line widths and profiles can provide clues as to means of improving analytical methods and also gives a better understanding of the physical processes taking place. The instrument normally used in scanning spectral line profiles is the Fabry-Perot interferometer. Although it has been

used for many years for this purpose and although much useful data have been obtained with it, it still has a number of disadvantages. First, to obtain high resolution on a Fabry-Perot interferometer, one needs mirrors having high reflectance dielectric coatings. However, these are not easily available for wavelengths below 300 nm. These coatings are also only usable over a relatively narrow wavelength region of a few hundred angstroms; therefore, if one wishes to look at widely separated regions of the spectrum, the mirrors must be changed. The Fabry-Perot is also complicated to operate, and it is frequently very difficult to achieve good stability. The échelle monochromator provides an alternative system for the measurement of spectral line profiles. As early as 1957, Walsh and coworkers (28) used an échelle monochromator for the direct measurement of the cadmium 228.8-nm line from a vapor discharge lamp to illustrate the unsuitability of that device in AAL. Self-reversal of the cadmium line was clearly observed. The échelle monochromator has been used by Stroke and coworkers to study isotopic shift and hyperfine structure of short-lived radioactive isotopes (29-35). This is one application where a high resolution, high luminosity monochromator is obviously important. The authors have used an échelle monochromator to obtain qualitative and semiquantitative results in scanning line profiles (17, 20). Unfortunately, an échelle system cannot provide the extreme high resolution of which a good interferometer is theoretically capable, and it was not possible to obtain the line profile directly without instrumental broadening, as is also the case with most Fabry-Perots. Attempts to correct for this broadening were unsuccessful.

338 A · ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976

The last, but perhaps the potentially most important application of échelle gratings, is as the dispersing device in a multielement spectrometer. TV-type image detectors are becoming popular in multielement applications (36-41), and the two-dimensional nature of these systems lends itself to adaptation with the échelle grating. In fact, what is most important when an image detector system is used is the two-dimensional nature of the échelle pattern, rather than high resolution and dispersion. When an image detector (of the ones currently available) is used with an échelle, some sacrifice in resolution and dispersion is necessary. Use of a high dispersion échelle spectrometer with such a system would require an image tube detector much larger and with higher resolution than those currently available (39, 40). Current vidicons are generally 25 mm in diameter and have on the order of 10 5 resolving elements. A square échelle pattern 17 mm on a side would fit into this 25-mm circle, and it could be generated covering a wavelength region of 200-600 nm using a 50 X 100mm, 79 groove/mm échelle grating (angle of diffraction 63° 26') in a spectrometer having a focal length of 160 mm. This spectrometer would have a reciprocal linear dispersion of approximately 3.0 Â/mm at 200 nm and 8.5 Â/mm at 500 nm. Since the resolution capabilities of the vidicon would be approximately 20 line pairs/mm, this means that each tube element would be looking at about a 0.25 Â window, depending upon wavelength. This type of resolution, however, should be adequate for many applications. The echelle-image tube combination in analytical spectrometry was first proposed by Margoshes in 1970 (42) for use with a vidicon detector. Some characteristics of the Margoshes

Table IV. Characteristics of Echelle-Vidicon System Proposed by Margoshesa

a

Order

Central wavelength, nm

Reciprocal linear dispersion. A/mm

Resolution, A

Equivslit width, μιπ

134 117 78 59 39 29

175.2 200.7 301.0 397.9 602.0 809.5

2.45 2.81 4.21 5.57 8.43 11.3

0.080 0.094 0.14 0.19 0.28 0.38

16 19 28 38 56 76

Taken from ref. 42.

s y s t e m are s h o w n in T a b l e IV. T h e t a b l e gives, for several o r d e r s , t h e cen­ tral wavelength, t h e dispersion, t h e resolution (controlled by t h e resolu­ tion of t h e vidicon), a n d t h e equiva­ lent slit w i d t h t h a t would have t o be used to achieve this resolution on a c o n v e n t i o n a l s p e c t r o m e t e r with 5 A / m m dispersion. T h e dispersion a n d resolution are p a r t i c u l a r l y good in t h e u l t r a v i o l e t region of t h e s p e c t r u m . T h i s is, of course, an a d v a n t a g e since t h e s p e c t r a of a t o m i c lines are general­ ly more c o m p l e x at s h o r t e r wave­ l e n g t h s . A working i n s t r u m e n t was first c o n s t r u c t e d by Danielsson a n d coworkers (43, 44) in S w e d e n . An image dissector t u b e was used. T h i s t y p e of image d e t e c t o r is basically a p h o t o m u l t i p l i e r t u b e in which t h e p h o t o c a t h o d e is s c a n n e d so t h a t only a small p o r t i o n is chosen for amplifica­ t i o n by t h e d y n o d e chain. T h i s d e t e c ­ tor has t h e a d v a n t a g e of lower noise a n d higher resolution t h a n t h e vidicon t u b e , b u t t h e r e a d o u t is i n h e r e n t l y se­ q u e n t i a l r a t h e r t h a n having t r u e si­ multaneous multiple-channel detec­ tion. T h e s y s t e m built by Danielsson a n d coworkers covers a s p e c t r a l range of 2 0 0 - 4 0 0 n m with a resolution of 0.023 Â a t 200 n m a n d 0.045 A a t 400 n m . T h e focal l e n g t h is 220 m m , a n d t h e display is i n t e r c e p t e d b y a 2 0 - m m d i a m e t e r image dissector. Wood a n d coworkers (45) h a v e recently designed an echelle-image t u b e c o m b i n a t i o n , b u t t h e i r s y s t e m uses an S E C vidicon r a t h e r t h a n an image dissector. T h e vidicon is p r e c e d e d by an image c o n v e r t e r t o overcome t h e lack of r e s p o n s e of t h e vidicon below ca. 300 n m . T h e i n s t r u m e n t described by W o o d and his colleagues h a s a resolut i o n of 0.7 A a t 810 n m a n d 0.19 A a t 230 n m a n d is able t o cover a wavelength r a n g e of 2 3 0 - 8 6 0 n m . T h e focal l e n g t h of t h e s y s t e m is 600 m m , a n d t h e vidicon used h a s a d i a m e t e r of 40 m m . A l t h o u g h t h e Wood s p e c t r o m e t e r w a s designed for t h e analysis of a t o m ic emission s p e c t r a , t h e versatility of t h e s y s t e m has suggested some applic a t i o n s in molecular s p e c t r o m e t r y . J o h n s o n h a s very recently r e p o r t e d (46) on t h e use of t h e W o o d echelle-

vidicon s y s t e m for molecular a b s o r p tion a n d fluorescence s p e c t r o m e t r y . A special high i n t e n s i t y m e r c u r y line source was used t o g e n e r a t e molecular fluorescence s p e c t r a . An a l t e r n a t i v e device for multielem e n t s p e c t r o m e t r y is t h e P M T . T h e P M T is t h e m o s t c o m m o n l y used d e t e c t o r in analytical optical s p e c t r o m e try, b o t h in single-channel s y s t e m s such as c o n v e n t i o n a l m o n o c h r o m a t o r s a n d in p o l y c h r o m a t o r d i r e c t r e a d e r s y s t e m s . Elliott (13) h a s , in fact, used a single c o n v e n t i o n a l P M T in conjunction with an optical encoding device for m u l t i e l e m e n t échelle s p e c t r o m e try. A m o r e desirable a p p r o a c h is t h e use of a P M T " b a n k " , i.e., an a r r a y of P M T ' s a r r a n g e d in a t w o - d i m e n s i o n a l p a t t e r n (47). A l t h o u g h t h e P M T ' s are fixed a n d n o t easily moved (as is t r u e with a c o n v e n t i o n a l direct r e a d e r ) , it is relatively easy to a r r a n g e certain lines to p a s s t h r o u g h t h e s y s t e m via a deflecting m i r r o r a r r a n g e m e n t . T h i s s y s t e m is described in detail elsewhere (48). T h i s p a p e r has given a general idea of w h a t a d v a n t a g e s échelle gratings h a v e over c o n v e n t i o n a l gratings a n d in w h a t a r e a s of analytical s p e c t r o m e t r y t h e s e a d v a n t a g e s m i g h t best be exploited. A l t h o u g h échelle gratings h a v e been in existence now for over 25 years, it is a p p a r e n t t h a t t h e i r use in analytical s p e c t r o m e t r y is only in its infancy, a n d t h a t future years will see t h e échelle g r a t i n g b e c o m i n g m o r e a n d m o r e i m p o r t a n t in t h i s area.

References (1) G. R. Harrison, J. Opt. Soc. Am., 39, 522 (1949). (2) G. R. Harrison, J. E. Archer, and J. Camus, ibid., 42, 706 (1952). (3) Spectrametrics, Inc., Andover, Mass., manufacturer's data. (4) D. Richardson, Spectrochim. Acta, 6, 61 (1953). (5) W. G. Kirchgessner and N. A. Finkelstein, Anal. Chem., 25, 1034 (1953). (6) G. R. Harrison and G. W. Stroke, J. Opt. Soc. Am., 45, 113 (1955). (7) G. W. Stroke, ibid., 51, 1321 (1961). (8) G. R. Harrison, Appl. Opt., 12, 2039 (1973). (9) D. J. Schroeder, ibid., 6, 1976 (1967). (10) R. Tousey, J. D. Purcell, and D. L. Garrett, ibid., ρ 365.

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