echelle grating spectrometers in analytical spectrometry

Peter N. Keliher and Charles C. Wohlers‘ Chemistry Department. Villanova University, Villanova, Pa. 19085

Echelle Grating Spectra ...,.,. in Analytical Spectrometry

Almost since its inception nearly 100 years ago, one of the major prob- lems 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 se- verely 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 important factors when de- signing spectrometers. For this reason, gratings replaced prisms, and grating spectrometers themselves began to be made with ever longer focal lengths for higher linear dispersion and with more finelyruled gratings for higher resolution. These solutions began to become exhausted in the 1940’s when focal lengths grew so long that light losses became too great even for such bright sources as the DC arc, and it became too difficult to rule gratings with finer and finer groove spacings and still keep efficiency reasonably high and ghost intensities reasonably low.

In the late 1940’9, Harrison and his group a t MIT decided to take a completely different approach in design- ing a high resolution and dispersion spectrometer, and in so doing developed the echelle grating (I, 2). They did this by applying the same hasic formulas for diffraction gratings that others had, hut instead came up with a different conclusion. The hasic for- mula for resolution, assuming that the angle between the incident and diffracted rays is small, is

-= h 2 N d . ~ i n f l = ~ ~ Ah h

where N is the number of grooves in the grating, d is the groove spacing, fl

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

Figure 1. Echelle grating: W, grating width: d, tion; i. angle of incidence

groove spacing; r, angle of reflec-

is the angle between the diffracted ray and the grating normal, and m is the

quite similar to a normal blazed plane grating except that the “short side”of

order in which-the grating is used. In- stead of using high groove densities (i.e., large nnmher of grooves) to achieve high resolution, the echelle grating increases the blaze angle (and the order) to achieve very high resolution.

The equation for dispersion, again assuming that the angle between the incident and diffracted rays is small, is

dl 2 f . t a n f l rnf d h h d . cos f l

where f is the focal length of the spectrometer. Here, the echelle grating spectrometer gives high dispersion again by using a high blaze angle and high orders instead of long focal lengths.

ure 1. The echelle grating appears

- = = _ _

The echelle grating is shown in Fig-

the grooves is used, i.e., echelle gratings are designed to be used a t blaze angles greater than 45”. In this way, a spectrometer using an echelle grating gives high dispersion without a very long focal length and high resolution without extremely fine groove spacings. The 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 echelle spec trometers.

Some advantages of the echelle grating are indicated in Table I which compares two “typical” conventional and echelle spectrometers of 0.5-m focal length. As may be seen, the echelle spectrometer has better than one order of magnitude higher resolution and dispersion with no sacrifice of optical speed.

Table I also emphasizes an important property of echelle gratings, the

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

Figure 2. Typical spectrometer blazed at 3000 A; 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 tha t it disperses wavelengths a t right angles to the echelle and thus effec- tively separates the orders. Typical echelle spectra are shown in Figures 4 and 5 . In the photograph of the 150-W xenon lamp continuum (Figure 41, one can see the different orders appearing as horizontal lines, with the lowest order (longest wavelength) appearing a t the bottom and the highest order (shortest wavelength) appearing a t the top. Here, the prism-cross dispersion element is, separating the wavelengths in the vertical direction, while the echelle grating separates them horizontal- ly. 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 phlotoelectric mode. When an echelle spectrometer is used as a monochrc;’mator in the photoelectric

Table 1. Comparison of Typical Conventional and Echelle Spectrometers

Conventional Echelle

Focal length Groove density Angle of diffraction Width, W Order (m) a t 300 nm Resolution at 300 nm Linear dispersion a t 300 nm Reciprocal linear dispersion a t 300 nm f-Num ber

0.5 M 1200 grooveslmm 10” 22’ 52 mm 1 62,400 0.61 mrnlnm 16 A/mm fj9.8

0.5 M 79 grooveslmm 63” 26’ 128 mm 75 763,000 6.65 rnrnlnm 1.5 &mm fl8.8

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 echelle monochromator are the same as any conventional monochromator. Early models of the only commercially available echelle 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 conve- nient 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 de- sired order with respect to relative signal intensity. Several examples of this are given in Table 11.

The two-dimensional pattern of wavelengths found with an echelle 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 tha t 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

BAtshWH * COS 13 f‘ a =

where Bh is the brightness of the source directed toward the entrance slit in the wavelength interval selected by the monochromator, t is the efficiency 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 spectral line width is smaller than the spectral slit width used, and that the

slit width is sufficiently larger than the critical slit width so that diffraction effects are negligible.

Therefore, if one wants t o compare the luminosity of echelle and conventional spectrometers having similar focal lengths, important factors to consider are the angle of diffraction 6, the slit height h , and the grating efficiency. If the echelle grating is physi- cally somewhat larger than a conventional grating, as is often t he case, this will make u p for any loss of luminosity due to the higher angle of diffraction. This leaves the slit height as the prin- cipal limitation on luminosity in an echelle spectrometer when the unit is used in the photoelectric mode. How- ever, balanced against this is the fa- vorable grating efficiency of the echelle as discussed earlier. For a given spectral resolution, a high resolution echelle spectrometer can have much wider slits than a medium resolution conventional spectrometer, thus increasing the relative luminosity of the echelle instrument.

Historical Background

As mentioned previously, the echelle grating was first developed by Harrison and coworkers in 1949 ( I , 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, Igratings 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 a t MIT, primaril) through the develop- ment of the interferometrically controlled ruling of gratings (6-8), which was designed specifically for ruling echelle gratings. In spite clf these im- provements, echelle gratings found lit- tle 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 echelle spec-

3 3 4 A * ANAL.YTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976

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Figure 4. Ec:helle spectrogram of 150-W xenon arc lamp. Note distinct orders

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

L- I

trometer was rntroduced by Spectra- metrics. 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 spectrographic to the photoelectric (monochromator) mode. Since 1970 a number of reports have ap- peared concerning the application of the Spectrametrics system in analytical spectrometry (15-22).

Applications

There are four major areas in which echelle spectrometers might be useful in analytical spectrometry: providing lower detection limits and greater se- lectivity 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. (241, 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 tha t source noise is the limiting factor. In emission spectrometry the two major sources of noisie which limit the sensitivity are from the source (flame flick- er, etc.) and from the photomultiplier tube (PMT) detector (dark current). Since P M T da.rk current noise will re- main constant, i t is important to decrease the ‘source noise from the background by decreasing the spectral

Table I I . Relative Intensities of Spectral Lines in Different Ordersa

Rei Line, nm Order intensity

Mercury, 253.7 99 12 100 34 ioi loo 102 40

Mercury, 434.8 57 13 58 100 59 82

Sodium. 489.0 42 5 1 _ - 43 100 44 27

a See refs. 13-1 7 and 21 for descrip- tion of instrumental system used.

bandpass until P M T noise predomi- nates 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 o f 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 echelle monochromator to have ten times the resolution and dispersion of conventional medium resolution monochromators of the type used in atomic absorption, significant im- provements 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 samar- ium 492.41 nm on neodymium 492.43-nm atomic emission, are elimi-

nated when a high resolution echelle monochromator is used (17). This important practical property of an echelle spectrometer (increase in speci- ficity) should be of great importance in emission spectrometry as high temperature and high background plas- mas (DC, microwave, or R.F) become more and more commonly used.

Atomic Absorption

Another application of echelle 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 consider- able time and expense when a number of different elements are to be ana- lyzed. Also, it would make background correction and multielement analysis in atomic absorption considerably eas- ier. 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. How- ever, with an echelle 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 echelle monochromator; some typical results are shown in Table 111. 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

3 3 6 A ANALYTICAL CHEMISTRY, VOL. 48, NO. 3, MARCH 1976

Table 111. Sensitivities Using Continuum and Line Sources

Sensitivity, AALa Sensitivity, AACa

Element and line, nm Ref. 25 Refs. 19, 22b Refs. 19, 22b Silver, 328.1 0.06 0.2 0.15 Ai u m in urn I 309.2 1 .o 2.7 4.1 Calcium, 42;I.J 0.08 0.15 0.20 Chrom i u in, 3 5 7.9 0.1 0.3 5 0.30 Copper, 324.7 0.09 0.065 0.20 Iron, 248.3 0.12 0.15 0.3 Potassium, 766.5 0.04 0.035 0.06 Magnesium, ,285.2 0.007 0.016 0.020 Sodium, 589.0 0.015 0.045 0.025 Lead, 283.3 0.5 1.1 1.5 Vanadiur1,318.5~ 1.7 2.7 13.0

dent l ight (0 0044 absorbance). All values are in ppm. Air-acetylene flame used for all elements ex iep t aluminum and vanadium where nitrous oxide-acetylene flame used. b Villanova results. C Multiplet, only one line o f which i s used in AAC.

QSensitivity i s defined as the concentration necessary to give 1% absorption o f the inci-

same echelle system using wide slits. I t was found possible to determine all elements having resonance lines at wavelengths greater than 225 nm, which includes the great majority of elements riorrnally determined by atomic absorption.

for AAC aire 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 echelle monochromator specifical1:y designed for AAC. Detec- tion limits, which compared less favor- ably for AAC than did sensitivities be- cause of higher source noise, could be improved by using a higher luminosity monochromator and/or a more intense and stable lamp; this would also ex- tend the lower wavelength limit below 225 nm. Very recently, Zander and coworkers (2’6) 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 rstios and detection limits. Detection limits in the echelle- AAC-wavelength modulation mode are three times better than in normal echelle-AAC. Working curves are linear over about three orders of magnitude of conceiitration.

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 improv- ing 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 i t has been

The sensitivities shown in Table 111

used for many years for this purpose and although much useful data have been obtained with it, i t 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 us- able over a relatively narrow wavelength region of a few hundred ang- stroms; therefore, if one wishes to look a t widely separated regions of the spectrum, the mirrors must be changed. The Fabry-Perot is also com- plicated to operate, and it is frequent- ly very difficult to achieve good stabil- ity. The echelle monochromator provides an alternative system for the measurement of spectral line profiles. As early as 1957, Walsh and coworkers (28) used an echelle monochromator for the direct measurement of the cadmium 228.8-nm line from a vapor discharge lamp to illustrate the unsuita- bility of tha t device in AAL. Self-re- versa1 of the cadmium line was clearly observed.

The echelle 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 echelle monochromator to obtain qualitative and semiquantitative results in scanning line profiles (1 7,20). Unfortunately, an echelle 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 di- rectly without instrumental broadening, as is also the case with most Fabry-Perots. Attempts to correct for this broadening were unsuccessful.

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 ( I 7 ) 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 h e profiles using an echelle monochromator would probably require an instrument of higher resolution than that used by the authors.

Multielement Analysis

The last, but perhaps the potential- ly most important application of echelle 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 echelle grating. In fact, what is most important when an image detector system is used is the two-dimensional nature of the echelle pattern, rather than high resolution and dispersion. When an image detector (of the ones currently available) is used with an echelle, some sacrifice in resolution and dispersion is necessary. Use of a high dispersion echelle 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 105 resolving elements. A square echelle 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 100- mm, 79 groove/mm echelle 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 A/mm a t :200 nm and 8.5 A/mm a t 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 a t about a 0.25 8, 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


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

order Central

wavelength, nrn

134 175.2 117 200.7 78 301 .O 59 397.9 39 602.0 29 809.5

a Taken from ref. 42.

Reciprocal linear dispersion, A h r n Resolution, A

2.45 0.080 2.8 1 0.094 4.21 0.14 5.57 0.19 8.43 0.28

11.3 0.38

system are shown in Table IV. The table gives. for several orders, the central wavelength, the dispersion, the resolution (controlled by the resolution of the vidicon). and the equiva- lent slit width that would have to be used to achieve this resolution on a Conventional spectrometer with 5 &mm dispersion. The dispersion and resolution are particularly good in the ultraviolet region of the spectrum. This is, of course, an advantage since the spectra of atomic lines are generally more complex a t shorter wavelengths. A working instrument was first constructed by Danielsson and coworkers (43, 44) in Sweden. An image dissector tube was used. This type of image detector is basically a photomult,iplier tube in which the photocathode is scanned so that only a small portion is chosen for amplifica- tion by the dynode chain. This detector has the advantage of lower noise and higher rewlution than the vidicon tube. but the readout is inherently sequential rather than having true simultaneous multiple-channel detection. The system built by Danielsson and coworkers covers a spectral range of 200-400 nni with a resolution of 0.023 8, a t 200 nm and 0.045 A a t 400 nm. The focal length is 220 nim, and the display is intercepted by a 20-mm diameter image dissector.

if'ood and coworkers ( 4 5 ) have recently designed an echelle-image tube combination, but their system uses an SEC vidicon rather than an image dissector. The vidicon is preceded by an image converi:er to overcome the lack of response of' the vidicon below ca. 300 nm. The instrument described by iVood and his, colleagues has a resolution of 0.7 280 nm and is able to cover a wavelength range of 230-860 nm. The focal length of the system is 600 mm, and the vidicon used has a diameter of 40 mm. A1thoug.h the Wood spectrometer was designed for the analysis of atomic emission spectra, the versatility of the system has suggested some applications in molecular spectrometry. Johnson has very recently reported ( 4 6 ) on the use of the Wood echelle-

a t 810 nm and 0.19 8, a t

Equiv sl i t width, prn

16 19 28 38 56 76

vidicon system for molecular absorption and fluorescence spectrometry. A special high intensity mercury line source was used to generate molecular fluorescence spectra.

An alternative device for multielement spectrometry is the PMT. The PMT is the most commonly used detector in analytical optical spectroine- try, both in single-channel systems such as conventional monochroma1;ors and in polychromator direct reader systems. Elliott (13) has, in fact. used a single conventional P M T in conjunc- tion with a n optical encoding device for multielement echelle spectrometry. A more desirable approach is the use of a P M T "bank", Le.. an array of PMT's arranged in a two-dimensional pattern ( 4 7 ) . Although the PMT's are fixed and not easily moved (as is true with a conventional direct reader), it is relatively easy to arrange certain lines to pass through the system via a deflecting mirror arrangement. This system is described in detail elsewhere (48 ) .

This paper has given a general idea of what advantages echelle gratings have over conventional gratings and in what areas of analytical spectrometry these advantages might best be ex- ploited. Although echelle gratings have been in existence now for over 25 years. it is apparent that their use in analytical spectrometry is only in its infancy, and that future years will see the echelle grating becoming more and more important in this area.

References

(1) G . R. Harrison, J . O p t SrJc.. Am., 39,

( 2 ) G. R. Harrison, J. E. Archer. and .J. 522 (1949).

Camus. ibid. , 42. 706 (1952). ( 3 ) Spectrametrics. Inc.. Andover. Mass.

manufacturer's data. (4) I). Richardson, Spectrochim. A c t a , 6,

61 (1953). (5) LV. G. Kirchgessner and N. A. Finkel-

stein, A n a ) . Chem., 25, 1 ( M (1953). (6) G. R. Harrison and G. IT. Stroke, J

Opt. .To(,. Am., 45, 118 (19%J. (71 G. LV. Stroke. i b id . , 51, 1321 (1961;l. ( 6 ) G. R. Harrison, A p p l . O p t , 12, 2039

(9) D. J. Schroeder. i b id . , 6, 1976 (1967). (10) R. Tousey. .J. D. Purcell. and D. L,.

(1973).

Garret t , ib id . , p 365.

(11) W. Liller, ibtd., 9, 2332 (1970). (12) D. J. Schroeder, Pub. As,!ron. SOC.

Pac., 82, 1253 (1970). (13) W. G. Elliott, Am Lab. , 2 ( 3 ) , 67

(1970). (14) G. J . Matz, ibid., 5 (3 ) . 75 (1973). (15) M. S. Cresser, P. N. Keliher, and C. C.

CVohlers, Spectrosc. Lett . , 21, 179 (1970). 116) F. L. Corcoran, Jr.. P. N. Keliher, and

C. C. Wohlers,Am. Lab., 4 l(3). 51 (1972). (17) hl. S. Cresser, P. N. Keliher, and C. C.

LVohlers. Ana/. Chem.. 35. 1.11 (1973). (18) S. S. Karacki and F. L.'Corcoran, hr.,

(19) P. N. Keliher and C. C . \Vohlers,

(20) P. N. Keliher and C. C. Wohlers.

Appl . Spectrosc., 27,41 (1973).

.4nai. Chem., 46,682 (1974).

Appl . Spectrosc., 29, 198 (1975). 121) M. S. Cresser, P. N. Keliher, and C. C .

iVohlers, Lab. I'ract.. 26, 335 (1975). ( 2 2 ) P . N. Keliher and C. C. \'Vohlers,

Anal. C'hem., 48, 140 (1976;i. (23) .J. D. LVinefordner and T. J . L'ickers,

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P.N.K. thanks the Villanova University Faculty Research Program for its support.


echelle grating spectrometers in analytical spectrometry

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