ICP emission spectrometer relative response by the branching-ratio method: branching ratios for Fe, Se, Te, Ge and Pd

Spectrochimica Acta Part B 54 Ž1999. 2167]2182

ICP emission spectrometer relative response by the branching ratio method: branching-ratios for Fe, Se, Te, Ge and Pd q P.S. Doidge Varian Australia Pty. Ltd., P.O. Box 222, Clayton Sth., Victoria 3169, Australia Received 18 July 1999; accepted 6 October 1999

Abstract The relative spectral response of a commercially available inductively coupled argon plasma ŽICP. emission spectrometer has been determined over a wide spectral range Žapprox. 190 to ) 900 nm. using overlapping sets of radiative branching ratios of several atomic and ionic species. Response curves were determined in two ways. In the first, calibrations were based on Ar II and Ar I lines emitted by Ar-filled hollow-cathode lamps used as line sources instead of the plasma torch. In the second, the ICP emission of selected lines of Ni and Fe was used. Branching ratios determined from the ICP emission of lines of Fe I, Se I, and Te I, using Ar lines for the intensity calibrations, were compared with previously published branching ratios or f-values for these atoms, and good agreement was found. The calibrations based on Ar II and Ni I were used to measure further branching ratios, and application to the measurement of branching ratios from selected levels of Ge I and Pd I is shown. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma atomic emission spectrometry; Branching ratios; Transition probabilities; Oscillator strengths; Argon

1. Introduction The variation in spectral response of a spectrometer may be of interest for various reasons: Ža. for optimising spectrometer design and to q This paper was published in the Special Issue in memory of Sir Alan Walsh, the premier pioneer of atomic absorption spectrometry. E-mail address: [email protected] ŽP.S. Doidge.

ensure consistent performance in the manufacture of spectrometers; Žb. for spectrochemical diagnostics, as, for example, in the use of Boltzmann plots to deduce temperature, when measured intensities of spectral lines must be reduced to a spectrometer-response-independent scale to allow reduction of intensities of lines of known transition probability Ž A-value. to a ‘slope’ temperature; Žc. when A-values are to be derived from emission-line intensities, the spectrometer’s

0584-8547r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 9 9 . 0 0 1 6 2 - 7

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spectral response must be known. In the last two cases, only the relative spectral response is needed. In the first case, when absolute efficiency may be of more interest, the relative response can also be used, if the relative efficiency curve includes a wavelength at which an absolute efficiency is known. Hinnov and Hofmann w1x used radiative branching ratios for spectrometer response calibration in the extreme ultraviolet ŽEUV. region, for which suitable calibrated sources are difficult to find. They used H and He II lines to cover the region down to 23.1 nm and the intensities were referenced to an absolute measurement in the long-wavelength region using a tungsten ribbon lamp. Adams and Whaling w2x and after them several groups w3]6x, have published branching ratio ŽBR. data of Ar, used in low-pressure discharge lamps. In Fourier transform ŽFT. spectrometry, for example, the spectrum of the fill gas of a discharge lamp containing the element of interest can be obtained at the same time as that of the spectrum of interest, and used, as internal calibrant, to establish a response calibration. Notwithstanding the obvious advantages of such methods, dispensing as they do with calibrated sources, the BR method seems to have received no attention from users of analytical plasmas, or manufacturers of emission spectrometric instrumentation. The main application to date of the method has been in determining spectrometer response in studies with an FT inductively coupled plasma ŽICP. spectrometer, to allow BRs, and thereby A-values, to be determined, as, for example, for V II w7x and Fe I w8x. The principle of the method is simple. In the radiative decay of a given excited atomic or ionic Žor diatomic molecular. level k, the ratio of Avalues, A k irA k j , for pairs of lines corresponding to transitions to lower levels i and j define branching ratios, and the measured intensity ratio of a branching pair is given by the ratio of transition probabilities, modified by the spectral response of the spectrometer, and is independent of conditions in the source emitting the lines Žprovided that neither line is self-absorbed.:

IŽl ki . A S s ki ki A k j Sk j IŽl k j .

Ž1.

where the spectral ‘sensitivity’ factor Sk n gathers all wavelength-dependent spectral response factors, such as those of the photomultiplier tube ŽPMT., grating or wavelength dispersive element, filters, lenses, etc. If the intensities are measured, and the branching ratio is known, then the relative spectral sensitivity at the two wavelengths can be determined, and if sufficient branching ratios are available for lines from a number of different upper levels, then families of lines can be overlapped to give the relative spectral response over a wide wavelength range. Whaling et al. w2,4x have reported branching ratios for Ar I and Ar II lines from 56 families in the region 206]4591 nm, from measurements with the National Solar ŽKitt Peak. Observatory Fourier transform spectrometer ŽFTS. of the spectra of Ar-filled hollow-cathode lamps, and reported uncertainties in the Ar branching ratios, relative to the strongest branch in each family, of as low as 1%; more typically, uncertainties for the strong branches Žwith BR ) 0.1. varied from 2]10%. For several levels, they listed data from the compilation of Hashiguchi and Hasikuni w5x who reported BRs of 389 Ar II lines belonging to 98 excited levels. More recently, Siems et al. w6x have reported BRs for Ar II that cover the important near-vacuum ultraviolet ŽVUV. region down to 160 nm, based on calibrations with secondary sources Žargon miniarc and deuterium lamp., and quoted uncertainties of G 5%. Comparison with BRs reported in w5x indicates good agreement with the latter, for levels common to both. Unfortunately, no direct comparison of w6x with the results of Whaling et al. w4x is possible, but it is clear that there exist three sets of independent measurements in Ar II, one set of which w5x agrees well with results in the other two w4,6x for common levels. Whaling et al. w4x also summarised some results of other authors on different species and drew particular attention to the comprehensive work on Fe I by the group at Oxford. ŽSee w9x, and

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references therein.. The results of these studies Žhereafter referred to as ‘the Oxford f-values’. are widely considered w8,10]12x to be the most reliable large-scale sets of f-values, on a relative scale, for several first-row transition metal spectra, as discussed in critical compilations w11,12x. The relative errors were claimed not to exceed 1%, though Whaling et al. w4x assigned errors of 3]5% to relative A-values derived from these f-values, and the claimed 1% accuracy has been questioned more recently w13x. A brief discussion of the Oxford f-values is now warranted, as some of these data were used for calibration, and to test calibration accuracy. The basis of the Oxford method for measuring relative f-values is high-resolution Žechelle . spectroscopy of the absorption profiles of atomic lines in a long graphite tube furnace whose temperature distribution is accurately known Žwithin 1 K.. The measurement of f-values of lines from different lower levels will be subject, at least, to uncertainties arising from the way in which the populations of the two lower levels are related one to the other. The Oxford data reduction process involves a scheme whereby the differences in f-values of pairs of lines linked through different pathways are compared and ‘relaxed’ through a least-squares routine so that the sum of nsn2

residuals,

Ý nsn1

DlogŽ gf . n Žwhere the pathway is

from n1 to n 2 ., is minimised, and independent of the path n1 ª n 2 . Reportedly, the method can give very low Ž- 1%. uncertainties in the resulting f-values, for lines from lower levels separated by up to ; 2 eV. The method does not then involve the direct measurement of emission branching ratios, but the high relative accuracy in the f-values means they can be used to derive relative transition probabilities for sets of lines involving common excited levels with high confidence; these relative A-values function as ‘branching ratios’. The Oxford f-values have been used to calibrate partially the response of the Fourier transform spectrometer ŽFTS. used to measure a large set of Fe I BRs w8x. The calibration of the spectral response over the range 190 to ) 900 nm of a commercially

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available ICP emission spectrometer will be described here. Procedures used to generate calibrations based on the Ar lines emitted by Ar-filled HCLs, and on lines emitted by an Ar ICP are described, and new BR measurements of lines of Fe I, Ge I, Pd I, Se I and Te I are presented.

2. Experimental Measurements were made on Varian Liberty Series II ICP emission spectrometers; the instrument has been described elsewhere w14x. Measurements reported here were made on a spectrometer with radially viewed plasma, though an instrument with axially viewed plasma was also used. A summary of procedures used has been given recently w15x, and the following discussion is limited to more detailed description of procedures, evaluation of the spectra and BRs used for response calibration, as well as to presentation of some new BR data. For measurements of Ar lines, the plasma torch was replaced by a hollow cathode lamp ŽHCL., with either Al or AsrCr cathodes, and filled to a pressure of 525 Pa of Ar. Lamps were modulated with a duty cycle of 50%, and the nominal average lamp currents, ranging from 6 to 24 mA, therefore corresponded to peak currents of 12]48 mA. The power supply was current-stabilised to better than 1%. Lamps were mounted in the sample compartment of the ICP instrument so that the cathodic discharge was focused on the spectrometer entrance slit. An Ar-filled AsrCr HCL was used for comparison with the Al]Ar lamp, to characterise HCL-window transmission changes with time. Comparisons of the results from these lamps also afforded checks on the response established from Ar II BRs. There were enough strong Ar lines for a calibration to be possible, although the Ar-filled HCLs used here were not optimised for Ar intensity and many of the lines reported previously w2]5x could not be measured with adequate intensity. The HCLs, with an extra optical element Žthe window., required a separate measurement of the quartz-window transmission. This was done on a Varian Cary 4 UV-visible spectrophotometer, over

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the range 188]500 nm; the transmission of the window decreased from 92% at ; 500 nm to ; 88% at 200 nm. All lines were measured in all possible orders of the 0.75-m monochromator; the product nl, with n the order number, was restricted to F 950 nm and orders up to fourth Žfor l - 237.5 nm. could be used. The instrumental resolution improved at a faster rate than order number as the diffraction limit was approached, as discussed by Wunsch et al. w16x. Thus, with the axial-view ¨ instrument, the instrumental width, as determined from the apparent width of narrow lines ŽBa II 230.424 nm or Cd II 214.44 nm w17x., was 5.5 pm in third order and 3.6 pm in fourth order, though still not good enough to resolve certain blends, such as Fe I l297.010rl297.012 nm. Measurements with the HCLs were limited to lines above ; 186 nm. The measurements could not be extended to 160 nm with the spectrometer used here, as no special provision was made for operating the Ar-filled lamps in a fully purged or vacuum-path enclosure. Most of the 98 Ar II levels, totalling 389 lines, listed in w5x were measured at least once, although for a number of the levels, especially those with branches in the region 220]240 nm, some lines were too weak to be useful. Many of the 56 Ar I and II levels listed in w4x were also measured. Eight of the nine near-VUV Ar II levels listed by Siems et al. w6x were measured. For Ar II, the number of usable lines per level ranged from two Žfor many UV levels. to a maximum of 11 Žfor the o . level listed w4,5x as 4 p9 2 P3r2 . An attempt was made to measure several groups of lines belonging to rotational levels of the v9s 1 state of the NO g bands, used for calibrating an FT ICP spectrometer w8x, these lines Žfrom P2 and Q2 rotational states . having been assigned BRs with accuracies of 4]26% w4,18x. Measurements were made with a lamp filled with an N2 ]O 2 mixture, and run at peak currents of c 50 mA, but the lines were not well resolved from their spin-doubled companions Žseparations were ; 10 pm. and could not be used for response calibration. For comparison with the present results, Fe I

and Ni I gf-values w9,19x were converted to relative A-values, using the relation g k A k i s g i f i k Ž 1.499= 10y8 l2 .

y1

Ž2.

˚ where l is the vacuum wavelength, in Angstrom, ¨ of the transition i ª k or k ª i. 3. Results and discussion 3.1. General The ‘intensity’ I Ž l . in Eq. Ž1. is the integrated relative signal HI Ž l . dl. If the spectral bandpass is somewhat greater than the physical width of the line being measured, the line area can be taken as proportional to the peak intensity w5x. With the spectrometers used here, the spectral bandpass was nearly always somewhat greater than the physical line width of the species used for calibration, e.g. the second-order bandpass was 9 pm, whereas, Fe I and Ni I lines with l - 400 nm typically have physical FWHMs, in the Ar ICP, of - 4 pm w20x. Among the lines measured, only Ar has ICP linewidths that approach or exceed the bandpass of the spectrometer used here, because of Stark broadening w20,21x. Because of this broadening, and because Ar II emission is not observable in the ICP, Ar lines for response calibration were measured in the emission of Ar-filled HCLs. For the present work, it was more convenient to measure peak heights, as a signal integrator was not available. 3.2. Summary of spectral-response calibrations deri¨ ed from branching ratios Measurements were made with Ar-filled HCLs of Ar II lines between 186 and ; 690 nm, and of Ar I between 416 and 922 nm. Fig. 1 covers the region from 300 to above 500 nm, in first order, using Ar I and II data; the peak near 500 nm corresponds to the grating blaze, and there is a filter change at 390 nm. Fig. 2 shows plots of second-order response from both Ar II w5x and Fe

P.S. Doidge r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 2167]2182 Fig. 1. Relative spectral response of the radially viewing spectrometer in the region 300 to ; 520 nm, in first order, derived from the branching ratios w4,5x of Ar II and Ar I lines measured in the emission of an Ar-filled HCL.

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2172 P.S. Doidge r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 2167]2182 Fig. 2. Relative spectral response of the radially-viewing spectrometer in the region 245]300 nm, in second order, derived from the branching ratios w4,5x of Ar II lines measured in the emission of an Ar-filled HCL, and, in the region ; 280]285 nm, from ICP emission of Fe I and Fe I BRs w8x.

P.S. Doidge r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 2167]2182 Fig. 3. Relative spectral response of the radially viewing spectrometer above 290 nm, in first order, derived from the relative A-values w19x of Ni I lines measured in the emission of Ni aspirated into the ICP, together with Fe I A-values w8x for three levels; wavenumbers of the levels are given to the right. The response is normalised to unity near 305 nm, where it is practically flat. The drop in apparent response below 300 nm by approximately 50% is due to a change of photomultiplier tube.

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I w8x lines, the latter to give greater detail near 280 nm. In the region 295]397 nm, calibration was made by means of Ni I lines emitted by Ni in the ICP, there being too few lines of Ar II to cover a critical region Ž; 340 nm. of the spectrometer’s response curve over which the response varied sharply; Fig. 3 shows results obtained using Ni I f-values w19x and some Fe I f-values w9x. 3.2.1. Interpretation of cusps in the spectrometer efficiency plots It seems reasonable to interpret certain of the cusp-like features of Figs. 1 and 3 in terms of the passing off of grating orders, as discussed by Hutley w22x. Malyj and Griffiths w23x have also given a discussion of this phenomenon in the context of results for holographic gratings with groove densities comparable to those used here. They derived an equation for the mth order-passing wavelength, l, of the observed diffraction order, n, expressed in terms of groove spacing, d, and the angle g between the incident and diffracted rays: Ž lrd . 2 w 2 Ž 1 q cosg .Ž m2 y nm . q n2 x q Ž lrd .w " Ž 1 q cosg .Ž n y 2 m .x q w 1 q cosg x s 0 2

Ž3.

where the upper or lower sign depends on whether m is positive or negative. For the spectrometers used here, g s 12.888, and the passing-off wavelengths for the n s y1 Žfirst. grating order are calculated to be lrds 0.7362 for m s y2 and lrds 0.5950 for m s q1. For the 1800rmm grating used here, the corresponding wavelengths are 409.00 and 330.55 nm. With reference to Fig. 1, based on Ar II and Ar I lines, the peak in efficiency, approximately 409 nm, probably corresponds to the m s y2 order-passing wavelength. Fig. 3, from Ni I BRs, shows a peak efficiency at 335]340 nm, and this is interpreted as the passing of the m s q1 order. ŽThe second-order minimum also occurs at ; 330]335 nm.. It is suggested that the BR method is suitable for characterising details of grating efficiency curves.

3.3. Summary discussion of specific experimental problems 3.3.1. Source intensity drift Intensity ratios were derived from levels measured at least twice; this provided a check on drift. With the single-channel, sequentially scanning spectrometer used here, source drift during measurement of the lines of a given level was a possibility that had to be considered. Drift was seldom a problem with the ICP, as modern instrumentation is capable of holding intensities stable to better than qry1% over periods of many minutes. The power supplies used with the HCLs were stabilised to minimise lamp intensity drift. Checks were made of HCL intensities on a periodic basis; the usual expedient adopted was to register a strong Ar II line Ž l s 427.752 nm. from time-to-time and remeasure levels only if the intensity of this line had drifted by more than a few percent from the last measurement. 3.3.2. Blends and order o¨ erlaps With Ar, blends were identified, as far as possible, from lists of Ar lines w24]26x. Sometimes, though, definitive assignment of blends was difficult, especially where order overlaps were involved. An example of an outlying result for an Ar I line, listed as 442.402 nm w4x, was found, for which apparent efficiencies were somewhat high. The BR Žs 0.0092. of this very weak line is assigned a quite large uncertainty w"15%, relative to the 427.219 nm reference ŽBRs 1. linex in w4x, and it is possible that the discrepancy for this line is due partly to the uncertainty in the listed BR w4x. However, it was also possible that this line was blended with the second order of a fairly strong Ar II line listed as 221.2083 nm by Kurucz w26x and also listed by Minnhagen w24x, but assigned there tentatively to Ar III. This line, in second order, would not have been resolved from 442.40 nm in the first order. With ICP measurements of Fe I and Ni I, problems with blends are potentially more numerous and severe, partly because of the more line-rich background of the ICP Ždue to Ar I and OH. in the region of the calibrant lines, partly

P.S. Doidge r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 2167]2182

because of extra line broadening, by comparison with lines emitted by HCLs and also because the predominance of ionised atoms over neutrals in the ICP means that neutral atom spectra become more difficult to measure against the Žoften. more intense ion lines. For Fe I, possible blends with Fe II lines had to be considered, and to aid in identifying possible blends, listings of Fe I and Fe II lines Že.g. w26,27x. were used. For some lines, there were blends with other Fe I lines; often these could be identified using data from w8x. For Ni, listings of Ni I w28x and Ni II w29x lines were used. Several Ni I lines w19x could not be used for calibration because of blends with other lines, mainly of OH Židentified from published wavelengths w30x.; blends with other Ni lines, with Ar, and through order overlap, were far fewer. 3.3.3. Accuracy of published Ar II BRs Anomalies in the w4x data include lines of the Ar II level 6 s 2 P3r2 Ž200,032.277 cmy1 ., for which the BRs of the 367.328 and 374.647 nm lines should be interchanged there, as suggested by the measurements here; the experimental intensity ratios were consistent with the BRs listed in w5x. Also, the Ar I line 552.498 nm, from the level 5d9 w5r2x 03 Žlisted as ‘level 19’ by Whaling et al. w4x. appears to have an incorrect BR listed there. Several anomalies were found in the w5x data. For the ‘400.114-nm’ line, belonging to the level o , the BR is greatly exaggerated. This 4 p9 2 P3r2 line was barely detected in the present measurements Žthough it was listed by Minnhagen w24x., and was omitted altogether in w4x, being replaced there by the line 404.57 nm. The intensities measured here are consistent with the BRs reported in w4x, and with the A-values listed in a critical study of Ar II w31x. Also listed in w5x, the BR of the line 232.2081 nm, from the level 5d 4 P5r2 , the BR w5x is of doubtful accuracy, as is that for l230.1825 nm of level 6 s 4 P3r2 . For this line, the measured intensity in this work was much too low for the line to have as high a BR as the one given w5x, and no evidence of blending was found for three other lines from this level. As to the Ar II BRs of w6x, the line 188.6386 nm appeared to be an outlier, and it is suggested that the BR for this line, as reported w6x, is somewhat

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high. Also, better consistency between level-tolevel efficiencies was found when the w5x data for o the level Ž 3 P0 .4 f w3x 7r2 were used in preference to w x those of 6 . Rather poor agreement is found between some BR values in w4,5x and those from a more recent study of Ar II w32x, and the latter were not further considered. Finally, regarding the experimental uncertainties in w2]6x, it is worth noting that quoted uncertainties of BRs in w5x do not incorporate the systematic errors associated with the calibration using the standard Žtungsten and deuterium. lamps, and reflect mainly the statistical scatter of the individual data. The statistical errors were inferred w5x to be small, by comparing with other data w2,3x, but non-statistical uncertainties for lines spanning a wide range remain unknown. 3.3.4. Self-absorption of the calibrant line emission, and argon metastability Self-absorption is not usually considered to be a problem with the ICP, and was easily detectable only for high concentrations of analyte, and with axial viewing: there was evidence that self-absorption at high concentrations with axial ICP viewing was high enough to cause errors in the response curves derived from Ni I BRs. For example, the peak intensity ratio, in second order, of the Ni I 300.2485 nm ŽBRs 1.r332.231 nm ŽBRs 0.0755. line pair Žwith the common excited level 4 p 3 D 3 . decreased from 36.5 to 30.8 when the Ni concentration into the ICP increased from 12.6 to 252 ppm, with axial viewing; the shorter wavelength line has both a larger f-value and a lower lowerlevel energy, so is more prone to self-absorption. The highest concentrations in the ICP consistent with negligible self-absorption were used throughout. Self-absorption can be a problem with HCLs, and problems may arise with Ar if the transition involves one metastable level w4x. Table 1 shows the results of measurements of line intensity ratios for Ar, for a range of lamp currents; clearly some of the ratios vary with lamp current by much more than the standard deviations, which are usually - 1%, on a relative basis. For the Ar I 4 p9w1r2x1 level, giving very intense lines, the

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ratios of l772 and l826 nm to l727 nm vary with lamp current; the former line has a metastable lower level, while the decrease in relative intensity of the 826-nm line probably reflects selfabsorption. Interestingly, l696 nm, also with a metastable lower level, seems constant in intensity, relative to l727 nm. By contrast, the much weaker w26x lines emitted by the higher Ar I 5 p w3r2x1 level show no such anomalies, even for l416 nm, with a metastable lower level. The absence of intensity anomalies for lines from this level is probably due both to lower excited-state number densities and lower A-values of the transitions. Several Ar I lines above 500 nm were finally rejected for reasons connected with selfabsorption or metastability. They included l772.4 and 826.4 nm lines, as listed in Table 1, as well as l706.725 nm from 4 p9w3r2x 2 .. This region was actually of little interest for the present study, and the loss of these calibration lines was not serious. With Ar II, there was no evidence that even the most intense of the lines measured, such as l427.75 nm, suffered from self-absorption ŽTable o 1.. For the 4 p 4 P5r2 level, there was an indication

ŽTable 1. of a slight increase in the intensity of l440 nm, involving a metastable lower level, relative to l480 nm, though even here the variation in ratio is less than 1%, over the range of lamp currents studied. Larger anomalies were observed for the lines l468.2299 and l471.0846 nm, from o o levels 4 p9 2 F7r2 and 4 p9 2 F5r2 , respectively. Both lines involved metastable lower levels and were discarded. It is concluded that Ar I lines for response calibration above 500 nm should be chosen with care, but that Ar II is less problematic in this regard, with HCLs of the kind used. 3.3.5. Changes in the spectrometer transmission with time The usefulness of the BR method depends, in part, on the ability to control spectrometer response variations while the measurement of the branching lines is being made. ŽConversely, BR measurements can be used to diagnose spectrometer response changes with time.. Usually, appreciable spectrometer response drift only occurs over quite long periods Žweeks or months., but with HCLs there is another problem that must be considered. Sputtered vapours can

Table 1 Measured hollow-cathode-lamp intensity ratios as a function of average lamp current for lines from selected levels of Ar I and Ar II a Spectrum excited level

lair , Žnm.

Ar I, 4 p9w1r2x1 Ž107496.417 cmy1 .

Intensity ratioa 6 mA

12 mA

24 mA

696.5430b 727.2935 772.4206b 826.4521

3.996 Ž6. 1 1.777 Ž4. 1.643 Ž4.

4.022 Ž8. 1 1.839 Ž4. 1.447 Ž4.

4.009 Ž5. 1 1.950 Ž5. 1.385 Ž3.

Ar I, 5 pw3r2x1 Ž117151.326 cmy1 .

416.4180b 427.2169 442.3994 459.6097

0.3559 Ž14. 1 Ž]. 0.1454 Ž7.

0.3577 Ž9. 1 0.01109 Ž4. 0.1462 Ž5.

0.3575 Ž14. 1 0.01151 Ž22. 0.1441 Ž15.

o Ar II, 4 p9 2 P3r 2 Ž172213.876 cmy1 .

294.2893 427.7528 473.2053

0.2569 Ž38. 8.809 Ž81. 1

0.2531 Ž19. 8.905 Ž85. 1

0.2540 Ž7. 8.871 Ž32. 1

o Ar II, 4 p 4P5r 2 Ž155043.158 cmy1 .

440.0986b 480.6021

0.3205 Ž8. 1

0.3221 Ž5. 1

0.3236 Ž7. 1

a b

Figures in parentheses are the measured standard deviations in the last digit; thus ‘0.1441 Ž15.’ means 0.1441" 0.0015. Denotes one level Žlower. of the transition as metastable Žsee text..

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deposit on HCL windows; the resulting light attenuation can be serious in the far UV or VUV w5x, and lead to errors and ambiguities in the interpretation of intensity ratios at different wavelengths. Its effect here was assessed by comparing the ratios of Ar II lines emitted by two HCLs, one the little-used AsrCr]Ar lamp, as mentioned above, the other the Al]Ar lamp, near the end of the work described here, when it had been used for c 100 mA-h. The ratios of two well separated lines Žat l ; 190 and l ) 400 nm. differed by no more than a few percent; it was concluded that effects of HCL window contamination were small over the study’s time frame.

gain at 1000 V to be - 1%. Notwithstanding this, many Ar II lines listed w4,5x could not be used as they were not intense enough. Near 240 nm, Ar II lines originate from high levels and have low BRs w4,5x: intensities were often low. In this region, and just below 300 nm, where the spectrometer efficiency was low and dropping sharply, a method based on interpolation, over intervals of a few nanometers, of the ICP continuum background between wavelengths at which relative responses were measured from Ar II BRs was used.

3.3.6. Signal-to-noise ratio considerations Signal-to-noise considerations were more important, and limiting, with the HCLs than with solutions of calibrating elements introduced into the ICP. The strongest Ar I lines from the HCL were observed with intensities of ) 10 6 counts sy1 , even in regions Ž) 750 nm. of low spectrometer efficiency, while Ar II lines were somewhat less intense. One of the most intense Ar II lines observed, l427.75 nm, was observed under ‘standard’ conditions with approximately 10 5 counts sy1 . Many far UV lines of Ar II gave count rates greater than 2000 sy1 ; the relative standard deviation of the shot noise corresponding to 10-s integrations of these count rates was calculated w14x from the known PMT gain and

The accuracy of the response calibration was checked by comparing BRs derived from the ICP emission of selected atoms, using a calibration against Ar II lines from the HCL, with those reported in the literature for the same lines. This was done for selected levels in Se I, Te I giving far UV lines, for comparison with previously published results for these elements, and for Fe I, above ; 295 nm, to compare with the Oxford relative f-values for Fe I.

3.4. Comparison with pre¨ iously published branching ratios and relati¨ e f-¨ alues

3.4.1. Comparison of BRs for Fe I Results for 10 transitions from two levels of Fe I are given in Table 2 and it can be seen that there is very satisfactory agreement of the BR values determined by measurement of peak ICP

Table 2 Comparison of branching ratios for the y 5 F1o Ž34692.14 cmy1 . and y 5 F3o Ž34328.75 cmy1 . levels of Fe I, measured using a response calibration derived from Ar II lines, with previous literature results w8,9x Excited level

Wavelength lair Žnm.

Measured BR present work

Relative A-value Oxforda

Measured BR w8x

y5 F 1 o

296.6898 373.4864 379.8511 460.2941 294.7876 297.3133 370.9246 375.8233 379.5002 459.2651

0.309 Ž15. 1 0.0360 Ž7. 0.00211 Ž10. 0.293 Ž15. 0.214 Ž11. 0.257 Ž13. 1 0.180 Ž9. 0.00256 Ž26.

0.3012 1 0.03584 0.00191 ] 0.2136 0.2468 1 0.1809 0.00253

0.317 Ž10. 1 0.0367 Ž11. 0.00190 Ž14. 0.275 Ž14. 0.203 Ž10. 0.251 Ž13. 1 0.180 Ž9. 0.00234 Ž19.

y 5 F3o

a

D.E. Blackwell et al. w9x and references therein.

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emission intensities, by calibration against Ar II lines from an HCL, with those derived from the f-values of Blackwell et al. w9x and the emission BRs of O’Brian et al. w8x. ŽSince the latter BRs were determined from an intensity calibration derived, in part, from the Oxford Fe I f-values, they do not represent an independent test of the Oxford values.. 3.4.2. Comparisons of BRs for Se I A comparison is given in Table 3 of BRs for six lines of Se I, measured by the present method, with those reported by Ubelis and Berzinsh w33x, who measured the emission of an electrodeless discharge lamp and used a standard lamp for intensity calibration. Two very weak branches expected at 259.4 and 350.3 nm w33x were not detectable in the ICP spectrum, even at the highest concentration of Se used Ž1000 mg mly1 .; their contributions to decays from these levels would be too weak to require consideration in calculations of normalised branching fractions. The agreement is good for most of the lines, and especially so for the three strongest lines from 3 o S1 , except for the weak 241.35 nm line from the 3 o S1 level, for which a somewhat lower BR is found by the present method. The value reported in w33x, 0.0054Ž18., relative to 196.09 nm, implies a lower bound to the BR of 0.0036, still somewhat higher than the result here. The present result may have some extra uncertainty, because of problems in measuring lines near 200 and 240 nm: the intensity measurement could not be made easily in the same order in both wavelength regions, and the calibration was effected by overlapping curves of different orders that necessarily have rapid response changes in the overlap region. 3.4.3. Comparisons of BRs for Te I Measurements were also reported on analogous levels of Te I w34x. Unfortunately, the 296.7 nm line from 6 s 5S2o listed there could not be measured in the ICP emission, even at a Te concentration of 1000 mg mly1 , partly because the spectometer response was too low in this region and partly because of this line’s weakness. Table 4 shows a comparison with their BRs. As

Table 3 Comparison of measured BRs for two excited levels of Se I Excited level

Wavelength lair Žnm.

Present results

Measured branching ratio w33x

5s 3S1o

196.0252 203.9842 206.2779 241.352

1 0.457 Ž23. 0.156 Ž8. 0.0028 Ž4.

1.00 Ž1. 0.46 Ž1. 0.155 Ž5. 0.0054 Ž18.

5s 5S2o

207.4784 216.4154

1 0.175 Ž14.

1.00 Ž5. 0.19 Ž1.

can be seen, there is excellent agreement between the two sets of results for the strongest three lines originating in the 6 s 3S1o level } results for the very weak 276.97 nm agree slightly less well } but the agreement is not as good for the two lines of the other level that can be compared. The reason for the poorer agreement for 6 s 5S2o is unclear: with the method used here, both lines lie on sensitive parts of the response curve, and are easily measured; neither was there any indication that the 225.9-nm line was blended with another line. A possible explanation is that the 225.9-nm line lies on a steep portion of the response curve derived here. In view of the excellent agreement in BRs for the stronger Se and Te lines by two independent methods} used in w33,34x and the method described here } the BRs and, consequently, the branching fractions of the strong lines can be regarded as well established. Table 4 Comparison of measured BRs for two excited levels of Te I Excited level

Wavelength lair Žnm.

Measured branching ratio Present results

w34x

6 s 3S1o

214.2822 238.3277 238.5792 276.9660

1 0.131 Ž13. 0.257 Ž26. 0.00359 Ž72.

1.00 Ž7. 0.13 Ž0.1. 0.26 Ž0.2. 0.0039 Ž12.

6 s 5S2o

225.9034 253.0738 296.6922

1 0.059 Ž7. Nda

1.00 Ž4. 0.085 Ž5. 0.00065 Ž25.

a

Nd, not detected.

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3.5. Comparison of emission and absorption methods for response calibration The question of the accuracy attainable by the methods used here is intimately connected to questions of the ultimate accuracy of emission and absorption methods of measuring relative fand A-values, and to which of these is the more accurate method. As noted in Section 1, the Oxford data are usually credited with being the most accurate large-scale sets of f-values, on a relative scale, and it has never been shown conclusively that the claimed relative accuracies of better than 1% in the Oxford f-values are questionable, though whether the claimed accuracies were always attained has recently been a matter of dispute w13,35x. Holweger et al. w13x have argued that the emission method using BRs is inherently more reliable than the absorption one Žsince both methods use radiative lifetimes for the normalisation to derive absolute f-values, the matter at issue concerns relative f- or A-values., and produces more accurate f-values. Blackwell et al. w35x, in response, have questioned this conclusion and have argued that the emission method using BRs is inherently more prone to temperature errors. As they point out, temperature errors can enter the emission BR method, at least in an indirect way, through the use of calibrated light sources, themselves calibrated against sources of known brightness temperature: any drift of the calibration lamp temperature Žwith lamp ageing, for example. will affect the calibration accuracy, on which the BR accuracy directly depends. Methods for spectral-response calibration that are free of this kind of problem are clearly desirable. The BR method satisfies the requirement that the calibration be independent of source intensity changes with time Žprovided the source is not drifting during measurement of the branching lines, and none of the lines is self-absorbed.. Since, however, the emission BR is ultimately traceable to a calibrated source, it is impossible to avoid the problem altogether. From the present perspective, the most that can be said is that the emission BR method is capable of yielding relative A-values accurate to some 1]5%. This accuracy is limited partly by the original radio-

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metric calibration with standard sources, and the accuracies improve as the lines’ wavelengths become closer. The absorption method, as refined by Blackwell et al., appears capable of better than 1% relative accuracy, to some extent independent of wavelength. wThere is, however, evidence w8x for systematic errors in the absorption values for lines with high Ž) 18000 cmy1 . lower levels and use of such lines was avoided here.x Unquestionably, the Oxford data are much better than many older emission measurements: with Ni I ŽFig. 3., there was better consistency in the efficiencies derived using the Oxford f-values w19x than when an older set of emission BRs w36x were used. This perspective on the general reliability of the Oxford values, compared with emission measurements, is also supported in a recent study of Fe I w37x. 3.6. Determination of new branching ratios The relative efficiency curve can be used for measuring new branching ratios, though with an accuracy limited, at least in part, by the accuracies of the original ratios used for the calibration. Of course, in as much as improved BRs lead to improved f-values, they help to advance towards the goal of absolute atomic absorption spectrometric ŽAAS. analysis proposed by Walsh w38x and further developed by L’vov w39x, as discussed previously w40x. The calibrations established here were used for determining new BRs of Ge I and Pd I, as well as for critical study of BRs in other spectra. Since the BRs, such as those of Ar II, used for response calibration are themselves determined using a standard lamp for response calibration, the efficiencies derived from a BR-based response calibration may not be as accurate as those derived more directly using such standard lamps for response calibration. The Ar BRs may, however, still be good enough to allow the accuracy of the BRs of other elements to be improved, with respect to existing data. 3.6.1. Germanium, Ge I The BRs measured for lines from 4 p5s and 4 p4 d levels of Ge I were recently combined with

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Table 5 Measured branching ratios of the 4 p6 s Ž1r2, 1r2.1o level Ž52148.73 cmy1 . of Ge I Wavelength Žnm.a

Measured BR

191.7591 193.8301 197.0879 222.0375 279.3925

0.733 Ž37. 0.498 Ž25. 1.00 Ž7. 0.0770 Ž100. 0.168 Ž22.

a

Wavelengths are vacuum-equivalent values below 200 nm and air-equivalent above.

new radiative lifetimes to give improved f-values for Ge I w15x. BRs of lines from another level of the 4 p6 s configuration have been measured and are listed in Table 5. Biemont has calculated, by a ´ relativistic Hartree]Fock method, the contributions of infrared branches to the decays from this level, and they amount, in total, to some 15% of the decays w41x. These lines Ž l ) 1100 nm. were not accessible to the spectrometer used here; therefore, it was not possible to normalise accurately the UV branches using the measured LIF lifetime of this level and results for these UV lines, thus, could not be included in w15x. Approximate A-values for the lines ŽTable 5. might be derived by assuming that the lines listed there contribute ; 85% of the total decays from this level with t s 10.5 ns w15x. Comparisons of Ge I BRs derived by the method used here with those of Lotrian et al. w42x are given elsewhere w15,43x.

3.6.2. Palladium, Pd I As has been discussed previously w40x, the Corliss and Bozman data w44x contain large systematic errors in gA-values, though the tables remain useful as a source of BRs: provided the listed gA-values are not subject to ‘wavelengthdependent’ errors, or radiation trapping, the relative gA-values from a common upper level, which are not affected by ‘excitation-dependent’ errors, can be used to determine BRs. When the CorlissrBozman gA-values are used to compute BRs for certain resonance lines of Pd I of interest in AAS, however, the BRs Žand thus the f-values. obtained are far too low to be plausible w40x. The correction to Corliss and Bozman’s intensities resulting from Corliss’s wavelength-dependent correction w45x is also too small to explain the discrepancy. The explanation is presumably connected with problems of radiation trapping in the copper arc w44x. It is therefore of some interest to improve upon the gA-values of Corliss and Bozman, and such revision of the f-values is described now. The calibrations based on Ar II and Ni I ŽFigs. 1]3. were applied in determining the BRs of lines from certain levels in the 4 d 9 5 p configuration of Pd I, the Pd measured in the ICP emission spectrum. Results for transitions from two of these levels are shown in Table 6, together with a comparison with BRs derived from w44x. As can be seen, the supposition w40x that the BRs of the resonance lines are much higher than implied by

Table 6 Branching-ratios for selected levels of Pd I } comparison of ICP results with previous results w44x Excited level

Transition, lair Žnm.

Present results

Corliss and Bozmana

4 d95 p 1 P1o

244.791 325.164 343.343

1.13 Ž11. 0.394 Ž20. 1

0.085 0.52 1

4d95 p 3 D1o

247.642 306.530 330.213 348.977

0.796 Ž80. 0.202 Ž10. 1 0.463 Ž23.

0.18 0.22 1 0.58

a

C.H. Corliss et al. w44x.

Measured branching ratio

P.S. Doidge r Spectrochimica Acta Part B: Atomic Spectroscopy 54 (1999) 2167]2182

Corliss and Bozman’s A-values w44x is correct: there is fair agreement Žwithin 25%. for the lines above 300 nm, while the 244.8-nm line has, relative to the 343.34-nm line from the same level, more than 10 times the BR. Unfortunately, it was not possible, with the spectrometer used here, to measure the infrared ŽIR. branches that contribute to the decays from the 4 d 9 5 p levels considered, and accurate determination of these lines’ BRs will depend on measurement of IR spectra of Pd HCLs. 3.7. Further applications Possible applications of BR data in plasma spectrochemical analysis include diagnostic uses, as in measurement of relative spectrometer response changes over the spectral regions used for such diagnostics as ion-to-atom ratios, and in the measurement of response changes with time, as discussed elsewhere w43x. Furthermore, reliably known BRs can be used to test the internal consistency of tables of ICP spectral-line intensities w43x, and could also be used to convert such tables of emission intensities to spectrometerresponse-independent intensities. Several sets of BRs are presently being tested for such an application, and among these a large set of Fe II data w46x can be singled out for mention as providing accurate BRs w43x.

4. Conclusions Spectral-response calibration of a commercially available ICP emission spectrometer has been achieved with radiative branching ratios. Line intensities were measured either as the emission of a hollow-cathode lamp used instead of the plasma torch, or as the emission of suitable calibrating atoms emitted by an ICP. As to the latter, Fe I and Ni I have been found to be suitable for coverage of large portions Žabove ; 225 nm. of the wavelength range of interest for ICP-AES. The BR method is applicable more generally, and should facilitate the reduction of self-consistent spectral-line intensities to spectrometer-responseindependent values, allowing tables of lines for

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one spectrometer to be converted to tables valid for another spectrometer. Acknowledgements I thank Graeme Plant and Jack Sullivan for the Ar-filled lamps, Alan Roper and Robert Loveridge for technical help, Melissa Quinn for Cary UV-Vis measurements, and Prof. W. Whaling ŽCaltech. for data w8x. I also thank the Guest Editor and reviewers who helped make this paper more concise than it might have been. References w1x E. Hinnov, F.W. Hofmann, Measurement of absolute radiation intensities in the vacuum]ultraviolet region, J. Opt. Soc. Am. 53 Ž1963. 1259]1265. w2x D.L. Adams, W. Whaling, Argon branching ratios for spectral-intensity calibration, J. Opt. Soc. Am. 71 Ž1981. 1036]1038. w3x K. Danzmann, M. Kock, Argon branching ratios for spectral-intensity calibration: a reply, J. Opt. Soc. Am. 72 Ž1982. 1556]1557. w4x W. Whaling, M.T. Carle, M.L. Pitt, Argon branching ratios for spectrometer response calibration, J. Quant. Spectrosc. Radiat. Transfer 50 Ž1993. 7]18. w5x S. Hashiguchi, M. Hasikuni, Experimentally determined branching ratios for transitions in Ar II, J. Phys. Soc. Jpn. 54 Ž1985. 1290]1298. w6x A. Siems, J.P. Knauer, M. Kock, S. Johansson, U. Litzen, ´ Ar II branching ratios for spectrometer response calibration in the near vacuum-UV, J. Quant. Spectrosc. Radiat. Transfer. 56 Ž1996. 513]516. w7x E. Biemont, N. Grevesse, L.M. Faires, G. Marsden, J.E. ´ Lawler, W. Whaling, Lifetimes and transition probabilities in V II and the solar abundance of vanadium, Astron. Astrophys. 209 Ž1989. 391]398. w8x T.R. O’Brian, M.E. Wickliffe, J.E. Lawler, W. Whaling, J.W. Brault, Lifetimes, transition probabilities, and level energies in Fe I, J. Opt. Soc. Am. B 8 Ž1991. 1185]1201. w9x D.E. Blackwell, A.D. Petford, M.J. Shallis, G.J. Simmons, Precision measurement of relative oscillator strengths-VIII. Measures of Fe I transitions from levels a3 F2 ] 4 Ž1.49]1.61 eV. with an accuracy of one percent, Mon. Not. R. Astron. Soc. 191 Ž1980. 445]450. w10x M.C.E. Huber, R.J. Sandeman, The measurement of oscillator strengths, Rep. Prog. Phys. 49 Ž1986. 397]490. w11x G.A. Martin, J.R. Fuhr, W.L. Wiese, Atomic transition probabilities scandium through manganese, J. Phys. Chem. Ref. Data, 17 ŽSuppl. 3. 1988. w12x J.R. Fuhr, G.A. Martin, W.L. Wiese, Atomic Transition Probabilities Iron through Nickel, J. Phys. Chem. Ref. Data, 17 Ž1988. Supplement 4.

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