Improvements in derivative-corrected nitrous oxide-acetylene flame emission spectrometry

0584-954?/86 s3.oo+o.M)

Specfrwhimh ..&a. Vol. 418, No. 1I, jq. 1139-l 150. 1986. Printed in Great

Persamoa Journals Ltd.

Britain.

Improvements in derivative-corrected nitrous oxide-acetylene flame emission spectrometry B. E. BUELL *Union Oil Company of California, Science and Technology Division, P. 0. Box 76, Brea, CA 92621, U.S.A. (Received ti July 1984; in revisedfornr 18 L&z&

1985)

Abstract-The use of repetitive optical scanning in the derivative mode has been thoroughly evaluated for flame emission speetrometry with a nitrous oxide-acetylene flame. Most of the detection limits obtained were the best reported for flame emission spectrometry. A comparison has been made between atomic absorption and inductively coupled plasma spectrometry. The flame emission technique was found to be complementary to and highly competitive with both techniques.

FOR FLAMEemission (FE)

spectroscopy there has been a gradual development through the years towards improved detection limits which has broadened the scope of FE making it competitive with atomic absorption (AA) and now inductively coupled plasma atomic emission spectroscopy (ICP). Another development has been the introduction of the nitrous oxide-acetylene (N-A) flame which has been shown by PICKETT and KOIRTYOHANN [l], CHRISTZANand FELDMAN [2l and BOUMANS and DEBOER f33 to produce good FE detection limits for many elements. Another reason FE improved was because better and higher dispersion monochromators were being used along with improved and specially selected photomultip~er tubes. Since the magnitude of flame background was a major factor, restricting detection limits as it became larger, techniques for improving line-to-background ratio (L/B), such as increasing dispersion, decreasing ,scattered light and modulating signals should lead to improved detection limits. me fact that flame background indeed was a major limiting factor was established by measuring detection limits for two equally rated manganese lines but in contrasting background areas. The measurements were made with the high dispersion spectrometer described by BUELL [43 in a com~ison of FE to spark spectrometry. The line at 445.76 nm in a low continuum background area has an excitation potential of 5.85 eV and the line at 428.11 nm in a spectral region with high CH peaks has an excitation potential of 5.82 eV, both have dvalues of 2.0. The Mn line in the high background region has a detection limit nearly ‘I-fold greater than that for the line in the low background region (1100 pg/l vs 160 %/l)* Thus using good, high dispersion spectrometers leads to improved FE results. For example, BOUMANS and DEBOER [3] obtained, for a limited number of elements, some of the best FE detection limits published using a one meter Czerny-Turner mon~hromator. In a spectrographic study concerning the influence of dispersion on detection limits, S~HN~DER [S] estabtished that if noise is equal (constant for a given source) detection limits will be proportional to the square of the reciprocal linear dispersion down to the limit imposed by the natural width of the line (0.75-2A/mm). Thus one can exploit the use of increased dispersion only to a certain point before reaching a mathematical limit and, more probably, a practical limit where increasing dispersion will lower flux transmitting power and lead to electronic noise restrictions which become as limiting as flame background fluctuations. Unfortu~tely many s~trometers used today, especially many of those used for AA, have *Present address-25125 Imperial Way, Hemet, CA 92345, U.S.A. (11 [2] [S] [4] [S]

E. E. PICKETT and S. R. KOIRTYOHANN, Spectrochim.Acra 23B, 235 (1968). G. D. CHRISTIAN and F. J. FELDMAN,Appi. Spectrosc. 25, 660 (1971). P. W. J. M. BOUMANS and F. J. DE&?OER, Speetroclrim.+a 2’7B,391 (1972). B. E. BUELL, AMi. C&em.38, 1376 (1966). T. SCHNIEDER, N. BASAL and CIE. BASEL,Spectrochim. Acto 17, 3OOO (1961).

1140

3.

E. BUELL

poor scattered light specifications and inadequate dispersion and resolution for good FE results; see BUELL [6] and LARSONet al. [7] for the importance of scattered light. As a consequence, and in coordination with the popularity and growth of AA, FE has become unjustifiably neglected; see FASSELet al. [S] comparing FE to AA precision for analysis of steel alloys. On the other hand, a few U.S. laboratories including Fassel’s, ours and others have exploited FE fully. In addition to using a good monochromator we also used a modulation technique consisting of repetitive optical scanning in the derivative mode (henceforth designated a.c. FE) as described by SNELLEMAN et al. [9]. BUELLpresented a paper on the performance of this spectrometer in 1972 [lo] and described it in Ref. [6] published detection limits ([6] Table 2.2 and pp. 79 and 138) that were better FE limits for many elements than previously reported. This instrument has performed excellently since 1970 in many applications and is the subject of this paper. In addition new detection limits have been obtained which have exceeded previous expectations, especially for lanthanide elements. A detection limit comparison is made with other instrumentation used in our laboratory. Modern instruments may provide improved detection limits. EXPERIMENTAL Apparatus

The instrumental system used consisted of a Jarrell-Ash 0.5-m monochromator equipped with a quartz refractor plate, similar to that described in Ref. [9], placed behind the exit slit which scanned a range of about 0.03-0.1 nm at 172 Hz. A PAR lock-in-amplifier was tuned to the same frequency. This prototype instrument was fabricated by Jarrell-Ash. A 35 m entrance slit was masked to a height of 7 mm and a 50 cun exit slit was used to provide an estimated spectral bandwidth of about 0.08 nm and a practical dispersion of about 0.06 nm in the derivative mode. A variable resistor box was placed across the signal input to the amplifier (4.7-15OOkohms). A Model 7127 A Hewlett-Packard strip chart recorder, a type Dl-30 Varian-Techtron digital indicator (4 digit meter with time averaging facilities) and a Perkin-Elmer variable nebulizer, S-cm nitrous oxide-acetylene burner were used. A specially selected R928 Hammumatsu photomultiplier tube was used. Two Perkin-Elmer Model 306 spectrometers were used. An Applied Research Laboratories Model 34000 ICP spectrometer, equipped with 32 channels mostly set up in the 2nd order to provide a dispersion of 0.46nm/mm and a resolution of 0.023 nm, a 20-pm entrance slit and SO-pm exit slit were used. It was also equipped with a one meter, scanning monochromator, with a resolution of 0.06nm in the first order. Spectrometric measurements and background

Burner height and flame parameters were optimized to provide the best detection limits. However, an a.c. FE system in the first derivative mode with the nitrous oxide-acetylene (N-A) flame, provided a more complicated situation than did conventional FE. These complications are summarized below. For cesium, potassium, rubidium and sodium an air-acetylene flame was used. (1) For each atomic line and sharp background peak (vibration-rotation fine structure for bandheads) a high and low wavelength wing occur extending each direction from the dark current position; see Fig. 1 for the a.c. FE spectrum for 2 rnd of barium at 553.5 nm. (2) A complex background spectrum of vibration-rotation, sharp peaks is provided by various radicals in the flame (C, CH, CHJ, NH, OH). The signals may be either negative or positive compared to analyte signals. Examples are given in Fig. 2 for C2 at 516.5 nm showing a large distorted negative peak, in Fig. 3 for a harmonic series of unknown species, (perhaps N2), near iron at 372.0 nm and in Fig. 4 for vanadium at 437.9 and the Cz background showing a negative blank for the positive V peak. A summary of relative background intensities for the low wavelength, derivative wings for various species is compared to those reported for d.c. FE by KIRKBRIGHT [11] in Table 1. For a more complete description of the background species in the N-A flame and their spectra see KIRKBRIGHT et al. [12].

[6] B. Z. BUELL,AppliedAtomicSpectroscopy,Ed. E. L. GROVES,Vol. 2, pp. 53-217.Plenum Press, New York (1978).

[7] G. F. LARSON,V. A. FASSEL,R. H. WINGEand R. N. KNISELEY,Appl. Spectrosc.30, 384 (1976). SLACKand R. N. KNISELEY, Anal. Chem. 43, 186 (1971). [9] W. SNELLEMAN, T. C. RAINS,K. W. YEE, H. D. COOKand 0. MENIS,Anal. C/tern.42, 394 (1970). [lo] B. E. BUELL,Pacific Conference on AnalyticalChemicaland Spectroscopy,p. 394 (1970). [l l] G. F. KIRKBRIGHT, M. SARGENT and T. S. WEST, Talanta16,245 (1969). [12] G. F. KIRKBRIGHT, M. K. PETERS and T. S. WEST, Talanta 14, 789 (1967).

[S] V. A. FASSEL,R. W.

1141

N-A emission flame spectrometry

Fig. 1. Barium spectrum at 553.55nm, a.c. FE.

I DARK CURRENT

e

9

9 v)

0

0

G

v)

vi

h

Fig. 2. C2 spectrum near 516.5 nm showing skewed line shape for a.c. FE in a complex background region.

(3) The optimum height in the flame is about 4mm higher for refractory elements compared to other elements, primarily due to the enlargement of the inner white cone with increasing acetylene flow. (4) For certain non-refractory elements, such as copper at 324.8 nm, OH and coinciding CH or NH background species may require compromise flame conditons. Although a lean flame produces maximum analyte signals for these elements it also causes the largest OH response. A small to medium pink cone produces less background and optimum detection limits. See PICKETT and KOIRTYOHANN [l] for responses of various background species vs fuel flow.

1142

B. E.

BUELL

Fig. 3. Tail end of a harmonic series of unknown background species near iron 372.0nm, a.c. FE in N-A flame.

120 BLANK (FLAME BANDS) !i

16 ppm

VANADIUM A

Fig. 4. Vanadi~

437.9 nm and Cz spectrum for a.c. FE. Dotted line is extrapolated below 16 mg/l vanadium to show a negative bIank compared to dark current.

(5) Because of the elimination of signals from continuum and broad bandheads the usual concept of L/B is not valid. Although no background signal is apparent in certain cases, the photodetector is still irradiated with background emission causing fluctuations to occur for nil signals that are far larger than dark current noise. For example, there was no or a very small background signal for iron at 372.0 mu and a very large background for vanadium at 437.3 nm yet the magnitude of background fluctuations at equal and high gains was similar for vanadium (5 %) and iron (6 YJ. Because of these complications some elements (holmium, molybdenum, neodymium, praseodymium and terbium) produced better detection limits and less background using the high wavelength derivative wing. For all other elements

the low wavelength

wing was used.

N-A

1143

emission flame spectrometry

Table 1. Relative intensities of bandheads in the N-A game

Species

Relative intensity Previoust Current+ d.c. FE[ 1l] a.c. FE

Wavelength

Mn (amp/l)*

403.08

400

C* C, C, CH OH

563.55 516.5 473.7 431.2 306.4

50 600 250 95 280 rich game 1600 lean Same

28 63 170 220 5900

*Given as a reference to reproduce relative intensities. tRelative intensities for the low wavelength, derivative wing. $Not on the same relative intensity scale.

Selection of wavelength The wavelengths used are the same as in Ref. [12] except for some of the lanthanides as listed in Table 2. Special, alternate wavelengths that are slightly less sensitive were also selected that eliminate mutual cobalt and nickel interferences. These wavelengths are 350.6 nm for cobalt and 349.3 nm for nickel. Limiting parameters for the N-A jiame Some comments on observations of trends vs dissociation energy (Do) and excitation potential (Ep) in the N-A flame may be of interest. The characteristic of this flame which makes it so valuable is its ability to dissociate monoxides that have relatively high Do. KIRKBRIGHT et al. [ 121 discuss this aspect of the flame concluding that a highly reducing atmosphere is provided in the rich N-A flame which contributes to the formation of atoms from refractory monoxides in the flame. Thus, for example, there appears to be little limitation due to Do below 8 eV for elements with low Ep. This is indicated by the list of elements in Table 3 which have excellent detection limits despite high Do. There does appear to be some limitation imposed by Do in the range 7-8 eV as Ep exceeds 3.4 eV and Do over 8eV definitely appears limiting for elements such as cerium and boron. The Ep appears to become more limiting as it exceeds 4 eV and the detection limit for zinc with an Ep of 6.6 eV is very poor, 2200&l. This is a classical case where AA is outstanding because the combination of a low Do, 2.8 eV, and a high Ep is not limiting for AA. Table 2. Selected wavelengths for a.c. FE Element

Gd La Nd Sm Tb

Wavelength 556.5 418.7 434.7 550.0 (740.3 band)* 492.45 (464.3)* 472.8 432.6

*Wavelength giving less mutual lanthanide interferences. Table 3. Elements with high dissociation energies provide good a.c. FE detection limits Element

Pr Nd Ti V

7.9 7.4 7.2 6.4

Ep(eV)

Detection limit (@ml)

2.5 2.5 3.1 3.1

5 2 7 2

B. E.

1144

BUELL

DISCUSSION OF RESULTS Detection limits The detection limits obtained for aqueous solutions using a.c. FE are given in Table 4 and are the pg/I of element providing a signal two times the standard deviation of the background fluctuations. A 10-s time constant was used on the lock-in amplifier and a 3-s averaging on the digital meter. At least two series of 10 readings were averaged to obtain the detection limits and many were verified on different days. These represent conservative detection limits that can easily be repeated. They are compared to values obtained by PICKE-I-T and KOIRTYOHANN [ 133 using equivalent instrumentation in the conventional d.c. FE mode (a 0.5 meter JarrellAsh h monochromator and a 5cm nitrous oxide-acetylene flame). The average improvement is 8-fold varying from 3 to 30-fold for 34 elements excluding lanthanides, Sc and Y. For the latter elements, a.c. FE provides outstanding gains averaging more than lOO-fold better and providing values competitive with the ICP. With a.c. FE the detection limit for L+aat the 441.8-nm band is equivalent to that for ICP but due to interferences a different wavelength is used for analysis; the detection limit for the 550.1-nm line is that recorded in Table 2. Table 4. Detection limits @gll) Element

43

Al As AU B Ba Be Bi Ca Cd Ce co :: CU DY Er EU Fe Ga Gd Ge Hf Hg Ho In Ir K La Li Lu Mg Mn MO

a.c.FE

d.c.FE

3 1.5

20 5 -

3: 1100 0.3 20

500 -

P 0.01 150 40 7 0.7 0.5 3 20 1 0.04 6 2 8 90 P P 0.7 0.5 250
(2oof 0.1 2000 50 5 10 70 40 0.6 50 10 2000 500 20 2 co.5 8000 0.03 1000 5 10 100

ICP

AA 2 30 100 20 2500 20 1 40 1 1

-

Element

P 10

2f 30+ 30 5 7’ 0.8* 0.5* 50 4+ 2.5* 9* 5*

Na Nb Nd Ni OS P Pb Pd Pr Pt Rb Re

5;

1

!! Sb Sc Se Si Sm Sn Sr Ta Tb Te Th Ti Tl Tm U V W Y Yb Zn Zr

2 3* 200 8 100 10 40 3 10 3’ 100 40 4000 8 1000 50 8000 10 500 50 100 8 50 40 2000 70 5 100* 2000 29 0.6 5’ 3000 10 0.1 0.2 2 0.5’ 30 9’

a.c.FE

d.c.FE

AA


0.5 1000 200 30 -

2 1000 2ooo 10 500 100000 20 20 10000 100 5 1000 3$4J’

40* 2* 5* 5* 200 90* 23. 40 9* 30 30 ;

100 100 100 80 2000 20 10 2000 3000 90 -

20 1 20 ll* 20’ 6 0.4* 50 20 20 30 4* 80 6* 302 4* 30 0.3* 2 3’ 5

6Ow 50 10 5 140 0.2 30 4 5 900 0.8 P 300 6 120 0.02 1400 0.5 P P 7 3 0.7 130 2 100 4 0.2 2200 300

*Union Oil. p = poor. tVia PO bandhead. :PE Mod1 403. [13] E. E. PICKETTand S. R. KOIRTYOHANN,Anal. Chin. 41, 28A (1969).

200 50 1000 2000 1 200 t: 30 200 500 0.1 5000 400 200 20 20 10 500 400 2 3000

90 30 200 30000 60 3000 300 40 2 5000

ICP

1145

N-A emission flame spectrometry

Detection limits for FE reported by BOUMANS and DEBOER[3] for 18 elements using a higher dispersion monochromator are better in many cases than in Ref. [ 131 and compared to a.c. FE are equal for nine elements and worse by 3.5 to 25-fold for nine elements. They also made a comparison to ICP but used a desolvation chamber to obtain some of the best detection limits reported for ICP. In Table 4, detection limits for the ICP are mostly from a bulletin reproduced in Ref. [ 143 that are guaranteed typical with commercially available instrumentation. They do not necessarily represent “state-of-the-art”, individually optimized limits. For the lanthanide elements ICP detection limits are those of NIKDEL et al. [15] who also reviewed those obtained previously. Values marked are those obtained in our laboratory. For comparison, values for AA are also given. They represent values ‘which were obtained with the PerkinElmer Model 306 spectrometer used in our laboratory. All values given in the table are for pneumatic nebulization, furnace detection limits are much more favorable. Detection limits for ICP and a.c. FE would also be improved using desolvation and solid sampling techniques. Cross comparisons of the technique providing best detection limits for 69 elements for all three techniques are summarized in Table 5 (detection limits are considered about equal if within a factor of 3). In Table 5, comparing all three techniques, the ICP is best for 13 elements providing outstanding improvements for highly refractory elements such as boron, hafnium, silicon, tantalum and zirconium. For 21 elements, including eight of the lanthanides, a.c. FE is best (AA using the Model 403 is equivalent for Rh). The best gains for a.c. FE are for alkali elements (which are relatively poor by ICP) and some of the lanthanides. For an additional 15 elements a.c. FE and ICP are equal and both are better than AA (column 3, Table 3). In column 4 are a list of 13 elements for which AA and ICP are equal and better than FE. Finally seven elements are equal for all three techniques. Thus ICP provides the best or equal results for 48 elements, a.c. FE for 43 elements and AA for 20 elements. It would seem therefore, that good instrumentation and the use of ax. FE places the technique of FE in a highly competitive position for trace analyses.

Table 5. Comparative detection limits for 69 elements (considered equal within a factor of 3)

ICP best (17 elements)

Hf Hg Ir La Mn P Sb

Ta Te Th Ti Y Zr

a.c. FE best (22 elements) Al Ca Cr Cs Er Eu Ga Ho In K Li

Na Nd Pr Rb Rh Ru Sr Tb Tl Tm Yb

ICP equals ICP equals All three a.c. FE AA equal (13 elements) (10 elements) (7 elements) Ba Ce Gd Ge Lu MO Nb Re Sc Sm V U W

As Au Be Bi Cd Mg OS Pt Sn Zn

Ag co cu Fe Ni Pb Pd

Total number of elements for which technique is best or equal ICP a.c. FE AA 47

44

17

[14] Adapted from Jarrell-Ash Bull., 434, ICP Infonnatiorc News&. 3, (10) 435 (1978). [15] S. NIKDEL, A. MASSOUMI and J. D. WINEFORDNER, Mikrochem J. 24, 1 (1979).

The use of organic solvents improves detection limits about 3-fold. A special solvent mixture of 80 % cyclohexanone-20 % special naphtholite, designed to dissolve heavy crude oil and shale oil samples as discussed in Ref. [6], has excellent nebulizing and burning characteristics in the N-A dame. In addition to approximately a 2-fold enhancement of signals due primarily to improved atomization efficiency, fluctuations in the &me are less compared with aqueous solutions thus resulting in a total 3-fold ~provem~nt in detection limits. Detection limits using this solvent mixture were previously reported for copper, iron, nickel and vanadium using both a.c. FE with the Jarrell-Ash spectrometer and AA and d.c. FE with a Perkin-Elmer Model 306 spectrometer [6]. Using identical nitrous oxide-acetylene burners the rug/l detection limits obtained are listed in Table 6, Note that the a.c, FE detection limits are best for all four elements and better by more than lo-fold for V.

The a.c, FE technique, in addition to providing better detection limits, also eliminates or minimizes certain interferences caused in d.c, FE by broad bandhead emission. Most of the elemental oxide ~ndheads are eliminate and, for example, neither boron nor calcium can be detected via the well known BO;? bandheads at 545 and 518 nm and CaO near 553.5 nm. The latter has been a classical case of interference for determing barium at 553.55 run. With a.c. FE even a IOO-fold excess of calcium does not interfere. Some interest~g data for calcium interference and flame back~ound specificity factors (SF) are given in Table 7. Note that although the SF for Aame background only improves 3-fold for a.c. FE compared to d.c. FE using high dispersion as discussed by &JELL [4j, that for barium improves 22-fold. Data previously obtained using an oxygen-hydrogen fIame are also given. From these data it can be calculated that the N-A flame produced a 50-fold improvement for the background SF and 20-fold improvement for calcium interference SF. Compared to technology of 1950 using a

Table 6. Comparison of detection Wits in organic solvent for selected elements

Element

Detection limits @8/l) Perk&Elmer a.~. FE FE AA

cu

1.3

-

5

Fe Ni V

1.6 1.8 0.7

-

I2 16 15

5 10

TabIe 7. Specificity factors for barium, 553.55nm

Instrumentation Jarrell- Ash a.~. FE Bausch and Lomb high dispersion Bausch and Lomb high dispersion Beckman DU low dispersion

Flame 5cmN,0-ClzHz

5cmN,O-C&,

Calcium* specificity factor

Flame background+ specificity factor

1300

.Ol

60

.53

0-H

3

O-H

0.f

1.5 16

*The ratio of Ca/Ba at a concentration of Ca that produced a signal equal to that for Ba (100% interference). $The concentration of Ba required to produce a signal equaf to the background.

N-A emission flame spectrometry

1147

Beckman DU monochromatnr and a total consumption, burner, the present a.c. FE system has provided a total improvement of 1600 in L/B for barium (background SF) and 13 000 for the calcium interference SF. The estimated improvement in detection limit is on the order of lOOO-fold.With a.c. FE there are some bandheads that are apparently sharper which can be detected. For example, the bandhead for La at 740 nm is analytically useful providing less interference and results equivalent to atomic lines. On the other hand, the elimination of most of the lanthanide oxide emissions bands is probably a major reason why a.c. FE is so outstanding for lanthanide elements. Precision

Although great emphasis has been given to detection limits, seldom has good precision data been reported for FE techniques. In the literature, claims unsupported by data have repeatedly been made that AA provides better precision than FE. This may be true as detection limits are approached where AA limits are best. On the other hand, FE or ICP may provide better precision for certain elements at low levels. Generally all techniques provide equivalent precision, in the range 0.3-l y0 RSD, at concentrations well above detection limits. ICP precision, however, is sometimes difficult to maintain at thidevel. Often precision at lower levels becomes quite important in selecting the technique to be used. For low levels of elements such as beryllium, cadmium and zinc good precision can be provided by AA where it is inconceivable to use FE. On the other hand, for example, an RSD of 0.6 % was obtained for 0.2 mg/l of vanadium in organic solvent using a.c. FE and the corresponding value for AA was 4.8 % RSD. Thus the table of detection limits can be used to deduce which technique will give the best precision at low levels (see Table 5 for the three techniques). Silver provides an example of an element giving equivalent precision for AA and FE at the 0.2mg/l level for which repeated measurements over a long time period gave an RSD of 0.88 % for AA and 0.77 % for a.c. FE. Additional data for precision using a.c. FE are given in Table 8. Calibration ranges

Emission techniques have the advantage, as opposed to AA, of a potentially greater linear calibration range comparing favorably with that for ICP, extending over 5 orders of magnitude. For a.c. FE, although curvature limits the range for the most sensitive wavelengths, a large linear calibration range (rivaling that for ICP) and extending to relatively high concentrations can be obtained for many elements by using secondary wavelengths with transitions above the ground state. The use of such lines extending calibrations to higher ranges can result in considerable time savings. For example, for analyses of solutions containing up to 4OOOmg/lcalcium and/or sodium, dilutions are not required. At 445.5 nm, calcium is linear to 4OOOmg/l and precise measurements can be Table 8. Precision data for the a.c. FE system

Time constant

Element

mg/l

Al

8 0.5 .02 2 2 2* 0.22 2*

10 1 10 1 3 1 10 1

0.32

10

Cr MO Ni

V

*Standards naphtholite).

prepared

Precision Peak-to-peak Ave. % dev. from mean fluctuations (%) 0.1-0.3 1 2-3 1.52 0.7-1.3 0.4-1.2 -

in organic

solvent

0.2 2 0.6 0.2 2.5 0.9 (Ave. for three 0.4 different I 1.1 days 0.8) 0.6 (1:l cyclohexanone-

B. E. BUELL

1148

made down to 2 mg/l (the approximate limit of linear calibrations for the 422.7 nm primary line). Combined with the primary calcium line at 422.7nm, a linear calibration range is possible from a detection limit of less than 0.00004mg/l to 4OOOmg/l over 8 orders of magnitude. Practical determinations with precisions better than 5 % are limited by blanks and flame fluctuations to a lower limit of about O.O04mg/l. (At a level of O.O2mg/l the precision was 1.3 % (RSD) using a 3-s averaging.) A nearly identical situation exists for sodium whereby the secondary line at 568.8 nm provides linear response and good precision in the range 4-4OOOmg/l. At this wavelength, using an air-acetylene flame for the range 80-4000 mg/l another advantage is that ionization effects do not cause calibration curvature and ionization buffers are not required. A particularly valuable secondary wavelength has been that for magnesium at 518.4nm which has a useful linear calibration range from 4 to 400 ms/l. For 8 mg/l magnesium using 3-s averaging, the average RSD was 1.7 % and the detection limit is 170mg/l (not in Table 1). Using AA the comparable RSD for 8 mg/l, magnesium was 0.9 %. Below 8 mg/l magnesium was linear for AA and was the preferred technique. Other useful secondary lines that extend the linear calibration ranges are 455.5 nm for Cs, 375.5 nm for Fe and 404.4 nm for K. Interferences Chemical and physical interferences. Both AA and a.c. FE using the same N-A flame have

nearly identical chemical and physical interferences. For determining elements such as Be, MO, V, etc. chemical interferences must be taken into account and accuracy approaching 2 % can be obtained by the use of appropriate buffers. In general, FE will provide fewer chemical interferences since the N-A flame is used for most elements whereas the air-acetylene flame is often used for AA and it is more susceptible to chemical interferences. The N-A, 5-cm flame can be used for AA but will provide poor sensitivity for many elements compared to the lOcm, air-acetylene flame. Chemical and spectral interferences for AA and FE have been reviewed by BUELL [6]. The ICP provides advantages over both techniques by eliminating most chemical interferences. Physical interferences involving nebulizing action such as viscosity effects, may be larger. Spectral interferences. Spectral interferences do occur for all three techniques but AA provides fewer interferences from spectral line overlap. For molecular, continuum and stray light interferences, AA provides no advantage, producing somewhat larger interferences in many cases than those encountered by a.c. FE. Examples are provided in Table 9 for some selected elements with approximately equal sensitivities by AA and a.c. FE. One advantage provided by a.c. FE is that spectral overlap from broad bandheads and continuum is essentially eliminated. For practical analysis overlap from atomic lines is infrequently encountered and usually can be corrected by calibration. Overlap from ion lines seldom occurs and is an advantage for a.c. FE compared to ICP. For ICP, in addition to spectral overlap from normal transitions, interferences as discussed by LARSON and FASSEL [16] can also be caused by line broadening and radiative-ion recombination contributing to spectral background. Table 9. Interference comparisons for AA and a.c. FE for elements selected to be approximately equal in sensitivity

Analyte Element

Ag cu Fe Ni Pb

Approximate sensitivity ratio AA/a.c. FE

2 1.5 0.4 0.5 2

Al AA 25 55 250

Interferences caused by 2000 mg/l expressed as analyte (kg/l) Ca Fe FE 0 <5 80

AA 40 50 70 200 180

FE 14 7 80 120 130

[16] G. F. LARSONand V. A. FASSEL,Appl. Spectrosc. 33, 592 (1979).

AA FE 20 0 40 80 120 180 (
Na

AA FE 20 0 100 50 5 25 200 0 (< 100)

1149

N-A emission flame spectrometry

Perhaps the greatest disadvantage for ICP is the larger number of spectral interferences that can be encountered. An interference problem, only occurring with the ICP, is encountered with ion lines. Because the ICP provides such outstanding sensitivity for ion lines (compared with atom lines) many interferences occur from spectral overlap for ion lines of elements rich in ion line spectra. Many of these interferences are caused by ion lines which are not listed in published wavelength tables, thus, one must generate one’s own correction data. Some idea of the prevalence of these ICP interferences is provided by data from our laboratory for the 32 channels: Ag, Al, B, Ba, Be, Ca, Cd, Ce, Co, Cr, Cu, Fe, K, La, Li, Mg, Mn, MO, Na, Nb, Nd, Ni, P, Pb, Pr, Si, Sn, Sr, Ti, V, Y and Zn. From interference data, U contributes the greatest number of interferences. For interferences expressed as mg/l interference per 1000 mg/l of interfering element, U caused interferences greater than 1 for 20 out of 32 channels; 3 of these were greater than 10. The number of interferences greater than 1 mg/I per 1000 mg/l for additional elements rich in ion line spectra are 12 for Sm, 12 for Pr, 12 for Nd, 11 for Th, 10 for Nb, 7 for Ta, 6 for Ce and 2 for Fe. Fe is included for reference as having a limited ion spectrum. In contrast to this, 21 of the elements included in the 32 channels had no or only one mutual interference greater than one on the other elements. These were mostly elements with simple spectra and relatively few ion lines. A specific application provided a better idea of how serious these ICP interferences were, the analysis of lanthanide samples. Samples analyzed could be divided into two types (1) pure compounds and (2) lanthanide ores such as bastnaesite and related products containing the lanthanides as mainly the four most abundant lanthanides Ce, La, Pr and Nd. Even for the analysis of type (2) samples for these four elements, four significant interferences occurred and had to be corrected for. Analyses for trace elements in type (2) samples was more difficult. For example analyzing such a sample containing lOOOmg/l of Ce in the final solution, nine interferences out of 28 channels were greater than 1 mg/l. In many of these cases a.c. FE provided less interference. The analysis for Th in bastnaesite ore is a good example of the severity of ICP interferences. Out of 40 useful Th lines tested, only three were found to have tolerable interferences and sensitivity adequate for trace analysis. For these three lines, all four of the most abundant lanthanides, Ce, La, Nd and Pr caused some interferences. The line at 346.99 nm was selected as best and provided a DL of 15 pg/l. Total interference from Ce, La, Nd and Pr in bastnaesite at a level of 1000 mg/l Ce was equivalent to about 3 mg/l of Th. Spectral scans for the four lanthanides revealed a total of six peaks within 0.03 nm of the Th line, none of which were listed in the 1975, 2nd Edition of NBS monograph 145. Most difficult was the analysis of certain type (1) samples such as Nd, Pr and Sm for trace elements. Lathanides such as La and Y caused few interferences and could be analyzed by ICP. Interferences caused by a lanthanide such as Pr, however, became prohibitive and a.c. FE backed by AA must be used for the analysis of most trace elements. An attempt was made to find alternate lanthanide analysis lines on the ICP for application to trace analyses of relatively pure Pr and Sm. Interferences occurred for all of the 55 lines tested and ranged from 0.5 to 840 mg/l per 1000 mg/l of interferent. In only two cases were improvements made. For Ce 22 lines were tested with interferences ranging from 5 to Table 10. Interference comparison table for Fe and ICP applied to analyses of pure lanthanides

Ce

Sample (interferent)

FE

Ce

--

La Nd Pr Sm Y

3 2 3 4 0.5

ICP

0.4 13 22 0.7 0

Mg/I per lOOOmg/l of interferent La Nd Pr FE ICP FE ICP FE ICP 0.7 0.2 0.1 0.2 0

0.2 2 2 0.5 0.04

0.4 0.04 0.6 0.2 0.3

0.5 0.4 12 16 0.07 <

2 0.04 1 0.1 0.1

1.7 0.5 13 1.6 0.15

1150

B. E. BUELL

840 mg/l per lOOOmg/l of Pr and only two were below 10 mg/l. Some interference factors for lanthanide analyses are compared in Table 10 for FE and ICP. Accuracy

All three techniques are capable of giving comparable precision and accuracies approaching 2%. In a long term study for a.c. FE determination of elements in catalysts, by comparison with other methods the accuracy for MO was within 2 % and the accuracy for Co and Ni was within 3 %.

CONCLUSION The use of repetitive optical scanning in the derivative mode for a.c. FE markedly improved detection limits and specificity. For 34 elements, excluding lanthanides, the improvement varied from 3 to 30-fold (averaging 8-fold) when compared with results reported by PICKETT and KOIRTYOHANN [ 133 using essentially identical equipment in the conventional manner. For the lanthanide elements improvements were outstanding, averaging more than Ml-fold better. By using a.c. FE with a good monochromator and a nitrous oxide-acetylene flame, detection limits were obtained that were better in most cases than any previously reported. Thus a.c. FE is a good alternative technique and highly competitive with both AA and ICP spectrometry. It is particularly attractive for the lanthanide elements. Comparing all three techniques, ICP gave equal or better detection limits for 47 elements, a.c. FE gave equal or better results for 44 elements and AA gave better results for 17 elements (see Table 3).