Elimination of spectral interferences in the determination of Se in nickel-base alloys using Zeeman background correction

Sprcrrahlmica Acra.Vol 398.Nor 2,3.pp 519-523. 1984 Rnud m Grc@tBnt~n

0581-8547 845300+ 40 c 1984PerymonPress Ltd

RESEARCH NOTE

Elimination of spectral interferences in the determination of Se in nickel-base alloys using Zeeman background correction F. J. FERNANDEZ and M. M. BEATY Perkin-Elmer Corporation. 901 Ethan Alien Highway, Ridgefield, CT 06877 U.S.A. (Received 8 August 1983) Abstract--Spectral interferences from the presence of nearby matrix lines can occur when usin continuum source background correotion. An interferena of this type exists when determining Se in nickel-base alloys using deuterium background corration. This interferenceinlroduees a significant analytic error at both the l%.O-and 204.bnm Se lines. By comparison, Zeeman effect background correction is free of any spectral interference problem. Zecman background correction, used in conjunction with the stabilized temperature platform furnace, permitted accurate analysis without the requiremenr for matrix-matched standards.

INTRODUCTION ACCUIIATE correction for background absorption is a significant problem in graphite furnace analyses. With continuum source correction systems several factors may affect the accuracy of background correction. It is widely recognized that if background levels are extremely high, inaccurate correction may result. Problems of a spectral nature may also occur since continuum source correction is achieved by averaging all spectral features of the background within the monochromator bandpass. The average background measurement thus obtained may differ from the background at the exact analyte wavelength. Spectral problems may result from the presence of molecular absorption having fine structure [l]. The presence of weak matrix element lines in the vicinity of the analyte resonance line may also produce spectral interferences [Z-S]. If these nearby matrix lines fall within the spectral bandpass, they will absorb radiation from the continuum source producing a negative background-corrected signal. MANNING [6] reported such a spectral interference when determining Se in the presence of high iron concentrations. Numerous authors [7-161 have demonstrated the analytical advantages of Zecman effect background correction. A Zeeman effect AA spectrophotometer employing a magnet positioned around the atomizer provides background correction at the exact analyte wavekngth. Such an approach provides virtual freedom from spectra1 interferences. MASSMANN [17] has shown the possibility of correction error exists if coinciding rotational lines of diatomic molecules show Zceman splitting. However, the likelihood of such an occurrence is extremely rare due to the small spectral shift of the &man components. MASSMANN [ 173 notes that the flame spectral interference he observed involves a line not normally used for AA analyses, and that he was unable to find a case of spectral interference using the graphite furnace. [1] [2] [3] [4] [S] [6] [7] [8] [9] [lo] [l l] [12] [13] [14] [lS] [la] [17]

K. SAEED and Y. THOMASSEN,Anol. Chim. Acto 130,281 (1981). F.

VAJDA, Anal.

Chim. Acra 128, 31 (1981).

F. J. FERNANDU and R. GIooINos, Atom. Specrrosc.3.61 (1982). B. WELZ,U. VOELLKOPF and Z. GROBENSKI,Anal. Chim. Acra 136,201 (1982).

H. KOUUMI,Anal. Chem. SO, 1101 (1978). D. C. MANNING, At. Absorption Newsletr. 17, 107 (1978). M. T. C. DE LOOS-VOLLEBREGTand L. DE GALAN, Specrrochim.Acta 37B, 659 (1982). K. YASUDA, H. KoUuMI, K. OHISHIand T. NODA,Prog. Amafyr.Atom. Specrrosc. 3,299 (1980). F. J. FERNANDEZ, S. A. MYERSand W. StivtN, Anal. Chem. 52, 741 (1980). P. R. LIDDELLand K. G. BRODIE, Anal. Ckm. 52, 1256 (1980). M. T. C. M Loos-VOLLEBREGT and L. DE GALAN,Specrruchim.Acra 33B, 495 (1978). H. KOIZUMI.K. YASIJDAand M. KATAYAMA, Anal. Chem. 49, 1106 (1977). H. KOUUMIand K. YASUDA, Specrrochim.Acra 318,237 (1976). E. PRNZKOWSKA, G. R. CAitNrtlcxand W. SLAVIN, C/in. Chem. 29,477 (1983). G. R. CARNRICK, W. SLAVIN and D. C. MANNING, Anal. Chem. 53, 1866 (1981). E. PRUSZKOWSKA,G. R. CARNRKKand W. SLAVIN,Anal. Chem. 55, 182 (1983). H. MASSMANN, Talanra 29, 1051 (19821. 519

520

Research note

The principal focus of this study was to compare spectral interferences observed in the determination of Se in nickel-base alloys using both continuum source and Zeeman background correction. Investigations were also made on the effectiveness of the stabilized temperature platform furnace [ 181 in reducing chemical interferences associated with this analysis.

EXPERIMENTAL Equipment

Two instrument systems were used during the course of this work. A Perkin-Elmer Model 4000 atomic absorption spectrophotometer equipped with a deuterium arc was used for contmuum source background correction experiments. Zeeman background correction results were obtained using a Perkm-Elmer Zeeman/5000 system. Both instruments were equipped with an HGA R -500 graphite furnace, a Model AS-40 autosampler, and a Se EDL. We have used the stabilized temperature platform furnace (STPF) as described by SLAvth et al. [ 19-221. Peak absorbance and integrated absorbances were calculated using a Perkin-Elmer Data System 10. Absorbance profiles were plotted using a Hewlett-Packard Model 7225 A Graphics Plotter. For the platform studies, solid pyrolytic graphite L’vov Platforms (Perkin-Elmer part number 029@2311) were used m coqunction with Perkin-Elmer grooved pyrolytic coated graphite tubes (part number BOlO9-322). When sampling from the tube wall. Perkin-Elmer pyrolytic coated gnapbite tubes (part number BOO91-504)were used. A sample volume of 20 ~1 was used. Instrument parameters and furnace conditions are given in Table 1. With platform samphng we found a 2-step drying process avoided any sample spattering. Table 1. HGA-500 graphite furnace conditions Sampling from the tube wall Dry

Char

Atomize=

Cool

Clean-out

120 20 10

1200 20 20

2650 0 5

100 1 5

2650 0 4

Dry

Dry

Char

Atomize”

Cool

Clean-out

180 2 5

250 25 10

1200 20 20

2400 0 5

100 1 5

2650 0 5

Temp. (“C) Ramp (s) Hold (s)

Samphng from the platform

Temp. (“C) Ramp (6) Hold (s)

PGas stop used during atomization step.

Procedure The NBS “Tracealloy” SRMs 897,898, and 899 were prepared usmg a HF: NHOJ dissolution procedure. After transferring a l-g sample to a lOO-ml Tdion beaker, 5 ml HzO, 5 ml HF. and 5 ml HNO, were added. After the Initial reaction subs&d the beaker waabeatcd gently and the solution evaporated to moist salts. The salts were then dissolved by adding 20 ml H1O, 2.5 ml HF, and 2.5 ml HNO, followed by gentle heating. The solutton was then diluted to volume in 50-ml T&on Basks. In addition to the Tracealloy SRMs, a seriesof synthetic mckel-base alloy standards spiked with known amounts of Se were prepared. The nominal base compositron of these synthetic standards was 55 “/;,Ni, 18 % Co, 12 “/. Cr, 5 % Al, 4 “/, Ti, and 3 7; MO. This base composition is typical for high temperature nickel-base alloys used in the aerospace industry.

RESJLIJAND DISWSSION Continuum source background correction

High-temperature nickel-base alloys used in aerospace applications typically contain high concentrations of Ni, Co, Cr, Al, Ti, and MO. Preliminary experiments using continuum source background correction indicated significant spectral interference problems when determining Se in a variety of alloys, including the SRMs. To characterize these interferences, individual solutions of the base elements were analysed at both the primary 196.0-111~and secondary 204.~nm Se lines. In addition to the Fe spectral interference reported by MANNING [a]. we observed a negative background corrected signal [18] W. SLAVIN,D. C. MANNINGand G. R. CARNRICK, Afom. Spectrosc. 2, 137 (1981). [ I93 W. SLAVIN,G. R. CARNRICK, D. C. MANNINGand E. PRUSZKOWSKA, Atom.Spectrosc. 4, 69 ( 1983) [ZO] W. SLAVIN,G. R. CARNRKK and D. C. MANNING,Anal. Chem. 54,621 (1982). [2l] D. C. MANNING,W. SLAVINand G. R. CARNRICK, Specrruchim. Acto 37B, 331 (1982). [22] D. C. MANNINGand W. SLAVIN,Appl. Spectrosc. 37, 1 (1983).

521

Research note M/4000

AA-BG

M/4000 8.25

AA-BG

a. 25

Se

196.0nm

Fig. 1. Spectral

Se

I

I

204.0nm

Interference signals obtained using conunuum source background correctton

when analysing Co solutions at the 196.~nm Se line. Similar spectral interference problems from Cr and Ni were observed at the 204.0-nm Se line. Figure 1 shows results obtained using a slit width of 0.7 nm. The data summarized in Table 2 show these spectral interences were evident using slit widths of 0.2, 0.7, and 2.0 nm. The secondary 204.0-nm Se line is approximately 5 times less sensitive than the primary 196.0-nm Se line. Due to the very low Se concentration m nickel-base alloys we did not investigate other secondary Se lines.

Table 2. Absorbance values obtained for mdivtdual base-element solutions Dt correction

Zeeman correctton

0.2

Slit width (nm) 0.7

2.0

20 pg Fe 20 pegco

-0.090 -0.040

-0.104 -0.047

-0.106 -0.047

ZOpgCr 200 pg Ni

- 0.014 - 0.025

- 0.022 -0.041

- 0.024 -0.044

0.2

Sht width tnm) 0.7 20

Se 196.0 nm 0.001 0.000

0.000 0.000

0.000 0.000

0.000 0.000

0.001 0.000

0.000 0.000

Se 204.0 nm

The fact that nearby matrix lines from Fe, Co, Ni, and Cr present spectral interference problems is not surprising considering the very rich lme spectra observed for these elements. Within a + 0.5-nm region about the 196.0-nm Se line there are 15 Fe lines and 9 Co lines present [23]. Near the 204.0-nm Se line there are 4 Ni and 4 Cr lines within a f 0.5-nm interval [24]. These potentially interfering lines are all extremely weakly absorbing and very low in intensity. However, the combined interference effect is significant due to the very high concentrations of these elements in nickel-base alloys. The resultant analytical error introduced is illustrated in Fig. ZA, which shows the Se peaks obtained for SRM 897. &cause the Se peak is superimposed over the negative spectral interference signal. the accuracy of both peak height and particularly peak area measurements is degraded. Values obtained for the SRMs based on sampling from the L’vov Platform and calibration using the synthetic alloy standards are given in Table 3. The synthetic alloy standards have a higher concentration of Ni and Co than the SRMs. The magnitude of the negative spectral interference signal is therefore less with the SRMs, accounting for the very large error in peak area values. Although closer to the certified NBS values the error using peak height values is still considerable. Results obtained sampling from the tube wall were not significantly different from those obtained sampling from the platform.

[23] An Ulwucwlet MulGplrr Table, NBS Circular 448, sec. 2, U.S. Dept. of Commerce (1952). [24] Mm Wovelengrh7hbles. John Wiley, New York (1956).

Research note

B

Rg. 2. Signals obtained for SRM 897 using (A) continuum source background correctton and (B) Zeeman background correction. Data obtained at the 196.0-nm Se line. Table 3. Se values @g/g) determined using commuum source background correctIona

SRM

NBS Vahleb

Peak height

Peak area

897 898 899

9.1 (0.1) 2.00 (0.02) 9.5 (0.1)

8.8 1.9 9.1

16.0 3.2 17.0

‘Calibration using synthetic alloy standards. Walue in parentheses is NBS “estimated uncertamty ” Zeeman

efecr background correction Using Zeeman background correction we observed no spectral interference problems at either the 196.G or 204.0-nm Se lines. Table 2 summarizes results obtained analysing individual solutions of the base elements using slit widths of 0.2,0.7, and 2.0 nm. Figure 2B shows the Se peaks obtained for SRM 89. With the spectral interferer&problem eliminated, the Se peaks are well-defined, permitting accurate quantification. Table 4 gives the values obtained for the SRMs based on sampling from the L’vov Platform and calibration with the synthetic alloy standards. Both the peak height and peak area values are in good agreement with the NBS certified values. Although the synthetic alloy standards permit accurate cahbration, preparation of these standards is timeconsuming. The possibility of using simple aqueous standards containing Ni was therefore investigated. Nickel is widely used as a matrix modifier for Se [25], allowing the use of higher charring

Table 4. Se values (pg/g) determined using Zeeman background correctionL

SRM

NBS Valueb

Peak height

Peak area

897 898 899

9.1 (0.1) 2.00 (0.02) 9.5 (0.1)

9.0 1.9 9.4

9.1 2.0 9.4

aCalibration using synthetic alloys standards. Waiue in parentheses is NBS “estimated uncertainty.”

(251 R. D. EDIGER, Atom. Absorprion Newslett.

14, 127 (1975).

523

Research note

temperatures. We found a maximum charring temperature of 1400°C was possible adding 200 pg Ni as a modifier vs 700°C without any modifier. Because Ni acts as a matrix modifier, direct calibration using simple aqueous standards is not practical. As part of this investigation we also compared results sampling from the tube wail versus sampling from the L’vov Platform. Values obtained for the SRMs using Se standards prepared in 1 “/, Ni are given in Table 5. With the SO-fold sample dilution employed, this concentration approximates the Ni level in typical nickel-base alloys. We observed that Se standards containing Ni yield a narrower, higher peak than alloy samples containing an equivalent Se concentration. As a result the error using peak height measurements is considerable, both sampling from the tube wall and from the platform. Peak area values obtained sampling off the tube wall are closer, but still below the NBS “estimated uncertainty*’ range. SWVIN[ 193 has reported that when all aspects of the STPF concept are employed, chemical interferences are often greatly reduced or eliminated. The STPF concept includes use of the L’vov Platform, integrated absorbance measurements, rapid heating of the graphite tube, and matrix modification. Using STPF conditions, we measured characteristic amounts of 28 and 138 pg/O.O044 A s for the 196.s and 204.0nm. Se lines, respectively. The very good agreement obtained using the L’vov Platform in conjunction with peak area measurements demonstrates the value of the STPF concept eliminating the chemical interferences encountered analysing nickel-base alloy samples.

Table 5. Se values @g/g) determined using Zeeman background corrections

NBS

Sampling from wall

SRM

Valueb

Peak height

Peak area

897 898 899

9.1 (0.1) 2.00 (0.02) 9.5 (0.1)

6.5 1.4 6.9

8.1 I.7 8.7

Samplmg from platform Peak height Peak area 6.9 1.6 7.5

9.1 2.0 9.4

*Calibration using aqueous Se standards contauung I “/, Ni. bValue in parentheses is NBS “estimated uncertainty.”

CONCLUSIONS

Spectral interferences occur when determining Se in nickel-base alloys using continuum source background correction. These spectral interference problems, due to the presence of weak, nearby matrix element lines, cause significant analytical errors at both the 196-O-and 204.@nm Se lines. No spectral interference problems were observed using Zeeman effect background correction applied to the atom source. Utilization of Zeeman background correction in conjunction with the STPF concept permitted the accurate determination of Se in NBS Tracealloy SRMs without the need to prepare carefully matched synthetic ahoy standards. Acknowledgement-We

thank Dr JOHN ZAVODJANCIKof the Pratt & Whitney Aircraft Group. who supplied the synthetic alloy standards and NBS materials.