Spectrochimica Acta, Vol. 368, No. 6, pp. 591 to 594, 1981.Printed irr Great Britain.

oo3 1-6987i 8 U06ose 1-04$02.00/o@ 1981 Pergamon Press Ltd.

RESEARCH NOTES

Spectral interference on the emission of sulphur I 180.73 nmin an inductively coupled plasma

(Receiued 23 December 1980)

INrRooucrroN

SUI-pHuR can be determined by emission spectroscopy in the vacuum ultra-violet region of the emissionspectrum at any of the three lines S I 180.73 nm, S I182.04 nm or S I 182.63 nm arising from the triplet statein the 'P-'Sn transition [1]. Of these lines it has been predicted that the S I emission line 180.73 nm is themost intense [2]. Consequently this is the line of first choice for the determination of S in aqrieous solutionsusing the Inductively Coupled Argon Plasma (ICAP).

This note describes a previously unreported spectral line overlap by Ca and a wing overlap by Mn on the S

I emission line at 180.73 nm, which was noticed during optimization procedures for S determinations in amulti-element standard.

INsrRuvmurATIoN

This study was undertaken on an Applied Research Laboratories 34000 inductively coupled argon plasmaemission spectrometer. The polychromator has a 1m Paschen-Runge mount, with a Bausch and Lombgrating (1080 lines/mm), equipped with a movable primary slit operated under computer control enabling theirnmediate vicinity of any line to be scanned for possible spectral interferences. This direct-readingspectrometer uses a PDP 1,U03 mini-computer with twin floppy disks for data acquisition, data treatment andinstrument control. A standard ARL torch of the Fassel design [3] was used in conjunction with a concentricglass nebulizer (Meinhard type) equipped with a tip washer. The spectrometer was operated under a partialvacuum (10-3 torr) pumped against an argon gas atmosphere. Conditions of analysis were not optimized for S

but are compromise conditions for simultaneous multi-element analysis. Conditions of analysis were:

RF PowerTorch viewing height

(unless otherwise indicated)nebulizer uptake rate

Ar flow ratesnebulizerplasmacoolant

1.6 kW18 mm above the coil

(*2 mm observation zone)2.3mlmin 1

1.51min-11 l min-1

11 I min-l

RESULTS

Scans from 180.69nm to 180.785nm in the region of the S line at 180.73nm were made at variousobservation heights of the plasma while aqueous solutions of aluminium, calcium, iron and manganese wereaspirated. All standard solutions were made from spectrally pure metals as the chlorides. Intensity counts forten seconds at each position were made at increments of 0.00225 nm for each scan. Scans are plotted in Fig. 1

as intensity against wavelength.Significant spectral overlap by the Ca II 180.74 nm emission line and a Mn line at 180.747 nm on the S line

is evident from Fig. 1. There is no overlap from the weak Fe line at 180.78 nm although an overallenhancement of the background from Fe emission was apparent when high concentrations of Fe wereaspirated into the plasma.

LansoN and Fasspt- [4] previously reported the presence of an Al continuum in the plasma produced bythe recombination of a free electron with an Al ion to yield an Al ground-state atom. This is also seen in Fig.1. Over the wavelength of interest very little fine structure from the Al continuum is seen. However, at high

[1] G. F. Krmenrcur, A. E. Wlno and T. S. Wesr Anal. Chim. Acta.62,24L (1972).

[2] W.L. Wrese, M. W. Stnrns and B. M. Mr-Bs AtomicTransitionProbablities. NSRDS-NBSZ2, Vol II.NBS Washington (L969).

[3] V. A. Fasssl and R. N. KNrselBy, Anal. Chem.46, 1155A, (1974).

[4] G. F. LensoN and V. A. Fesser Appl. Spec.33, 6, 592 (1979).

591

592 Research notes

180.70 180.72 180.74 180.76 180.78

Wavelength (nm)

Fig. 1. wavelength proflles of Al (4000 pgml t), ca (500 p.gml-1), Mn (500 pgml-1), and Fe(5000 pg ml-l), about the S I 180.73 nm emission line. For the sake of clarity the S emissionprofile has been omitted from the figure. On the same scale however a2.5 pt,gml-l solution of Saspirated into the plasma gave an intensity peak of 26mY and a full width at half maximum of0.0075 nm. The centre profile position of S I 180.73 nm is shown as a vertical dotted line. Allsolutions were prepared from high-purity chemicals made up from the respective chloridesdissolved in distilled deionised water. Background emission scans were obtained from scanning

the spectrum while aspirating a water blank.

molar ratios of Al/S, the contribution of the Al continuum to the S intensity reading could well be analyticallysignificant.

What appears to be a weak Mn line at L80.72 nm superimposed on the background flne structure rs

unlikely to be caused by line reflection, as no intense lines to promote this effect were seen in the vicinitywhen scans were made from 180.69 nm to 180.785 nm.

The major Mn line at 1.80.745 nm thus appears to be flanked by two weak Mn emission lines at 180.72 nmand 1"80.77 nm. However only the line at L80.72 overlaps the S significantly and the effect of this line iseclipsed by the larger Mn line.

In Fig. 2 the line to background ratios for Ca, Mn and S are plotted against the height of observation in theplasma at a constant power setting.

It can be seen that Mn and Ca II give maximum emission higher in the plasma than does S. Hence thespectral interferences of Ca and Mn may be reduced slightly by observing the S emission at a lowerobservation height in the plasma.

Several other elements of analytical interest, including Cr, Cu, Ni and Se werc also aspirated into theplasma while the emission spectrum was scanned in the region of the S I emission line. No emission lines ofany consequence were seen even when concentrations as high as 1000 u,gml-1 were used. The Se I

12 20 2Aobservation height above coil (mm)

Fig.2. Line to background ratios of the emission intensity versus observation height above theRF coil in the plasma for aqueous solutions of ca (500 pg ml-t), Mn (500 pg ml-1), and S(5 g,gml-1), at the S I 180.73nm emission line. Average detection limit for S I 180.73nm at

the 1,4 nm observation height was 0.02 g.g ml-1.

36

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-Research notes

Table L. Sensitivity ratios and critical concentrationratios for A1, Ca, Fe and Mn at the profile position for

S I L80.73 nm

593

InterferentSensitivity ratio

interferent/analyte CCR

CaMnAIFe

11.1 x 10-33.3 x 10-3

0.30 x 10 3

0.04 x 10-3

90303

33302-5000

180J4 nm emission line reported to be present in the arc was not detected at all in this region of the plasma

tsl.For practical analysis the contribution of the interferent to the S emission signal is the most important

parameter to be determined. If a linear relationship between apparent analyte concentration and interferentconcentration can be established, correction factors for each interferent can be calculated and applied to theanalyte emission signal at each analysis.

Wing overlaps of spectral lines often produce non-linear curves when apparent analyte concentrations areplotted against interferent concentrations. Consequently for a series of interferent concentrations in solution,intensities were measured at the S I profile position and apparent S concentrations were obtained from the Sstandard calibration curve.

Over four orders of magnitude of concentration all the interferents studied (Al, Ca, Fe and Mn) gave a

linear relationship between apparent S concentration and interferent concentration. Table 1 gives the ratiosof the sensitivities of interferent to analyte signals in the spectral window of the S line.

BouuaNs [6,7] proposed the critical concentration ratio as a convenient criterion for deciding when a

correction of an analyte signal for spectral overlap is required and when such a correction is still feasible. Thecritical concentration ratio (CCR) is defined as the ratio of the concentrations of the interferent to analyte atwhich the ratio of the intensities of the interfering line and the analysis line within the spectral window of theanalysis line is equal to unity. If the actual concentration is substantially larger than the CCR, a correction isvalueless, because the result becomes highly uncertain. If the actual concentration is essentially smaller than theCCR, the interference can be neglected and a correction omitted.

The interferent concentration level which produces an intensity signal equivalent to the least quantitativedeterminable amount of analyte (e.g. five times the'2o detection limit) can be obtained in this case bymultiplying the CCR value by 0.L. The CCR thus obtained is then the maximum acceptable concentration ofthe interferent which can be tolerated in the matrix of the sample before correction at the limit ofquantitative determination becomes cumbersome.

According to BouuaNs, the definition of the CCR (see in particular equation 4 in ref t6]) is the reciprocalof the ratio of the sensitivities of the interferent to analyte signals in the spectral window of the analysis lines.These values are also listed in Table L. Concentration corrections may be calculated for A1, Ca, Fe and Mninterference on S from either the sensitivity ratios or the CCRs and the known concentrations of theinterferents (see e.g. ref. [8]).

KrRreRrcHr et al. LLl reported significant interferences in the plasma on S I (180.73 nm) from Ni, Mn andFe at 50-fold concentration ratios. Although there is a Ni II line at 180.45 nm, this is well outside thewavelength range for S interference. The higher resolution of the spectrometer used in the present studycould account for the different result for Ni. The linear reciprocal dispersion (LRD) of this spectrometer is

b.gt nm/mm with a bandpass of 0.02 nm compared with an LRD of 3.3 nm/mm for the monochromatordescribed by KmrnnrcrT et al. Although Krxrsnrc:ur et al. reported quantitative analytical values for Mnand Fe interferences on S, the spectral character of the interferences was not described. The rather largerinterference effect they reported for Fe when compared with this study could be due to their observing a

contribution to the apparent S concentration from the Fe line which we observed at 180.78 nm. This couldagain be a consequence of using a relatively low-resolution monochromator not specifically designed toovercome the problems of stray light and residual far scatter which can be particularly significant in ICAPemission.

It would appear that insufficient energy is generated in the analytical region of the plasma viewed in thisstudy to promote the emission of the Ca III line reported to be present in the Ca spectrum from the arc [5].

[5] A.N. Zxroer-, V. K. PnoroFJEV, S. M. Rersrrr, V. A. Sle.vuvt and E. A. Snn-eroeR, Tables of SpectralLines. IFI/Plenum, New York (1970).

t6] P. W. J. M. BourtaNs, Spectrochim. Acta 358, 57 (1980).

[7] P. W. J. M. BouuaNs, Line Coincidence Tables for Inductitsely Coupled Plasma Atomic EmissionSpectrometry. Pergamon Press, Oxford (1980).

[8] P. W. J. M. BouuRNs, Spectrochim. Acta 3lB, t47 (1976).

594 Research notes

StiN,lM,lRY

Spectral interferences on the S I 180.73 nm emission line by Al, Ca, I\[n and Fe in an inductively coupledplasma are described. The critical concentration ratios for Ca, Mn, Al and Fe with respect to S are 90, 300,

3300 and 25000 respectively (full width at half maximum:0.0075 nm). Intensity measurements at various

observation zones in the plasma show that the spectral interference from Ca and Mn emission linp overlaps

may be reduced by observing the S emission at a lower observation height in the plasma.

Applied Biochemistry and J. T.ne

Grasslands Dirsisions M. W. PxrrcsanoD.S.r.R.Prhsate BagPalmerston NonhNew Zealand