Anal. Chem. 1988, 60,2505-2508
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Coincidence Profiles for Phosphorus Emission at 178.287 nm Observed in the Third Order by Inductively Coupled Plasma Emission Spectrometry Khudre M. Attar Central Analytical a n d Materials Characterization Laboratories, Research Institute, King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
Spectral Interference profiles for 11 concomitants on phosphorus emission at 178.287 nm observed In the third order were acqulred by scannlng the phosphorus channel of a simultaneous vacuum argon lnductlvely coupled plasma emission spectrometer, using the polychromator prlmary slit. Vacuum ultraviolet (vacuum UV) as well as UV emission lines were observed, despite the fact that an Interference filter with less than 2 % transmission above 250 nm was located before the channel photomultiplier. Spectral interference from Si (1000 mg/L) was attributed to a vacuum UV emission line at 178.32 nm, from Mn (200 mg/L) and Cr (200 mg/L) to UV lines at 267.42 and 267.36 nm, respectlvely, observed in the second order. Fe (1000 mg/L), AI (1000 mg/L), and Mg (1000 mg/L) exhibited background enhancement. V (200 mg/L), Ti (200 mg/L), NI (200 mg/L), Cu (200 mg/L), and Ca (1000 mg/L) dld not interfere wlth titanlum and nickel showing vacuum UV emlsslon lines at 178.26 and 178.34 nm, respectively, that may wing-overlap the phosphorus line at higher concentratlons. A simple technlque for distinguishing vacuum UV from UV llnes by using the spectrometer argon purge nozzle of a vacuum Inductively coupled plasma atomic emission as an alr optical filter was demonstrated. The technlque Is useful for facilltles where a scannlng monochromator Is not available.
The most sensitive lines for phosphorus determination by air-path inductively coupled plasma emission spectrometry are the ultraviolet (UV) lines a t 213.62 and 214.91 nm (1). Spectral interferences on these lines have been documented (2). Copper, chromium, iron, vanadium, and titanium can interfere a t 213.62 nm, whereas copper, iron, aluminum, and vanadium interfere a t 214.91 nm. Interferences can be overcome by appropriate interelement corrections, but such corrected measurements are less accurate than those made at spectrally clean lines. They can, also, be avoided by indirect methods such as complexing the phosphorus with a molybdenum-containing ligand and analyzing for molybdenum, but strict adherence to sample preparation procedures is necessary ( 3 ) . The vacuum ultraviolet emission lines (vacuum UV) of phosphorus a t 178.29,177.50, and 178.77 nm have also been investigated. The line a t 178.29 nm was found to be free of interferences from elements normally found in steel matrices (4).
The objective of this study is to investigate the interferences of a wider range of concomitant elements, on the most sensitive vacuum UV line of phosphorus (i.e. 178.29 nm), and provide coincidence profiles for the phosphorus line, with the profiles of 11prevalent elements, superimposed. Interference profiles are necessary for the appropriate selection of correction procedure. For example, a minor line overlap can be avoided by dilution rather than spectral correction provided
that the dilution reduces the spectral interference below the detection limit of the interferent without considerably sacrificing analyte signal-to-noise ratio; for wing overlaps, background correction may be adequate. The vacuum argon inductively coupled plasma emission spectrometer used for this study wm equipped for both sequential and simultaneous analyses with the phosphorus channel located a t 178.287 nm, in the third order. The task of identifying interferent lines is augmented when one observes vacuum UV lines in the third order, because UV lines in the second order may overlap the analytical line. Manufacturers minimize such problems by placing a UV interference filter before the channel photomultiplier. Intense UV lines can pass through, if the filter had some residual transmission to radiation above 200 nm. This situation was encountered in the present study but resolved by the use of the argon purge nozzle as an air optical filter to distinguish vacuum UV from UV lines. The results are presented along with the interference profiles.
EXPERIMENTAL SECTION Apparatus. The argon inductively coupled plasma emission spectrometer used for recording the scans was an Applied Research Laboratory Model 3580 vacuum spectrometer. It was equipped with a polychromator scanning accessory that allowed automatic scanning of a selected channel through stepper motor control of the primary slit drive with a range of 0.6 nm in 1600 motor steps. It was also fitted with a nozzle (snout),purged with argon to allow observation of emissions in the vacuum ultraviolet region. The interference filter located before the phosphorus channel photomultiplier was type 180B manufactured by A R C with maximum transmission of 38% at 184.8nm. The specification and operating conditions employed are listed in Table I. The system software allowed retrieval, scaling, identification, and subtractions t o be performed on a maximum of 4 scans per channel at a time. Spectral Scan Parameters. Spectral scans were performed by using the polychromator assembly of the spectrometer, in a range of 0.3 nm centered at the fixed phosphorus channel (178.287 nm) (third order). This was possible by first profiling the channel with a standard (10 mg/L) solution of phosphorus and setting the rest position of the scanning accessory at the peak, before scanning the test solutions. Scanning parameters were as follows: scan start position (steps before peak), 400 (0.15 nm); scan end position (steps after peak), 400 (0.15 nm); scan increment (steps), 25 (0.0094 nm); integration time (per step), 2 s; total rinse and flush time, 20 s. An additional 3-min interval was allowed between spectral scans for flushing, cleaning, and drainage of the nebulizerspray chamber assembly with deionized water. Blank scans were conducted at regular intervals to observe memory effects. None was observed for any of the test solutions. Test Solutions. Eleven single-element standard solutions selected for the interference study were prepared from NBS spectrometric solutions. The element concentrations were as follows: vanadium (200 mg/L), titanium (200 mg/L), nickel (200 mg/L), copper (200 mg/L), chromium (200 mg/L), manganese (200 mg/L), iron (1000 mg/L), aluminum (1000 mg/L), calcium (1000 mg/L), and silicon (1000 mg/L). Solutions were prepared in glassware that has been soaked overnight in 10% HCl, 10%
0 1988 American Chemical Society 0003-2700/88/0360-2505$01.50/0
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table I. Experimental Facility instrument polychromator
vacuum snout argon flow scanning facility plasma rf generator torch observation height incident power reflected power coolant Ar gas flow plasma Ar gas flow sample delivery system peristaltic pump nebulizer aerosol carrier flow sample uptake rate data acquisition system software
Applied Research Laboratories 3580 ICP spectrometer, vacuum version, equipped with a monochromator and a simultaneous polychromator for 48 channels 1-m focal length in a paschen-runge mounting, 1080 grooves/mm holographic grating with a useful range of 170-270, 170-410, and 340-820 nm in the 3rd, 2nd, and 1st orders, respectively; reciprocal linear dispersion of 0.31 nm/mm and band-pass of 0.006 nm in the 3rd order; 20-pm primary slit and 37.5- or 50-pm secondary slits; fitted with 48 phototubes and filters maintained at 27.3 pmHg with argon 1.48 L/min scanning accessory for multielement instrumentation (SAMI); stepper motor control of the primary slit drive permits automatic scanning of selected channels; 0.004 &'step as supplied with ARL 3580 ICP-AES, operating at 27.12 MHz quartz with a 3-turn load coil 15 mm above the load coil 1200 w 5w 12.8 L/min 0.8 L/min Ismatec Mini4840 variable flow Meinhard type, concentric all glass 0.8 L/min 2 mL/min digital Equipment Corp. PDP 11/23+ computer with 256K memory, Winchester/floppy storage system, VID 240 graphics display unit, LA50 printer SAS/DPS-11 System automation software with GRAPHICS option
I
0.43
178.133
WAVELENGTH
178.4 4 1
178 133
WAVELENGTH
178 441
0 I2O
F
043 178.133
WAVELENGTH
.L
_ -,
178.441
Figure 1. Coincidence profiles of test solutions on the phosphorus channel: A, P(-), V (.-), Mg (--), AI (---), Fe (---): D, P(--), Si (--), Ca (-. -), blank (---).
Cr (---); C,P(-),
"OB, and deionized water, respectively. The analyte phosphorus standard was 1mg/L. All test solutions were prepared to contain (1%HC1 + 1% "OB) deionized distilled water (v/v) matrix, except for titanium, which contained (1.6% HCl + 0.4% "OB). The acids used were Baker Instra-Analyzed reagents. The potential interferent solutions are labeled throughout the paper by the element symbol followed by the concentration value in parentheses.
RESULTS AND DISCUSSION Interference Profiles. Interference profiles of the 11 test elements are presented in Figure 1. They are divided into four parts A, B, C, and D. Each includes the 1 mg/L phosphorus scan with the scans of three elements superimposed.
Ti (-. -), Ni (- - -); B, P(-),
.
Mn (.....), CU (- -),
The fourth scan D includes a blank scan. The observed lines are numbered for identification purposes. Some distorted line shapes and sliced peaks resulted from the large scan increment per step (0.0094 nm). Employment of a smaller scan increment was not warranted because of the expensive NBS materials used. Distinguishing Vacuum UV from UV Lines Using the Snout. Only vacuum UV lines should have been observed, with the interference filter located before the phosphorus channel photomultiplier. But the filter was found to have some residual transmission to radiation above 200 nm. The percent transmission for the filter a t 246, 303, and 351 n m was determined to be 1.87%,0.45%,and 0.27 %, respectively.
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
0.30
t
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TITANIUM
4
-
10
1
''1
000
- /
178 133
'
n
-. A & , ,,
WAVELENGTH
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178 LL1
I
178 133
WAVELENGTH
178 441 1
I 1
NICKEL
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1
I
'
o'80
f
IRON
0.00 178.133
WAVELENGTH
178,441
178.133
WAVELENGTH
178,441
WAVELENGTH
178.441
$t I-
z
178.133
WAVELENGTH
178.441
178.133
Figure 2. Profiles of test solutions showina the vacuum UV lines that were absorbed by air in the optical path: profiles with argon in the optical path (-*-); profiles with air in the optical path (-).
Intense emission lines in the region (267.20-267.66 nm) will be expected to appear in the scans, in their second order, because they will fall at the range investigated (178.133 X 3 to 178.441 X 3). This occurred and complicated the element wavelength identification. Order sorting, before line identification, became a necessity. The problem was resolved by disconnecting the argon supply to the purge nozzle and allowing air to fill it, at ambient condition. The element scans were repeated with air in the optical path. The intensity of the base line was decreased by 30.9%. The reduction in the intensity of the UV lines above 200 nm was in the range 33% to 57 70,probably due to the reduction in the plasma plume height when the argon purge was cut off, whereas vacuum UV lines below 200 nm were dramatically diminished and some emission lines completely disappeared, because of absorption by water vapor and oxygen. The percent reduction in the net intensities of the observed lines is presented in Table 11. Those lines for which the reduction in intensity was greater than 80% were considered as the vacuum UV emission lines below 200 nm. Scans that contained vacuum UV lines with argon in the optical path are compared to the same with air in the optical path in Figure 2, after blank (with argon in the purge nozzle) and blank (with air in the purge nozzle) subtractions. A scan of 200 mg/L chromium is also shown for comparison purposes. Confirmation of the lines above 200 nm for the elements that showed no vacuum UV lines was done by scanning the sequential spectrometer in the range 267.200-267.662 nm in the first order, using the same scan parameters. Line Assignments and Interferences. The use of the air snout for distinguishing UV from vacuum UV lines can be appreciated when one realizes the fact that line 17, for instance, would have been mistaken for the iron line at 267.22
Table 11. Reduction in Net Intensity of Lines due t o Airo element blank P (1mg/L)
v (200)
Ti (200) Ni (200) Fe (1000) Si (1000)
phosphorus channel % reduction
line no.*
178.287 nm (3rd order) 3 4
5 6 7 17
18 19
all other lines
30.9 100.0 100.0 89.3 100.0 97.6 85.3 96.0 97.9 95.5 33-57
a Obtained after blank subtraction. *Denotes emission line numbers from Figure 1.
nm observed in the second order, and separation can only be achieved by a sequential monochromator. The listing of observed emission lines and spectral interferences from the 11 test solutions are presented in Table 111. The following lines (3, 4, 5, 6, 7, 17, and 19) were not found in the available references. All UV lines were confirmed from the MIT wavelength tables (5). Other vacuum UV lines were found in ref 6. The low intensity shoulder on wing of line number 18 for silicon may be attributed to either a phosphorus impurity in the NBS standard or contamination of the used solution. The types of spectral interferences are described in the table and appropriate corrective action is recommended. It should be borne in mind that only spectral interference, not chemical, is considered in this study. Same order (third order) spectral interference was observed only for Si (1000
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table 111. Line Assignments and Interferences on the Phosphorus Channel (Detection Limit (DL)O = 0.029 mg/L) test element (mg/L) V (200)
spectral line, nm (order)
(3)
V I1 267.28 (2nd) V I1 267.40 (2nd) V 178.22 (3rd)
Ti (200)
(4)
Ti 178.26 (3rd)
Ni (200)
(5)
Ni 178.15 (3rd) Ni 178.34 (3rd) Ni 178.39 (3rd) Mn I1 267.34 (2nd) Mn I1 267.42 (2nd) Mn I1 267.52 (2nd) Mn 267.55 (2nd) Mn I1 267.58 (2nd)
(1)' (2)
Mn (200)
c u (200) Cr (200)
Cr Cr Cr Cr
I1 267.26 (2nd) I1 267.32 (2nd) I1 267.36 (2nd) I1 267.65 (2nd)
Mg (1000) Fe 178.15 (3rd)
Fe (1000) A1 (1000)
Si 1178.32 (3rd) Si 178.42 (3rd)
Si (1000) Ca (1000)
type of interference
apparentbconcn
slight wing overlap from 267.40 (2nd) there might be wing overlap no interference
