Quality assurance in operating a multielement ICP emission spectrometer

0584-8547184S3.00+ .W ‘0 1984.Pergamon Press Ltd.

SpecrrochimicoArm. Vol. 398, No. 1. pp. 95-113, 1984. Printed in Great Britain.

Quality assurance in operating a multielement

ICP emission spectrometer*

ROBERT I. BOJTO AnalyticalResearchLaboratory,Exxon Research and Engineering Company, P. 0. Box 4255, Baytown, TX 77522,

U.S.A. (Received 4 March 1983; in revised form 16 May 1983)

Abstract-In the industrial laboratory environment, quality assurance in the operation of a multielement inductively coupled plasma emission spectrometer (ICPES) must often be entrusted to laboratory technicians with little or no technical background in spectrochemical analysis. Therefore, to be successful, a quality assurance program must be reduced to a simple, routine practice. Essential components of the quality assurance program described in this paper are (1) An atom-to-ion emission intensity ratio for multielement optimization and for reproducing optimum analysis conditions. (2) A concise, easily applied specification for sensitivity and for precision. (3) A regimen for monitoring of, and correcting for, calibration and background drift. (4) A set of comprehensive spectral interference calibrations maintained using the emission intensity ratio. (5) A high resolution spectrometer for minimizing spectral interferences. (6) A program of long term performance monitoring and maintenance/record keeping. Each of these components is described in detail. Adherence to this program enhances analytical reliability by helping to ensure that raw concentrations are generated consistently under optimum instrumental conditions, and that corrections for spectral interferences are applied accurately even though interference calibrations may be several months old. The importance of adequate resolution and the proper choice of positions for off-line background measurements is borne out by a detailed study of the determination of toxic trace elements in National Bureau of Standards fly ash samples. As, Be, Cd, Pb, Sb, and Se were determined accurately without isolation/preconcentration from the aluminosilicate matrix. Several determinations required corrections for residual spectral interferences amounting to 100-500~ of the resultant concentration, underscoring the accuracy of the interference correction procedures.

1.

INTRODUCTION

QUALITY assurance in the day-to-day operation of a multielement inductively coupled plasma emission spectrometer (ICPES) must often be entrusted to persons with little or no technical background in spectrochemical analysis. This is particularly true in an industrial environment where capable technicians operate the analytical instruments while technically trained personnel supervise the laboratories. To be implemented successfully, a quality assurance program for ICPES operation must be reduced as much as possible to a simple, routine practice. Quality assurance protocols should be incorporated into the routine ICPES operating procedures so that they are regarded as essential and are followed rigorously. Simple, convenient operations are less likely to invite short-cutting. Frequent, effective communication between ICPES operators and their supervisor is an important part of this quality assurance program. Good record keeping is an essential part of any quality assurance program [l]. Concise, easily retrievable records enhance the quality of the information conveyed, serve to ensure that the procedures were performed properly, and document irregularities for future reference. Quality assurance in operating a multielement ICPES may be achieved through: (1) Careful optimization of instrumental analysis conditions using an atom-to-ion intensity ratio as a reference.

*Extension of a paper presented at the “Symposium on inductively-coupled plasma spectroscopy: quality assurance” during the Eastern Analytical Symposium, New York, November 18, 1982. Guest-editor: RAMON M. BARNES.

[l] J. K.

TAYLOR,

Anal.

Chem. 53, 1588A (1981).

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96

Monitoring and correcting for calibration and background drift. (3) Comprehensive spectral interference corrections applicable to a wide variety of sample types. (4) Maintenance of accurate interference calibrations and optimum analysis conditions using the intensity ratio calibration technique. (5) Long term performance monitoring and spectrometer maintenance. Although the procedures described herein have evolved significantly during the past six years, the intensity ratio calibration technique has been in use throughout this period. The objective of this quality assurance program is to ensure that ICPES data are generated consistently under optimum conditions and that corrections applied to these data are always valid. As an important benefit, this regimen provides an early warning of instrumental problems and aids in troubleshooting. (2)

2. INSTRUMENTATIONAND PROCEDURES 2.1. Instrumentation The dual spectrometer ICPES system consists of a single ICP source viewed by a 0.75-m Paschen-Runge spectrometer and a 0.75-m echelle spectrometer. The spectrometers are operated simultaneously, and raw concentration data are transmitted on a Hewlett-Packard HP-1000 computer for corrections/report generation. This ICPES system has been described in detail in a previous paper [2]. For convenience, information relevant to the present discussion appears in Tables 1 and 2. Exit slits for the echelle lines listed in Table 2 are installed in a multielement cassette. The plasma is imaged onto the entrance slit of the echelle spectrometer by means of a plane mirror used in conjunction with a concave mirror to accomplish a 5 : 1 image demagnification. The concave mirror is fitted with micrometer adjustments for precise X-Y positioning. The interior light path of the echelle spectrometer is purged with nitrogen to increase the sensitivity of the As 193.696 nm line. The wavelengths used for off-line background correction are shown in Table 2. Where no entries appear, no off-line measurements are made. Background correction for the

Table 1. Dual spectrometer ICPES system ICP source/sample introduction Equipment Rf generator: Plasma-therm model HFS 2000D Plasma torch: Jarrell-Ash #/19O4OO54 Nebulizer: Jarrell-Ash variable cross flow $9&974 Ar flow control: Tylan model FC 260 (aerosol carrier) and FC-202 (coolant)

Spectrometers Paschen-Runge Model Resolution Entrance slit width Entrance slit height Exit slit width Background correction

Jarrell-Ash AtomComp 750 0.035 nm (first order) 25 pm 10mm 50 pm 0.04 nm on high wavelength side of peak

[Z] R. I. Bono, Spectrochim.Acta 38B, 129 (1983).

Typical operating conditions Forward rf power: 1200 W Reflected rf power: < 5 W Coolant Ar flow rate: 18 Imin-’ Aerosol carrier flow rate: 600 ml min-’ Auxiliary flow rate: 0 1min-’ Sample uptake rate: 1.2 ml min-’

Echelle SpectraMetrics (Beckman) SpectraSpan IIIB O.OO3-0.01nm 1OOpm 0.3 mm 50pm, 100~ Either side of line, variable distance from peak

97

Quality assurance in operating a multielement ICPES Table 2. Analytical wavelengths (nm)

Element

Ag Al As B Ba Be Ca Cd co Cr cu Fe K Li Mg Mn MO Na Ni P Pbe Pt Sb Se Si Sn Sr Ti Tl U V W 2%

Peak

Paschen-Rungea Background or alternate

(I) 338.289 (I) 308.215 (I) 193.696 x 2 (I) 249.773 (II) 455.403 (I) 234.861 (II) 393.366 (II) 214.438 (II) 228.616 (I) 357.869 (I) 324.754 (II) 259.940 (I) 766.490 (I) 670.784 (II) 279.553 (II) 257.610 (II) 202.030 (I) 588.995 (I) 341.476 (I) 214.914 x 2 (II) 220.353 (I) 265.945 (I) 231.147 x 2 (I) 196.026 (I) 288.158 (II) 189.980 (II) 407.771 (II) 334.941 (I) 377.572 (II) 367.007 (II) 292.402 (II) 207.911 x 2 (II) 206.200

Peakd

Echelle Background

338.329

(I) 237.324 x 2b 193.716 x 2 249.813 234:901 (II) 315.887b 214.478 228.656 357.909

(I) 193.696 (100)

193.721

(I) 234.861 (50)

234.890

(II) 214.438 (50)

214.465

(II) 202.030 (100)

202.002

(II) 231.604 (100)

231.633

(II) 220.353 (100)

‘220.323

(I) 206.833 (100) (I) 196.026 (100)

206.805 195.999

(II) 32;.77Sb c (I) 383;31 b 202.co70 (I) 330.298b 341.516 214.934 x 2 220.393 265.985 196.co66 (I) 298.765b 190.020 c 377.c612 367.047 292.442 207,931 x 2 206.240

(I) 377572 (50) (II) 409.014 (50)

371.520 408.958

“Lines observed in the second order are indicated by x 2, line spectrum designations are in parentheses. bLess intense line for higher concentrations, no background correction. CNo background correction employed. dExit slit widths (,umfare given in parentheses, roman numerals are line spectrum designations. cThe Paschen-Runge spectrometer also uses Pb (I) 283.306 nm with background correction at 283.346 nm.

Paschen-Runge spectrometer is accomplished using a two-position rotating refractor plate behind the entrance slit that allows off-line measurements to be made on the high wavelength side of the peak [3]. The echelle spectrometer employs a 33-position refractor plate permitting off-line m~surements on either or both sides of the analytical line, at variable distances from the peak [2]. For multielement data acquisition, the same high and low wavelength positions of the refractor plate must be applied to all elements, however. Water-saturated argon is used for aerosol generation to minimize nebulizer fouling and associated calibration drift due to salt deposition at the tips of the cross-flow nebulizer needles [4,5]. Venturi aspiration into the nebulizer is usually chosen for particulate-free aqueous samples. [3] G. F. LARSON,R. T. GOODPASTURE and R. W. MORROW,in Applications ofPlasma Emission Spectrochemistry,

Ed. R. M. BARNES.Heyden,Philadelphia (1979). [4] R. I. Bono, in Developments in Atomic Plasma Spectrochemical Analysis, Ed. R. M. BARNES,p. 506. Heyden, Philadelphia (1981). [S] J.-O. BURMAN,in Applications ofPlasma Emission Spectrochemistry, Ed. R. M. BARNES.Heyden, Philadelphia (1979). SA(B) 39:1-G

98 2.2. reality

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assurance procedures

2.2.1. ~uit~ele~nt opti~izatjo~. M~tielement optimization for the Pa~hen-Rungs spectrometer is accomplished using the intensity ratio of Cu (I) 324.754 nm/Mn (II) 257.610nm as a reference [6]. During the optimization procedure, rf power, plasma observation height, and Ar coolant flow rate are held constant while the aerosol carrier flow rate is varied. Detection limits and other figures of merit are determined for all elements in the spectrometer array as a function of the Cu/Mn intensity ratio using a 1 ,ug/ml solution of both metals in dilute acid as a reference. A mass flow controller on the aerosol carrier stream facilitates accurate measurements of the Cu/Mn intensity ratio by maintaining the flow rate to within a few tenths of one percent [7]. Upon selecting the optimum value for the Cu/Mn intensity ratio, the cross-ffow nebulizer needles are adjusted to maximize the Cu (I) 324.754 nm and Mn (II) 257.610 nm intensities without altering their ratio. This tuning exercise usually requires further adjustments to the aerosol carrier flow rate. When the optimization process is complete, a final set of detection limits is acquired and replicate measurements of the Cu/Mn intensity ratio are made to determine a two-sigma precision tolerance specification for this intensity ratio. Finally a minimum intensity specification for Cu (I) 324.754 nm is set. The value selected is approximately 70% of the highest intensity obtainable by fine tuning the nebulizer. Multielement optimization for the Paschen-Runge spectrometer with the selection of a value for the Cu/Mn intensity ratio is always followed by interference calibration for both spectrometers (2.2.4). 2.2.2. Reproducing optimum conditions. Daily preparation for the analysis of samples or for interference calibration involves the following operations: Paschen-Runge

spectrometer

1. Optical alignment 2. Cu/Mn intensity ratio adjustment 3. Analytical calibration

Echelle spectrometer 1. Optical alignment/multielement optimization using MO 2. Analytical calibration

Optical alignment within the Paschen-Runge spectrometer is maintained using a mercury vapor lamp. The plasma torch is aligned with the spectrometer entrance slit by maximizing the signal from Cu while aspirating a dilute Cu solution and traversing the plasma image across the slit. The height of the emission zone above the load coil (16 mm) is never altered. Resetting the Cu/Mn intensity ratio at the preselected value serves to reproduce optimum analysis conditions for the Paschen-Runge spectrometer. At least three consecutive measurements of the Cu/Mn intensity ratio must fall within the predetermined tolerance range and meet the minimum Cu intensity specification, otherwise the ZCPES cannot be operated. Failure to obtain the proper Cu/Mn intensity ratio or meet the precision and/or intensity specifications warns the operator to check the operation of the sample introduction system and the optical alignment of the ICP source and Paschen-Runge spectrometer. The Cu/Mn intensity ratio optimization procedure is repeated after the problem is located and corrected. The echelle spectrometer is optimized by maximizing the signal obtained from MO (II) 202.030 nm while aspirating a lOpg/ml MO solution. The multielement cassette is aligned first, using the MO charmel as an ali~ment reference. The plasma image is then positioned on the entrance slit of the echelle spectrometer to obtain the maximum MO intensity. The vertical and horizontal adjustments are made using the Pitch and Yaw micrometers on the concave mirror. Analytical calibration of both spectrometers follows using the aqueous reference solutions [6] R. I. Burro, in Developments in Atomic Spectrochemical Analysis, Ed. R. M. BARNES,p. 141. Heyden, Philadelphia (1981). [7] G. F. LARSON,R. T. GOODPASTURE and R. W. MORROW,in Developments inAtomic Plasma Spectrochemical Analysis, Ed. R. M. BARNES,Heyden, Philadelphia (1981).

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a multielementICPES

99

described in Table 3. A daily log is maintained containing: (1) Operator’s initials. (2) Mercury profile setting (Paschen-Runge alignment). (3) Paschen-Runge spectrometer “A” frame temperature. (4) Mass flow controller setting at proper Cu/Mn intensity ratio. (5) Cu and Mn intensities. (6) MO intensity (echelle). (7) Problems encountered/maintenance performed. Items 2-5 are normally checked twice daily and double entries are logged. 2.2.3. Calibration, background drijt. Samples and reagent blanks are analysed following the sequence: blank, sample, sample, blank, sample, sample, blank, etc. Reagent blanks from specific sample preparations are substituted for the calibration blank as required. Calibration standards, samples, and blanks are contained in polyethylene bottles of equal size filled to approximately the same depth to avoid differences in Venturi aspiration rate. One or more calibration standards are analysed by both spectrometers after every eight to twelve samples to check for calibration drift. Alternately, a “check standard” containing lower concentrations of all of the elements in the analytical program may be employed [7]. If one or more elements have drifted more than f 3 % (excluding Cr, .Na and Tl which are allowed + 5 % drift and K, Li which are permitted + 10 % drift), the Cu/Mn intensity ratio is determined. If it has drifted outside the tolerance range, the Cu/Mn is reset by adjusting the aerosol carrier flow rate. The calibration standard or “check standard” is reanalysed. Often calibration accuracy has been adequately restored for both spectrometers, and the analysis of samples may resume. If not, one or both spectrometers must be recalibrated. The number of recalibrations required per day (usually just one) is entered in the operator’s log. Raw concentrations for all elements must be determined within their respective linear calibration ranges. Analytical limits for each element are posted for easy reference. Upper concentration limits for the echelle spectrometer are lower than the corresponding limits for the Paschen-Runge spectrometer. When the upper concentration limit is exceeded for any element channel in the echelle spectrometer, the operator is warned with an audible signal. The HP-1000 computer program automatically defers to the Paschen-Runge spectrometer data file to obtain the raw concentration for this element, if possible. Concentrations above the upper limits of the Paschen-Runge spectrometer must be redetermined using diluted samples. These concentrations (corrected for dilution) are entered as substitutions for out-ofrange values before final analysis reports for the undiluted samples are calculated. Elements inadvertently determined out-of-range are identified with an asterisk on the final analysis reports. Background drift corrections are performed by the HP-1000 computer, which subtracts the adjacent blank analysis from each sample analysis. If reagent blanks from specific sample preparations are used, they are analysed again as samples using the calibration blank for comparison. This practice serves as a routine check for unusual contamination in the sample preparation procedures. Table 3. Referencesolutions for calibrations Standard number b

Matrix

I II

2.5 % HNO, 2.5 % HNO,

111 IV V VI

2.5 % HCL H,Q 2.5 % HCL 2.5 % HNOJ

Elements containedC Calibration blank Ag (l), Be (l), Ca (lOO),Cd, Co, Cu (I), Li, Mn (l), Pb, Sr (l), Tl, Zn Al (lOO),Ba (I), Ca, Fe (lOO),K (20) Mg (50), Na, Ni, Pt B (5), P (50), Si (lOO),W As, Cr, Fe, MO, Na (200), Sb, Se, Sn, Ti, U, V As, Be (0.2), Cd (2), MO, Ni (5), Pb, Sb, Se, Tl, U

sPrepared from ultra-pure reagents and 18 megohm deionized water. bI-V are used to calibrate the Paschen-Runge Spectrometer, I and VI are used to calibrate the echelle spectrometer. CConcentrations are 10 &ml unless indicated otherwise in parentheses.

100

ROBERTI. BOTTO

Similar samples are grouped together as much as possible to avoid errors caused by the incomplete washout of an element concentrated in one sample augmenting trace levels of that element in an adjacent sample or blank. In addition, a brief wash is provided between samples followed by a minimum of 60-s delay between the start of aspiration and data collection for samples and blanks. At the end of each analytical run of up to 30 samples, a diagnostic printout is obtained which compares the precision of the background measurements from blank analyses with the current detection limits. Large differences may indicate washout problems or element channels performing below specification. The raw concentration data from both spectrometers, together with a copy of the finished analysis reports (for samples and reagent blanks) and the diagnostic printout, are filed for one year. 2.2.4. Calibration for spectral interferences. Spectral interference calibrations for both spectrometers are maintained continuously for correcting raw data obtained from the analysis of aqueous solutions. Recalibration for spectral interferences is mandatory if: (1) Reoptimization for the Paschen-Runge spectrometer has resulted in a new value for the Cu/Mn intensity ratio. (2) The Cu and/or Mn photomultiplier tubes have been cleaned or replaced (or associated electronics repaired). (3) One or more exit slits in the multielement array have been reprofiled (either spectrometer). (4) The current calibration is no longer accurate. Interference calibration begins with the analytical calibration of both spectrometers after reproducing optimum analysis conditions using the Cu/Mn intensity ratio and MO optimization procedures (2.2.3). Thirty-three reference solutions prepared from ultra-high purity materials each containing 1000 pg/ml of one element are then analysed simultaneously by both spectrometers. The analyses are separated by one or more reagent blanks with adequate time allowed for washout. Spurious concentration data obtained are weeded for contaminants [6], and organized into two 34 x 34 matrices (there are two entries for Pb) of interference correction factors (coefficients). One matrix includes interference data from the Paschen-Runge spectrometer only, and the other matrix contains substituted data from the echelle spectrometer. The interference coefficient matrices are mathematically inverted and are stored in the memory of the HP-1000 computer for correcting raw concentration data acquired from the Paschen-Runge spectrometer alone or from the dual spectrometer system. 2.2.5. Monitoring and maintenance. The accuracy of current spectral interference calibrations is monitored on a monthly basis [8]. Interferencecoefficients are redetermined for several elements, including Cr, MO, U and V which are together responsible for 88 spectral interferences. The decision to recalibrate is reached when comparisons with the calibration currently in use reveal important differences, compromising the accuracy of interference corrections. Interference calibrations may remain acceptably accurate for 6-9 months or longer [8]. The operator’s log contains a continuous record of performance with regard to the sensitivity and stability of the dual spectrometer ICPES system. An attempt is made to adhere to a regular schedule of preventive maintenance for the ICPES. Nebulizer and torch conditions are checked approximately biweekly and replacement parts are installed as necessary. Every six to nine months, or when it becomes necessary to recalibrate for spectral interferences, the following are performed: (1) All optical surfaces are examined for fogging and cleaned/replaced if necessary. (2) All photomultiplier windows are cleaned. (3) Exit slit alignments are checked and reprofiled if necessary. (4) Optimum compromise conditions, Cu/Mn intensity ratio and analytical limits are redetermined. (5) Recalibration for spectral interferences is performed. Photomultiplier tubes in the Paschen-Runge spectrometer develop an exterior fog where light impinges on them over a period of months. Most may be cleaned in place using 15-cm

[S] R. I. Borro, Anal. Chem. 54, 1654(1982).

Quality

assurancein operatinga multielementICPES

101

cotton-tipped applicators dipped in isopropanol. Those that cannot be cleaned in place are withdrawn for cleaning. The cause of this fogging is unknown to the author. However, a 20-40”/1, increase in sensitivity after cleaning is not uncommon. Maintenance items are logged as they are performed and a record is kept of all maintenance expenses. 2.3. Fly ash preparation and analysis Samples of NBS coal fly ash SRM’s 1633 and 1633 A were air dried and digested using HF/HCl/HN& mixtures in open Teflon @ dishes, sealed Teflon@ bottles, or sealed polyethylene bottles. The sealed Teflon@ bottle preparations were heated for at least 4 h at 80°C. The sealed polyethylene bottle preparations followed the method of SILBERMAN and FISHER[9] except the bottles were placed in an ultrasonic bath for at least 6 h. For the sealed bottle preparations, boric acid was used to complex the HF remaining after digestion and to dissolve water-insoluble fluorides. The open dish preparations required no boric acid addition. Ultra-pure reagents and acid-leached containers were used throughout. The fly ash pre~rations were only diluted enough to achieve a dilution factor ~/W) of approximately 100 before being filtered to remove insoluble residue (primarily unburned carbon). A 1: 10000 dilution was used for determination of the major elements. Replicates were prepared from individual fly ash weighings: l l

SRM 1633 (11 total): 3 open dish, 5 Teflon@ bottle, 3 polyethylene bottle. SRM 1633A (8 total): 3 open dish, 2 Teflon@ bottle, 3 polyethylene bottle.

Calibration standards and blanks having acid and/or boron concentrations matching the fly ash solutions were prepared. The fly ash solutions were analysed by ICPES using the matrixmatched blanks for background drift correction. Raw concentration data were collected from both spectrometers simultaneously. Corrections for spectral interferences were performed using the currently stored matrix calibrations (approximately eight months old). 3. DISCUSSION 3.1. Intensity ratio optimization/calibration procedure A key feature of the ICPES quality assurance regimen is the use of an atom-to-ion emission intensity ratio for multielement optimi~tion and for reproducing optimum plasma excitation conditions. Cu (I) 324.754 nm and Mn (II) 257.610 nm were chosen because of their sensitivity to plasma excitation conditions [6] and because both lines were already a part of the analytical array. Other atom/ion line combinations should function as well. The precision and accuracy improvements noted by ~DEGARD[lo] in the analysis of acid extracts of stream sediments using Li (I) 670.784 nm and Y (II) 371.030 nm as internal standards suggest that the Li/Y intensity ratio would provide a sensitive indicator of plasma conditions. LARSON[l l] employs a magnesium ion-to-atom intensity ratio in conjunction with the hydrogen beta line and an argon atom line as diagnostic aids. The hydrogen and argon lines are sensitive to nebulizer performance and rf power consumption, respectively. The Cu/Mn intensity ratio and tolerance specification provide a simple, readily measured definition for a set of compromise conditions for multielement analysis which may involve the optim~tion of many instrumental variables. The ICPES operator maintains the Cu/Mn ratio within the required tolerance throughout every analytical run, resulting in the continuous maintenance of optimum conditions. This method is preferable to passively monitoring the emission intensities of one or more diagnostic lines, and subsequently using this information to reject poor quality data. Poor quality data are less likely to be generated while actively maintaining optimum conditions. Maintaining a predetermined atom-to-ion emission intensity ratio is also preferable to maintaining constant emission intensity of any single line (from internal reference spike, external reference solution or plasma/aerosol[9] D. SILBERMAN and G. L. FISHER, Anal. Chim. Acta 106,299 (1979). [lOI M. J~DEGARD, 1. Geochem. Explor. 14, 119 (1981). [ll] G. F. LARSON, Eastern Analytical Symposium, New York (1982), Paper No. 117.

102

ROBERTI. Borro

derived emission). The latter procedure does provide close control over excitation conditions for short periods. However, as necessary instrumental changes are made (nebulizer adjustment, torch replacement, etc.), optimum compromise conditions will be lost and must be redetermined. Changes in plasma parameters or in the efficiency of aerosol generation/transport will cause the intensities of atom and ion lines to vary independently. In contrast, an atom-to-ion emission intensity ratio can remain fixed for many months and should serve as a valid indicator of optimum conditions despite minor changes in instrumental variables. Used as a reference, the Cu/Mn intensity ratio minimizes guesswork involved in reoptimizing after making changes in any of these variables. Failure to obtain the proper intensity ratio, or meet the tolerance or sensitivity specifications, warns the operator of trouble and aborts the analytical run before poor quality results are produced. The intensity ratio also aids the operator in troubleshooting (Table 4). The MO optimization procedure for the echelle spectrometer is based on the observation of a well-defined peak in the vertical emission intensity profile of MO (II) 202.030 nm in the region approximately 16-20 mm above the load coil. BLADES and HORLICK [12] observed such intensity maxima for similar “hard” lines noting that many peaked in the same region of the plasma. Therefore, optimum excitation conditions for MO should apply to eight of the ten echelle elements; those having lines with excitation potentials higher than about 5 eV: As, Be, Cd, Ni, Pb, Sb, and Se. MO was chosen to represent this group of elements because its maximum was the one most easily and reproducibily located. 3.2. Calibration and background drift corrections Correcting for calibration drift by first resetting the Cu/Mn intensity ratio has the following advantages: (1) Optimum analysis conditions are maintained. The ICPES is not being continuously recalibrated at an altered set of operating conditions. (2) Spectral interference corrections are likely to be more accurate (3.3). (3) Experience has shown that if calibration drift is due to minor changes in rf power, the entire calibration (50 analytical lines, 2 spectrometers) will probably be restored to acceptable accuracy. The Cu/Mn adjustment effectively shortcuts the recalibration procedure. The Paschen-Runge spectrometer is not thermostated. Therefore, significant changes in room temperature often presage episodes of excessive calibration drift [7]. The operator is provided with a readout of the Paschen-Runge spectrometer “A” frame temperature, the spectrometer interior temperature, and the room temperature in the vicinity of the spectrometer. A sudden change in room temperature (caused by an air conditioning failure, for example) may not alter the spectrometer “A” frame temperature for an hour or more. In the interim, there may be time to correct the problem. Keeping a record of the “A” frame Table 4. Use of the Cu/Mn intensity ratio as a troubleshooting aid Problem

Possible cause(s)

Proper ratio cannot be obtained

Nebulizer needles badly out of alignment, partially plugged or broken. Serious gas leak.

Proper ratio obtained but aerosol carrier flow rate higher than usual

Leak in gas lines between mass flow controller and nebulizer.

Proper ratio obtained but Cu intensity too low

Nebulizer needs retuning. Sample needle partially blocked. Plasma not precisely aligned with spectrometer entrance slit.

Precision tolerance cannot be met

Loose nebulizer needle. Intermittent leak.

[12] M. W. BLADES and G. HORLICK, Spectrochim. Acta 36B, 861 (1981).

gas

Quality assurance in operating a multielement ICPES

103

as well as the number of recalibrations required per 8-h day, aids in the identification of causes of excessive drift. The use of water-saturated argon for sample nebulization and a mass flow controller for the aerosol carrier stream serves to minimize vibration drift [4,5]. The two-point method is employed for analytical calibration. The low standard for all elements is the calibration blank. Working only within the linear calibration range of each element channel avoids the complication of calculating and storing individual calibration curves. Maintaining duplicate channels for high concentrations (Table 2) effectively extends the linear range for “major” elements, minimizing the number of dilutions required. LARSON et al. [7] discuss the advan~ges of interspersing blanks with samples for reducing errors due to memory or carry-over from previous samples, and for background drift correction. They alternate samples and blanks, whereas in the present procedure, two samples back-to-back are followed by a blank. The former procedure, though less efficient, is preferred if widely different samples are analysed in a random manner. The present procedure depends on grouping similar samples together and allowing su%cient washout time to minims memory errors. temperature,

3.3. Spectral interference corrections Ref. [6] contains a detailed discussion of the spectral interference calibration and correction procedures. A comprehensive approach to spectral interference corrections was chosen because of its applicability to a wide variety of sample types. The multielement array is assumed to include “all” of the elements encountered at signifi~nt concentrations in the samples analysed. The inverted matrix of interference correction factors contains positive or negative matrix elements quantifying all of the observed spectral interferences, including those which are mutual (element X and element Y interfere with each other). The inverted matrix method avoids the necessity of slower, iterative calculations. Empirically determined correction factors for spectral interferences are extremely sensitive to changes in certain ICP operating parameters [6,13], particul~ly the aerosol carrier flow rate (Table 5). If only a few correction factors are required for a certain type of analysis, these factors may be determined immediately after analytical calibration, before the analysis of samples begins [ 133. This approach may also be used to update critical elements in a matrix of interference correction factors as an alternative to complete recalibration. The Cu/Mn intensity ratio calibration procedure was developed to improve the accuracy of interference corrections for the Paschen-Runge spectrometer and to obviate the need for frequent updating and recalibration. Using the Cu/Mn intensity ratio calibration technique, interference correction factors for

Table 5. Sensitivity of interferencecoefficients to changes in aerosol carrier flow rate (Paschen-Runge spectrometer) Element

Cr Cr MO MO MO Ti Ti u acorresponding

Interfering with

Al Pb Ca Ni Pb P Tl Sb

(308.215 nm) (283.306 nm) (315887nm) (341.476 nm) (283.306nm) (214.914nm) (377.572 nm) ~231.147nm)

“/”Change in coefficient with 37 oA Change in aerosol carrier flow rat& 141 224 108 167 218 177 196 101

to changing the Cu/Mn intensity ratio from 1.0 to 6.0.

[13] J. W. MCLAREN,S. S. BERMAN,V. J. BOYKOand D. S. RUSSELL,Anal. Gem. 53, 1802 (1981).

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ROBERTI. Borro

88

COEFFICIENTS

18 MEASUREMENTS AVG.

= 10.6 % RSD

I 5-8

8.12

12.16

16.20

20.24

24.28

28-32

I

II 32.36

% RSD Fig. 1. Distribution of RSDs obtained from 18 measurements of 88 interference coefficients performed during a fifteen-month period for the Paschen-Runge spectrometer.

the Paschen-Runge spectrometer can be determined within 4-5 % RSD on a day-to-day basis. A number of instrumental changes, including minor changes in forward power and observation height, can be made without significantly changing the interference calibration [6]. Use of the Cu/Mn intensity ratio reference improves the long term stability, as well as the short term precision of interference calibrations. Results ofa recent study [8] are summarized in Fig. 1 in which an interference calibration for the Paschen-Runge spectrometer, consisting of approximately 200 correction factors (coefficients), was maintained for 15 months using the Cu/Mn intensity ratio. Eighty-eight coefficients were selected for monitoring as described in 2.2.4, and were measured 18 times during this period. The distribution of RSDs for these measurements is plotted in the figure. The coefficients showing relatively poor long term stability ( > 20 % RSD) were attributed to two channels the exit slits for which were found to be misaligned at the end of the 15 months. After completing the dual spectrometer ICPES system, it was realized that the intensity ratio calibration technique is not easily extended to the echelle spectrometer: Optimum spectral lines for intensity ratio measurements were not installed in the echelle multielement cassette, partly in an effort to minimize the cost of the project. Attempts to use the lines installed (MO (II) 202.030 nm and Tl (I) 377.572 nm, for example) revealed how cumbersome and inconvenient intensity ratio measurements were using the readout devices available with the echelle spectrometer. In addition, the echelle spectrometer was supplied with fixed mirror transfer optics which provided no simple method for intensity ratio adjustment. During the first six months of dual spectrometer operation, an attempt was made to maintain the alignment of the ICP source and echelle spectrometer entrance slit as best as possible without changing the observation height above the load coil. Interference monitoring during this period revealed poor reproducibility of interference coefficients for the echelle spectrometer (Fig. 2, middle graph), but excellent reproducibility of interference coefficients for the Paschen-Runge spectrometer (Fig. 2, lower graph). The concave mirror for the echelle spectrometer was then fitted with micrometer adjustments and the MO optimization procedure described in 2.2.2 was instituted. The upper graph in Fig. 2 shows the reproducibility of the echelle interference coefficients measured during six months the MO optimization procedure was in effect. The reproducibility of the echelle interference coefficients shows much improvement although the excellent reproducibility of the Paschen-Runge interference coefficients has not yet been achieved. ICPES reports which our clients receive contain a diagnostic section showing percentage

105

Quality assurance in operating a muItielement ICPES aECHELLE 15 COEFFICIENTS 6 MEASUREMENTS &lo ~TIMI~TION

AVG. = 12.5% RSD

64-

ECHELLE 15 COEFFICIENTS 6 MEASUREMENTS

4-

AVG. = 22.0 % RSD

“0 t5

30

2 1

2c PASCHEN-RUNG6 60 COEFFICIENTS 9 MEASUREMENTS

AVG. = 8.0 % RSD 1(

I

n 2-5

5-8

12-16

16-20

20.24

24.26

‘ 28-32

32-36

44 RSD

Fig. 2. Long term interference coefficient repr~u~biiity for the Paschen-Runge and echelle spectrometers. The Paschen-Runge data were collected during a nine-month period. The echelle data were collected during twelve months. During the last six months, the MO optimization procedure was in use.

of interference correction, relative to the corrected concentration, applied to each element determi~tio~. It should be noted that this is a different diagnostic formula than given in Ref. [6-J:

Percent interference = correction

Concentration corrected for spectral interference

-

Uncorrected (or raw) concentration x loo‘

Concentration corrected for spectral interference

Thus, a 100% interference correction results when corrected to 1 pg/ml. Negative and positive corrections operation. In recognition that spectral interference corrections our clients are given the following yardstick to judge

a raw concentration of 2 ,ug/ml is are applied, hence the absolute value may be a significant source of error, the reliability of each ICPES result:

106

ROBERTI. Rorro Percent interference correction

Reliability of results

o-loo

Reliable results expected. Interference correction should introduce little error (< 10% relative).

10&500

Less reliable results. Use for semiquantitative estimation purposes.

>5OO

Results generally unreliable.

This reliability index is based on mounting evidence from long term monitoring that interference calibrations updated no more often than biannually contain few coefficients which differ more than + 20 % with corresponding values determined on the spot. On the average, a long term accuracy of + 10 % is maintained. It should be possible to improve the short term precision of interference coefficient measurements for the Paschen-Runge spectrometer further by installing an autoprofiling system such as the one constructed by LAYMAN [14]. 3.4. Importance of adequate resolution andflexible background correction capability The importance of adequate resolution for the analysis of complex materials by ICPES has been stressed in several recent papers [2, 15-l 81. Equally important is the ability to perform off-line background corrections on either or both sides of the analytical line, at variable distances from the peak [16, 191. Adequate resolution and flexible background correction capability are especially important when making measurements near the detection limit in the presence of high concentrations of Al, Ca, Fe, Mg, Si, Ti and other abundant elements [2,16]. The reason for adding the echelle spectrometer to the original ICP source and PaschenRunge spectrometer system was that the Paschen-Runge spectrometer lacks sufficient resolution for accurate and reliable determinations of As, Be, Cd, Pb, Se and other trace elements in complex sample matrices. The analysis of toxic trace elements and other metals in NBS fly ash samples will be used to illustrate the importance of spectral resolution and flexible background correction capability and to demonstrate the accuracy and limitations of the spectral interference correction procedures. ICPES results from the three sample preparation techniques used for SRM’s 1633 and 1633 A were found to agree closely except: As and Cd results from the open dish preparations were significantly low, indicating partial loss; Si was not quantitatively recovered (as was hoped) from the sealed bottle preparations, and Ag, Pt, Sn, Tl, U and W were not detected. Boron could not be determined because it was volatilized from the open dish preparations. The ICPES results for major, minor and non-toxic trace elements in 1633 and 1633 A are compared in Tables 6 and 7 with the certified values or literature values. All of the ICPES data in these tables exhibit acceptable precision. Indeed, all but five determinations yielded RSDs < 5 %. The precision data are particularly good considering that they were obtained from three different sample preparation techniques using three sets of acid-matched calibration standards and blanks. Excellent agreement with the NBS certified values was obtained for all elements except Ni. The ICPES values for Ni in 1633 and 1633A are 10 and 6% low, respectively. Agreement with the literature is excellent also (when sufficient literature data are available to permit an evaluation), except in the case of Mg in 1633. Literature values for Mg in 1633 display a wide variation; the higher numbers are attributed to neutron activation. [I41 L. R. LAYMAN,1982 Winter Conference on Plasma Spectrochemistry, Orlando, FL, Paper No. 50 (1982). Cl51 J. M. MERMETand C. TRASSY,in Developments in Atomic Plasma Spectrochemical Analysis, Ed. R. M. BARNES, Heyden, Philadelphia (1981). WI J. W. MCLARENand S. S. BERMAN,Appl. Spectrosc. 35,403 (1981). Cl71 A. STRASHEIM,N. M. WALTERSand A. R. OAKES, XXI Colloquim Spectroscopium Internationale-8th International Conference on Atomic Spectroscopy, Cambridge, 1979, Abs. in ICP InJ Newd. 5, 146 (1979). WI A. W. BOORN,1982 Winter Conference on Plasma Spectrochemistry, Orlando, FL, Paper No. 43 (1982). Cl91 H. R. S~BELand R. L. DAHLQUIST,Am. Lab. 13, 152 (1981).

107

Quality assurance in operating a multielement ICPES

[ZO] reports that Mg results he has obtained are in good agreement with this study. The Ni and MO analyses reported in Tables 6 and ‘7were performed using the echelie spectrometer. The remaining elements were determined using the Paschen-Runge spectrometer. The amount of interference correction applied to each of these results was small or none at all. Reasonably accurate MO analyses were obtained from the Paschen-Runge spectrometer (1633, 32 pg/g MO; 1633 A, 44 pg/g MO). However, an average of 400% interference correction was applied to arrive at these values due to interference from an Fe (II) line at 202.070 nm that coincides with the MO background measurement position. The Ni results from the Paschen-Runge spectrometer were grossly in error due to a line overlap interference from Zr, an element not included in the analytical array. The Paschen-Runge and echelle spectrometer data for the toxic trace elements in NBS SRM’s 1633 and 1633A are given in Table 8. Accurate As results were produced by both spectrometers despite substantial amounts of spectral interference correction. Results produced by the echelle spectrometer are somewhat more precise and less interference correction was applied to them. Figure 3 shows a wavelength scan in the vicinity of the As line obtained while aspirating a 1% solution of 1633 A. The As peak rests upon a steeply inclined background due to broadened Al lines at lower wavelength. An Fe line is located 0.03 nm from the As peak on the low wavelength side. The single-point high wavelength correction used for both spectrometers cannot completely compensate for the Al interference, and a residual correction must be applied. A two-point background correction is possible with the echelle spectrometer, but the best low wavelength correction point for As is not optimum for the other elements. The inferior resolution of the Paschen-Runge spectrometer necessitates a sign&ant correction for Fe interference. To obtain accurate Be results with the P~chen-Runge s~trometer was not possible. The interference situation is shown in Fig. 4. The Be peak is adjacent to a relatively intense Fe line. LARSON

Table 6. ICPES analysis of SRM 1633

Elementa

Concentration

Al (%I

&%I co Cr cu Fe (%I R (%I Li Mg (%I Mn MO Na (%I Ni P Sf

Ti (%I V Zn

12.5 2720 4.6 39 131 128 6.1 1.70 80,140,300 1.57 493 27 0.32 98 880,120O 1430 0.72 214 210

Reported values UncertaintyC % RSD (number)d 0.5 190

0.3 3 2 5 0.3 0.09 0.22 7 6

250 0.06 8 20

4.0 7.0 6.5 7.7

(16) (16) (20) (18)

4.9 5.3 14

;21j (17) (3) (16)

22 f (8) 9.4 (15) -f(2) 17 (24) 8.3 (21) f f

Mean

This studyb S.D. % RSD

12.6 2860 4.54 32 125 128 6.14 1.67 1.61 1.29 490 26s 0.32 88s 910 1240 0.63 222 199

0.2 70 0.06 1 9 6 0.07 0.07 14 0.03 14 2 0.02 2 30 30 0.02 3 7

*VaIues are in ,ugJg unless y0 is indicated. bData from 11 determinations. CEstimate from NBS certilicate or standard deviation of literature values. dNumber of literature values in parentheses. ePercent interference correction applied to each result. fCertified by the NBS. sDetermined using the echelle spectrometer.

[20] G. F.

LARSON,

personal communication (1982).

1.2 2.4 1.4 3.9 6.8 4.6 1.1 4.3 8.4 2.0 2.8 6.7 5.9 2.4 3.3 2.7 3.8 1.4 3.4

%IC.e 0 0 0 50 0 4 0 0 0 0 6 0 0 0 0 0 0 5 0

108

ROBERT I. Ekxro Table 7.

Elementa

Al (%I Ba Ca (.%) co Cr CU Fe (%) R (%) Li Mg (%) Mn MO Na (%) Ni P Sr Ti (%) V Zn

Concentration 14.0, 14.0 1500 1.11 46,37 196 118 9.4 1.88 0.455 190, 190 29,37 0.17 127 830 0.84, 0.80 300,360 220

ICPES analysis of SRM 1633 A

Reported values uncertaintyc 0.01 6 3 0.1 0.06 0.01 0.01 4 30 10

Sourced NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS NBS

Inf., Lit. Inf. Cert. Inf., Lit. Cert. Cert. Cert. Cert. Cert. Inf., Lit. Inf., Lit. Cert. Cert. Cert. Inf., Lit. Inf., Lit. Cert.

Mean

This studyb SD. % RSD % I.C.e

14.1 1540 1.08 37 187 120 9.38 1.86 151 0.452 189 32f 0.176 119’ 1320 790 0.77 305 222

0.2 30 0.02 1 8 4 0.07 0.06 15 0.008 5 2 0.008 2 30 30 0.03 5 2

1.5 2.1 1.4 3.0 4.2 3.3 0.7 3.3 10 1.8 2.8 4.8 4.5 1.3 2.4 3.4 3.7 1.6 0.8

Walues are in pg./g unless % is indicated. bData from eight determinations. CEstimate from NBS certificate. dNBS Inf. = NBS information value; Lit. = value from literature; NBS Cert. = NBS certified value. ePercent interference correction applied to each result. Determined using the echelle spectrometer. Fe1

193.666 “In

SRM 1633A/lllO

BROADENED AIII LINES AT

LOWER x

Fig. 3. Echelle spectrometer wavelength scan for 1% solution of 1633A centered at As (I) 193.696 nm. Arrows indicate peak (P) and background (B) measurement positions.

0 0 0 47 0 5 0 0 0 0 19 0 0 0 0 0 0 9 1

ROBERT I. Bono

110

FeII 234.630 nm

SRM

-

1633/1’X’

REAGENT BLANK

-

Fig. 4. Echelle spectrometer wavelength scan for 1% solution of 1633 centered at Be (I) 234.861 nm. Arrows indicate peak (P) and background (B) measurement positions.

Again, both spectrometers employ single-point, high wavelength ~ckground corrections. The resolution of the Paschen-Runge spectrometer is insuficient to prevent the Fe line overlap from overwhelming the Be signal. In contrast, Be results from the echelle spectrometer are both precise and accurate. The amount of interference correction applied was negligible. The Cd determinations were performed within three times the detection limits of both spectrometers, which are equivalent [2]. The echelle spectrometer is unable to resolve the Cd line from a weak Fe line that rests on a sloping background due to a broadened Al line at higher wavelength (Fig. 5). Background measurements must be performed on the high wavelength side of the peak to avoid strong interferences from Pt, Cr, MO and W (not a factor in this fly ash analysis). This situation results in the necessity of applying miniscule, but important interference corrections for Al and Fe to the echelle data, the coefficients for which were updated just prior to the analysis. The Cd determinations performed by the echelle spectrometer are not precise when the concentration is close to the detection limit, but are reasonably accurate. The amount of interference correction required (63 % for 1633,90 “/;,for

Fe

BROADENED Al LINE NEAR

214.54 “Ill

REAGENT BLANK

p

4-h Fig. 5. Echelle spectrometer wavelength scan for 1% solution of 1633 centered at Cd (II) 214.438 nm. Arrows indicate peak (P) and background (B) measurement positions.

Quality assurance in operating a multielement ICPES

111

1633 A) appears moderate. However, these figures are somewhat misleading because they are net interference corrections resulting from comparable negative and positive contributions from Al and Fe, respectively. The total amount of interference correction applied was roughly 300 % of the corrected concentrations. The Paschen-Runge spectrometer produced raw Cd concentrations requiring corrections for Al and Fe approximately 10 times greater, hence the inaccurate results from this spectrometer. Pb (II) 220.353 nm is also located on the broadened wings of Al lines at higher wavelength (Fig. 6). Background correction on the low wavelength side of the peak is preferred. The high wavelength position for background correction utilized by the Paschen-Runge spectrometer results in the application of large interference corrections for Pb due to Al. Consequently, the Paschen-Runge results for Pb in 1633 and 1633 A are imprecise and inaccurate, whereas the echelle data are both precise and accurate. The Paschen-Runge and echelle spectrometers employ different wavelengths for Sb. The P

BROADENED AL I LINES AT

220.467. 220.462 “ill

SRM

y

1633/100

REAGENT BLANK

Fig. 6. Echelle spectrometer wavelength scan for 1% solution of 1633 centered at Pb (II) 220.353 nm. Arrows indicate Peak (P) and background (B) measurement positions.

FelI 206.792

“In SRM lB33/190 I

AL

RECOMBINATION CONTINUUM

Fig. 7. Echelle spectrometer wavelength scan for 1% solution of 1633 centered at Sb (I) 206.833 nm. Arrows indicate peak (P) and background (B) measurement positions.

112

ROBERT I. Bono Fe1 196.059 nm

SW1 1633AllOO

e

REAGENT BLANK

Fig. 8. Echelle spectrometer wavelength scan for 1% solution of 163349centered at Se (I) 196.026 nm. Arrows indicate peak (P) and background (B) m~surement positions.

former uses Sb (I) 23 1.147 nm, a line so beleaguered by spectral line overlaps that it is useless for all but “clean” sample matrices [2]. Sb (I) 206.833 nm is barely resolved from a weak Ti line and is 0.04 nm from an Fe line. In 1633, all three lines rest on an elevated background caused by Al recombination continuum (Fig. 7). Background correction on the low wavelength side of the Sb peak, between the peak and the Fe line, is preferred to avoid Cr, MO, Ni, Sn, U, V, and W lines on the high wavelength side. As a result, Sb concentrations close to the detection limit can be determined in the fly ash matrix with reasonable accuracy and with moderate amounts of interference correction (Table 8). Selenium cannot be determined accurately in Fe-bearing sample matrices using the Paschen-Runge spectrometer. An Fe_ line on the high wavelength side of the Se peak coincides with the position for background correction (Fig. 8). The echelle spectrometer combines high resolution with background correction on the low wavelength side of the peak to obtain accurate Se analyses for SRM’s 1633 and 1633A. The Se determinations were performed close to the detection limit with moderate amounts of interference correction attributed to Al recombination continuum. Table 9. Precision of fly ash solution measurements compared with “pure water” detection limits

Element

“Pure water” detection limit (2 Sigma/*

Concentrations measured in fly ash soiutions (% RSD)

Cd

O.~~grn~-’

0.020 figrnl-’ (27) 0.012 pg ml- ’ (SO)

Sb

0.035 pg ml- *

0.084 pg ml- ’ (36) O.lOjfgml-l (31)

Se

0.045 pg ml- 1

0.092 ,ugml-’ (28) 0.10 ~gml-’ (52)

aThe ,/? factor results from the averaging of two “exposures” per determination. Fly ash solution composition: I g lOOmI-’ fly ash; 2 g lOOmI-’ HOBOs; 24 ml IOOml-’ concentrated acids.

Quality assurance in operating a multielement ICPES

113

The precision of the fly ash solution measurements of Cd, Sb and Se is compared in Table 9 with the detection limits for these elements determined in deionized water. Since the RSD values in this table approach those expected for the 20, detection limit, the results strongly suggest that the Cd, Sb and Se detection limits in the fly ash solution matrix are similar or identical to those determined in pure water. Thus pure water detection limits may be cautiously applied to analyses performed in complex sample matrices, provided that interference correction is moderate and applied accurately and that optimum conditions are employed for background corrections. 4.

CONCLUSIONS

A simple, convenient quality assurance program including effective maintenance and record keeping practices has been devised for multielement ICP emission spectrometry. The key features of this program, the intensity ratio calibration procedure for example, have proven their worth during six years of continuous use. The intensity ratio calibration procedure: provides a means of reproducing optima analysis conditions; warns the operator of problems and serves as a troubleshooting aid, improves the precision and maintains the long term accuracy of spectral interference calibrations. Critical interferences are reduced or eliminated through the use of a high resolution echelle spectrometer. The Paschen-Runge and echelle spectrometers have been shown previously to be comparable in terms of sensitivity, stability and speed [2]. The intensity ratio calibration procedure is not easily extended to the echelle spectrometer, but the MO optimi~tion procedure now in use functions nearly as well for maintaining the accuracy of the echelle interference calibrations. The importance of adequate resolution and the proper choice of positions for off-line background measurements has been borne out by a detailed study of the determination of toxic trace elements in coal fly ash. The observation that accurate trace element dete~inations are possible with interferen~ corrections as high as 1~5~~, underscores the accuracy of the interference correction procedures. Acknowledgements-The author is grateful to R. M. BARNES,F. L. FRICKE,G. F. LARSON,R. A. NADKARNI and R. B. helpful discussions and encouragement. K. R. BOTFO,D. G. PORTERand M. C. LAFUENTEkindly assisted in preparing the manuscript.

WILLIAMS for

SA(B)

39:1-H