JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, AUGUST 1987, VOL. 2 491

Rapid Determination of Eight Elements in Cement and its Raw Mixes by Inductively Coupled Plasma Atomic Emission Spectrometry

M. L. Fernandez Sanchez, J. Palacio Suarez," E. Fernandez Molina" and Alfredo Sanz Medel Department of Analytical Chemistry, Faculty of Chemistry, University of Oviedo, Oviedo, Spain

An investigation was carried out on the influence of the operating conditions and of the analytical performance characteristics of inductively coupled plasma atomic emission spectrometry for the determination of eight elements (Si, Al, Fe, Ti, Ca, Mg, Na and K) in the raw components of cement. The aim of this investigation was to develop a rapid method of quality control for such raw materials. The results showed that this technique is sensitive enough for each element of interest and no serious interferences were detected after a detailed inter-element interference study. Rapid determination by automatic sequential analysis of the eight elements in a single solution of the sample proved to be highly satisfactory. A suitable fusion method for non-clinkerised material, normally difficult to dissolve, is also given. Keywords: Cement raw components; inductively coupled plasma; atomic emission spectrometry

The cement industry today requires a large number of determinations, particularly for the elements Si, Al, Fe, Ti, Ca, Mg, Na and K in a large and varied number of real samples, e .g . , raw materials and/or their mixtures (non- clinkerised materials) and final clinkerised products. At present the quality control of these elements in the cement industry is usually performed by X-ray fluorescence(XRF) and/or atomic absorption spectrometry. It is important to note, however, that these two common techniques may be plagued by severe interferences, which limit the versatility of the techniques, or they may lack the required sensitivity or speed for rapid and convenient determinations of all the relevant elements. Thus it is clear that new analytical techniques for such analyses should be investigated in detail, with attempts to combine the multi-element capability, rapidity and convenience of X-ray fluorescence with the versatile calibration, relatively low cost and reliability of the low concentrations possible with atomic absorption (AA).

Inductively coupled plasma (ICP) atomic emission spec- trometry (AES) seems to be especially suited to these purposes, considering its exceptional analytical characteris- tics: high power of detection for many elements, relative freedom from matrix interferences, wide linear range of concentrations, good precision and intrinsic ability for multi- elemental analysis. In fact, the suitability of ICP-AES for major, minor and trace element analysis has been widely shown for siliceous geological materials. 1-3 Moreover, Degre4.5 has already reported the successful application of the ICP to the analysis of cements giving practical information on standards and sample preparation along with instrumental conditions for 14 elements encountered in non-clinkerised and clinkerised cement industry materials. The performance of this technique was checked against the results obtained by classical methods4 with excellent results (with the exception of the determination of Na). The simultaneous multi-element analysis of the samples utilised the simple addition of Ni to all solutions as an internal standard to improve the precision.4>5 A comparison of atomic absorption and ICP-AES for the determination of Si, Al, Fe, Ca and Mg in cement has been reported by Casetta and Giaretta.6 According to these authors ICP-AES provides a technique as accurate as AA but more convenient for cement analysis because of the superior sample throughput and freedom from typical chemical interferences by Si and A1.h

Analysis of cements has attracted comparatively little attention over the past few years, in terms of publications.7~8

* Cementos Tudela Vequin Factory, Oviedo, Spain.

In this paper we examine in detail the analytical performance characteristics of ICP-AES for the rapid and reliable determi- nation of eight elements of practical interest (Si, Al, Fe, Ti, Ca, Mg, Na and K) in non-clinkerised materials. A single solution of the sample using a fusion with Li2C03 - B2035 is used for all the eight elements under study and a comparison (of accuracy, precision and speed or sample throughput) is made between the manual analysis, element by element, and the automatic sequential analysis available with our commer- cial instrument.

Experimental Apparatus

A Perkin-Elmer Model ICP 5000 inductively coupled plasma spectrometer was used. The source and spectrometer charac- teristics and the operating conditions finally selected are described in Table 1.

Reagents Stock standard solutions, (1.000 g 1-1) of each of the eight elements of interest were prepared from pure salts or by dissolution of high-purity metals in HC1. The standard silicon solution was prepared by fusion with Li2CO3 and BzO3 and the final dissolution of the melt in re-distilled water; this solution was standardised gravimetrically (as Si02).

The solutions were stored in polyethylene flasks to avoid any possible contamination by glass (e.g., alkali metals). For determining low levels of alkali metals in particular it is necessary to ensure that distilled water, reagents and glass- ware do not contaminate the sample solutions. Hence, ultrapure water (e.g., that obtained from a Milli Q system) and high-purity reagents were used for preparing the calibrat- ing standards.

Table 1. Instrumentation and plasma operating conditions

Spectrophotometer Plasma operating conditions Sequential scanning Coolant flow-rate . . 16 I min-1 C0.03 band pass Auxiliary flow-rate . . 0.8 1 min-1 Holographic UV grating Injector flow-rate . . 0.55 1 min-1

ICPsource Observationheight . . llmm Nebuliser pressure . . 32 p.s.i.

2 kW, 27.12 MHz Plasmapower . . . . 1.00kW Fused silica torch Cross-flow nebuliser

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492 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, AUGUST 1987, VOL. 2

Table 2. Optimum experimental conditions for each element of interest

Element hlnm H*/mm Ft/p.s.i. WIIkW 16 26 1.20 16 24 1.05 17 25 0.95 18 20 1.25 19 23 1 .00 17 26 1.10 11 32 1 .oo 11 32 1 .oo

Si . . . . . . 288.15 Ca . . . . . . 317.93 Mg . . . . . . 279.54 Fe . . . . . . 259.94 A1 . . . . . . 396.15 Ti . . . . . . 334.94 K . . . . . . 766.49 Na . . . . . . 589.54

* H = viewing height. t F = nebuliser pressure.

W = plasma power.

Table 3. Background equivalent concentration (BEC), detection limits (DL) and dynamic range for each analytical line

Element Si . . . . Ca . . . . Mg . . . . Fe . . . . A1 . . . . Ti . . . . K . . . . Na . . . .

BEC/pg 1-l . . 410 . . 920 . . 30 . . 680 . . 400 . . 430 . . 121 . . 300

Linear DL/pg 1-1 range/mg 1-l

20 500 14 500 0.3 100 4 500 8 500 1.2 500

18 500 10 500

Sample Dissolution and General Procedure The sample (previously homogenised, sieved to 200 mesh and dried) is accurately weighed (250-300 mg), mixed with 1 g of Li2C03 and placed in a Pt crucible. The mixtures is covered with 1 g of B203 and heated to ca. 1000 “C for 1 h. After cooling, the melt is dissolved carefully in 5% HCI. Finally, the solution is diluted to the mark with 5% HC1 in a 500-ml calibrated flask. When the solution is homogeneous it is transferred into a polyethylene flask. Care should be exercised in determining alkali metals, especially if the dissolution is carried out in the cement works, to avoid contamination from air dust. A blank and synthetic samples are prepared by the same procedure as used for real raw component samples. The final solutions are introduced directly to the plasma.

Calibration is accomplished by using very pure synthetic samples or, preferably, with NBS standard reference materials 633-639 , treated as above.

Results and Discussion Influence and Optimisation of Instrumental Conditions After a literature searchgJ0 and some preliminary experi- ments, the analytical emission lines finally selected for the determination of Si, Al, Fe, Ti, Ca, Mg, Na and K in non-clinkerised material by ICP-AES are those detailed in Table 2. We studied the influence of the more critical operating parameters of the ICP on the analytical emission signal for every single element (line). Thus, the effect of variations in the viewing height above the load coil, the plasma power applied and the nebuliser gas pressure were investi- gated. As the above mentioned instrumental parameters are inter-related, we chose to use a Simplex optimisation methodll-13 where the criterium adopted for maximising the function was the ratio INIIB ( I N being the net emission signal observed for each analyte at its relevant wavelength and IB the corresponding “blank” emission). The optimum conditions observed in each instance are given in Table 2 for each element under study. As expected, there are important differences in optimum operating conditions depending upon the emission line considered (in particular for the “soft” lines

of K and Na, where to obtain optimum intensity to back- ground ratios the observation height is lower than normal).

We were aiming at a rapid automatic method for determin- ing the eight elements that are economically important in non-clinkerised material. It was clear that we had to consider using a single dissolution of the sample and “compromise” operating conditions that were identical for all elements under study. Considering the composition of the non-clinkerised material, it is apparent that the low alkali metals content of these samples determines what the “compromise” operating conditions should be. Therefore we selected an observation height of 11 mm, a nebuliser pressure of 32 p.s.i. and a radiofrequency power of 1.0 kW (see Table 2).

Using the above mentioned compromise operating condi- tions we investigated the detection power of the ICP for each element in pure aqueous solutions. We then determined experimentally the background equivalent concentration (BEC) in order to evaluate the detection limits [DL = 2@.(BEC), being the relative standard deviation of the background emission] with compromise conditions. The results obtained for each element are given in Table 3. Considering the high dilution of the sample in the dissolution process (see Sample Dissolution and General Procedure) it is clear that very low detection limits have to be attainable for the minor elements (Ti, Na and K). As shown in Table 3, ICP-AES provides sufficient detection power for the determi- nation of the lowest concentration levels to be expected in non-clinkerised material after the recommended dissolution procedure. The linear ranges of the calibration graph for each element were also investigated (up to 500 mg 1-1) and are given in the last column of Table 3.

Investigation of Potential Interferences

The high temperature of the plasma provides an exceptional wealth of lines in the emission spectrum. For this reason spectral interferences are highly likely to occur, which thus constitute perhaps the most serious problem when analysing real samples with complex compositions. A spectrometer with a high resolving power along with automatic background correction facilities is extremely helpful in dealing with this problem. As the literature documentation on possible over- laps by weak emission lines in the ICP is not completel4J5 we considered it important to study in detail the possible spectral interferences caused by the presence of the individual elements present in non-clinkerised material on the analytical emission signal of each element under study. Using as the reference the emission intensity of pure aqueous solutions of a given element, we measured the intensity of the emission observed for solutions containing this element and the foreign ion under study at similar concentrations and twice the “normal” level usually encountered in the non-clinkerised samples.

Two different approaches were followed for possible interference studies: graphic display of the emission line profile 0.5 nm either side of the analytical line and automatic single analysis of the desired element in the presence of the foreign ion. The graphic display experiments (essential for investigating the real nature of a possible interference) showed that the only spectral problem that occurs is at the 396.15-nm emission line of aluminium where there is a partial overlap by the wing of the 396.85-nm line of calcium. The effect of such an overlap is a spectral background shift (see Fig. 1) in the determination of aluminium in the presence of the high levels of calcium that are present in non-clinkerised material. This spectral interference, however, is easily over- come by automatic background correction , 0.09-nm displace- ment at both sides of the analytical line (see Fig. 1). After monitoring the display on the screen for the influence of each possible matrix element on each individual element, we proceeded to quantify possible analytical errors in the

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, AUGUST 1987, VOL. 2 493

Table 4. Inter-elemental interference study (results expressed as YO recovery)

Foreign ion concentration/mg 1-1

Concentration Analyte testedlmg 1-l Ca . . 50 Si . . 10 Fe . . 5 A1 . . 5 Mg . . 2 K . . 1 Na . . 1 Ti . . 1

Ca Si Fe A1 Na Ti

150 300 50 100 97 96

96 100 102 102 100 99 107 106 98 98 94 101

103 96 106 115 117 129 155 204 101 101 101 108

- - 96 101 - -

30 60 98 96 94 95

99 99 97 97

102 98 100 106 98 99

- -

20 40 102 100 101 101 101 103

99 97 105 102 105 97 98 100

- -

15 30 5 10 100 101 101 96 99 96 100 94 97 101 102 101

105 104 97 99 99 104

130 156 100 96 101 101 95 98

- - 101 98 - -

1 2 102 loo 101 99 103 102 loo 101 99 100 98 103

101 98 - -

1 2 97 94

105 103 97 103

105 103 105 103 104 103 103 105 - -

I 396.15 nrn -1

t - m C 0 v) .-

I I B I

I I I I I A I I 1 I

k ---- &--- 4 0.09 nm 0.09 nrn

Wavelength - Fig. 1. Spectral interference from Ca2+: A, 5 mg 1-1 of Al; and B, 5 mg 1-1 of A1 and 300 mg 1-1 of Ca

determination of each element under study brought about by the presence of the other ions in the matrix. Thus, each element was determined in the presence and absence of the other ions. The results obtained are summarised in Table 4 (where automatic background correction was used for the determination of aluminium) along with the concentration levels used. As can be seen, no serious interference problems were detected except in the determination of sodium in the presence of Ca, Mg or Si. The observed enhancements of the sodium emission caused by these elements (Table 4) proved to be due to sodium impurities present in the calcium and magnesium salts or silica used to prepare the solutions for the interference studies. In fact, subsequent experiments showed that analogous relative enhancements of the analyte signal were observed when shifting to the 588.99-nm line of sodium (it is clear that any possible spectral interference from Ca, Mg or Si should change by using a different analytical line of sodium).

Accuracy Studies In order to ascertain the quality of the analytical results obtainable by the ICP method proposed here for the eight elements of interest, we analysed a standard cement sample from the National Bureau of Standards (the composition of this certified cement sample is given in Table 5). The cement was dissolved according to the general procedure proposed and analysed manually element by element (“single” utility software) and using calibration standards prepared individu- ally for each element by simple addition of the corresponding pure metal solution to a “fusion blank” (in order to avoid possible nebulisation and transport effects caused by the

Table 5. Accuracy and precision of the recommended ICP-AES procedure in the analysis of a certified cement

ICP-AES Component NBS*,% procedure,% RSD,%

SiOz . . . . 21.48 20.80 0.9 CaO . . . . 62.05 61.78 0.8 MgO . . . . 3.83 3.86 0.8

A1203 . . . . 4.45 4.39 0.7 Ti02 . . . . 0.25 0.24 0.6

Na20 . . . . 0.12 0.13 4.1

Fez03 . . . . 3.55 3.41 1.2

K20 . . . . 0.59 0.54 1.8

* National Bureau of Standards, certified values.

different salinities and hence viscosities of standards and sample). The results are given in Table 5 which show that the ICP-AES values obtained are in very good agreement with the certified values from the NBS. The short-term precision observed for these ICP-AES determinations (each result is the mean of three determinations, with an integration time of 0.5 s of the corresponding atomic emission) was very good (ca. 1%) for most of the elements, as can be seen in the last column of Table 5. The only exceptions are the alkali metals as precisions of +_2% for potassium and -t4% for sodium were achieved for the real analyses. It must be stressed, however, that even the precision for sodium is satisfactory if one considers that its concentration in the final solution (see the General Proce- dure) is ca. 0.3 mg 1-1 and that the high energy ICP source is not particularly suited for the alkali metals due to their low ionisation potentials.

Automatic Sequential Analysis of Non-clinkerised Materials Having verified the quality of the ICP-AES results for the determinations element by element using simple Cali bration, we investigated the suitability of the ICP for determining the eight elements of interest in an automatic sequential manner (“multi-utility”) which should provide a faster and more convenient quality control method for the raw components of cement.

To take full advantage of the software available for automatic sequential multi-element analysis and considering that the ICP is essentially a technique for liquid samples, perhaps the main requirement is to be able to dissolve quantitatively all the elements of interest in a single and stable solution. Moreover, this single sample solution should contain the eight elements at concentration levels well above the ICP-AES detection limits (Table 3) in order to provide reliable results. Finally, if a conventional pneumatic nebuliser is used for sample introduction, care must be taken over the excessive salt content of the final aspirated solution (otherwise

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494 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, AUGUST 1987, VOL. 2

Table 6. Analysis of raw mixtures by ICP-AES using manual and automatic utilities

Non-clinkerised material*

M- 1 M-10 M-14

Content RSD Content RSD Content RSD Method of (accuracy), (precision)$, (accuracy), (precision)$, (accuracy), (precision)$,

Component analysist Yo Y O Y O O/U YO YO

SiOz . . . . Single 8.11 1.7 13.30 0.8 14.67 1.6 Mu1 ti 8.00 1.2 13.25 3.2 14.79 3.2

CaO . . . . Single 47.16 1.1 40.78 0.4 40.11 0.7 Multi 46.70 1.7 40.79 1.6 40.31 1.2

Multi 1.06 6.3 1.51 3.9 1.63 2.4

Multi 1.85 5.4 3.85 2.6 2.63 4.2 A1203 . . . . Single 2.28 0.8 3.57 1.6 4.32 3.6

Multi 2.28 2.1 3.51 4.0 4.27 4.2

Multi 0.11 4.9 0.17 4.9 0.20 2.7 K 2 0 . . . . Single 0.57 1.9 0.93 1.8 1.09 1.9

Multi 0.58 6.7 0.93 7.3 1.02 3.1 Na20 . . . . Single 0.05 14.1 0.09 6.1 0.08 14.2

Multi 0.05 21.9 0.08 10.4 0.08 10.4 * Supplied by Cementos Tudela Vequin Factory, Oviedo, Spain. t Single, single element analysis; multi, multi-element analysis. $ Relative standard deviations, as percentages, of five independent determinations.

MgO . . . . Single 1.09 0.9 1 S O 1.4 1.64 2.0

Fe203 . . . . Single 1.78 4.5 3.73 2.8 2.66 4.7

TiOz . . . . Single 0.11 2.5 0.17 3.1 0.20 1.8

M-23

Content RSD (accuracy), (precision)$,

YU Y O

23.10 1.3 22.62 2.7 30.14 1.4 30.31 2.4 2.33 0.5 2.28 2.6 4.72 5.1 4.44 7.8 6.71 1.3 6.67 2.7 0.32 1.7 0.32 1.7 1.62 0.8 1.62 1.4 0.16 2.9 0.13 3.5

nebuliser clogging is likely to occur). With all the above considerations in mind, we carried out some preliminary fusions of non-clinkerised materials with different fluxes containing boron,16 particularly LiB02 or mixtures of Li2CO2 + Bz03. The sample dissolution procedure finally selected for the non-clinkerised samples was fusion with lithium carbonate and boron oxide as described under Sample Dissolution and General Procedure.

The preparation of standard solutions for calibrating the ICP instrument requires some comment: we tried to use a single synthetic solution prepared from the corresponding “pure” salts of the eight elements of interest in the non- clinkerised materials. Unfortunately this synthetic solution failed to provide a suitable standard for calibration of the ICP-AE spectrometer because of the impurities contained in the eight different salts used to prepare the solution. These impurities alter the known added concentrations, particularly of the elements existing at low levels (e.g., sodium and potassium), by significant unknown amounts. For this reason, and considering the excellent results obtained for the NBS certified cement (Table 5 ) using individual standard solutions, a more accurate calibration is obtained by using the NBS cement, dissolved as are the non-clinkerised samples, as the one single-solution calibration standard for the eight elements of interest in the raw materials.

The results obtained, taking the precautions mentioned, for the automatic sequential analysis (“multi-element” software) of the eight elements in four different non-clinkerised materials (covering the normal concentration ranges found for these samples) are given in Table 6. The figures are compared, in terms of accuracy and precision, with those obtained by analysing the same samples for the eight elements but using the previously validated method of analysis element by element (“single” software). In Table 6 each figure is the mean of five independent determinations performed according to the general procedure (five independent fusions). The good precision observed, particularly using the single modes, indicates that reproducibility of the sample dissolution method used is satisfactory as is the final ICP-AES determina- tion. For sodium, the standard deviations observed were +10-15%. The poorer performance for this element is primarily due to the low sodium concentration in the final solutions: at such low concentrations not only is the ICP

technique too close to the detection limit for this element but there is also a high risk of Na+ contamination of the sample solutions (from the glassware or even from atmospheric dust). Therefore, great care has to be taken in order to prevent any kind of sodium contamination.

If one compares the means of the two analyses, as shown in Table 6, good agreement is observed between the results obtained by both methods (manual, single element and automatic sequential analysis). It has to be noted, however, that generally the precision was worse when automatic sequential analysis was used. However, when the non-clinker- ised samples are taken into solution (still the time-limiting step of the analysis) the ICP-AES determination of the eight elements in the non-clinkerised sample can be performed by automatic sequential analysis in less than 10 min, while it takes more than 1 h to perform these analyses manually element by element. The decrease in precision with automatic analysis is, however, marginal and it is more than compensated for by the speed and sample throughput attainable with routine analysis.

Conclusions The exceptional analytical characteristics of the ICP allow the reliable determination of major, minor and trace elements that are of economic interest in non-clinkerised materials in a single solution without any previous separations. Adequate precision and accuracy are achieved (even for the low concentrations of the minor components that result from the dilution of the samples) without resorting to internal standar- disation techniques. However, not only does the use of a polychromator allow simultaneous multi-element analysis,4+5 but using a simpler sequential ICP spectrometer (having adequate automatic sequential software) also allows the rapid determination of all the elements of interest without signifi- cant losses in precision. The ICP-AES technique proves to be clearly superior to AA for this type of analysis and provides a valuable alternative technique to XRF with the advantage of the higher sensitivity inherent to ICP-AES. Most of the results given here could be easily extended to the ICP-AES analysis of any siliceous materials encountered in the cement industry.

We gratefully acknowledge the financial support given by Fundaci6n para la Investigaci6n Cientifica y la Tecnologia (FICYT) de Asturias.

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, AUGUST 1987, VOL. 2 495

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Paper J6J124 Received December 19th, 1986

Accepted March 11 th, 1987

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