Water Analysis by Atomic Absorption and Flame Emission

13 Water Analysis by Atomic Absorption and Flame Emission Spectroscopy E D W A R D A. B O E T T N E R and F R E D I. G R U N D E R

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Department of Industrial Health, School of Public Health, University of Michigan, Ann Arbor, Mich.

Several elements (Zn, Pb, Cu, Ni, Ca, Mg, Fe, and Mn) are determined routinely in water samples using atomic absorption spectroscopy. Sodium and potassium are determined by flame emission. The preparation of the samples, the analytical method, the detection limits and the analytical precisions are presented. The analytical precision is calculated on the basis of a sizable amount of statistical data and exemplifies the effect on the analytical determination of such factors as the hollow cathode source, the flame, and the detection system. The changes in precision and limit of detection with recent developments in sources and burners are discussed. A precision of 3 to 5% standard deviation is attainable with the Hetco total consumption and the PerkinElmer laminar flow burners.

THIS

laboratory is concerned with the analysis of a wide range of materials which include a variety of biological specimens, mineral samples, air particulate matter, and water samples. The information obtained is used to evaluate various individual and environmental prob lems. T w o of the techniques which have been used for water analysis are atomic absorption (1, 4) and flame emission (2) spectroscopy, and a study of factors affecting these methods is described here. The samples came from a number of sources which included well water and city water. Consequently, the concentration ranges of some of the elements were quite wide. Five determinations were made daily on both the samples and standards for each element over a period of several months to provide sufficient data for an adequate evaluation of precision. A t present, ten elements ( N a , K , C a , M g , Zn, Pb, M n , C u , Fe, and N i ) are being deter mined quantitatively. 236

In Trace Inorganics In Water; Baker, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


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237

To carry out the analysis in the best manner possible, a study was conducted on the various hollow cathode lamps, burners, and optical systems to assess them as to their advantages and disadvantages i n the analysis of water. Particular emphasis was given to the comparison of the total consumption and premix burners (6, 7). The results are pre sented here in the form of conservative values attainable of sensitivity and precision, and a discussion of the differences noted. The first portion of the study was carried out on natural and treated water samples. W h e n the statistical data showed a need to examine several parameters more thoroughly, we shifted to laboratory water standards containing known amounts of five elements which are repre sentative minor and major contaminants in waters. Experimental

Procedure

Water samples were collected in large plastic bottles in the field direct from kitchen spigots. These bottles were acid cleaned and flushed thoroughly with distilled water before use. A 500 m l . sample of this water was placed in an evaporating dish and 15 ml. of concentrated nitric acid added. This was evaporated on a steam bath to approximately 25 ml., transferred to a 50 m l . acid washed volumetric flask, and brought up to volume with double distilled water. This portion was then analyzed for K, Zn, Pb, M n , C u , Fe, and N i , using standards containing the elements of interest. For the analysis of C a , M g , and N a , 10 m l . of the water samples were added to an acid washed 50 ml. volumetric flask. One m l . of a 10% solution of lanthanum, prepared from lanthanum oxide ( 3, 8 ), was added and brought up to volume with double distilled water. Standards were prepared containing the elements of interest and the lanthanum. The analyses were performed on a Jarrell-Ash Model No. 32-360 multipass atomic absorption spectrophotometer equipped with a Beckman total consumption three burner set operated on a hydrogen-air mixture, and Westinghouse single element hollow cathode lamps. N a and Κ were done by flame emission and the rest of the elements by atomic absorption. The samples and standards were poured into porcelain combustion boats and placed in position for aspiration. Dis tilled water was aspirated alternately with the solutions to provide a baseline. A l l data was recorded on a Sargent Model SR recorder with 1 millivolt sensitivity. Results Samples were collected, prepared, and analyzed once a month. Every month five determinations for each element were made on both the samples and standards. To obtain precision data eight samples, selected on the basis of their determined concentrations, were prepared and analyzed each month for five months. A large quantity of these eight samples was collected to eliminate natural variations which may occur.



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The samples were first run on the Jarrell-Ash instrument, with the three burner set and the same instrument was used as a flame emission spectrophotometer for the determination of sodium and potassium. A statistical summary of analytical precision for eight elements i n the eight samples are shown i n Table 1. Here the precision is expressed as the % standard deviation (coefficient of variation), which is defined as one hundred times the ratio of standard deviation to the mean concentra tion ( 9 ) . It can be seen from this data that the last three elements, which are present in a quantity near the limit of detection, have large deviations. It is clear that these figures could be lowered if the analysis were r u n using higher concentrations. But our purpose was to evaluate the useful ness of the technique for routine determinations. Table I.

Precision of Analyses Using Original Three-Burner Set Statistical Summary

Element

Range (p.p.m.)

Mean Cone. (p.p.m.)

Std. Dev. (p.p.m.)

% Std. Dev.

Na Κ Mg Ca Zn Pb Mn Cu

0.7 -30.0 16.2 -40.2 4.8 -11.2 0.4 -29.9 0.3 - 9.1 0.10- 0.39 0.02- 0.36 0.06- 0.21

8.3 29.7 8.8 10.3 1.6 0.25 0.10 0.15

0.9 3.7 1.0 2.3 0.3 0.12 0.04 0.08

10.8 12.4 11.3 22.3 18.8 48.0 40.0 53.0

To investigate some of the factors affecting precision, lead and mag nesium were chosen, as the first represents elements present i n concentra tions near the limit of detection and the second those elements well above this limit. Figure 1 shows a calibration curve for lead using the Beckman three burner set. The standard deviation of the water standards is indi cated at each concentration. The calculations for this curve were based on data taken over a period of four months which would consist of a total of twenty determinations with one set of samples being prepared and analyzed each month. T o illustrate the effect of some of the instrumental parameters, Figure 2 shows the % standard deviation plotted versus concentration. The amount of variation caused b y instrumental factors, such as the flame, hollow cathode lamps, and detecting system, was expected to be a function of the concentration, and such an effect is shown i n Figure 2, where the deviation for the lowest concentration is 50% and for the highest, only 7% Figure 3 shows the calibration curve for magnesium with the standard deviation of the standards indicated. Here, an air flow that aspirated a smaller amount of sample into the flame was used to permit the analysis of the high concentrations of



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0.25

0.50

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Water Analysis

0.75

ΙΌ0

1.25

1.50

CONCENTRATION : g / m l M

Figure 1.

Calibration curve for lead, showing standard deviation of standards

70 r

0.25

0.50

075

1.00

1.25

1.50

CONC. g / f a f M

Figure 2.

Variation with concentration of the standard deviation of lead

magnesium present without further dilution. B y maintaining an optimum fuel-air ratio at this reduced sample consumption condition, no deterio rating effect was noted i n the precision values. This figure summarizes work done over a period of six months with a total of thirty determina tions. The variance curve shown i n Figure 4 indicates results similar to that found with lead with the highest percent standard deviation for the



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lowest concentration. The other elements, including those analyzed by flame emission, showed much the same type of characteristic. One can trace the cause of the variations by isolating and examining the different portions of the analytical system. The electronic noise from the detection and amplifier system was found to be very low and its effect on precision or detection limit can be neglected for the elements included i n this study. The cathode lamp noise was generally less than 1 or 2 % of the signal, but for an analysis at or near the limit of detection, this noise was a factor in both the limit obtained and the precision. B y far the greatest contribution to the variations in the analyses was found to be the flame. F o r example, with the Beckman three burner set, large variations i n the data were encountered with iron and so this element was used to evaluate the burner system, using water standards. These variations were appar ently caused by large fluctuations i n sample consumption, inasmuch as the sample flow rate was varying 20% or more on both a short term (25 minutes ) and a day-to-day basis.

Figure 3.

Calibration curve for magnesium, showing standard deviation of standards

A t this point it became apparent that: 1) The precision attainable was relatively poor, even in cases well above the limit of detection for several elements. 2) The burners appeared to be the principal factor affecting the precision, inasmuch as the results were about the same in using flame emission as i n atomic absorption.


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30

CONC.

Figure 4.

^g/mi

Variation with concentration of the standard deviation of magnesium

To investigate this matter of precision further, as well as the limit of detection, two other types of burners and various hollow cathode lamps were tested under similar conditions. The primary objective was to compare a new single flame total consumption burner, operated on a hydrogen-air fuel mixture, with a premix burner using acetylene and air for fuel. The total consumption burner is the Hetco type, and was used with the Jarrell-Ash spectrophotometer. The premix burner is the Perkin-Elmer type and it was tested as a part of the Perkin-Elmer M o d e l 290 Atomic Absorption Spectrophotometer. Synthetic standards for C a , M g , Fe, Pb, and C u were analyzed in varying concentrations. The data was assessed on both a short-term (daily) basis, where new standard curves were prepared daily, and on a long-term basis, where the shift i n the calibration curve was included i n the data. The results obtained in the analysis of lead, comparing the total consumption and premix burners on a short-term basis, is shown i n Figure 5. The same hollow cathode lamp was used with both burners. It w i l l be noted that the premix burner exhibits better precision at high con centrations of lead but becomes poorer below 2 p.p.m. A t less than 7 p.p.m., the original Beckman three-burner system appears superior to the other two. This was the only element where the three-burner system showed an advantage. Figure 6 shows the precision obtained i n analyzing for copper. In this case, the two types of burners perform similarly with the premix burner slightly better when used with the same hollow cathode lamp.



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2

4

6

8

10

12

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14

I N

16

18

W A T E R

20

CONCENTRATION : g / m l M

Figure 5. Variation with concentration of the standard deviation of lead for three types of burners

CONCENTRATION : μ g / m l

Figure 6. Variation with concentration of the standard deviation of copper for two types of burners Figure 7 shows the results i n the analysis of magnesium i n water, where the precision with the premix burner is slightly better. However, a different lamp was used here with each burner so some or all of this difference could be attributable to a difference i n the lamps. The older three burner set was rechecked with no improvement i n precision. T h e effect that the hollow cathode lamp can have on precision can be seen in Figure 8, where two different lamps were used with the premix burner


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PRECISION MAGNESIUM

\\

• - Original Burner 30

\\\\

° - Total Consum. Burner χ - Pre-Mix Burner

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ζ

\\ \ \ \ N χ

10

1 .5

°

Λ

t 1.0 CONCENTRATION :

*

J

1-5

20

/mi

Figure 7. Variation with concentration of the standard deviation of magnesium for three types of burners

5

10 CONCENTRATION :

Figure 8.

15

20

/ml

Variation with concentration of the standard deviation of iron for two types of burners and two hollow cathode L·mps

in the analysis of iron. Variation i n energy output of one lamp affected the precision obtained on the premix burner, such that the values were approximately twice as poor as with a second lamp. T h e second lamp was used with both flames and the precision data showed the total con sumption burner to be about a factor of two better than the premix. The precision obtainable for the two burners i n the analysis of calcium is


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shown in Figure 9. Here the two burners show about the same perform ance except for the superiority of the total consumption type at the lower concentrations of calcium. 5 0 ft? PRECISION : CALCIUM

ζ ο Downloaded by UNIV QUEENSLAND on April 14, 2013 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0073.ch013

>

x - P r e - M i x Burner

30

UJ

o

ο - T o t a l Consum. Burner

Ω

or

ζ< *

10

15

10 CONCENTRATION

Figure 9.

M

30

g/mi

Variation with concentration of the standard deviation of calcium for two types of burners

The data in Figures 5 through 9 were computed on the basis of a single days run of ten determinations, and then averaged over several days. This then gives the precision obtainable for the case where one checks his calibration curve each day with known standards and then adjusts the curve if any shift is observed. To determine the importance of this procedure, the same absorption data were converted to concen tration using a single calibration curve over the period of the tests, with the result that there was generally a decrease in precision. This informa tion is summarized in Table II, where the precisions attainable are given on both the short and long term basis. Several points are apparent in Table II. First, the precision data on these two burners are considerably better than that shown in Table I for the older three-burner system. This was true both for flame emission and flame absorption. Second, the two burners are about equal on an overall basis for the five elements examined. The other information summarized here are the detection limits obtainable. It w i l l be noted that these figures are not as low as some cited (6) in the literature for a signal-to-noise ratio of two while com parable to others (5). For the first three elements, it was neither neces sary nor desirable to set up conditions for maximum sensitivity because of the large amount of these elements in water. However, with the last



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three elements, instrumental conditions were optimized, except that scale expansion was not used to the maximum. But the most important factor here appears to be the quality of the hollow cathode lamps, i n that the limit of detection for any one element w i l l vary from lamp to lamp. Secondly, the sensitivity and stability attainable with a particular lamp w i l l frequently deteriorate with age. A n example of this was noted with a particular lamp for lead, where the detection limit dropped from 0.2 p.p.m. to 0.5 p.p.m. within the first few hours of operation. The ability to produce hollow cathode lamps in a uniform and reproducible manner has improved considerably since atomic absorption equipment became available commercially, but the quality control of these lamps is still a problem. Table II. Limits of Detection and Effects on Overall Precisions for Six Elements When Using Single Calibration Curves and Curves Prepared Daily Precision, % Std. Dev.

Element

Total Cons.

Pre-Mix

Total Cons. Short Term/ Long Term

Na Ca Mg Fe Pb Cu

0.2 0.5 0.05 0.8 0.5 0.04

0.2 0.05 0.5 1.5 0.04

5.0 2.5/2.5 4.0/8.5 3.5/3.5 5.0/9.0 6.0/6.0

Limit of Detection, p.p.m.

Pre-Mix Short Term/ Long Term

2.5/4.0 2.5/4.0 7.0/10.0 4.0/4.0 5.0/e.o

Conclusions This study was conducted for the purpose of evaluating atomic absorption and flame emission spectroscopy for the routine quantitative analysis of large numbers of water samples for elements present in quanti ties ranging from large to trace amounts. The conclusions reached are as follows: 1. The alkali metals and alkaline earth elements are generally present in sufficient quantities to be analyzed directly without concentration. Flame emission is preferable for the alkalies because of its good sensi tivity and few interferences i n the analysis of this class of elements i n waters and because the technique eliminates the need for hollow cathode lamps. For the alkali earth elements, the flame emission method is still applicable, but with some problems owing to chemical interferences i n some waters. Atomic absorption is preferred for this class of elements and although the chemical interference problem still exists, it is not as serious as in flame emission. Those elements present in amounts less than



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their limit of detection require concentration by evaporation or extraction before analysis by atomic absorption. 2. The two instrumental components which have the most significant effect on the limit of detection and precision of analysis are the burners and the hollow cathode lamps. For the two instruments used i n this study, there was no evidence that other components (monochromator, detector, amplifier, etc.) were limiting factors in the detection limit or precision. 3. Of the two types of burners available (total consumption and pre-mix burners ) neither has an over-all advantage for the five elements compared. 4. The state of the art in the production of hollow cathode lamps still leaves much to be desired, both as to uniformity between lamps and changes within single lamps with time. 5. The precision of analysis can be within 3 to 5 % standard devia tion if care is taken to check calibration curves frequently. Also, fre quent maintenance of the burners is necessary to attain this precision consistently. 6. Even with the difficulties cited above, the combination of flame emission and atomic absorption spectroscopy has become in a very short time one of our better methods for the analysis of waters for cations for the following reasons: limited sample preparations necessary, high sensi tivity, good analytical precision, low cost, and simplicity of equipment. Literature Cited (1) Biechler, D . G., Anal. Chem. 37, 1054 (1965). (2) Dean, John Α., "Flame Photometry," McGraw-Hill Book Co., New York, 1960. (3) Dickson, Richard E., Johnson, C. M . , Appl. Spectr. 20, No. 4, 214 (1966). (4) Fishman, M . J., Downs, S. C., U. S. Geol. Surv. Water Supply Papers 1540C (1966). (5) Mavrodineanu, R., "Encyclopedia of Industrial Chemical Analysis," F. D . Snell, C. L . Hilton, eds., p. 160, Interscience, New York, 1966. (6) Robinson, James W., "Atomic Absorption Spectroscopy," Marcel Dekker, Inc., New York, 1966. (7) West, T. S., Nat. Bur. Std. (U. S.) Monograph 100, 266 (1967). (8) Yofe, J., Finkelstein, R., Anal. Chim. Acta 19, 166 (1958). (9) Youden, W . J., "Statistical Methods for Chemists," John Wiley & Sons, Inc., New York, 1951. April 24, 1967. This investigation was supported in part by USPHS Research Grant OH-00232, Division of Occupational Health, Bureau of State Services. RECEIVED


Water Analysis by Atomic Absorption and Flame Emission

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