Performance of a simple atomic absorption spectrophotometer

Spectrochimica actn, 1961,Vol. 17,pp. 710to 718. Pergnmon PressLtd. Printedin NorthernIreland

Performance of a simple atomic absorption spectrophotometer B. M. GATEHOUSE and J. B. WILLIS Division of Chemical Physics, CSIRO, Chemical Research Laboratories, Melbourne, Australia (Received 23 January 1961) Abstract-The scope of a simple atomic absorption spectrophotometer in the determination of metal ions in solution is illustrated by the results obtained for thirty-six elements. Appropriate selection of the experimental conditions for each element has yielded lower limits of detection than

those

reported

lines permits

previously

the method

for the atomic

to be extended

absorption

method.

to the determination

The use of second

resonance

of high concentrations.

the early work of WALSH [l] and RUSSELL et al. [2] there has been a rapidly increasing interest in atomic absorption methods for the determination of metal ions in solution. Recent progress has been such that the scope of these methods is now much wider than that described in the most recent reviews [3, 41, and it appears opportune to give a detailed survey of the method as exemplified by the performance of a simple atomic absorption spectrophotometer [5]. This type of equipment has been used by DAVID [&9] for the analysis of soils and plants, by WILLIS [lo-131 for the analysis of blood serum and urine, and by RAWLING et al. [14] for the analysis of lead concentrates. SINCE

Experimental The apparatus was that developed by Box and WALSH [5] except that the powerpacks and amplifier were standard production models. * Similarly many of the hollowcathode tubes were commercially madet to the design of JONES and WALSH [15]. The solution to be measured was sprayed into the flame through an EEL atomizer and spray chamber; the uptake of solution was about 3 ml/min. The IO-cm burner, which could use either air-coal gas or air-acetylene, was of massive construction in stainless steel in order to avoid distortion due to heat. Details of construction are shown in Fig. 1. The consumption of air was about 4 l/min, that of coal gas 2 l/min, and that of acetylene about 1.2 l/min. This burner could of course be replaced by the water-cooled type described by CLINTON [ 161 and Techtron Appliances, Ransley Glass

Market Street, 25 Islington

Melbourne, Australia. Collingwood, Victoria,

[l] A. Spectrochim. Acta 108 (1955). 121BARBARA J. RUSSELL, J. P. SHELTON and A. WALSH, Spectrochi,m. Actn 8, 317 (1957). 13j J. W. ROBINSON, A&Z. Chem. 32, 17A (1960). _ [4] A. C. MENZIES, Anal. Chem. 32, 898 (1960). [5] G. F. Box and A. WALSH, Spectrochim. Acta 10, 255 (1960). [Sl D. J. DAVID, Analyst 83, 655 (1958). [7] D. J. DAVID, Analyst 84, 536 (1959). [8] D. J. DAVID, Analyst 85, 495 (1960). [91 D. J. DAVID, Nature 187, 1109 (1960). lo] J. B. WILLIS, Spectrochim. Acta 16, 259 (1960). 111 J. B. WILLIS, Spectrochim. Acta 16, 273 (1960). 121 J. B. WILLIS. Spectrochim. Acta 16, 551 (1960). 131 J. B. WILLIS, Anal. Chem. 33, 556 (1961). 141 B. S. RAWLING, M. C. GREAVES and M. D. AMOS, Nature 188, 137 (1960). 151 W. G. JONES and A. WALSH, Spectrochim. Acta 16, 249 (1960). 161 0. E. CLINTON, Spectrochim. Acta 16, 985 (1960).

710

Performance of a simple atomic absorption spectrophotometer

used extensively by ALLAN [17-201 in agricultural interest. Calibrating solutions for each metal solution containing 1000 p,p.m., which hydrochloric acid or a pure stoichiometric were acidified with acid where necessary

his work on the analysis

of samples

of

were made up by dilution from a stock was obtained by dissolving the metal in salt of the metal in water. The solutions to prevent hydrolysis.

Fig. 1. Diagram illustrating construction of IO-cm burner. The two halves are dow&ed and screwed together.

Results and discussion Table 1 summarizes the best experimental conditions found for the determination of each metal, both for very .dilute and for more concentrated solutions. The limits of detection are based on the assumption that the smallest detectable absorption was 1 per cent, which corresponded to the maximum noise level, although it will be seen from Table 1 that in many cases the noise was well below this figure. With a monitored hollow-cathode source and a scale expansion unit the limits of detection could certainly be reduced somewhat, as the noise introduced by irregularity in the atomizer was only about &0*25 per cent. The limits of detection given here refer to solutions containing only the salt of the ion being determined; in the presence of Iarge concentrations of other ions the sensitivity in some cases may be less than that shown. Five factors are involved in the choice of optimum conditions for the estimation of a given metal. 1. H&w-cathode

tube filler gas

Most of the hollow-cathode [li’] [18] [19J [ZO]

3

J. E. J.E. J. E. J. E.

tubes used in this investigation

ALLAN, Analyst 83, 466 (1958). A~~~~,&ectrochim. Acta 15,800(1959). A~t~~,Spect~ochi~~ Acta 17, 4.59 (1961). ALLAX, Spectrochi~la. Acta I?, 467 (1961). 711

were filled with

B. M. GATEHOUSEand J. B. WILLIS Table 1. Optimum conditions and limits of detection of metal ions in solution by a simple atomic absorption spectrophotometer [5] Lamp current (mA)

Line

Slit width*

Noise

(11)

(A)

:Zk.%

12 30

6708 3233

110 3

Na

750 830

5890 3302

K

900 900

Rb

Metal

Flame?

-__

Concentration (p.p.m.) giving 50% absorption

:oncentration (p.p.m.) giving 1% absorption

Notes

1

nil 1

C C

2 1000

0.03 15

13 3

0.25 1

C C

2 350

0.03 5

a,b,l

7665 4044

30 6

0.5 0.25

C C

2 350

0.03 5

8.b.J

800 800

7800 4202

150 6

0.6 0.25

C C

9 700

0.1 10

8-J

Cs

650 900

8521 4556

400 8

1 nil

C C

9 1500

0.15 20

a.1

cu

4 18

3248 2228

3 0.5

nil 2

C C

6 100

0.1 2

a,1

Ag

4 4

3281 3383

3 3

0.5 0.5

C C

4 12

0.05 0.15

a.1

Au

6 6

2428 2676

1 1

1 1

C C

20 100

0.3 1.3

a,l,n

Be

20

2349

1

1

A’

Mg

6 6

2852 2796

2 1

0.25 0.25

A A

0.6 1500

Ca

10 20

4227 2399

6 1

0.25 1

A A

5 2000

0.08 20

Sr

20 20

4607 4078

8 6

nil nil

A A

10

0.15 3.5

Ba

36

6536 7911 3262

14

1

A A

600

Zn

6

2139 3076

2 2

1 0.5

C C

Cd

2

2288 3261

2 3

1 1

2537

1

20

3962

4 4 4 4

2874 4172 2944 4033

Li

Hg Al Ga

>lOO 0.01 5

k L,C.l

0

b,c,l

8 > 1000

c.m

2 10,000

0.03 150

a.d.k.0

C C

2 1400

0.03 20

0.5

C

1000

10

5

0.25

A

1 4 1 4

0.5 0.25 0.5 0.25

A A A A

See 1 k,notes

712

> 1000 250 400 300 500

3 5 3 7

e k s.k

Performance of a simple atomic absorption spectrophotometer Table I-(cant&f .

-

Metal

L-P current (m‘W

Line

Slit width*

(A)

(A)

lJoise

: &%I

Flame?

1

_760 750

2768 3776

7 4

0.25 0.5

C C

Ti

10

4382

10

0.25

A

Sn

8

2863

3

1

Pb

4 4

2833 2614

2 1

0.5 1

V

10

2832

2

0.5

Tl

Concentmtion

Concentration

(p.p.m.1 giving 50% absorption

(p.p.m.1 giving 1% absorption

60

0.8 3

200

> 1000

A (very rich)

350

C C A

5 0.5 50

Notas

SJ

1

%@.I

%l

1

’

I(very rich)

10

2909

2

A

0.5

/[very rich)

> 1000

A I[very rich) A /(very rich) A A A A)

10

3184

2

0.25

10

4379

7

0.25

10 10 10 10

4053 4080 4124 4137

6 6 6 6

0.25 0.25 0.25 0.25

Sb

6 12

2311 2176

4 1

1 2

c C

100 75

1-5

b.S.1

Bi

8

3068

2

1

C

150

2

a,b,C,I

cr

10 4

3579 4254

4 7

nil 0.5

A A

10 60

0.15 0.5

C,l

6

3133

4

0.25

40

0.5

cm

4

3170

4

1

A (very rich) A

20

4234 4302

7 7

0.25 0.25

A A

MIl

5 6

2735 2801

1 1

0.75 1

C C

4 10

0.05 0.2

8.1

Fe

5 10

2483 3720

1 4

0‘75 0.25

A A

7 75

0.1 1

b3.k

co

25 25 25

2407 3454 3527

1 3 3

1 0.5 0.5

A A A

15 300 400

0.2 4 3

Ni

23 23

2320 3415

0.5 3

1 0.5

C c

15 40

092 0.5

b,d,hk

Rh

4

3435

3

o-5

A

25

o-3

8,CJ

Pd

6

2476

2

1

c

55

0.8

8,b.l

Pt

7

2653

1

A

600

5

a,bJ

Nb

X0

w

-

1 -

713

> 1000

150

2 >lOOO

I

B. M. GATEHOUSEand J. B. WILLIS

argon, which is removed by “clean-up” more slowly than helium or neon. However, we have found that with zinc and nickel tubes a higher signal-to-noise ratio, for a given line width, is obtained by the use of helium. Similarly CROSSWHITE et al. [21] have shown that neon is to be preferred for iron tubes. We have not made a systematic study to determine the optimum filler gas for each tube. In Fig. 2 are shown calibration curves obtained with two zinc hollow-cathode tubes filled with argon and helium, respectively. 2. Choice of lamp cwrent To obtain the highest sensitivity it is essential that the line whose absorption is to be measured should not exhibit either self-reversal or self-absorption. For some tubes, such as silver, zinc and cadmium, whose cathodes sputter profusely, this condition can be achieved only by using tube currents as low as 2 mA. At such low currents the discharge is extremely weak, and tends to become unstable, so that it is necessary to strike a compromise between high sensitivity and a tolerable noise level. One of the advantages of helium as a filler gas is that it permits the use of higher currents for the same amount of sputtering. Fig. 2 also shows the effect of hollowcathode current on the calibration curves for zinc. 3. Type of flame It is not generally realized that a versatile atomic absorption spectrophotometer for measurement of a wide range of metal ions in solution requires provision for at least two types of flame. The burner used here is designed to burn either air-coal gas or air-acetylene mixtures, and although for many metals the type of flame has FOOTNOTES

TO TABLE

1.

* The spectral slit-width quoted is the half-intensity band-width, which is taken from the dispersion data supplied with the Beckman DU monochromat,or. The mechanical width was checked for various settings of the slits. t C = coal gas-air; A = acetylene-air. 8 Markedly dependent on lamp current. b Markedly dependent on slit width. c Markedly dependent on flame type. d Helium-filled hollow-cathode tube used. e “Mineralite” mercury lamp used as source. r Neon-filled hollow-cathode tube used. s The line at 2176 A is slightly more sensitive than that at 2311 A for low concentrations of antimony, but the calibration curve flattens out rapidly above about 100 p.p.m. The noise level is much higher than for 2311 A. h The calibration curve for the line at 2407.3 A flattens out rapidly above about 50 p.p.m., owing to the presence of a weak line near 2406.7 A (probably Cu 2406.67 A, arising from the copper cathode containing the pellet of cobalt). 1 The calibration curve for the line at 2320.1 A flattens out rapidly above about 50 p.p.m., owing to the presence of a weak line near 2319.8 A (probably Ni 2319.76 A). 1 Osram or Philips spectral lamp as source. k Hollow-cathode tube made by Mr. W. G. JONES. * Hollow-cathode tube supplied by Ransley Glass Instruments, Melbourne. m Hollow-cathode tube supplied by Mr. D. J. DAVID. n LOCKYERand HAMES[22] found that, for a given concentration of gold, the absorption decreased as their Meker-type burner warmed up. This effect does not occur with our burner. 0 GIDLEY and JONES [23] found that halogen acids caused spurious absorption near 2139 A; this effect does not occur with our equipment. [21] H. M. CROSSWRITE,G. H. DIEKE and C. S. LE~AGNEUR,J. Opt. Sot. Am. 45, 270 (1955). 1221 R. LOCKYERand G. E. HAMES, Amlyat 84, 385 (1959). [23] J. A. F. GIDLEY and J. T. JONES, Amlyst 85, 249 (1960). 714

Performance

of a simple atomic absorption

spectrophotometer

little effect on the sensitivity there are some for which the appropriate choice of flame is essential. Barium, chromium, rhodium, tin and molybdenum all require an acetylene flame if satisfactory sensitivity is to be obtained; molybdenum indeed shows almost no absorption at all in the coal-gas flame. Furthermore, some types of chemical interference are more easily overcome in the air-acetylene flame than in the air-coal gas flame [IO]. Several metals, on the other hand, show appreciably higher absorption in the coal gas than in the acetylene flame, notably IO

0.8

06

&

6

si :: a

/ / 0.4

/ /, / 0.2

C

Fig. 2. Calibration

2.5

curves for zinc, using helium-filled and argon-filled tubes run at different currents.

hollow-cathode

potassium and bismuth. Choice of the optimum air-gas mixture and height of the absorption path above the base of the burner is also essential, especially for calcium [7, IO], strontium, barium, gallium, tin, chromium, manganese, molybdenum [9] and rhodium. Fig. 3, which shows calibration curves for tin and bismuth, illustrates the importance of choosing a suitable flame, while Fig. 4 shows the effect, first reported by DAVID [9], of varying the acetylene-air mixture and the height of the absorption path in the flame when determining molybdenum. 4. Slit width The function of the monochromator is to isolate the resonance line(s) whose absorption is to be measured from all other radiation emitted by the light source, and the slit width should be as wide as possible consistent with this condition being 715

B. M. GATEHOUSEand J. B. WILLIS Bi.

PPm

125

boo

IT

OS

06

__

P

x

3

6 ._ 8 ._

Fig. 3. Calibration curves for tin and bismuth using different types of flame.

%

04

0.2

-

I

1 ‘““‘ye_..

_

i

500

0

Sn,

P

IO00

750

p.m.

IC

OE

Fig. 4. Effect of variation of acetylone-air mixture and height of absorption path in the flame on the sensitivity for molybdenum. The air flow wae kept constant at 5.5 l/min and the acetylene flow was varied as shown.

3 i 51 2

--I--

---T--T

I

0.E

0,

Heaght

716

in flame

above

burner

top,

mm

Performance of & simple atomic absorption spectrophotometer

fulfilled. Indeed, in some cases it is advantageous to sacrifice some sensitivity by opening the slit and thereby obtaining increased signal/noise ratio. In cases where complete isolation of the selected resonance line is not achieved there is a resulting loss in sensitivity and the calibration curve becomes markedly curved. A typical example is the curve for the nickel line at 2320.1 A shown in Fig. 5. An echelle spectrum obtained by our colleague Dr. G. R. HERCUS shows that the marked curvature in this case is due to the monochromator passing the line at 2319.8 A I

0

i

0 ::

f 0

0

>

50

Ni.

w.m

Fig. 5. Calibration curves for nickel 2320& showing effect of incomplete separation of resonance line.

which is not a resonance line and therefore does not exhibit any absorption. Similar effects are produced by general background in the neighbourhood of the line (see ALLAN [18]). 5. Choice of absorbing line As has recently been pointed out by ALLAN [18], the most sensitive lines in absorption do not necessarily coincide with the strongest emission lines. The lines listed in Table 1 include those found to give the highest sensitivity for the elements concerned and lying in the wavelength range above 2100 A. It is possible that more sensitive lines lie below this wavelength. For example it is known that the oscillator strength of the mercury line at 1849 A is fifty times greater than that of the 2537 A line. Table 1 also lists less sensitive lines for many of the elements. These facilitate the application of the absorption method to the determination of high concentrations. 717

B.M.GATEHOUSE

and J.B.

WILLIS

Table 1 summarizes the capabilities of a single-beam atomic absorption spectrophotometer. It should be stressed that this instrument is of simple design and the results obtained should not be taken to represent the limits of the absorption method. It is obvious that further improvements are to be expected from double-beam methods or from other systems for monitoring the light source. Similarly the full possibilities of scale expansion and multiple traversals of the flame have yet to be explored. However, we are of the opinion that the most spectacular advances in technique will come from new and improved methods of sample atomization. Acknowledgement-We course of the work.

wish to thank Dr. A. WALSH

718

for his interest and advice throughout the