A Study of Atomic Absorption Spectroscopy A. C. MENZIES Hilger & Watts Ltd., London, England
Flame photometry has been used for many years to analyze materials which may b e brought into solution. For certain metals, notably the alkali and alkaline earth metals, the method has become standard. However, its scope is restricted and not until 1955 did Walsh suggest analyzing by flame absorption instead of emission. The newer technique is called atomic absorption spectroscopy. The technique and some of the difficulties in its application, as well as some examples of its use, are presented.
T
of atomic absorption for analytical purposes was first proposed and developed by Walsh (15). There had previously been applications of other kinds. The earliest observation of atomic absorption was probably that of Wollaston (16), who first observed the non famous absorption lines in the spectrum of the sun-the “Fraunhofer lines.” They were first explained by Brewster ( 2 ), who in 1832 was working on another phenomenon-passing white light through nitrous anhydride and observing the absorption which took place. He suggested that the Fraunhofer lines were also due to the passage of white light through a n absorbing vapor. In 1860 Kirchhoff ( 7 ) systematically investigated the reversal of spectral lines due to lithium, sodium, potassium, and calcium, strontium, and barium, in the nonluminous flame of the burner n-hich his collaborator Bunsen had invented. Liveing and Dewar (8)also made many investigations of the absorption of spectral lines by vapors. The reversal of spectral lines has been applied frequently to determining the temperature of flames or similar bodies of luminous vapor, as in the vapors surrounding stars. The basis of the method is that when light from a source is passed through a hot vapor, the lines will appear in emission or in absorption, according to whether the temperature of the source does or does not exceed that of the vapor. When the two are equal, the line is not observable. (It is assumed that thermal equilibrium has been reached in the flame.) An example of such a n application is that of Huldt (6), who measured the temperature of an acetylene flame. HE USE
898
ANALYTICAL CHEMISTRY
RESONANCE SPECTRA
I n flames, the great majority of the atoms of a metallic element introduced into the flame are in the ground stateLe., that of least electronic energy. When the atoms receive energy, either from the heat of the flame, or froni radiation of suitable n-ave length incident in the flame, they may reach some higher allowed state. The word “allowed” here means that the transitions should be such that the spectral rules are not broken-viz., in terms of L-S coupling, that L should change by =t1, and that J should change by & I or 0 (0 +- 0 excluded). Also the two spectral terms should have the same multiplicity. Unfortunately, theqe rules, especially the last one, may on occasion be broken, especially for heavy atoms-for example, the 2536-A. line of mercury, which is a strong line in absorption, although it is a singlettriplet intercombination. The lines corresponding t o all such allowed transitions together constitute the resonance spectrum of the element. One may speak of the line of longest m-ave length (lowest transition energy) in the resonance spectrum as the resonance line. This term should be used with care, however, because repeated use may give the impression that this line is the only one which may be used in observing absorption by atoms in the ground state. Receiving energy due t o the heat of
Zinc 2138 A. Potassium 7665 A.
In an aeseinbly of atoms and electrons, in thermal equilibrium a t teniperature T , having two states, of energy El and E2, respectively, the numbers of atoms in the two states will be given by n,
=
a2 = C
statistical weights of the respective states. For a spectral term having the total quantum number J , p is 1). J has (2J 1) equal to (25 orientations in the field of the atom, and the magnetic quantum number niJ has (2,J 1) different values. For example, if J = 1, VLJ may be -1, 0, or +1-that is, these are three different possible values for w-and the statistical weight. p . is 3. The ratio n,/nl will be
+
p2/pl
e - ( E i E d kT
For the line 2138.56 A. of zinc, corresponding to the transition ’So - ’p1, J of the lower term is zero, so that p l is unity, n-hile J for the upper term is 1, so that p , is 3. T l i ~ for . this line, p 2 / p l is 3. and n2,nl is 3e-(E1-E?)i T , S o w (E2 - E l ) is h V , if I, is the frequency of the spectral line Thus, n2/n1 = 3e-hp K T and this lead* to the folloning values of n2 nl for the temperatures of flames used in flame photometry.
3.79 x 10-14 2.57 X 10-4
1.13 X 10-9 4.67 x 10-3
The theory of resonance radiation and excited atoms has been treated by Mitchell and Zemansky (IS). As Walsh (16) pointed out, only a slight acquaintance with the theory is sufficient t o bring out the salient features of atomic absorption.
+
+
Oxyhydrogen (3100” K.)
RELATIVE NUMBERS OF ATOMS IN THE GROUND STATE AND A N EXCITED STATE
ple-E2’k*
Here C is a constant independent of E , while p l and p z are numbers, called
Air/Coal Gas (2100’ K.)
the flame, with consequent emission of the energy as a spectral line, corresponds to what happens in flame photometry; taking the energy from another source of radiation corresponds t o atomic absorption.
C ple-E1 kT
and
Oxycyanogen (4700” K.) 1.85 X los6 3.66 X l o u 2
The low values of nz,’nl are mainly due t o the exponential factor, nhich greatly outweighs the iveight factor. One should expect a larger value for nZ/nl hen the wave length is long, so that E z - El is relatively smaller. The potassium line 7665 A. for IThich p z / p l is 2 has been added in the above table for coniparison: n2/nlis still small. I n flames the large majority of the metal atoms are in the ground state. ABSORPTION CONTRASTED WITH EMISSION
Because in the flame the great majority of the atoms are in the ground
state, to obtain a strong emission line, the temperature of the flame should be high enough to cause enough atoms to he excited to the higher level, and the spray should he as concentrated as possible, so as to introduce a large number of atoms without unduly cooling the flame. For emission the temperature of the flame is very important, because it decides how many lines are excited. On the other hand, as shown by the preceding paragraph, most of the atoms are in the ground state, the initial state for absorption, and the temperature of the flame makes little difference to this. Hence, the absorption process is more efficient than the emission process, provided the atoms are present in the flame. This explains one of the chief sources of poor performance in the absorption method: Some elementsfor example, calcium-tend to form refractory compounds in the flame, so that molecules rather than atoms are present. Thus, for the ahsorption process, the flame temperature should he that which brings about dissociation of the solute in the spray, into as great a proportion of atoms in the ground state as possible. This explains why zinc,. for example, with a short wave-length resonance line, is easily determined in absorption, with a sensitivity of 0.1 p.p.m., while in
emission it is difficult to detect, let alone measure it. Even in the oxycyanogen flame, only about 1 atom per million is in the excited state necessary to emit the resonance line, while practically ail are in the ground state, ready to absorb. (So far, the atoms have been considered to have a singlet ground state, hut the general considerations are not affected if the ground state should he multiple: It is necessary formally only to put 11,
=
C Zpie-EiikT
where the summation is carried out over the components of the multiplet.) TECHNIQUE
The general principle is to treat the flame into which the spray is injected as if it were a trough of absorbing solution in spectrophotometry, and to measure the absorbance of the flame. The absorbance of the flame for light of a resonance wave length is a direct measure of the concentration of absorbing atoms in the flame, and hence of the concentration of atoms in the dissolved material. SPECIAL EQUIPMENT
Source. The absorption line in the flame is very narrow. Various factors cause broadening of the line, the chief being Doppler broadening and pressure broadening, and these are described by Mitchell and Zemansky (15). The most important is the Doppler broadening, caused by the thermal motion of the atoms. I n fact, just as in the acoustical Doppler effect, the change in frequency depends upon the velocity of the atom concerned. Putting the kinetic energy proportional to the temperature Mu'a
T
so that Y
Figure
1.
Hollow-cathode
lamp
ad/TIM
and
Photographed during constiuction to show detail
An a
m
where A v is the Doppler broadening. So heavy atoms in a low temperature flame give rise to very narrow ahsorption lines. If, therefore, the measurement is to he efficiently carried out, the source must yield very sharp emission lines. This is an important point in the technique, to provide a source giving sufficiently sharp lines-otherwise absorption will not he observed efficiently. Pressure broadening is due to collisions between the atoms with one another, and with the atoms and molecules constituting the flame itself. The sources which are most suited to the purpose are sources stimulated by either microwaves or hollow-cathode lamps, and the latter have been found generally more convenient. Figure 1 shows the construction of the Hilger hollow-cathode lamps. For the alkali metals and other volatile metals, enclosed axe lamps such as the Osram or Philips type may be used, hut as pointed out by Russell, Shelton, and Walsh (14) they should he considerably underrun to reduce self-reversal. This consideration makes clear the need for sharp lines in the source and that attention has to he paid to the conditions under which the hollow-cathode lamp is operated-a higher operating current may produce more light, but the lines may he insufficiently narrow. A second requirement is that the source should have a steady output. Flame. The flame should have a long path length, and t h e effective length of t h e flame may he increased, as has been done by Russell, Shelton, and Walsh (14, by multiple passages through the flame. Even so, a long path length is advantageous. This was stressed by Allan (1) in his work on the determination of magnesium in soil, in which he found that the sensitivity was approximately proportional to the length of the flame. Figure 2 shows a cutaway view of the burner in the Hilger flame housing. The flame mill itself emit some light of the wave length which is being measured, and if great, this would falsify the results. Walsh (16) eliminated this emission by the ingenious device of modulating the source, and having a tuned amplifier
.-
Figure 3. Equipment for atomic absorption spectroscopy Figure 2.
Cutaway view of Hilger burner and house
From left to right, spectrophotometer, hdio*-cothoda In front, atomizer over rink ond hmet for sampler
lamp, burner house
VOL. 32, NO. 8, JULY 1960
899
in the receiving circuit: The flame emission may be regarded as a steady output, so that the detector is blind to it. Fortunately, in many cases the emission is very low, so that often the emission from the flame may be ignored. I n a long flame, some light emitted from the back of the flame directed towards the spectrograph may be absorbed as it travels forward, and be re-emitted in random directions. Spectrophotometer. The absorbence of the flame may be conveniently measured by an instrument of the photoelectric type, such as the Uvispek spectrophotometer; apparatus is available commercially which may be added t o such a n instrument, and Figure 3 s h o w the apparatus used in the author’s laboratory. From left to right are the lamp and flame housing, the Uvispek spectrophotometer, and the power pack to operate the spectrophotometer from the town mains. The latter may be put
below the bench. Other forms of spectrophotometric apparatus may be used; Allan (1) used a Hilger medium quartz spectrograph with a photomultiplier placed so as to receive the 2852-A. magnesium line. I n the author’s laboratory, a direct-reading medium quartz spectrograph has been used with good results; using a brass hollow-cathode lamp, slits were set to receive copper and zinc lines. 06I
40
60
BO
(Percentage composition) Copper Zinc ChemiChemiSample A.A. cal A.A. cal I 70.1 70.07 30.0 29.93 I1 63.0 63.00 36.9 37.0 85.2 85.15 1 5 . 0 14.85 I11 5 8 . 7 58.73 3 8 . 8 39.31 IV 63.1 63.22 36.9 36.78 V Manganese VI
1.36
Nickel
1.34
0.07
Lead 1.26
0.08
Copper 1.28 56.5
56.31
Zinc
Table
II.
A.A.
Chemical
38.4
39.5
Determination of Zinc in Copper Alloys
Procedure Gravi- Atomic Polaro- metric absorpgraphic zinc, yo tion
Alloy
Copper/zinc
0.51 1.01 1.56
99.5/0.5 99.0/1.0 98.5/1.5
Phosphor bronze sample 168 158 512 152 403 169 173 177
Gildin metal (85h)
900
0.72 0.32
0.72 0.31 0.020 0.16 0.097 0.096 0.079 0.29 14.8
ANALYTICAL CHEMISTRY
0.50
i.00 1.53
0.72 0.32 0.015 0.17 0.096 0.084 0.062 0.27 14.8
The resolution requirements in this technique are met by the relationship of the narrowness of the emission l i e in the source to that of the absorption line in the flame. The purpose of the monochromator used is not to provide fine resolution; rather, it has two tasks: to reject resonance lines of other elements, and to prevent the receiver from being overloaded with light-e.g., from the carrier gas in the discharge tube. If it does these, it can be fairly coarse-in fact, for some purposes a filter may suffice, as in the Sirospec designed by Walsh (15) for sodium determinations.
100
GPrn C”
Analysis of Bronze
RESOLUTION OF THE SPECTROPHOTOMETER
, 20
Table 1.
taneous currents or integration Kith condensers. The method was briefly mentioned by Menzies (11). The drawback is the need for doubling receivers, while the gain is in speed and accuracy.
Figure 4. Working curve for copper concentrations
0 Copper alone X Copper with added lead
A special technique devised in this laboratory uses two slits and receivers for one element, so arranged that light of an absorbed wave length passes through one, and light due to a line which is not absorbed passes through the other. When the solution is sprayed in the flame, the ratio of the intensity of the absorbed line to that of the unabsorbed line falls. Two receivers are necessary, but only one beam of light, so that the need for two beams is avoided, or the need for successive measurements, one without and one with the solutions being sprayed. The lamp must be stable for accurate n-ork so that the intensities of the two lines from the lamp stay in constant proportion. If the two lines were homologous (ratio of intensities sufficiently independent of electrical conditions in the lamp) the lamp would not need to be stable. However, a truly homologous line pair is not to be expected, because the excitation potential of the upper state of the resonance line will lie below the upper state of the unabsorbed line in the energy diagram of the spectrum. Consequently a higher potential will be needed for the unabsorbed line t o be emitted than for the resonance line. This makes it clear that the best unabsorbed line to choose is one with a low excitation potential for the upper state. Successful use has been made of this technique, using a two-channel direct reader, designed in this laboratory, employing alternatively instan-
MODEL
OF
THE PROCESS
A working model of the process is as follows: The solution is sprayed, the spray consisting of drops. These drops, which form the cloud entering the flame, are remarkably uniform in size. Each drop contains some of the solute, and under ideal conditions each drop would contain the same mass of solute. On entering the flame, a drop is evaporated, forming a roughly spherical mass of the solid solute. For want of a better name, and to distinguish it from a drop, let us call this a “clot.” The clot contains in solid form whatever is in the solution when dried, and it consists of an intimate mixture of the salts. The clot is then acted upon by the heat of the flame, decomposed, and dissociated, a t least partially. The rush of gas meanwhile carries i t away, and, therefore, the time during which a clot may be effective is limited. The greater the concentration of the solution, the larger the clot, and so the more limited is the time during which decomposition may occur. If the flame has only a small region of temperature hot enough to decompose a particular clot, the shorter will be the available time, so that one may think of an extreme case when only the outer layer of a clot may be effective-i.e., the efficiency will be low, and the absorption will be limited to a small region of the flame. An interesting experiment made by Gidley and Jones (5) tends t o confirm this view. They found that the interference between magnesium and aluminum, when these are sprayed into the flame, disappeared when two atomizers were used, one for the solution of magnesium, and the other for the solution of aluminum. Thus the magnesium and the aluminum were contained in separate clots and interference disappeared.
APPLICATIONS TO ANALYSES
Copper-Zinc Ratio in Leaded Brasses. The copper-zinc ratio in leaded brasses was determined by atomic absorption using a brass hollow-cathode lamp as source. The copper line was at 3247 A. and the zinc line a t 2139 A. A standard solution made t o contain 1000 p.p.m. of copper and of zinc was diluted t o give solutions of 100 p.p.m. downward. Three solutions containing 50 p.p.m. of lead were made up to show the effect of lead. These contained 50, 40, and 20 p.p.ni. of copper and of zinc, respectively. Figure 4 shows the m-orking curve for copper. I n no solution did the points lie off the working curves for copper or for zinc. The emission flame photometric procedure was equally applicable for the deter mination of copper in leaded brasses i f the temperature of the flame were not hotter than that of the oxyhydrogen flame. Although the separation of lead is a problem, this was eliminated by dissolving the material. However, no zinc line could be detected by this method, even when a concentrated solution of zinc sulfate was sprayed into an oxyhydrogen flame. Analysis of Copper Alloys. Bronze is difficult t o analyze by flame photometry because there are interferences of a kind not present in atomic absorption spectroscopy. I n this laboratory, five elements were determined in one bronze, and copper and zinc were determined alone in five differmt solutions (Table I). The souIces used were hollow-cathode lamps for copper, manganese, and nickel. a brass cathode for zinc, and a leaded biass cathode for lead. The bronze contained 15% tin. Determination of Zinc in Copper Alloys (Gidley and Jones) (4). Zinc, n hirbh is nctoriously difficult t o determine by emission flame photometry, is easy by absorption spectroscopy. Zinc was determined in copper alloys (4) using a hollow-cathode lamp as source and the 2138-A. line. -4valve voltmeter bridge circuit of Coheur and Hans (3) Kas used for an integration period of over 30 seconds. Results are given in Table 11. Archeological materials were examined in weights of 50 mg. for zinc content down to concentrations of 0.2 to 0.001%. No metallic interference was found for 25 elements. There was a slight interference from large quantities of magnesium as well as strong absorption of 2138 A. by halogen acids. Zinc in Aluminum Alloys (Gidley and Jones) (4). Using the same apparatus as for copper alloys, the typical results obtained are shown in Table 111. Zinc in Zirconium Based Alloys. Gidley and Jones (4) determined zinc in zirconium based alloys over a wide range of concentrations. The results viere consistent and appeared reliable, but were difficult to check because a sufficiently accurate determination was
impracticable by an alternative procedure. Determination of Noble Metals. Certain noble metals were determined quantitatively in the following concentrations in parts per million: Ag, 0.1; Au, 1; Pd, 2; Pt, 10; and Rh, 2. The sources used were hollow-cathode lamps in which the cathode was lined with foil of the appropriate metal. The lamps were powered with a stabilized Hilger supply unit FA 41. The H909 atomic absorption attachment was attached to the standard Uvispek spectrophotometer H700. Air consumption was 7.8 liters per minute a t 20 p.s.i. and the gas consumption was 4.3 liters per minute a t mains supply pressure (ea. 5 inches of water). Silver solutions were obtained from AnalaR silver nitrate (British Drug Houses) and standard rhodium solution from Johnson, Matthey and Co. The others were prepared by dissolving the metal in aqua regia. There was no interference from one metal with another nor from iron or lead if, in the case of gold, the iron were fully oxidized; if not, the gold salt was reduced. It was also advantageous to cool the top of the burner with water when gold was being determined. It the burner top became hot, the unstable gold salts in the solution tended to decompose to form metallic gold before entering the observed part of the flame. Working curves for silver and for gold are shown in Figure 5 . The metals could be quantitatively determined down to the concentrations in solution as given above. INTERFERENCE BY CATIONS
Emission flame photometry suffers from interferences of various kinds; from some of these absorption is free. I n emission, the presence of atoms of other elements may affect the temperature of the flame, and may affect the number of free electrons in the flame, so that various states are excited in different relative amounts. Absorption is free from this, as the state important for absorption, the ground state, is so highly populated that little difference in the population can be made by variations in the flame due to other elements. Interference of this kind has been looked for, and not observed in absorption. As an example, in emission flame photometry the percentage of sodium measured is affected considerably by the presence of potassium-the potassium is said to enhance the sodium. One explanation n-hich has been advanced is that the sodium becomes “lost” through ionization, but that when potassium is present also, some of this is ionized, so that the electron pressure in the flame is increased, which in turn partially inhibits ionization of the sodium. However, this interference has been studied by Margoshes and Vallee (10) who, using
Table Ill. Determination of Zinc in Aluminum Alloys Procedure Volu- Atomic Gravi- metric absorpmetric zinc, % tion Sample No. 16
0.08
19 ~. 21
0.10 0.26 0.76 1.01
24 30
0.087 0.093 0.13 0.25 0.77 0.96 1.02 4.44
Complex alloy 4.49
4.37
1428 1429
0.10 0.14 0 63 0.60 0.64 0.42 0.44 0.66 0.67
0.10 0.63 0.41 0.64
a direct-reading scanning method, s h o w d that the interference is mainly due to increase in the level of the background. They found that foreign cations increased the level of the background, but the height of the line being investigated above the background remained constant over a wide range. For example, they found that 10 p.p.m. of calcium gave rise to a line with the same height above background, whether sprayed by itself, or together with 100 p.p.m. of sodium, 30 p.p.m. of potassium, and 100 p.p.m. of magnesium, all at the same time. Such interferences should not affect absorption. If ionization were a t work, the number of atoms ionized would be negligible compared with the number of atoms in the ground state. If background n-ere enhanced, the absorption would not be greatly affected, the increased background in the flame coming under the same heading as flame emission, already discussed. Cation Interference by Combination. Another type of cation interference, which may affect absorption as well as emission, is when a n element present may form a compound with the element being determined. Gidley and Jones (4) made copious measurements on the effect of other cations on the apparent concentration of zinc, using the Hilger atomic absorption apparatus. Zinc a-as present as 5 p.p.m., and 1000 p.p.m. of other metals were added. These metals had no effect: Li, Na, K, Ca, Sr, Ra, Cu, Ag, Cd, Pb, Cr, Mn, Fe, Ni, Co, Ti, Zr, P, Sb, B, Bi, As, Th, and Sn. They found a slight effect from magnesium and aluminum when quantities on VOL. 32, NO. 8, JULY 1960
901
4i-
2 5;
I S
_3
20 A
0
, 50
)
60
70
80
90
100
ppm
Working curves for concentrations of noble
the order of 10,000 times as much magnesium or aluminum as zinc were present. A curious effect nhich they did find was that the halogen acids absorbed in the region 2100 to 2200 A. BO that they had to avoid these acids, and use the nitrates. The measurement of magnesium in solution mas different. On adding large quantities of aluminum, silicon (as sodium silicate), titanium, zirconium, and hafnium, magnesium content as measured was lessened, while with thorium it was practically unaffected, and with strontium the apparent quantity of magnesium was enhanced. I n this experiment about 500 times as much of the added element was present as the magnesium. The actual results were: Added
Element A1 Si Ti Zr Hf Sr Th
Apparent
hk,
70
3 8 26 60 50
150 106
Allnn ( I ) found that the determination of magnesium by absorption was upset by the presence of aluminum, which was probably due to the formation in the flame of a niagnesiumaluminum compound. K i t h 2.6 p.p.m. of magnesium in the spray, Allan found that the apparent concentration of magnesium decreased as the aluminum concentration was increased. Figure 6 sho~vsthe results found by Allan. A more surprising result was obtained in this laboratory by a beginner ~ v h omas determining magnesium in small quantities, in the presence of aluminum. The result was so surprising that it Ivas
902
40
OPT
Figure 6. Depressionof apparent magnesium concentration in a solution of magnesium chloride by addition of aluminum
' / Figure 5. metals
30
ANALYTICAL CHEMISTRY
Ordinates, apparent concentration of magnesium in p.p.m. Abscissas, concentration of added aluminum in p.pm
checked by another technician with considerable experience in work involving atomic absorption, and the findings were confirmed. The curve of Figure 7 shows a maximum interference, followed by partial recovery for two different concentrations of magnesium. There mas thus an apparent discrepancy between the results now reported, and those obtained by Allan. Happily this has been resolved: Allan's results were fully confirmed in this laboratory when the magnesium was present as the chloride. The curious curves of Figure 7 n-ere obtained when the beginner had used magnesium sulfate. Allan's case is one of cation interference; ours is due to anion interference as well. ANION INTERFERENCE
This affects absorption as \\-ell as emission, if it is due to the removal from the flame of atoms in the ground state. A notable example is calcium; it may be determined by absorption, but the sensitivity is not good, for there is a tendency to form refractory compounds, which are dissociated with difficulty in the flame. Some guide is furnished by the melting points of the conipounds-CaCl5 770" C., Ca(KOa)* 560" C., tertiary calcium phosphate, 16iO" C. Calcium oxalate decomposes, forming highly refractory CaO (possibly the hydroxide). The heat of formation is even more important because it indicates the stability of the compound. The depression of calcium, therefore, is brought about by molecule formation, so that the number of atoms nhich are required for the process are diminiqhed. This formation of stable compounds can sometimes be turned to advantage in emission-e.g., the CaO band head used for calcium is better than 4226 -1.
VARIATIONS OF SENSITIVITY IN DIFFERENT PARTS OF THE FLAME
A flame such as is used in flame photometry or in atomic absorption has a definite structure, and various zones are recognized. (A familiar example is the blue cone of the Bunsen burner.) These zones vary in temperature, and as is t o be expected, different elements are more abundant as atoms in the ground state in different zones. This variation of abundance is more marked for some elements than for othersfor example, calciuni atoms are abundant in one small region of the air-coal gas flame (just above the blue cone) while the variation in abundance of the alkali atonis is less marked. For the greatest sensitivity in determining calcium, it is advisable to select a region of the flame at this particular height. The abundant height even varies somen h a t for different salts of the same cation-e.g., for the chloride, bromide, iodide, and nitrate of calcium. CURVATURE OF WORKING CURVES
When working curves of absorbance z's. concentration in the solution sprayed
are constructed, for low concentrations -roughly up to about 20 p . p m but varying with different materials-the curres depart little from straight lines. (The straightest curve n e have found EO far is for gold, which is straight up to 50 p,p,ni.) At higher concentrations they become curved, in a direction corresponding to absorbance not keeping pace with concentration. The degree of curvature may depend upon the conditione, because the curves shoivn by Russell, Shelton, and n'alsh ( 1 4 ) appear to be straight, folloTved by a sharp curvature. Many of the curves obtained in the Hilger laboratory have
the appearance of never being absolutely straight, but start R-ith a very slight curvature which increases as the concentration rises; a good example is zinc. Russell, Shelton, and Walsh (14) consider t h a t the curvature they find is probably due t o increasing pressure broadening as the concentration rises. As the broadening increases, so the absorption a t the middle of the band will get less intense-i.e., the “broadening” is accompanied by ‘%hallowing.”
Let I bc the intensity t o ahich Io falls, after absorption by the flame. Then instead of the expected absorbance A = log Io/I,what would actually be measured is A’ = log (104- i o ) / ( l +io)
By observation of A’ a t tlTo different concentrations, io/lo may be calculated. This has been done for three Hilger working curves, and for one from Allan (l)-viz., curve A for magnesium of Figure 4 in his paper. The results obtained are as follows:
C 6-
I
Concn., P.P.M.
Calcd.,
iolla
Copper (Hilger) 100 80 60 50 40
0.081 0.084 0.077 0.082 0.085
Zinc (Hilger) i
1
~ _ _ _ 3 ~_____ - c I
4
-ppm
5
41
Figure 7. Variation of apparent concentration of magnesium in solutions of magnesium sulfate with added aluminum
--- 5
p.p.m. of magnesium made from MnSOa.7Hz0, AI as chloride 2.5 p.p.m. o f magnesium made from MgSOa.7H20, AI as chloride Ordinates, absorbance of solution for light of the magnesium line Abscissas, concentration of added aluminum in p.p.m.
100 80 60 40
Magnesium (Hilger) 5 4 3 2‘ / z
-
Curvature may also arise through some of the incident light falling outside the region of strong absorption, either because the emission line of the source is too broad, or as a consequence of the fact mentioned in the previous paragraph-Le., the regions of flame accepted Iiy the monochromator are all equally absorptive. In either case the absorption is “diluted” by some light nhich relatively escapes absorption; in the first case due to spectral width, in the second to spatial height. Some calculations of the curvature of the Hilger curves fit in with this suggestion. Thinking in terms of differently efficient zones of the flame, one may derive a simplified model in the following way. Suppose two zones enter the nionochromator, one absorptive, the other nonabsorptive (in actual fact there is probably a gradation). Suppose Io is the intensity of the light incident on the absorptive region, in such directions t h a t it would be accepted by the detecting system. Let iobe the intensity of the light similarly accepted, which falls on the nonabsorptive system.
0 131 0 131 0.138 0.127
0 095 0 100 0 100 0.095
eniploying a line nhich starts in absorption from a metastable state, not the ground state, but from nhich a n atom cannot, without breaking selection rules, go directly to the ground state-it must be excited first to some higher state, from which it may then be dropped t o the ground state. Atoms in a metastable level may go t o the ground state as a result of collisions with other atoms. For a particular element, conditions in the flame may be such that a considerable metastable population exists, and a line starting in absorption from a metastable level may be good for analysis of that element. For rxample, in this laboratory, the lines so far found best for cobalt and nickel start from metastable levels. It is morthwhile to try different lines for a n element to see m-hat line gires the bcpt results. Another reason for choosing a line may be that the apparatus is more sensitive in the region in which the line lies-for example, the resonance lines of some elements are outside the region in 11-hichthe photocell is sensitive. Lines 17-hich have been found good for a number of elements are tabulated below. These are not claimed to be the best lines to use with any arpnratus n-hatevrr-they are the bcst found so far for the coinbination of the T7vispek and the HDOS absorption flallle attacbliment.
Magnesium (Allan) 32 24 16
0.112 0.117 0.112
The approximate constancy of iO/Io for each of the different curves shows t h a t the curvature could be due to a measure of dilution of the light, which might be due either t o nidth of the source emission line or to inclusion of a height zone of poor absorption. Calculation of ioU0 for a calcium working curve provided a much larger value, consistent with the smallness of the region over which calcium atoms in the flame are abundant. Another causr, when the variations in concpntration are great, can arise because the solution increase. in viscosity n i t h concentration, so that the actual flow of solute is lessened. The curvature discussed in this paragraph, although interesting, is not of great importance for analysis, because a t low concentrations the curves are sensibly straight, and in any case standards can be used.
Elenient
Line
2 E:; Ca Cd
co Cr Cs Cu
Fe
2
4227 2288 3633 3579,4264 8521 3248 2483 2536 7665
Element Mg
Mn Na Ni Pb Pd Pt Rh Sn Sr Zn
Line 2852 4034,2704 5890 3415 2833 2476 2659 3435 2863 4607,5535 2138
Some of the aboTe lines are not the resonance lines, in the sense of being the smallebt allowed transitions from the ground state, apart from the cases already discussed of transitions from metastable leT-els. For euample, the Pd line 2476 A. is a singlet-triplet combination. Similar anomalies are shown by the most persistent lines of the elements. The nickel line begins in a metastable state in absorption. The best lines are not necessarily the niost persistent lines. SENSITIVITIES I N SOLUTION
SUITABLE SPECTRAL LINES
Earlier it mas stated that atoms in the ground state are required. This is normally, but not invariably, true. Sometimes good results are obtained by
The sensitivities so far obtained, measured in parts per million in the solution, are tabulated below. As a general rule, a spray of about 5% strength can certainly be used, so t h a t VOL. 32, NO. 8, JULY 1960
903
in the solid the sensitivities obtained will not be more than 20 times worse. Using DisUsing- Hollow-Cathode charge Lamps Lamps 0 . 1 Mn 1 Na 0.1 0 . 5 Ni 5 K 0.1 2 Pb 50 Rb 2 Ca cs 10 Pd 2 co 5 Pt 10 Cd 2 Cr 50 Rh 2 T1 10 1 cu Sr 2 2 Fe Sn 500 Hi2 50 0.1 Mg 0 . 1 Zn
62”,
Unsuccessful attempts were made to determine d l , Ir, Mo, Ti, Ta, W, and V, using the direct methods already described. Zirconium probably belongs to this category. Some success in special circumstances might be obtained by using a n indirect method-e.g., by employing the depressing effect of aluminum on magnesium to determine aluminum. CONCLUSIONS
The absorption technique is advantageous under the following circumstances: when one element is to be
determined in the presence of a large amount of another one, which may give strong interference in emission flame photometry, not due to actual chemical combination; when an isotope is to be analyzed in the presence of another isotope of the same element. Clearly if the source is a single isotope source, only that isotope in the flame can absorb the line. Electrodes intended t o be used as standards for ordinary emission arc or spark techniques may be analyzed by this method, and so be able to function as “standard electrodes” because they can be analyzed with respect to AnalaR reagents. ACKNOWLEDGMENT
The author acknowledges helpful discussions with, and experimental assistance from, R. A. Lockyer and G. E. Hames, and help from Wanda Jordan, also of this laboratory, who devised the arrangement of two slits and one recorder for one element and the twochannel direct reader. He thanks J. A. L. Gidley and J. T. Jones of Imperial Chemical Industries for early communication of results of their investigations and permission to quote.
REFERENCES
(1) Allan, J. E., Analyst 83, 466 (1958). (2) Brewster, Sir D., Report of 2nd Meeting, British Association, p. 320, 1832. ( 3 ) Coheur, P., Hans, H., Rev. uniuerselle manes 92, 63 (1949). (4) Gidley, J. A. F., Jones, J. T., Analyst 85,249 (1960). (5) Gidley? J. A. F., Jones, J. T., private
communication.
(6) Huldt, L., dissertation, Uppsala, 1948. ( 7 ) Kirchhoff, G., Pogg. Ann. 109, 275 (1860). (8) Liveiy, G., Dewar, J., “Collected PaDers, Cambridge University Press, Ckmbridge, EnglaGd, 1915. (9) Lockyer, R., Hames, G. E., .Inalyst 85, 385 (1959) (10) hlargoshes, Marvin, Vallee, B. L., ANAL.CHEM.28, 180 (1956). (11) Menzies, A. C., Actm do Congress0
XV Internacional de Quimica Pura e Aplicada, Vol. 11, p. 2, Lisbon, 1958. (12) Menzies, A. C., Colloquium Spectroscopicum Internationale VI, Amsterdam, 1956, Pergamon Press, London. (13) hfitchell, A. C. G., Zenianeky, M. W., “Reson:nce Radiation and Excited Atoms, Cambridge University Press, Cambridze. Eneland. 1934. (14) Russe&’B. Shelton, J. P., TF7alsh, A., Spectrochim. Acta 8, 317 (1957). (15) Walsh, A., Zbid., 7, 108 (1955). (16) Wollaston, W.H., Phil. Trans. Roy SOC. London. Ser. A 92,365 (1802). RECEIVED for review February 29, 19GO. Accepted May 5, 1960.
x,
Analytical Distillation by Gas Chromatography Programmed Tempera tu re Operation F. T. EGGERTSEN, SIGURD GROENNINGS, and J. J. HOLST Shell Developmenf Co., Emeryville, Calif.
A gas chromatographic method is described for obtaining information on hydrocarbon samples analogous to that from an analytical distillation. The gas chromatographic distillation employs a simple technique for the temperature programming of a short nonselective column. Boiling points of the components are determined from times or temperatures of emergence according to a general relationship which is adequately valid for all hydrocarbons. The effluent hydrocarbons are oxidized and detected as carbon dioxide by thermal conductivity; this makes response factors unnecessary for interpretation of peak areas. The method, which is inherently general in nature, has been applied to petroleum distillates ranging from -40” to 400” C. The results agree well with those obtained by timeconsuming conventional precision distillations, and also give a better ac-
904 *
ANALYTICAL CHEMISTRY
counting of light and heavy ends as well as more detailed information, particularly of minor components or fractions. An important feature of the method is that a few milligrams of sample are sufficient to yield a good distillation curve.
I
N CHARACTERIZING hydrocarbon mixtures for specification or other purposes, a precise analytical distillationfor example, 20 t o 50 plates-is sometimes needed. Such a distillation requires several hours and large samples. Gas chromatography can be employed to obtain essentially a boiling point analysis, although, with the separating column at a constant temperature, the analysis is restricted to a rather narrow boiling range. Lighter components emerge too soon and tend to overlap, while heavy components emerge
very late, producing relatively wide bands or remaining in the column. The new technique of temperature programming of the separating column (1-4, 6-9) makes a wide-range, singlestage analysis possible. hloreover, it appears t o be well suited for obtaining the gas chromatographic equivalent of a conventional analytical distillation. By using a column packing nliich separates according to boiling point, and by precise programming of the column temperature, the boiling range for various peaks can be determined from times or temperatures of emergence. In such a gas chromatographic (GC) distillation, detailed separations are of secondary concern, and only a short separating column is required. The method described here was designed to yield in about hour of operating time the information equivalent to that obtained in an approximately 20-plate conventional distil-
