ANALYTICAL CHEMISTRY
1654 The data in Table I tend to confirm this mechanism. Whereas the reaction is apparently not rigidly quantitative, such a condition is not necessary as long as reproducible comparisons are obtained. Table I1 presents calculations from the data in Table I, showing the greater probability of the sulfite oxidation to dithionate rather than the oxidation t o sulfate. Actualh the agreement between the sodium chromate concentrations found and those calculated for the dithionate mechanism (Equation 3) are in fairly good agreement u p t o about 175 p.p.m. of sodium sulfite. The agreement beyond this point becomes poor and may indicate that the oxidation of sulfite t o sulfate is also taking place, becoming more predominant in this higher concentration range.
ACKNOWLEDGMENT
The authors wish t o acknowledge the constructive criticism of Wrinslow H. Hartford, which has added greatly t o the value of this paper. LITERATURE CITED
(1) Am. Public Health Assoc.. Sew York, “Standard Methods for the Examination of Kater and Sewage,” 9th ed.. pp. 86 7 , 1946.
(2) Hartford, W.H., private communication. RECEIVED for review February 27, 1952. Accepted June 13. 1952. Presented at the Meeting-in-hfiniature of the Yew Tork Section, A M E R I C A S CHEMICAL SOCIETT.February 8. 1952.
Increased Sensitivity for the Perkin-Elmer Flame Photometer Use of Fine-Spray Hot- Chamber Aspiration CLYDE A. DUBBS Metabolic Unit, General Medical Research Program, Veterans Administration Center, Los Angeles 25, Calif.
HE sensitivity of the Perkin-Elmer flame photometer can increased remarkably, without sacrifice of over-all performance, by the simple substitution of a fine-spray hot-chamber aspiration system for the coarse-spray cold-chamber aspiration system supplied a i t h the photometer. A quantitative comparison of sensitivities for these two aspiration systems is reported in this paper. C04RSE-SPRAY COLD-CHAMBER 4SPIRATION SYSTEM
The coarse-spray cold-chamber aspiration system (Figure l ) , employing a glass atomizer, similar to that described by Katelson (?), is standard equipment for the hfodel 52C and some Model 52A Perkin-Elmer flame photometers (8). The sample solution
ER
-
sage of the air supply tubing). The inside diameter of this grommet is just right to give a snug fit with the fine-spray glas.atomizer (Beckman Catalog No. 10696). When properly dimensioned, as illustrated, the horizontally aligned train, atomizer-grommet-chamber-rubber connector-burner, is readily installed Lvithin the unaltered Perkin-Elmer cabinet. The heating jacket about the chamber has a 90-ohm winding of No. 26 Chrome1 wire. Operating a t 70 volts, it gives a temperature of approximately 230” C. a t the center of the chamber when no air is flowing. A flexible support for the sample beaker, screwed on the outside of the panel, completes the conversion. Several potential weaknesses in the particular system described should be mentioned. The length of the chamber (11 em.) is considerably less than that of the commercial Beckman chamber (19 cm.). Although the shorter chamber has performed satibfactorily in this laboratory, the possibility exists that, under different aspiration conditions, the length may not allow for adequate vaporization of the spray. .41so the relatively narrou inlet neck of the chamber must be sufficiently short to permit free course of the spray into the chamber, for erratic bphavior occurs if the spray is alloJwd to strike the neck. Therefore, to assure
Figure 1. Coarse-Spray ColdChamber Aspiration System (Perlcin- Elmer)
is poured into the funnel and atomized as a relatively coarbe spray against the top of the cold chamber. Large droplets, condensing in the chamber and helical tube, flow off to the drain, while the air stream carries tine spray along to the burner,
Figure 2. Fine-Spray Hot-Chamber .4spiration System (after Gilbert et a l . )
FINE-SPRAY HOT-CHARIBER ASPIRATION SYSTEM
The fine-spray hot-chamber aspiration system (Figure 2) is a copy of that supplied with the original Beckman flame attachment (@,modified slightly to permit convenient installation within the Perkin-Elmer cabinet. JVith this system, the sample solution is drawn up through the fine-bore capillary and completely atomized as a fine spray, which vaporizes aithin the hot chamber to give an aerosol of very fine particles which are swept into the burner. I n the right front panel of the Perkin-Elmer instrument there is a small hole, fitted with a rubber grommet (intended for pas-
satisfactory performance in other laboratories, the author recommends use of a full-length, wide-necked aspiration chamber, such as described by Gilbert, H a w s , and Beckman (5, Figure 6). Installation would then require cutting a larger hole in the PerkinElmer cabinet. PROCEDURE
For comparison purposes, all operating conditions, except aspiration conditions, are held constant unless otherwise stated, Propane pressure of 4 pounds per square inch is used, and the
1655
V O L U M E 2 4 , NO. 10, O C T O B E R 1 9 5 2 Table I.
Comparisons of Aspiration System Sensitivities Meter
Response
Re
Cb
Ca
RW
With Fine-Spray .4tomizer 30.1 ; ( r b l = 0.54); Hot Chamber 9.4 450 50.0 5.3 32 0 9.1 440 40.0 4.4 28.0 10.0 480 30.0 3.0 21.5 10.5 510 20 0 1.9 15.0 With Fine-Spray .4tomizer No. 2 ; ( r b 2 50.0 11.5 32.0 40.0 9.9 28.0 30.0 7.5 21.5 20.0 5.0 15.0 With Fine-Spray .Atoinizer No. 1; ( r b l 50.0 20.0 32.0 40.0 17.0 28.0 12.0 30.0 21.5 8.0 20.0 15.0
C
=
=
0 . 1 6 ) ; Hot Chamber 4.3 700 4.0 650 4.0 650 4.0 650
SODIUM CONCENTRATION(PPM), FINE-SPRAY HOT-CHAMBER ASPIRD; 0 100
3
'I-
gF
0 . 5 4 ) ; Cold Chamber 2.5 120 2.4 11.5 2.5 120 2.5 120
sodium concentration iD.D.rn.) to nive the indicated meter _ . reauired . response. Subscript a refers to coarse-spray cold-chamber aspiration Subscript b refers t o fine-spray aspiration. = sensitivity ratio on concentration basis = ca/cb. = sensitivity ratio on weight basis = C o / C b X I'o/Tb. = rate of sample consumption (ml. per mtn.) ; ra = 26.0. Correcponding Cn and Cb values are taken from Figure 3.
=
The coarse-spray cold-chamber aspiration system con~umes 26 ml. of sample per minute, but actually more than 99% of this omount goes down the drain. Diskant (2) has confirmed this abservation independently, and Bills and coworkers (1) have reported 95 to 97% waste for an earlier aspiration system of similar design. Fine-spray hot-chamber aspiration consumes much less sample, 0.54 ml. per minute for atomizer S o . 1, 0.16 ml. per minute for atomizer No. 2, but a substantial proportion
I
80
I
w 60
2
100
90
-
Figure 1. Effect of ;ispiration System on JIeter Kespnnse (Different Sodium Concentrations) Gain setting 4:44 * 0 : l U in both cases See footnote h, Tnhle 11. for abbreviations
of this amount is carried into the burner. The greatly increased sensit,ivity thereby olitained by fine-spray hobchamber aspiration is emphasized by- the following ohservations:
flame IS adjusted as described in the instruction manual (8). Air prebsure of 25 pounds per square inch is used for fine-spray hot-chainher aspiration; 10 pounds per square inch for coarsespray cold-chamber aspiration. (Since the burner receives it3 air supply through ports open to the atmosphere, its flame characteristics are independent of aspiration pressure, as confirmed by observation.) The photometer is operated as a direct intensity instrument, but conclusions hold equally well for internal standard operation. -411 samples are aqueous solutions of reagent grade sodium chloride.
1. To obtain a given meter response, coarse-spray cold-charntier aspiration requires from four to 10 times the sodium conccntration required by fine-spray hot-chamber aspiration (Figure 3 arid R, values in Table I). In confirmation, a standard curve for fine-spray hot-chamber aspiration, covering the concentration range up to only 5 p.p.m., closely approximates a standard curve for coarse-spray cold-chamber aspiration, covering the tenfold greater range up to 50 p,p.m, (Figure 4). 2. To obtain a given meter response for a given period (sufficiently long to permit, a reliable meter reading), coarse:.pray cold-chamber aspiration requires 50 to 150 times the volume of solution containing 450 to 700 times the weight of sodium required by fine-spray hot-chaml)er aspiration (R, values in Tahle I). For example, coarse-spray cold-rhamber aspiration of 500 niicrograms of sodium (10 ml. of 50 p.p.m.) gives the same mrter response (100) for the sanir prriod (23 seconds) as fine-spray hotchamber aspiration (\\\-ithaspirator S o . 1) of only 1.05 micrograms of sodium (0.21 ml. of 5 p,p.ni,). This specific case was confirmed experimentally.
SEN SlTIVITY
OTHER PERFORMAXCE CHARACTERISTICS
Several sensitivity characteristics, determined for each aspiration system, are plotted in Figures 3 and 4 and tabulated in Tahle 1. Results for two fine-spray atomizers are included.
Aspiration Pressure. For proper comparison, each type of atomizer must be operated a t its own optimum aspiration pressure (Figure 5 ) . T h r coarsr-spray atomizer cannot be operated
Figure 3. Effect of Aspiration System on Meter Response (Same Sodium Concentrations) Gain setting 7 : 35 f 0 : 10 i n all cases See footnote b, Table 11, for abbreviations
ANALYTICAL CHEMISTRY
1656 above 10 pounds per square inch because the increasing air stream begins to affect the flame, even eliminating its small cones. The fine-spray atomizer is not operated above 25 pounds per square inch because any slight further increase of sensitivity is offset by practical difficulties-for example, blowing off of tubing connections. With decreasing pressures, aspiration rate for either type of atomizer becomes increasingly dependent on the pressure head of the solution. Thus with fine-spray aspiration of a sodium solution a t 10 pounds per square inch, the meter reading falls from 48 to 31 as the solution level above the capillary tip of the atomizer falls from 1.0 to 0.1 cm. .It 25 pounds per square inch, this effect is not measurable. Temperature of Aspiration Chamber. The aspiration chamber is heated sufficiently to vaporize all water, thereby increasing the amount of material carried to the flame. This effect is evidenced by a fourfold sensitivity increase over cold-chamber aspiration with the same atomizer (Table I). I t has been stated that n i t h the commercial Beckman finespray hot-chamber system there is no precipitation on the chamber walls ( 5 ) . However. with the present system, it was found that aspiration of a 50 p,p.m, sodium solution for 15 minutes deposited about 50% of the aspiiated sodium on the chamber walls (determined by analyses of chamber rinse water). This result suggests the possibility of further sensitivity increase by additional improvements of the aspiration process. This deposit is substantially fixed to the walls and does not contaminate samples aspirated later. as evidenced by no measurable background increaw upon aspirating distilled 1%-ater. Of course, visible accumulations that may build up over a period of days may raise background and hhould he cleaned out periodically.
‘oor-l-
50
$40
4
-
:f 10
Table 11.
5
10
OO
ASPIRATION PRESSURE (LB. PER
20
25
sa. IN.)
Figure 5. Effect of Aspiration Pressure on Meter Response
Atomizer Clogging. Clogging. an important practical difficulty, is accentuated in fine-spray fine-bore atomizers, although the author believes that this effect is often overemphasized. This laboratory has had 2 years’ experience with fine-spray aspiration of 10- to 100-fold dilutions of blood serum, urine, and ashed samples, many of which contain suspended solid material. Although clogging may occur intermittently, it is usually rapidly and completely dispelled hy a simple 3- to &second operation
Instrumental Precision under Various Conditions
Gain Setting 0 : 3 0 * 0:13
Meter Instabilitya Flame Aspiration alone into flame flame i.0.5 i.l.O i.l.5
4 : 4 4 zc 0 : l O
10.0
zto.2
6:22 i. 0 : 1 0 7 : 3 5 i. 0 : l O
i.O.O ztO.O
10.0
a b
So
10.0
So diu In
concn,
Aspirated,
+1.5 1 0 7 11.0 11.0 zt0.6
P.P.hI. J
0 3 59 > 30 50
Type of rtspirationb CSCC FSHC CSCC FSHC FSCC FPIIC
Average meter reading is set to 100 in all cases CSCC, coarse-spray cold-chamber aspiration. FSHC, fine-spray hot-chamber aspiration. FSCC, fine-spray cold-chamber aspiration
It is certainly the author’s opinion that the advantages of finespray aspiration considerably outweigh any disadvantage of the clogging problem. Precision. Fine-spray hot-chamber aspiration can give greater precision than coarse-spray cold-chamber aspiration (Table 11). Precision is limited by meter instability, which refers to the continuous rapid fluctuations of the meter needle about an average position. Better precision can be obtained by fine-spray hot-chamber aspiration of a given sample because a gain setting that permits greater instrumental stability can be chosen. Essentially the same precision can be obtained by coarse-spray cold-chamber aspiration of a given sample or by fine-spray hotchamber aspiration of the sample after tenfold dilution. Thus a sodium concentration as low as 0.5 p.p.m. (instead of 5.0 p.p.m.) can he determined with a precision of 1.5%. I t is suggested that the aspiration system described in this paper could increase the sensit,ivities of certain other instruments-for example, the Barclay (4).Fox (3, 6 ) , Process and Instruments (9), and White (10) flame photometers.
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“‘“L 0 / P
( E ) which involves placing a finger over the outlet tip of the atomizer to reverse the air flow down the solution capillary, thereby bubbling off the material that clogs the constricted inlet tip. Occasionally obstinate cases may require repetition of this operation several times. Diskant ( 2 ) has adopted the proposed system, although he has chosen to use a cold chamber with the Beckman fine-spray atomizer on his Perkin-Elmer flame photometer, He reports that for the past year his laboratory has been satisfactorily using this system for routine sodium and potassium analyses of natural waters, even though in his laboratory sample size limitation is never a problem.
LITERATURE CITED
(1) Rills, C. E., McDonald, F. G., Siedermeier, W., a n d Schwartz, hI. C., h x a ~CHEM., . 21, 1076 (1949). (2) Diskant, E. M., private communication. (3) Fox, C . L., ANAL.CHEY.,23, 137 (1951). (4) General Scientific Equipment Co.. H a m d e n , Conn., “‘Barclay Flame Photometer.” ( 5 ) Gilbert, P. T., Jr.. Hawes. R. C., a n d Beckman. A. O., i l x a ~ . CHEM.,22, 772 (1950). (6) Janke a n d Co., I n c . , Hackensack, S . ,J,, “ I n t e r n a l S t a n d a r d Flame Photometer.” (7) S a t e l s o n , S.. Am. J . Clln. Path., 20, 463 (1950). (8) Perkin-Elmer Corp.. Glenbrook. Conn.. “Instruction Manual Model 52.1 Flame Photometer,” 1949. “P. a n d I. Flame (9) . . Process a n d Instruments, Brooklyn, S . T.. Photometer.” (10) White, J. E., ANAL.CHEM..24, 394 (1952).
RECEIVED for review August 2 , 1951. Accepted July 24, 1952. Reviewed by the Veterans Administration and published with the approval of the Chief Medical Director. The statements and conclusions of the author are the result of his own study and do not necessarily reflect the opinion or policy of the Veterans Administration.
