Table 1.
Reproducibility of Determination of 500 p.p.m. of Carbon Dioxide in Nitrogen with Precut Column System“
Av . peak height, Std. dev., Sample No. of vol., cc. samples 70 cm. 5.82 f 2 26.0 8 8.77 f 2 95 9 Conditions of analysis are given in Figure 2 A .
Average peak area, cm.2 3.63 13.55
Std. dev,
%
f 2 =t5
0
“vent time” for air was ascertained. Since the analytical column must be continually purged, the flow rate through the compensation column was adjusted so that helium stream divides equally between these two columns a t the original flow rate through the analytical column. CALIBRATION AND DISCUSSION
Where the COZ content was reasonably large, a detectable signal was obtained as is shown in Figure 2A for a 500-p.p.m. CO, utilizing the 26-cc. inlet loop. The T/C detector was calibrated with a standard gas blend containing 500 p.p.m. of C o n in N z obtained from the Matheson Co., Inc. The typical results presented in Table I illustrat,e the relative merits of peak height us.
peak area for this analysis. A comparison of the area data for 26.0-cc. and 95-cc. samples indicated a 3.7 increase in signal area for the 3.7-fold increase of sample. However, the peak height data showed only a 1.5 increase in signal for the 3.7-fold sample increase. Although the peak height calibration may be more convenient, the peak area data, having shown excellent agreement with the sample size, was used for calibration purposes. The peak height was an aid in the determination of the method detection limits. Therefore, with the apparatus conditions as previously described, the calibration values were 138 1i.p.m. of COz per sq. cm. and 37 p.p.m. of COS per sq.cm. for the 26- and 95-cc. loops, respectively. The deviations observed during the calibration are shown in Table I.
I n the analysis with the 26-cc. loop sample, the average COZ peak height was 5.82 em. for approximately 500 p.p.m. Because the lowest measurable peak height is about 0.24 cm., the minimum detectability would be 20 p.p.m. of CO,. With the 95-cc. gas blend sample, the average C o n peak height was 8.77 em. for approximately 500 p.p.m. As the lower limit of readable peak heights is again about 0.24 cm., 13 p.p.m. of COZwould be the minimum concentration detectable. ACKNOWLEDGMENT
The authors are pleased to acknowledge the valuable assistance of James R. Huff in preparing this manuscript, and R. L. Steil and J. Fuchs for construction of instrument modifications. LITERATURE CITED
(1) Brenner, Nathaniel, Ettre, Leslie S., ANAL.CHEM.31, 1815 (1959).
J. N. MURRAY J. B. DOE
Research Division Allis-Chalmers Mfg. Company Milwaukee, Wis. 53201 WORKsponsored by the U. S. Army Research and Development Laboratories, Fort Belvoir, Va., under Contract No. DA-44-009-AMC-240(T).
Nitrite Interference in Spectrophotometric Determination of Atmospheric Sulfur Dioxide
a
SIR: The West-Gaeke ( 7 ) modification of the Schiff reaction is used extensively to measure concentrations of sulfur dioxide in the atmosphere. Monitoring instruments utilizing other principles for the determination of sulfur dioxide are quite commonly standardized, by assay ( 7 ) of known concentrations prepared for calibration. To eliminate NOz interference in the method, West and Ordoveza (8) proposed a modified sodium tetrachloromercurate (11)collecting solution containing 0.06% sulfamic acid. Zurlo and Griffini (9) have described the use of o-toluidine to eliminate this interference. We have found that the use of sulfamic acid in the collecting solution produces small but significant errors upon aeration. The increased use of this method for monitoring over long periods of time and the attachment of significance to small trends over the same periods of time require that the aeration effect be eliminated. A project involving atmospheric sampling in Denver ( 2 ) enabled us to evalu942
ANALYTICAL CHEMISTRY
ate the West-Ordoveza modification for measuring sulfur dioxide. Parallel samples were collected during the study, using the unmodified West-Gaeke method and the West-Ordoveza modification. The results from these samples are shown in Figures 1 and 2. These figures have a dual ordinate. The lower gives the concentration found for nitrogen dioxide, which was determined by concurrent sampling. The upper ordinate shows the results obtained by the unmodified West-Gaeke method and the West-Ordoveza modification. In the first day’s sampling (9 January), the two methods used for SOn gave parallel results but showed a difference in magnitude. The WestOrdoveza modification gave lower values than did the unmodified method. This was contrary to expectations since nitrogen dioxide interferes with the development of the color and thus leads to lower results in any method which does not compensate for this interference. The figure also shows that as the concentration of nitrogen dioxide decreased,
the values for the sulfur dioxide concentration obtained by the two methods approached more closely to each other. The West-Ordoveza method still gave a lower value and, in the case of three samples, actually yielded negative values for the concentrations. Figure 2 gives the results obtained from samples collected on 18 and 19 March. I n this case, the WestOrdoveza method was modified by increasing the concentration of sulfamic acid in the collecting solution. With the exception of the one aberrant value obtained at five o’clock in the afternoon, the values for the concentration of sulfur dioxide given by the two methods were relatively divergent. The modified West-Ordoveza technique gave large negative values for the sulfur dioxide concentration, which could not be explained on the basis of errors in the experimental procedures. The methods proposed by WestGaeke ( 7 ) , West-Ordoveza (8),and Zurlo and Griffini (9) were evaluated in a laboratory study. The reagents used
0 :
6 -
SO,,WEST-GAEKE
COLL S O L N
x = SO, , W E S T ORDOVEZA COLL SOLN
\
0 -7
I
= SO2 , W E S T - G A E K E COLL. S O L N
x
= 902 . W E S T - O R D O V E L A COLL. SOLN
0
= NO2
!
6 -
e
pphm5
o
5 0 2 4c
4
5 4 3 -
whm2 so2 I -
4
-
0I -
-2
1/9/64
Figure 1.
:
:
pphm NO2 1 3 4 2/21/84
1/28/64
Denver-1
-
-3 -4 -
/9/64,
1 /28/64, 2 / 2 1 / 6 4
5
6
7
8
9
IO II
RESULTS
Effect of Addition of Sulfamic Acid and o-Toluidine. Samples were prepared which contained the equivalent of 20 pg. of sulfur dioxide and varying amounts of sulfamic arid or o-toluidine, as shown in Table I. The results show the magnitude of color suppression from the addition of the amino compounds. As indicated by West-Ordoveza and by Zurlo-Griffini, the loss of sensitivity resulting from this suppression of color is acceptable in view of the great sensitivity of the method and the further increased sensitivity obtained by t'he modified reagent preparation detailed by Pate, Lodge, and Wartburg ( 5 ) . The 137, decrease for sulfamic acid or layo decrease for o-toluidine shows, however, that identical treatment of samples, blanks, and standards is required to eliminate major systematic error. Effect of Varied Concentrations of Sulfur Dioxide. Sulfamic acid or otoluidine was added to samples containing 10 t o 50 pg. of sodium metabisulfite. The color developed (86%, RSD = 0.O2yGand 91y6, RSD = 0.017,, respectively) was independent of the amount of equivalent sulfur dioxide present. Effect of Nitrite. A series of samples were prepared containing 20 pg, of sulfur dioxide as sodium metabisulfite in TCRI. I n each set of three samldes, one contained sulfamic acid in the concentrations specified by West and Ordoveza, and a second contained o-toluidine in the concentration recommended by Zurlo and Griffini. The
I
2
3 4
5
6
7
6
9
IO II I 2
MIDNIGHT
3/19/64
3/18/64
Figure 2.
were those specified by these authors except that concentrations were varied ab indicated, and the bleached pararosaniline reagent was prepared by the method of Pate, Lodge, and Wartburg ( 6 ) . One-em. glasa cells and a Zeiss PMQII spectrophotometer were used for all abqorbance measurements.
I2
sets of samples were prepared so that they contained varying amounts of nitrite (as potassium nitrite). The color was read against blanks and standards which did not contain sulfamic acid or o-toluidine, in order that the color suppressing effect of these compounds would be obtained. Figure 3 confirms the suprisingly large interference by nitrite in the colorimetric determination of SO2 as reported by West and Ordoveza. Sulfamic acid eliminated the nitrite interference satisfactorily. o-Toluidine, on the other hand, is not generally satisfactory, since the efficiency of elimination is dependent on the concentration of nitrite which is present. I n the presence of moderate amounts of nitrite, the addition of o-toluidine as specified by Zurlo and Griffini would only partially compensate for the interference. Aeration of Dichlorosulfitomercurate Solutions. Ten-ml. samples were aerated a t 1 1.p.m. for 30 minutes, as shown in Table 11. T h e first section of the table gives results where the sulfamic acid concentration in the samples was held a t 6 mg. per 10 ml. of sample, corresponding to the concentration suggested by West and Ordoveza. I n this group of samples, the sulfur dioxide content of the samples was varied. The samples containing sulfamic acid consistently showed a onethird loss of color upon aeration. The equivalent samples which did not con-. tain sulfamic acid gave the normal amount of color. The per cent or ratio of color suppression due to the aeration is therefore independent of the sulfur dioxide concentration of the sample. The second section of the table gives results where the sulfamic acid concentration of the samples was varied. Upon aeration, the per cent of color suppression varies with the concentra-
Denver-3/18/64
Table I. Color Suppression upon Addition of Sulfamic Acid and o-Toluidine
SulColor famic suppresacid, mg. sion, 70 0
0
o-Toluidine, mg.
Color suppres-
sioii, 70 0 7
0 1
17 23 28
9 12
18
Sample
=
ml. of TCM.
20 pg.
12
48 61 67 SO, (as Na2S205) in 10 18 24
a Reagent concentration Ordoveza method. * Reagent concentration Grifini method.
Table II.
in
West-
in
Zurlo-
Aeration of Dichlorosulfitomercurate Solutions"
Amino Per cent recover@: SO, with N withorit N compd.* comconimg./ content, pg. pound* pouiidb sample SA 6 5 65 (3) 105 SA 6 10 67 99 64(6) YY(2) SA 6 15 SA 6 25 96 SA 3 15 .~~ SA 12 15 27 15 103 TOL 2 15 100 SA 6d TOL 2d 15 102 a Sample volrime of 10 ml., aerated at 1 1.p.m. ( p = 630 mm.) for 30 minutes. * SA = Sulfamic acid. TOL = o-Toluidine. c Yalries are mean of nrimber of sets shown after valiie. Each set consists of folir irideperideiit aerations of two samples i n parallel. \-allies have been corrected for color snppressioii by iritrogeii compoiinds. d Amino comporind added after aeration but prior to addition of reagents.
:3(4)
~
VOL. 37, NO. 7 , JUNE 1965
943
100-
90-
I
I
I
I
-
I
IOU
x o
70
-
NO;
= NO;,
I
901
*
I
I
I
I
I
I
I
I
I
80 SULFAMIC ACID
4
IO
I
I
20
30
I
I 40
50
I 60
I
I
I
70
80
90
I00
Y No;
1
Figure 3. Interference by NOz- and effect of added amino compounds 15 pg. of SOl/lO ml. of TCM
tion of sulfamic acid in the collecting solution. The third group of samples listed in the table contained o-toluidine in a concentration of 2 mg. per 10 ml. of sample, corresponding to the concentration recommended by Zurlo and Griffini. Aeration resulted in no additional color suppression over and above that due to the presence of the o-toluidine. The fourth section of the table gives results from samples which did not contain an amino compound when aerated. The corresponding amino compound was added subsequently in the manner specified by Zurlo and Griffini. KO color suppression resulted from this test. Two additional sets of samples containing 15 mg. of sulfur dioxide in T C M
Table 111. Effect of Aeration Volume on Dichlorosulfitomercurate(1l) Solutions Containing Sulfamic Acid
Flow Aeration rate time (1.p.m.) (rnin.)
Air
vol.
(liters)"
Color loss,
1 5 wg. SO2 and 12 mg. Siilfamic Acid in 10 ml. of TCM 0 47 16 7 5 24 0 47 16 7 5 21 0 49 16 7 8 22 1 72 24 41 3 60 1 72 30 51 6 73
At 630 mm. pressure.
944
ANALYTICAL CHEMISTRY
20
40 50 60 70 80 VOLUME OF AIR ( L I T E R S )
30
90
100
Figure 4. Effect of aeration volume on dichlorosulfitomercurate solutions containing sulfamic acid 15 pg. of SO2 and 6 mg. of sulfamic acid in 10 ml. of 0.1 M sodium tetrachloromercurate
and 6 or 12 mg. of sulfamic acid, respectively, were aerated with varying combinations of flow rates and aeration times. The results in Table I11 indicate that there is a direct relationship to the volume of air sampled-i.e., to the amount of oxygen to which the sample is exposed. The interference produced by aeration of the sulfamic acid-containing collecting solution is proportional to the amount of air passed through the sample. The potential interference is therefore created prior to the addition of pararosaniline or formaldehyde. Sampling Known Concentrations of Sulfur Dioxide. As pointed out by Urone, Evans, and Noyes (6),sulfur dioxide standards prepared from sodium
metabisulfite are not completely comparable with actual concentrations of sulfur dioxide and air. Therefore, known concentrations of sulfur dioxide and nitrogen dioxide were prepared in Mylar bags for direct sampling. The apparatus and techniques used for preparation and sampling of these concentrations were, with minor modifications, those described in earlier publications ( 4 ) . The results of these determinations shown in Table IV confirm the results obtained from the previous tests. DISCUSSION
The reaction between decolorized pararosaniline and the formaldehyde adduct of sulfur dioxide has been given ( 1 , s )as:
&H,6i I
%
15 wg. SO2 and 6 mg. Sulfamic Acid in 10 ml. of TCM 0.90 15 13.5 16 0 48 30 14.4 20 1 85 15 22.5 28 0.49 60 29.4 37 1.85 30 45.0 45 0.88 60 52.8 60 1.72 35 60.2 62 1.72 47 80.8 74
a
IO
+ CH30S0,H
4
II +
-
NH3Cl I
NHCH,-O-S03H
c1
+ di-& t ri- Subat ituted m
'
The interference from sulfamic acid and the amino compounds evaluated by Zurlo and Griffini (9) is logically explained in terms of a simple competition for I1 between the primary amino groups of pararosaniline I and the amino groups of the interfering compounds. The difference in magnitude of the interference can then be ascribed to differences in reactivity of the various amino compounds with 11. The results further indicate that a n oxidation product of sulfamic acid must be formed which is stable and has a high potential for either combining with I1 to form a compound which will not react with the pararosaniline, or which reacts directly with I11 to form a new uncolored compound. Investigations are currently under way in our laboratory to identify this compound. Air Sampling Data. The anomalies present in the air sampling data in Figures 1 and 2 may now be explained by reference to the experimental d a t a obtained above. The concentrations given for 9 January in Figure 1, and determined by the West-Gaeke method, closely approach the actual sulfur dioside concentrations. I n the absence of appreciable concentrations of nitrogen dioxide, the aeration effect gave low values when obtained by the WestOrdoveza method. When the concentrations of sulfur dioxide became quite low, negative values resulted. In the remaining samples, shown in Figure 1 for 28 January and 21 February, the decrease due to nitrogen dioxide interference in the West-Gaeke method is comparable to the decrease due to aeration interference in the West-Ordoveza method. The data in Figure 2 (18, 19 March) show that the increased concentration of sulfamic acid in the collection solution increased the aeration interference to such an extent that negative values were commonly obtained. The nitrogen di-
Direct Comparison of Methods by Sampling Known Concentrations 9% SO, recovereda
Table IV.
p.p.m.b SOz 1 1
NOz
West-Gaeke %b
Std. dev.
West-Ordoveza Std. %b dev.
Zurlo-Griffini Std. %b dev.
Post-sampling addition of sulfamic acid Std. %b dev.
102 0.7 4.5 95 3.2 1.5 102 3.5 90 1.5 = Based on nominal concentration prepared in Mylar bags at 630 mm.!atmospheric pres0 10
103 55 56
7.5 1.6 0.9
90 85 94
3.5 3.8 0.5
io0 90
sure. b Mean of three determinations.
oxide concentrations were not large enough to lower the sulfur dioxide values obtained by the West-Gaeke procedure to zero levels. Obviously, the negative values obtained have no real meaning in terms of actual air concentrations. The postulated aeration product of sulfamic acid must, therefore, react with substances contributing to the reagent blank of the method. It is assumed that this is unbleached pararosaniline (mono- and dihydrochlorides). When the regular blank is subtracted mathematically or spectrophotometrically from the sample reading, a negative value can be obtained. Modiiied Procedure. We recommend t h a t two modifications be adopted when the West-Gaeke procedure is used. T h e pararosaniline reagent should be prepared as previously detailed (6) and 0.5 ml. of a 1.2% sulfamic acid solution should be added to the samples after sampling and prior to the analysis. LITERATURE CITED
(1) Huitt, H. A., Lodge, J. P., Jr., ANAL. CHEM.36, 1305 (1964). (2) Lodge, J. P Jr., Pate, J.. B., Crow, L. W., “A Spe2al Study of Ax Pollutiof:
in the Denver Metropolitan Area.
Presented a t the Annual Meeting, Rocky Mountain Section, American Industrial Hygiene Association, Albuquerque, N. M., August 1963. (3) Nauman, R. V., West, P. W., Tron, F., Gaeke, G. C., Jr., ANAL.CHEM.32, 1307 (1960).
(4) Pate, J. B., Lodge, J. P., Jr., Neary,
M., Anal. Chim. Acta 28, 341 (1963). (5) Pate, J. B., Lodge, J. P., Jr., Wartburg, A. F., ANAL. CHEM. 34, 1660 /rnL-n\
1UU.G).
( 6 ) Urone, P., Evans, J. B., Noyes, C. M.,
“Tracer Techniques in the Evaluation of Analytical Methods for the Determination of Sulfur Dioxide in Air I. Apparatus and Preliminary Studies of the Colorimetric Method.” Presented a t the Annual Meeting, Rocky Mountain Section, American Industrial Hygiene Association, Albuquerque, N. M., August 1963. ( 7 ) West, P. W., Gaeke, G. C., Jr., ANAL. CHEM.28, 1916 (1956). (8) West, P. W., Ordoveza, F., Ibid., 34, 1324 (1962). (9) Zurlo, N., Griffini, A. M., Med. Lavoro 53, 330 (1963). JOHN B. PATE BLAIRE. AMMONS GLENDA A. SWANSON JAMES P. LODGE,JR.
National Center for Atmospheric Research4 Boulder, Colo. a Operated by the University Corporation for Atmospheric Research with sponsorship of the National Science Foundation.
Quantitative Determination of Monosaccharides by Gas Liquid Chromatography SIR: During the past few years a considerable amount of work has been reported on the gas chromatographic analysis of carbohydrates. One important area of application of this technique is in the quantitative determination of carbohydrates that occur in biological materials. The lack of volatility of carbohydrates has been overcome by converting them t o volatile derivatives such as polymethyl ethers, polytrimethylsilyl ethers, or polyacetate esters. Reviews have been published which describe the
utilization of such derivatives for the separation of sugars (1,3,6). Among the various derivatives, the trimethylsilyl (TMS) ether derivative appears to be ideally suited for routine determination and gives spectacular qualitative results. However, the resolution has not been adequate to permit accurate quantitative analysis of commonly occurring mixtures of sugars, such as glucose, galactose, and mannose. Stationary phases useful in the separation of T M S ethers are the polar ethylene glycol succinate ester (EGS) and
the nonpolar silicone gums SE-52, S E 30, and XE60. The nonpolar silicone phases give symmetrical peaks of T M S ethers but give incomplete resolution, and columns of exceptionally high efficiencies are necessary to achieve a satisfactory separation. Successful separation and quantitation of glucose and galactose have been reported using a nonpolar silicone column (7‘) only by converting the hexoses to their methyl glycosides before trimethylsilylation. On the other hand, the polar EGS phase gives better resolution but is severely VOL. 37,
NO. 7, JUNE 1965
945
