Anal. Chem. 1983, 55. 2434-2436
2434
Though handling of the parts of the mill and of the materials a t low temperatures is inconvenient, the Teflon disk mill is recommended as an effective, contamination-freedevice for size reduction and homogenization of biological tissue. Operation a t cryogenic temperatures reduces loss of volatile components and changes in composition during the size reduction step. Samples as large as 1000 g can he homogenized with this technique. The mill uses a commercially available drive unit and is fast and simple to operate. The quality and quantity of the samples produced should make this technique useful for the sample preparation for the analysis of biological tissues.
LITERATURE CITED
40
so
so
Id0
sieves
1o;
( mesh
260
I
LIVER, ball mill LiVER
ADiPOSE
$%6
MUSCLE
Flgw 2. Pallick size UWibulbn of frozen Wue after
hcmogenizatbn In Teflon milk approximately 150 g of tissue, grinding time 4 min. liver in ball mill B, and liver, adipose, and muscle tissue in disk mill no. 2. and less then 2% for several essential trace elements, for which large differences among 1-g test portions were observed previously (2). This suggests that subsampling errors due to inhomogeneity can be confined to less than 2%.
(1) Kratochvii. 8.: Taylor, J. K. Anal. Chem. 1981, 53. 924A-938A. (2) Lievens. P.: Versieck. J.; Comeiis, R.: Hoste, J. J . Radioanal. Chem. 1977. 37. 483-496. (3) lyengar, 0. V.: KaSperek, K. J . Radloanal. Chem. 1977. 39.
301-316. (4) Bailey, J.; Filrpalrick. K. A,; Harrison. S. H.: Zeisler, R. NBS Spec. Publ. ( U S . )1983,No. 656.
RECEIVED for review June 20,1983. Accepted August 25,1983. This work was supported in part by the Office of Research and Development, U.S. Environmental Protection Agency. Certain commercial equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endoresement by the National Bureau of Standards nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Coulometric Determinatlon of Sub-Part-per-Million Levels of Sulfur in Volatile Liquids I. J. Oita Standard Oil Company (Indiana), Research Department, Analytical Services Diuision, Naperuille, Illinois 60566 In petroleum refining technology, the presence of subpart-per-million levels of sulfur has become increasingly important. The Wickbold method ( 1 ) requires complete volatilization and comhustion of a large sample over a period of several hours. It has not heen widely accepted because of the hazards involved. Coulometric methods (2) are generally not precise enough below 1ppm. Drushel(3) reports a precision of about 10% for samples containing less than 1ppm, using the Houston-Atlas analyzer, which is a reductive method. The newly developed method described in this paper is a modification to oxidative coulometry which permits sulfur analysis below 0.1 ppm. In oxidative coulometry, the sample is burned in oxygen and the generated sulfur dioxide is coulometrically titrated with iodine. However, there are three main problems with this method. First, although the S02/S08equilibrium in comhustion reaction is generally 90% in favor of SO, a t the 1000 OC comhustion temperature, it varies with temperature, flow rate, and the type and amount of sulfur. For trace samples, this ratio has been observed to he as low as 70%. For this reason, it was essential to develop a method that would convert all of the sulfur in the sample to SO,. The second protilem with the roulometrir approarh is that reaction of iodine with SO? is nut selective. I f olefins are formed during the comhustion, they cnnsume iodine leading to high results. In order to minimize olefin formation, the sample must be burned slowlv. Howrver, this is unmtistkctog. in two respects: la) the large snmple size required ior trace analysis makes the combustion time excessive and rb) the iodine generation is so drawn out that accurncy of its measurement is greatly reduced. On the other hand, tw, rapid 0003-270018310355-2434SO1 5010
comhustion can lead to soot formation which can adsorb SO, and cause low results. Thus, depending upon the comhustion conditions and type of sample, results could be high or low. Although many of these errors are not important above the 1ppm sulfur level, they can become quite significant helow 1ppm. The present development ensures that all sulfur enters the coulometric cell as SO, and is based on the following reactions (4):
SO,
f
- + - + so, +
3CuO
Cu,O
so, f CUO
CuSO,
cuso,
cuso, (900 "C) CUO so, + 1/,o2 so,
-
(1) (2)
1/20,
(3) (4)
Reaction 3 was incorporated into a recent gas chromatography method for sulfur (5) wherein combustion products were passed over CuO at 900 "C in a single step yielding only SO,. Reaction 4 does not take place to any appreciable degree since SO, is effectively removed by the CuO. An excellent study of this reaction was published hv Robinson and Kusakabe (6). I n order to apply these rearfions to rnulometry, a two-step process was devi4oped. f i r g t , the sample is burned i n oxygen and the combusti~nprodurts arc passed over ('110 at 700 "C. a temperature nt which both SO2 and 5 0 , fbrm stable C'iiSO,. Partially oxidized hydrocarhons IP oxidized hy the CuO that eliminntps hoth d d i n and soof formation. In the second step. the CuO CuSO, zone is rapidly heated to YO0 "C which hherates unly SO? according to reaction 3 nnd ns R slug u,hich is easily measured. The 700 "C trupping tempcrnturc was Z 1983 American Chemical Soc ely
ANALYTICAL CHEMISTRY, VOL. 55, NO. 14, DECEMBER 1983
700 - 9 0 0 o c
n
T - 3OOP cell
I
2435
S -300 Furnace
I
Center 900°C
Quartz copper oxide tube
Rate selector
Recorder
Figure 1.
Oxidative coulometric system for sulfur determination. 1"-
-"46/16
1 1
.r
Quartz wool
Table I. Sulfur Content of Solvents (ppm)
-
1 Copper oxide 6"
L l c o n e "0"ring
Figure 2.
Quartz copper oxide tube.
chosen since CuO will trap SO2 and SO3according to Dugan (4) and also oxidize hydrocarbons. CuO at 700 "C will prevent the formation of soot on the exit end of the tube. Also, olefins which can be formed in the pyrolysis step are oxidized to noninterfering products by CuO at 700 "C. Thus, the problems associated with oxidative coulometry for trace sulfur determination have been resolved.
EXPERIMENTAL SECTION Apparatus. A schematic diagram of the system is shown in Figure 1. The Dohrmann S-300 furnace, T-300Ptitration cell, and C-300 coulometer were used. A Sage Model 341 syringe p u p was used to inject the sample slowly into the inlet zone. The Quartz oxidizing tube that contains the copper oxide is shown in Figure 2. A small plug of quartz wool is placed at the tube constriction followed by 1in. of copper oxide and a second retaining plug of quartz wool. This tube is larger than the oxidizing tube supplied by Dohrmann as part no. 511-605. The O-rings are Dohrmann part no. 511-762. Reagents and Materials. Wire of reagent grade cupric oxide cut into 2 mm lengths was used. Since different lots of cupric oxide from the same manufacturer had varying degrees of effectiveness, a number of cupric oxide batches should be tested and the one producing the sharpest peak should be used. The cupric oxide section must be conditioned at 900 'C to remove any original sulfur on the copper oxide section. Special distilled in glass isooctane and n-heptane from Burdick and Jackson were used to prepare known samples. These solvents were purified by percolation through 24 in. of Davidson grade 70 silica gel followed by 6 in. of activated charcoal in a 1in. column. Enough 0.1 N AgNO, was added to the silica gel so that the gel just began to stick to the wall of the container. The wet silica gel was dried overnight at 90 "C in a vacuum oven and poured into the column after cooling. A 1 ppm range sulfur standard was prepared by diluting the 145 ppm Dohrmann standard, No. 511-865, with purified isooctane. The Dohrmann standard contains n-butyl sullide in isoodane. The 1ppm standard was further diluted to prepare standards containing less than 1ppm. Using the method of standard addition, corrections were made for the small amount of sulfur left in the purified solvent. Procedure. The oxygen flow rate is set at 80 mL/min and the helium flow rate is 30 mL/min. The initial furnace temperatures are set at 800 "C inlet, 900 "C center, and 700 "C outlet. Set the coulometer at 300 gain, 130 bias, 100 range, and 999 time. All of these coulometer settings are approximate and small variations can be made for individual samples. The sample size should vary high with the estimated sulfur content of the samples. About 1mL is used for samples containing less than 0.5 ppm and about 0.1 mL is used for samples in the 1 ppm range. The rate selector on the Sage pump is set at the maximum rate that provides combustion without the formation
a
solvent
found"
range
std dev
isooctane n-heptane n-heptane (purified)
0.100 0.079 0.042
0.087-0.107 0.078-0.087 0.035-0.044
0.008 0.006 0.003
Average of five determinations.
Table 11. Sulfur Content of Standards (Butyl Sulfide in n-Heptane) (pprn) .range founda calcd 1.53 0.122 0.350
1.55 0.123 0.336
1.49-1.63 0.120-0.128 0.321-0.365
std dev 0.06 0.003 0.019
" Average of six determinations. of soot on the outlet end of the combustion tube. For most samples, the rate can be set at 100 pL/min. The sample introduction rate of 100 pL/min actually requires more oxygen than the 80 mL/min as set in the procedure above. The excess hydrocarbon is oxidized by the copper oxide which is regenerated as soon as the sample introduction step is completed. Injection of the sample is started with the outlet furnace temperature at 700 "C. After all of the sample is injected into the combustion tube and the recorder shows a zero base line, the temperature of the outlet furnace is increased to 900 OC, as fast as pwsible. The outlet furnace is kept at 900 OC until d the sulfur is driven off of the copper oxide. The integrater on the coulometer is turned on just before the start of the sulfur peak and turned off when the zero base line is reached. After the zero base line is reached, the power to the outlet furnace is cut off to allow the furnace to cool down to 700 OC as rapidly as possible. This work was carried out on the Dohrmann S-300 furnace which was not designed for rapid temperature changes. It takes about 8 min to go from 700 O C to 900 OC in the outlet section. Usually, the integrator of the coulometer is turned on when the outlet furnace is at 800 "C. The response curve would have a sharper peak if the outlet section could be heated faster. The shape of the tail of the desorption curve depends to some degree on the purity of the copper oxide. Some copper oxide gave a very prolonged tail while others did not. The prolonged tail is probably due to impurities in the copper oxide which form stable sulfates. Usually, the desorption step is completed in 15 min after the intergrator is turned on.
RESULTS AND DISCUSSION Table I shows the results obtained on two special glass distilled solvents. Because n-heptane contained slightly less sulfur than isooctane, it was used for the purification tests. The sulfur level in n-heptane was lowered from 0.079 to 0.042 ppm by passing it over a silver nitrate impregnated silica gel column. The sulfur content of the effluent was not reduced after heating the column to 80 OC. Samples containing a known amount of sulfur were prepared as shown in Table 11. Good results on the sample
2436
Anal. Chem. 1983, 55, 2436-2439
Table 111. Sulfur Content of Naphtha Samples (ppm) sample
a
found
range
167 0.32Za 0.312-0.337 168 0.077' 0.074-0.084 Average of six determinations.
std dev
0.010 0.004
Table IV. Sulfur Content of Reagent Grade Solvents sample xylene toluene acetone
PPm ofS
0.213 0.091 0.145
sample isopropyl alcohol methanol
ppm of
s
0.327 0.188
Table V. Sulfur Content of Solid Samples sample wax sample (original) wax A wax B polypropylene
m m of S calcd found 8.0 f 0.5
10.0 2 0.5
0.2,0.5 8.0,8.4 0.8,9.4 0.6,0.7
containing 1.53 ppm sulfur indicate that recovery was complete. Once it was established that the method worked for samples in the 1 ppm range, standards containing less than 1 ppm were prepared and the results are shown in Table 11. Over the range of 0.1-1.5 ppm sulfur, recoveries of 96-104% were obtained. For all three samples, the relative standard deviation was no greater than 6%. These data were obtained with the Dohrmann coulometric system which measures the sulfur in absolute nanograms of sulfur. Table I11 shows data obtained on ,two refinery naphtha samples. Sample 167 was found to contain 0.332 ppm of sulfur and sample 168,0.077 ppm of sulfur. In the normal coulometric method, these two samples would be considered equivalent in sulfur content since the method cannot discriminate between them.
A number of reagent grade laboratory solvents were analyzed for trace sulfur and the data are shown in Table IV. All of the solvents analyzed contain less than 0.5 ppm of sulfur. This method can be used for samples that contain chlorine or nitrogen since these interfering gases are not trapped on the copper oxide but pass through the coulometric cell while the sulfur dioxide is held on the copper oxide. The sulfur is measured after the interfering gases are swept out of the system. Although most of the work has been on volatile liquid samples, preliminary work indicates that solid samples can be analyzed. The data in Table V were obtained on solid samples. The wax samples were prepared by mixing a known high sulfur wax with a low sulfur wax. The value for the high sulfur wax was 80 ppm as determined by an X-ray method. The sulfur in the low sulfur wax could not be detected by the X-ray method which means that it is probably less than 0.5 ppm. The mixture was heated to form a homogeneous liquid and then cooled to form a solid wax sample. Wax A and wax B were prepared by using different amounts of the high sulfur wax and the low sulfur wax. The recoveries are within 0.6 ppm in the 10 ppm range. A high molecular weight polypropylene was analyzed and an average value of 6.5 ppm sulfur was obtained. Registry No. CuO,1317-380;sulfur, 7704-34-9; polypropylene,
9003-07-0.
LITERATURE CITED (1) (2) (3) (4)
Wickbold, R. Anal. Chem. 1957, 2, 530. Coulson, D. M.; Cavanagh, L. A. Anal. Chem. 1980, 32, 1245. Drushel, H. U. Anal. Chem. 1978, 50. 76. Dugan, G.; Carre, J. F. Report N 27201; Hercules Research Center,
1972. (5) Pella, E.; Columbo, 8. Mlkrochim. Acta 1978, 271-276. (6) Robinson, 6. W.; Kusakabe, M. Anal. Chem. 1975, 47, 1179.
RECEIVED for review May 9,1983. Accepted August 29,1983. This paper was presented by I. J. Oita in August, 1980,at the 180th National Meeting of the American Chemical Society in Las Vegas, NV.
Chemical Ionization Mass Spectra of 2,4-Dinitrophenylhydrazones of Carbonyl and Hydroxycarbonyl Atmospheric Pollutants Daniel Grosjean' Environmental Research & Technology, Inc., 2625 Townsgate Road, Westlake Village, California 91361 Carbonyl compounds are currently receiving increasing attention as major pollutants in the indoor and outdoor environment. As a source of free radicals, carbonyls play an important role in ozone formation in urban smog as well as in the photochemistry of the unpolluted troposphere ( I ) . Scavenging of carbonyls by fog, clouds, and rain (2) and the subsequent formation of carbonyl-bisulfite adducts may have implications for precipitation acidity. Formaldehyde is of major concern as an indoor pollutant ( I ) , and unsaturated carbonyls (e.g., acrolein) may cause eye irritation in outdoor, residential, and workplace atmospheres ( I ) . Thus,quantitative information regarding airborne carbonyls is critical to a number of important environmental issues. Present address: Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91125.
We have recently reported (3, 4 ) on a high-performance liquid chromatography (HPLC)method for trace level analysis of carbonyls as their 2,4-dinitrophenylhydrazones(DNPH)
(N02)2CGHsNHNH2+ O=CRIR2 HzO + (N02)2CeH3NHN=CR1R2 (1) -+
and applied this method to carbonyl measurements in urban air (5, 6), industrial atmospheres (7), precipitation samples (2), and environmental chamber studies of hydrocarbon photochemistry (8, 9). Because of the complexity of these environmental samples, molecular structure confirmation is often required following HPLC analysis. Spectroscopic methods such as infrared and ultraviolet analysis are not suitable for differentiating between carbonyl DNPH homologues. In contrast, molecular information could be readily obtained by collecting the appropriate
0003-2700/83/0355-2436$01.50/00 19S3 American Chemical Society
