914
Anal. Chem. 1981, 53, 914-916
ACKNOWLEDGMENT I thank J. M. Bewers and P. A. Yeats of the Atlantic
Table 111. Analysis of Zinc in Open Ocean Water trial
b(seawater l , fig L-I)
1 2 3 4 5 6
0.39 0.28 0.44 0.27 0.33 0.36
av
std dev est lower limit of analysis
Oceanographic Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, for providing a sample of seawater collected near the mid-Atlantic ridge. I also thank our colleagues R. E. Sturgeon and A. R. Kean for helpful suggestions and discussions.
LITERATURE CITED Thibaud, Y. Report of ICES Intercallbratlon Exercise on Cadmlum in Sea Water, 1980, Institut Scientlflque et Technique des PBches Marltlmes, Nantes, France, In press. Segar, D. A.; Gonzalez, J. G. At. Absorpt. News/. 1971, 10, 94. Segar, D. A. J. Environ. Anal. Chem. 1973, 3 , 107. Manning, D. C.; Siavln, W. Anal. Chem. 1978, 50, 1234. Sturgeon, R. E.; Berman, S. S.; Russell, D. S. Anal. Cbem. 1979, 51, 2364. Edlger, R. D.; Peterson, G. E.; Kerber, J. D. At. Absorpt. News/. 1974, 13, 61. Sperllng, K. R.; Fresenius, 2. Anal. Chem. 1977, 287, 23. LeBihan, A.; Courtot-Coupez, J. Analusis 1975, 3 , 59. Hoenlg, M.; Vanderstappen, R.; Van Hoeyweghen, P. Analusis 1979, 7, 17. Hydes, D. J. Anal. Chem. 1980, 52, 959-963. Guevremont, R.; Sturgeon, R. E.; Berman, S. S. Anal. Chim. Acta 1980, 115, 163. Guevremont, R. Anal. Cbem. 1980, 52, 1574. Slavin, W. At. Specfrosc. 1980, 1 , 66. Keuhner, E. C.; Alvarez, R.; Pauisen, P. J.; Murphy, T. J. Anal. Cbem. 1972, 4 4 , 2050.
0.34 0.06 0.12
concentration was 1.24 pg Zn L-l with a standard deviation of 0.09 for six analyses. The lower limit of analysis (twice the standard deviation) was 0.18 pg Zn L-l in seawater. A sample of open ocean seawater with much lower zinc concentration was analyzed to assess the accuracy and precision of direct GFAAS analysis near the detection limit (Table 111). An exact determination of reagent and instrumental blanks was carried out by using a sample of seawater previously extracted by use of the APDC, DDC/MIBK procedure. The result of the direct analysis was 0.34 f 0.06 pg Zn L-l in good agreement with 0.29 Fg L-l using the APDC, DDC/ MIBK method. The lower limit of analvsis is estimated to be 0.12 pg Zn L-l in seawater, not differing greatly from 0.18 pg Zn L-’ using the coastal seawater sample.
RECEIVEBfor review September 25,1980. Accepted December 6, 1981. Issued as NRCC No. 19049.
Quantitative Thin-Layer Chromatography of Sedimentary Organic Matter Alaln Y. Huc” CNRS, Laboratoire de G6ologie Apliquge ERA 60 1, 45046 Orlgans, France
Janine 0 . Roucachs Institut FranGais du P h o l e , Synth6se GGologique et Geochimie, 92506 Rueil Malmalson, France
A standard method of analyzing the bitumens in sedimentary rocks is to extract the ground rock with a solvent mixture such as chloroform or dichloromethane, benzene, and methanol and then to remove the solvent at a temperature around 40 “C, leaving mainly bitumens in the c16+molecular range. These are then analyzed by column chromatography. The size of the available sample sometimes prevents the study of the CIS+ fraction of some bitumens by this method. For instance, column chromatography usually is not suitable when only a few well cuttings are available for extraction or when “Fisher” pyrolysis products have to be analyzed. The thinlayer chromatography (TLC) provides an alternative, allowing a quantitative separation of 10-90 mg of bitumens (I).
EXPERIMENTAL SECTION The chromatography is carried out on 20 X 20 cm precoated silica gel plates (Merck F254). The required coating thickness is related to the amount of sample and is about 0.25 mm for 10 mg up to 20 mg of sample and 2.5 mm for 20 mg up to 90 mg of sample. A known amount of sample, dissolved in chloroform, is applied as a thin uniform line on one side of the layer, using a commercial applicator (CAMAG). Several successive applications will normally have to be made, and the layer should be allowed to dry between applications and before the chromatography itself, The layer is then placed vertically in a developing chamber containing cyclohexane as the solvent. The chromatography has to be carried out in a chamber which has been as nearly saturatad as possible with the developing solvent. For this purpose it is recommended that filter paper be used which partially lines the wall of the chamber. The upper centimeter of adsorbent has been
previously scraped off and the solvent is allowed to reach the top end of the layer. After this, the chromatogram is allowed to remain in the chamber 10 min more. This “overrunning” will allow the solvent to be more evenly distributed throughout the layer. The development time ranges from l to 5 h according to the thickness of the layer and the room temperature. The layer is then removed from the chamber and allowed to dry. A nondestructive visual method is used in order to locate three fractions corresponding to the main structural groups of compounds usually occurring in oils and bitumens, namely, (1)the saturated and the unsaturated hydrocarbons (SHC + UHC), (2) the aromatic compounds (ARO), including aromatic hydrocarbons, naphtenoaromatic, and thiophenic compounds (2), and (3)nitrogen, sulfur,and oxygen containing compounds, including resins and asphaltenes (NSO). The half upper surface of the layer is sprayed with a 0.5% solution of berberine sulfate in methanol. Then the layer is examined in daylight and in ultraviolet light (Figure 1). The saturated and unsaturated hydrocarbons can be located just after removing the layer from the developing chamber while the adsorbent is impregnated with cyclohexane, They appear in the daylight as a faint translucent area. They can be seen more accurately in ultraviolet light as a well-separated yellow band, dyed by the berberine sulfate, with an R, 2 0.7. The NSO compounds stay near the origin (Rf 0-0.2). They occur as a dark line right at the place of the application and as a yellow to green band in ultraviolet light for the most migrated fractions. The aromatic compounds show an intermediate migration behavior (RfE 0.2-0.5). A part of them can be seen in ultraviolet
0003-2700/81/0353-0914$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53. NO. 6. MAY 1981 VI5
SP'OYCd
b
"ELLOW a !
Berberine sulfola
I
........ ""'")SHC.UHC .........
........
" E m
K"E*
-
PURPLE*
Flgure 1. Schematic picture of a developed layer, summarizing the optical properties of the different compounds. Asterick designates colors under UV light.
----CmBON
Flgure 2. Recovery of the compounds.
light as bright purple to blue colors. Usually the upper part of the aromatic zone has to be dyed with berberine sulfate to be seen in ultraviolet light. Once the three zona (SHC + UHC, ARO, NSO)are located, the adsorbent containing the compounds is separately scratched off the glass plate and collected in beakers where the samples are mixed with chloroform. The recovery of the compounds is carried out by a subsequent solvent wash through a fritted funnel set with a fiberglass filter (Whatman GF/C). The washing solvents used are chloroform for SHC UHC, chloroform and then benzene for ARO. and chloroform and then methanol-benzene (L1) for NSO The volume of these recovered fractions (-100 ans)is reduced to less than 2 cm3with a rotaevaporator,then carefully dried in a tared aluminum crucible at room temperature under a nitrogen flow, and weighed on a microbalance (3). If properly applied, using very clean vessels and distilled high-purity solvent, the recovered fractions can be subsequently analyzed by gas chromatography and mass spectrometry.
,R E M B U N
Flgure 3. Capillary gas chromatography, comparison of saturated hydmcarbons and aromatic armpounds in a Mumen separated by both methods: mC)thin-layer chromatography; (CC)column chromatography: (FID) flame ionlzation detector; (FPD) flame photometry
detector.
+
RESULTS AND DISCUSSION The quantitative recovery of the depcaited sample has been checked by using a wide range of soluble organic matter: an artificial mixture (including n-alkanes, isoprenoid alkanes, aromatic hydmcarbons, NSO compounds), some topped crude oils (2210 "C), rock extracts, and oils obtained from pyrolysis of shales. In all cases the final recovery ranged from 91% to 108% (Figure 2). The variation is due to analytical errors, losses by evaporation, and minor contaminations. Separation of the different structural types in oils and rock extracts is usually performed by liquid chromatography on a column (4). A quantitative and qualitative comparison of the TLC method and of the conventional column chromatography has been made on a large number of samples (I). The two procedures provided data which result in the same geochemical interpretations, so that both techniques might he simultaneously used in a same geochemical problem. In all samples studied the gross compositions are very similar by the two techniques (Table I). The general shapes and the detailed patterns of the capillary gas chromatograms of the
30
35
90
45
cc
Flgure 4. Chromatogrems of saturated hydrocarbons rich In hlgh molecular weight compounds (blunmn): mC)thin-layer chromatog raphy: (CC) column chromat@graphy.
different groups of hydrocarbons are in good agreement (Figure 3) as well as the mass spectrometry data. Minor differences might occur due to the different procedures. For instance when using column chromatography the saturated hydrocarbons are usually recovered d k l v e d in hexane which
916
Anal. Chem. 1981, 53, 916-917
Table I. Comparison of Thin-Layer Chromatography (TLC) and Column Chromatography (CC) Results Aa SHC + UHC ARO %
NSO % recovery % a
Crude oil.
BQ
Db
Ca
EC
TLC
CC
TLC
cc
TLC
cc
TLC
cc
TLC
cc
66.7 21.7 5.6 94.0
54.7 21.4 3.8 80.9
60.0 30.0 8.8 99.7
59.1 30.6 6.7 96.4
63.9 20.2 14.9 98.5
62.6 14.7 13.7 91.0
9.9 28.2 62.2 100.3
6.0 32.5 48.5 87.0
21.9 25.0 48.6 95.2
20.4 27.6 50.4 98.4
Pyrolysis oil.
Rock extract.
SHC+UHC
I
b
C
the column chromatography is not quantitative for SHC + UHC since the heaviest alkanes are eluted with the aromatic fraction. In such a case the TLC technique is more suitable because there is no overlap between the saturated hydrocarbons and the aromatic compounds bands (Figure 4). Another advantage of the technique is that the TLC can be performed without a previous deasphalting which is required when carrying out a column chromatography on petroleum compounds. On occasion, in recent sediments and sometimes even in ancient sediments, unsaturated hydrocarbons are present. As previously quoted, in the described procedure the unsaturated hydrocarbons (UHC) show the same chromatographic behavior as the saturated hydrocarbons and thus provide an interference with the latter. A way to solve this problem is to carry out a second quantitative TLC in order to discriminate between the two types of hydrocarbons (5) (Figure 5). The layers are prepared by using a mixture of silica gel (50 g of silica gel, G Type 60, Merck, for five layers with a coating thickness of 0.5 mm, 5 g of silver nitrate, and 120 mL of water). When dried the plates are activated (1 h at 140 “C) and stored in a dark container until use. Cyclohexane is used again as developing solvent. The resulting bands of saturated hydrocarbons (Rf= 0.7) and unsaturated hydrocarbons (Rf 0-0.5) appear after spraying the layer with berberine sulfate in methanol solution. The recovery of the compounds is carried out by using the method previously described. In this case the adsorbent is extracted with ethyl ether.
LITERATURE CITED CARBON NUMBER-
Figure 5. Discrimination between saturated (SHC) and unsaturated (UHC) hydrocarbons (bitumen).
has a higher boiling point than the chloroform. So, in many cases we have observed a more important loss of the lighter hydrocarbons with the column chromatography procedure. Another difference is seen when samples are rich in high molecular weight saturated hydrocarbons. In some samples
(1) Huc, A. Y.; Roucach6, J.; Bernon, M.; Calllet, G.; da Silva, M. Rev., Inst. Fr. Pet. 1976, 3 1 , 67-98. Chem. Abstr. 1976, 85, 145438~. (2) Castex, H.; RoucachB, J.; Boulet, R. Rev., Inst. Fr. Pet. 1974, 29(1), 3-40. Chem. Abstr. 1074, 8 1 , 17253111. (3) Monln, J. C.; Peiet, R.; FBvrler, A. Rev., Inst. Fr. Pet. 1078, 33, 223-240. Chem. Abstr. 1978, 89, 165852e. (4) Oudin, J. L. Rev., Inst. Fr. Pet. 1970, 25, 3-15. Chem. Abstr. 1970, 73, 68661k. (5) RoucachB, J.; Bouiet, R.; da Silva, M.;Fabre, M. Rev., Inst. Fr. Pet. 1977, 32(6), 981-994. Chem. Abstr. 1078, 89, 11346111.
RECEIVED for review September 29,1980. Accepted January 22, 1981.
Chromatographic Identification and Quantitation of Dimethyl Sulfoxide Wayne A Morris Sanford Regional Crime Laboratory, Florida Department of Law Enforcement, Sanford, Florida 3277 1
As of October 1,1980, the State of Florida has allowed those people licensed to prescribe medication to prescribe dimethyl sulfoxide (Me2SO)for bladder cystitis. As of this time, only one product is approved by the Food and Drug Administration (FDA) for such use. However, so-called “Arthritis Clinics” have begun to dispense MezSO as arthritis pain relievers.
Some of these clinics are dispensing the approved MezSO product improperly, while others are dispensing Me2S0 which is not approved for human use. The author received several such submissions at the Sanford Regional Crime Laboratory with a request to differentiate the approved product from the other products.
0003-2700/61/0353-0918$01.25/00 1981 American Chemical Society
