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ANALYTICAL CHEMISTRY, VOL. 51, NO. 1 1 , SEPTEMBER 1979
the materials that have been used, but at times erratic results were obtained with all probe extensions. This effect was considered to be due to localized charges on films on the extension. A metal probe tip was then employed, with source and probe circuitry changes so that the ion source voltage was applied to the probe extension (9). No further difficulties were encountered. The currently preferred method of operation is to use an unheated source with a gold or copper probe extension for both thermal and electrical conduction, and to heat the sample by probe heating. The source may be heated if desired. I t should be noted that the controversy over field desorption mass spectra obtained without an applied field is not related to plasma desorption ionization (IO),although both techniques are used for studies of nonvolatile compounds. Compounds of type c will probably not yield ionic products under our conditions. The high energy conditions of Macfarlane ( 1 1 ) or the laser ionization techniques described by Meuzelaar and his colleagues (12) may be required in these instances. In this work, and in earlier related studies, it has been considered that ionization occurs by reaction a t a gas-solid or gas-liquid interface. The establishment of methods for the ionization of compounds that are nonvolatile or that undergo partial or total decomposition on heating should significantly extend the range of mass spectrometric bioanalytical applications. I t should be possible to carry out the analysis of liquid chromatographic effluent streams without pre- or post-column derivative formation. Further, if samples need not be volatilized in order to obtain CI spectra, new approaches can be made to many bioanalytical problems. The nature of the supporting surface is important in some instances; this is suggested by this work and by the studies of Friedman and his colleagues (13, 14) and Munson (5) on Teflon surfaces. A siloxane film has the advantage of providing an adhesive surface that will hold solids but that will not lead to adsorption losses of polar compounds.
LITERATURE CITED 61) Baldwin. M. A.: McLaffertv. F. W. Ora. Mass Soecbom. 1973. 7. 1353-1356 (2) Holland, J F , Soltmann, B Sweeley, C C B~omedMass Specfrom 1976, 3, 340-345 (3) Soltmann, B , Sweeley, C C , Holland, J F Anal Chem 1977, 49, 1164-1166. (4) Hunt, D. F.; Shabanowitz, J.; Botz, F. K.; Brent, D. A. Anal. Chem. 1977, 49, 1160-1163. (5) Hansen, G.; Munson, €3. Anal. Chem. 1978, 50, 1130-1 134. (6) Cotter, R. J.; Fenselau, C. C. American Society for Mass Spectrometry, 26th Annual Conference, St. Louis, Mo., May 28-June 2, 1978; Abstracts, p 672. (7) Cotter, R. J. Anal. Chem. 1979, 5 1 , 317-318. (8) Thenot, J-P.; Nowlin, J.; Carroll, D. I.; Montgomery, F.; Horning, E. C. Anal. Chem., 1979, 5 1 , 1101-1104. (9) Carroll, D. I.; Nowlin, J. G.; Montgomery, F. E.;Stillwell, R. N.; Horning. E. C.American Society for Mass Spectrometry, 27th Annual Conference, Seattle, Wash., June 3-8, 1979. (IO) Beckey, H.D.;Rollgen, F . W. Org. Mass Specfrom. 1979, 14, 188-190. (11) Macfarlane, R. D.;Torgerson, D. F. Science 1976, 191, 920-925. (12) Posthumus, M. A.; Kistemaker, P. G.; Meuzelaar, H. L. C.: Ten Noever de Brauw, M. C. Anal. Chem. 1978, 50, 985-991 (13) Beuhler, R. J.: Flaniqan, E.; Green, L. J.; Friedman, L. Biochemistw1974. 13, 5060-5068 (14) Beuhler, R J , Flanigan, E , Green, L J , Friedman L J Am Chem SOC 1974, 96, 3990-3999
D. I. Carroll I. Dzidic M. G. Horning F. E. Montgomery J. G . Nowlin R. N. Stillwell J-P. Thenot E. C. H o m i n g * Institute for Lipid Research Raylor College of Medicine Houston, Texas 77030 Received for review December 15, 1978. Accepted June 20, 1979. This work was supported by Grants GM 13901 and GM 24092 of the National Institute for General Medical Sciences, and by Grant Q-125 of the Robert A. Welch Foundation.
AIDS FOR ANALYTICAL CHEMISTS Representation of Extraction Efficiencies Winston K. Robbins Exxon Research and Engineering Company, P.O. Box 121, Linden, New Jersey 07036
Extractions are widely used for analytical sample preparation. This paper presents a simple technique which can aid the analyst in the selection of extraction conditions. Theoretically, the best way to compare different extraction systems is through the use of distribution coefficients (KD) of the compound of interest. In a liquid-liquid extraction, a solute is distributed between two immiscible phases-the solvent (S)and the extractant ( E ) . The equilibrium may be expressed mathematically by the Nernst distribution law:
where K D is the distribution coefficient for the solute and CE and Cs are the concentrations in the two phases. The efficiency for an extraction is measured by E , the fraction extracted. From separations theory, E is related to K D by the equation 0003-2700/79/0351-1860$01 OO/O
where VE equals volume of the extractant, Vs equals volume of the solvent, and n equals the number of times the solvent phase is equilibrated against fresh extractant. Thus, a knowledge of the distribution coefficients allows comparison of different extraction procedures. Distribution coefficients for many aqueous systems have been tabulated in an extensive review by Leo ( I ) . The K D values for other systems may be conveniently determined by the techniques of Bowman and Beroza (2-4) or Berezkin and co-workers ( 5 ) . Since the relationship between K D and % E is cumbersome and difficult to visualize, most extraction conditions are 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
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If Equation 4 is plotted on log-log paper, the result is a series 5 0
1
r(
of straight lines intersecting a t a point (1,l)with slope of -n. On this plot, (1 - E ) decreases as KD increases. But as (1E ) decreases from 1to 0, its complement E is increasing from 0 to 1. Thus, E may be plotted as a linear function of [(l KD)(VE/ Vs)] on the inverted log scale, but this plot is awkward because it is in a negative quadrant, Le., E increases ad KD increases but the slopes are negative. This relationship may, however, be transformed to positive coordinates by reflecting through the plane where E = 0. With this transformation, one obtains the set of lines with slope of +n and intercept a t (l,O), (Note, however, that %E is plotted on the inverted log scale obtained by turning a sheet of loglog paper over from top to bottom.) This relation is then off-set by -1 so that the abscissa now becomes (KD)(VE/ V s ) . In this plot, a series of straight lines with slope of n (the number of extractions) relates the %E on an inverse log scale in the product of K,(VE/ Vs) on a log scale. With this plot, it is possible to: (1)make rapid comparisons between systems with known KD’s, (2) select volume ratios necessary to use with systems with known KD’s, (3) chose a system with an adequate KD to give a desired % recovery ( % E ) ,or (4)determine the number of steps necessary for “complete” (99%) extraction. In past practice, the analyst generally varied (VE/ Vs)and n but selected the extraction pair somewhat arbitrarily on the basis of experience. However, with the plot given, selection of extraction pairs and conditions can be made systematically from KD values.
+
30 20 10
0 0
-1
1
I
I
1
2
3
I
DISTRI6LTI0\1 RATIO K D
Flgure 1. Graph of
I
I
r:j
3
5
I
.
b 764
YO extracted vs. the distribution coefficient
selected on the basis of experience or semi-empirical considerations. Even workers who have measured distribution coefficients have sought alternate expressions for comparing different extractions. For example, Bowman and Beroza (2-4) have defined a “p-value” for several pesticide related extraction systems which in effect is the percent extracted under a fixed set of conditions. Suffet and Faust (6--8)have extended this p-value approach to calculate an (F,) value (total fraction extracted) as a more general term. These new terms were created because it is difficult to visualize the relationship between the distribution coefficient (ITD),the volume ratio (VE/Vs), and the number of extractions ( n ) on the percent efficiency ( % E ) when plotted in normal coordinates. The difficulty in relating the effect of extraction parameters on percent efficiency has been overcome by the derivation of the plot shown in Figure 1. The plot was derived in the following topological exercise. Equation 2 may be re-written in the form,
LITERATURE CITED (1) A. Leo, C. Hansch, and D. Elkins, Chem. Rev, 71, 525 (1971). (2) M. C. Bowman and M. Beroza, J . Assoc. Off. Anal. Chem., 48, 943 i,i a f i m “VI,.
(3) M. C. Bowman and M. Beroza. J . Assoc. Off. Anal. Chem., 48, 358 (1965). (4) M. C. Bowman and M. Beroza. Anal. Chem., 38, 1544 (1966). (5) V. G. Berezkin, A. G. Pankov, and V. D. Losbshilova, C:hromatographia, 9, 490 (1976). (6) I. H. Suffet and S. D.Faust, “Chapter 2, Fate of Organic Pesticides in the Aquatic Environment”, Adv. Chem. Ser., 111, 1972. (7) 1. H. Suffet, J . Agric. Food Chem., 21, 288 (1973). (8) I. H. Suffet, J . Agric. food Chem., 21. 591 (1973).
RECEIVED for review March 9,1979. Accepted May 10, 1979.
Sample Cleanup and Concentration Apparatus for the Determination of Chlorinated Hydrocarbon Residues in Environmental Samples John Solomon Department of Fisheries and Oceans, Freshwater Institute, 50 1 University Crescent, Winnipeg, Manitoba, Canada R3T 2N6
In toxicological studies of pesticides in the aquatic environment there is often a need to analyze large numbers of small animal tissue samples (fish organs, insects). Rapid methods of extraction of small samples for the determination of chlorinated pesticide residues using a specially designed 0003-2700/79/0351-1861$01.OO/O
ball-mill have been developed ( I , 2) but the cleanup and concentration of the extract for gas chromatographic (GLC) analysis has remained somewhat time-consuming. Recommended multiresidue procedures for cleanup of extracts containing chlorinated hydrocarbons (3,,4) use large quantities 1979 American Chemical Society
