Anal. Chem. 1980, 52, 1313-1318
1313
Identification of Explosives at Trace Levels by High Performance Liquid Chromatography with a Nitrosyl-Specific Detector Arthur L. Lafleur‘ and Brian D. Morriseau Thermo Electron Corporation, Analytical Instruments Division, 10 1 First Avenue, Walfharn, Massachusetts 02 754
A method is presented for the identification and determination of explosives and other related compounds possessing thermally labile nitro or nitroxy groups. It involves the use of high performance liquid chromatography (HPLC) with a nitrosylspecific detector (TEA Analyzer). Results are presented for EGDN (ethanediol dinitrate), NG (glycerol trlnitrate), PETN (2,2-bis( nitroxymethyl)-l,3-propanediol-l,3-dinitrate), Petrin (2-hydroxymethyl-2-nitroxymethyl-l,3-propanediol-l,3-dinitrate), PEDN (2,2-bis( hydroxymethyl)-l,3-propanediol-l,3dinitrate), RDX (hexahydro-l,3,5-trinitro-s-triazine), NGu (1nitroguanidine) and HMX (octahydro-l,3,5,7-tetranitro1,3,5,74etrazocine). Calibration curves covering a concentration range of three orders of magnitude are presented for RDX and PETN. The 90 YOconfidence interval for quantiication of RDX is f1.6% at the 4-ng level and that for PETN is f3.2% at the 6-ng level. Retention behavior with silica and NH2bonded phase HPLC columns is presented. Use of retention times on the two different columns coupled with a response on the nitrosyl-specific detector serves as a convenient method for identification. Although standard explosive preparations were analyzed at nanogram levels without purification, no interferences were presented by ancillary components such as plasticizers and stabilizers.
Present techniques for the identification of small aniouiits of explosives usually involve the use of thin-layer chromatography (TLC) (1-3). Some improvements in speed and accuracy are possible through the use of spectrophotometric methods with the T L C techniques (4-6). Despite their widespread use, these techniques possess several disadvantages, including relatively large sample requirements that preclude their use for t h e analysis of trace quantities of explosive substances (3, 7). Recause of its speed, sensitivity, and simplicity, gas chromatography (GC) has also received attention as a method for the analysis of explosive preparations. This technique, however, is limited by the fact t h a t many explosive compounds are incompatible with the usual GC conditions because of their inherent thermal instability and other problems (8-1 I). Satisfactory results have been obtained only for the analysis of thermally stable nitroaromatic compounds such as the nitro derivatives of toluene (12,13) and such ancillary constituents as plasticizers and stabilizers (9, 11, 14, 25). In an attempt t o increase sensitivity and selectivity, GC has been employed with a variety of detectors. Limited success has been achieved using an electron capture detector (EC). However, great care must be taken to avoid overloading or contamination, and special techniques must be used to ensure linearity of response (12, lti, 17). Electron-impact mass spectrometry (MS) seemed to be advantageous for the analysis of explosives a t trace levels; but apart from its denionstrated potential for the identification of nitroarelies (It?), it shows little promise for the analysis of 0003-2700/80/0352-13 13$01 OO/O
other types of explosives (19-21). Chemical ionization mass spectrometry has also been investigated for explosives analysis (22-26) b u t showed promise only when used in conjunction with HPLC and with special reagent gases (26). Other more specialized techniques have also been investigated b u t none has been shown t o be superior t o those currently in use (3). One very promising technique relatively new to explosives analysis is high performance liquid chromatography (HPLC). Application of it t o the analysis of explosives was reviewed in 1977 (13). It is apparent from this review and recent work from other laboratories (3, 7,27) that common explosives can be separated from one another using a variety of HPLC conditions. T h e major limitation with trace analysis using HPLC has heen the use of spectrophotometric detectors which are sensitive to all UV absorbing substances such as stabilizers and plasticizers, which may have absorptivities two or more orders of magnitude larger than those of the explosives to be analyzed. In spite of significant drawbacks regarding HPLC detectors, the technique itself is ideal for the separation of thermally unstable compounds such as explosives. Recent work from our laboratories has demonstrated the feasibility of identifying trace quantities of explosives with a high degree of selectivity using HPLC with a nitrosyl-specific detector, the T E A Analyzer (28). T h e instrument itself and its principles of operation have been described previously (29, 30).
EXPERIMENTAL Equipment. A system consisting of two Mfi000 series HPLC
pumps and a Model 660 solvent programmer (Waters Associates, Milford, Mass.) was employed for solbent programmed operation. A syringe-loading sample injector equipped with a 20-pL sample loop (Model 7120, Rheodyne, Inc., Berkeley Calif.) was used for sample introduction. The columns used were 25 cm long and had an inside diameter oi 3.2 mm. One was packed with Lichrosorb Si-60 (10-pm particle size) and the other with Lichrosorb NH2 packing (10-pm) (EM Laboratories, Darmstadt, FRG). The detector was a TEA Modell 502A Analyzer (Thermo Electron Corp., Waltham, Mass.) coupled to a computing integrator (System I, Spectra Physics, Santa Clara, Calif.). Chemicals. The solvents used for ’HPLC were distilled in glass grade (Burdick and Jackson Laboratories, Muskegon, Mich.) and acetone. ‘The ethanol used isooctane (2,2,4-trimethylpentane) in the solvent programmed chromatography was a specially denatured preparation designated as “Fleagent Alcohol” (Scientific Products, McGraw Park, Ill.), consisting of ethanol with approximately 5% each of methanol and isopropanol. Procedure. The solvent programining with the YH2 columns employed a mixture of isooctane (2 2,4-trimethylpentane) and ethanol as the mobile phase. The alcohol concentration was programmed from 1.5% to 9070 over a period of 20 min and held at the upper level for the remainder of the analysis. Curve No. 7 of the model fi60 solvent programmer was wed. It produces a slightly concave gradient. The solvent programming began immediately after injection. The same mobile phase was used with the Si-60 column but the optimum programming parameters were slightly different. The alcohol concentration was varied from 1.5 k to 80% over a L? 1980 American Chemical Society
1314
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
501
TIMI: !MINUTES)
TIME [MINUTES)
Figure 1. HPLC-TEA analysis of four common explosives: (1) nitroglycerine, (2) PETN, (3) RDX, and (4) HMX. (a) silica column,
column
Table I. Analysis of Explosive Preparations explosive preparation detonating cord letter bomb composition C-1 1/2 military Flex-X sheet explosive composition C-4 dynamite sample
amount no. of injected, ng peaks 60 1 60 1 60 1 60 1 60 1 60 2 80
2
HPLC retention times, s silica 340 340 350 355 350 900 1095 305 455
-NH, 660 660 660 650 655 980 1445 300 630
iden ti t y (see text) PETN PETN PETN PETN PETN
RDX HMX EG DN NG
peak areas' silica
147 5 155 13
2"
205 6 166 14
' Arbitrary units. period of 20 min and held at 80% for longer analyses. The same programming curve was used and programming began immediately after injection. A solvent flow rate of 1.5 mL/min was maintained, and the injection volume was 20 pL in all cases. The TEA Analyzer was operated at a pyrolyzer temperature of 550 "C with argon as carrier gas at a flow rate of 25 mL/min. The oxygen flow to th6 ozonator was approximately 5.0mL/min, and the reaction chamber pressure was 2.3 mbar. The cold traps contained an ethanol/liquid nitrogen slurry maintained at -80 to -90 "C.
RFSULTS AND DISCUSSION Chromatography. T h e result obtained for the HPLCTEA analysis of a 20-pL aliquot of a 10.0 pM mixture of four common explosives is shown in Figure 1. Figure l a shows results obtained using a silica column and Figure l b shows the same compounds on a NH2 column. The identity of the peaks and the amounts of material they represent are as follows: (1) nitroglycerine, 45.4 ng; (2) P E T N , 62.4 ng; (3) RDX, 43.8 ng; (4) H M X , 59.2 ng. T h e small peaks between nitroglycerine and R D X are decomposition products of nitroglycerine. All of four components had increased retention on the NH2 column compared with silica. This is particularly evident in the case of P E T N which elutes after nitroglycerine on the NH2 column but before it on silica. This shift can serve as a valuable method for the identification of components in certain mixtures of explosives. The silica column is preferable to the NH2 column, however, for quantitative analyses where both P E T N and nitroglycerine could be present.
Figure 2 shows results obtained in the HPLC-TEA analysis of three nitrate esters of pentaerythritol. In all cases, peak 1 is the tetranitrate (PETN), 2 is the trinitrate (Petrin), and 3 is the dinitrate ester (PEDN). Figures 2a and 2c were obtained from a 20-pL injection of a 10.0 pM solution of a Petrin standard on NH2 and silica columns, respectively. A peak for the tetranitrate is clearly evident in both cases, showing incomplete purification of the Petrin. Figures 2b and 2d were produced by a 20-pL aliquot of a solution 10.0 1 M each in P E T N , Petrin, and P E D N , representing 62.4, 54.2, and 45.2 ng each, respectively. A silica column was used for 2d; for 2b, a NH2 column was used. Figure 3 shows results obtained for the HPLC-TEA analysis of an 80-ng sample of a standard dynamite having a nitrate ester composition of nine parts EGDN to one part NG. T h e dashed curve was obtained with a NH2 column while the solid curve was obtained with silica. T h e two peaks labeled 1 represent EGDN and those labeled 2 represent NG. From the data in Figure 3, i t is readily apparent t h a t nitroglycerine shifts to a longer retention time on an N H 2 column whereas EGDN does not. Again, this behavior can be very useful for the identification of dynamite compositions. In order to determine the practical value of the method under investigation, several unpurified explosive preparations were analyzed using the HPLC-TEA technique. T h e results are listed in Table I. T h e identity of P E T N in the first group of explosives is confirmed by the fact t h a t the peak which appears a t a retention time of 6 min on silica is shifted to 11 min on the NH2
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
1315
60’ 50
I
I
0
5
I5 TIME (MINUTES) IO
I .
20
0
5
IO
I5
20
15
20
TIME (MINUTES)
s
5
(c’
m t
-w
30
zol IO
0 1
t I
0
5
IO
TIME (MINUTES)
Flgure 2. HPLC-TEA analysis of nitrate esters of pentaerythritol using HPLC with NH, (a, b) and silica (c, d) columns: (1) tetranitrate (PETN), (2) trinitrate (Petrin), (3) dinitrate (PEDN)
column (see Figure 1). It should be noted that the plasticized PETN-based explosives are known to contain approximately 15% to 40% elastomeric binder of various proprietary compositions (31). The presence of these impurities did not affect the HPLC performance nor were any spurious signals recorded on the T E A Analyzer. RDX-based compositions comprise a large class of explosives of which composition C-4 is a principal member. T h e results of its analysis is also presented in Table I. Composition
C-4 is a plasticized explosive containing 91% RDX, 5.3% dL(2-ethylhexyl) sebacate, 1.6% SAE 10 engine oil, and 2.1% polyisobutylene (32). The added plasticizing ingredients did not interfere in any way with the analysis. T h e additional peak, not expected from the given formula, has the same retention time as H M X on both columns. I t is well known that H M X frequently is an impurity in explosives containing RDX since both compounds are prepared from essentially the same starting materials (33). Therefore, it is very probable
1316
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
Table 11. HPLC Retention Data for Eight Explosives silica compound EG DN PETN
“2
Sr,
chemical identity
%
n
shift,= %
306 657
i7 +9
2.3
2
1.4
9
-1.6 +90
s
311 346
56
1.8
2
-r9
2.7
9
NG
1,2-ethanediol dinitrate 2,2-bis(nitroxymethy1)-l,3propanediol-l,3-dinitrate glycerol trinitrate
3.3
7
t38
1.4
2
216
3.3 2.0
6
+9
64 0 763
i22
2-hydroxymethyl-2-nitroxymethyl-
463 627
215
Petrin
3
+22
PE DN
1,3-propanediol dinitrate 2,2-bis(hydroxymett1yl)-l, 3-
755
i7
0.9
2
878
i28
3.1
2
+16
i2
0.2
____
4 1
+31
3.1
____
4 1
+12 +28
+1
0.1
3
1014 1230 1467
ir20
1.3
4
+31
RDX NGU HMX
propanediol-l,3-dinitrate hexahydro-l13,5-trinitro-s-triazine
1-nitroguanidine octahydro-1,3,5,7-tetranitro-
904 960 1116
---
___
1,3,5,7-tetrazocine a
r.t.
(F): mean values for retention
percent.
n : number of samples.
e
times, s. s: standard deviation, s . s r : relative standard deviation expressed as Percent change in retention time relative t o silica column.
90r
*o
.
‘~
’
.~...---, ~
0
”:,
l0Fd’LlL
0‘
5
IO I5 TIME (MINUTES1
20
Figure 3. Nitrosyl-specific detection of dynamite components using HPLC-TEA with NH, (---)
and silica
(---)
columns
t h a t the additional peak results from the presence of a small amount of H M X in t h e RDX. T h e third section of Table I lists results for a commercial dynamite preparation referred to as “giant gelatine”. I t is a gelled dynamite and thus incorporates a small amount of nitrocellulose in its formula. Although it also contains other components such as binders, fillers, and stabilizers, no interferences were encountered. Although it is widely thought that dynamites are merely stabilized nitroglycerine, and indeed this has been t h e case historically, this particular dynamite is shown to consist principally of EGDN. This is not too surprising since most dynamites of recent manufacture incorporate some EGDN in order to prevent the freezing of nitroglycerine during cold weather ( 3 4 ) . T h e quantification of the EGDN and NG in dynamite samples could serve as a means of determining the origin of a particular sample of dynamite where the original container is not available. This would be very useful in the case of post-blast investigations where the amounts of sample are very small and other evidence may be lacking. However, the incorporation of tagging materials referred to as “taggants” in all commercial dynamites in the United States could make chemical analysis less crucial if the tagging materials can be recovered (35). Analysis of Retention D a t a . The results for 62 HPLC-
TEA analyses of eight different explosives is presented in Table 11. Since these analyses were performed as much as 10 to 15 weeks apart, the data are significant with regard to long term reproducibility of the HPLC conditions. Only one column of each type was used for these analyses. All of the components are clearly resolved on the silica column. With the NH2 column, however, there is a problem in the resolution of P E T N and NG. The retention times obtained for these two compounds overlap significantly; thus one could not assign a priori an identity t o a peak having a retention time in the 645-665 s range, for example. However, whenever both compounds were present, the peaks were always resolved and their retention order remained unchanged. T h e last column shows the shift in retention time obtained with the NH2 column. In all cases, except for EGDN, the shift was quite large, especially for PETN. This difference can serve as a convenient means of confirming the identity of a compound. All of the compounds studied could be detected at the 1-ng level with a signal-to-noise ratio greater than 3 except for NG which required 20 ng. T h e difference in retention behavior for the compounds studied is consistent with the differences in surface molecular structure for the two different adsorbents employed. T h e explosive compounds all contain -X-N02 groups where X is either N or 0. For these structural moieties, the electronic distribution can be represented by the nitrogen atom being slightly positive and the oxygen atoms slightly negative. These groups behave normally with silica-based adsorbents whose surfaces contain weakly acidic silanol groups; however, with NH, columns, there is a possibility of a much stronger solute-sorbent interaction. T h e sorbent used in the N H 2 columns is produced by bonding -R-NH2 groups to the surface of a silica adsorbent; therefore, the basis for sorbent-solute interaction and compound separation lies in the chemical properties of the amino group. One of the reasons why the NH2 column was investigated for the separation of explosives was the well-known formation of addition complexes between aromatic nitro compounds and amines (4, 36). Although the original intent of the investigation was the separation of 2,4,6-trinitrotoluene (TNT) and N-methyl-N,2,4,6-tetranitroaniline (Tetryl) from complex mixtures, these compounds were bound so strongly to the NH2 sorbent that no suitable solvents could be found for elution. The mechanism for the intermolecular bonding between amines and nitro compounds can be thought of as a n attraction between the electron-rich amino nitrogen atoms and those moieties made electron-deficient by the electron withdrawing properties of the nitro groups (36). It is thought that a similar interaction is responsible for the behavior observed for the explosive compounds investigated
ANALYTICAL CHEMISTRY, VOL. 52, NO. 8, JULY 1980
1317
L O G ~ ~ L A M O U N TINJECTED (ng)] 0,NO-
-w
H2C-F - CH20N02
i
NITROSYL GROUPS (MOLE NO) 1
3~
3 4 5 LOGIOICONC (ng/mL)]
Figure 4. Plot of detector response as a function of concentration for RDX and PETN
in this study, although it is much weaker than that observed for aromatic nitro compounds. Quantification. Precision and accuracy are important criteria for analytical techniques and linearity of response will enhance their usefulness. With this in mind, a series of experiments were performed to obtain statistical data relevant t o the applicability of the HPLC-TEA technique to the identification and quantification of explosives. T h e results are shown in Figure 4. RDX was chosen to represent the explosive nitramines and P E T N was selected as a typical nitrate ester. T h e points for the graphical data were obtained from a series of experiments involving ten determinations a t each of three concentration levels. For RDX, the concentration levels were 219,2190, and 2 1 900 ng/mL. The normalized integrator responses a t these concentrations were 3.62, 32.1, and 348 units, respectively. T h e 90% confidence intervals ( N = 10) for the determinations were f 1 . 7 % , f 0 . 9 3 % , and 11.470, respectively. For P E T N , the concentration levels were 3.2, 3120 and 31 200 ng/mL. The normalized integrator response was 5.68, 57.3, and 640 units, respectively. T h e 90% confidence intervals ( N = 10) for the P E T N studies were f3.170, f2.4%, and f5.2 %, respectively. The linearity of the response as indicated by the correlation coefficients, was greater than 0.999. A linearity determination was also performed whereby the molar response was determined for two types of compounds, nitrate esters and nitramines, as a function of the number of nitrosyl groups per molecule. For the nitrate ester experiments, 0.100 m M solutions of the following compounds were employed: Isosorbide-5-nitrate, isosorbide dinitrate, glycerol trinitrate, and pentaerythritol tetranitrate ( P E T N ) . Ten determinations were performed for each of the compounds. T h e relative standard deviations were 1.570,0.7%, 1.970, and 8.9%, respectively. The plot of response vs. number of nitrosyl groups is shown in Figure 5 . For the nitramine experiments, 1-nitroguanidine, RDX (a trinitramine), and HMX (a tetranitramine) were used. As with the nitrate esters, a 0.100 mM solution of each nitramine was made up and ten determinations were performed. The relative standard deviations for the determinations were 2.9%, 5.670, and 2.6%, respectively. T h e results are also shown in Figure 5 . T h e linearity of both plots is good, as the correlation coefficients greater than 0.99 indicate.
CONCLUSIONS T h e methods developed in this work provide a useful technique for the identification of explosive compounds a t trace levels a n d for the analysis of explosive compositions. Even with nanogram quantities of analytes, the signal-to-noise ratios were consistently large, exceeding 100 for most analyses.
Figure 5. Plot of detector response vs. the number of nbosylcontaining
functional groups per molecule for nitrate esters and nitramines The linearity of response and dynamic range were satisfactory and contributed to the simplification of sample preparation procedures. HPLC retention behavior on both silica and NH2 columns coupled with a response on the nitrosyl-specific detector enables confident identification of explosive compounds. These factors along with the technique's immunity to interferences, should make it useful for the analysis of difficult samples such as blood and tissue which contain many compounds and generally require complex clean-up procedures before analysis. It is anticipated that the results of this work can be applied to the analysis for cardiac vasodilators such as NG, P E T N , and related compounds which appear in the blood and other body tissues in trace amounts.
ACKNOWLEDGMENT We thank Christopher Barrett for his efforts in maintaining and operating the analytical instrumentation, and David Fine and Brian Challis for their valuable technical assistance. LITERATURE CITED Beveridge, A. D.; Payton, S. F.; Audette. R. J.; Lambertus. A. J.; Shaddick, R. C. J . Forensic Sci. 1975, 20,431-454. Parker, R. G.; Owen, J. M.; Cherolis, J. H. J . Forensic Sci. 1975, 20, 254. Yinon, J. Grit. Rev. Anal. Chem. 1977, 7(4), 1-35. Parihar, D. 9.; Prakash, 0.; Bajaj, I.;Tripathi. R. P.; Verman, K. K. Mikrochim. Acta 1971, 393-398. Macke, G. F. J . Chromatogr. 1988, 38,47-53. Hoffsommer. J. C.; McCullough, J. F. J . Chromatogr. 1988, 38, 508-514. Camp, M. J. "Proceedings, New Concepts Symposium and Workshop on Detection and Identification of Explosives"; National Technical Information Service: Springfield, Va., 1978; p 579. Camera, E.; Pravisani, D. Anal. Chem. 1984, 36, 2108. Trowell, J. M.; Philpot, M. C. Anal. Chem. 1969, 41, 166. Norwitz, J. M.; Apatoff, J. B. J . Chromatogr. Sci. 1971, 9 ,682. Trowell, J. M. Anal. Chem. 1970, 42, 1440-1442. Hoffsomer, J. C.; Rossen, J. M. Bull. Environ. Contam. Toxicol. 1978, 10, 78. Dalton, R. W.; Kohlbeck, J. A.; Bolleter, T. J . Chromatogr. 1970, 5 0 , 219. Kohlbeck, J. A.; Dalton, R. W. Presented at 25th ICPRG Working Group on Analytical Chemistry, Dover, N.J., 1968. Tunstall, F. I.H. Anal. Chem. 1970, 42, 542-543. Hoffsommer, J. C.; Rossen, J. M. Bull. Environ. Contam. Toxicoi. 1972, 7 , 177. Hoffsommer, J. C.; Glover, D. J. J . Chromatogr. 1971, 67,417. Meyerson, S.;VanderHaar, R. W.; Fields, E. K. J . Org. Chem. 1972, 37,4114. Vol. F.; Schubert. H. Explosivsroff 1988. 76, 2 . Bulusu. S.;Axenrod. T.; Milne, G. W. A. Org. Mass Spectrom. 1970, 3, 13. Stals, J. Trans. Faraday SOC. 1971, 67, 1768. Gillis, R. G.: Lacey, M. J.; Shannon, J. S. Org. Mass Spectrom. 1974, 9 ,359. Zitrin, S.;Yinon. J. "Proceedings 2nd Int. Symp. Mass Spectrom. Biochem. Med."; Raven Press: New York, 1974. Zitrin, S.; Yinon, J. Org. Mass Spectrom. 1978, 7 1 , 388. Safferstein, R.; Chao, J.-M.; Anura, J. J. J . Assoc. Off. Anal. Chem. 1975, 58,734. Vouros, P.; Petersen, B. A,; Colwell, L ; Karger, B. L.; Harris, H. Anal. Chem. 1977, 49, 1039. Meier, E. P.; Taft, L. G.; Graffeo. A. P.; Stanford, T. B. "Proc. 4th Joint Conf. Sensing Environ. Pollutants, New Orleans, La."; American Chemical Society: Washington, D.C.. 1978. Lafleur. A. L.; Morriseau, 9. D.; Fine, D. H. "Proceedings New Concepts Symposium and Workshop on Detection and Identification of
1318
Anal. Chem. 1980, 52, 1318-1322
Explosives", National Technical Information Service: Springfield, Va., 1978; p 597. (29) Fine, D. H.; Lieb, D.; Rufeh, F. J , Chromatogr. 1975, 107, 351. (30) Fine, D. H.; Rufeh, F.; Lieb, D.; Rounbehler, D. P. Anal. Chem. 1975, 4 7 , 1188. (31) Federoff, B. T.; Sheffield, 0. E., Eds., "Encyclopedia of Explosives and Related Items", Vol. 3; Picatinny Arsenal: Dover, N.J., 1966; p D99. (32) Ref. 31, p C484. (33) Ref. 31, p C613. (34) Ref. 31, p D1584.
(35) Parker, W. L. Proceedings, "New Concepts Symposium and Workshop on Detection and Identification of Explosives"; National Technical Information Service: Springfield, Va., 1978; p 441. (36) Urbanski, T. "Chemistry and Technology of Explosives", Vol. I; Pergamon Press Ltd.: Oxford, England, 1964; p 310.
RECEIVED for review September 17, 1979. Accepted March 26, 1980.
Comparison of Semi-Integral, Semi-Differential, Direct Current Linear Sweep, Direct Current Derivative Linear Sweep, Pulse, and Related Voltammetric Methods by Computerized Instrumentation A. M. Bond Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 32 17, Victoria, Australia
Computerized instrumentation is used to demonstrate the conslderable dependence of the semidifferential, semi-integral, and derivative techniques on the nature and data manipulation procedures undertaken on the dc current-voltage curve from which these measurements are derived. Provided that the dc linear sweep data are corrected for background current, differences in limits of detection in semi-differential, semi-integral, and derivative approaches are very small. However, altering the dc potential format from linear to staircase or pulsed ramps significantly improves the limit of detection. These techniques and those derived from them are therefore inherently superior to those derived from the linear potential-time ramp.
T h e technique of semi-integral electroanalysis or deconvolution voltammetry, which utilizes the semi-integral of the current time curve has been available for several years (see ref. 1-5 for example). More recently, semi-differential electroanalysis or convolution voltammetry has been developed (6-10) and applied to problems in analytical chemistry. Semi-differential and semi-integral electroanalysis are closely related techniques. Semi-differential electroanalysis, in fact, is simply the derivative of the semi-integral method. Linear sweep vGltammetry is a method intermediate between both of these techniques; however, experimentally it is the simplest to implement because the displayed and directly measured quantity is simply the current, i. T o obtain the semi-integral, m, or semi-differential, e, requires that the current, i, be first measured and then semi-differentiated or semi-integrated via either analog or digital methods. Clearly, these different methods operate on the same electrochemical time scale and in no sense of the word can they be envisaged as independent methods of electroanalytical chemistry. Derivative (first or second) linear sweep voltammetry (11-15) is also another closely related electroanalytical technique derived from the current-voltage curve in linear sweep voltammetry. This approach has been reported as being very successful in improving the analytical usefulness of linear sweep voltammetry. In view of this result, it is not surprising that Smith (16)has suggested that semi-differential techniques (Le., derivative of semi-integral) should prove advantageous 0003-2700/80/0352-1318$01 O O / O
to the semi-integral ones in analytical work. Linear sweep techniques, using a linear potential-time voltage ramp, have less favorable faradaic to charging current ratios than equivalent techniques using staircase or pulsed (17-21) potentials to generate the dc potential. Staircase techniques are therefore inherently more sensitive than their linear sweep counterparts. In principle, semi-derivatives, semi-integral, derivative, etc., methods derived from these staircase or pulse techniques could also be developed. However, when the dc ramp format is changed from linear to pulsed versions, the time scale of the measurement is altered and indeed new considerations apply. When analog instrumentation has been used to generate and process data, many of the above closely related techniques have been developed and discussed almost completely in isolation. While it is true t h a t unique and technique dependent electronic developments were required in the analog work, it is also equally clear from the theory that very few of these techniques and/or concepts are independent ones. Indeed, for a given dc waveform and electrode process, all semi-integral, derivative, and semi-differential responses can be generated from exactly the same experimental data and, in principle, differences should therefore be confined to the method of data manipulation. With analog circuitry, experimental assessment of this hypothesis is difficult to implement. However, with the advent of computerized instrumentation and digital electronics, it becomes very easy to record and store the raw dc data in memory. Subsequently, it then becomes a simple matter to generate sequentially, semi-derivative, semi-integral, first derivative, second derivatives, etc., curves by applying the appropriate mathematical manipulation to the raw data. The requirement, or temptation to treat or report data from any of these methods in isolation or even the need to view them as anything other than closely related techniques therefore vanishes when using computerized versions of instrumentation. With a view to providing a systematic approach of all the above techniques and t o optimizing the performance of all approaches in analytical work, a computerized version of electrochemical instrumentation has been constructed which can collect the raw data and undertake all the required data manipulations and provide the required readout in the areas of semi-integral, semi-differential, and derivative electroanC 1980 American Chemical Society
