Evaluation of Tritiated Scandium for the Electron Capture Detector C. Harold Hartmann Varian, lnstrument Division, Aerograph Operations, 2700 Mitchell Dr., Walnut Creek, Calif. 94598
A new high temperature tritiated source utilizing a scandium substrate has been evaluated. From a comprehensive 3H emanation study, the maximum operating temperature has been set at 325 "C. Extensive field evaluation in a pesticide residue laboratory has shown a significant improvement in electron capture detector performance. The source contamination problem has been greatly reduced. The detectivity to pesticides has been demonstrated to be greater by a factor of three than the standard titanium tritide source.
The electron capture detector (ECD), as thousands of analysts will testify, is very susceptible to the chronic problem of gradual performance deterioration. This problem is believed to be due primarily to foil contamination, probably from the deposition of chromatographic effluent on the surface of the radioactive source. Of the two most popular radioactive sources used in the ECD, the 63Ni with its 400 "C upper temperature limit is less prone to this contamination problem than the tritiated titanium with its 225 "C upper temperature limit. However, on the basis of other chromatographic performance factors, the G3Ni isotope is a much poorer choice than the 3H, as described in comprehensive comparison by Fenimore ( I ) . The principal subject of this work by Fenimore was a new prototype tritiated rare earth source which showed feasibility for a 300 "C maximum temperature. A complete description of this product could not be given a t that time (1971) since the manufacturer, U.S. Radium Corp. of Bloomsburg, Pa., wished the exact nature of the experimental source to be held proprietary. During the first part of 1970, Varian Aerograph entered into a cooperative program with U.S. Radium toward the development and evaluation of a new radioactive source which utilizes scandium as the substrate for entrapping the 3H. This present work will describe this new source, the general method of fabrication, and the resulting chromatographic performance. Evidence will be presented to support the proposed maximum operating temperature of 325 "C. The most important factor contributing to this upper temperature limit is the 3H emanation. The release rate of 3H must not exceed a certain tolerance in order to meet regulatory agency requirements. An ion chamber method is utilized and described. With certain precautionary measures taken, the ion chamber method provided reliable, continuous 3H emanation information, which resulted in better knowledge of the tritium release characteristics than had been available with previous procedures. This knowledge in turn has resulted in a product of much safer radiological performance because of a special process instituted in the fabrication process. Coincident with safer operation is the unexpected accomplishment of a source with an approximate four times longer life expectancy. Best GC performance is obtained Fenimore, P. R. Loy, and A. Zlatkis, Anal. Chem., 43, 1972. (1971).
( 1 ) D. C.
from this new 3H source as incorporated into a new low volume electron capture cell, which will also be described. The effectiveness of the high temperature operation of this new ECD to resist contamination will be demonstrated with the data obtained from field testing in the San Francisco laboratory of the California Department of Food and Agriculture. Data were obtained throughout a 5-month evaluation, during which time over 750 extracts of various fruits and vegetables were analyzed. This extensive field trial showed that the new 325 "C temperature operation significantly reduces the 'contamination problem. The decrease in the contamination factor was to be expected. A completely unanticipated result was that this new scandium source has significantly better sensitivity and detectivity than the previous titanium source.
EXPERIMENTAL Apparatus. The Scandium Source. The new tritium sources evaluated in this study had foil dimensions of 'li in. X 'h in. X 0.002 in. Two types of foil material were evaluated: Hastelloy C a nickel-base alloy, and 302 stainless steel. The latter has been the most popular foil for ECD sources; however. under high temperature operation the stainless steel foil was found to be unsatisfactory. The reason for this stems from the much lower modulus of elasticity a t the elevated temperatures which causes the foil to have poor electrical contact to the inside wall of the Kovar cell which, in turn, is polarized to the desired detector potential. Poor electrical contact of the source to the cell voltage will cause the standing ionization current to drop u p to tenfold or more. This in turn will cause a similarly adverse effect on chromatographic performance factors, e.g., linearity, sensitivity, and detectivity. A better choce of foils for high temperature operation is Hastelloy C, which has a higher modulus of elasticity a t 325 "C than 302 stainless steel a t 225 "C; it has therefore been selected as the foil for the new source. One consequence of this change in foil material was t h a t the deposition procedure of applying the new substrate to the foil had to be reoptimized from those used with 302 stainless steel. The most essential substance of this new source is the substrate, scandium, which holds the radioactive isotope 3H. 'There is incomplete agreement a s to the exact means of 3H attachment. Some authorities believe that most of the 3H is held to the substrate by chemical bonds of the hydride: Sc3H3, (Ti3H2 for the previous source). The proposed formula Sc3H3 is questionable because of the reported difficulty in forming scandium trihydride (2, 3). Others believe that a major percentage of 3H is held by physical entrapment of the gas. There is reasonable agreement that both mechanisms hold some 3H; of greatest uncertainty is the relative percentage of 3H a s the hydride us. the occluded gas. The fabrication of the source begins with a thin film deposition of the rare earth, scandium, on one of the two foil surfaces. The thickness of the scandium layer is on the order of 1 micron. In practice, the actual thickness of the rare earth deposit depends on the total activity level desired, on the metallurgical characteristics of the hydride, and on the specific activity ofthe rare earth. Radiochemists usually reserve the term "specific activity" as the disintegrations per second per gram of the radioactive species. Therefore, the rare earth substrate should not have specific activity because a s an element it does not exist as a radioactive isotope. However, when the chemical bonds form between the rare ( 2 ) W. M . Mueller. J. P. Blackledge. and G . G . Libowitz. "Metal H y drides,'' Academic Press, New York and London. 1968, p 480 (3) A . C. Switendick. Int J . Quantum Chem.. 5 . 459 (19711
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
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Table I . Tritiated Substrate Comparison Property Ti3H2 Specific activity, theoretical Specific activity, achievable Specific activity, practical Foil material Max. temperature limit Nominal 3H loading
Sc3H3
2
3
13i4
2 Y2
1-1 112 Stainless Steel 225 "C 250 mCi
2 Hastelloy C 325 "C 1000 mCi
earth and 3H, the molecular product becomes radioactive and, in this case, specific activity will refer to the number of 3H atoms per atom of rare earth. This specific activity derives from chemical principles and hence will be referred to as the theoretical specific activity. The theoretical specific activity of scandium is 3 atoms 3H per atom of Sc. As a practical reality, this number is only approached. With extensive exposure to 3H a t elevated temperatures, a specific activity of about 2.5 is gradually approached; however, this is not of practical value since the scandium becomes very brittle and friable, which causes the source to be seriously damaged during subsequent handling operations. In production a specific activity of about 2 is realized for scandium. These and other physical properties for both scandium and titanium are compared in Table I. The next procedure in the fabrication process is reacting the 3H with the rare earth, a similar procedure for either scandium or titanium. This is accomplished by placing the foil in a low volume vacuum chamber. The general process is as follows: air evacuation, exposure to 3H, then a series of heating cycles to temperatures in the range of 400 to 450 "C. An indication of the actual amount of 3H held by the rare earth is obtained from the decrease of pressure in the heating vessel from beginning to end of the process. The resulting Sc source has a nominal activity level of 1000 mCi per Y4 in.2 which is four times the loading used previously with the Ti sources. The higher loading for scandium was suggested originally by the manufacturer, as a fabrication expedient, but was subsequently proved acceptable with 3H emanation data. With the 50% higher specific activity of scandium, the corresponding thickness of the scandium deposit to achieve a comparable activity to the titanium source should be 50% thinner. This would have meant a more costly fabrication process. An alternative to a thinner rare earth deposit was to use a dilution of 3H and deuterium before exposing the gas to the foil. This alternative is not an economically attractive solution either. Therefore, prototype 1000-rnCi sources were fabricated and tested. The resulting 3H high temperature emanation was compared to similar data from the 250-mCi Sc sources. The comparison yielded one of several unexpected findings as a consequence of this work: the 3H emanation from the 1000-mCi loading was about the same as the 250-mCi loading, not four times greater as might be expected. This unusual result has been a t least qualitatively confirmed in a recent publication by Niemeyer (4) a t the Oak Ridge National Laboratory. He showed with titanium sources that a 1000 mCi/in.2 source yielded less than $!3 as much 3H emanation as a 500 mCi/in.2 source. It seems that for some unknown reason, the thicker the rare earth deposit (within certain limits as yet undefined), the more temperature-stable is the source. With the confirmation from ORNL of this phenomenon and with the realization that the extra 750 mCi was a n insignificant cost consideration of the source, a source loading of 1000 mCi was established. The final step in the manufacturing process is a source stabilization (patent pending) which enables the new source to meet acceptable 3H emanation standards. Without this process an overtolerance of 1.5 to 100 times the acceptable rate can occur. Overtolerance of titanium sources was also observed in Niemeyer's (4) work. The total 3H lost during this overtolerance cycle, however, is typically small. Subsequent reshaping or mechanical deformation of the foil will cause a reoccurrence of this overtolerance but of a lesser magnitude. Therefore, all scandium sources were preshaped and then stabilized in the fabrication process to avoid this emanation burst. Nierneyer, ORNL-TM-3633, oak Ridge National Laboratory, Tenn., Dec. 1971.
( 4 ) R. G .
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
Figure 1. Scandium Electron Capture Detector, cross section assembly 1 Cell cap, 2 Cell heater, 3 Radioactive source 4 Ceramic insulator 5 Collector electrode
The Detector Cell. To achieve optimum chromatographic performance from this new higher operating temperature tritium source, a redesign of the detector cell was required. This new detector is shown in cross section in Figure l . The special components of this detector are the ceramic insulators, the Kovar ionization chamber, easily removable cell cap, and source heater. The material used to electrically insulate the cathode from the anode and the anode from chassis ground must be able to limit the leakage current under high temperature operation and should also be able to sustain possible impact. A high alumina ceramic, such as AD 99 from Coors Porcelain Company, Golden, Colo., fully satisfies both of these objectives. By observing special handling procedures during and after the fabrication process, the leakage current a t 325 "C will be less than lo-" A . Because the peak-to-peak noise level from a tritium source (Ti or Sc) is about lo-" A, the magnitude of current leakage from the ceramic is easily hidden in the randomness of the ion current generated in the ionization process. Kovar is the preferred material to bond to the ceramic since the two materials have similar coefficients of expansion. Kovar. however, has one serious disadvantage under high temperature operation: it oxidizes readily which causes electrical contact problems. This difficulty is overcome by plating the exterior Kovar surfaces. A significant design factor of the ionization chamber is the small internal volume of 0.9 ml.The chromatographic advantages of a small volume ECD have been dramatically demonstrated with capillary column applications by Fenimore (1). With a larger volume detector, a purge gas is needed to avoid peak band broadening due to the detector itself. A makeup gas can be added to the column effluent to minimize band spreading in larger volume detectors; however, this remedy results in lower sensitivity of the ECD since it is a concentration type detector. The cell cap serves an important purpose. It minimizes back diffusion of oxygen from the air into the ion chamber with an 0.011-in. X 3/1,3-in. exit tube. This is important because oxygen can be detected with an ECD a t about 1 ppm. This means that oxygen entering the cell at this concentration or greater would noticeably reduce the standing current and, hence, deteriorate other performance factors. The special radioactive source heater localizes heat in the zone of greatest susceptibility for contamination buildup. This minimizes overheating electrical components located in the vicinity of
-5OOVdc
I
Figure 2. Schematic of ion chamber apparatus for monitoring 3H emanation
Inlet for t h e purge gas air (specificionization,S = 34 eV): 8. Silica gel drying bed: C. Inlet for NZ purge: D. Heated chamber r tritium source: E. Filter paper type particulate trap assembly: F. Electrostatic precipitation: G. Ion chamber with volume, V = 1 . 2 liters: H. Electrometer input with sensitivity, 1 = 10-l2 A / m V ; 1. Grounded guard ring: J. Rotometer indicator of flow rates, F = 2 liters/min: K . Needle valve restrictor for flow adjustment; L. Carbon vane p u m p ; M. Vent to fume hood
A.
the source. A special case ground surrounds the exterior surfaces of the heater to minimize electronic interference in the electrometer circuit. The interior surface is ceramic, which prevents the cell polarization voltage from being grounded out. The Tritium Monitor. There are two recognized methods for measuring low concentrations of gaseous 3H. One method is to convert the 3H to the oxide (3HzO) and then dissolve the oxide in appropriate fluid for scintillation counting. The other method is the direct approach of measuring the resultant electrical current as the gas is passed through an ion chamber. Both methods can yield accurate results if particular precautions are observed. With the scintillation method there are three major requirements: Efficiency in converting 3H to the oxide; solubilization between the aqueous phase and the organic scintillation fluid; and an efficient calibration, such as by the internal spiking technique. One basic disadvantage with the scintillation method is that the technique requires batch sampling. For relatively constant 3H concentrations, batch sampling is quite acceptable: for rapidly changing 3H concentrations (as was observed in this work), batch sampling could easily lead to a distorted 3H emanation profile. With the ion chamber method, there are also several precautions to be observed, which will be enumerated in the following discussion. The components of the 3H monitor are shown schematically in Figure 2. Ambient air is utilized and serves as the gas to be ionized in the ion chamber. Large interferences can be experienced from water moisture in the air on humid days. A silica gel dehumidifier helps to minimize this problem. All released radioactive species (gaseous and particulate) are purged from the vicinity of the heated source by a 40 ml/min nitrogen flow and then purged into the air stream. Released particulates of Ti3Hz or Sc3H3 dust are collected in a filter paper trap and counted by a 2 T proportional counter. The activity level from particulates is typically 100 times less than the activity from the gaseous activity and, hence, will not be reported separately in this work. The main purpose of this trap is to prevent the particulates from passing into the ion chamber where they tend to accumulate and gradually elevate the background ionization of the ion chamber. The air stream is next passed through an electrostatic precipitator, which is necessary to minimize anomalous responses from gaseous ion interferences, such as from cigarette smoke ( 5 ) . The air stream then enters a 1.2-liter cylindrical ion chamber. This ion chamber volume and associated air flow rate represents a compromise between sensitivity of response and speed of response. Only 100 V dc is applied to the ion chamber to collect the generated ion current-this relatively low potential is recommended to minimize the pickup and measurement of erroneous insulator leakage currents. This 100 volts is efficient enough to collect over 99% of the ion current. Special grounded guard rings are a t each end of the ion collector electrode to also minimize erroneous leakage currents. A coaxial electrical cable is needed to carry the signal to the electrometer AjmV sensitivity). A (5) J. R. Waters, Presented at the 5th Annual Meeting of the Health Physics Society, Chicago. HI.. July 2 , 1970.
rotometer continuously monitors the stability of the air flow rate. As the carbon vane gradually wears on the air pump, a needle valve restrictor is employed to reestablish the desired flow rate of 2 liters per minute. As there will be great importance placed on the accuracy of the 3H determinations made here, there have been two different type calibrations performed on this ion chamber system. ( a ) Theoretical calibration-as has been shown by Shoemake (6)
I S*F
R
=
(6.3 X
[y] ( 5 103)(3.7 X
["-'* R
=
X lo4)
34 l.2
1
(0.34 x 10")
(1)
1.9 pCi /min.
( b ) Certified Calibration Standard. A tritiated methane calibrator, Model CL-1, from Johnston Laboratories, Inc. of Cockeysville, Md., was utilized for closed loop calibration of the ion chamber system. By connecting the calibrator (which simply injects a known aliquot of radioactive methane) between A and L in the flowpath of Figure 2 and by measuring the entire volume of the pneumatic system, an accurate calibration is obtained. The sensitivity of the ion chamber instrument was thus determined to be 2.1 pCi/min for a full scale deflection of A. With the average from (a) and ( b ) above, the sensitivity of the 3H monitor was taken a t 2 pl/min. The Field Evaluation. The effectiveness of this new ECD to minimize the contamination effect was evaluated with the cooperation of the California Department of Food &Agriculture. Although both the San Francisco and Sacramento laboratories participated, only the data from the San Francisco Lab will be reported in the present work since the evaluation in Sacramento is still in progress. The procedure was simply to expose the new ECD to the routine of sample analyses normally encountered by a pesticide residue laboratory. A separate chromatographic channel was made operational incorporating the new ECD so that every sample analyzed with the standard apparatus could also be injected into this specially equipped instrument. The extraction and sample cleanup procedure was as follows: Finely dice vegetable or fruit. Tumble with equal weight/volume of benzene for one hour. Remove pigment with Attagel 40. Decant solvent from solids. The chromatographic conditions were as follows: Column: 370 QF-1 and 1% DC 200 on Gas Chrom Q 100j120 mesh. N2 flow: 40 ml/min. Temperatures: Injector, 200 "C; column, 200 "C; detector, 215 "C (for comparison to the titanium source) and 325 "C (for the evaluation a t maximum operating temperature). Detector volume: 0.9 ml for both Ti and Sc ECD. (6) G R. Shoemake, J F Lovelock, and A Zlatkls, J Chromatogr 12. 316 (1963)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4 , APRIL 1973
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735
Ti%
ii 230'C
Ti3H @230°C
,
3
I
6 9 TEMPERATURE EXPOSUPE TIME, hoilrs
f
12
Figure 4. Complete 3 H emanation profile
8
16 24 TEMPERATURE EXPOSURE TIME, mln
Figure 3. initial 3H emanation profile
RESULTS A N D DISCUSSION Evaluation of 3H Emanation. Two major questions were raised soon after U.S. Radium made available the prototype high temperature scandium sources. The most obvious question concerned the actual decrease in the source contamination problem a t the higher temperature. Before that answer could be obtained, the second question had to be answered, which concerned the establishment of the acceptable maximum operating temperature. Although the maximum operating temperature for a titanium tritide source is 225 "C, the regulatory agency suggested that 230 "C be used to obtain a n emanation control on Ti3H2. The purpose of this 5 "C higher temperature was t o hopefully obtain better readabiity of the emanation rate, which is only about 1 pCi/min. During this evaluation, 44 new Ti3H2 sources were tested. They were randomly selected from six different groups of sources representing different manufacturing batches. The sources were heated to 230 "C with the resulting 3H emanation continuously monitored with the ion chamber system described in Figure 2. An emanation profile for a typical Ti3H2 source is shown in Figure 3. It could be easily assumed t h a t after 24 minutes at 230 "C, the emanation rate had equilibrated at 90 pCi per minute. As it happens, this represents only the initial portion of the more complete 3H emanation profile which is recorded a t a 22.5 times slower chart speed, as shown in Figure 4. Observing the complete emanation profiles from the 44 Ti3H2 sources yields the following results: A new Ti3H2 source should be heated about 12 hours t o allow the 3H emanation to reach reasonable equilibration. The 3H emanation rate continues to gradually decrease even after 12 hours and until all the 3H has escaped from the foil; therefore, there is no real equilibrated emanation rate. During the first hour of temperature exposure, a peak 3H emanation will occur which can be from 1.5 to 10 times higher than the "equilibrated" emanation occurring at 1 2 hours. The equilibrated emanation can vary over a range of 20-fold because o f Batch t o batch variations. It was common to find the total source radioactivity to be a factor of 2 or more higher than the nominally stated amount. This was deduced from stripping the 3H from the sources. An explanation could be attributed in variations in the fabrication process. Also because variation within a particular batch was found to be on the order of &5070. 736
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 4 , APRIL 1973
Average 3H emanation after 12 hours a t 230 "C for new Ti3H sources was 10 pCi/min with a low of 1 pCi/min and high of 16 pCi/min. The interpreted results from previous investigators of 1 pCi/min (S), 6 pCi/min (7), and 2 pCi/ min (4) fall within the range of emanations found in this study. A good comparison of these previous results (6, 7 ) is not possible because very few sources were tested and no detail is given about previous temperature exposure to the time of the data taking. Forty-six scandium sources were tested a t elevated temperatures. The emanation rates of six preproduction sources a t 335 "C were (in the order tested) 6, 2, 3, 11, 5 , and 4 pCi/min, yielding a corresponding average of 5 pCi/ min. To obtain a conservative upper temperature limit, the regulatory agency suggested that this temperature be decreased 10 "C. Therefore, a n upper temperature of 325 "C is recommended for these new tritiated scandium sources. Hydrogen Carrier Gas. Two previous reports (4, 6) have shown t h a t the 3H emanation rate was not significantly different for the four most widely used chromatographic carrier gases; N2, He, Ar, and Ar plus 5 7 ~CHI. It has been shown (6) t h a t if H2 carrier was used, the emanation rate from a Ti3Hz source was about four times greater than N2 carrier a t 225 "C. In the present work. a factor of ten times greater emanation was found when using H2 carrier. The new Sc3H3 source was similarly tested with H2 carrier a t the proposed maximum operating temperature of 325 "C. Another unexpected result ensued: 9570 of the 3H was released from the source during the first sixteen hours. This result suggests that H2 readily exchanges with the 3H. Of course these results strongly suggest that H2 not be recommended as the carrier gas with the Sc3H3 source a t 325 "C. Chromatographic Performance. The most significant discovery of the present work occurred during the comparison of response between the titanium source and the scandium source. The following work was conducted at the San Francisco pesticide laboratory of the California Dept. of Agriculture. Both sources were operated about 200 "C in two different gas chromatographic channels with the conditions described in the experimental section. A seven-pesticides standard was injected into each channel with simultaneously recorded results as shown in Figure 5 . All seven pesticides yielded a factor of 3 greater response or sensitivity on the channel equipped with the scandium ECD source. Since all analytical conditions for all comparisons were very nearly identical, the sensitivity also represents a n improvement of a factor of 3 in detectivity (the smallest amount of sample readable above the noise level). Similar work was conducted in our laboratory, ( 7 ) L. Kahn and M C . Goldberg, J Gas Chrornatogr
3, 288 (1965)
W 3
a > Sc SOURCE TEMPERATURES 225'C 0 325OC
0:
..
0 0
$ E
O
a
k
10
20
30
0
DURATION OF TEST, days
Figure 6. Standing current decay during 34- day test of Sc ECD
Figure 5. Ti3H2 vs. Sc3H3response comparison 1 . Lindane. 0 5 ng: 2. Heptachlor. 0.5 ng: 3. Aldrin, 0.5 ng; 4 . Heptachlor epoxide, 0.5 ng. 5. p p ' - D D E . 1 . 0 ng: 6 . p.p'-DDD, 1 . 0 ng: 7. p.p'-DDT. 1 .O ng
which yielded a factor of 5 greater sensitivity but because of a greater noise level, only a factor of 3 increase in detectivity. Both laboratories found that this factor of 3 deteriorated rapidly for 200 "C operation, as will be further described in the following section on contamination evaluation. A t a 323 "C operating temperature, however, the factor of 3 increase was maintained throughout a 34day testing period. Since contamination changes tritiated source performance with time, a gas-solid chromatographic system (molecular sieve 5A and gas sampling valve) was utilized to further evaluate this unexpected performance improvement in the Sc source. Two inorganic gases were tested: 0 2 and SF6. The sensitivity rsults showed no significant difference between the Ti and Sc sources. It therefore seems that whatever unexplained mechanism is responsible for the response increase of the Sc source does not operate on the relatively non-dissociative capturing species as it did on the dissociative pesticide molecules. The 90 \.' dc mode of operation was utilized to evaluate the linearity comparison of the two sources. Both yield an approximate linear range of 500-fold. Contamination Evaluation. Of primary interest to the analyst is the best chromatographic performance under actual analytical conditions. The new Sc source was thus evaluated in a cooperative program a t the Sacramento and a t the San Francisco pesticide residue laboratories of the California Department of Food & Agriculture. Only a portion of results from the San Francisco Lab will be reported at this time. These results come from two test periods, both with 19 days of analysis (typically 15 analyses per day) which were distributed throughout a 34-day period of evaluation. The purpose of the first test period was to establish control data from which improvements could be compared. A new Sc3H3 source was in continuous operation during the 34-day period a t near the maximum operating temperature of the previous 3H source, i . e . , 215 "C. During the second test period, another new Sc3H3 source was operated continuously for another 34-day peri-
od a t a temperature of 325 "C. At the beginning of each day of analysis, a standing current measurement was taken, which was followed by an analysis of a seven-component pesticide standard. The performance a t 325 "C is compared to the performance a t 215 "C with respect to rate of standing current loss, change in pesticide sensitivity, and other contamination symptoms. The standing current of an ECD is a vital necessity of operation. In general, as the standing current decreases with age so does the sensitivity, the range of linearity, and the base-line stability. Both Sc3H3 sources in these tests started with about the same standing current of 3 x 10-8 A. Both sources gradually, but steadily, lost standing current during the 34-day periods as shown in the plots of Figure 6. This was a 30% loss for the 215 "C operation and a 21% loss for the 325 "C. It had been believed previously that standing current loss with the Ti3H2 sources was due to a condensation of column effluent since the source cannot be operated much higher than the column temperature, usually about 200 "C for most pesticide residue applications. Since the operation of the Sc3H3 a t 325 "C still caused standing current losses, it does not seem reasonable that condensation is the only contamination mechanism. It is interesting to note that during the first fifteen days of operation both sources lost about the same amount of standing current, i e . , lo%, yet changes in pesticide sensitivity were remarkably different a t the two operating temperatures. Three parameters influencing pesticide sensitivity were evaluated. These were source temperature, expose time a t the temperature, and source substrate. Previous investigations (8, 9) have reported the effect on sensitivity of increasing the detector temperature of an ECD. Pesticide sensitivity data from the 325 "C operation of a Sc source were compared to the data taken a t 215 "C with another Sc source. The data taken a t 215 "C were obtained on a source before significant contamination was evident (rate of decrease in lower curve of Figure 7 . ) As seen by Table 11, the change in sensitivity of the pesticides Lindane, Aldrin, Heptachlor-Epoxide, and p , p ' DDE are not significant. There were three pesticides that showed a noticeable change in sensitivity due to the 110 "C hotter temperature: Heptachlor, 40%; p,p'-DDD, 30%; p,p'-DDT, 60%. Changes in peak height with time are illustrated in Figure 7 for the pesticide Lindane. There was a similar effect (8) E. C. Pettitt, P. G . Simmonds, and A. ZlatKis, J . Chrornaiogr Sci 7 , 645 (1969). (9) A . ZlatKis and E. C. Pettitt, Chrornatographia. 2. 484 (1969)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 4, APRIL 1973
737
W
4
::8C A .J
c
S c SOURCE TEMPERATURES
0
215'C 0 325°C
0
3
0
+
Pesticides by order of elution
0
W
Lindane Heptachlor
I
W
60 X
0
B W
z
a0 W
Table I I. Sensitivity Changes with Temperature for Chlorinated Pesticides Change in response,
Aldrin
Heptachlor epoxide p,p'-DDE
p,p'-DDD P.P'-DDT
%, 325 "C compared to 215 "C
-6 -40
$6 -3 +7 - 30
- 60
cc 40
z a
z _1
0
0
I
I
I
IO 20 DURATION OF TEST, days
30
Figure 7. Response decay during 34-day test of Sc ECD
T
I
I
I 1
Sc E C D
3
215OC
!--A---A
CL-
Sc E C D @ 325°C
0 I
+----
____
l l m i n --
Figure 8. Contamination effects from head lettuce extract on 34th day
Figure 9. Spinach overtolerance, 2 1 ppm ethyl parathion
experienced by the other six pesticides in the standard. The first 15 days a t 215 "C operation caused the response to drop about 50%; however, there seems to be an unexplained gradual recovery of about Y4 of that loss during the next 15 days. The Lindane response a t 325 "C operation increased (also unexplained) about 15% during the first 15 days; then gradually lost that increase over the next 15 days. It seems that under analytical conditions of pesticide residue analysis, the higher operating temperature around 300 "C should be applied to the new source so that the maximum response improvement will be maintained as long as possible. The 325 "C operation of the Sc3H3 not only reduced the loss of standing current and peak response, but it also minimized two additional contamination effects having to do with base-line stability. One of these effects is the familiar upset in the base line following elution of the various sample components. As illustrated in Figure 8, a lettuce extract analyzed on the 34th day of the 215 "C test period is compared to a similar lettuce extract analyzed on the 34th day of the 325 "C test period. The trace a t the top of the figure indicates two forms of base-line instability: a sharp drop in the base line after the solvent elutes and a subsequent sag in the base line with gradual recovery after eight minutes. The 325 "C chromatograph of another lettuce sample showed no indication of such effects.
Another base-line instability effect is the standing current decrease during the working period. It has already been shown in Figure 6 how the day-to-day variations appeared over the 34-day period. During each day, however, there was another more temporary loss experienced which progressively got worse with time. During the last ten days of the 215 "C test, the standing current loss during the day was as much as 20%. Most of this was recovered during the "idle" overnight operation, but with that much change in standing current during the working days, increased frequency of calibration is required to maintain good quantitation. The 325 " C operation reduced this loss during the day to 2-370. One month after the start of the Sc source evaluation at 325 "C, an unusual sample was analyzed. This was a produce extract that contained a pesticide overtolerance which was the first overtolerance incident to occur in the previous nine months. It occurred in a particular shipment of spinach soon to be distributed to the retail outlets. The GC analysis is shown in Figure 9 with the 2.1 ppm ethyl Parathion occurring a t a retention time of about five minutes. The published tolerance for ethyl Parathion is 1.0 ppm. It is significant to note the stability of base line during this important analysis, which was obtained after 31 days of continuous operation during which time there had been 240 produce extracts analyzed. Since
738
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 4 , APRIL 1973
contamination effects are of small consequence after even 34 days of operation, further studies will be needed to determine the operation period before contamination becomes a serious effect.
CONCLUSIONS Comprehensive emanation data have revealed several useful characteristics of tritiated sources: that without adequate precautions, new titanium sources can yield emanation rates in excess of the accepted tolerance; that 3H emanation rates are not directly proportional to source loading; that titanium sources require about 12 hours a t the maximum operating temperature before the emanation rate has reached reasonable equilibration. This information has been utilized in developing the new high temperature tritiated scandium source for ECD application. The 325 "C maximum operating temperature of the new Sc source has been proved effective a t minimizing the historic contamination problem of the electron capture de-
tector. Another interesting result of this works was the discovery that the detectivity to pesticides was better by a factor of three than the previous tritiated (titanium) source. Future study will be needed to reveal the explanation for this phenomenon. This explanation notwithstanding, the scandium tritide ECD will offer a great benefit to analysts in many fields in addition to pesticide residues.
ACKNOWLEDGMENT The author wishes to express particular appreciation for the technical assistance from the following: Felix M. Err0 and Wm. R. Lewis of the California Department of Food and Agriculture ( S . F. Laboratory]; Elmer Trone and Roger S. Hill of Varian Aerograph Research Department; and Donald B. Cowan of U.S. Radium Corporation. Received for review November 22, 1972. Accepted January 8, 1973.
ANALYTICAL CHEMISTRY, VOL. 45, N O . 4, APRIL 1973
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