It is interesting to note that PAH compounds analyzed on isotropic liquid phases (12,14,16,28) resulted in elution patterns of chrysene < triphenylene and benzo[a]pyrene < benzo[e]pyrene < perylene. As shown in Figures 3 and 4, elution patterns on a nematic liquid crystal phase are, in contrast, consistent with the degree of solute rod-like character for a given set of geometric isomers. Figure 5 demonstrates the temperature dependence of selected separation factors (a),based upon the retention data and operating conditions given in Table I. While a decreases with increasing column temperature, the rate of decrease varies significantly for different isomeric pairs and is greater, the larger the difference in length-to-breadth ratio between two isomers. In addition, the larger this difference is, the larger is the value of a. The surprising selectivity of the nematic liquid crystal phase employed in this study results in large separation factors for pairs of the most troublesome PAH isomers. This is especially significant since a considerable reduction in the number of theoretical plates is possible for practical separations, and makes the use of conventional laboratory columns of 0.6-1.0 X lo3 platedfoot feasible. The broad utility of this liquid phase is illustrated in the programmed temperature separation of 16 of the most significant 3-5 ring PAH isomers as shown in Figure 6. This separation should be of immediate interest to workers in air pollution measurements of PAH components. An investigation of the thermodynamics of PAH solutes in this liquid crystal solvent is presently under consideration to quantitatively discern the relative contributions of the PAH molecular characteristics of size, shape, and n-type interaction to retention behavior. ACKNOWLEDGMENT The authors express their appreciation to Delmo P. Enagonio of the National Bureau of Standards, and R. B. Ashworth of the United States Department of Agriculture, Ag-
ricultural Research Center, for their donation of several of the compounds reported in this study. LITERATURE CITED (1) P. Shubik, Proc. Mat. Acad. Sci. USA, 69, 1052 (1972). (2) H. W. Gerade, "Toxicology and Biochemistry of Aromatic Hydrocarbons," Eisevier, Amsterdam, 1960. E. C. Miller and J. A. Miller, "Chemical Mutagens," A. Hollaender, Ed., Vol. 1. Plenum Press. New York. 1971. D 105. E. Sawicki, Chemist-Analyst, 53, 24, 56, 88 (1964). R. Schaad, Chromatogr. Rev., 13, 61 (1970). 0. Hutzinger, S. Safe, and M. Zander, Analabs Inc. Res. Notes, Vol. 13, No. 3 (1973). E. Sawicki, R. C. Corey, A. E. Dooiey, J. B. Gisclard, J. L. Monkman, R. E. Neligan, and L. A. Ripperton, Health Lab. Sci., 1, 31 (1970). N. F . lves and L. Guiffrida, J. Ass. Offic. Anal. Chem., 55, 757 (1972). E. Clar, Spectrochim. Acta, 4, 116 (1950). V. Cantoti, G. Y . Cartoni, A. Liberti, and A. G. Torri, J. Chromatogr., 17, 60 (1965). L. DeMaio and M. Corn, Anal. Chem., 38, 131 (1966). K . Bhatia. Anal. Chem., 43, 609 (1971). J. Frycka, J. Chromatogr., 65, 341, 432 (1972). D. A. Lane, H. K. Moe, and M. Katz. Anal. Chem., 45, 1776 (1973). A. Zane, J. Chromatogr., 38, 130 (1968). N. Carugno and S. Rossi, J. Gas Chromatogr., 5, 103 (1967). K . Grob. Chem. ind. (London), 248 (1973). G. Grimmer, A. Hildebrandt, and H. Bohnke, Erdoel Kohle. 25, 442, 531 (1972). G. Grimmer, Erdoel Kohle, 25, 339 (1972). UlCC Technical Report Series, Vol. 4 (1970). IARC Internal Techn. Rep., No. 711002 (1971). W. L. Zielinski. Jr., D. H. Freeman, D. E. Martire, and L. C. Chow, Anal. Chem., 42, 176 (1970). G. Chiavari and L. Pastorelli, Chromatographia, 7, 30 (1974). H. 2. Kelker, fresenius' Z.Anal. Chem., 190, 254 (1963). A. 8. Richmond, J. Chromatogr. Sci., 9, 690 (1971). H. Keiker and E. von Schivizhoffen, Advan. Chromatogr. 6, 247 (1968). L. C. Chow and D.E. Martire, J. Phys. Chem., 75, 2005 (1971). R. C. Lao, R. S. Thomas, H. Oja, and L. Dubois, Anal. Chem. 45, 908 (1973). H. Kelker, B. Scheurle, and H. Winterscheidt, Anal. Chin?. Acta, 38, 17 (1967).
RECEIVEDfor review July 29, 1974. Accepted October 30, 1974. This study was sponsored by the National Cancer Institute under Contract No. N01-(20-25423 with Litton Bionetics. Inc.
Gas-Liquid Chromatography System with Flame Ionization, Phosphorus, Sulfur, Nitrogen, and Electron Capture Detectors Operating Simultaneously for Pesticide Residue Analysis H. A. McLeod, A. G. Butterfield, D. Lewls, W. E. J. Phillips, and D. E. Coffin Food Research Laboratories, Health Protection Branch, Health and Welfare Canada, Ottawa, Ontario
A gas chromatograph with one column, a three-way effluent splitter and five dlfferent detectors operating slmuitaneously is described. The column was 42 In. of 5 % OV-17 on Chromosorb W HP. One outlet of the splitter directed approxlmateiy 5 0 % of the effluent to a Coulson electrolytic detector operating in the nitrogen mode; the second directed 45% to a Meipar flame photometric detector (FPD) operating as a trimdetector (flame ionization, sulfur emisslon at 394 nm, and phosphorus emisslon at 526 nm); the remaining 5 % of effluent was monitored by a Nickel-63 electron capture detector (EC 83Ni). The system determines different pesticide compounds such as organochlorine, organosulfur, organophosphorus, carbamate, triazines, and compounds such as piperonyl butoxide from a single injection of sample extract representing 50 mg of sample. Data from analysis of 674
ANALYTICAL CHEMISTRY, VOL. 47, N O . 4 , APRIL 1975
spiked plant and animal tissues are used to Illustrate the application of the system as part of a multiresidue screening procedure.
Multiple detector systems are used with gas-liquid chromatography to improve the selectivity, increase the number of compounds that can be detected, aid in confirming identity, and reduce the time of analysis. Configurations that use gas splitters with combinations of detectors such as flame ionization, electron capture, and the mass spectrometer have been reported for pesticide (1,2)and toxicological analysis (3). Two or three detectors that function simultaneously around one or two flame ionization burners have been developed in the last decade. These are the flame photometric detector (FPD) introduced by Bowman
1
I1 ON
1
GLC
5 % OV I 7 CHROMSORB W H P
1
, PSR'I
FS?
* ~ o c p r sir p
rec
7
PSR3
PSR4
-__
__ I h C V A
800 C C N P J T E R
r,c-
e
REPCRTl .-_.__J
r3i-r
Figure 3. Diagram showing the relationship of the computer processing equipment
P
~
IF
1
.-i____
1 ANALYTICAL --__
,der
I
~~
~
PSR5
Figure 1. Diagram showing the relationship of the major GLC components
MU L T ' P L E X E Q UNIT .~
,_ A_ N A L O G TO D I G I T A L C O N V E R T E R _ ~ ~ ~ _ _ _ _ - -
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Table I. Operating Parameters for the Five-Detector System GLC Components
1
Detector gas flows, m l per ~ i i i n u t c NELPAR F1 D , S 3 9 4 P 5 2 6 DETEC'ORS
1
w
~
Gas
FPD
EC 63xi
Helium Nitrogen Hydrogen Air
44
8 90
...
... ...
200 180
~oulson
Column
54
106
...
... ...
...
...
...
Temperatures O C and program times
Oven, initial Oven, temperature increase rate Oven, final Oven, cool down time Inlet Transfer lines
U
,
0 030"
,
5.0.8 C
1 " Y 0 250" 0 3 X
2 3
3
5 X 0 250' 0 D X 0 030" I D
E
i 7 9 " Y 0 028''W I T H
S
F
110'X0025''
5 6
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3
0 125 R A N G L E
''
BEND
I.
EC 63Ni F PD
Pyrolysis furnace Coulson cell water
Figure 2. Dimensions and construction details of the 3-way gas splitter
Electrometer settings Detector
and Beroza (4),that detects phosphorus and sulfur, and a tri-FPD reported by Versino and Rossi ( 5 ) that detects P526, S394, and C1 as InCl a t 360 nm. For nitrogen compounds, the Coulson electrolytic conductivity detector ( 6 ) is used extensively. A five-detector system that simultaneously detects the presence of pesticide residues containing active groups such as organochlorine, organosulfur, organophosphorus, organonitrogen, and compounds such as piperonyl butoxide has been developed in our laboratory. This report describes the GLC apparatus, thk operating parameters and illustrates the analytical potential of the system with data obtained from analysis of spiked samples. EXPERIMENTAL Apparatus. A diagram illustrating the relationship of major components in the system is given in Figure 1. A three-way effluent splitter was constructed from a 1:l stainless steel splitter as outlined in Figure 2, and installed in a Micro-Tek 220 gas chromatograph. The FPD and EC e3Ni detectors were relocated to accommodate the gas transfer lines and 3-way splitter. The Coulson electrolytic conductivity apparatus was modified to improve base-line stability a t the most sensitive attenuator and voltage settings by lowering the temperature of the de-ionized water to approximately 16 "C. This was accomplished by circulating tap water through a stainless steel coil installed in the de-ionized water reservoir. Flame out of the FPD with each injection was circumvented by
125", hold 2 minutes 10" per minute 280". hold 10 minutes 16-18 minutes 250" 2 80" 290" 280" 780" 16"
F ID s 394 P 526 EC 63Ni Coulson
Input current (amperes full scale)
2 x 2 x 4 x 4 x 1x
10-1' 10-9
lo-'@ 30 volts
Bucking range (amperes full scale)
2 2 2 2
x x x x
lo-? 10-8 10-8
10-8
...
changing the order of mixing combustion gases with the GLC effluent gas. Normally oxygen and air are mixed with the GLC effluent before the introduction of hydrogen. By reversing this procedure, that is mix the GLC effluent gas and hydrogen first, the flame is sustained for injection aliquots as large as 10 ~ 1 . The operating temperature range of the FPD was increased and dark current of the photomultiplier tubes reduced by replacing the aluminum housings with water cooled housings. Construction details of the water-cooled housings have been described by Dale and Hughes (7). In addition, the photomultiplier tubes were cooled by circulating tap water through coils of aluminum tubing wrapped around their casings. A GLC column was prepared using Pyrex glass tubing 42 in. X 0.25-in. 0.d. X 0.17-in. i.d. and 5% OV-17 on Chromosorb WHP. The column was conditioned for 24 hours a t 320 "C while sweeping with nitrogen carrier gas a t 15-30 ml per minute. Two dual-pen and one single-pen Westronic recorders with 1 millivolt (mV) scales were used to record the detector responses. In addition, all five detector outputs were recorded on a 9-channel incremental magnetic tape for off-line processing of data with a NOVA 800 computer (Figure 3). Area calculations and retention times were obtained through computer processing of the taped data using a program to be described a t a later date. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4 , APRIL 1975
675
MILK CONTROL
B A S E LINES
,
s394
7
1
1
7
P526 L
'
EC
A
COULSON
1 FID
1
I
Flgure 4. Simultaneous response patterns of the five detectors for a complete (48 minutes) temperature programming and cool down cycle
a
1'
Figure 6. Response of four detectors to a cleaned up milk extract
new FPD and EC 63Ni rates from the total flow through the system. Second, the hydrogen flow through the pyrolysis furnace was adjusted to 30 ml per minute with the low pressure regulator set to 10 psi. (CAUTION: carrier gas pressure in system must always exceed hydrogen.) With the Coulson cell in position, the helium flow rates through the other detectors were redetermined and that for the Coulson cell was calculated as before. These data were examined and if the splitter flow rates did not fall within the ranges of 5-10 ml per minute for the EC 63Ni, 45-50 for the FPD and 50-55 for the Coulson, then the density of the Fiberfrax plug and/or hydrogen gas flow were adjusted to give the desired splitter ratio. In extreme deviation of flow rates. readjustment of the splitter inserts was required also.
RESULTS AND DISCUSSION
MLS
PER M I N U T E
Figure 5. Effect on Melpar P 526-nrn and S 394-nm response of varying rates (mllmin) of oxygen or air for two different rates of hydrogen
Reagents. Standard pesticide stock solutions of 1 mg per ml were prepared from manufacturers reagent quality chemicals. Mixtures to be used as reference standards and for spiking samples were prepared by serial-diluting aliquots of the stock solutions. Procedure. The various operating conditions including electrometer settings are given in Table I. All effluent gas flows were determined using a soap bubble flowmeter. The oxygen, air, and helium rotameter flow controllers were calibrated over a 10 to 200 ml per minute range with the low pressure gauges set at 50 psi. The helium carrier gas flow through the GLC column (200 "C) was adjusted to approximately 107 ml/min with the helium flow controller. After connecting the gas splitter to the column, a gas flow ratio of approximately 1 part EC 63Ni, 8 parts FPD, and 13 parts Coulson was established by inserting various sized stainless steel wire or hypodermic needles into the splitter arms as in Figure 2. Operating temperatures of transfer lines, detectors, etc. were adjusted to equilibrate at the values given in Table I. A Fiherfrax plug 1.5 cm long, loosely packed, was inserted into the outlet end of the quartz pyrolysis tube and helium gas flows determined under operating conditions. First, the flows through the FPD, EC 63Ni and pyrolysis furnace (Coulson cell disconnected) were determined and the total flow through the system calculated as the sum of the three flow rates. After connecting the Coulson cell, the flow rates of the other detectors were determined and the Coulson flow rate was calculated by subtracting the sum of the 676
*
ANALYTICAL CHEMISTRY, VOL. 47, N O . 4, APRIL 1975
T h e operating parameters and GLC OV-17 column were selected o n t h e basis of previous experience a n d published reports (8).OV-17 is a moderately polar liquid phase t h a t is h e a t stable t o 350 "C a n d bleeding is minimal for t h e 5 different detectors. Figure 4 illustrates t h i s stability for t h e t e m p e r a t u r e programming parameters given in T a b l e I. T h e total time for t h e chromatograms is 48 minutes. T h e S394 and P526 base lines exhibit moderate levels of electronic noise, primarily from t h e photomultiplier tubes. A nine-point, least squares t y p e of mathematical smoothing (9) is used in t h e computer program t o eliminate t h i s type of interference. T h e EC G3Ni, a n d FID detector base lines have response peaks from unknown compounds t h a t occur during t h e oven t e m p e r a t u r e final hold period. T h e small peaks a r e 5 1 0 % of full scale (1 mV, 2 X lo-" ampere), t h e largest 25% a n d can be allowed for when interpreting results from sample analysis. T h e Melpar S 394-nm a n d P 526-nm response was optimized by determining t h e effect of different hydrogen, oxygen, a n d air gas ratios. Figure 5 illustrates t h e response of t h e two detectors t o parathion for hydrogen plus oxygen a n d hydrogen plus air. Hydrogen flow of 200 ml per m i n u t e is o p t i m u m for oxygen or air. Air is superior t o oxygen with hydrogen. A combination of air, oxygen, a n d hydrogen ( n o t shown) was as good as air plus hydrogen, b u t n o t superior. Gas flows of 180 ml/min of air, plus 200 ml/min of hydrogen were selected for maximum S 394 response while retaining approximately 90% of t h e P 526 response. T h e response of t h e 5 detectors t o biological samples was determined for milk a n d carrots. Carrots were spiked with a mixture of s t a n d a r d pesticides at half Canadian tolerance levels. Milk was spiked with a mixture of pesticides used for control of cattle pests, Samples were spiked prior t o ex-
Table 11. Summary of Results for Milk Spiked with a Mixture of the Pesticides Listed a n d Analyzed by the 5-Detector System Detector, peak area per ng and recovery ( % )
Compound
Dichlorvos Lindane Simazine (s) Dioxathion Ronnel Malathion Crufomate Piperonyl butoxide Methoxychlor Coumaphos a
RRT
EC - P A
7 57(3 7) 142 2 6(90)
710
948 1000 1011 1089 1182 1262 1570 1618 1857
pi -PA
P -PA
S -PA
a
1672(49)
-
-
-
-
617
-
-
1761 2( 89) 63 52 (90)
-
.-
+
-
+
718(23) 1217(129) 1512(120) 1432(92)
-
-
-
11797( 96)
-
-
-
-
-
510(92)
+
.-
292(95) 846(80) -
A dash indicates no response expected, a plus indicates response expected but not obtained a t the concentration used.
and malathion by the FPD in the phosphorus mode is not known. The paper strip recordings of detector responses for milk sample extracts and standard pesticide mixtures illustrate the complex data that must be interpreted. Figure 6 is the response pattern of milk only for S 394, P 526, EC, and Coulson detectors. There are no significant response peaks for S 394 but there are for the P 526 and EC detectors. The P 526 peaks are 5 to 10% of the 1mV, 4 X ampere full scale and the 11or so major peaks for the EC range from 10 to 6W of the 1 mV, 4 X ampere full scale. The nature of the compounds detected in the milk control were not determined because our reference file of relative retention times and pesticides has not been completed. The one major peak for the Coulson is simazine used as a reference compound for relative retention times. I t should be noted that the detectors are operating a t or close to maximum sensitivity and the sample extracts have had minimal cleanup. Figure 7 is the response pattern of pesticide standards in the spiking mixture. The majority of the peaks exceed the 1-mV scale of each recorder and cannot be measured manually. However, a 9.999-mV scale is used in digitizing the data and recording on magnetic tape for computer processing. Providing the detector’s linear range is not exceeded,
traction and the low temperature cleanup described previously by McLeod and Wales (IO).The equivalent of 50 mg of sample in 5 ~1 of hexane was injected for each sample. Table I1 is a summary of the pesticides, their retention times relative to simazine, computer-calculated area per nanogram of each compound and per cent recovery for spiked milk samples. Simazine and piperonyl butoxide were used as internal standards to calculate relative retention times. The nitrogen detector and FID are usually void of residue or extraneous response peaks a t the retention time of these compounds. Retention times were measured to the nearest second. Relative retention times are expressed in whole numbers to facilitate programming the small, 16-bit, 8-K, Nova Computer. Peak areas are expressed as the number of integration units for a nanogram of the pesticide passing through the detector. A value of at least 400 area units can be taken as detectable. The per cent recoveries are given in brackets immediately following the peak areas. These data indicate it is possible to resolve the 10 compounds in the mixture including the overlapping pair, simazine and dioxathion. The low recovery for dichlorvos can be explained on the basis of its high volatility and for dioxathion on its instability and poor GC column characteristics. The reason for the high recoveries of ronnel
Table 111. Summary of Results for Carrots Spiked with a Mixture of the Pesticides Listed a n d Analyzed by the 5-Detector System Detector, peak area per ny and recovery ( 6 ) Compound
Trifluralin Botran Simazine Diazinon Linuron Parathion M a1athi on Carbaryl CY- Chlordane Dieldrin Captan Piperonyl butoxide (P.B ,) Methoxychlor
RRT
472 590 606 606 712 729 733 792 808 808 820 1000 1041
Ii -PA
EC -PA
99 27 (96) 1008(15) 84 8(152) 11976b 11976b -
309(76) 40682b 40682b 40682b 2615? 26157b -
+
P-PA
S -PA
FID-PA
-a -
-
3954(98)
74 6(93)
0.s:
0s. 0s.
-
0.S.c -
-
-
5814b 5814b 5814* 5 14(70)
A dash indicates no response expected, a plus indicates response expected but not obtained at the concentration used. These areas are based on a single overlapping peak for two or more compounds and it was not possible to calculate area per NG or 90recovery. O.S. indicates the response exceeded the scale of 9.999 mV and calculations could not be made. (1
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
677
MlLKStSTDS
M I L K STDS. I
1
COULSON i / /
COULSON
' d ---
Figure 8. Response of 4 detectors to an extract of milk fortified with the standard pesticide mixture listed in Figure 7
2
Figure 7. Response of four detectors to a mixture of dichlorvos, lindane, simazine(s), dioxathion, ronnel, malathion, crufomate, methoxychlor, and coumaphos. Piperonyl butoxide was also present but is not detected
recovery calculations are made without resorting to dilution and re-running the diluted extract. Note also that these peaks greatly exceed those observed in the milk control results (Figure 6). Figure 8 is the response pattern of each detector for 5 p1 of milk (50 mg) extract containing the equivalent of pesticides illustrated in Figure 7 . Although the response peaks for all pesticides, except vapona and coumaphos, are off scale, it was possible to calculate peak areas and relative retention times. The FID response for milk analysis is shown in Figure 9. FID, standards only, has piperonyl butoxide (indicated by arrow) as its major peak, the remaining peaks are column background, illustrated previously in Figure 4. The remaining two chromatograms, milk and milk with standards, have a multiplicity of responses from unknown extractives. The majority of these compounds elute from the GLC during the last 5 minutes of the final temperature hold and the first 15 minutes of the cool down cycle. This is advantageous as interference to early eluting pesticides is minimized and column life extended by the later elution of coextractives. Analytical data for a 10-compound spiking mixture added to carrots are shown in Table 111. Compounds are grouped together where overlap of their response occurs. Relative retention times are based on piperonyl butoxide because simazine nitrogen response could not be distinguished from that for botran and diazinon. The need to use two reference standards in this type of screening procedure is demonstrated by this overlapping of compounds. Trifluralin (0.6 ppm) resolved as a distinct peak on the EC and Coulson detectors. Recoveries were satisfactory. It was difficult to use the EC 63Ni data over the R R T ~ B range 590 to 1000 because of the excessive background from unknown co-extractives. These compounds may have contributed to the low recovery (15%) for botran (2 ppm) because of saturation and/or physical interference to the electron absorption process. Recovery of 5 ppm of methox678
ANALYTICAL C H E M I S T R Y , VOL. 4 7 , N O . 4 , APRIL 1975
r
rl K
r - 1r i
r-i
71
Figure 9. FID response (5th detector) to the standard pesticide mixture, milk and milk plus standards (STDS). Piperonyl butoxide is indk cated by an arrow
ychlor (70%) was low but considered satisfactory for a screening procedure. The interference experienced with carrot extracts, and the narrow linear response range of the EC "Ni detector indicates a need for a more specific organochlorine detector, e.g., Coulson (6) or Hall (11) electrolytic conductivity detector functioning in the HCI mode. Alternately, a more extensive cleanup of extracts is indicated but this would increase greatly the screening analysis time and narrow the scope of the procedure. It was possible to separate diazinon (1 ppm) from simazine and botran with good recoveries (93-98%) using the P 520 and S 394 responses. Linuron (0.9 ppm) was separated from parathion (0.5 ppm) by EC 63Ni not by nitrogen response characteristics. Recovery of linuron by EC 63Ni was excessively high because of interference from co-extractives whose identity was not determined. The P 520 and S 394 responses for malathion (0.5 ppm) and parathion overlapped and were
off scale ( O S ) a t spiking level. The 9.999-mV scale of the data recording unit was exceeded and area calculations were not usable. However, the data did indicate that residue levels were of a magnitude that warranted a sample rerun at higher signal attenuation to bring the response within the 9.999-mV limits. A similar O S . reading was obtained with the S 394 response for captan (20 ppm) a t half tolerance. CONCLUSIONS A 5-detector GLC system has been described that has potential as the basis of a rapid screening procedure for chemical residues in biological samples. To be fully exploited, various specific detectors with large response ranges are required and their responses should be recorded for computer processing and interpretation. An extensive computer-based reference library of different chemical detector response characteristics is needed. The EC 63Ni detector with its limited dynamic range and non-specificity was satisfactory for milk extracts with minimal cleanup, but not for carrots.
ACKNOWLEDGMENT The authors are grateful for the assistance given by workshop services in modifying equipment and by drafting and photographic services in preparing diagrams. LITERATURE CITED (1) D. M. Daks, H. Hartmann and K. P. Dimick, Anal. Chem., 36, 1560 (1964). (2) J. R. Wessel, J. Ass. Offic. Anal. Chem., 51, 666 (1968). (3) H. Brandenberger. Pharm. Acta Helv., 45, 394 (1970). (4) M. C. Bowman and M.Beroza, Anal. Chem., 40, 1448 (1968). (5) 6.Versino and G. Rossi, Chromatographia,4, 331 (1971). (6) D. M. Coulson, J. Gas Chromatogr.,3 , 134 (1965). (7) W. E. Dale and C. C. Hughes, J. Gas Chromatogr.,6 , 603 (1968). (8) M. C. Bowman and M. Beroza. J. Ass. Offlc. Anal. Chem., 53, 499 (1970). (9) A. Savitzky and M. J. E. Golay, Anal. Chem., 36, 1627 (1964). (10) H. A. McLeod and P. J. Wales, J. Agr. FoodChern., 20, 624 (1972). (11) R. C. Hail, J. Chromafogr. Sci., 12, 152 (1974).
RECEIVEDfor review July 19, 1974. Accepted November 26, 1974.
Mass-Spectrographic Determination of Hydrogen Thermally Evolved from Titanium G. L. Powell, F. W. Postma,' C. Cook, H. Tucker, and A. L. Williamson Union Carbide Corporation, Nuclear Division, Oak Ridge Y- 12 Plant, Oak Ridge, Tenn. 37830
A technique for determining trace amounts of hydrogen in metals using mass-spectrographic detection of hydrogen thermally evolved from a metal sample and calibrated with an expansion of hydrogen gas has been used to determine the quantity of hydrogen evolved at 930 OC from National Bureau of Standards Type 352 hydrogen-in-titanium standards. These standards are rated at 32 pg H/g Ti and were found to deliver 30 f 6 pg H/g Ti in these experiments. At 930 OC, the hydrogen pressure within the vacuum system and the concentration of hydrogen in the sample approach the equilibrium relationships throughout the analysis. These near-equilibrium conditions limited the hydrogen evolution rate from the sample resulting in very long extraction times, difficulty in determining when extraction was complete, and prevented all the hydrogen from being evolved from the sample. These difficulties were sufficiently great that the hydrogen-In-titanium standards co'uld not be used as control samples for thls method of analysis. The proportlonallty constant between the hydrogen concentration In the tltanlurn sample at 930 "C and the square root of the hydrogen pressure at equlllbrlum was directly measured using thls hydrogen analysis technique and yielded good agreement with published values. A method for preparing control samples from uranium or nickel that deliver 1 to 20 pg H/g sample and that do not experience near-equlllbrlum hydrogenmetal lnteractlons during analysls and related dlfflcultles is described.
In the course of developing a hydrogen analysis method based on the thermal evolution of hydrogen from a metal sample in vacuum, and the mass-spectrographic detection of this evolved hydrogen ( I ) , experiments were carried out Present address, Oak Ridge Gaseous Diffusion Plant, Oak Ridge, Tenn.
to reconcile the ideal gas law calibration of the instrument to hydrogen-in-titanium standards supplied by the National Bureau of Standards (Standard Sample 352) ( 2 ) .These 352 samples are rated to contain 32 bg H/g Ti. The unique metal-hydrogen relationships for a particular metal or alloy determine the hydrogen evolution characteristics a t elevated temperatures in vacuum. This has been demonstrated in the case of solid uranium alloys (3) and solid tungsten-nickel-ion alloys ( 4 ) . The efficiency with which the sample surface converts contaminants to hydrogen during analysis generates a unique lower limit of detection for hydrogen in a particular alloy in a particular condition of surface contamination. At sufficiently high temperatures, evolution from a metal sample is ultimately limited by the diffusion of hydrogen from within the sample. Titanium behaves differently from uranium alloys and tungstennickel-iron alloys, in that the hydrogen pressure over the titanium sample during analysis a t 930 "C approaches the equilibrium hydrogen pressure for microgram per gram concentrations of hydrogen in titanium. This reduces the hydrogen evolution rate from the titanium sample and increases both the time required for an analysis and the uncertainty of the end point of the analysis. The quantity of hydrogen not extracted from the sample, Le., the hydrogen dissolved in the sample in equilibrium with the hydrogen pressure in the extraction chamber, is significant. This report describes the experiments that were necessary to establish a procedure for the routine determination of hydrogen in titanium and the development of working standards of nickel and uranium that are more compatible with this method of analysis. EXPERIMENTAL Instrumentation The instrument used for these experiments has been described by Condon et al. (I). The instrument is an ultra-high vacuum system consisting of an extraction chamber A N A L Y T i C A L C H E M I S T R Y , VOL. 47, NO. 4 , A P R I L 1975
679
