CONCLUSIONS The aim of our study was to find the rate determining step of the overall electrode process, on the basis of the response time curves. For precipitate-based electrodes, it could be stated that the ionic diffusion through the adhering laminary layer is insignificant, provided that the adhering layer is incm). In this case, the response time finitely thin (6 I curves can be described with a first-order kinetic equation and only the initial slope of the response time curves was evaluated, which may give further information for the electrode kinetics. Accordingly the initial slope of the response time curves of a silver iodide-based electrode was found steeper in silver solutions than in iodide ones. This fact cannot be explained with the difference in the relative ionic mobilities which means that, under these conditions, the response time values characteristic for the electrode kinetics may be measured. A t slow flow-velocities, the response time curves are explained with diffusion kinetics. However, in this case and that of electrodes of large surface area, the thickness of the adhering layer is a function of time and place, and it results in an inhomogeneous concentration distribution. According to this, the potential-time function of the electrode after introducing a concentration jump can be explained with the resultant potential value of the elementary potentials formed a t different points of the electrode surface being in contact with a solution of different concentrations. A relatively simple replacement circuit (Figure 9) has been used for explaining the effect of various types of resistances on the resultant potential value. With the help of this model, the differing character of the response time curves on the direction of concentration jump can be explained. I t must also be stated that if the rate of the ionic diffusion process through the adhering laminary layer is much greater than the next kinetic step, then the evaluation of the transient phenomena of the electrode is not influenced by the nonhomogeneous concentration distribution at the electrode surface. This is in accordance with our findings obtained with neutral carrier and covered surface electrodes, a t which the effect of flow-rate on the rate of response is not so significant. Accordingly the response time curves can be explained by the
theory based on the ionic diffusion in the bulk of the electrode membrane (Equation 4).
ACKNOWLEDMENT The authors are greatly indebted to GBza Nagy for his contribution to this work concerning the study of the multielectrode system.
LITERATURE CITED (1) G. A. Rechnitz and H. F. Hameka, Fresenius' 2.Anal. Chem., 214, 252 (1965). (2) G. Johansson and K. Norberg, J. Nektroanal. Chem., 18, 239 (1965). (3) G. A. Rechnitz and G. C. Kugler. Anal. Chem., 39, 1682 (1967); G. A. Rechnitz and M. R. Kresz, Anal. Chem., 38, 1789 (1966); G. A. Rechnitz, M. R. Kresz and S. B. Zamochnick, Anal. Chem., 38, 973 (1966). (4) Bo Karlberg, Anal. Chim. Acta, 66, 93 (1973). Bo Kariberg. J. Electroanal. Chem. lnterfacial Electrochem., 42, 115 (1973). (5) R. P. Buck, J. €lektroanal. Chem., 18, 363-380 (1968). (6) R. P. Buck, J. Elektroanal. Chem., 18, 381-386 (1968). (7) K. Toth, I. Gavaller, and E. Pungor, Anal. Chim. Acta, 57, 131 (1971). (8) K. Toth and E. Pungor, Anal. Chim. Acta, 64, 417 (1973). (9) W. E. Morf and W. Simon, "ion-selective Electrodes", M. S. Frant and J. W. Ross, Ed., in press, 1975. (10) W. E. Morf and W. Simon, paper presented at International Workshop on Ion Selective Electrodes and on Enzyme Electrodes in Biology and in Medicine, Sloss, Reisenburg near Ulm, Germany September 15-18. 1974. (11) W. E. Morf, E. Lindner, and W. Simon, Anal. Chem.. 47, 1596 (1975). (12) P. L. Markovic and J. 0. Osburn, AlChE J., 19, 504 (1973). (13) K. Toth and E. Pungor in "Ion Selective Electrodes Symp. 1972", E. Pungor Ed. Akad. Kiado, Budapest, Hungary, 1973, pp 145-164. (14) R . Rangarajan and G. A. Rechnitz. Anal. Chem., 47, 324 (1975). (15) K. Toth and E. Pungor paper presented at the International Symposium on Selective ion-Sensitive Electrodes, Cardiff, Wales, 1973. (16) V. G. Levich. "Physicochemical Hydrodynamics," Prentice-Hall, International, Inc., 1962. (17) W. Vieistich, 2.Electrochem., 57, 646 (1953). (18) J. T. Davies. "Turbulence Phenomena", Academic Press, New York, 1972. (19) Yamada and H. Matsuda, J. Electroanal. Chem., 44, 189-198 (1973). (20) B. Fleet, T. H. Ryan, and M. J. D. Brand, Anal. Chem., 46, 12 (1974). (21) G. Kortum and J. O'M. Bockris, "Textbook of Electrochemistry", Elsevier Publishing Company, New York, Amsterdam, London, Brussels, 1951. (22) J. Pick, K. Toth, E. Pungor, M. Vasak, and W. Simon, Anal. Chim. Acta, 64, 477 (1973). (23) E. Pungor, J. Havas, K. Toth, and G. Madarasz, French Patent 1. 402. 34, 1965. (24) G. J. Moody, P. B. Oke, and J. D.R . Thomas, Analyst (London),95, 910 (1970); G. J. Moody and J. D. R . Thomas, "Selective Ion Sensitive Electrodes", Merrow, Watford, England, 1972. (25) H. Malissa, M. Grasserbauer, E. Pungor, K . Toth, M. K. Papay, and L. Polos, Anal. Chim. Acta, 80, 223 (1975).
RECEIVEDfor review October 23, 1975. Accepted February 23, 1976.
Gas-Liquid Chromatography/Ultraviolet Photoelectron Spectroscopy and the Photoelectron Spectra of Nitrosamines D. Betteridge' and Syed Khaja Hasanuddin Department of Chemistry, University College of Swansea, SA2, BPP, United Kingdom
David 1. Rees J. Lyons & Co. Ltd., London W. 14, United Kingdom
The effluent from a gas-liquid chromatography (GLC) column has been introduced directly to a uv photoelectron spectrometer. The GLC column can serve as a heated inlet system for the spectrometer, In which It permits measured amounts of sample to be introduced and provides a check on sample purity. Alternatively, the spectrometer can be' used to monitor the effluent of the GLC column. A complete spectrum of the sample can be obtalned or the spectrometer can be operated at a fixed potential as a selective GC 1078
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
detector. The operating parameters and conditions are described and the value of the combination Is critically dlscussed. The photoelectron spectra of several nitrosamines are reported and briefly discussed.
A gas-liquid chromatograph may be coupled to an ultraviolet photoelectron spectrometer with two objects in mind. First, the chromatograph might serve as a superior
p
( a ) 15 mtorr.
H e CARRIER GAS I O r n l rnin-’
N
T lO-3-1O-‘torr.
C
Figure 1. Schematic arrangement of GLC-PES link up GC. gas chromatographic column: I, insulating tube PVC, 2 cm long; F. FID: V, on/off valve; A , “Swagelok” T-piece; T. target chamber of PE spectrometer; N, stainless steel tube 0.075 or 0.025 cm i.d., approximately 90 cm long; C, constriction
3 W
E
10
2
4
zc3 1
6
2
VOL.)Jl
inlet system for the spectrometer. A measured amount of sample could be introduced, the purity of the sample would be checked, and a wide variety of compounds could be introduced. It would thus provide a basis for quantitative analysis, and by removing volatile impurities would make qualitative spectral information more reliable. Second, the spectrometer could serve as a novel detector for the chromatograph. It could either provide photoelectron spectra of compounds emerging from the chromatograph or be set a t some predetermined potential and function as a selective GC detector. There is a similarity between the layout and functioning of GC-MS and GC-PES systems. However, the target chamber of the spectrometer into which the sample is introduced is maintained within a pressure range of 10-lTorr and there is a throughput of helium from the ionizing source of 0.6-1.0 ml min-’. Consequently, the pressure differential between the chromatograph and spectrometer is much less with GC-PES than GC-MS. We have already reported that the combination is practicable ( I , 2 ) . In this paper we report the results of preliminary experiments to establish the important parameters and to evaluate the analytical importance of GC-PES.
4
6
VOL.yL (b) 10 rntorr.
18-
t 16:
14-
c
-
E 12-
w
5
m W
10-
-
86:
-
4-
2
2
6
4 V0L.pL
4
6
VOL.)JI
(c) 5 m t o r r .
EXPERIMENTAL Apparatus. The photoelectron spectrometer constructed in our laboratory has already been described (1, 3, 4 ) . It was designed with the GLC link in mind, having an easily accessible, heatable, target chamber and Helmholtz coils which provide a magnetic field for the analysis of electron energies. The coils also compensate for the Earth’s magnetic field and other magnetic disturbances around the spectrometer. It was possible to obtain a spectrum of 18-20 meV resolution with the chromatograph operating adjacent to the spectrometer. The scan circuit of the spectrometer has a wide range of settings. A Perkin-Elmer Model 800 gas chromatograph with a flame ionization detector (FID) was used. A stainless steel column (1 m X 0.126 cm) containing either 15% Carbowax 20M or 2% E301 on Chromosorb W DMCS 60-80 mesh was used as appropriate with helium as the carrier gas. Calibrant gases were admitted to the target chamber by a separate port. Pen recorders were used in the main, but a fast-scanning uv recorder was occasionally used. The connection of the chromatograph and the spectrometer was made with a stainless steel needle tube, N (cf. Figure 1). Electrical insulation of the spectrometer was achieved by using a short piece of PVC tubing, I, as a link between the needle tube and the target chamber. The needle tube was heated ohmically and the target chamber by a hot air blower or heating tape. (The heated target chamber ( 4 ) was constructed after the experiments described below had been completed.) Control of sample throughput was provided by varying the size of the constrictions and by having the “T” piece, A, open or closed. Some compromise over chromatographic or spectroscopic performance or both were necessary since the spectrometer requires 20 s to produce an identifiable spectrum and 7 min to yield one of good quality, and, during the run, the sample pressure in the target chamber must be constant. The spectrometer response a t fixed potential, however, was virtually instantaneous. According to the object of a particular run, one of three options was selected: (a) the effluent from the column was passed directly to the spectrometer; (b) the effluent was split to varying degrees between the FID and spectrometer, the “T” piece being closed; (c) the “T” piece was
2
4 V0L.uI
6
2
4
6
VOL. 4 L
Figure 2. Effect of the interfacial and FID constrictions on the maxi-
mum sample pressure and total time spent by the sample in the tar-
get chamber Interface constriction: (a) 15 mTorr (b) 10 mTorr (c) 5 mTorr. FID constriction: nil ( 0 )2.0 ; Torr ( 0 ) ;1.0 Torr (A): 0.5 Torr (0)
opened until the sample was detected by the FID, whereupon it was closed and the effluent split between the FID and the spectrometer. Chemicals. Alcohols were obtained commercially and used directly. Nitrosamines were characterized by GLC and mass spectrometry. The pesticides were those used earlier ( 5 ) .
RESULTS AND DISCUSSION Operating Parameters. An improvement in sensitivity was achieved as the pressure drop along the column was increased (6). However, because of the comparatively low pumping speed, the flow rate of carrier gas had to be reduced to 1 ml min-l in order to keep the pressure in the target chamber within permissable limits. It proved feasible to pass the total effluent from the chromatograph into the spectrometer and to obtain good P E spectra, but it was too time consuming for routine application. Usually, the GC effluent was split between the FID and the spectrometer. The time the sample spent in the target chamber and the pressure it exerted therein obviously depended upon several factors. The effect of relative size of the FID and interfacial constrictions and the total amount of the sample were investigated. For convenience, the constrictions were made by crimping the tubes. The size of the ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, J U N E 1976
1079
I
3.
,
;\*
20 21 eV
2.
4.
Figure 3. Photoelectron spectra of
various nitrosamines,obtained via GLC inlet system
constriction was defined as the pressure observed in the target chamber when one end of the tube was connected to the target chamber and the other to the atmosphere, i.e., the lower the stated pressure, the greater the constriction. The carrier gas flow rate before connection to the spectrometer and FID was 10 ml min-l. The results are shown in Figure 2. From them it may be concluded that (a) the most favored conditions for the measurement of a high-resolution spectrum were provided by severely constricting the outlets to the FID and spectrometer and using a sample of a t least 5 pl, (b) the truest response to pressure in the target chamber was obtained when there was no constriction to the FID, (c) for samples of less than 1 pl, the balance of the constrictions was less critical. Thus, chromatographic efficiency had to be lowered in brder to allow a high-resolution spectrum. Some improvement in the flow rate was obtained if the effluent was vented via the "T" piece until a peak was sensed by the FID, but undesirable loading was inevitable. The spectrometer can function as a detector a t a fixed setting and low-resolution spectra may be obtained under more satisfactory chromatographic conditions if smaller samples are taken. The concentration profile of the sample passing through the target chamber was approximately Gaussian. In order to obtain a satisfactory high-resolution spectrum, a high and constant concentration of the sample had to be maintained in the interfacial system for over several minutes. This was achieved by using a narrow connecting tube of 0.175-cm i d . , and large sample volumes of 1-10 111. T h e GLC as a n Inlet for the Photoelectron Spectrometer. The high-resolution spectra of a wide variety of compounds have been obtained with the column operating a t temperatures up to 280 "C. The spectra of volatile compounds, such as benzene, were identical with those obtained normally, under static conditions. Some examples of those of the nitrosamines obtained with column temperatures of 150-200 "C are shown in Figure 3. Memory effects were rarely encountered so that a steady throughput of samples could be maintained. 1080
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
The chromatographic trace from the FID was obtained simultaneously and usually confirmed the purity of the sample, but with two pesticides impurities were detected. The sample of dimethoate gave rise to a chromatogram of several peaks, which could be assigned to minor impurities and dimethoate. The spectrum of pure dimethoate was then obtained. By contrast, dichlorvos decomposed in the column. A number of chromatographic peaks and associated low resolution P E spectra were obtained. The latter corresponded to HCl, methanol, a small halogen-containing compound (dichloracetone?), and a phosphorus-containing compound. These are consistent with the known hydrolysis products of dichlorvos. When this compound was first examined by PES, many hours of pumping were needed to remove the impurities and to obtain the spectrum of the pure compound ( 5 ) .Only HCl was identifiable a t that time, the system being too complex for identification by spectrum stripping, but it seems in retrospect that the compound was being hydrolyzed by adsorbed water in the inlet system. When the sample was placed directly in the target chamber of the spectrometer used in this study, a spectrum of pure dichlorvos was obtained immediately with slight heating. (The spectrum obtained first of all from moistened dichlorvos was very similar to that obtained in the earlier study, and confirms that the inlet system of the PS15 photoelectron spectrometer brought about sample decomposition.) Thus, GLC is a useful inlet system, especially for stable samples of dubious purity, for quantitative work and for repeated examination of the same sample. The time taken to run a "one-off" spectrum is determined by the relevant GLC parameters and the time taken to establish them. If the sample is pure, its spectrum may be obtained a t much lower temperature by direct insertion in the target chamber, as with our instrument (3, 4 ) or the Perkin-Elmer PS18. However, it is less convenient to change samples with these instruments, and, if a pen chart recorder is used, there is often a fall off in intensity from start to finish as the sample evaporates. With the direct insertion inlet, the time taken to establish the optimum temperature for an
G L C - P E S LINK SEPARATION OF E T H A N O L ( 5 p l ) AND P Y R I D I N E ( 5 p l ) SEQUENTIAL 5 0 S E C P E S SCANS
12 1.
2.
-
16
EtOH
Pyridine
20
12
16
20
12
16
-
20
12
I6
20
12
16
eV
Figure 4. GC-PES separation and identification of ethanol and pyridine: sequential spectral scans
-
20 eV
12
16
20
-
5 pI of each taken. Scan time: 50 s. The oxygen lone-pair peak of ethanol at 10.6 eV and the peaks of pyridine at eV are shaded to aid identification. ArlXe mixture as calibrant
unknown sample is the major time factor in running a “one-off’ sample. For some samples only a few minutes are required, but the most difficult may take up t o 2 ,h. However, far more sample is required than for the the comparable GLC pilot experiments. In order to check the efficiency of separation, it is desirable to obtain the PE spectra of successive parts of a peak emerging from a GLC. The successive spectral scans of ethanol and pyridine as they emerged from the chromatograph, having been clearly separated, are shown in Figure 4. The concentration profile of the GC peak was reflected in the variation of intensity of the P E spectra. The shaded bands make it easy to see this and also the differences in the spectra. The resolution is low, but adequate for qualitative identification in many instances. In one experiment the GLC efficiency was reduced so that a mixture of ethanol and acetone appeared to the FID as one compound. Spectral scans through the peak clearly showed the partial separation of the two compounds and gave spectra which enabled the components of the mixture to be clearly identified. Spectrometer as Selective GC Detector. Often in the interpretation of P E spectra, particular peaks are assigned as characteristic of a particular atom or molecular grouping, e.g., “lone-pair” peaks, T bands, etc. Thus, a spectrometer tuned to a particular ionization potential (IP) might serve as a selective GC detector. The possibilities are exemplified by consideration of acetone and alkyl alcohols, whose spectra are shown in Figure 5. It is seen that while the oxygen “lone-pair” peak (the one a t lowest IP) is a characteristic feature of all these compounds, its IP, eV, varies: 9.6 (acetone), 11.1 (methanol), 10.6 (ethanol), 10.4 (1-propanol, 1-butanol), and 10.1 (1-pentanol). At higher IP’s, e.g., 15.75 eV. there are broad bands in all the spectra whose intensity, relative to the oxygen lone-pair peak, increases with increasing number of carbon atoms in the molecule. These values suggest that it is impossible to set a P E spectrometer as an oxygen-sensitive detector, but it might be set as a methanol detector or an alcohol-other-than-
methanol detector or a general detector, where sensitivity would vary with the molecular weight of the sample. Mixtures of alcohols of varying concentrations were injected onto the column, the effluent being monitored by the FID and the spectrometer a t some fixed setting. The results for one mixture at different spectrometer settings are shown in Figure 6. At a setting of 11.1eV methanol was detected with great sensitivity, whereas it was not detectable when the setting was a t 10.6. At higher IP, the higher alcohols were detected with enhanced sensitivity. The chromatographic peaks obtained with both FID and P E spectrometers were virtually identical in width and retention volume (Figure 7 ) . There was only a very slight time lag between the FID and the spectrometer responses. Samples of 0.05 p1 were easily detected and the areas under the GC-PES peaks were proportional to the amount of sample injected. Similar results and discriminations were obtained with mixtures of acetone, methanol, and ethanol. Figure 7 also shows that there are exceptions to the generalizations made above. Pentanol, with an alkyl band that encroaches on the oxygen lone-pair band and a trough in its spectrum a t 14.75 eV, exhibits similar responses to methanol at settings of 11.1and 15.75 eV. A number of interesting possibilities exist for use as a selective detector. There are fine differences in I P for a group as shown by the alcohols, but characteristic peaks in a series occur over a relatively narrow band, e.g., for oxygen C=O ca. 9.5, ROH, 10.6, CH3, 11 eV; for aromatic T 8-9 eV (two-bands); for et,helynic H 10 eV. Unfortunately, there is a severe limitation, in that no one I P can be uniquely associated with one molecular grouping in the way that correlations are commonly made in infrared and NMR spectroscopy. For example, as the above figures suggest, the carbonyl oxygen peak coincides with one of the aromatic peaks in the spectra of benzaldehyde. Spectra of Nitrosamines. The spectra of some nitrosamines have been shown earlier ( 2 ) , but they were not interpreted. The IP’s of the first three bands are given in Table I. The main features are two very distinctive bands ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1081
Table I. Ionization Potentials of N-Nitrosoamine Dilithylnitrosoamine Diethylnitrosoamine Dipropylnitrosoamine Methylethylnitrosoamine Methylpropylnitrosoamine Ethylisopropylnitrosoamine
Dipentylnitrosoamine Nitrosopyridine a
1
2
3
8.88
9.58 9.2 9.34 9.61 9.62 9.57 9.27 9.4
12.6" 12.1" 11.76"
8.56 8.77 9.00 9.0 8.96 8.78 8.77
4
5
6
7
8
9
12.57
13.22
14.0
15.0"
16.0
16.8"
12.70 12.0 12.4" 11.2" 11.6
Broad band.
,
f
19
21 eV.
Methanol C H , O H
I
I
/4u iy 19 19
21 21 eV eV
result in the peak due to Nz being a t slightly lower I P than that due to N1. The oxygen lone-pair IP, by analogy with C=O oxygen lone pair, would be a t ca. 9.5 eV. Thus we would predict two peaks in the ratio of 2:1, the reverse of what is actually seen. Accordingly, INDO calculations were performed for dimethylnitrosamine. The highest occupied molecular orbitals and associated eigenvalues (eV), may be described as: N2(pz) lone pair (-11.56); 0 2 lone pair, N1 lone pair N-Nu (-12.08); 0 2 lone pair, N lone pair, C-Nu (-16.55). The N-Oa orbital is the fifth in the sequence a t -18.50 eV. The only significant difference from the simple interpretation is that the 0 and N lone pairs are mixed, different combinations accounting for the two "lone-pair bands". The first two orbital energies are close together and are well separated from the third. The calculated values, even when 2-3 eV have been subtracted, to allow for the systematic error made in the calculations, are not in particularly good agreement with the actual IP's, assuming these to be the negative of the eigenvalues (Koopmans' theorum). There are two possible ways of reconciling the deductions with the experimental observations. First, it would be assumed that N1 and 0 lone-pair orbitals are almost degenerate and have higher I P than the N2 lone-pair orbital; i.e., the electron effects noted above have a much greater effect than assumed. Second, and more probably, there is a considerable reorganization of the molecular ion consequent upon ionization. This is likely because the assumed planarity is strongly dependent upon the electronic configuration. Extensive rearrangement would invalidate Koopmans' theorum and hence all the deductions made above. The bands above 11 eV are broad and have the character associated with alkyl groupings mixed in with appropriate u C-N, C-0, and N-0 combinations. There are differences from compound to compound and the total intensity relative to the nitroso bands increases with molecular weight.
CONCLUSIONS Figure 5. UP spectra of acetone and methanol and ethanol. (Note improvement in intensity at higher IP compared with spectra obtained on the PS15 (7), a consequence of using a fixed energy ana-
lyzer) with areas in the ratio 1:2 associated with the nitroso group in the region 8.5-9.6 eV. I t is assumed that the nitrogen and oxygen atoms are coplanar, in the x-y plane. The nitrogen atom adjacent to the oxygen is labeled N1 and the other Nz. The simplest interpretation of the three bands in the spectrum a t lowest I P would be that they resulted from ionization from the three "lone-pair'' orbitals on N1(pxy),Nz(p,), and O(pxy).The nitrogen peaks would be a t ca. 9 eV and almost degenerate; the inductive effect of the alkyl groups adjacent to Nz and the electron-withdrawing effect of the oxygen on N would 1082
ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
The GLC functions well as an inlet system for a photoelectron spectrometer. I t is especially valuable for compounds of dubious purity and quantitative studies. Its drawbacks are the time taken to establish satisfactory conditions of separation and the possibilities of sample decomposition on the column and/or interfacial system. I t could be used with more advantage and under far more favorable operating conditions if the sensitivity of the spectrometer were improved. The P E spectrum can usefully complement a low resolution mass spectrum for the purposes of identification, particularly where isomers are concerned, and there are interesting if rather esoteric possibilities in using the spectrometer in conjunction with an element sensitive GC detector (e.g., for P, N, S). The most serious limitations are sensitivity and cost.
P E S AS SELECTIVE G C DETECTOR
t
MpJ&
FI D
Pen1 O H
10 3 4 5 6 7 8 9 10 11 12 13 m i n
-
20 30 40 50 CONCEN "/e
( b ) 11.1 eV
PES
1.P 11.1 eV
t
12'0
2 10.0 2 8.0 U
(L
2
6.0
U
40 2.0
t C
I
CONCEN."/o --+ PES
1 P 10.6 eV
( c ) 15.75eV
12.0 n
E 10.0 U
5
8.0
[L
U y
6.0
U
E PES
I P 15.76eV
3 4 5 6 7 8 9
loll
12 13 m i n
Figure 6. Comparison of FID and spectrometer as a GC detector at different settings of the spectrometer Amounts of components, 1: MeOH, 0.4; EtoH: 0.3; PrOH; 0.15: BuOH, 0.1; PentOH, 0.05. Detector settings: (a) FID; (b) 11.1 eV; (c) 10.6 eV; (d) 15.76 eV
In this feasibility study we used the apparatus available to explore the potential of the combination. We conclude with a few notes as to how the spectrometer sensitivity and hence the overall performance of the GC-PES combination could be improved. (a) Reduction of Helium Flow into the Target Chamber. This would permit a greater proportion of the sample to be admitted to the target chamber with an improvement of sensitivity by an order of magnitude. The newly designed target chamber ( 4 ) has greatly reduced he-
4.0
2.0 10 20
-
30 GO 50 CON CE N ."10 PLOT OF THE PEAKS AREAS OF THE P E RESPONSE VS CONCENTRATION O F THE MIXTURE OF THE FIRST FIVE ALCOHOLS. MAGNETIC FIELD WAS ADJUSTED TO 10-4,ll.l AND 15.75 eV.
Figure 7. Photoelectron spectrometer as selective GC detector. Plot of peak areas of PE response vs. concentration in mixtures of alcohols 1 @I of mixture injected. Spectrometer setting, eV: (a) 10.6; ( b ) 11.1; ( c ) 15.75
lium pressure in the target chamber. A molecular separator would also achieve the same result. (b) Improved Design of the Spectrometer. The newly designed lamp of the Perkin-Elmer PS18 has led to an increased intensity of signal by about an order of magnitude, and might be adapted to the system described here. Similar improvement could be made by using a smaller electron orbit, Le., increasing the luminosity at the expense of resolution (9, 10). The use of an electrostatic hemispherical analyzer with improved vacuum and lamp design has brought ANALYTICAL CHEMISTRY, VOL. 48, NO. 7, JUNE 1976
1083
about ca. 1000-fold improvement ( I I ) , but a t considerable cost. (c) Data Processing. We have recently added a microcomputer with signal averaging facilities and a simple spectral stripping and integrating programs. The spectrometer scan has been reduced to 30 ms. Thus, fluctuations in sample pressure are insignificant and spectral purity can be checked easily by comparison of spectra obtained at different times. By using the data more efficiently, it should enable the sample size to be reduced by an order of magnitude. (This is also, of course, a valuable addition to the direct-insertion heated inlet system.) In our future work we shall use the GC-PES combination when warranted and take advantage of the greater sensitivity and ease of quantitative measurement provided by the microcomputer and the newly designed target chamber. ACKNOWLEDGMENT We are grateful t o the Lyons Central Laboratories for the loan of the gas chromatograph.
LITERATURE CITED (1) D. Betteridge, A. D. Baker, P. Bye, S.K. Hasanuddin, N. R. Kemp, D. I. Rees. M. S. Stevens. M. Thompson, and B. J. Wright, 2.Anal. Chern., 263, 286 (1973). (2)D. Betteridge. Analyst (London), 99, 994 (1974). (3)D. Betteridge, A. D. Baker, P. Bye, S. K. Hasanuddin, N. R. Kemp. and M. Thompson, J. Electron Spectrosc., 4, 163 (1974). (4) P. Bye and D. Betteridge. J. Electron Spectrosc., 7 , 355 (1975). (5)D. Betteridge, M. Thompson, A. D. Baker, and N. R. Kemp, Anal. Chem., 44,2005 (1972). (6) D. Ambrose, "Gas Chromatography". 2nd ed, Butterworths, London, 1961,p 159. (7)A. D. Baker, D. Betteridge. N. R. Kemp. and R. E. Kirby, Anal. Chern.,
44, 2005 (1972). ( 8 ) H. Lempka, private communication, April 13,1975. (9) N. R. Kemp, P h D Thesis, University of Wales, 1971. (10) D. W. Turner and D. P. May, J. Chem. Phys., 45,471 (1966). (11) D. Jones and D. Betteridge, unpublished studies, July 1975.
RECEIVEDfor review December 10, 1975. Accepted January 12, 1976. Grants for instrumentation were provided by the SRC and ARC.
Determination of Trace Aluminum in Urine by Neutron Activation Analysis A. J. Blotcky,' D. Hobson, J. A. Leffler, E. P. Rack, and R. R. Recker General Medical Research, Veterans Administration Hospital, Omaha, Neb. 68 105; Department of Chemistry, University of Nebraska, Lincoln, Neb. 68588; and Department of Medicine, Creighton University School of Medicine, Omaha, Neb. 68 102
Trace aluminum determination in biological material is difficult because of the very low levels generally present and the fact that significant quantities of aluminum are present in all reagents and the laboratory environment. A cation-exchange chromatography procedure is outlined for the simple and quantitative determination of trace amounts of aluminum (Le., 20.05 pg Al/ml urine) in biological material, employing 28AI neutron activation analysis. The procedure, utilizing a lowpower nuclear reactor (-3.1 X 10" n/cm2-s), consistsof a) wet digestion of the biologlcal material, b) cation-exchange chromatography employing 1 M nitric acid to remove the major radiocontaminants sodium, chloride, silicate, and phosphate ions, and c) irradiation and radioassay of the aluminum contained in the resin. The aluminum concentration in normal urine samples varied from
