position 2 (turned W), the solvent reservoir is isolated from both the helium and vent lines. Valve N permits purging of the line from the solvent reservoir (position 3) and allows the use of a stop-flow injection technique by controlling the flow of the mobile phase to the column (positions 1 and 2). Valves M and P are not essential, since vacuum and solvent inlet lines can be attached directly to unions H and K ; however, these valves facilitate replenishing and degassing of the mobile phase. Residual mobile phase is removed from the reservoir by closing valve N (position 2), venting valve G (position 3) and drawing the solvent into the vacuum trap (7‘)(valve M , position 3). The reservoir is refilled by closing valve G (position 2) and drawing the new mobile phase from an external container into the reservoir through the filling tube (valve M , position 1; valve P, position 1). Most mobile phases are readily degassed by closing valve P (position 2) and applying low vacuum (ca. 100 m m ) to the reservoir for approximately 30 seconds (valve M , position 1). The solvent line from reservoir J to valve N is purged by opening valve G (position 1) and slowly opening valve N to position 3 to expel a few milliliters of solvent into a waste container.
Table I. Solvent Delivery System: Modes of Operation
Position of valves (Figure 1 ) Mode
a
Vent reservoir Empty reservoir Fill reservoir Degas solvent Purge Inject
G 3Q 3 2 2 1 1
M 2 3
Run
1
N
P
1 1 3Q
2 2 2 2 3Q
2 2 1 2 2
2
2
2
1
2 2
Open valve slowly.
To introduce a sample directly onto the column, the mobile phase flow is stopped by closing valve N (position 2), the sample is injected through the high pressure “septumless” injection port with a conventional microliter syringe, and the carrier flow re-established by reopening valve N (position 1). These functions are summarized in Table I. Received for review November 30, 1972. Accepted January 24, 1973.
Improved Double Detection Gas Chromatograph-Mass Spectrometer Interface for the Analysis of Complex Organic Mixtures Fabrizio Bruner, Paolo Ciccioli, and Silvana Zelli Laboratorio lnquinarnento Atrnosferico de/ C.N.R.,c / o lstituto di Chirnica Analitica, Citta Universitaria, 00785 Rorna, Italy
Coupling of gas chromatogrphy and mass spectrometry has been a well established technique for several years, and most of the mass spectrometers commercially available are sold with a standard coupling device to be employed for the analysis of the effluent from a chromatograph. However, the two instruments, once coupled, hardly give their best performance. In particular, the gas chromatographic columns do not work in optimum analytical conditions, mainly where linear gas velocity is concerned. Furthermore, “dead volumes,” due to the gas lines connecting the two instruments, very often affect the chromatographic efficiency. This is particularly true when the mass spectrometer is not originally constructed with the aim of coupling it with a gas chromatograph. In the past few years, several apparatus became available that partially overcome the defects cited above. But in the majority of cases, the instrument is a more highly sophisticated detector for gas chromatography than an instrument for organic mass spectrometry. This is so because sensitivity and resolution often give unsatisfactory results in practical work. Usually, the detection of the effluent from the chromatographic column is made by means of the Total Ion Monitor (TIM) which, to eliminate the high background coming from ionized helium atoms, has to be operated with an ionizing energy of the electron beam around 20 eV. When a spectrum has to be taken, the electron energy is suddenly increased to the usual 70-eV value, and this, provocates the base line going off scale during all the scanning time. In the case of complex mixtures, when many spectra 1002
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6 , MAY 1973
should be taken during the chromatogram, the original trace is completely lost and one has usually to make two chromatograms, the first being used as reference. This is quite time consuming in cases of very complex mixtures. Moreover it may be difficult to repeat the chromatogram twice with exactly the same analytical conditions, especially if temperature programming is used. From the above considerations we thought that double detection FID and TIM might be of great help in routine work, more so than using it occasionally to test the efficiency of the connecting lines. This paper reports the results obtained by modifying the mass spectrometer presently available in our laboratory, an AEI MS12, for better GC-MS operation.
EXPERIMENTAL Preliminary Experiments. The mass spectrometer was originally equipped with a stainless steel capillary tube (0.25-mm i.d., 1.50 m long) with one end soldered to the entrance of a standard Biemann-Watson separator ( I ) , while the other end was connected to the outlet of the chromatographic column through a rubber septum. The line connecting the separator with the source was a glass tube (2-mm i.d., 30 cm long), sealed to t h e proper source reentrance. With this device, poor chromatograms were obtained compared with those carried out with the same column but using the conventional FID. Without changing anything in the lines connecting the GC with the mass spectrometer, a double detection system was set up to look a t the differences between the two chromatograms in the same conditions. The chromatogram shown in Fig(1) J. T. Watson and K. Biernann, Anal. Chem., 36, 1135, (1964).
I '12'26
F.I.D. T.I.M.
+ -
c
. i d
b'
c
I
1
1
2
I
3
4
5
I
I
6
7
I
a
I
1
9
10
time (min)
L
Figure 1. Double trace chromatogram prior to modifications of the coupling system
Column: 2-m, 5% Apiezon Lon Chromosorb W. T = 180 "C; P = 1.5 kg/cm2. Carrier gas: helium, Temperature of the coupling line: 280 "c
ure 1 was obtained using a double channel recorder (Leeds and Northrup Speedomax Model XL 682). The column used was a 2-m long, ys-in. i.d. tube packed with Chromosorb W coated with 5% Apiezon L. A Carlo Erba Model G.I. 450 gas chromatograph was used for this and all other experiments. From this chromatogram, two defects can be seen in the coupling system: first, broadening of the peaks detected by the TIM, not very important for scarcely retained peaks, becomes very large for retained compounds; second, the retention time of retained peaks increases with respect to the FID signal. Raising the temperature of the gas lines much above the boiling point of the highest molecular weight compound did not improve the results a t all. Cutting the metal capillary to a shorter length did not help either. All these facts showed the presence of dead volumes and that the connecting lines were acting as a second column with bad results for chromatographic efficiency. Note that the two pens are shifted relative to each other 3 mm on the time axis. Modifications. The changes introduced in the gas lines have been the following: Elimination of every metal part except the three-way connection a t the end of the column, and substitution of the metal capillary with a glass rod (0.5-mm i.d., 5-mm 0.d.) to connect the column to the separator. The necessary pressure drop is realized by inserting a t the beginning of this line a glass leak of the same outer diameter as the glass rod, but with an internal diameter of 0.07 mm. The length of the leak was 40 mm. The total length of the glass rod from the column to the separator was 40 cm. This, because of the position of the source in the mass spectrometer, is the shortest way to connect the column outlet to the instrument. The glass rod is bent a little less than 90" to make the connection. The separator exit, after its standard leak, is connected t o the source reentrance by a glass rod of the same size as before but 5 cm long. The part of the chromatograph usually employed for the inlet splitter of capillary columns injection is used to make the connection with the mass spectrometer. In this way the injection port, FID, and the first part of the line connecting the GC to the M S are placed very close to each other and heated a t the same temperature using the original heating system given by the manufacturer. The 3-way manifold and the capillary connecting the FID to the column are kept to a n intermediate temperature between those of the column and gas lines.
k
1 0 THE SEPARA TOR
Figure 2. Schematic of the double detector system
The column outlet is connected through the brass 3-way manifold to a stainless steel capillary (0.25-mm i.d.) and to the glass rod. The stainless steel capillary serves to divert part of the column effluent to the FID. ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
1003
/I c16H34
FIE TIh 5
0
I
10
,
I
,
I
25
15
,
,
I
50
45
80
-time
a5
90
(min)
Figure 3. Double trace chromatogram with the modified coupling system
Experimental conditions as in Figure 1, but T = 130 "C
r511
>
FI D
I
I
,
.
.
,
1
,
0
,
,
.
.
.
,
I
,
,
1
15
,
,
s
,
,
I
,
,
,
,
,
,
,
I
,
30'
1
45
,
,
!
,
, , I
60
,
75
,
,
.
>
,
,
II
I
Figure 4. Double trace chromatogram of an artificial tea aroma.
Column: 5-m, 2-mm i.d.. s.s., Sterling FT coated with 1.5% FFAP. Temperature: isothermal initial 50 "C for 5 min, programmed at 1 "C/min u p to 215
Table I . Flow Distribution at Various Gas Velocities A . Sample: n-hexane. Column temperature: 90°C
Inlet Linear gas YO sample pressure (He), velocity, to kg/cm2 cmfsec FID 5.7 10.8 3.5 6.7 12.4 5.4 7.0 13.2 6.5 8.0 14.8 9.0 9.0 16.0 11.0 10.0 18.0 14.0 11.0 19.0 16.0 B. Sample: n-decane. Column temperature: 190°C
6.5 7.0 8.0 9.0 10.0 11.0
1004
9.2 10.0 1x.5 12.4 14.5 15.4
1.5 1.7 5.0 6.5 10.8 11.5
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
For the present work with the column used, the length of the capillary was 6 cm. With this length, a column flow rate between 40 and 120 mljmin can be used. By changing the length of the capillary, this range can be very easily and properly shifted. Rotating parts, sooner or later sources of leaks, have been avoided in this way. The final part of the glass rod is inserted in a metal guide connected to the 3-way manifold. A neoprene "0" ring ensures a vacuum proof connection. The apparatus is schematically shown in Figure 2. The glass rod, separator, and source lines were heated by means of a nickel-chrom wire protected with the well known asbestos strip. The temperature controls furnished by AEI for their original coupling system were used t o maintain the various line parts at the desired temperature. If the mass spectrometer has to be operated without the gas chromatograph, the 3-way manifold is disconnected from the glass rod and substituted with a rubber septum. This, together with maintaining the rotary pump of the separator working, makes the pressure in the source a very low value, about 2 x 10-7 Torr. Testing the Operating Conditions. The first test made was to elute the same mixture of hydrocarbons of Figure 1, and the results are reported in Figure 3. Here the column temperature was 130 "C. This chromatogram shows a decisive improvement because the two detectors yield the signal of all the peaks a t the same time, and no peak broadening can be observed on the TIM
signal even for strongly retained compounds. The number of theoretical plates was the same calculated on both traces. The gas line temperatures were the same for the chromatogram of Figures 1 and 3, and kept a t a value of 280 'C. Further experiments were made to check the relative amounts of sample going to the FID and separator, respectively, a t different linear gas velocities. These were made by injecting into the column a certain amount of a compound with a reasonable retention time. By measuring the peak areas obtained when all the amount injected was sent to the FID and those corresponding t o the same amount of sample eluted with the MS line open, we obtained the percentage going to the FID for each linear gas velocity. Particular attention was paid to the working conditions of the detector so that they would be the same for all the measurements. The results, obtained using the 5-cm column described below, are collected in Table I. The changes in the percentages to FID and TIM are significant but not dramatic with the change in the inlet pressure and linear gas velocity. Not more than 10% of the sample is wasted in any way for the FID detection in normal operating conditions. No significant changes are observed by changing the column temperature in a range of 100 'C. Chromatographic Column. The column used for the analysis shown in Figure 4, (5-m, 2-mm i.d. stainless steel), was of the gas-liquid-solid chromatography type (2-4). Sterling FT, a graphitized carbon black with a surface area of about 15 m2/gram, obtained from Elettrocarbonium, Narni, Italy, sieved a t 60-80 mesh range, and was coated with 1.5% FFAP. a n acidic liauid phase.
RESULTS AND DISCUSSION In Figure 4, a double trace (FID and TIM) chromatogram is reported. The sample was a standard mixture representing a tea aroma which was supposed to consist of 23 major components. The resolving power of the column allowed us to distinguish 88 compounds. The properties of this type of column and their application to the analysis of aromas and essential oils will be the object of further work. Here, we would like only to point out that this'type of column gives very large separation factors for compounds of similar structure and that, due to the strong adsorption of the liquid phase on the support, these columns give a negligible bleeding even if the temperature is increased very close to the limit of the liquid phase. The chromatogram of Figure 4 is reported to show the efficiency of the coupling system, that allows operating the column in its analytical conditions. Sixteen ~l of the pure sample were injected into the column. As an example, in Figure 5 , the mass spectrum obtained from the peak indicated in Figure 4 (a-ionone) is reported. CYIonone is well separated from its @ isomer. The drawing is the exact reproduction of the spectrum displayed on the UV light sensitive paper of the oscillographic recorder (not normalized). The relative intensities
"C; isothermal at 215 "C to the end. P = 7 kg/cm2. Carrier gas: helium
ia
M
60
7o
m
90
100
tX,
IZa
130
1 0
160
Ibo
170
180
190
mle
Figure 5.
Unnormalized mass spectrum of a-ionone as it appears on the oscillographic paper
Scanning rate: 8 sec/dec multiplier;voltage 50% (2) A. Di Corcia. D. Fritz, and F. Bruner, Anal. Chem., 42, 1500 (1970). (3) F. Bruner. P. Ciccioli, and A. Di Corcia, Anal. Chem., 44, 894 ( 1972). (4) A. Di Corcia and F. Bruner, J. Chromatogr., 62,462 (1971).
of the peaks in the spectrum are the same as those obtained by taking the spectrum directly from the mass ANALYTICAL CHEMISTRY, VOL. 45, NO. 6 , MAY 1973
1005
spectrometer, as reported in the literature ( 5 ) . The spectrometric resolution is about 1500 (10% valley), the total amount of the peak corresponding to a-ionone is about 0.7 pg (0.02% of the sample). In the scanning conditions described in the caption of Figure 5, this corresponds to about 10 nanograms of a-ionone to get the spectrum. Nevertheless, the spectrum is still very intense and the trace of the intermediate galvanometer is sufficient to unequivocally identify the compound. The spectrum of Figure 5 has been taken in routine work. Chromatographic efficiency and mass spectrometric sensitivity and resolution are both nearly as good as possible. The number of theoretical plates is about 6500. The possibility of using a wide range of flow rates is helpful for obtaining mass spectra of minor components in complex organic mixtures. In fact, columns of high loading capacity can be exploited. This allows the major peaks not to be overloaded and the minor ones to be present in sufficient amount to take interpretable mass spectra. The (5) K. Biernann, "Mass Spectrometry," Organic Chemical Applications,
McGraw-Hill, New York, N.Y., 1962.
main advantages of this system are that one can get mass spectra from a very complex chromatogram without the need of doing it at least twice, and that a very large range of linear gas velocities can be used without problems for the operating conditions of the mass spectrometer. With the apparatus here described, it is possible to eliminate the TIM recording of the chromatogram using for this scope the FID trace. By working with the repetitive scanning device operating continuously during the chromatogram and watching the output of the mass spectrometer on an oscilloscope, one has only to check the intensity of the spectrum, and when it reaches the right value, push the button to record the spectrum of a given chromatographic peak on the UV light oscillograph to get it displayed on paper. The results of the present work show also that many defects of coupling MS and GC, attributed to the separator, are actually dependent on the inefficiency of the gas lines of the interface. Received for review September 19, 1972. Accepted January 29, 1973.
Carbon Paste Electrode with a Wide Anodic Potential Range Jorgen Lindquist Department of Analytical Chemistry, University of Uppsala, S-75727 Uppsala 7 , Sweden
Increasing interest in electroanalysis and study of electrode processes at potentials more positive than can be reached by mercury has resulted in a great variety of electrode materials. The useful potential range and the magnitude and reproducibility of residual currents in different media are the most important parameters governing the selection of an electrode material. The need for a reproducible, easily renewable stationary electrode for the anodic region is great because the surface film formation and adsorption of reactants, intermediates, or products are all known to influence the reproducibility of peak current measurements. For this reason the carbon paste electrode ( I , 2 ) seems to be the most practical electrode for analytical work in the anodic region and it has also become widely used. The anodic limit for this electrode is about +1.3 volt (us. SCE) in acid aqueous media ( I ) , which is about the same as for glassy carbon, pyrolytic graphite, and wax impregnated electrodes (3-5). However, microcrystalline carbon products adsorb oxygen in different forms. This can be removed in different ways depending on which surface compound is of concern. One form can be removed as oxides a t very high tempera(1) C. Olson and R. N. Adarns. Anal. Chim. Acta, 22, 582 (1960). (2) R. N. Adarns, "Electrochemistry at Solid Electrodes," Marcel
Dekker, New York. N . Y . , 1969. (3) H . E. Zittei and F. J . Miller, Anal. Chem., 17, 200 (1965). (4) J. F. Aider, B. Fleet, and P. 0. Kane, J. Electroanal. Chem., 30, 427 (1971). (5) J. H . Morris and J . M. Schernpf, Anal. Chem., 31, 286 (1 959),
1006
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
tures; another can be removed as oxides by evacuation a t ordinary temperatures, and other forms can be pumped off as oxygen, etc. Such surface compounds might have an influence on the properties of carbon powder when used as a material for electrodes. Activated carbon and carbon black have a marked quinone-hydroquinone character ( 6 ) , which should affect the background current. Activated carbon is not suitable for preparing electrodes but graphite powder, which is mostly used, probably contains similar compounds but to a lesser extent. By removing the oxygen in a vacuum at high temperature and then blocking the surface of the carbon against further adsorption of oxygen, it was possible to obtain an improved carbon paste electrode.
EXPERIMENTAL Ten grams of graphite powder (Ringsdorff-Werke RW-A) was placed in a quartz tube 45 x 1.7 cm. The tube, closed a t one end, was bent to a right angle and connected to a 50-ml Erlenmeyer flask with a side arm for connection to a pump. In the Erlenmeyer flask were 5.6 grams of ceresin wax and 1.4 grams of paraffin oil (Merck's paraffin liquid for spectroscopy). The part of the quartz tube containing the graphite powder was placed in a tube furnace, the pressure reduced to less than lo-* mm Hg, and the temperature increased to about 1000 "C. At the same time, the Erlenmeyer flask was heated for a while to melt (6) V. A. Garten and D. E. Weiss, Aust. J. Chem., 8, 68 (1955)
