Estimation of the Carbon/Hydrogen Ratio of Organic Compounds by Laser Pyrolysis Plasma Stoichiometry Analysis Nicholas E. Vanderborgh and William T. Ristau Department of Chemistry, University of New Mexico, Albuquerque, N. M. 87106
The laser degradation of materials results in two distinct types of product ( 1 , 2). Intense laser pulses pump a fraction of the sample under investigation into a plasma, the laser-induced plume. These atomic fragments quench into a series of low-molecular-weight molecules. Other products emanate from the edges and bottom of the laserproduced crater. These sections of the sample experience less vigorous pyrolysis and products often result which are typical of less intense thermal methods. For instance, laser-induced degradation of polystyrene includes styrene as one of the products with a series of low-molecularweight fragments, benzene, etc. These must result from thermal blow-off near the site of the laser pyrolysis. Thus, laser degradation yields two types of products-plume quenching products and thermal blow-off products. Both of these molecular distributions yield information about the sample. Thermal blow-off products give a molecular fragmentation pattern while the plume quenching products give insight into the atomic composition of the molecule if the chemical composition of the plume parallels that of the sample under study. Consider now the plume quenching products. The approach taken here is similar to that reported previously (3). This discussion, for simplicity, is limited to compounds containing only hydrogen and carbon. If the laser energy is entirely thermal-i. e., no photochemical considerations are important-then free-energy considerations allow an estimation of the expected product distribution if the temperature a t which the products form is known. That is, the most stable product ensemble a t any particular temperature can be calculated. This approach has been reported previously and was shown to be of good utility if the product distribution is assumed to form a t a temperature close to 3500 "K( 3 ) . The overall laser degradation process may be represented as
with the cooling process ( 4 ) . This is a reasonable assumption when the plasma is contained within a small diameter quartz chamber under pressures higher than atomospheric typically found on the inlet side of a gas chromatograph. The atomic C and H fragments can quench into several different product distributions
where a sample is rapidly heated by a pulsed laser zap (process A), atomic fragments are pumped into a plasma and a t the termination of the pulse (pulse width of the order of milliseconds) products rapidly form a t some temperature T Q , (process B), and then this ensemble cools to ambient temperature (process C) and is subsequently separated and detected with gas chromatographic instrumentation. Process B can be visualized as an isothermal one. This assumes that the quenching reactions are rapid compared
Statistical mechanical considerations lead to calculations as shown in Figure 1. Here the standard free energy of formation of these various hydrocarbons is presented as a function of temperature. (Actually AG"/T is plotted us. temperature; this is a convenient format.) Since previous work has indicated that TQ must be in the vicinity of 3500 "K, the product distribution a t that temperature should also be the most probable for these C-H mixtures. Figure 1 predicts that the free energy of gaseous carbon is very high while that of CzHz starts with a large positive AG"/T a t lower temperatures and then rapidly decreases as the temperature is increased. From these data. it appears that ethylene would be the most probable product. Next, the distribution would be rich in methane. These data do not, however, consider the stoichiometry of the plasma. For aromatic molecules, there clearly must be a hydrogen deficiency for the production of ethylene and an even greater deficiency for the production of methane. Therefore material balance dictates that, for aromatic compounds, the most probable product is acetylene. Thus, the principal degradation product from a compound with a hydrogen:carbon mole ratio of 1:l should be acetylene even though this compound is not the one with the lowest free energy of formation. On the other extreme, for aliphatic compounds where the hydrogen: carbon ratio is in the vicinity of 2:1, these results would predict that the principal product should be ethylene. Thus, the product distribution should shift from an ensemble rich in acetylene to one rich in ethylene as the sample is varied from an aromatic to an aliphatic one. A careful quantification of the components emanating from the plume quenching products should give a measure of the aromaticity of the sample under study. This communication reports experimental data which support that conclusion. Wiley and Veeravagu ( 5 ) showed the utility of this idea. They, however, tried to include high-molecular-weight fragments, say four carbon compounds, as products from
(1) W. T. Ristau and N . E. Vanderborgh, Anal. Chern., 43, 702 (1971) (2) W. T. Ristau, Ph.D. Dksertation. The University of New Mexico, AIbuquerque 1971 (3) W . T. Ristau and N. E. Vanderborgh, Ana/. Chern., 44, 359 (1972).
(4) 0. F. Folmer, Jr.. and L. V. Azarraga, J. Chrornatogr. Sci., 7, 665 (1969). ( 5 ) R. H.Wiley and P. Veeravagu, J , Phys. Chern.. 72, 2417 (1968).
[sample]
l a ~ e rpulse 4
[plasma]
TQ B
[product ensemble]
cool
[products]
C
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 8, JULY 1973
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Table I. Plume Quenching Products from the Laser Pyrolysis of Hydrocarbonsa Compound
Empirical formula
Benzo[d,e,f ] phenanthrene o-Terphenyl m-Terphenyl p-Terphenyl Naphthalene Biphenyl 1,2-DiphenyIethan Polystyrene Durene Paraffin a
H/C
A
Methane, YO
Ethane ethylene, %
0.63 0.78
1.oo 1 .oo
0.80 0.83
1 .oo 1 .oo 0.71
1.6 f 0.8 0.8 f 0.5 1.4 f 0.5 0.2 f 0.1
1 . 2 f 0.8 0.8 f 0.5 1.5 f 0.5 0.2 f 0.1 1.3 f 0.5 1.0 f 0.5 2.5 f 0.8
1 .oo 1 .oo 1.40 2.00
1.3 f 0.5 1 .O f 0.5 4.6 f 1.8 5.7 f 1.5 12.8 f 2.0 17.2 f 1.5
0.63 0.14 0.00
9.0 f 1.5 8.4 f 1.8 39.1 f 2.0
Acetylene,
YO
97.2 f 1.5 98.4 f 1 .O 97.2 f 1.0 99.6 f 0.2 97.4 f 1.0 98.1 f 1.0 92.9 f 2.5 85.3 f 3.0 78.7 f 2.5 43.8 f 3 . 0
Errors are sample standard deviation based on a minimum of four separate analyses.
I
I O
-40
-20
0
-
20
2
.
j 5
40
I
1.0
r
:
HYDRffiEN:CARBON 60
2.0
Figure 3. Plasma stoichiometric analysis Curve A-% methane in low-molecular-weight gases vs. hydrogen/carbon ratio. Curve B-% acetylene in low-molecular-weight gases
80
100
TDIPERATURE
(K x
Figure 1. Standard free energy of formation for a series of lowmolecular-weight hydrocarbons For convenience AGOIT is shown as a function of temperature
0
2
4
MIMJTES
Figure 2. Flame ionization detector response for low-molecularweight hydrocarbons Peak identification: ( A ) methane, (B) combination of ethylene and ethane, (C) acetylene. Separation achieved on 0.5-m molecular sieve 5 A column
the degradation process. These must result from thermal blow-off of solid sample degradation and should not be considered as part of the plasma products. Biscar also 1530
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
showed the proportionality between the per cent of acetylene coming from an oil shale sample and the "Fischer assay," the oil content of the shale (6).
EXPERIMENTAL E x p e r i m e n t a l details for these experiments h a v e been r e p o r t e d previously (1, 2, 3, 7). B r i e f l y , t h e sample was c o n t a i n e d in a q u a r t z t u b e connected o n - l i n e t o t h e i n l e t p o r t of a gas c h r o m a t o graph. A p u l s e d ruby laser was used t o t h e r m a l l y degrade t h e sample d e p o s i t i n g 2 J ( a p p r o x i m a t e l y ) w i i h a pulse width of 1msec.
RESULTS AND DISCUSSION Figure 2 shows a typical analysis of the plasma quenching products. This separation was made on a molecular sieve 5A 0.5-m column operated a t 25 "C. High-molecular fragments were trapped on this column so only the volatile plume quenching products are visible. One other product results from this degradation process, molecular carbon. Almost every shot produced a black deposit on the inside of the sample holder. This had the chemical properties of elemental carbon. Hydrogen, a possible product, could not be detected with the FID. Experimental data are shown in Table I for a wide variety of hydrocarbons. Here the compounds are listed by the hydrogen/carbon ratio of the sample. It can be seen immediately that two types of compounds are evident, those which give a high-acetylene yield and those which give a low-acetylene percentage. Inspection shows that all those compounds that have a H/C ratio below 0.8 give predominately acetylene, just as predicted from consideration of (6) J. P. Biscar, J. Chrornatogr., 56, 348 (1971).
(7) W . T. Ristau and N. E. Vanderborgh, Anal. Chern., 42, 1848 (1970).
the plume quenching process. This result was also noted by Wiley and Veeravagu ( 5 ) . These data can be treated in several ways. Figure 3 shows the information plotted as the C / H ratio us. the per cent of methane in the product mixture and again as the per cent of acetylene. The column used did not permit separation of ethane from ethylene. Another way that these data may be treated is to consider the extent of aromaticity in these test compounds. Here we define a function A , the number of aromatic hydrogens/total number/hydrogens per molecule, this value A is shown graphically us. the per cent of methane in Figure 4. These data clearly show that the composition of the products coming from the plume-quenching process reflects the elemental composition of the sample. This happens even though a residual carbon film is left on the walls of the degradation chamber. Either the carbon is but a small fraction of the total amount of the quenching products or the percentage of carbon formed increases as the H/C ratio decreases. From visual inspection, we suspect that the second eventuality is occurring although no successful attempts to quantitate this conclusion can be reported. The data in Figure 3 show a realistic estimation of the error in these analyses. These errors result from two causes, nonreproducibility of the degradation event and from the graphical data analysis used to obtain these numbers. Experimental evidence indicating that degradation by laser radiation can be quite precise; one specific series obtained with electronic integration shows a standard deviation of less than *2.0'% (2). Therefore, we conclude that much of the error found in these values results from the method of data interpretation.
Ln Y
3
S
I
I
0.00
I
I d 20
AROHATIC!:I
Figure 4.
1
I
1
L
0.40
0.60
1 0.80
I
I
1 .oo
FUUCTIOII, A
Plasma stoichiometric analysis
YO
methane yield piotted vs. the aromaticity function, defined in text. Compound identification: ( A ) paraffin, (E) durene, (C) polystyrene, (D) 1,2-diphenyIethane, (E) naphthalene, terphenyl, benzo[d,e,f]phenanthrene
These results show that this method does offer a useful route for the characterization of the type of molecule under study. Certainly the case of hydrocarbons is the most elementary. Similar estimations on compounds containing carbon, hydrogen, and oxygen must be done. These data should be, in theory, useful for an estimation of the oxygen composition. (For instance the amount of carbon monoxide might be monitored.) Such studies are now under way. We term this technique Plasma Stoichiometric Analysis and feel that it may serve as a useful technique for the routine and rapid characterization of materials. Received for review October 20, 1972. Accepted February 5 , 1973.
Use of Newer Amino Group Reagents for the Detection and Determination of Kanamycin David M. Benjamin, John J. McCormack, and Dieter W. Gump Departments of Pharmaco'ogy and Medicine, University of Vermont College of Medicine, Buriington. Vt. 05407
Kanamycin (KM) is a polybasic aminoglycoside antibiotic which possesses clinically useful activity against a variety of pathogenic bacteria ( I ) . KM, however, is capable of producing serious adverse effects such as renal damage and auditory and vestibular dysfunction; the potential toxicity of KM, particularly in patients with preexisting renal impairment makes it highly desirable to monitor levels of this antibiotic during therapy (2). A number of empirical guides for controlling KM therapy have been used and these guides have become increasingly sophisticated ( 3 ) , but it is generally agreed that direct measurements of drug levels in serum would serve more usefully to control KM therapy. Many commonly used assays for K M in clinical laboratories have been based on inhibition of the growth of sensitive bacteria by this antibiotic; until recently, such as-
says required up to 24 hr for completion and this time factor was a definite disadvantage with such biological assays. Recently Sabath ( 4 ) and his colleagues have described a rapid microbiological method for the determination of KM but the results obtained with this method may be difficult to interpret when a patient is receiving additional antimicrobial agents concurrently with KM. The limitations of conventional microbiological assays for K M have prompted several investigations which have as their objective the development of a clinically useful method for the analysis of aminoglycoside antibiotics in biological fluids. D. H. Smith and his colleagues ( 5 ) have described very recently an elegant and rapid enzymatic assay for the aminoglycoside antibiotic gentamicin in clinical samples and this method, in principle, is applicable for the determination of kanamycin. The present report
(1) P. A . Bunn, Med. Clin. N . Amer.. 54, 1245 (1970) (2) S. M . Finegold, A n n . N . Y.Acad. Sci., 132, 942 (1966). (3) G . E. Mawer. B R . Knowles, S. B. Lucas, R . M . Stiriand, and J. A . Tooth, Lancet, 1 , 12 (1972).
(4) L. D. Sabath. J. I . Casey: P. A. Ruch, L. L. Stumpf, and M . Finland, J . Lab. Clin. M e d . , 78, 437 (1971) (5) D. H. Smith, E. Van Otto, and A . L. Smith, New Eng. J . Med.. 286, 583 (1972) A N A L Y T I C A L C H E M I S T R Y , VOL. 45,
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