The analysis of volatile organic peroxides by gas-liquid chromatography

The analysis of volatile organic peroxides by gas-liquid chromatography

The Analysis of Volatile Organic Peroxides by Gas-Liquid Chromatography C. F. CULLIS and E. FERSHT Department of Chemical Engineering and Chemical Tec...

702KB Sizes 35 Downloads 123 Views

The Analysis of Volatile Organic Peroxides by Gas-Liquid Chromatography C. F. CULLIS and E. FERSHT Department of Chemical Engineering and Chemical Technology, Imperial College of Science and Technology, London (Received November 19621 Organic peroxides of moderate stability and volatility can be satisfactorily identified and estimated by gas-liquid chromatography, provided that suitably elevated column temperatures and reduced column pressures are employed. The effects of column length and of the nature of the stationary phase have also been investigated in an attempt to and concentration establish the optimum conditions for separation and analysis of various alkyl peroxides Hydrogen peroxide suffered extensive decomposition on all the derived from isopentane. columns used. chromatographic

have been based on the fact that these compounds are frequently thermally unstable and of low volatility. However, since H. R. WILLIAMS and H. S. MOSHER~ have shown that most primary and secondary alkyl hydroperoxides are stable up to at least 9O”C, no appreciable decomposition would necessarily be expected to take place on gas chromatography columns operated below this temperature. In the same way, although organic peroxides generally have high boiling points, they nevertheless have sufficiently large vapour pressures to enable the minute quantities required to be carried on to the column in a stream of carrier gas. This paper describes an attempt to develop a method for the analysis of the peroxides which might be formed as primary products of the combustion of a given hydrocarbon. isoPentane was chosen as the fuel since it is the simplest hydrocarbon in which initial attack can take place at four different positions and thus lead to the formation of one tertiary, one secondary and two primary alkyl monohydroperoxides The object of this work was to find the optimum conditions under which these four peroxides, as well as di-tert-amyl peroxide and hydrogen peroxide (which are also possible initial products

THE development of gas chromatography has enormously facilitated analysis of the complex mixtures of products which may be obtained on combustion of hydrocarbons and other organic compounds. Volatile aldehydes, ketones, alcohols, acids, paraffins, olefins and epoxides can now be analysed rapidly and easily. The method has, however, hitherto been relatively little used for the detection and estimation of organic peroxides, which may play an important role in certain combustion systems. M. H. ABRAHAM, A. G. DAVIES, D. R. LLEWELLYN and E. M. THAIN’ have shown that the fairly stable tert-butyl and tert-amyl hydroperoxides can readily be identified and determined by gas chromatography and M. H. ABRAHAMand A. G. DAVIES later succeeded in analysing the somewhat more reactive n-butyl and isobutyl hydroperoxides in this way. H. S. MOSHER and his co-worker? have used gas chromatography to estimate ally1 hydroperoxide and K. 0. KUTSCHKE and R. T. B. RYES have successfully separated a number of unstable low molecular weight peroxides including methyl, ethyl and isopropyl hydroperoxides and diethyl peroxide. Previous objections to the use of gas chromatography for the analysis of organic peroxides 185

C.

lS6

F. Cullis and E. Fersht

of the combustion of isopentane) could be analysed by gas-liquid chromatography. The application of the method devised to the study of the mechanism of the combustion of isopentane will be described in a later paper. Experimental Materials

Hydrogen peroxide was kindly supplied by Laporte Chemicals Limited; it was free from inhibitors and was 99.6 to 99.7 per cent pure, the only impurity being water. tert-Amy1 hydroperoxide and di-tert-amyl peroxide were both prepared by treating tert-amyl alcohol with 70% w/w sulphuric acid and then allowing the resulting tert-amyl hydrogen sulphate to react in situ with the appropriate amo’unt of 30% w/v hydrogen peroxide6. Z-Methyl-but-l-hydroperoxide and 3-methyl-but-1-hydroperoxide were prepared by causing the corresponding alcohols to react with methane sulphonyl chloride in the presence of pyridine? and then allowing the methane sulphonic esters formed to react with alkaline hydrogen peroxide in aqueous methanolic solutiorP. Attempts to prepare 3-methylbutZ-hydroperoxide from the corresponding methane sulphonate were unsuccessful, although several other secondary alkyl hydraperoxides have been prepared in this ways. This peroxide was, however, eventually prepared by autoxidation of the appropriate Grignard reagent*. Since its preparation has not apparently hitherto been described, the procedure adopted is outlined below. Preparation

of 3-methyl-but-2-hydroperoxide

The Grignard reagent was prepared in vessel A (Figztre 1) and the autoxidation was carried out in vessel B. 10 ml of dry ether, 0.9 g of dry magnesium and a crystal of iodine were placed in A, 20 ml of dry ether were placed in B and the whole system was flushed with nitrogen. 5 g of dry 3-methyl-but-2-bromide were placed in the hypodermic syringe, H, and were slowly added to A through the serum cap, S. The mixture was then stirred magnetically and the bromide was run in at a rate such that the reaction proceeded smoothly. When all the hromide had been added and bubbling had

Figuve I. Apparatus for preparation of 2-methyl-but-2-hydroperoxide

ceased, sufficient dry ether was added to give a solution cu. 0.5 N with respect to the reagent and the contents of A were forced under nitrogen pressure into the drop-funnel, C. Meanwhile, a stream of dry oxygen, sufficient to agitate the ether, was passed through B, which was cooled to -80°C. After the gas had been flowing for 5 min, addition of the Grignard reagent was started and was continued over a period of two the oxygen flow being maintained hours, throughout. The contents of B, which now included appreciable amounts of a white precipitate, were allowed to reach room temperature and were then poured on to crushed ice. The solution was made faintly acid with 6~ hydrochloric acid and was extracted with ether (6 x 50 ml). The ethereal extracts were combined and the ether was evaporated off under reduced pressure, leaving ca. 1 ml of a liquid residue which was carefully fractionated, the fraction boiling at 42.1-43.O”C/l mm being collected. Gas chromatographic apparatus was of conventional (a) The flow system--This

June 1963

The analysis of volatile organic peroxides by gas-liquid chromatography

design. The flow rate of the carrier gas was controlled by a vapour pressure controller and was measured by a capillary flowmeter. The pressures on the inlet and outlet sides of the column were indicated by mercury manometers and could be adjusted by needle valves. A vacuum pump at the outlet end of the flow line was used to maintain sub-atmospheric column pressures. (b) Cawier gas-Experiments with nitrogen, hydrogen and helium as carrier gases showed little difference in the efficiency of separation of peroxides on a diglycerol column, although the last two gases are believed to enhance axial diffusion in the column. However, nitrogen was used in all subsequent work on account of its ready availability and since the explosion hazard makes it undesirable to pump hydrogen into the atmosphere. (c) Partition coZzc,mm-The partition columns were Pyrex glass U-tubes, of 0.5 cm i.d., and were fitted with ground glass sockets at each end. They were mounted vertically in a widebore Pyrex glass tube which was surrounded by a constant temperature vapour jacket. 60-100 mesh, acid-washed Embacel was used throughout as the solid support and MS 550 silicone oil, and polyethylene glycol dinonyl phthalate, (mol. wt = 400) were used as stationary phases. (d) Introduction of samples-With columns operated at sub-atmospheric pressure, introduction of samples by means of a microsyringe generally results in more than the required amount of material for analysis being sucked on to the column, It was therefore decided to use a crushing technique, although this normally involves stopping the flow of carrier gas between successive analyses. To overcome this difficulty, a glass inlet system was devised in which as many as six samples could be passed through the column without interruption . of the nitrogen flow. The aparatus used is shown diagrammatically in Figure 2. Nitrogen enters at A and can be directed, by opening the appropriate

tap, T, into

any one of six radially distributed lines each of which leads to a crushing chamber, C, containing the sample tube, G, and heated to 80°C. At

187

any given time, this tube, which is filled with an appropriate ,weight (usually 2 to 5 mg) of the material to be analysed, may be crushed whereupon the sample vapour is carried on to

Carrier gas

\

I column I

Figure 2. Schematic diagram of apparatus for introduction of samples for gas chromatographic analysis

the column. After the analysis of one sample is complete, the nitrogen flow is simply directed through the chamber containing the next sample to be analysed. The necessity of having six inlets means that there is a considerable deadspace immediately above the top of the column. This tends to decrease column efficiency but the volume was reduced as much as possible by packing with glass beads of 1 mm diameter. The sample tubes were thin-walled glass spirals which rested on a bed of glass wool. A serum bottle cap, S, was placed over the open

C. F. Cullis and E. Fersht

188

end of each crushing chamber and through each cap passed a stainless steel rod, R. This rod had sealed to its end a glass disc of ca. 0.5 cm diameter, manipulation of which was used to crush the sample tubes. (e) The detector-The katharometer detector used contained detecting and compensating platinum filaments mounted in channels drilled in an aluminium block. The two filaments formed opposing arms of a Wheatstone bridge circuit and carried a current of 120 mA. Any off-balance current resulting from the appearance in the detecting channel of a substance other than the carrier gas was fed into a microvoltmeter amplifier capable of giving an output 10, 100 or 1000 times the input. The output passed first through a smoothing system and then through a 50 ohm potentiometric resistance, 20 ohms of which were used to develop a voltage which was applied to a 10 mV potentiometric recorder. Procedure

Systematic measurements were made of the effects of the following variables on the chromatographic behaviour of the peroxides studied: column temperature, column pressure, column length, concentration of stationary phase, nature of stationary phase. Control‘experiments were also carried out to determine whether any of the peroxides suffered decomposition on the various columns used. of the resulting These involved : (a) examination chromatograms for peaks other than that due to the initial peroxide, (b) comparison of the quantity of iodine liberated from acidified potassium iodide by the effluent gases from the column with that liberated directly by an amount of peroxide equivalent to that introduced on to the column. Results

and Discussion

(1) Influence of column telnperatwe out on all the Experiments were carried peroxides studied with column temperatures of 56”, 76” and 100°C but under otherwise identical Some typical chromatograms for conditions. tert-amyl hydroperoxide are shown in Figure 3.

Vol.

7

At 56”C, a single broad peak was obtained while at 76”C, the peak was quite sharp. At lOO”C, a large negative peak was obtained and this was followed by several positive peaks. It is clear from this observation and from measurements of iodine liberation that all the peroxides undergo

Figure 3. Influence of temperature on chvomatogram peak shape. Column inlet pressure, 43 cm Hg; column length, 1 ft; column packing, 5 per cent by flow rate. 27 weight of silicone oil on Embacel; ml I min

considerable decomposition at this temperature. IJnder most conditions, however, all the peroxides (except hydrogen peroxide-see section 6) pass through a column at 76°C substantially unchanged. On 4 ft long silicone oil columns and with inlet pressures close to atmospheric, slight decomposition was sometimes observed but under the conditions used during most of this work, the peroxides passing through columns maintained at 76°C suffered no appreciable breakdown. This latter column temperature was therefore used for all subsequent measurements . (2)

ZnfEtience of column presswe

When the column inlet and outlet pressures are varied, the gas flow rate changes and so, in consequence, does the retention time. If the retention time is measured under certain pressure conditions, its value under another set of pressure conditions may readily be calculateds. Figures 4(a) and 4(b) show the variation of

June

The analysis of volatile organic peroxides

1963

by gas-liquid

189

chromatography

experimental and calculated retention times with inlet pressure for three peroxides, the observed and theoretical values being arbmitrarilyassumed

?? ‘\

$- 2-.(a)

6 ,

r.-

I

‘.

?? ? ?? ? .I_

?? L_ ---_ --L__ I

--.-____

--_

-m-

30

40

50

60

--? d-

161 (b) 30

I

I

I

I

60

70

40

Column

50 inlet

pressure,

Figure 5. Influence of column inlet pressure on chromatogram peak shape for 3-methyl-but-l-hydroperoxide. Column temperature, 76°C; column length, 1 ft; column packing, 20 per cent by weight of silicone oil on Embacel

fragments of the sample tube after chromatograms had been obtained at inlet pressures near atmospheric.

cm Hg

Figure 4. Influence of column inlet pressure on retention time. -, experimental curves; - - - - -, calculated curves. (a) column temperature, 76’C; column length, 1 ft; column packing. 20 per cent by weight of silicone oil on Embacel. 0 tert-Amy1 hydroperoxide; ?? 3-methyl-but-I-hydroperoxide. (b) column temperature, 76°C; column length, 2 ft; column packing, 10 per cent by weight of dinonyl phthalate on Embacel. @ 2-methyl-but-l-hydroperoxide to be the same at the lowest inlet pressure investigated. On this basis, the experimental retention times at higher column inlet pressures are always greater than the calculated values, although the difference is less marked when a longer column is used [Figure 4(b)]. The chromatogram peaks show broadening at low and high column inlet pressures and are comparatively sharp at intermediate pressures (Figure 5) whereas sharpening would normally be expected to increase continuously with pressure owing to the resulting enhanced flow rate. The anomalous behaviour observed at high inlet pressures is probably due to the slow evaporation of the peroxides on to the column under these conditions; this results in abnormally broad peaks and abnormally long retention times. In support of this view, tiny droplets of peroxide were found to b’e still adhering to the glass

(3) InfEzGenceof column length Observations were made of the effect of column length on the retention times and the shapes of the chromatogram peaks for several peroxides. Some typical chromatograms are shown in Figure 6. The influence on column efficiency is shown in Figure 7 where the number of theoretical plates [calculated as (4 d/~)~ when d is the distance of the peak apex from the point of injection and w is the peak base width] is plotted against column length for tert-amyl hydroperoxide and 2-methyl-but-1-hydroperoxide. Although the efficiency of separation increases with column length, so in general does the broadening of the peaks. A compromise length must therefore be employed which will give both good separation and a peak shape which allows easy measurement of the area. Thus, for example, with column packings containing 20 per cent by weight of silicone oil, the optimum column length is 4 ft, whereas for packings containing 10 per cent by weight of dinonyl phthalate, a 2 ft column is in general most satisfactory. (4)

InlfEuence of concentration phase

An increase

of stationary

in the percentage

of stationary

190

Vol.

C. F. Cullis and E. Fersht

7

- Amy1 roperoxide

2- Methyl-but-I-hydroperoxide

Figure 6. Influence of column length on chromatogram peak shape and retention time. Column temperature, 76OC: column inlet pressure, 47 cm Hg; column packing, 20 per cent by weight of silicone oil on Embacel; flow rate, 22 mllmin

phase has an effect similar to that of column length, and both column efficiency and peak broadening are increased. Some typical chromatograms obtained on silicone oil columns are shown in Figure 8 and in this case 20 per cent of the stationary phase appears to give the most satisfactory results. (5) Influence of natwe of stationary Phase Studies were made of the retention times and chromatogram peak shapes obtained on columns containing polyethylene glycol, silicone oil and dinonyl phthalate respectively as the stationary phase. The results in Figure 9 show clearly that the order of retentivity is: dinonyl phthalate (SIL) > polyethylene (DNP) > silicone oil glycol (PEG). This is also the order of decreasing column efficiency, the numbers of theoretical plates being cu. 180, 60 and 16 respectively. With polyethylene glycol columns, only a slight increase in column efficiency was obtained by increasing the percentage of stationary

phase and at became very thus not a separation of

the same time the peak broadening marked. Polyethylene glycol is convenient stationary phase for the peroxides studied in this work.

(6) Behaviour of hydrogen peroxide Hydrogen peroxide vapour was passed throug!r the different chromatographic columns under a wide variety of operating conditions. In every case a broad negative peak was obtained but little or no peroxide could be detected iodometrically in the effluent gases, even when a trap containing acidified potassium iodide was placed immediately adjacent to the exit from the column. It appears therefore that hydrogen peroxide decomposes more or less completely under all conditions used and that it cannot therefore be detected or analysed by gas-liquid chromatography. (7) General conclusions Systematic variation of the factors

described

in

June 1963

The analysis of volatile

organic peroxides

200

by gas-liquid

chromatography

191

5,020

30 % Silicone oil 5 x)20

30

3.8 65

12.7Retention time 5”7.49S

19.0

3-Methyl-but-I-hydroperoxidt

Figure 8. Influence of concentration of stationary peak shape and phase (silicone oil) on chromatogram retention time. Column temperature, 76°C; column inlet pressure, 47 cm Hg; column length, 2 ft: pow rate, 21.3 ml/min

# t 50 -

I

I

1

0

Column

I

I

2

3

4

Length,ft

Influence of column length on column Figure 7. Column temperature, 76’C; column inlet efjiciency. pressure, 47 cm Hg; column packing, 20 per cent by pow rate, 20 weight of silicone oil on Embacel; mllmin. C tert-Amy1 hydroperoxide; ?? a-methylbut-1-hydroperoxide

sections

1 to 5 shows that the most favourable conditions for separation of the peroxides investigated involve the use of a column temperature of 76”C, a column inlet pressure of

45 to 55 cm of mercury (and an outlet pressure of 18 to 22 cm of mercury) and either a 4 ft. column containing 20 per cent by weight of silicone oil (I) or a 2 ft column containing 10 per cent by weight of dinonyl phthalate (II). The retention times for the various organic peroxides on the two types of column are given in Table I. It will be seen that it is not possible to separate the two isomeric primary hydroperoxides from one another but a good separation can be obtained between these two compounds (which appear as a single peak on chromatograms) and 3-methyl-but-2-hydroperoxide, tert-amyl hydroperoxide and di-tert-amyl

Stationary phase: PE%,DNP

PEG SIL

DNP

PEG SIL

DNP

Relent Ion tlme(mln):l.8

2.6

17.1

4.0

28.4

2 2 6.8

5.3

74

3-Methyl&t-I-hydroperoxide

Figure 9. Inpuence of nature of stationary phase on chvomatogram peak shape and retention time. Column temperature, 76’C; column inlet pressure, 47 cm Hg; column length, 2 ft; column packing, 10 per cent by weight of stationary phase; flow rate, 21.2 ml/m&

C. F. Cullis and E. Fersht

192 Table

1.

Retention

times of some organic

Substance

I

Retention

peroxides time II

Di-tert-amyl peroxide tert-Amy1 hydroperoxide 3.Methyl-but-2-hydroperoxide 2-Methyl-but-I-hydropevoxide 3-Methyl-but-1-hydrofieroxide

_I_

5.3 11.5 14.0 17.3 17.3

6.9 17.3 20.8 28.4 28.4

Vol.

7

(Figwe 10). In all cases, a linear relationship exists between the weight of peroxide introduced on to the column and the chromatogram peak area, and this makes quantitative analytical data easy to obtain. The present work thus shows that under carefully selected conditions, certain organic peroxides (but not hydrogen peroxides) can be satisfactorily identified and estimated by gas-liquid chromatography.

peroxide

This work was sponsored in the early stages by the United States Air Force under contract AF61 (sI4)-929.

References

1 ABRAHAM, D.

I1

R.

and

M. H., DAVIES, THAIN, E. M.

A.

G.,

LLEWELLYN,

.4nalyt.

chim.

Acta,

J.

them.

1957, 17,499

A

2 ABRAHAM, M.

H.

and

DAVIES,

A.

G.

sot. 1959, 429

s DYKSTRA, S. and MOSHER, H. S. J. Amer. &em. sot. 1957,79,3474 WURSTER, C. F., DURHAM, L. J. and MOSHER, H. S. J. Amer. them. Sot. 1958, 80, 327

4 KUTSCHKE, K. communication 5 WILLIAMS, H.

Ftgure 10. Typical chromatograms for mixtures of pevoxides. Colzcmn temperature, 76°C; column inlet I. Column pressure, 47 cm Hg; flow rate, 22 mljmin. length, 4 ft; column packing, 20 per cent by wezght of silicone oil on Embacel. II. Column length,, 2 ft: column packing, IO per cent by wezght of drnonyl A, Di-tert-amyl peroxide; phthalate on Embacel. tert-Amy1 hydroperoxide; C. 3-Methyl-but-2B, hydroperoxide; D, Z-Methyl-but-I-hydropevoxzdef 3-methyl-but-I-hydvoperoxide

them.

0. R.

and

RYE,

R.

T.

and MOSHER, H.

B. S.

SOC. 1954, 76, 2984

6 MILAS, N. A. and SURGESOR, D. them. Sot. 1946, 68,643

M.

Private J. Amer. J.

Amer.

7 SEKERA, V. C. and MARVEL, C. S. sot. 1933,55,345

J. Amer.

them.

s. A.

J. Amer.

them.

* 2c~~$,

(;:;a;~33ucKLen,

’ , 9 KEULEMANS, A.

I. M. Reinhold : New York,

Gas Chromatography, 1959

p 150.