Rapid Determination of Volatiles in Alcoholic Beverages by Precolumn-Backflush Gas Chromatography1) Gunnar Hagman and Johan Roeraade" Royal Institute of Technology, Department of Analytical Chemistry, S-10044 Stockholm, Sweden

Key Words: Gas chromatography, GC Precolumn-backflush Alcoholic beverages Selectivity change caused by water Direct analysis

Summary A method for quantitative determination of volatiles such as ethyl acetate, acetaldehyde, and methanol in alcoholic bever- ages is described. A short packed precolumn coupled to a capillary column via an effluent splitter is employed. The less volatile part of the sample is retained on the precolumn and back-flushed using a 10-port rotary valve, while the volatiles are separated on the capillary column.The advantage of the system is that the analysis time per sample can be reduced to a few minutes. Moreover, the capillary column can be kept at low temperature, since it is not contaminated with material of low volatility.

The water in the sample was found to have a significant influence on the retention of the compounds (as also observed earlier by Gmb und Habich [l]). This effect was exploited to increase the resolution between ethyl acetate and methanol, by injecting a portion of pure water prior to sample injection. Apart from the better separation, the water vapor also has a deacti- vating effect.

The packed precolumn tolerates large amounts of non-volatile material. Even after several hundred direct injections of wine and spirits, the chromatographic performance of the system was still satisfactory.

1 Introduction Volatile compounds like ethyl acetate, acetaldehyde, and metha- nol play an important role in the quality control of alcoholic beverages. Determination methods employing packed columns are frequently utilized 121. In view of the necessary sample throughput in routine product control, a rapid capillary method is desirable,' Unfortunately, most alcoholic beverages also contain compounds of low volatility, which cause an unnecessary length- ening of the analysis time. Moreover, the presence of non-volatile material complicates the chromatography with frequent cleaning of injector inlets, exchange of retention gaps and replacement of capillary columns. In certain cases (e.g., liqueurs with a high sugar content) an off-line sample workup may even be more practical.

Previous workers have utilized conventional split and splitless techniques for the determination of volatiles and higher alcohols. Grob, Jr., et al. [3] employed a Carbowax-400 capillary column for the direct analysis of wines and liqueurs, by frequently changing the glass liner, combined with a removal of the first part of the column. De Nijs and de Zeeuw [41 as well as MacNamara 151 utilized a chemically bonded Carbowax column.

For the determination of the most volatile compounds, it would be advantageous to carry out an on-line prefractionation to time optimize the separation. Commercially available two-dimension- al GC systems could be utilized for this purpose, but automated versions of such systems can become rather complex. In the present paper, a simple backflush system, employing a short packed precolumn, is described.

2 Experimental

2.1 System Description

A schematic view of the precolumn-backflush system is shown in Figure 1. A V4"packed column injector was employed as a heated inlet, kept at 200 "C. The packed precolumn with the 2-port effluent splitter was kept in the GC oven. The effluent splitter consisted of a simple 1/4''-1/i6" Swagelok union with two drilled sideholes, onto which '/16" stainless steel capillary tubes (i.d.: 0.03") were brazed. The 10 port valve (Valco ClOWP) was also mounted inside the GC oven. v 1 - V ~ are variable restIictors

$53 Carrier gas in

I

Presented at the 10th International Symposium on Capillary Chromatog- raphy, Riva del Garda, Italy, 1989.

Figure 1

Schematic view of the precolumn-backflush setup.

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3

d C C

(needle valves, Nupro). In the foreflush mode (where the Valco valve is in the position shown in Figure l ) , the carrier gas pressure and the setting of V2 controls the effluent split ratio as well as the flow rate through the precolumn. In backflush position, the setting of V1 controls the flow rate through the precolumn, while V3 is employed to purge the split line in the foreflush direction.

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2.2 Equipment and Materials

Gas Chromatograph: Varian, model 3700, FID detector (temp: 240 "C). Integrator: Hitachi. D-2000. Capillary column: CP-Wax-

A

6 i I

C

Figure 2

58 CB, 50 m, i.d.: 0.32 mm, df: 0.20 pm. Carrier gas flow: 1.15 ml/min. Split ratio: 1 : 65. Packed precolumn: V4" o.d., 2 mm i.d., length: 12 cm, packed with 5 % Carbowax 1000 on Chromo- sorb W, AW DMCS, 60-80 mesh. Carrier gas: Helium.

3 Results and Discussion

3.1 Considerations Regarding the Precolurnn

An important system requirement is to keep extra band broaden- ing, caused by the packed precolumn, to a low level. A loss of

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li L c Chrornatograrns of a standard solution containing 50 mg/l of acetaldehyde (1), ethyl acetate (2), methanol (3), and 1-propanol(5) in 5 % ethanol (4)lwater. Conditions: Oven temperature 50 "C (isothermal), att. 4 x sample size lpl. (A) Split injection on the capillary column without a precolurnn. (B) Conditions as in A. 2 pI of pure water were injected 2 rnin before the sample injection. (C) Split injection as in A, but with the precolumn installed. (D) Conditions as in C. 2 pi of pure water were injected 2 rnin before the sample injection.

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chromatographic efficiency of no more than 10-15 % can be considered as acceptable. A very short precolumn would be optimal, but a distinct prefractionation would then not be obtained due to an insufficient number of theoretical plates.

In striving to keep the system as simple as possible, initial experiments were carried out with a standard W' injector liner which was filled with packing material and mounted in the hot inlet. However, this system has an insufficient degree of freedom. When kept at low temperature (80-100 "C), unacceptable band broadening was observed as a result of too slow an evaporation of the sample. At higher inlet temperatures, (200-250 "C) band broadening was negligible, but partition coefficients were too low and no useful preseparation was obtained. The problem was solved by keeping an empty part of the precolumn in the hot injection port, while the section with the packing material was kept in the oven at the same temperature (50 "C) as the capillary column. A packed precolumn in a separate oven would have provided the most flexible prefractionation system, but at the expense of more complicated equipment.

The choice of split ratio is critical for several reasons. Since the flow rate through the precolumn is equal to the sum of the flows through the splitter and the capillary column, a low split ratio will lead to a slow sample transport through the precolumn and result in an excessive band broadening. A high split ratio will lead to problems with the detectability of compounds at low concentra- tion levels. Moreover, if the flow through the precolumn becomes very high, the efficiency of the prefractionation will suffer.

In the present design, a split ratio of 1 : 65 was employed. With sample injections of 1 ~ 1 , this provided sufficient sensitivity for a reliable quantitation of methanol, acetaldehyde and ethyl acetate down to 10 mg/l while the loss of theoretical plates for these compounds, compared to chromatography without a precolumn was calculated to be only of the order of 10 % (cf. Figure 2A and 2C).

3.2 Effect of Water on the Separation

As can be observed in Figure 2A, the separation of ethyl acetate and methanol on the capillary column was not completely satisfactory under the conditions chosen. This could be improved, e. g., by choosing a column with amore selective stationary phase. However, the choice of suitable phases is limited. Carbowax 400 would be of interest [31 but this phase has high bleed character- istics and limited long-term stability. In this study we have pursued another route:

On several occasions we observed retention shifts depending on the time lapse between injections and the water content of the samples. This effect must be attributed to a "phase soaking" [6-71 of the stationary phase with water, causing a temporary selectiv- ity change. This has also been pointed out by Grob and Habich [ 1 I . Polar compounds are retarded while non-polar compounds are accelerated. Thus, the effect can be exploited to improve the separation between ethyl acetate and methanol.

When we studied the effect on a system using a capillary column and a regular split injector without precolumn, we noted that the retention shifts were increasing when a sample was repeatedly injected with short intervals. After the first few injections, retention became stable. Obviously, the strongly retarded water from the sample has not yet left the column before the water from a second injection is added. Eventually, a steady state is obtained.

The magnitude of the final retention shifts depends on the water concentration in the sample and the time between the repetitive injections. Of course the split ratio as well as other operating and column parameters (column length, stationary phase and phase ratio, sample size and gas velocity) are also of decisive impor- tance.

Instead of a repeated injection of sample, pure water can be preinjected. The result is shown in Figure 2B. A significantly better resolution between methanol and ethyl acetate is obtained with the prewetted stationary phase (cf. with Figure 2A). Under the present chromatographic conditions, injection of '2 pl of water 2 min before the injection of the sample was found to provide a satisfactory priming of the column. For other operating conditions and/or columns optimal priming parameters have to be experi- mentally re-established.

When the packed precolumn was utilized to backflush compounds eluting after ethanol, it was not possible to evoke retention shifts by repetitive sample injections. Obviously, the water from the sample is backflushed and never reaches the capillary column. In this situation it is very useful to combine the procedure with a pre-injection of water in foreflush mode. The selectivity change can be accurately reproduced, since it is independent of the water content of the sample.

It can also be noted that the water injections have a deactivating effect (cf. the tail of the ethanol peak in Figure 2A vs 2B and 2C vs 2D). Also the shape of the methanol peak is slightly improved, allowing better quantitation. However, for samples with a large ethanol content, the methanol peak broadens due to solvent effects. This has also been observed by Grob, Jr., et aI. 131.

3.3 System Performance

The performance of the system was evaluated for both wines and spirits. Figure 3 shows two chromatograms of Malt Whiskey, where the upper chromatogram was obtained using a capillary column and a normal split injector without a precolumn. and the lower chromatogram with the precolumn-backflush system. The proportions between the compounds eluting before ethanol proved to be identical (confirmed by separate runs with acetone as internal standard), yet a strong cut-off is obtained already after propanol with the prefractionation system. Satisfactory repeat- ability was obtained, as is shown by the results in Table 1. Ca. 300 injections of various wines and spirits (including sweet vermouth wines) did not notably affect the chromatographic performance. After 50 additional injections of Swedish Punch (sugar content > 30 %), the repeatability started to deteriorate. However, this is quickly remedied by replacement of the pre- column.

Table 1

Repeatability of 10 consecutive injections on the precolurnn- backflush system. Conditions as in Fig. 3B.

Mean amount: rng/l (RSD %)

Sample Acetaldehyde Ethyl acetate Methanol

Standard 50 (2.5) 50 (1.6) 50 (1.5) Red wine 41 (5.1) 57 (2.7) 152 (1.8) Whiskey 76 (3.1) 2'26 (3.0) 24 (5.6)

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0 10 20 30min

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Figure 3

Chromatograms of Malt Whiskey. For component identity, see Fig. 2. (A) Split injection on the capillary column without precolumn. Conditions: att. 4 x 2 pI of pure water were injected 2 min before 1pI of sample, oven temperature 50°C for 10 min, then 10°/min to 200°C. (B) Split injection where the precolumn-backflush system is employed. Conditions: As in A but the precolumn was backflushed 5 s after the sample injection.The oven temperature was kept at 50 "C (isothermal).

4 Conclusions 0 Pre-injection of water can be exploited to increase the resolu- tion between the polar and non-polar compounds. This creates the possibility for a further reduction of the analysis time.

0 Rapid quantitative determination of acetaldehyde, ethyl ace- tate, and methanol in alcoholic beverages by capillary GC is possible by direct injection, using a simple packed precolumn-

Alarge number of samples can be analyzed before replacement of the precolumn is necessary.

backflush system. The capillary column is protected from contam- inants and can be kept at a low temperature.

0 The use of an auto-injector should lead to still better repeat- ability. Such work is currently in progress.

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Acknowledgements

We are indebted to the Swedish Wine and Spirits Corporation for financial support-Thanks are due to N. Clarke for reviewing the English.

[31 K. Grob, Jr., H. P. Neukom, and H. Kaderli, HRC & CC 1 (1978) 98.

[4] R. C. M. de Nijs and J . de Zeeuw, HRC & CC 5 (1982) 501

151 K. MacNamara. HRC & CC 7 (1984) 641.

References

111 K. Grob and A. Habich, HRC & CC 6 (1983) 34

121 Official Methods of Analysis (1984) 14th Ed., AOAC. Arlington, VA, USA, Sections 9, 10, & 11.

Journal of High Resolution Chromatography

I61 K. Grob, Jr. and B. Schilling, J. Chromatogr. 259 (1983) 37.

171 K. Grob, Jr and B. SchiUing, J. Chromatogr. 260 (1983) 265

Ms received: May 25, 1989 Accepted: November 3, 1989

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