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Determination of the isomers of sorbic acid by gas chromatography
253
SOTES
CE-IROM.
5470
Determination
of the
isomers
of sorbic
acid
by gas chromatography
Sorbic acid (2,4-hexadienoic acid) can exist in four possible steric configurations. The commercially available acid, which is used as a food preservative, leas the trnns-tmm configuration (I). Several worliers1-4 11ave developed spectrophoto
CH3,
y
,C=C\
,H
,c=c,
H
CO2 H
H
trans
H
H
H, H c-c’
H
,c=c;
( ~~3) -tram
m
(r,d I- C;S ( %P, It
H
CHs' C;S
bans
- trans I
‘C=CI
CH3, H ,c=c. ,a2H ,c=c H H ‘H
COzH
(0)
/ CH3
CO,
‘WC H’ ‘H
H
cis - cis lx
metric methods of determining sorbic acicl content in foods. 19.0Yu5, using UV spectrophotonietry, clcterminecl o.oo5°/0 sorbic acid with an accuracy of o.4o/o. Gas-liquid chromatograpl~y U--H and paper chromatography 0 llave also been succesful in isolating and determining sorbic acid content. FIo~vever, metllocls have not been described for the determination of tile individual isomers of sorbic acid. A gas clwon~atographic metllocl has been clevelopzd for the quantitntivc cletermination of the sorbic acid isomers. The proceclure involves the formation of the volatile trimetliylsilyl (TMS) esters of the acicls.
The gas cliron~atogral~l~ that was used was a Varian Aerograpli Model xSG0 wit11 thermal conductivity cletectors. The recorcler was an Aerograph Model 20. The mass spectra were obtained 0~1 a Consoliclatecl Electrodynamics Corporation Model 21-104 rapid scanning mass spectrometer. The gas cliromatograph-mass spectrometer (CC-MS) coupling was carried out using the metering valve techniquel”. COZUWUn& cow~?‘tio~~~.The chro~~~ato~rapl~ic column was r/4 in, Q.D. by I-) ft. aluininum tubing containing IG% OV- 210 on acid-wasllecl, DMCS-treated, 45-Go mesh Clironiosorl~ TV. ,Before use, the colunin was conclitionecl overiiiglit at 230’. The injection port awl cletector temperatures were 230°. The oven temperature was 150~ and was prograinn~ecl, after 7.5 inin, to 230 o at a rate of Io”/min. The helium flow rate was 40 ml/min. sL?~~L&OU~ ;h~ocedw. Weigh I00 mg of sampIe and 15 nig of sorbitol (internal standarcl) into a reaction vial. Add 0.5 ml of pyridine ancl heat until complete dissolution. Aclcl 1 nil of Iiesariletli~tlclisila~a~ie (I-IMDS) ancl 0.5 ml of trinietl~ylchlorosi-
254
NOTES
lane (TMCS). Allow to react with stirring for IO min at room temperature. IO min on steam bath. Centrifuge and inject 8 ~1 into gas chromatograpll. Xt?sacl?ts and
Heat
for
discussio~z
A typical gas chromatogram obtained for a rnixture of the TMS derivatives of the isomers of sorbic acid is shown in Fig. 1. The reagents used for the silylation procedure were completely eluted from the gas chromatograph after 3 min. Next followed three peaks which, by GC-MS, NMR and IR, were identified. as the cis (y,b)-tm~s (c@), tram (y,b)-cis (oz,p) and tmns-tmlzs isomers of sorbic acid-TMS. The TRIS derivative of the internal standard, sorbitol, eluted after 22 min.
5
L I
5
Time
20
CA%1
.
25
I. GLC separation of the TMS clcrivativcs of the isomers of sorbic acid. (I) TMCS. (2) HILIDS. (3) pyridine, (4) lrans(y,d)-cis (a,P)sorbic acid, (5) cis @,a)-harts (fx,P, sorbic acid, (G) fvms-baws sorbic acid, (7) sorbitol, Fig.
A response factor* of 1.13, with a relative standard derivation of I,S~I&, was found for a sorbic acid (tmlzs-tm?ts) to sorbitol ratio of I :I. The response and the precision were found to be independent of the concentration ratio. However, it is expec-
l
j.
Response
factor
=
mm
mea sorbitol/g sorbitol sorbic acid (Ivans-tmlzs)/g sorbic
ChrO9l~atO@‘., 60 (X971) 253-2_=,;5
acid
__-..
“w-1.
_.
.__..
.~
NOTES
255
ted that the precision of the method will be poorer when a sample contains the four sorbic acid isomers. That would be due to the imperfect separation of the isomers.
I.
D.
&‘~ELNICIC
2
D.
MELNICK
T-L LUCKMANN, 1700d h!t?S., 20 (195-j) G.&g. I-I.LUCKMANN, Food Rss., 19 (Jg54) zo. 3 G. k\tDI~l
Received
AND
AND
April
1’.
1’.
zoth,
rg7r J. CJwo~togr~.,
60 (197’)
=53-2.55
SOTES
CE-IROM.
5470
Determination
of the
isomers
of sorbic
acid
by gas chromatography
Sorbic acid (2,4-hexadienoic acid) can exist in four possible steric configurations. The commercially available acid, which is used as a food preservative, leas the trnns-tmm configuration (I). Several worliers1-4 11ave developed spectrophoto
CH3,
y
,C=C\
,H
,c=c,
H
CO2 H
H
trans
H
H
H, H c-c’
H
,c=c;
( ~~3) -tram
m
(r,d I- C;S ( %P, It
H
CHs' C;S
bans
- trans I
‘C=CI
CH3, H ,c=c. ,a2H ,c=c H H ‘H
COzH
(0)
/ CH3
CO,
‘WC H’ ‘H
H
cis - cis lx
metric methods of determining sorbic acicl content in foods. 19.0Yu5, using UV spectrophotonietry, clcterminecl o.oo5°/0 sorbic acid with an accuracy of o.4o/o. Gas-liquid chromatograpl~y U--H and paper chromatography 0 llave also been succesful in isolating and determining sorbic acid content. FIo~vever, metllocls have not been described for the determination of tile individual isomers of sorbic acid. A gas clwon~atographic metllocl has been clevelopzd for the quantitntivc cletermination of the sorbic acid isomers. The proceclure involves the formation of the volatile trimetliylsilyl (TMS) esters of the acicls.
The gas cliron~atogral~l~ that was used was a Varian Aerograpli Model xSG0 wit11 thermal conductivity cletectors. The recorcler was an Aerograph Model 20. The mass spectra were obtained 0~1 a Consoliclatecl Electrodynamics Corporation Model 21-104 rapid scanning mass spectrometer. The gas cliromatograph-mass spectrometer (CC-MS) coupling was carried out using the metering valve techniquel”. COZUWUn& cow~?‘tio~~~.The chro~~~ato~rapl~ic column was r/4 in, Q.D. by I-) ft. aluininum tubing containing IG% OV- 210 on acid-wasllecl, DMCS-treated, 45-Go mesh Clironiosorl~ TV. ,Before use, the colunin was conclitionecl overiiiglit at 230’. The injection port awl cletector temperatures were 230°. The oven temperature was 150~ and was prograinn~ecl, after 7.5 inin, to 230 o at a rate of Io”/min. The helium flow rate was 40 ml/min. sL?~~L&OU~ ;h~ocedw. Weigh I00 mg of sampIe and 15 nig of sorbitol (internal standarcl) into a reaction vial. Add 0.5 ml of pyridine ancl heat until complete dissolution. Aclcl 1 nil of Iiesariletli~tlclisila~a~ie (I-IMDS) ancl 0.5 ml of trinietl~ylchlorosi-
254
NOTES
lane (TMCS). Allow to react with stirring for IO min at room temperature. IO min on steam bath. Centrifuge and inject 8 ~1 into gas chromatograpll. Xt?sacl?ts and
Heat
for
discussio~z
A typical gas chromatogram obtained for a rnixture of the TMS derivatives of the isomers of sorbic acid is shown in Fig. 1. The reagents used for the silylation procedure were completely eluted from the gas chromatograph after 3 min. Next followed three peaks which, by GC-MS, NMR and IR, were identified. as the cis (y,b)-tm~s (c@), tram (y,b)-cis (oz,p) and tmns-tmlzs isomers of sorbic acid-TMS. The TRIS derivative of the internal standard, sorbitol, eluted after 22 min.
5
L I
5
Time
20
CA%1
.
25
I. GLC separation of the TMS clcrivativcs of the isomers of sorbic acid. (I) TMCS. (2) HILIDS. (3) pyridine, (4) lrans(y,d)-cis (a,P)sorbic acid, (5) cis @,a)-harts (fx,P, sorbic acid, (G) fvms-baws sorbic acid, (7) sorbitol, Fig.
A response factor* of 1.13, with a relative standard derivation of I,S~I&, was found for a sorbic acid (tmlzs-tm?ts) to sorbitol ratio of I :I. The response and the precision were found to be independent of the concentration ratio. However, it is expec-
l
j.
Response
factor
=
mm
mea sorbitol/g sorbitol sorbic acid (Ivans-tmlzs)/g sorbic
ChrO9l~atO@‘., 60 (X971) 253-2_=,;5
acid
__-..
“w-1.
_.
.__..
.~
NOTES
255
ted that the precision of the method will be poorer when a sample contains the four sorbic acid isomers. That would be due to the imperfect separation of the isomers.
I.
D.
&‘~ELNICIC
2
D.
MELNICK
T-L LUCKMANN, 1700d h!t?S., 20 (195-j) G.&g. I-I.LUCKMANN, Food Rss., 19 (Jg54) zo. 3 G. k\tDI~l
Received
AND
AND
April
1’.
1’.
zoth,
rg7r J. CJwo~togr~.,
60 (197’)
=53-2.55