An evaluation of TLC systems for opiate analysis

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ABSTRACT
Introduction
TLC systems

Details

Author: V. RAJANANDA, N.K. NAIR, V. NAVARATNAM
Pages: 35 to 47
Creation Date: 1985/01/01

An evaluation of TLC systems for opiate analysis

V. RAJANANDA
N.K. NAIR
V. NAVARATNAM
National Drug Research Center, a UN/WHO Collaborating Center for Research and Training, Universiti Sains Malaysia, Minden, Penang, Malaysia

ABSTRACT

The authors selected 38 thin-layer-chromatography (TLC) systems described in the available literature published over the last 10 years and evaluated those systems with respect to their suitability for detection and identification of opiates in urine, opium and heroin, as well as adulterants in heroin. A total of 14 substances: 8 opiates (morphine, 6-monoacetylmorphine, diacetylmorphine, codeine, acetylcodeine, noscapine, papaverine and thebaine) and 6 adulterants (ephedrine, quinine, methadone, caffeine, cocaine and strychnine) were used as test samples for this research. Using laboratory-coated plates and pre-coated plates, 15 and 13 TLC systems, respectively, were found to be able to detect and identify morphine and codeine in urine without interference from the remaining 12 substances. For the detection of opiates in opium samples as well as opiates and adulterants in illicit heroin samples the TLC system: chloroform-n-hexane-triethylamine (9 : 9 : 4) which was developed by the National Drug Research Centre, Penang, Malaysia, was found to be most suitable on both laboratory-coated and pre-coated plates. In addition, the following two systems, one on laboratory-coated plates - hexane-chloroform-diethylamine (50 : 30 : 7) - and the other on pre-coated plates - benzene-dioxane-ethanol-ammonia (50 : 40 : 5 : 5 ; T-7) - were also found to be among most suitable TLC systems for the analysis of opiates in opium samples. The article also presents the relative cost of each of the 38 evaluated TLC systems.

Introduction

Various techniques have been developed and used for the detection and identification of drugs of abuse including colour tests; spectroscopy such as infra-red, ultraviolet (UV) and mass spectroscopy; immunoassay techniques such as radio-immunoassay and enzyme multiplied immunoassay; and chromatography such as thin-layer-chromatography (TLC), high-performance liquid chromatography, gas chromatography (GC) and gas chromatography/mass spectrometry. Of these, chromatographic techniques are the most widely used for drug detection and identification in both solid and fluid samples. TLC is commonly used in routine screening procedures, and subsequent confirmatory analysis usually involves a more sensitive and sophisticated chromatographic technique such as GC. In less well-equipped laboratories, TLC may serve for confirmatory analysis of samples screened by a colour test (spot test) where the identification of a drug class is sought rather than a specific substance. The wide use of TLC for drug screening procedures can be attributed to the low cost of equipment required, including the cost of TLC tanks and UV-viewer; the low cost of material used, such as solvents and silica gel; the low operational cost as this technique requires a minimum level of technical skill; the high output with as many as 15 samples per plate; and its reasonable sensitivity and accuracy.

An illicit heroin sample may contain a number of natural and synthetic opiate alkaloids as well as adulterants and diluents. The major chemical components that are likely to occur in illicit heroin samples are natural alkaloids, such as morphine, codeine, noscapine, papaverine and thebaine; synthetic alkaloids, such as 6-monoacetylmorphine (MAM), diacetylmorphine (DAM) and acetylcodeine; adulterants, such as caffeine, cocaine, methadone, strychnine, quinine and ephedrine; and diluents, such as sugars, talc, starch and dyes.

This article summarizes the results of research to assess the capability of each of the 38 TLC systems for the detection and identification of opiates and adulterants listed above as well as to determine whether any of the evaluated systems can be used for the analysis of opiates in urine, opium and heroin, the three types of samples that are most commonly required to be analysed.

The opiates most often found in urine (of opiate users), opium and heroin samples, as well as adulterants in heroin samples, are listed below.

Urine. Glucuronide conjugate of codeine and morphine are the main forms of opiates found in urine after the use of opiates. Thus, the analysis of codeine and morphine in urine by TLC techniques is essential for the detection and analysis of opiates in this body fluid.

Opium. In addition to morphine and codeine, opium contains smaller amounts of other opiate alkaloids, mainly noscapine, thebaine and papaverine, and the analysis of opiates in opium samples requires TLC systems capable of resolving these five substances.

Illicit heroin. Heroin manufactured by the incomplete acetylation process of opium or crude morphine contains: morphine, MAM, DAM, codeine, acetylcodeine, noscapine, papaverine and thebaine. The most commonly found adulterants are ephedrine, quinine, methadone, strychnine, caffeine and cocaine. TLC systems for the analysis of illicit heroin samples should be capable of resolving not only major opiates but also the common adulterants listed above.

TLC systems

A number of TLC systems have been described in professional literature. For the purpose of this assessment the authors have selected from the available publications 38 TLC systems [ 1 - 16] , which are presented in table 1.1

Table 1

TLC systems for the analysis of opiates selected from the available literature

Code No. of TLC System

TLC System

Reference

1
Chloroform-n-hexane-triethylamine (9:9:4)
[ 1]
2
Diethylether-acetone-diethylamine (85:8:7)
[ 2]
3
Ethylacetate-cyclohexane-dioxane-methanol-water-ammonia (50:50:10:10:0.5:1.5)
[ 3]
4
Ethylacetate-cyclohexane-dioxane-methanol-water-ammonia (50:50:10:10:1.5:0.5)
[ 3]
5
Ethylacetate-cyclohexane-ammonia-methanol-water (70:15:2:8:0.5)
[ 3]
6
Ethylacetate-cyclohexane-methanol-water (56:40:0.8:0.4)
[ 3]
7
Ethylacetate-cyclohexane-methanol-ammonia (70:15:10:5)
[ 3]
8
Ethylacetate-cyclohexane-ammonia (50:40:0.1)
[ 3]
9
Methanol-ammonia (100:1.5:T-1)
[ 4]
10
Benzene-dioxane-ethanol-ammonia (50:40:5:5:T-7)
[ 4]
11
Ethanol-aceticacid-water (6:3:1:T-8)
[ 4]
12
Dioxane-chloroform-ethylacetate-ammonia (60:25:10:5)
[ 5]
13
Ethylacetate-benzene-acetonitrile-ammonia (50:30:15:5)
[ 5]
14
Ethylacetate-benzene-acetonitrile-ammonia (25:30:40:5)
[ 5]
15
Acetonitrile-chloroform-ethylacetate-ammonia (40:30:25:5)
[ 5]
16
Ethylacetate-n-butanol-ammonia (60:35:5)
[ 5]
17
Ethylacetate-methanol-ammonia (85:10:5)
[ 6]
18
Chloroform-cyclohexane-diethylamine (8:10:3)
[ 7]
19
Chloroform-ethanol-acetone-ammonia (20:20:5:1)
[ 7]
20
Chloroform-methanol-diethylamine (16:3:1)
[ 7]
21
Toluene-acetone-ethanol-ammonia (20:20:3:1)
[ 8]
22
Chloroform-diethylether-methanol-ammonia (75:25:5:1)
[ 8]
23
Ethylacetate-isopropanol-ammonia (40:30:3)
[ 10]
24
Chloroform-methanol (9:1)
[ 11]
25
n-butylether-diethylether-diethylamine (45:45:10)
[ 11]
26
Benzene-ethylacetate-methanol-ammonia (80:20:6.5:0.1)
[ 12]
27
n-hexane-ethylacetate-ammonia (60:40:0.1)
[ 12]
28
n-butanol-aceticacid-water (35:3:10)
[ 12]
29
n-butanol-n-butylether-ammonia (25:70:2)
[ 12]
30
Methanol-n-butanol-benzene-water (60:15:10:15)
[ 12]
31
Ethanol-pyridine-dioxane-water (50:20:25:5)
[ 13]
32
t-amylaIcohol-n-butylether-water (80:7:13)
[ 13]
33
n-hexane-chloroform-diethylamine (50:30:7)
[ 14]
34
Toluene-acetone-ethanol-ammonia (8:6:0.5:2)
[ 14]
35
Toluene-acetone-ethanol-diethylamine (30:60:7:3)
[ 15]
36
Toluene-acetone-ethanol-diethylamine (45:45:7:3)
[ 16]
37
Cyclohexane-benzene-diethylamine (70:25:10)
[ 16]
38
Chloroform-acetone(9:1)
[ 16]

1Additional references on chromatographic techniques for the analysis of opiates are also suggested to the reader [ 17 -25] .

Materials and methods

Laboratory-coated TLC plates were prepared from Merck silica gel 60 GF 254 to a thickness of 0.25 mm. Pre-coated plates were obtained from Merck (20 x 20 cm aluminium backed; silica gel 60 GF 254; 0.2 mm layer thickness). Developed plates were visualized under UV and with spray reagents. The samples were spotted individually and as a mixture.

The individual standards were dissolved in methanol to a concentration of 5 mg/ml. The mixture was prepared by mixing equal volumes of the standards. A spotting volume of 2 µl was applied. The TLC tanks were pre-equilibrated with the solvent system 15 minutes before use. The laboratory coated plates were activated at 100°C prior to their use. Spray reagents were prepared according to E. C. G. Clarke [ 4] .

The relative cost of each solvent system was based on prices presented in the British Drug House Catalogue (1984) and compared with the cost of the TLC system developed in the laboratory of the National Drug Research Centre, Penang, Malaysia.

Results and discussion

Table 2 shows the colours observed with the use of Marquis, acidified iodoplatinate or Dragendorff reagents. Since the opiates and adulterants listed in table 2 are visible in the UV (254 nm) with quinine strongly fluorescent (light blue) at 366 nm, the substances were first located by UV spectroscopy and subsequently confirmed by the colours produced with the reagents.

Evaluation of TLC systems

The Rf values of the opiates and adulterants listed in table 3 were obtained by using laboratory-coated plates for 38 TLC systems listed in table l; and the results are presented in table 3.

Table 4 shows the Rf values of the opiates and adulterants listed in table 3 that were obtained using pre-coated plates for the 38 TLC systems.

Laboratory-coated plates

Urine.Table 5 shows that 15 TLC systems with laboratory-coated plates were found to be able to detect and identify morphine and codeine in urine without any interference from the remaining 12 test substances (6-monoacetylmorphine, diacetylmorphine, acetylcodeine, noscapine, papaverine, thebaine, ephedrine, quinine, methadone, caffeine, cocaine and strychnine). Without the presence of these 12 substances, the number of suitable systems increases to 25.

Table 2

Colours observed with the use of acidified iodoplatinate, Dragendorff and Marquis reagents for the identification of opiates and adulterants

Reagent

Morphine

6-mono acetyl morphine

Diacetyl morphine

Codeine

Acetyl codeine

Noscapine

Papaverine

Thebaine

Ephedrine

Quinine

Methadone

Caffeine

Cocaine

Acidified iodo-platinate
Blue
Dark blue
Dark blue
Blue
Purple
Purple
Purple/brown
Purple/brown
Brown
Black
Green/brown
Yellow
Green/black
Dragen-dorff
Orange
Orange
Orange
Red/orange
Orange
Orange
Orange
Orange
No colour
Black
Black
No colour
Black
Marquis
Violet
Black
Black
Dark blue
Black
Green/black
Maroon
No colour
Yellow/brown
No colour
Orange/red
Brown
No colour

Table 3

Rf values of opiates and adulterants using laboratory-prepared TLC plates

(Rf x 100)

Code no. of TLC System

MOR

MAM

DAM

COD

AcCOD

NOS

PAP

THE

EPH

QUI

MET

CAF

COC

STR

1 5 34 46 22 56 74 59 66 12 8 88 27 82 25
2 20 71 83 57 90 97 67 87 60 75   42 96 53
3 0 4 6 0 7 64 39 7 1 4 18 17 18 0
4 4 18 29 13 33 72 53 30   28 64 32 67 11
5 18 44 50 28 53 78 68 51 19 74 74 43 72 21
6 0 3 3   4 51 28 3   10 10 9 9 0
7 23 42 50 35 55 81 73 57 30 55 86 53 84 36
8 0 2 4 2 6 49 27 44 1 2 26 10 29 0
9 38 46 47 39 47 71 71 45 30 53 45 62 63 22
10 10 36 47 22 48 80 71 43 8 35 73 52 79 20
11 53 57 50 50 54 63 63 54 68 61 68 72 75 47
12 17 39 43 27 44 74 68 41 12 32 68 55 74 20
13 6 34 38 18 48 77 64 46 12 23 74 41 74 18
14 12 47 51 29 59 82 73 59 19 39 76 57 79 31
15 10 42 44 28 58 82 75 60 15 34 73 55 76 28
16 1 10 12 5 17 65 42 15 5 5 61 18 62 3
17 22 49 50 23 48 75 68 46 20 41 68 52 71 23
18 6 37 53 34 63 66 59 67   23 76 32 73 47
19 62 88 90 77 90 95 95 89 40 83 88 83 94 60
20 69 82 90 83 89 93 92 89 66 76 83 84 90 80
21 28 54 57 43 64 81 69 62 28 54 79 54 78 43
22 7 24 31 20 43 76 75 50 9 17 50 49 75 23
23 30 55 46 39 56 79 76 54 26 57 74 61 77 28
24 16 42 61 47 65 84 84 62 15 50 52 72 67 57
25 6 30 40 26 52 55 29 41   25 90 12 72 27
26 3 9 15 5 16 61 45 16 3 5 26 20 42 3
27 0 1 1 0 3 27 19 1 0 0 10 4 12 0
28 18 33 30 18 30 76 76 27   44 47 56 37 13
29 5 13 13 5 17 60 47 17 5 23 50 13 53 3
30 38 48 49 37 49 82 83 42   51 24 77 53 22
31 38 47 45 38 47 78 77 40   55 56 68 71 24
32 5 10 8 4 10 68 66 9   17 33 64 76 4
33 3 28 50 24 58 62 54 64   14 82 28 76 37
34 22 51 62 40 66 80 63 60   36 77 35 70 36
35 34 69 72 57 75 92 84 72   68 88 72 89 52
36 41 64 70 57 72 83 74 72 50 65 81 63 81 64
37 2 14 26 12 37 37 17 40   9 77 6 62 19
38 0 3 8 2 11 64 58 8 0 0 5 29 21 3

Note: stationary phase is silica gel 60 GF 254.

MOR = morphine NOS = noscapine MET = methadone

MAM = 6-monoacetylmorphine PAP = papaverine CAF = caffeine

DAM = diacetylmorphine THE = thebaine COC = cocaine

COD = codeine EPH = ephedrine STR = strychnine

AcCOD= acetylcodeine QUI = quinine

Table 4

Rf values of opiates and adulterants using commerically-prepared TLC plates

(Rf x 100)

Code no. of TLC System

MOR

MAM

DAM

COD

AcCOD

NOS

PAP

THE

EPH

QUI

MET

CAF

COC

STR

1 6 32 42 19 51 69 54 59 11 7 87 25 82 25
2 19 59 70 43 82 85 55 72 5 59   39 91 49
3 0 3 3 2 4 42 21 4 0 16 18 9 18 9
4 3 11 17 7 18 59 39 14   33 36 19 39 3
5 14 31 34 20 36 68 56 34   27 64 34 65 14
6 0 1 2 0 1 34 16 1   3 3 5 5 0
7 17 43 49 29 54 77 67 52 24 44 77 45 77 25
8 0 1 2 1 3 34 13 2 0 1 12 5 19 1
9 27 34 34 27 34 55 56 32 20 39 33 49 52 16
10 8 23 32 15 34 73 61 28 4 20 56 42 65 13
11 37 43 36 33 40 56 56 40   55 62 36 63 39
12 10 28 31 19 36 70 63 33 8 22 60 47 71 12
13 3 21 23 10 32 70 54 32 7 12 65 32 68 11
14 6 30 35 16 41 75 63 42   21 67 43 72 48
15 7 33 38 18 46 76 69 47 11 23 67 49 74 23
16 1 8 9 3 13 56 32 10   3 52 13 53 2
17 12 33 34 19 38 72 62 35 14 29 63 43 69 14
18 6 35 52 31 58 61 54 62   21 77 30 72 43
19 50 74 77 63 79 90 89 79   72 82 77 88 58
20 67 87 87 87 90 95 95 94 83 95 96 91 92 89
21 21 40 45 28 46 71 58 44   35 65 42 68 24
22 4 20 32 13 37 73 71 41 4 8 42 43 72 17
23 18 43 42 27 45 73 68 43   43 68 50 71 28
24 5 22 47 23 48 80 80 43 6 18 23 63 41 26
25 4 20 29 17 38 43 20 39   16 78 10 62 21
26 1 3 7 2 9 50 34 8 1 3 12 14 28 2
27 0 0 0 0 1 15 6 1 0 0 3 2 3 0
28 9 14 19 9 20 57 57 13   23 35 38 13 8
29 2 7 7 2 9 42 30 11 4 14 35 9 40 3
30 19 27 31 20 29 74 74 22 13 27 48 65 28 12
31 31 39 37 29 37 68 67 29 11 37 32 57 51 17
32 2 5 5 2 5 45 42 4   10 9 19 5 2
33 2 18 36 15 42 47 40 51   8 75 22 71 34
34 9 30 34 22 43 63 48 43   24 68 34 67 32
35 18 47 52 33 53 71 62 51   41 68 53 70 36
36 23 49 55 40 57 69 60 56   47 30 51 69 46
37 1 5 18 8 25 25 11 27   5 61 4 46 12
38 0 1 3 1 6 47 40 5 0 0 2 20 40 1

Note: stationary phase is silica gel 60 GF 254.

MOR = morphine NOS = noscapine MET = methadone

MAM = 6-monoacetylmorphine PAP = papaverine CAF = caffeine

DAM = diacetylmorphine THE = thebaine COC = cocaine

COD = codeine EPH = ephedrine STR = strychnine

AcCOD= acetylcodeine QUI = quinine

Table 5

Classification of the TLC systems according to their resolution capability

 

Resolution category b

 

Laboratory coated plates

Pre-coated plates

Code no. of TLC System a

I

II

III

I

II

III

1
+
+
+
+
+
(+)
2
+
(+)
(+)
     
4
+
(+)
(+)
 
(+)
 
5
(+)
(+)
 
(+)
(+)
 
7
(+)
(+)
(+)
+
(+)
(+)
10
+
(+)
 
+
+
 
11
(+)
   
(+)
   
12
+
(+)
 
(+)
(+)
(+)
13
(+)
(+)
(+)
(+)
(+)
 
14
+
   
+
   
15
(+)
(+)
(+)
+
(+)
 
17      
(+)
(+)
 
18
+
   
(+)
   
19
+
   
+
(+)
 
20
(+)
         
21
(+)
(+)
 
+
(+)
 
22
+
         
23
+
(+)
 
+
(+)
 
24
(+)
         
25
(+)
   
(+)
(+)
 
26
(+)
         
31      
(+)
   
33
+
+
(+)
+
   
34
+
   
+
(+)
 
35
+
   
+
(+)
 
36
+
   
+
(+)
 
37
+
   
+
(+)
 

a TLC systems that do not fall under any of the three resolution categories have been omitted.

b I = morphine and codeine in urine: II = morphine, codeine, thebaine, noscapine and papaverine in opium; III = morphine. codeine. thebaine, noscapine. papaverine, acetylcodeine, 6-monoacetylmorphine and diacetylmorphine in heroin.

Note: + = system suitable without interference from the rest of the opiates and/or adulterants tested: (+) = system suitable only in the absence of the renaming tested substances.

Opium. Although 12 systems can resolve the opiates morphine, codeine, noscapine, thebaine and papaverine, in samples of opium only 2 systems, 1 and 33, were found to be capable of doing so without interference from the other tested substances (table 5).

Illicit heroin. Seven systems were found to be capable of resolving all opiates in samples of illicit heroin (table 5) when no adulterants were present. However, with the successive addition of the potential adulterants, the number of suitable systems dropped correspondingly. When all 14 substances were included, only system 1 was found suitable (table 5).

Pre-coated plates

Urine. Table 5 shows that 21 TLC systems with pre-coated plates could detect and identify morphine and codeine in urine samples, but only 13 systems were found to be capable to do so without interference from the other 12 tested substances.

Opium. Although 17 TLC systems with pre-coated plates were found to be able to resolve the five major opiates in samples of opium, only 2 systems, 1 and 10, could do so without interference from other tested substances (table 5).

Illicit heroin. Systems 1, 7 and 12 were capable of resolving all of the opiates, but could not do so in the presence of the adulterants listed in table 3. TLC system 7 was unable to resolve any of the adulterants. Cocaine and strychnine interfered with morphine and noscapine in system 12. In system 1, caffeine interfered only with strychnine. Compared with system 12, where the two adulterants interfered with two opiates, system 1 was found more favourable since the two adulterants in this system interfered with each other only.

Table 6 shows the TLC systems that are capable of resolving some of the opiates and adulterants listed in table 3.

Table 6

TLC systems that are capable of resolving some of the opiates and adulterants

listed in table 3

 

Adulterants

 

Laboratory coated plates

Pre-coated plates

Code no. of TLC System

EPH

QUI

MET

CAF

COC

STR

EPH

QUI

MET

CAF

COC

STR

1
+
+
+
+
+
+
+
+
+
(+)
+
(+)
2
+
+
NT
+
 
+
           
7
+
 
+
+
+
             
12            
+
+
+
+
   
13
+
+
 
+
               
15
+
+
+
+
               
33
NT
+
+
 
+
+
           
Note:
EPH = ephedrine
COC = cocaine
QUI = quinine
STR = strychnine
MET = methadone
NT = not tested
CAF = caffeine
(+) = cross interference

Table 7 Relative cost of the TLC systems

Code No. of TLC system

Relative cost a

Code No .of TLC system

Relative cost a

1 1 20 0.57
2 0.68 21 0.41
3 0.57 22 0.59
4 0.58 23 0.38
5 0.46 24 0.56
6 0.51 25 0.70
7 0.40 26 0.38
8 0.52 27 1.07
9 0.50 28 0.32
10 0.68 29 0.58
11 0.89 30 0.40
12 0.82 31 1.33
13 0.50 32 0.79
14 0.68 33 1.13
15 0.68 34 0.38
16 0.49 35 0.42
17 0.43 36 0.48
18 0.65 37 0.52
19 0.82 38 0.54
a Compared with system 1.

Table 8

Assessment of TLC systems for opiate analysis in urine,

opium and illicit heroin samples

Type of sample

TLC plate

Most suitable systems

System with lowest cost

Total

Urine
Laboratory-coated plates
1, 2,4,10,12,14,18,19,22, 23, 33-37 23, 34 15
 
Pre-coated plates
1, 7, 10, 14, 15, 19, 21, 23, 33 -37 23, 34 13
Opium
Laboratory-coated plates
1, 33 1 2
 
Pre-coated plates
1, 10 10 2
Illicit heroin
Laboratory-coated plates
1   1
 
Pre-coated plates
1   1

Costing of TLC systems

The relative cost of the solvent systems used are listed in table 7. The TLC systems suitable for opiate analysis are arranged in terms of cost as follows (see also table 8):

Laboratory-coated plates

Urine: 23 and 34; 35; 36; 37; 4; 22; 18; 2, 10, 14; 12 and 19; 1; 33

Opium: 1; 33

Illicit heroin: 1

Pre-coated plates

Urine: 23 and 34; 7; 21; 35; 36; 37; 10, 14 and 15; 19; 1; 33

Opium: 10, 1

Illicit heroin: 12, 1

Although system 1 is relatively more expensive than most of the others, its resolutive capability in heroin analysis outweighs this disadvantage.

Concluding remarks

The authors selected 38 TLC systems from the literature available over the last 10 years and evaluated their capability for the analysis of opiates in urine, opium and heroin samples. Resolution was found to be slightly better on laboratory-coated TLC plates than on commercially pre-coated plates.

A total of 14 substances - 8 opiates and 6 adulterants - were used as test samples. From the Rf data the systems were divided into three categories based on their ability to resolve opiates in samples of urine, opium or illicit heroin. The relative cost of solvents used for each of 38 TLC systems was identified and presented in table 7. The most suitable TLC systems for the analysis of opiates in samples of urine, opium or illicit heroin are presented in table 8.

Acknowledgement

The authors are grateful to the United Nations Fund for Drug Abuse Control and the Government of Malaysia for the joint funding of this research project.

The authors wish to thank several of their colleagues working at the National Drug Research Centre for their constructive criticism and assistance in carrying out this project.

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02

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03

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04

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05

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08

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09

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10

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11

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12

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13

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14

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15

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17

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18

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19

T. A. Gough and P. B. Baker, "The selection of gas chromatographic stationary phase and operating conditions for the separation and quantitation of heroin and structurally related compounds", Journal of Chromatographic Science, vol. 19, 1981, pp. 227-234.

20

H. Neumann, "Analysis of opium and crude morphine samples by capillary gas chromatography: Comparison of impurity profiles", Journal of Chromatography, vol. 315, 1984, pp. 404-411.

21

S. T. Chow and others, "A comparison of chromatographic methods for estimation of the diacetylmorphine content of illicit heroin", Journal of Chromatographic Science, vol. 21, 1983, pp. 551 - 554.

22

P. B. Baker and T. A. Gough, "The separation and quantitation of the narcotic components of illicit heroin using reversed-phase high performance liquid chromatography", Journal of Chromatographic Science, vol. 19, 1982, pp. 483 -489.

23

B. C. Pettitt and C. E. Damon, "Analysis of poppy straw concentrate by high performance liquid chromatography", Journal of Chromatography, vol. 242, 1982, pp. 189 - 192.

24

C. J. C. M. Laurent, H.A. H. Billiet and L. DeGalan, "High performance liquid chromatography of heroin samples on alumina by ion exchange in mixed aqueous-organic mobile phases", Journal of Chromatography, vol. 285, 1984, pp. 161 - 170.

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