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Screening Methods for Drugs of Abuse Compounds Using GC

10 Apr 2025

GC has long been used for screening and quantitation purposes in the analysis of drugs of abuse. Choosing the right detector for your GC analysis can be imperative for obtaining the best quality data. In this blog we will discuss detectors for GC and their applicability as it relates to the analysis of drugs of abuse. In this work, a variety of compounds were chosen to demonstrate the selectivity and versatility of a Restek Rxi-5Sil MS GC column for underivatized drugs.

There are a variety of GC detectors to select from. Universal detectors can be a great, cost-effective tool in any lab. For GC, the most common universal detector is a flame ionization detector (FID). To accurately quantitate compounds using a universal detector, all compounds must be chromatographically resolved.

Virtual Method Development

To assist in method development, Restek’s free method development tool, Pro EZGC Chromatogram Modeler (Restek Pro EZGC Chromatogram Modeler), was utilized. This tool can help develop methods virtually for a variety of compounds and includes many drugs of abuse compounds. The following is an example method modeled in EZGC for the analysis of drugs of abuse compounds with full resolution of analytes.

Figure 1. Example of chromatogram and conditions modeled using Pro EZGC.

Image 1 Image 2

 

EZGC can be a great tool in helping with method development, but in order to model compounds of interest, those compounds must already be present in the library and are only available for Restek specific column chemistries. In this case we were able to find adequate resolution using a “5-type” phase in EZGC and chose the 5-Sil since the selectivity would be similar. As stated above, to use a universal detector like FID, all compounds need to be fully resolved using unique phases and selectivity like the Rxi-5Sil MS column. Depending on the number of compounds, this can pose some challenges with compound co-elutions resulting in longer run times. In addition to the FID lacking specificity, it is also not an appropriate detector for drugs of abuse applications in biological matrices because of the lack of sensitivity.

Liners

Liner choice is important for optimizing injector performance. The first thing that should be considered before choosing a liner is the mode in which the analytes will be introduced. Split mode is when only a portion of the injection is directed to the analytical column and is preferred because it offers less resonance time in the injection port. If compounds are active, this causes the analytes to transfer faster to the head of the analytical column and leaves less time for activity to occur. The downside of using split mode is it dilutes the sample, so sample concentration can be a factor when deciding which mode is best for analysis. Spitless mode is when the majority of the injected sample is transferred to the GC column and is advantageous if your analyte is in low concentration or sample amount is limited but the downside can be a bigger solvent effect that can adversely affect early eluting compounds. Overall, peak shape is often better in split mode, so this is typically the general recommendation. This is also the preferred method for underivatized drugs of abuse. Restek’s Topaz liners offer a premium deactivation coating and are an excellent choice for active compounds, especially those that are acidic or basic in nature. By adding wool to the liner, it helps with sample volatilization and transfer to analytical column resulting in a sharp, less narrow solvent peak, as well as more narrow peak shapes for analytes of interest and less solvent effects in general. For these reasons, the single taper inlet liner (cat#23303) with wool was chosen for this work.

Column Selection

Another major consideration in method development is column selection. The Rxi-5Sil MS (cat#13623) is a superior choice for underivatized drugs of abuse due to its low polarity characteristics. Additionally, this column has proven to have inertness for active compounds including acids, bases, and neutrals. This phase is engineered for low phase bleed, specifically for the use with MS, and phenyl groups are incorporated into the polymer backbone which helps improve thermal stability making it less prone to oxidization.

Drugs of Abuse by GC-MS

To demonstrate the applicability and selectivity of the Rxi-5Sil MS column, 34 common drugs of abuse compounds from a variety of classes including opiates, barbiturates, sedatives, anxiolytics, antitussives, stimulants, anti-depressants, hallucinogens, anesthetics, and antihistamines were analyzed using a GC-MS. Mass spectrometry can be a great tool to increase specificity and sensitivity for an application. Single quadrupole mass spectrometry allows the user to develop faster, more robust, sensitive, and selective methods when compared to universal detectors, such as the FID. Here a method was developed for the drugs of abuse panel at a 25 ppm concentration using a 10:1 split, with the MS operating in scan mode. Table 1 shows the GC-MS method parameters for analysis. Figure 2 shows the chromatogram of the 34 common drugs of abuse baseline resolved.

Table 1: Method conditions.

Analytical Column: Restek Rxi-5Sil MS (cat#13623)
Inlet: 250 °C
Split: 10:1
Flow: 2 mL/min
Transfer line: 280 °C
Liner: 4 mm Topaz Single Taper Liner with wool (cat#23303)
MS Mode: Scan 40- 500 m/z (+)
Rate °C/min Temperature °C  Hold Time (min) Run Time (min)
- 150 1 1
4 210 0 16
30 320 2 21.67

 

Figure 2: 34 compounds analyzed using Table 1 conditions.

Table 2: Analyte list and retention times in minutes.

Analyte Number Compound Name Retention Time (mins)
1 propofol 2.45
2 benzocaine 4.61
3 butabarbital 6.11
4 butalbital 6.18
5 phenacetin 6.37
6 cotinine 6.45
7 amobarbital 7.01
8 pentobarbital 7.44
9 secobarbital 8.36
10 Norketamine 8.88
11 prilocaine 9.14
12 Ketamine 9.52
13 Diphenhydramine 9.82
14 phencyclidine (PCP) 10.33
15 Doxylamine 10.75
16 Tramadol 11.48
17 phenobarbital 11.92
18 Chlorpheniramine 12.7
19 procaine 13.17
20 Venlafaxine 14.42
21 Brompheniramine 14.79
22 Methadone 15.39
23 Dextromethorphan 15.44
24 Amitriptyline 16.54
25 cocaine 16.66
26 Imipramine 16.89
27 tetracaine 17.05
28 Cyclobenzaprine 17.11
29 Sertraline 18.06
30 codeine 18.2
31 6-acetyl-morphine 18.87
32 heroin 19.3
33 Zolpidem 19.89
34 alprazolam 20.44

 

At a concentration of 25 ppm, all 34 compounds are detected with adequate sensitivity. When the concentration is lowered to 1 ppm, some of the compounds are no longer detected. To demonstrate the capabilities of the MS, a much faster method was developed. Using the same 34 compounds and an optimized SIM mode, all compounds were analyzed within an 8-minute overall cycle time, resulting in a much faster analysis and much lower detection limits. When the MS runs in SCAN mode it measures a full mass range, but when in SIM mode the instrument focuses on specific ions of interest. The result of this is that longer dwell times can be achieved for ions of interest resulting in better signal to noise ration. Table 3 shows the optimized GC-MS SIM method used. Figure 3 shows the chromatogram of the 34 compounds with much faster analysis time and lower concentration.

Table 3: GC-MS conditions for SIM method.

Analytical Column: Restek Rxi-5Sil MS (cat#13623)
Inlet: 250 °C
Split: 10:1
Flow: 1.4 mL/min
Transfer line: 280 °C
Liner: 4 mm Topaz Single Taper Liner with wool (cat#23303)
MS Mode: Scan 40- 500 m/z (+)
Rate °C/min Temperature °C  Hold Time (min) Run Time (min)
- 200 1 1
30 320 3 8

 

Table 4: Scheduled SIM parameters.

Time Group Dwell (ms) Mass (m/z)
1.00 1 100 163, 178
1.75 2 100 120,165
2.10 3 90 98, 108, 137, 141, 156, 168, 176, 181, 197
2.50 4 90 58, 86, 87, 167, 168, 180, 182, 195
2.95 5 70 58, 71, 117, 200, 218, 242
3.27 6 100 86, 99, 134, 203
3.61 7 60 58, 71, 72, 82, 182, 202, 223, 235, 247, 249, 271, 275
4.30 8 60 162, 262, 268, 274, 299, 327
5.10 9 60 204, 235, 279, 307, 327, 369

 

Figure 3: GC-MS SIM chromatogram of the 34 drugs of abuse in under 7 mins.

Table 5: Analytes and their retention time in minutes.

Analyte Number Compound Name Retention Time (mins)
1 propofol 1.5
2 benzocaine 1.95
3 butabarbital 2.19
4 butalbital 2.24
5 phenacetin 2.24
6 cotinine 2.32
7 amobarbital 2.32
8 pentobarbital 2.4
9 secobarbital 2.55
10 Norketamine 2.72
11 prilocaine 2.78
12 Ketamine 2.82
13 Diphenhydramine 2.87
14 phencyclidine (PCP) 2.98
15 Doxylamine 3
16 Tramadol 3.12
17 phenobarbital 3.12
18 Chlorpheniramine 3.31
19 procaine 3.36
20 Venlafaxine 3.55
21 Brompheniramine 3.65
22 Methadone 3.72
23 Dextromethorphan 3.79
24 Amitriptyline 3.89
25 cocaine 3.94
26 Imipramine 4.01
27 tetracaine 4.02
28 Cyclobenzaprine 4.08
29 Sertraline 4.49
30 codeine 4.58
31 6-acetyl-morphine 4.94
32 heroin 5.2
33 Zolpidem 5.72
34 alprazolam 6.29

 

The faster cycle time is due to the specificity of the detector. Now, only isobars are required to be resolved, and this allows for much faster methods to be developed. In addition to this, lower limits of detection can be reached; often orders of magnitude below the capabilities of an FID. To gain additional sensitivity and specificity, a GC-MS/MS can be used.

Conclusions

So which detector is right for you? For seized drug cases, the use of GC-FID may be applicable with a method that can resolve common drugs of abuse. For biological samples such as blood, urine, and oral fluid, an MS is usually needed to reach appropriate limits of detection and a triple quad can also be used for added specificity/sensitivity. This work has shown Rxi-5Sil MS GC column has demonstrated robustness for the analysis of underivatized drugs of abuse with unique selectivity that is able to resolve compounds for detection by FID or even faster methods with MS detection.

Are you interested in implementing a drugs of abuse methods in your lab or struggling with certain analytes in your current method? Our EZ Software Suite can help with method development for both GC and LC methods. Check it out here: EZGC & EZLC Online Software Suite. All that is needed to use the free software is a restek.com login. We are constantly updating our online EZ libraries with novel compounds, but if you are interested in a compound that is not currently listed, please reach out to us and let us know! E-mail Corby Hillard or Jamie York to ask about new psychoactive compounds (NPS) or for help with your method development.