HPLC Stationary Phases to Have in Your Toxicology Toolbox
2 Jan 2025When it comes to stationary phases in HPLC, there are a ton of options to choose from. At Restek alone, we offer a catalog of 10+ stationary phases. Each of these phases has its own strengths, weaknesses, and capabilities depending on your intended analysis. When you start working on a new method, you might investigate several different stationary phases to see which one will best suit your list of analytes and their chemical properties. This will be especially important if that analyte list includes critical isomer pairs that need to be resolved.
In a toxicology lab setting, the scope of analytes will usually include drugs of abuse, therapeutic drugs, over the counter drugs, and alcohol metabolites. Varying polarities and structural differences can largely impact how analytes within these large scopes will interact with different stationary phases. Unfortunately, there is no “catch-all” phase that can effectively analyze all the compounds involved in toxicology testing. Most often, method developers need to employ several different stationary phases to meet their analysis goals for different analyte classes. To find out which ones work for you, you could try them all out one by one—or you can keep reading as we break down what stationary phases all toxicology labs should have in their method development toolbox.
C18
Most method developers see C18 columns as a “general-purpose” or “go-to” column in reversed-phase chromatography. This stationary phase is established, reliable, and is well-suited to handle the large, multi-drug class analyte lists that are common in toxicology testing. C18 phases work through hydrophobic interactions, which makes them a good choice when working with a large list of analytes that have varying polarities. While C18 phases can be used to analyze a wide range of compounds, they lack the selectivity needed to fully resolve some critical isomer pairs that have highly aromatic characteristics.
There are some difficult separations, however, where the C18 is still a good fit. Differentiating between the d- and l-isomer forms of methamphetamine and amphetamine by LC-MS/MS typically requires a chiral column, which are expensive and do not have broad utility. For labs that do not wish to go this route, this troublesome separation can be performed on a standard C18 column following a simple derivatization procedure.
Even if C18 cannot separate every analyte in a toxicology testing scope, it is a crucial stationary phase that every method developer should have on hand.
Figure 1: d- & l- Amphetamines in urine on a Raptor C18 column. Both sets of enantiomers are baseline resolved without the use of a chiral column.

Peaks | tR (min) | Precursor Ion | Product Ion 1 | Product Ion 2 | |
---|---|---|---|---|---|
1. | l-Methamphetamine* | 3.11 | 400.3 | 339.0 | 323.8 |
2. | d-Methamphetamine* | 3.38 | 400.3 | 339.0 | 323.8 |
3. | l-Amphetamine* | 4.16 | 386.1 | 325.0 | 308.0 |
4. | d-Amphetamine* | 4.55 | 386.1 | 325.0 | 308.0 |
Column | Raptor C18 (cat.# 9304A12) | ||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dimensions: | 100 mm x 2.1 mm ID | ||||||||||||||||||||||||||||
Particle Size: | 2.7 µm | ||||||||||||||||||||||||||||
Pore Size: | 90 Å | ||||||||||||||||||||||||||||
Guard Column: | Raptor C18 EXP guard column cartridge 5 mm, 2.1 mm ID, 2.7 µm (cat.# 9304A0252) | ||||||||||||||||||||||||||||
Temp.: | 35 °C | ||||||||||||||||||||||||||||
Standard/Sample | |||||||||||||||||||||||||||||
Conc.: | 500 ng/mL in urine | ||||||||||||||||||||||||||||
Inj. Vol.: | 10 µL | ||||||||||||||||||||||||||||
Mobile Phase | |||||||||||||||||||||||||||||
A: | 0.1% Formic acid in water | ||||||||||||||||||||||||||||
B: | 0.1% Formic acid in methanol | ||||||||||||||||||||||||||||
|
Detector | MS/MS |
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Ion Mode: | ESI- |
Mode: | MRM |
Instrument | HPLC |
Sample Preparation | A 500 ng/mL standard (d- and l-amphetamines and methamphetamines) was prepared in pooled urine. 50 µL of the standard was aliquoted into a microcentrifuge tube. 10 µL of a working internal standard (20 µg/mL (±)-amphetamine-D11 and (±)-methamphetamine-D11 in water) and 20 µL of 1M NaHCO3 was added and vortexed at 3000 rpm for 10 seconds. After vortexing, 100 µL of 0.1% (w/v) Marfey's reagent (1-fluoro-2-4-dinitrophenyl-5-L-alanine amide) in acetone was added, vortexed, and heated at 45 °C for 1 hour. Samples were allowed to cool to room temperature before the addition of 40 µL of 1M HCl in water. The sample was then vortexed and evaporated to dryness under nitrogen at 45 °C. Samples were reconstituted in 1 mL of 40:60 water:methanol (v/v) and filtered using Thomson SINGLE StEP standard filter vials cat.# 25893 prior to analysis. |
Notes | Thomson SINGLE StEP standard filter vials cat.# 25893 were used to produce this chromatogram, but have since been discontinued. For assistance choosing a replacement for this application, contact Restek Technical Service or your local Restek representative. |
Strengths: C18 is a versatile, established phase that can handle large lists of analytes with varying polarities.
Weaknesses: C18 does not possess the selectivity to adequately resolve many critical isomer pairs in drugs of abuse testing.
Biphenyl
While not as widely used as C18, the Biphenyl stationary phase brings many advantages to the table for method developers. The Biphenyl phase’s biggest strength is its ability to separate aromatic compounds, which makes it particularly adept at resolving critical isomer pairs. This enhanced selectivity comes from π-π interactions between the electrons in the aromatic rings of the Biphenyl ligand and the π-electrons within the structures of the analytes of interest. The true potential of the Biphenyl phase is unlocked when used in conjunction with methanolic mobile phases, as the structure of methanol contains no π electrons. This allows the π-π interactions within the stationary phase to occur uninterrupted, resulting in improved separation. The Biphenyl stationary phase can resolve compounds that are difficult to separate or lack adequate retention on C18 or other phenyl phase columns. In some cases, the enhanced selectivity of the Biphenyl phase may also help to minimize matrix interferences.
Figure 2: 18 Drugs of abuse in urine on a Raptor Biphenyl column. The critical isobars (morphine/hydromorphone; codeine/hydrocodone) are fully resolved from each other, as well as from matrix interferences present in human urine. When analyzed by C18 or other phenyl phase chemistries, early eluting isobaric compounds may experience poor resolution and peak shape.

Peaks | tR (min) | Precursor ion | Product ion 1 | Product ion 2 | |
---|---|---|---|---|---|
1. | Morphine* | 1.34 | 286.2 | 152.3 | 165.3 |
2. | Oxymorphone | 1.40 | 302.1 | 227.3 | 198.2 |
3. | Hydromorphone* | 1.52 | 286.1 | 185.3 | 128.2 |
4. | Amphetamine | 1.62 | 136.0 | 91.3 | 119.2 |
5. | Methamphetamine | 1.84 | 150.0 | 91.2 | 119.3 |
6. | Codeine* | 1.91 | 300.2 | 165.4 | 153.2 |
7. | Oxycodone | 2.02 | 316.1 | 241.3 | 256.4 |
8. | Hydrocodone* | 2.06 | 300.1 | 199.3 | 128.3 |
9. | Norbuprenorphine | 2.59 | 414.1 | 83.4 | 101.0 |
Peaks | tR (min) | Precursor ion | Product ion 1 | Product ion 2 | |
---|---|---|---|---|---|
10. | Meprobamate | 2.61 | 219.0 | 158.4 | 97.2 |
11. | Fentanyl | 2.70 | 337.2 | 188.4 | 105.2 |
12. | Buprenorphine | 2.70 | 468.3 | 396.4 | 414.5 |
13. | Flurazepam | 2.73 | 388.2 | 315.2 | 288.3 |
14. | Sufentanil | 2.77 | 387.2 | 238.5 | 111.3 |
15. | Methadone | 2.86 | 310.2 | 265.3 | 105.3 |
16. | Carisoprodol | 2.87 | 261.2 | 176.3 | 158.1 |
17. | Lorazepam | 3.03 | 321.0 | 275.4 | 303.1 |
18. | Diazepam | 3.31 | 285.1 | 193.2 | 153.9 |
Column | Raptor Biphenyl (cat.# 9309A5E) | ||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dimensions: | 50 mm x 3.0 mm ID | ||||||||||||||||||||||||||||
Particle Size: | 2.7 µm | ||||||||||||||||||||||||||||
Temp.: | 30 °C | ||||||||||||||||||||||||||||
Standard/Sample | |||||||||||||||||||||||||||||
Diluent: | Urine:mobile phase A:mobile phase B (17:76:7) | ||||||||||||||||||||||||||||
Conc.: | 10-100 ng/mL | ||||||||||||||||||||||||||||
Inj. Vol.: | 10 µL | ||||||||||||||||||||||||||||
Mobile Phase | |||||||||||||||||||||||||||||
A: | Water + 0.1% formic acid | ||||||||||||||||||||||||||||
B: | Methanol + 0.1% formic acid | ||||||||||||||||||||||||||||
|
Detector | AB SCIEX API 4000 MS/MS |
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Ion Source: | TurboIonSpray® |
Ion Mode: | ESI+ |
Instrument | API LC-MS/MS |
Notes | Lorazepam was prepared at 100 ng/mL; all other analytes are 10 ng/mL. |
Strengths: Biphenyl columns offer superior selectivity over C18 and other phenyl phase columns, allowing for the resolution of critical isomer pairs and matrix interferences.
Weaknesses: Biphenyl is not as commonly used as C18, so method development may involve more optimization.
FluoroPhenyl
For method developers working in a toxicology setting, the use C18 and Biphenyl phases is likely a familiar concept. In general, the compounds included in toxicology testing scopes can be successfully analyzed using a combination of C18 and Biphenyl columns. One of the exceptions to this is the analysis of cannabinoid isomers. Typical toxicological testing to determine cannabis use involves analyzing for the major psychoactive component in cannabis, Δ9-THC, and its two metabolites, 11-OH-Δ9-THC and Δ9-THC-COOH. The emergence of Δ8-THC, an isomer of Δ9-THC, has necessitated that labs also include this compound and its metabolites in their scope of testing. This updated compound list of six analytes includes three pairs of isomers which require full resolution for accurate reporting. C18 and Biphenyl columns do not exhibit the selectivity to adequately resolve all three pairs of isomers, but a FluoroPhenyl phase column does.
As we discussed earlier, C18 columns work through hydrophobic interactions and Biphenyl columns through π-π interactions. FluoroPhenyl phases use both mechanisms in addition to dispersion, shape selectivity, cation exchange, and dipole interactions. By using multiple retention mechanisms, FluoroPhenyl phase columns are capable of unique separations. For toxicology labs that encounter THC isomers in their casework, a FluoroPhenyl column is a must for these tough separations.
Figure 3: Δ8-THC, Δ9-THC, hydroxy- and carboxy-metabolites in whole blood on a Raptor FluoroPhenyl column. All three sets of isomers are fully resolved from each other to ensure accurate reporting.

Peaks | tR (min) | Conc. (ng/mL) | Precursor | Product 1 | Product 2 | |
---|---|---|---|---|---|---|
1. | 11-OH-Δ8-THC | 4.66 | 50 | 331.0 | 313.0 | 201.1 |
2. | 11-OH-Δ9-THC | 5.00 | 50 | 331.0 | 313.0 | 201.1 |
3. | Δ8-THC-COOH | 5.04 | 250 | 345.1 | 327.0 | 299.2 |
4. | Δ9-THC-COOH | 5.85 | 250 | 345.1 | 327.0 | 299.2 |
5. | Δ8-THC | 10.88 | 50 | 315.0 | 193.0 | 123.2 |
6. | Δ9-THC | 11.26 | 50 | 315.0 | 193.0 | 123.2 |
Column | Raptor FluoroPhenyl (cat.# 9319A1E) | ||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dimensions: | 100 mm x 3 mm ID | ||||||||||||||||||||||||||||||||||||
Particle Size: | 2.7 µm | ||||||||||||||||||||||||||||||||||||
Pore Size: | 90 Å | ||||||||||||||||||||||||||||||||||||
Guard Column: | Raptor FluoroPhenyl EXP guard column cartridge 5 mm, 3 mm ID, 2.7 µm (cat.# 9319A0253) | ||||||||||||||||||||||||||||||||||||
Temp.: | 40 °C | ||||||||||||||||||||||||||||||||||||
Standard/Sample | |||||||||||||||||||||||||||||||||||||
Δ8-Tetrahydrocannabinol (Δ8-THC) (cat.# 34090) | |||||||||||||||||||||||||||||||||||||
Δ9-Tetrahydrocannabinol (Δ9-THC) (cat.# 34067) | |||||||||||||||||||||||||||||||||||||
(±)11-nor-9-carboxy-Δ-9-THC (Δ9-THC-COOH) (cat.# 34068) | |||||||||||||||||||||||||||||||||||||
Other compounds obtained separately. | |||||||||||||||||||||||||||||||||||||
Diluent: | 40:60 Water:methanol, both with 0.1% formic acid (v/v) | ||||||||||||||||||||||||||||||||||||
Inj. Vol.: | 5 µL | ||||||||||||||||||||||||||||||||||||
Mobile Phase | |||||||||||||||||||||||||||||||||||||
A: | Water, 0.1% formic acid | ||||||||||||||||||||||||||||||||||||
B: | Methanol, 0.1% formic acid | ||||||||||||||||||||||||||||||||||||
| |||||||||||||||||||||||||||||||||||||
Max Pressure: | 390 bar |
Detector | Shimadzu 8045 MS/MS |
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Ion Mode: | ESI+ |
Instrument | Shimadzu Nexera X2 |
Sample Preparation | Five hundred microliters of whole blood was transferred to a 12 mL glass test tube. Fifty microliters of internal standard and 50 µL of control material were transferred to the test tube and vortexed. Five hundred microliters of HPLC grade water was added to each sample and vortexed. One hundred microliters of 10% acetic acid was added to each sample and vortexed. Two and a half milliliters of 80:20 hexanes:ethyl acetate was added to each sample and vortexed until visibly combined. Samples were centrifuged at 2800 rpm for 15 minutes. The top layer was transferred to a new glass test tube and dried down under nitrogen. Samples were reconstituted with 100 µL of 40:60 methanol:water, both containing 0.1% formic acid, and vortexed. Samples were transferred to 2 mL screw-thread vials (cat.# 21143) with glass inserts (cat.# 21776) and capped with short-cap, screw-vial closures (cat.# 24498). |
Notes | The column was stored in 100% acetonitrile when not in use. |
Strengths: FluoroPhenyl columns offer unique selectivity that can achieve the degree of separation needed for THC isomers in toxicology casework.
Weaknesses: FluoroPhenyl columns may not provide an effective solution for analyzing large analyte lists like C18 and Biphenyl.
EtG/EtS
While the analysis of volatiles like ethanol is performed using gas chromatography (GC) methodologies, labs may also choose to monitor for the compounds ethyl glucuronide (EtG) and ethyl sulfate (EtS), which are biomarkers for monitoring alcohol use. The analysis of EtG and EtS can be performed by LC-MS/MS, though there are some significant challenges associated with it. High polarity, matrix interferences, and analyte suppression all complicate this analysis. Many methods suffer from poor retention, co-elution with matrix interferences, and long run times. The Raptor EtG/EtS column is a unique, proprietary phase developed by Restek specifically to combat these pain points. The EtG/EtS column offers robust, consistent retention of both compounds and maintains resolution from matrix interferences. While this column does require some additional conditioning for the most optimized results, it offers a unique selectivity that any lab performing alcohol metabolite analysis should be sure to have.
Figure 4: EtG/EtS in urine on a Raptor EtG/EtS column. Both analytes are adequately retained on the column and are well resolved from matrix interferences.

Peaks | tR (min) | Conc. (ng/mL) | Precursor Ion | Product Ion | Product Ion | |
---|---|---|---|---|---|---|
1. | Ethyl-β-D-glucuronide-d5 (EtG-d5) | 1.21 | 200 | 226.2 | 85.0 | - |
2. | Ethyl-β-D-glucuronide (EtG) | 1.23 | 500 | 221.2 | 75.1 | 85.1 |
3. | Ethyl sulfate-d5 (EtS-d5) | 2.36 | 50 | 130.1 | 98.0 | - |
4. | Ethyl sulfate (EtS) | 2.38 | 500 | 125.1 | 97.1 | 80.0 |
Column | Raptor EtG/EtS (cat.# 9325A12) | ||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dimensions: | 100 mm x 2.1 mm ID | ||||||||||||||||||||
Particle Size: | 2.7 µm | ||||||||||||||||||||
Pore Size: | 90 Å | ||||||||||||||||||||
Guard Column: | UltraShield UHPLC precolumn filter, 0.2 μm frit (cat.# 25809) | ||||||||||||||||||||
Temp.: | 35 °C | ||||||||||||||||||||
Standard/Sample | |||||||||||||||||||||
Ethyl- | |||||||||||||||||||||
Ethyl- | |||||||||||||||||||||
Ethyl sulfate-d5 sodium salt (EtS-d5) (cat.# 34104) | |||||||||||||||||||||
Ethyl sulfate sodium salt (EtS) (cat.# 34103) | |||||||||||||||||||||
Diluent: | 0.01% Formic acid in water | ||||||||||||||||||||
Conc.: | 500 ng/mL (40x acetonitrile precipitation sample prep) | ||||||||||||||||||||
Inj. Vol.: | 10 µL | ||||||||||||||||||||
Mobile Phase | |||||||||||||||||||||
A: | 0.01% Formic acid in water | ||||||||||||||||||||
B: | 0.1% Formic acid in acetonitrile | ||||||||||||||||||||
|
Detector | MS/MS |
---|---|
Ion Source: | Electrospray |
Ion Mode: | ESI- |
Mode: | MRM |
Instrument | HPLC |
Sample Preparation | A 500 ng/mL standard was prepared in urine. A 50 μL aliquot was mixed with 10 μL of internal standard (20 μg/mL EtG-d5 and 5 μg/mL EtS-d5 in water) and 150 µL of acetonitrile by vortexing at 3000 rpm for 10 seconds and centrifuged at 4300 rpm for 10 minutes at 10 °C. After centrifugation, 100 µL of the supernatant was diluted with 900 µL (40x dilution) of 0.01% formic acid in water. The sample was then vortexed at 3000 rpm for 10 seconds and injected for LC-MS/MS analysis. |
Strengths: This column offers unique selectivity specifically catered to the analysis of alcohol metabolites.
Weaknesses: For optimal results, this column requires conditioning with matrix. See “Successful Strategies for the Analysis of EtG and EtS in Urine” for more information. Additionally, this column has a specific intended use and is not amendable to large analyte lists.
Summary
The list of compounds toxicology labs must test for can be extensive and contain analytes that have very different chemical properties. Most of these compounds will be successfully analyzed by using a combination of C18 and Biphenyl columns, which offer complementary selectivity. However, challenging separations like THC isomers and EtG/EtS will require method developers to utilize alternative column chemistries like FluoroPhenyl or specialty phases like EtG/EtS. While there is no perfect solution for all analytes, with these four HPLC stationary phases in your toxicology toolbox, you’ll be in great shape to handle any separation that comes your way.