HPLC Interview Questions and Answers for Pharma QC

In short: HPLC is the most used instrument in a pharma QC lab, so almost every QC interview includes HPLC questions. This guide gives model answers on the principle, columns, mobile phases, detectors, system suitability, method validation, troubleshooting and data integrity. Learn the reasons behind each answer; panels often follow up with “why?”

Why interviewers focus on HPLC

Assay, related substances, dissolution and content uniformity tests in most formulation and API labs rely on HPLC. An analyst who understands the instrument can spot a bad run before it becomes an out-of-specification (OOS) investigation. Interviewers therefore test three levels:

1. Concepts: principle, retention, resolution and the role of each part of the system.
2. Daily practice: column care, mobile phase preparation, system suitability and sequence set-up.
3. Judgement and integrity: what you do when a peak looks wrong, and how you handle integration and reprocessing.

For broader QC questions, see our top 50 QC interview questions.

Principle and components

1. What is the principle of HPLC?

Model answer: HPLC separates components of a mixture by pumping a liquid mobile phase at high pressure through a column packed with a stationary phase. Each component interacts differently with the stationary and mobile phases, so they travel at different speeds and leave the column at different retention times. A detector measures each component as it elutes.

2. What is the difference between normal-phase and reverse-phase HPLC?

Model answer: In normal-phase HPLC the stationary phase is polar (for example silica) and the mobile phase is non-polar. In reverse-phase HPLC, the most common mode in pharma, the stationary phase is non-polar (for example C18 bonded silica) and the mobile phase is polar, such as water or buffer mixed with methanol or acetonitrile. In reverse phase, more polar compounds elute first.

3. Name the main parts of an HPLC system.

Model answer: Solvent reservoirs, degasser, pump (isocratic or gradient), autosampler or injector, column oven, column, detector and the chromatography data system (CDS) that controls the instrument and processes data.

4. What do the numbers on a column label mean, for example “C18, 250 × 4.6 mm, 5 µm”?

Model answer: C18 is the bonded phase (octadecylsilane). 250 mm is the column length, 4.6 mm is the internal diameter, and 5 µm is the particle size of the packing. Smaller particles give higher efficiency but higher back-pressure.

5. What is the difference between isocratic and gradient elution?

Model answer: In isocratic elution, the mobile phase composition stays constant throughout the run. In gradient elution, the proportion of the stronger solvent increases over time, which helps elute strongly retained compounds and sharpen late peaks. Gradient methods need re-equilibration time before the next injection.

Mobile phase and column care

6. Why do we filter and degas the mobile phase?

Model answer: Filtering removes particles that could block frits and damage the column or pump seals. Degassing removes dissolved air that can form bubbles in the pump or detector, causing pressure fluctuation, noise and baseline drift. Our example SOP for mobile phase filtration assembly shows a typical lab procedure.

7. Why should buffer not be left in the column after use?

Model answer: Buffer salts can crystallise when organic solvent is introduced or the system stands idle, blocking the column and damaging seals. After use, the column is flushed with a buffer-free mixture and stored in the solvent recommended by the manufacturer or the method.

8. What is the effect of pH on reverse-phase separations?

Model answer: For ionisable compounds, pH changes the degree of ionisation and therefore retention. Small pH errors can shift retention times and resolution, which is why buffer pH must be measured and adjusted exactly as the method states, before adding organic solvent if the method says so.

9. What is a guard column?

Model answer: A short column placed before the analytical column to trap particles and strongly retained sample components. It protects the more expensive analytical column. It must be compatible with the method and allowed by the procedure.

Detectors

10. Which detector is most common in pharma QC, and why?

Model answer: The UV or photodiode array (PDA) detector, because most drug molecules absorb UV light. It is robust, sensitive enough for most assays and impurity tests, and a PDA also gives spectra that help check peak purity.

11. When would you use an RI or fluorescence detector?

Model answer: A refractive index detector suits compounds without a UV chromophore, such as some sugars, but it cannot be used with gradients and is temperature-sensitive. A fluorescence detector is very sensitive and selective for naturally fluorescent or derivatised compounds. Our HPLC calibration guide part 3 covers these detectors.

System suitability

12. What is system suitability, and why is it done?

Model answer: System suitability tests check, at the time of analysis, that the complete chromatographic system — instrument, column, mobile phase and method — performs adequately for the intended analysis. If system suitability fails, the run is not used for reporting results.

13. Name common system suitability parameters.

Model answer:

The acceptance criteria come from the method or the pharmacopoeial monograph, together with the general chromatography chapter. There is no single universal limit that applies to every method, so always quote the criteria written in the method you are running.

14. Can you adjust a pharmacopoeial HPLC method?

Model answer: Pharmacopoeias allow certain limited adjustments, such as small changes in column dimensions, particle size, flow rate or mobile phase composition, within defined ranges, as long as system suitability is met. Changes beyond those ranges need method validation or verification. The allowed ranges are listed in the general chromatography chapter.

Method validation

15. What parameters are evaluated in HPLC method validation?

Model answer: Under ICH Q2(R2), the typical characteristics are specificity, linearity and range, accuracy, precision (repeatability and intermediate precision), detection limit and quantitation limit (for impurity methods), and robustness, which is studied during development. The parameters needed depend on whether the method is an assay, an impurity test or an identification test. See our analytical method validation guide.

16. What is the difference between method validation and method verification?

Model answer: Validation proves that a new or modified method is suitable for its intended purpose. Verification shows that an already validated method, such as a pharmacopoeial one, works as expected in your own lab, with your equipment and analysts.

17. What is a forced degradation study?

Model answer: The drug substance or product is deliberately stressed with acid, base, oxidation, heat and light. This shows that the method can separate degradation products from the main peak, which makes it “stability-indicating”.

Troubleshooting

18. What could cause high back-pressure?

Model answer: A blocked inline filter, frit or guard column, a clogged column, buffer precipitation, or a blocked tubing or injector part. I would isolate the cause step by step, for example by removing the column and checking system pressure. Our HPLC troubleshooting steps give a structured approach.

19. Why might retention times drift?

Model answer: Changes in mobile phase composition or pH, column temperature, flow rate, a leak, insufficient equilibration or column ageing. Checking each of these systematically usually finds the cause.

20. What causes peak splitting or fronting?

Model answer: A void or damaged column inlet, a partially blocked frit, sample solvent stronger than the mobile phase, or column overload. Matching the sample diluent to the mobile phase is a common fix.

21. Why does the baseline drift or show noise?

Model answer: Air bubbles, a dirty flow cell, a failing lamp, poor mobile phase mixing, temperature changes or contaminated solvents. Degassing, flushing the flow cell and checking lamp energy are first steps.

Data integrity in chromatography

22. What is a “trial injection”, and why is it a problem?

Model answer: It is an unofficial injection made to “see” the result before running the official sequence. Regulators have treated unreported or deleted injections as serious data integrity failures. Every injection must be part of the official, traceable record, and any test or conditioning injection must be allowed by the SOP and documented. Our article on an FDA warning letter about unofficial HPLC injections shows the consequences.

23. When can you manually integrate a peak?

Model answer: Only when the SOP allows it, with a documented scientific reason, using the audit trail, and with second-person review. Manual integration must never be used to make a failing result pass.

24. Why must analysts have individual logins in the CDS?

Model answer: So that every action — injection, processing, reprocessing, approval — is attributable to one person, as ALCOA+ requires. Shared logins break attributability.

Frequently asked questions

Do freshers need hands-on HPLC experience to get a QC job?

It helps a lot, but it is not always required. If you used HPLC during your project or training, describe exactly what you did. If not, show that you understand the principle, system suitability and good documentation.

Which software should I learn?

Labs use different chromatography data systems, such as Empower, Chromeleon and OpenLab. The concepts — sequences, processing methods, audit trails — are similar, so understand the concepts first.

How do I explain resolution simply?

Resolution tells you how well two peaks are separated. Higher resolution means the peaks overlap less, so each can be measured accurately.

Is gas chromatography asked in the same interview?

Often yes, especially for residual solvents. Know the basics of GC columns, carrier gas and FID detection. See our GC troubleshooting guide.

References

• ICH. Q2(R2) Validation of Analytical Procedures (2023).
• United States Pharmacopeia. General Chapter <621> Chromatography (harmonised text, official 1 December 2022; includes the allowed adjustments of chromatographic conditions).
• European Pharmacopoeia (current edition). 2.2.46 Chromatographic separation techniques (harmonised with USP <621> through the Pharmacopoeial Discussion Group).