Liquid Chromatography Questions and Answers

Answer: C

Loading a large volume of sample and eluting into a fraction of it raises the concentration by the ratio between them, often a hundredfold. Detector noise is unchanged, column retention is a separate property, and the injection volume is limited by the column. Concentrating before measuring is more effective than any improvement to the measurement itself when the analyte is simply too dilute.

Answer: D

Components extracted alongside the analyte can suppress or enhance its detector response, particularly in electrospray, so a calibration in clean solvent no longer applies. Irreversible retention, absorbance and viscosity are separate problems that also exist. Because the effect varies between samples of different origin, it is measured with a post-extraction spike rather than assumed negligible.

Answer: A

Spiking a known amount into an already extracted blank matrix and comparing the response with the same amount in solvent gives the suppression or enhancement directly. A blank alone shows interference rather than suppression, duplicates show precision, and two columns test selectivity. Measuring an effect by constructing the two conditions that differ only in it is the general design, and here the difference is the matrix itself. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Answer: B

Adding a reagent between column and detector converts the analyte into something the detector sees far better, without the derivative having to survive the chromatography. Eluent removal, concentration and neutralisation are other functions. Derivatising after the separation rather than before avoids changing the analyte's chromatographic behaviour, at the cost of extra plumbing and the band broadening it brings. The shared properties explain why these belong to one family, and the differences explain why more than one exists.

Answer: C

If the underivatised analyte is poorly retained or tails badly, converting it first can transform the chromatography as well as the detection. Reactors are readily available, reaction speed varies, and detectors tolerate reagents routinely. Choosing between pre- and post-column derivatisation means asking whether the problem lies in the separation or only in the detection, and the answer settles it immediately. A single measurement chosen to respond to the one asymmetry beats several that respond to shared behaviour.

Answer: D

Injecting a standard and checking the plate number, tailing factor, resolution and repeatability confirms that column, instrument and eluent are together fit to run the batch today. Validation is done once for the method, and wavelength and pressure checks are instrument qualification. Testing the whole system on the day, rather than trusting a validation performed months earlier, is what catches a degraded column before a batch is wasted. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: A

Tailing factor, plate number, resolution from the nearest peak and the repeatability of replicate injections are the standard four, each reporting on a different aspect of the system's condition. Lamp hours, room temperature and eluent age are good practice to record but are not performance measurements. Choosing criteria that respond to the failures actually experienced is what makes such a test worth running. Reading the definition literally, rather than recalling a formula, is the reliable route through questions of this shape.

Answer: B

The first two are correct and the third confuses two problems: filtration removes particulates and does nothing about dissolved co-extracted material, which is what suppresses or enhances the response. Matrix effects are addressed by clean-up, by matrix-matched standards or by a labelled internal standard. Each preparation step solves a specific problem, and assuming one covers another is how an uncorrected bias survives into the result.

Answer: C

The plate number has fallen below its limit, which points at a degraded or partly blocked column, and the batch must not proceed until it is resolved. Passing two criteria out of three is not passing, wavelength does not affect efficiency, and a larger injection would reduce the plate count further. A suitability test that is overridden when it fails serves no purpose at all, which is the point of setting the limits in advance. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Answer: D

This is a system suitability test. Validation establishes a method's performance once, qualification demonstrates that an instrument is installed and operating correctly, and calibration relates a response to a concentration. Four activities on four different timescales, and using the right term matters because a regulator will ask which of them was performed.