Why do your sample peak areas keep fluctuating? It could be down to the capping process.

 **Author:Orginal from Ms Xiaoyue Lü | HAMAG

You prepared a batch of samples in the morning, and the first injection went perfectly. When I tested them again in the afternoon, however, the peak areas had shrunk considerably.

The next day, you prepared the samples afresh, and the results returned to normal.

So you began troubleshooting the instrument: checking the column pressure, changing the flush solution, inspecting the injection system, and even running the method all over again.

After all that, the problem still remained unresolved.

In fact, when encountering fluctuations in peak area, apart from the instrument and the sample itself, there is another factor that is easily overlooked: the sealing of the sample vial.

This is particularly true for samples that need to be stored for long periods, or for analyses using headspace injection; the seal between the cap, the gasket and the vial opening is also worth investigating.

Today, let’s discuss this seemingly simple yet easily overlooked step—capping.

PART 01Peak areas are fluctuating – don’t just focus on the instrument

When faced with unstable analytical results, many people’s first instinct is to check the instrument.

You would typically check:whether the column pressure is stable · whether the column temperature has reached the set point · whether the baseline is normal · whether the injection needle is blocked · whether the injection volume is stable · whether there have been any deviations in sample preparation

These checks are, of course, necessary.

However, if the problem persists after routine troubleshooting, it is worth taking a closer look at what has happened to the sample between its preparation and actual injection.

During this period, the sample is stored in a sample vial. Whether the cap fits correctly, the gasket is suitable, and the seal is reliable can all affect the condition of the sample.

For samples containing volatile components, poor sealing may lead to loss of components during storage; for headspace sampling, the preservation of volatile components within the vial and the state of headspace equilibrium also warrant attention.

Therefore, when peak areas fluctuate, the integrity of the sample vial seal is certainly an area worth investigating.

It should be noted that variations in peak area are not necessarily caused by the capping process. Sample stability, sample preparation, the injection process and the condition of the instrument may all influence the final results.

The key is not to rush to conclude what has gone wrong, but to ensure that no potential contributing factor is overlooked.

PART 02 The cap looks fine, but is it really sealed?

First, let me ask you a question: how do you usually tell if a cap has been crimped properly?

It doesn’t wobble or come off; it looks flat and feels reasonably secure to the touch.

These observations can help identify some obvious anomalies, but just because it looks acceptable on the surface does not mean it is properly sealed.

The effectiveness of a sample bottle’s seal depends on whether the bottle mouth, cap, gasket and sealing method are compatible, as well as on whether the capping process is carried out correctly.

There are three main types of common anomalies:

① Incomplete capping

For screw-top bottles, if the aluminium cap has not been correctly crimped into place, or if the capping head does not match the bottle neck specifications, this may result in an unreliable seal.

If the sample contains volatile components and there is a sealing defect at the bottle neck, component loss may occur during storage.

② Over-tightening the cap

Tightening the cap is not necessarily better the tighter it is.

If the cap is tightened too much, or if the cap head does not match the bottle neck specifications, this may cause abnormal deformation of the aluminium cap, and may even affect the condition of the gasket and the reliability of the seal.

The material, structure and chemical compatibility of the gasket are equally important. Even if the aluminium cap appears to be tightly secured, one must not assume that the seal is reliable based on appearance alone.

③ Inconsistent capping conditions within the same batch of samples

When processing samples in batches, manual operations may be affected by operational habits, fatigue levels and individual differences amongst staff.

In the absence of clear operational standards, capping conditions within the same batch of samples may vary.

Whilst these variations may not necessarily be directly identifiable from the chromatograms, they are nonetheless worthy of attention in situations where sample preservation and analytical reproducibility are of concern.

This is not to say that manual capping is necessarily unreliable, but rather that as the number of repetitions increases, it becomes more important to establish a consistent and repeatable method.

PART 03Why is consistency so important in capping?

Anyone who has processed samples in batches will know that capping a single sample is no great feat; it is only when capping dozens or even hundreds in succession that the task becomes truly repetitive. When performed manually, there may be variations between different operators, and even the same operator’s performance may vary from one occasion to the next.

If one relies solely on feel to judge each time, it is difficult to establish a consistent standard for the capping process.

For laboratories conducting method validation, stability studies or those requiring batch sample processing, operational consistency is a key concern. Of course, this does not imply that variations in capping will necessarily lead to changes in peak area, nor should all analytical errors be attributed to the capping process.

What we truly need to do is minimise unnecessary operational variations in sample preparation and capping as much as possible, thereby making the entire analytical process easier to control.

We would also like to emphasise one point here the sealing mechanisms and dimensions vary between different sample vials. Threaded vials require matching caps and a specific screw-on method, whilst crimp-top vials require aluminium caps of the appropriate specification, crimping tools and equipment that meet the relevant requirements.

When selecting a model, you should base your choice on the actual bottle shape and the equipment’s compatibility requirements. If the specifications do not match, applying considerable force during operation does not guarantee a reliable sealing result.

PART 04Standardising Repetitive Capping Operations

Given that consistency is key in capping operations, is there a way to minimise human variation in repetitive tasks?

This is also a practical reason why laboratories opt for electric cappers.

Hamai electric cappers are primarily designed to meet the capping requirements of laboratory sample vials. Compared to methods that rely entirely on manual force, electric capping helps to minimise human variation in repetitive tasks, whilst also reducing the physical strain involved in capping large batches.

More Consistent Force Application

When capping repeatedly by hand, the force applied may vary. Using electric capping equipment helps to ensure greater consistency in the capping process.

More Standardised Sealing

Reliable sealing involves more than simply pressing the aluminium cap down firmly; it also requires the bottle neck, cap, gasket and capping equipment to be properly matched. Selecting equipment according to specifications helps to minimise sealing issues caused by mismatched specifications or improper operation.

Less labour-intensive for batch operations

If large numbers of samples need to be processed daily, repetitive capping can be time-consuming and physically demanding. Electric capping machines can perform repetitive capping actions, reducing the burden of sustained manual effort.

Please note when selecting equipment, you should first confirm the actual bottle types and specifications to be used. Different specifications may require corresponding equipment or capping components; specific requirements are subject to product compatibility.

An electric capping machine cannot replace sample stability assessments, nor can it guarantee that all fluctuations in peak area will be resolved. Its value lies in making it easier to maintain consistency in the repetitive task of capping.

PART 05 Peak areas fluctuating again?

You might want to keep this checklist to hand.

The next time you encounter fluctuations in peak areas, try troubleshooting step by step using the following guidelines.

1.Check the seal on the sample vial

Examine the vial cap, the crimped edge of the aluminium cap and the gasket for any obvious abnormalities. When verifying airtightness, use a validation method suitable for the specific vial type and application; do not rely solely on visual inspection.

2.Confirm that the cap and gasket are compatible with the sample vial

Check that the specifications match, that the gasket material is suitable for the sample and the solvent used, and that the sealing method is correct.

3.Check the consistency of capping operations

Compare the capping condition of different sample vials, noting any obvious differences in the crimp, abnormal deformation or inconsistencies in the procedure. Where necessary, further validate the sealing performance.

4.Verify sample handling conditions

Pay attention to the time elapsed between sample preparation and injection, storage temperature, sample tray conditions, and the stability of the sample itself.

5.Review sample preparation

Verify steps such as weighing, volumetric dilution and dilution, and observe whether the sample exhibits precipitation, separation or other abnormalities.

6.Check the injection system

Depending on the instrument type, check the injection needle, injection volume, injection repeatability and needle flushing.

7.Check instrument status and analytical conditions

In accordance with the specific method, check column pressure, flow rate, column temperature, baseline, detector and other relevant conditions. If retention times have also changed, a further investigation should be carried out, taking into account the flow path and analytical conditions.

The above is a practical troubleshooting framework; not all methods need to be followed in a fixed order. In practice, the order of checks should be adjusted according to the symptoms of the fault, the requirements of the method and the available evidence.

When experimental results show fluctuations, we tend to start by checking the instrument, the sample injection system and the sample preparation process.

Whilst these are certainly important, there is another step—often overlooked—between the preparation of the sample and its introduction into the instrument: capping.

Capping is not the sole cause of peak area fluctuations, but it is a crucial aspect of the sample preparation process that warrants attention.

For laboratories handling large volumes of samples, carrying out frequent capping procedures, or with high requirements for sample storage and operational consistency, selecting suitable capping