Issues with Scaling and Method Transfer

There are many challenges associated with moving a method whether that be on the same nominal scale or indeed from one scale to another to ensure that a successful separation is obtained. There are a variety of parameters that have to be considered, and not all of them are obvious since some of them will relate to the system that is employed. Examples are given which demonstrate the importance of considering all the aspects of the method transfer.
In the first example the helpdesk was initially asked to develop the separation for some very polar compounds for a pharmaceutical purity study. The role of the helpdesk was to improve the resolution between the active pharmaceutical compound and the impurity to allow a real sample to be analysed. The method development was successful, and a method was developed; An example chromatogram is shown in Figure 1, and although not from the original method, confidentiality issues prevent this, it does highlight the issues of method transfer.
The method was validated with standards and subsequently used with real samples with no issues. Eventually the customer looked to outsource the product with the analytical method, this is where issues occurred. The contractor received the method and reported back to the client that the method ‘was not working’, as ‘there was a shift in the retention times for some of the components’. Initially the column utilised was suggested as introducing variability into the assay, so an identical replacement column was purchased which also exhibited the same retention time shifts.
The method returned to the customer to confirm that the assay was working, no issues were observed. An investigation into the root cause determined that the issue related to the types of pump that were being used, specifically the dwell volumes of the two pumps were quite different.  Figure 2 demonstrates the effect that the two systems with different dwell volumes can have on the retention times for some of the peaks.

This example, which was based on a column packed in all cases with the same fully porous particles, was interesting since it can easily be avoided. However, the helpdesk considered if there were other situations where changes to the assay (in the first example it was the type of instrumentation) occur which might not have such an obvious effect. In the example given previously a fully porous particle was being used, however this poses an interesting question regarding changing the substrate material. One of the major differences associated with the superficially porous material packed column is that the column has less pore volume than its corresponding fully porous material packed column. Does this then mean that it may not be feasible to transfer some gradient based methods from fully porous to solid core technology because of the issues associated with the effective dwell volume of the column? For an understanding of this, it is important to understand the relative volume differences that are present in the two different packing material formats of columns.
In a fully porous column the amount of space available for mobile phase is about 70%, which means that in a 150 x 4.6 mm column the void volume of the column is:..............

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