In this article I discuss the dichotomy between the often touted benefits of fast GC methods and the proportion of GC methods actually in use.
Chromatographers should be aware that one of the two primary advantages of GC over HPLC is its speed (the other is its detector choices and performance). Commercial GC instrumentation has continuously evolved to take better advantage of the potential speed but practitioners have been slow to implement. Why is that?
In conversations that I have had with chromatographers and students in GC courses that I taught, I asked, “What is the definition of fast GC?” My favourite answer is “anything significantly faster than what I do now.” I like this from a practical sense because the benefits of changing a behavior are always measured relative to the current behaviour. Even if someone is already using a method that is faster than average, there may be compelling justification to move even faster.
So what is the actual proportion of the methods that are currently in use? Since the speed is related to the column internal diameter (i.d.), I thought I’d ask the marketing managers of two major column manufacturers what the general distribution of column diameters was that were sold. Their answers were, of course, general, since the actual numbers are considered confidential, but the answers were fairly consistent: 320 µm > 250 µm > 530 µm > 100 µm.
This was a bit surprising to me, especially since the default columns shipped with new instruments are typically 250 µm i.d. That alone should exaggerate not only the current sales numbers of 250 µm columns but should also influence the development and subsequent adoption of new methods that would then be implemented and require replacement purchases. So I was expecting 250 µm columns to be in first place. If customers picked the column they wanted to be shipped with their new instruments, would they pick 250 µm or would they want larger sizes?
If the instruments can easily accommodate smaller columns, what keeps people from using them? Most obvious is that many of the methods in use were originally developed using larger columns. The analytical body of results and company experience are all tied to those original methods. The time, cost and risks associated with changing a method, especially in a regulated environment, can far outweigh the potential advantages. In fact, many methods are “prescriptive” and changes cannot be made.
However, plenty of time has passed since instrumentation could easily use smaller i.d. columns. There has been plenty of time to adopt small i.d. columns and fast GC methods when developing and validating new methods. So what is the attraction of larger column diameters? The most significant limitations that often steer people away from using small bore columns (in my opinion) are capacity and ruggedness, which are related. When a column’s capacity is exceeded, it is “overloaded”; peaks become broader and distorted. This degrades resolution and causes retention times to shift.
In Table 1, important characteristics of columns of the same efficiency are compared including relative speeds and capacities. The stated peak widths are calculated based on isothermal elution at k’ = 1, which is similar to what the width would be in a typical temperature programmed run.
The reason one uses narrower columns in fast GC is to take advantage of the fact that efficiency is related to the ratio of length/diameter. One can achieve the same separation power with a 2.5X shorter 100 µm column compared to a 250 µm column. However, capacity is directly related to the mass of stationary phase in the column. That in turn is related to the square of the radius and the length. The capacity, and, therefore, usable dynamic range of the100 µm column, would be (125/50)2 × 2.5 = 15.6 times less than the corresponding 250 µm column.
Smaller capacity affects the usable dynamic concentration range that a given method can cover. It is more about the high concentration end than the low end of the range. Even though narrower diameter columns can theoretically improve detection limits (improve S/N) because they yield narrower peak widths (higher peaks for the same sample amount), much of the potential improvement is reduced because detectors need to operate faster for the narrower peaks, lowering S/N (faster acquisition = less signal averaging = lower S/N).
The most significant limitations to using narrow bore columns for trace analysis, however, are practical ones. It is much more problematic to get the same sample amount into narrower columns as can be effectively introduced into a larger column. The lower capacity of small diameter columns not only affects individual analyte capacities but also impacts the amount of condensed solvent that it can safely accommodate. This is important for splitless and cool on-column sample introduction techniques typically used for trace analysis. The dramatically decreased capacity and lower internal surface area per length cause a given condensed solvent volume to spread out much farther into the column (broader initial band widths) and also increase the tendency of the condensed film to break up, causing split peaks. This potential problem requires that much more time and care be spent during method development (solvent selection, injection conditions, addition of retention gap or pre-columns…) with small bore columns than when larger columns were used.
The lower capacity also makes the columns much less rugged. Harsh solvents and dirty sample matrices can much more quickly affect the performance of small diameter columns than larger ones. And the shorter columns show the degradation sooner.
As far as some of the stated motivations for adopting fast GC, I think that many financial calculations showing lower cost per sample with fast GC methods are a bit too simplistic. True, more samples can be run in a given time period on a given instrument but the cost of analysis is often dominated by the labour involved on the front end (sample prep) and back end (data review), making the costs associated with running the instrument (and, therefore, any savings) less significant. Calculations of instrument operation costs are also often overstated. For example, instrumentation cost/sample in temperature-programmed analyses is dominated by power consumption and carrier gas use. The power required to heat and cool per analysis is the same (or more) for temperature-programmed fast GC methods in conventional GCs because they heat over the same temperature range and require faster ramp rates (less power-efficient). In addition, the use of carrier gas is dominated not by the column flow-rates (inversely related to column diameter) but by the split flow. Since small bore columns have lower capacities and problems with condensed solvents, split injection is usually used. Due to the lower capacities, higher split ratios and, therefore, higher split flows are used for small bore columns, making total carrier gas use higher than with larger bore columns.
Any of the above explanations can explain the slow adoption of fast GC methods and small bore columns. Potential benefits do exist. Most of the issues in implementing fast GC methods can be dealt with (e.g., use a retention gap when doing splitless or cool on-column injections and to protect the column from aggressive solvents and sample matrix, use Gas Saver mode to reduce split vent flow after sample introduction to reduce carrier gas use), however, one has to become informed about the fundamentals and pay attention to what is going on as one develops and validates. My goal is to continue providing useful context and information through these monthly articles that help you to make good decisions regarding development and troubleshooting of GC methods and increases your likelihood of success whatever path you choose.
References
1. M. S. Klee, MS & L. M. Blumberg, J. Chrom. Sci. , (40), 234-247 (2002).
This blog article series is produced in collaboration with Dr Matthew S. Klee, internationally recognized for contributions to the theory and practice of gas chromatography. His experience in chemical, pharmaceutical and instrument companies spans over 30 years. During this time, Dr Klee’s work has focused on elucidation and practical demonstration of the many processes involved with GC analysis, with the ultimate goal of improving the ease of use of GC systems, ruggedness of methods and overall quality of results. If you have any questions about this article send them to techtips@sepscience.com
