EPA Method 552.3

Question

EPA Method 552.3 uses a liquid-liquid micro-extraction and runs the extracts through a GC-ECD in order to quantify haloacetic acids in drinking water. I have been the primary analyst on this method for about 6 months. While I have an operating procedure that works well, I am always looking to optimize and fine-tune certain aspects.

In every chromatography forum I have ever seen, a constant problem with Method 552.3 is the co-elution of monochloroacetic acid with other contamination peaks. I was thinking about attaching a liquid nitrogen dewar to the oven in order to change the start temperature of the temperature program from 30 degrees Celsius to 0 degrees Celsius. Does anyone have any experience with this? Is this worth my time and effort?

Also, I use two different Restek columns. Stx-CLPesticides 30 meter x 0.32 mmID x 0.32 um df, and Stx-CLPesticides2 30 meter x 0.32 mmID x 0.25 um df. My predecessor favored these columns, but said Agilent columns were very similar. Does anyone have any experience with the Agilent columns, and how much success do you have using them?

Any insight would be great! Even if it does not pertain to these two questions, any good tips anyone can think of would be helpful.

timstrutt · Accepted Answer

Hi,

Method 552.3 uses narrower bore columns than yours:

"PRIMARY GC COLUMN – DB-1701, 30-meter length, 0.25-mm i.d., 0.25-μm film"

"CONFIRMATION GC COLUMN – DB-5.625, 30-meter length, 0.25-mm i.d., 0.25-μm"

The phase ratios are the same (0.25mm/0.25µm and 0.32mm/0.32µm) so relative retentions should be the same but the narrower-bore "official" columns will give you greater resolving power (~88,000 Plates against ~69,000). This will allow better separations at the expense of maximum sample analyte capacity (~50ng against ~200ng).

On modern instruments with a "clean" matrix like drinking water (when you don't normally inject large amounts of material) you can reduce the ID of the column and keep the same film thickness to give a similar effect - If the resolution is adequate you can also save money by using a shorter column (its efficiency will be higher) and your analysis time will be shorter for a similar resolution. The peak will also be narrower which helps counteract the significant decrease in maximum sample capacity (the same amount of analyte injected on a 0.18mm id column gives roughly twice the peak hight compared to a 0.25mm id column).

As you say, using a lower initial column temperature will allow better separation of early eluting compounds like MMCA, but cryogenic cooling is usually expensive to run (and you can run out of Liq-N2!) so it is not ideal for a routine analytical method. It is likely that increasing the column film thickness (hence changing the phase ratio) will give better resolution of early eluters, but can also give a high bleed for late eluters, and increases the analysis time.

Your current columns are "tuned" for this type of analysis and are often also used for general pesticide work - They can also be purchased in different lengths and diameters.

Agilent have their own tuned columns for this work: DB-CLP1 & DB-CLP2 - They both have a 0.32mm ID, but the CLP1 has a film of 0.25µm and the CLP2 has a film of 0.5µm: https://www.agilent.com/cs/library/applications/5991-0615EN.pdf

If you have a lot of interfering coeluters, it may be worth checking the purity of your solvent, Na2SO4, NaHCO3, sulfuric acid, vials, glassware, etc.