Meet Low ng/L Detection Limits for Volatiles in Drinking Water Using Coconut Charcoal Cartridge SPE and a 50 µL Injection in an Unmodified Agilent Split/Splitless inlet - Find out how at the NEMC Drinking Water Section Thursday Afternoon

I've been working on combining EPA methods 521 and 522 for a while. Both methods use the same SPE cartridge and very similar extraction procedures. In order to meet extremely low PPT detection limits required by 521 on my 7890A-5975C, I had to resort to a 50 µL injection. Generally,a special injection port such as a PTV injector is required for this type of injection. With the PTV inlet temperature set near the boiling point of the solvent, the sample is introduced at a high split ratio and as the solvent evaporates the analytes of interest are concentrated in the inlet. After a predetermined time, the split valve is closed and the inlet temperature is increased to transfer the concentrated sample and remaining solvent onto the column. This solvent-venting, analyte-concentrating step requires a relatively large difference in boiling points between solvent and solute, more than 100 ⁰C, in order to prevent loss of analytes of interest to the split vent. This rules out using a PTV type injection port for the analyte list covered here due to inadequate differences in boiling points.

I made a few trial runs with my MMI operating in PTV solvent vent mode. I was not able to recover a detectable amount of THF-D8 (the 522 internal standard) when injecting a total mass of 50 ng until I set the oven temperature below the boiling point of dichloromethane, solvent focusing the analyte in question. The following figures are of 50 µL injections of 1.0 µg/mL standard

Now lets look at a 50 µL CSR-LVSI injection of the same 1 µg/mL standard in an unmodified split/splitless inlet on the same 7890A-5975C

If we overlay the MMI and CSR-LVSI runs, the difference in mass transfer to the column is significant.

Finally, the labeled chromatogram of the high point of my calibration curve.

 

 

[ng/mL]	ICAL 1	ICAL 2	ICAL 3	ICAL 4	ICAL 5	ICAL 6	ICAL 7	ICAL 8	R
THF-d8	100	100	100	100	100	100	100	100	--
THF	0.10	0.20	0.50	1.0	2.5	5.0	25	50	0.998
1,4-Dioxane-d8	200	200	200	200	200	200	200	200	1.90%
1,4-dioxane	--	--	0.50	1.0	2.5	5.0	25	50	0.999
n-nitrosodimethylamine-d6	20	20	20	20	20	20	20	20	3.60%
n-nitrosodimethylamine	--	--	0.025	0.050	0.13	0.25	1.3	2.5	0.998
n-nitrosomethylethylamine	--	--	--	0.10	0.25	0.50	2.5	5.0	0.998
n-nitrosodiethylamine	0.010	0.020	0.050	0.10	0.25	0.50	2.5	5.0	0.998
n-nitrosodi-n-propylamine-d14	10	10	10	10	10	10	10	10	--
n-nitrosopyrrolidine	0.010	0.020	0.050	0.10	0.25	0.50	2.5	5.0	0.997
n-nitrosodi-n-propylamine	--	--	0.050	0.10	0.25	0.50	2.5	5.0	0.997
n-nitrosomorpholine	--	0.010	0.025	0.050	0.13	0.25	1.3	2.5	0.994
n-nitrosopiperidine	0.010	0.020	0.050	0.10	0.25	0.50	2.5	5.0	0.997
n-nitrosodi-n-butylamine	--	0.020	0.050	0.10	0.25	0.50	2.5	5.0	0.997

I'll be presenting the details at 3:30 on Thursday in the drinking water section at NEMC. I'll be posting the details here on the blog over the next few weeks too.