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Comparison of Cryotrap desorbing temperature and the time of transfer to GC to analyte response and peak shape

Author: Mike Hiatt
Affiliation: U.S. Environmental Protection Agency, National Exposure Research Laboratory Environmental Sciences Division, P.O. Box 93478, Las Vegas, Nevada 89193-3478.
Phone: 702 798 2381. Fax: 702 798 2142.

Purpose. A study was conducted to evaluate the impact cryotrap desorb temperature and transfer time have on analyte response (using the surrogate analogues) and the peak shapes of 1,4-dioxane and pyridine. Our belief was that if too much distillate was transferred from the cryotrap to the GC the column, the column would be overloaded with water and polar compounds’ chromatography would be seriously impacted. Therefore we wanted to investigate how changing cryotrap conditions related to good chromatography and analyte response.

Instrument conditions. We have tuned the condenser to maximum analyte response (regardless of peak shape) where mass transfer (water) is between 0.3 and 0.6 g over a 7.5 min distillation. The GC conditions for this study is -25 deg for 6 min, ramp 50 °C/min to 40 °, ramp 5 °C/min to 120°C, final ramp 22°C/min to 220 and hold 9 min. Flows were approximately 4mL/min. The column was a VOCOL megabore interfaced to an Agilent 5972 through a jet separator. Note: This study is only an indication as to how cryotrap conditions may be affected when configured with a split injection GC/MS.


Measurement of impact from cryotrap conditions. In this study we only looked at 1,4-dioxane, pyridine, nitrobenzene, and naphthalene labeled surrogates. Fifty five distillations were performed using combinations of transfer time (between 2 and 10 min) and desorb temperatures (between 20 and 150 deg C). Two factors were being measured, intensity of analyte response and chromatography.

Maximum analyte response for each compound was determined and responses of the analytes were compared to their maximum as a fractional response. For each of the 55 distillations the average fractional responses (for the four compounds) were recorded.

Chromatography was measured by assessing peak shapes of both 1,4-dioxane and pyridine. The measured evaluation resulted in, poor (not easily distinguished), marginal (could be manually integrated with difficulty), acceptable (could be manually integrated easily) and good (automatically integrated by data system). If one of the compounds was rated marginal or less the chromatography was considered “bad”. If both compounds were good the chromatography was good (labeled “int” for integrated).

Cryotrap conditions were selected to identify where a maximum for the average fractional response occurred and where chromatography for both 1,4-dioxane and pyridine would at least be easily integrated (manually).

The resultant data was then reduced to maximum, 50%, and 20% of maximum fractional response for a desorb temperature (identified in figure as max, 50% and <20% respectively). Also recorded were conditions that produced poor chromatography as well as conditions that produced good chromatography (labeled as ‘bad’ and ‘int’ in figure).


Results. To simplify interpretation of collected data we graphed maximum, 50%, and 20% fractional averaged responses on a grid where y-axis is the cryotrap to GC transfer time and the x-axis is the cryotrap desorb temperature (target). Also included are those conditions that produced automatic integrations of both 1,4-dioxane and pyridine (labeled ‘int’) and conditions that produced one or both compounds to be have poor chromatography.

The best peak shapes for 1,4-dioxane and pyridine lie below the line ‘max’. Balancing good chromatography and fractional response, the zone between ‘max’ and ‘50 %’ indicate acceptable cryotrap conditions. From this graph the trade-offs are evident. The shorter transfer times have maximum, good peakshape, 50% maximum and low response all converging and likely will make results more erratic. The greater difference between maximum and 50% maximum is for longer transfers (at lower desorb temperatures). This gives the greates potential for consistent responses and good peak shapes but has the drawback of longer distillation times.

The 2 minute transfer time limitation is likely the ‘dead’ time associated with warming the cryotrap and the minimal helium flushing (helium flow(4 mL) × 2 min= 8 mL He) of the trap volume (~7 mL).

Cryotrap conditions; desorb deg vs. transfer time

For example (after a condenser is ‘tuned’) an analyst were to find good responses of all analytes but poor chromatography it would be useful to evaluate selecting a shorter transfer time and/or a cooler desorb temperature. Also if the analyst finds response poor but chromatography good a higher desorb temperature and/or longer transfer time should be investigated. If both low response and poor peak shape are experienced, the desorb temperature is too warm and transfer time is too long. If there is poor fractional response for all compounds and chromatography cannot be evaluated, the analyst should investigate both warmer desorb temperatures and longer transfer times.

The actual maximum at each desorb temperature is highest at 20 °C and lowest at 150 °C (see table below). The table shows the maximum average fractional response by desorb temperature. Again as a reminder we could get somewhat higher responses but they were not considered if either 1,4-dioxane or pyridine had poor peak shape.

desorb temp Average fractional response
20 .76
40 .53
50 .72
60 .44
70 .45
80 .65
90 .63
100 .58
110 .44
120 .51
130 .43
140 .49
150 .16

 


Conclusion. There is a range of cryotrap conditions that can be selected to provide good results for a given system (GC and vacuum distiller). The trade-off is time of analysis. It appears the lower desorb temperatures give additional chromatographic separation on the cryotrap (possibly separating or limiting water being transferred with analytes) and provide a greater ruggedness. The cryotrap settings should be between the “max” and “50%” lines where we also find the peak shapes of 1,4-dioxane and pyridine to be good.

Note: We do not expect this data will be identical for systems with a different configuration (flow split, column dia, phase and thickness). If after this tuning (max response and getting .3-.6g water) chromatography is still undesirable for 1,4-dioxane or pyridine cryotrap conditions should be evaluated. The above is a guide to evaluate modifying cryotrap conditions to improve chromatography if necessary.


 

 


 

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Author:  Mike Hiatt / Email:  Hiatt.Mike@epa.gov
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