Residual Solvents by Headspace - Repeatability issue

[christenseng]

Has anyone experienced high %RSD when performing residual solvents analyes by headspace using a static headspace autosampler such as the CTC-PAL? In particular, I'd like to know if you have encountered injections that are higher than normal. We've experienced this using DMSO as a diluent, both neat and with added water. Analytes span the range of polarity and bp. Incubation temps have ranged from 50-80C, time 10 min. Thanks.

[Axar]

Ten minutes are not enougth to reach equilibrium. And headspace always gives hight %RSD. My SOP determines RSD as not more than 15% for HS.

[Peter Apps]

Off the shelf, both valve and loop and syringe based equilibrium headspacers will give RSDs of about 3% for raw peak areas. Any worse than that is a symptom of poor setup or inapproproate conditions. With tweaking and optimisation you can get 1% RSDs for raw peak areas, with unconventional operating conditions and/or hardware modification you can get better than 0.5 %.

The weak point in the CombiPal clones is the low thermal mass of the sample oven. Syringe injections are also vulnerable to flow and pressure conditions during injection, and headspace sample volume and injection rate have to take column flow and split ratio into account.

There is a lot more the repeatable headspace analysis than changing the sample oven temperature.

Peter

[vajjara]

Dear Christen,

the CTC-PAl is a syringe technique. In normal conditions it has a quenching effect as the syringe needle shifts from the hot region (Vial) to cold ( lab condition) again back to injector ( Hot condition).

Moreover the syringe shaft is heated. So what happens? You cant use the same HSS syringe at a higher temperatur, then at a lower temperature. Meaning there will be a leak, or the volume of sample taken will be less. Hence for a better % RSD you had to use transfer technique.

Try with Teledyne Tekmar they have both Static and Dynamic HSS in one.

Vajjara

[krickos]

Hi

First, it is possible to obtain an accepteble equillibrum on 10min if the low sample volume approach that Rodney often mention is used.

As Peter touched at, injection speed etcetera is also a point, if you search the forum you should find a few topics on that with recommendations.

When it comes to recondensation issues it is a bit surprising how often analyst overlook the diluent. In this case the diluent has a bp of 189°C. Initially if I recall right the combi pal could heat syringe to 125, nowdays 150°C if I recall right. Still this is at least 50°C less that what most loop instruments offer and 40°C below the bp of the diluent. So I have always been a bit suspicious about the syringe technique when high boilers are involved.

So if the diluent/analyte do recondensate in the syringe or a loop for that matter it will sooner or later cause you sensitivity issues and precision issues.

If you post more detail on RSD values and instrument parameters, those more familiar with the combi pal might be able to help you better.