ADVANCES IN GC VALVES continued
certain applications. Diaphragm valves typi- cally last longer than rotary valves. Also, with their smaller size and built-in actuation, they give equipment designers more flexibility and reduce detector upset and valve artifacts that can corrupt the data.
However, problems with traditional diaphragm valves limited their acceptance in the industry. They were subject to leaks, including diffusion through the diaphragm. The temperature range was somewhat limited compared to rotary valves. Also, diaphragms could stick to the valve head, restricting flow and making it difficult to get good repeatability. Several ad- vances in diaphragm valve design overcome these challenges.
Again, the big breakthrough is the integrated purge feature. This not only prevents leaks around the valve body, but also prevents leaks due to diffusion through the diaphragm, unlike the earlier optional external purge ap- proach (see Figure 4).
Updates in diaphragm materials and coatings eliminate the problem of sticking, even after the diaphragm has remained unmoved for months. This means no unscheduled main- tenance or lengthy downtimes waiting for replacements.
High-performance materials and compo- nents extend the temperature range, and cutting-edge diaphragm valves can be used under vacuum. All of this makes diaphragm valves a better choice than ever for applica- tions requiring high cycle injections and/or higher sensitivity.
Stream selection valves Diaphragm technology has also improved the performance and lifetime of stream selec- tion valves. The diaphragm-based selection valves have multiple ports that can manage various sample inputs and outputs, extending the application range to isolation and flow diversion. Integrated purge, similar to that in the diaphragm valves discussed above, will reduce the outport and diffusion leak rate of multiposition diaphragm-based stream selec- tion valves. This in turn enables high sample integrity, leading to better results.
Figure 4 – Diaphragm valve with purge.
Figure 5 – Example of two-stream selection schematic.
These new stream selection valves have a fully swept flow path and zero dead volume, elimi- nating carryover between sample streams to deliver higher-quality data (see Figure 5).
In addition to integrated purge, the latest development in diaphragm stream selection valves is the introduction of independently controlled ports. These allow users to select sample streams in whatever sequence they require, optimizing the analysis routine. For example, these sample stream selection valves can be used to isolate samples dur- ing thermal desorption prior to injection. Independent ports also facilitate autocalibra- tion from a fixed position without contact with other streams.
AMERICAN LABORATORY • 14 • SEPTEMBER 2014 Conclusion
With the analytical industry trending to smaller sample sizes and lower detection limits, advances in rotary and diaphragm GC valve technology that extend life and reduce leakage mean the valve is no longer the constraining factor in GC equipment design or performance. Automated stream selection allows better use of any instrument with less operator activity.
Joseph Rotter is Director of Business Develop- ment—AFP, Norgren, Inc., 5400 S. Delaware St., Littleton, CO 80120-1663, U.S.A.; tel.: 720- 283-5706; e-mail:
joseph.rotter@
norgren.com;
www.norgren.com/norgrenafp
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52