How to Select Your Sample Interface


Perhaps the most critical decision in designing a successful online analyzer system is the selection of the sample interface.

Guided Wave pioneered in-line fiber optic sample probes more than two decades ago, with the introduction of the SST (Single Sided Transmission) Probe. This probe permits precise spectroscopic analysis of products directly in the process line without the need for sample systems and their associated investment and installation costs, lag times, failures, and constant maintenance requirements. When sample systems fail, reliable measurements stop, resulting in system downtime that is not the fault of the analyzer. Sample systems have their place and should be used when samples are particulate-laden, wet (2-phase), or overly temperature sensitive. Otherwise, in-line probes are the most cost-effective and trouble-free solution to real-time process spectral analysis.

While an insertion probe approach can often reduce installation costs, sometimes safety, servicing, and/or sample conditioning requirements make it necessary to integrate a sampling loop or slipstream. Flow cells are preferred whenever:
(a) direct insertion probes are inappropriate and
(b) the required optical path length is 30 mm or greater.

Another consideration is usability. All probes and flow cells must be referenced periodically. The period will depend upon the instrument and the process and may range from one hour to several months. Depending on the process, windows may become fouled in time and require cleaning. Therefore it is convenient if some accommodation has been made to either remove the probe from the process for these required services or to be able to perform them in place. Guided Wave offers a probe extraction device and/or cleaning ports on the flow cell to facilitate this maintenance.

In Situ Sampling:
• No Sample Prep
• No Lag Time
• Representative Sample Assured Service
Sample Loop Required:
• Sample Prep Possible
- Thermostating
- Filtering
- Drying
• Lag Time (depends on fast loop design)
• Probe must be removed from process for cleaning and
• Extractor Mechanisms available
• Can be cleaned and zeroed in place
• Excellent
• Excellent
• Sample Systems require maintenance

Selecting Flow Cells

Insertion probes can be directly dipped into a laboratory beaker or inserted into a process pipe, so that the light is truly brought to the sample. However, in some applications, it may be more convenient to compromise and meet the sample halfway by using a fl ow cell. When it is not possible to insert a probe directly into a process line due to safety reasons, service requirements or the need to precondition the sample, a fl ow cell is the best choice. Optically establishing and maintaining alignment of the two halves and ensuring that the pathlength is constant and reproducible are key for continuous performance. Flow cells are convenient for their serviceability. They are usually mounted in a process side loop for maintenance reasons. Often, the seal technology is simple o-rings which require periodic service. Hence, unlike most insertion probes, fl ow cells are designed to be disassembled in the field. On re-assembly, it is critical that the flow cell design
is such that the pathlength returns to its original value. Some common convenient auxiliary features of flow cells may be thermal control, clean-out ports, and adjustable pathlengths.

Selecting Insertion Probes

Selecting the proper probe is crucial to the success of the
application. NIR and UV-Vis spectroscopy both cover many
varied applications, which require a varied array of probes.
While probes are predominantly passive devices in an
otherwise active measurement system, their physical and
optical properties can signifi cantly add to or detract from
the desired measurement. Effective pathlength, optical
properties such as throughput, collimation, aberrations,
sensitivity to vibration, sensitivity to thermal changes, and
the quality of fi ber connectors are important parameters
in a probe design.

The probe is contained in a housing that must be sealed to
the process to prevent escape of the sample. The housing
is usually metallic but may be glass, ceramic, polytetrafl
uoroethylene (PTFE), polymer or any material compatible
with the process. The probe-to-process seal may be
o-rings, energized seals, compression fi ttings, chevron
seals, welded fl anges, etc. The process conditions and
parameters will mandate the seal and safety requirements.

Servicing Probes and Flow Cells

No matter the manufacturer all probes and flow cells used for absorption spectroscopy must be referenced (or zeroed) periodically. Furthermore, the windows may need periodic cleaning plus o-rings and other elements may need service.

Therefore practical methods for maintenance of the probe and flow cell, access to the windows, or complete disassembly and reassembly abilities must be considered when choosing new sample interfaces.

See Guided Wave’s In-line Probe Extractor Mechanism (data sheet #1046) which allows for quick and easy probe cleaning without interruption of the

The Smart Choice

Guided Wave’s sample interfaces deliver accurate, real-time process measurement results. Durability, as well as the low maintenance requirements make Guided Wave sample interfaces a cost effective, smart choice to help optimize production, improve yields, ensure consistent product quality and enhance profitability.

Guided Wave has been a leader in online, fi ber-coupled, process probe and fl ow cell design for over 35 years. While Guided Wave’s probes and fl ow cells are optically matched for use with our NIR or UV-Vis analyzers to provide top system performance, we also supply sample interfaces for use with many other analyzer brands.

Our reputation, experience and design compatibility have made Guided Wave the “go-to” provider worldwide for direct insertion probes and sample loop flow cells:
• For liquid or gas streams
• Optical measurement pathlengths from 1mm to 1 meter
• Customized, innovative designs to meet users’ specifi c process and safety needs
• Optically effi cient designs which deliver more light, ultimately providing more chemical specifi city and lower detection limits
• Rugged designs built to withstand harsh process conditions, while maintaining safety, ease of maintenance and longer service life
• Intrinsically safe designs (white paper #2029) - meets ANSI/ISA dual seal requirement and IEC 60079-28 standard
• CRN Certification available
• Common integration with most analyzers; manufactured to facilitate full integration with any fi ber optic system configured with SMA905 connectors and fi bers having a core diameter from 400 to 600 micron

probe monitoring

The sample interface is a virtual "windows into the process" and is a critical component of any optical analyzer system. Guided Wave sample interfaces (probes, flow cells) are designed for use with NIR or UV/Vis fiber optic coupled analyzers. They are designed to be rugged and reliable in harsh chemical and physical process conditions while providing optimal transmission for long-term reliable measurements.

More information is available about specific products can be found on each specific products page.