Which LC Detector Should I Buy?

You might not like the answer…

So the other day, I was with a customer when they asked me, “Viet, there are so many detectors out there! How do I know which one I need?”  To which I answered with the one comment that all of you will love to hate, “It depends!”

Detectors for chromatography instruments come in all shapes and sizes. There is not one type that fits all applications, and sometimes more than one detector may be needed to perform complete analysis of your sample.  Optical detectors fall into two types:

1. Selective Property Detectors that measure concentration by using a unique attribute of the sample (e.g., UV/Vis detectors)
2. Bulk Property Detectors that measure changes
   in the solvent and solute as a whole (e.g., Refractive Index Detectors)

My hope is to eliminate some confusion brought about by the wide array of available detectors to help you make a more educated decision on which detector is right for your project.

So which LC detector should I pick?

UV/Visible (UV/Vis) –UV/Vis detectors come in a variety of forms: fixed wavelength, variable wavelength.  These detectors are traditionally limited to one or two wavelengths, and as their name suggests they need to have the ability to absorb in the UV/Visible range, typically around 190-800 nm range.  They  tend to be used in routine analysis where the detected wavelength does not change and the  most common usage is limited to one wavelength at a time.  Because the wavelength is not being changed during an analysis, the baseline noise tends to be very low.  The combination of good sensitivity, broad range of application, excellent linear dynamic range, and ease of use makes the UV/visible absorbance detector an excellent general-purpose detector.  UV/Vis detectors are the most common in the industry and have specific functionality that typically makes them the cheapest detectors on the market.

Photodiode Array (PDA) – A PDA detector is similar to the UV/Vis detector because it requires a sample with a chromophore that absorbs in the UV/Visible range.  The distinct advantage of these detectors over UV/Vis detectors is that entire spectrums are collected, which allow the absorbance at multiple wavelengths to be monitored simultaneously.  The collection of spectra also allows the user to assess the homogeneity (a.k.a. purity) of a peak by comparing the spectra within a peak.  The ability to assess peak homogeneity and to monitor multiple wavelengths simultaneously makes the PDA detector especially attractive for methods development and analysis of complex samples.  PDA detectors bring a great depth of information and extra detail in the data. As a result these detectors tend to command the highest price of all detectors until you get into mass detection.

Fluorescence (FLR)  – The FLR detector is typically selected when there is a need for selectivity or more sensitivity than can be provided with the other detectors.  The most important aspect when selecting this detector is that the sample you wish to analyze must fluoresce or you must derivatize the sample with a fluorescing agent.  The desirable attributes of the FLR detector include specificity towards certain compound classes and high sensitivity.  FLR detectors tend to be priced higher than a UV/Vis detector because the detector needs two of everything, but you should not expect to pay double the cost of a UV/Vis detector. Common uses: amino acids, aflatoxins, vitamins, polyaromatics

Refractive Index (RI) – The RI detector is considered a universal detector.  The RI detector was the first type of detector created for Gel Permeation Chromatography (GPC).  It is dependent on the fact that all samples will change the refractive index of the solvent, and thus it can detect any compound.  The refractive index detector monitors and measures the bulk refractive properties of the mobile phase passing through by comparing against a reference of unadulterated mobile phase.  As other compounds are carried along by the mobile phase, the change in refractive index, when compared to the reference, is registered.  The drawback when using a refractive index detector is that you can only run isocratic methods. Because it is a universal bulk property detector, its sensitivity is limited.  Due to the limited capabilities and functionality of these detectors they tend to be around the same price as a UV/Vis Detector. Common uses: sugars, polymers, fatty acids

Evaporative Light Scattering (ELS) – Scientists who use the ELS detector will typically be employing the detector for samples that have low or no UV/Vis response and do not ionize well for mass spectrometry.  ELS detection has some limitations, in that it is a destructive technique and does not work well when the sample volatility is similar to the mobile phase and the ELSD does not have a large linear range.  However, the ELS detector does have broad applicability by virtue of the detector also being a bulk property or “universal” detector.  Because of the unique capabilities which an ELS detector provides a chromatographer, the price for these detectors can sometimes rival that of a PDA.  Common uses:  triglycerides, lipids, natural products, sugars and oils.

Mass Detection – Mass detection provides confirmation, by mass assignment, thus mass spectra contain more qualitative information that is unique to the structure of the target molecule than UV/visible and fluorescence spectra.  MS provides both confirmation of peak identity and sensitive detection.   The mass detector’s biggest challenge is the detection of neutral samples.  When the samples cannot pick up a charge (positive or negative), the mass detector cannot properly move the sample through the detector to analyze it.

Even so, detection by MS is becoming increasingly popular due to technological advancements that have led to a smaller benchtop detector footprint and user-friendly startup and maintenance features, which make it more accessible to chromatographers who are often newer users of MS detection. As more and more scientists explore the capabilities of MS detection, new ways and means of ionizing compounds are being discovered and exploited.  Once a detection scheme predominantly sought for qualitative information, MS is now being adopted for quantitative purposes especially for assays that require extreme sensitivity.  When dealing with the pricing to gain mass analysis for a lab, the prices can vary wildly depending on the type of instrument that’s selected.

Even with the detectors we covered today, there are still a variety of niche detectors to consider, which include but are not limited to: electro-chemical, conductivity, multi-angle light scattering (MALS), corona aerosol discharge (CAD), and optical rotation detectors — just to name a few.

Additional resources:

• How do LC detectors work? Read an overview in this section of our primer on HPLC instrumentation.
• See the range of HPLC and UPLC detectors available from Waters.

Follow me on this blog as I will next look at the differences between binary and quaternary LC instruments, and how to choose the right one for your laboratory’s needs.