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How to Determine the Size of 1/4” and 1/8” National Pipe Thread tapered (NPT) Fittings

Restek’s Technical Service team gets quite a few questions from customers about fittings. There is a bewildering assortment of fitting types and sizes as you can see here on the Restek web site. A blog post titled, “I need a fitting, but which one?” is where we often direct customers for help with these questions. One of the things discussed in that blog post is National Pipe Thread tapered (NPT) fittings as one of the main types used, but many people have difficulty identifying the correct size NPT fitting for their needs. The confusion comes from the fact that the outside diameter (OD) of an NPT fitting does not match the “name” of the fitting. For an example let’s look at Restek catalog # 23187 shown below, which is a 1/4″ to 1/8″ NPT Male Connector.


The 1/4″ designation in the name refers to the compression fitting side of the fitting (on the left in the picture above) which has a nut and ferrules to accept and connect to 1/4″ OD tubing. Makes sense, right? However, the threads on the right side of the fitting in the picture above are called 1/8″ Male NPT, but if you measure the OD of the 1/8” NPT side you will find it is about 0.4″ in diameter…certainly not very close to 1/8″ (0.125”). As a general “rule of thumb” an NPT thread is approximately 1/4″ (0.25”) larger than its “name.” For a 1/4″ NPT fitting the “nominal” OD is 0.533”.


NPT fittings are slightly tapered so the “nominal” diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads. This is a bit confusing, but NPT threads are made to the ANSI B1.20.1, SAE AS71051 standard and anything complying with a standard with a name like that is bound to be bewildering. Hopefully the image below will help.


The charts below are from the Swagelok Thread and End Connection Identification Guide with the first chart (from page 12) showing dimensions for male NPT threads and the second chart (from page 13) containing the dimensions for female NPT threads.


A few other helpful blog posts related to NPT fittings are:

Don’t forget the end fittings when you purchase an inline gas regulator”

How to connect 1/8 inch tubing to a Restek gas regulator

Swagelok® and Parker® Tube Fitting Manuals


Finally, you should always use PTFE tape when making a connection with an NPT fitting.


Thanks for reading!

The main source of artificial trans fatty acids is banned on June 18, 2018. Are we ready?

One of the leading causes of deaths in the world is heart disease, accounting for approximately 24% of deaths in the USA in 2015.1 One of the factors linked to cardiovascular diseases is consumption of artificial trans fatty acids (TFAs). Artificial TFAs are a by-product of partially hydrogenated vegetable oils, which were introduced as a replacement for saturated fatty acids found in butter, after concerns about adverse health effects. The TFAs have a benefit of a higher boiling point than their cis counterparts allowing for denser packing leading to a more solid form of hydrogenated oils.2 However, this is not the only way that cis and trans fatty acids differ. While cis fatty acids help maintain a healthy balance between low-density and high-density lipoprotein cholesterols (LDL and HDL, respectively), the artificial TFAs increase LDL levels and decrease the HDL levels. In addition, TFAs increase total levels of cholesterol, effectively increase the risk of cardiovascular disease.2

There is a push to eliminate the partially hydrogenated oils (the main source of artificial TFAs) globally. In Europe Denmark was the first to execute legislative implementation in 2004, followed by Austria, Sweden, Hungary and Latvia, while other European countries implemented guidelines for voluntary self-regulation.3 In the United States, the FDA declared partially hydrogenated oils as not “generally recognized as safe” in June of 2015 and food manufacturers have been given three years to phase them out (unless otherwise approved).4 On May 14, 2018, World Health Organization (WHO) together with the non-profit Resolve to Save Lives released a step-by-step guide for the elimination of industrially produced TFAs with the goal of completely phasing out partially hydrogenated oil and artificial TFAs by 2023.5

Less than a month away from the US ban of artificial TFAs, we have decided to test products that traditionally contain TFAs– margarine stick, shortening and butter flavored popcorn. The margarine and shortening were purchased at a local grocery store, while the popcorn was bought at a dollar store. The fatty acids were trans-esterified using sodium methoxide, following a method by Ichihara et al6 and analyzed on multiple Restek columns, namely Rtx-2330, Rt-2560, and FAMEWAX. All these columns can be used for fatty acid methyl esters (FAMEs) analysis, where Rtx-2330 is a general use polar columns with cyano-polymer stationary phase, while FAMEWAX is formulated to deliver comparable elution order to other Carbowax columns with better FAMEs resolution. RT-2560 is specifically formulated to ensure accurate quantification of critical cis/trans FAMEs.

Let’s start with the individual products. Margarines previously contained 10-20 % of saturated, 30-40 % monounsaturated, 20-30% polyunsaturated fatty acids and approximately 15% TFAs.7 We tested the new margarine and the composition was approximately 40% saturated (peaks 1-6, Fig. 1), 23% monounsaturated (peaks 7a- 8, Fig. 1), 37% polyunsaturated fatty acids (peaks 9-10, Fig. 1) and no TFAs (peak 7b, Fig.1). This shift is caused mostly by addition of palm and palm kernel oils, which contain more saturated fatty acids (approximately 50% and 80%, respectively).8-9 In Figure 1 we can see chromatograms of the trans-esterified margarine stick without any alterations (red), and a sample spiked with methyl elaidate (methyl trans-9-octadecenoate), the most common artificial TFA added.

Figure 1: GC Analysis of trans-esterified margarine stick. Red – original sample, blue – sample spiked with methyl elaidate (peak 7b).

Figure 1: GC Analysis of trans-esterified margarine stick. Red – original sample, blue – sample spiked with methyl elaidate (peak 7b).

The differences between the individual columns are apparent. Both cyano polymer-based columns (Rtx-2330 and Rt-2560, Fig. 1A and B, respectively) elute trans isomers before the cis. This is particularly advantageous because even when using GC-MS we cannot distinguish the isomers based on their EI spectra. This helps to unambiguously determine the identity of peak 8 as methyl cis-11-octadecenoate. When it comes to Carbowax polymer-based columns, trans isomers elute right after its cis counterparts. Nevertheless, FAMEWAX column is capable of separating the cis/trans isomers (we can see two distinct peaks, Fig. 1C peak 7a and 7b).

The next product we analyzed was shortening. Traditionally shortening can contain as much as 40% of industrial TFAs.10 As with the margarine, the shortening we bought had no significant TFAs present (Fig. 2). In contrast to the margarine, the polyunsaturated fatty acids in shortening accounted for almost 50% of FAMEs, which was also in direct contradiction with the product’s label of 20.8% polyunsaturated fatty acids.

Figure 2: GC Analysis of trans-esterified shortening

Figure 2: GC Analysis of trans-esterified shortening

It is worth noting that the relative content saturated of fatty acids’ in shortening is lower than in margarine. This is most likely due to the absence of palm kernel oil, however, the palm oil was still present. Our third sample, butter-flavored popcorn (Fig. 3), contained no palm or palm kernel oil, resulting in the least amount of saturated fatty acids, approximately 16%. In addition, the popcorn contained the most monounsaturated fatty acids (mainly oleic acid) – approximately 67% of total FAMEs. This can be attributed to the oil source – while fats in both margarine and shortening are comprised of soybean oil, palm oil and palm kernel oil (margarine only), popcorn’s source of fat is canola oil and fully hydrogenated cottonseed oil.

Figure 3: GC Analysis of trans-esterified popcorn’s butter flavoring

Figure 3: GC Analysis of trans-esterified popcorn’s butter flavoring

Overall, our evaluation confirmed that US food producers ditched the artificial TFAs. On the other hand, the introduction of palm and palm kernel oil means increases in the saturated fatty acids’ content. When it comes to column selection, FAMEWAX provides great separation of the majority of FAMEs with shorter analysis times (especially for longer chains), however, if you need to quantitatively determine the TFAs, Rt-2560 is the best choice.


  1. https://www.cdc.gov/nchs/fastats/leading-causes-of-death.htm
  2. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3955571/
  3. https://ec.europa.eu/food/sites/food/files/safety/docs/fs_labelling-nutrition_trans-fats-report_en.pdf
  4. https://www.federalregister.gov/documents/2015/06/17/2015-14883/final-determination-regarding-partially-hydrogenated-oils
  5. http://www.who.int/news-room/detail/14-05-2018-who-plan-to-eliminate-industrially-produced-trans-fatty-acids-from-global-food-supply
  6. https://www.sciencedirect.com/science/article/pii/S0003269715005357
  7. https://ndb.nal.usda.gov/ndb/foods/show/04629
  8. https://ndb.nal.usda.gov/ndb/foods/show/299928
  9. https://ndb.nal.usda.gov/ndb/foods/show/299949
  10. https://ndb.nal.usda.gov/ndb/foods/show/04667

Next time we’ll look into how does margarine compare to other fats, such as butter, coconut oil, palm oil and olive oil.

Stabilwax Pro-EZGC Library Updated

Recently I blogged about the expansion of Rxi-624 EZGC library, this time I want to focus on an update of Stabilwax. At 462 compounds, this library covers the most searched, such as methanol or xylenes, and requested compounds, such as acetic and formic acid.*
Similar to my last blog, I’d like to show you a comparison between the model and the real analysis:

Fragrance Allergen Standard (#33104) on Stabilwax

One thing you can see (apart from great retention time match between those two) is that there is room for improvement. Let’s run it through EZGC!

The analysis time was reduced from 23 minutes to 7 minutes. That’s the power of EZGC.

*We recommend using Stabilwax-DA for the acid analysis

Rxi-624Sil MS Pro-EZGC Library: 591 Compounds to Choose From

Five years ago Chris English showed in his blog post “I Can’t Drive 55” — The Pure Power of EZGC all 233 compounds that were in the Rxi-624Sil library. While that is a lot, we didn’t want to stop there. This weekend we added over 350 new compounds with various functionalities – e.g. alcohols, aldehydes (first time on EZGC!), amines, esters and more!

Just to show you the power of EZGC, here are few real chromatograms with their modeled counterparts. Let’s start with the hot topic of the day, cannabis terpenes:

The second example is one of the completely new compound sets in the EZGC library – aldehydes:

And last (but not least) the EPA 8260 method:

Volatiles by EPA Method 8260 on Rxi-624Sil MS (30m, 0.25mm ID, 1.40µm)

This huge update was made possible in collaboration with Becca Stevens, Amanda Rigdon, and Megan Burger.

A better way to configure your EZ No-Vent GC-MS Connector Part II

Last time, I wrote a blog (here) that showed a better way to configure the EZ No-Vent in the software so that the column length didn’t need to be manipulated. I kept the volumetric flow the same, and showed different ways to minimize the negative chromatographic effects of the reduced linear velocity. Today, I’m going to show you what happens when you keep the linear velocity the same after installing the EZ No-Vent.

Figure 1: 8270 Chromatogram on a 30 m x 0.25 mm x 0.25 µm Rxi-5Sil MS (cat# 13623) by Split Injection


Figure 2 – 30 m column configured as a composite column in MassHunter


Figure 3: 8270 Chromatogram on a 30 m x 0.25 mm x 0.25 µm Rxi-5Sil MS (cat# 13623) by Split Injection with the carrier gas linear velocity matched to that of the chromatogram with no EZ No-Vent installed.


Figure 4: 30 m column configured as a composite column in MassHunter. Note the volumetric flow and average linear velocity.

As you can see, the tailing that plagued the EZ No-Vent chromatograms in the previous blog (here) has been eliminated by increasing the speed of the analytes through the column. When a GC-MS is run without an EZ No-Vent (Figures 1 & 2), the vacuum extends quite a long way into the analytical column, causing an increase in linear velocity as analytes approach the end of the column. When the EZ No-Vent is installed, the 100 µm restrictor greatly reduces this effect, causing an overall drop in average linear velocity under the same volumetric flow, reducing efficiency. Restoring the original average linear velocity appears to be the solution to the negative chromatographic effects (Figures 3 & 4).

Dilute, Shoot, and Elute – am I missing anything? YES!


Everyone’s lab is different in terms of how many samples per day are processed, but they all share the common pain point of sample preparation. Some samples like blood and plasma need a significant amount of prep to remove proteins, phospholipids, and salts, whereas labs running urine or drinking water samples can “get by” with a 5x or greater dilution.

No matter how limited or extensive your sample prep, the one thing that is critical to prolonging the lifetime of both your column and instrument is particulate removal, and you know what that means: filtering.

We’ve blogged about filtering mobile phase before, and recently gave you a behind-the-scenes tour of column construction in the Clog Blog to emphasize how important it is to remove particulates from samples. You also want to keep your HPLC or UHPLC up and running, and the downtime plus parts and labor expense for replacing any or all of the sample needle, injection port, valve rotor, and outlet tubing of your autosampler is far greater than sample filtration supplies.

I like to use “the paint example” when talking about sample prep. Chances are you or a colleague have done some home remodeling that includes painting, so you know you don’t just go get the color you like, roll it on, and you’re done. For best results, you have to fill in any holes or scratches, sand, tape, prime, and finish with your carefully chosen color from the selection of 500 shades of beige. Sure it takes extra time, but it turns out looking great. It’s the same with HPLC sample prep: the more care you take with filtering, the longer your column and instrument will last. My colleague and frequent blogger Nancy from Tech Services has a great post about making your HPLC columns last longer and filtering is high on her list too!

The easiest method for manual sample filtration is to use a Thomson Filter Vial. Anyone who knows me will tell you that this is one of my favorite products EVER. There are different membranes depending on whether your sample has a high aqueous or high organic content, and the 0.2µm membrane is ideal for small ID UHPLC tubing, which is typically 0.1mm ID or less and is prone to clogging. There’s even an eXtreme version of the Thomson vial that has a multi-layer filter for samples with 10-30% particulates. Both vial types are very simple to use with a lot less mess, hassle, and waste compared to a separate syringe, disc filter, and collection vial. Here’s an example using a 0.2µm PVDF filter vial for the analysis of fentanyl in urine.

Another effective way of removing particulates is through centrifugation. After a protein crash or dilution, you can place your vials or multi-well plate into a centrifuge and spin away to pull particulates into a pellet at the bottom of the vial or well. This analysis of immunosuppressive drugs used a precipitation solution vortexed with whole blood, then 4,300 rpm in the centrifuge to remove particulates.

After filtering or centrifugation you can put your vial or plate into the autosampler for analysis. Make sure you adjust the needle setting so it stops 3-5mm above the pellet so it doesn’t suck up any of the particles you just pulled out of the sample solution!

A simple filtration or centrifugation step will allow you to make the most of dilute-and-shoot sample prep’s high-throughput capabilities while helping to keep particulates out of your instrument and column. This reduces instrument downtime and prolongs column lifetime so it’s a win-win for your lab’s productivity. You can also double up on column protection with good sample prep and the use of a guard column, especially if the sample has fine particles that can pass through a filter membrane or not form a good solid pellet. Here is a great starting point for guard column selection.

Thanks for reading!










EXP Fittings: Which Nut Do I Need?

Our brilliant friends at Optimize Technologies are continually innovating and making our lab lives easier. We love their patented EXP Titanium Hybrid Ferrule technology that is truly universal. You can use it on any tubing, any 10-32-port, any instrument—wrench-tight to 20,000 psi. This fully UHPLC-capable EXP ferrule is used with all EXP nuts. Restek offers three EXP nuts: hand-tight, hex-head, and EXP2.

The question is, which nut do I need? I’ve tried them all and found that it is truly situational. Ask yourself: how much space do I have, how often will I make-and-break the connection, and am I connecting PEEK or stainless steel (SS) tubing? Here are two examples.

The EXP hand-tight nut is ideal for column connections, especially on systems operating </=600 bar. Most column ovens in HPLC or intermediate-HPLC instruments are large enough to allow the hand-tight nut. Its larger and more-ergonomic head allows easy hand-tightening for repeated connections up to 8,700 psi. No wrenches, just fingers. While you certainly can wrench-tighten the hand-tight nut to provide leak-tight seals up to 20,000psi, the EXP2 nut is perfect for UHPLC applications. The torque driver allows a wrench-tight seal that’s as easy as finger-tight.

Alternatively, in the super-tight space of a 6-port injection valve, the EXP hand-tight nut won’t fit. The EXP2 nut is ideal in this situation. Its slim profile and torque driver allow flawless, fast, zero-dead-volume connections in places that traditional wrenches don’t fit.

If you’re using PEEK tubing, move to a hex-head nut. EXP2 fittings aren’t recommended for use with PEEK tubing. This is because the torque driver that’s included with the fittings is so effective, that’s it’s easy to crush your tubing. So if you’re using PEEK tubing and have minimal access or space, use the EXP hex-head fittings. If you just can’t fit a traditional wrench in there, try using the socket side of the ValvTool Wrench (find it here http://www.restek.com/catalog/view/960 ). It’s slotted to fit over the tubing and slimmer than traditional wrenches.

Also, consider staggering the height of the nuts around your 6-port injection valve. EXP Hex-Head fittings come in three lengths: short, standard, and long. Varying the height allows breathing room for your wrench. An example of that is shown below.

Most analysts who try EXP fittings keep using EXP fittings, because they’re truly universal, extremely well-designed and durable, and easy. Please let us know what you think of them!

A better way to configure your EZ No-Vent GC-MS Connector

If you are using the EZ No-Vent for Agilent mass spectrometers (cat.# 21323), you are probably tricking your instrument into working properly by inflating the length of your column in your acquisition software. If you have MassHunter, or a recent version of MSD ChemStation  (G1701EA), you don’t have to do this. Instead of configuring your 30 m x 0.25 mm ID column as a 112 m column, you should configure a composite column (see Figure 1). Even if you aren’t using the EZ No-Vent with your mass spectrometer, you should be configuring your column as a composite column because the 17 cm transfer line is heated by the transfer line heater, not the oven.

Figure 1 – 30 m column configured as a composite column in MassHunter (without the EZ No-Vent)

Figure 2 shows a composite column configured with the 17 cm x 0.10 mm ID vacuum restrictor, which is the part of the EZ No-Vent that allows you to disconnect your analytical column without turning off your MSD. You’ll notice the average linear velocity is significantly reduced when the EZ No-Vent is used (39 cm/sec drops to 26.8 cm/sec) even though the volumetric flow stays the same. This has some negative chromatographic effects.

Figure 2 – 30 m column configured as a composite column in MassHunter (with the EZ No-Vent)

Figure 3 is an example chromatogram collected on a 30 m x 0.25 mm x 0.25 µm column without using the EZ No-Vent. Figure 4 shows the chromatographic effect of installing the EZ No-Vent while keeping all acquisition parameters the same (aside from configuring the restrictor in the composite column).

Figure 3 – Example Chromatogram collected without the EZ No-Vent

Figure 4 – Same acquisition parameters as Figure 3, but collected with the EZ No-Vent installed

The good resolution of the first eluting compound (1,4-Dioxane) and the dichloromethane solvent peak has collapsed, in Figure 4, and Aniline and bis(2-Chloroethyl)ether are now coeluting. The resolution for the two PAH pairs has also degraded. Lowering starting oven temperature 20°C improves the separation of 1,4-dioxane and solvent peak, though 1,4-dioxane is still on the solvent peak’s tail (see Figure 5). The PAH resolutions have also improved, almost to the level of performance when the EZ No-Vent isn’t installed.

Figure 5 – Chromatography is improved by reducing the starting oven temperature.


The fix to the peak tailing can be found in the follow-up blog: A better way to configure your EZ No-Vent GC-MS Connector Part II

355 compounds have been added to the Rxi-XLB library for the Pro EZGC Chromatogram Modeler

The Rxi-XLB is a popular column for the analysis of PAHs and persistent environmental pollutants (POPs) such as PCBs and Pesticides. We’ve updated the library of 209 PCB congeners, expanded the PAH library to 47 compounds (including 6 deuterated isotopologues commonly used as internal standards or surrogates), and added the 203 compounds in the GC Multiresidue Pesticide Kit, 35 of the 75 polychlorinated naphthalene congeners (PCNs), and 43 phthalates.

Click here to explore the Pro EZGC Chromatogram Modeler. A free website account is required.


How do I Replace my LC Tubing?

For those that are experienced with LC, you probably have this all figured out.  If that is the case, this might serve as a good introduction to Restek products that you can use for this.  For those that are newbies to liquid chromatography, welcome and I hope you find this useful!

The best way to replace tubing in an LC system largely depends on where the tubing is located, how it is used and the material it is made from.  In all cases, tubing must be cut straight and square on the end to achieve a good seal.  This is even more critical when you are operating at higher pressure ranges.

PEEK (polyether ether ketone) tubing is used for lower pressure applications.  We recommend using PEEK tubing with ID of ≤0.007 inches for applications up to 7,000 psi and the tubing with an ID of ≥0.010 inches for applications that go up to 5,000 psi.  Our product listing for PEEK tubing is located at this link (click here).  These are color coded with a stripe so you can tell what the ID (internal diameter) is.  Here are the codes:

Clear= 0.0025”

Red= 0.005”


Blue= 0.010”

Orange= 0.020”


We suggest to use our Clean-Cut Tubing Cutter catalog #25069 to cut PEEK (and other plastic) tubing. Here’s what it looks like:


Stainless steel tubing can be used at just about any pressure range, including the higher ranges for UHPLC.  It is also considered more permanent, so most internal LC connections are usually stainless steel. Although most durable, stainless steel is harder to work with, in general.  I mentioned earlier that it is critical to have tubing cut straight and square on the ends to achieve a good seal.  This is very true with stainless steel, especially since it is used at higher pressure ranges.  It is also more difficult to ensure the tubing is smooth and very straight on the end, so for these reasons, we do not recommend for customers to cut their own stainless steel tubing for LC.  It is much better to order the tubing that is already professionally cut to the desired length.  We offer stainless steel tubing in pre-cut lengths of 5, 10, 20 and 30 cm and ID sizes ranging from 0.005” – 0.020”.  These are shown at this link (click here) on our website.  We use the same color code for these as we do for the PEEK.  Instead of a stripe along the tubing, there is narrow band around the outside of the tubing.


While it is nice to have generic lengths of tubing available with no fittings attached, you may be looking for something more specific.  In some cases, you may need to obtain tubing with pre-swaged end fittings attached or you may be looking for specific dimensions to fit a certain location for replacement within your instrument.  Some instrument parts may also contain a specialized end fitting to serve a certain purpose that is specific for that instrument model.  This is why we often suggest that customers replace the entire tubing assembly that comes with the proper fittings, as specified by the instrument manufacturer.  The only reliable way to do this is to identify the manufacturer’s catalog or part number and seek a direct replacement for that.  Fortunately, for our customers using Agilent LC models, we do sell a few of the more popular tubing assemblies to fit Agilent LC models, located here on our website:   Capillary Stainless Steel Tubing Assemblies

If you select an individual item from the link above on our website, you can see which Agilent part number the item corresponds to.  Feel free also to contact our Tech Services department to see if we have a replacement, if you know the part number and cannot find it.

To help confirm your selection, I have listed the primary location of usage and dimensions for each of these tubing assemblies in the table below.  Please note that some of these may be used for general purposes in other instrument models also, but the location I have specified here in the indicated model does require these particular dimensions.  Generally for most Agilent HPLCs, the tubing used in the system flowpath between the solvent module and the autosampler sample injection valve has an ID of 0.25 mm.  Likewise, the tubing downstream from the sample injection valve to the detector has an ID of 0.17 mm or smaller to minimize band spreading.


Here are some links for additional reading on related topics:

How do I know what the internal diameter of my LC tubing is?

Which fitting do I use for which tubing?

How are stainless steel fittings for HPLC different from polymer-based fittings?

Finding the right fitting for HPLC or UHPLC

How can I use Raptor Columns and the EXP Direct Connect holder with stainless steel fittings?

Need parts for your Agilent 1100? HP 1050? Restek has you covered.

Also, be sure to check out our Youtube video “LC Tubing and Fitting Choices”.

I hope you found this post helpful.  Thank you for reading!