Quick & Painless Email Subscription

Use the “Subscribe” link at right. It just takes a few seconds!

What can I use as a cap for my mobile phase bottles?

Our choices for mobile phase bottle tops are based primarily on two different products.

The first product is the Hub-Cap Bottle Top, catalog #26541. This is designed to fit a 4-Liter solvent bottle. :

26541solvent-bottle

 

 

The body (also called insert) of the Hub-Cap Top is cap is made of inert PTFE and has three threaded holes. The exterior ring is composed of high density polyethylene and does not usually come in contact with the mobile phase. Threads for all three holes in the body are for ¼-28 fittings, two of them for 1/8 inch OD fitting nuts and the other is for 1/16 inch nuts. However, the intention is really to run 1/8 inch OD tubing through the two larger holes without the use of a fitting nut. Using it this way without a nut creates a slight vent that can be beneficial, since it avoids the creation of a vacuum as solvent is being pumped from the bottle. The 1/16 inch OD hole comes with a PEEK plug that can be removed, if needed for ventilation, or can be replaced with a ¼-28 thread fitting nut if desired.  If needed, catalog numbers 25794 and 25975 can purchased as a nut and ferrule for the 1/8 inch OD holes.  However, at this time, we do not sell nuts and ferrules for 1/16 inch OD ¼-28 fittings, only the PEEK plug that is mentioned.

 

The other product is the Opti-Cap Bottle Top, catalog #25300. This is designed to fit a 1 or 2-Liter GL-45 bottle:

25300

gl-45-bottle

 

 

 

 

 

 

 

 

 

 

 

The body of the Opti-Cap is made of inert PTFE and the exterior ring is made of polypropylene. Similar to the Hub-cap, the body has three holes that are threaded for ¼-28 fittings. Two of them are for 1/8 inch OD fitting nuts and the other is for 1/16 inch nuts and comes with a PEEK plug installed.  Again, similar to the Hub-Cap Top, the intention is really to run 1/8 inch OD tubing through the two larger holes without the use of a fitting nut when used as for mobile phase. As is the case for the Hub-Cap, catalog numbers 25794 and 25975 can purchased as a nut and ferrule for the 1/8 inch OD holes if needed. Again, we do not sell nuts and ferrules for the 1/6 inch OD hole.

You may notice that we also sell the Eco-cap, catalog number 26395, which is a simplified version of the Opti-Cap. It has three holes like the Opti-cap, but none of the holes are threaded, so it cannot be used with fittings.  A luer-type polypropylene plug is provided for the 1/16 inch hole and the body of the Eco-Cap is composed of a translucent polymer called ETFE.

eco-cap

 

 

What to do you do when you have the wrong type of top for the bottle type that you are using?

Here is where it gets tricky.

If you have an Opti-Cap Bottle top already (see above), but you need to use it with a 4-Liter bottle, what you need is a Hub-Cap Adaptor, catalog #26538:26538

You can purchase the Hub-Cap Adapter with the Opti-Cap Bottle Top together as a kit as catalog # 26540.  I would suggest purchasing this kit if you expect to be using GL-45 bottles most of the time, but you might want to use a 4-Liter bottle occasionally and want to be prepared for either. You can also use the Hub-Cap Adapter with the Eco-Cap, to adapt it for use with 4-Liter bottles, if desired, although it will not produce an air-tight seal.

 

If you have a Hub-Cap Bottle Top already (see above), but you need to use it with a GL-45 bottle, what you need is an Opti-Cap Adaptor, catalog #27197:27197

You can purchase the Opti-Cap Adapter with the Hub-Cap Bottle Top together as a kit as catalog #26551.  I would suggest purchasing this kit if you expect to be using 4-Liter bottles most of the time, but you might want to use a GL-45 bottle occasionally and want to be prepared for either.

I hope you have found this informative.

Thank you for reading.

 

How Dirty Are You? Part 5…Micropipette Tips

Mouth Pipetting Source: http://collections.nlm.nih.gov/catalog/nlm:nlmuid-101446257-img

Mouth Pipetting
Source: http://collections.nlm.nih.gov/catalog/nlm:nlmuid-101446257-img

Pipettes are common tools for drawing and dispensing precise volumes in chemical, biological and medical labs. Thanks to the invention of micropipettes and other techniques, we don’t do mouth pipetting anymore.

Positive displacement pipettes are particularly useful because the air gap in standard micropipettes are replaced by capillary pistons, which greatly improve the accuracy for handling aggressive liquids, such as organic solvents, and viscous solutions.

It sounds like the perfect way to transfer problematic solvents, however, the question of cleanliness has not be addressed. This reminds me of the blog series from my colleague Julie Kowalski, “How Dirty Are You? So maybe it is time to add another subject to it.

To mimic the worst case scenario, I soaked the positive displacement pipette tips (50-250 µL) into 10 mL of various organic solvents in glass centrifuge tubes and shook them for 30 min. The organic solvents are toluene, acetone, dichloromethane, hexane, and acetonitrile. The 0.5 ppm triphenylphosphate (TPP) was prepared as the internal standard. The solvents were then completely dried under N2 at 50 °C after extraction. Finally, 100 μL of the corresponding solvent was added for reconstitution using glass syringes.

For comparison, another set of samples were prepared by pipetting 250 μL of solvents 10 times. Blanks went through the entire preparation without soaking the micropipette tips.

Each sample was tested on a Rxi-5ms (30 m x 0.25 mm x 0.25 µm) using a Shimadzu GC-MS-QP2010 Plus instrument.

The 30 min soaking results are shown below. I was not surprised to see a contamination peak at higher intensity than the internal standard (TPP) for all solvents. What really surprised me was the absence of phthalate.

So, what is the contamination? It was identified as “Erucylamide” in the NIST MS search library with a relatively good match (>90%). Erucylamide is often employed as an antiblocking/smoothing agent for plastic products, such as PE, PP, and PVC. Therefore, it is a very common contaminant in plastic packages and products.

erucylamide-chrom

So, what is the worst solvent? Toluene produced the most intense signals. Acetonitrile and acetone generated relatively similar backgrounds which were cleaner relative to toluene. It pretty much followed the polarity trend except for hexane (see the picture below).

From the chromatograms below soaking for 30 min produced much higher signals than 10-time pipetting. For disposable pipette tips, many people only do one aspiration for each sample, sometimes two if doing sample priming. Therefore, you will get a cleaner background less times you use the micropipette.

picture1

polarity

The good news is the major ions of erucylamide (m/z 59/72/55) are relatively small, which may not interfere with your target in either full range scan or specific transitions. For anyone who may still be concerned about this contamination, I hope this blog will make you feel better.

 

RIP Philae

The exact resting place of the Philae lander on Comet 67P/Churyumov-Gerasimenko has finally been pinpointed, in new photographs by the Rosetta orbiter. Rosetta did a close pass of the comet on September 2nd and the survey caught the first clear glimpse of Philae’s position.

Philae's resting place on Comet Churyumov-Gerasimenko

Photograph shows Philae wedged into a deep crack on the comet’s surface.
Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

 

I’m so proud that Restek was able to contribute columns to this astounding mission!

To find out more about the Rosetta mission and Restek’s contribution, read Chris English’s interview with Robert Sternberg, head of the GC space instrumentation group at Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA). There is also an in-depth paper on the COSAC instrument, COSAC, the Cometary Sampling and Composition Experiment on Philae, by Fred Goesmann, Helmut Rosenbauer, and Reinhard Roll of the Max Planck Institute for Solar System Research, if you’re interested in finding out more about the development of the instrument and the columns that accompany it.

Oh, and did you know that Restek has columns on three other space missions?! That’s right, four Restek columns can be found on the Curiosity rover (MSL 2011 mission) in its SAM instrumentthree columns are on the ExoMars 2018 mission in its MOMA instrument, and Restek’s MXT®-1701 column was the first capillary column that ever flew in space, on the Cassini/Huygens mission! And the venerable Pro ezGC application was crucial to the column selection for Curiosity, ExoMars, and Rosetta! (See the Sternberg interview, and this article by C. Szopa and R. Sternberg, for more details on Pro ezGC’s use.)

 

What is the pressure limit for my LC column?

This is a question we get in Tech Services fairly often.  Unfortunately, we are not able to provide a published limit for every column that we produce, but here are a few of them that we have specified:

  • For 3 and 5 um fully porous particle ROC LC columns, the maximum operating pressure is 400 Bar/ 5800 psi, which is also the limit for older or more conventional HPLC systems and close to the limit for column hardware. My personal preference is to operate below 200 Bar/ 2900 psi if possible, although the 3 um particle columns will tend be towards the upper range in terms of pressure.
  • For Raptor 2.7 um (superficially porous particle) columns, the maximum pressure rating for these columns and for many mid-pressure range HPLC instruments is 600 Bar/ 8700 psi. I would prefer to operate below 500 Bar/ 7250 psi as much as possible.
  • For Raptor 5.0 um (superficially porous particle) columns, the maximum pressure rating for these columns and for older and more traditional HPLC instruments is 400 Bar/ 5800 psi. I would prefer to operate below 300 Bar/ 4350 psi as much as possible.
  • EXP guard holder (catalog #25808) and EXP fittings that are used with Raptor columns and guard cartridges are rated up to 20,000 psi when tightened by wrench, 8,700 when hand-tightened.
  • UltraShield and Inline filters (catalog #24993), that are used with our Pinnacle DB UHPLC (1.9 µm) columns, are rated up to 1000 Bar/ 14700 psi.

The difficulty in giving an absolute pressure limit for various columns is that the expectations are different depending on the application and the column dimensions.  I usually refer customers to the table from my blog post Building up pressure on HPLC?, which was originally intended for fully porous particle columns.  Please see below for an updated version of this table. As you can see, the predicted pressure is affected by the column dimensions, flow rate and the specific solvents that are used.  Aqueous mobile phases usually produce a backpressure similar or slightly higher than what is shown here for methanol.  Please keep in mind that because backpressures typically increase over the lifetime of the column, flow conditions for a new column should be established towards the lower end of the operating range if possible.

predicted pressures LC

Although this table was intended for fully porous particle columns, the predicted pressure is not too far off from what it would be for a superficially porous column with the same particle size.  For example, the results above for 3.0 µm particle columns can be used also to give a rough estimate of pressures for a 2.7 µm Raptor SPP column.

In all cases, it is important to stress that for best chromatographic results, all LC and UHPLC columns should be used as close as possible to the optimal flow rate.  Operating close to the optimal value also helps to ensure that the pressure stays within a range appropriate for instrumentation and for the specific column hardware.

Optimal flow rates for all of our LC columns are shown here in this table from our catalog:

optimal flow rates

Here are some links to other related blog posts and articles you might read , if interested:

Should I use a 2.7 or 5.0 µ, Raptor™ column?

What is SPP and when should I use a Raptor™ column?

Technical Service “Red Flags” –LC

Resurrecting an old reverse phase LC column…

Frequently Asked Questions: HPLC and UHPLC

 

I hope this post has helped to clear up some questions you might have had.

Thank you for reading.

Impact of pH on Derivatization of Nitrophenol Compounds

Our goal of evaluating the Normalized French Environmental Method 12673 required testing the method’s ability to derivatize nitrophenols and alkylphenols in addition to the chlorophenols the method was designed for. Unfortunately, nitrophenols proved challenging to derivatize at low concentrations. In an effort to improve the derivatization of nitrophenols, we experimented with the pH of the reaction mixture.

The original method requires 5 mL of 1 molar potassium carbonate (0.7 g solid potassium carbonate) to be added to 50 mL of water sample, followed by the addition of 1 mL acetic anhydride, then extraction with hexanes. In an attempt to improve the conversion of nitrophenols to acetic acid esters, the amount of potassium carbonate being added to the solution was increased, or replaced with sodium hydroxide. Our hope was that increasing the basicity of the solution would create a better environment for the leaving group, promoting the nucleophilic attack process.

A small experiment was designed in order to evaluate the effect of pH on derivatization of nitrophenol compounds. This experiment included both increasing and decreasing the pH of the derivatization reaction in order to observe the relative response from compounds, which were analyzed on the Rxi-5Sil MS column.

blog 2 - ph 9 11.66 13.5Figure 1: Three overlaid chromatogram showing peaks for (1) 2-nitrophenyl acetate, (2) 3-nitrophenyl acetate, and (3) 4-nitrophenyl acetate. Each overlaid chromatogram represents a derivatization in a different pH solution.

Only nitro-containing phenols were determined in this experiment. Nitrophenyl acetates performed significantly better than dinitrophenyl acetates, and were chosen to reflect to effect of pH. The pH 11.66 derivatization performed best for two of the three nitrophenyl acetates, followed by pH 9, and pH 13.5. The increasing pH, therefore, did not enhance the derivatization process. There appears to be a point at which the pH can impede the derivatization process; the increased pH was also observed to inhibit the derivatization of the alkyl and chlorophenol compounds.

blog 2 - tableTable 1: Responses of chloro- and alkyl phenyl acetate compounds after derivatization in pH 11.6 and pH 13.44 conditions.

The increased pH had a negative effect on the derivatization of these compounds. Despite being a higher concentration, the responses of the compounds derivatized at pH 13.44 are significantly lower than the responses of those derivatized at pH 11.66.

Improved Peak Symmetry as a Result of Derivatization of Phenols

Peak symmetry is important for determination and quantification of compounds. In order to create a symmetric peak, the compound must be able to elute all at more or less the same time. If a compound interacts with the column itself (ex: by temporarily adsorbing to the fused silica surface), it delays travel and effects peak morphology, causing “tailing”.

When comparing EPA method 525.2 to NF EN 12673 for the extraction of phenols, peak symmetry was notably improved using NF EN 12673.

The hydroxyl group on phenol compounds is highly active and able to hydrogen bond with the silanol groups present to some extent in all fused silica GC columns. This is one possible causes of phenol peak tailing. Phenyl acetates, however, have an acetic acid ester group rather than a hydroxyl group, which greatly reduces compound activity,  resulting in cleaner, symmetric peaks.

Relative response factors (RF) were calculated for each of 10 calibration points for pentachlorophenol and 12 calibration points for pentachlorophenyl acetate, to illustrate the effect of peak tailing. Ideally, relative response factors should be equal at each calibration point because they are normalized by concentration. Peak tailing, however, complicates the peak integration, which pushes the RF further from the “ideal”. The relative standard deviation (RSD) shows how close the RFs are to one another. The higher RSD in Figure 1 shows that the RFs deviated more from one another in the pentachlorophenol calibration, than they did in the pentachlorophenyl acetate calibration.

blog 3 - pentaCP RF graphs - 2

Figure 1: A graphical representation of RRF’s for pentachlorophenol (right) and pentachlorophenyl acetate (left) peaks used for calibration purposes. Concentrations used for calibration represent the concentration of phenols in the original water sample.

Accordingly, the peaks in the pentachlorophenol calibration exhibited more tailing than the peaks in the pentachlorophenyl acetate calibration.

blog 3 - pentaCP chromatogramFigure 2: A chromatogram showing peak shapes for pentachlorophenol (left) and pentachlorophenyl acetate (right).

Nobel Laureate Talks and Restek Champagne Reception

From July 19th – 22nd 2016 Restek Corporation with the collaboration of the German Restek Office (Restek GmbH) attended the 39th ISEAC conference, organized from the “International Association of Environmental Analytical Chemistry” (IAEAC) at the Hamburg School of Food Science, lead by  Prof. Dr. Markus Fisher. The central subject of the ISEAC-39 conference is the innovative use of analytical methods for the investigation of environmentally and food relevant questions.

The interesting concept of this conference series is to show the close connection between Food Safety Analysis and Environmental Analysis, as was mentioned by the vice-president of the Federal Institute for Risk Assessment (Bundesinstitut für Risikobewertung, BfR), Prof. Dr. Reiner Wittkowski, talking about “What goes in must come out – environmental impact on food safety”. In fact, both communities are often facing the same challenges, as shown by the discussion about the Glyphosate issue or about MOSH/MOAH, both of them related to food safety as well as to environmental aspects. The same co-incidence can be recognized in observing polyfluorinated surfactants like Perfluorooctane Sulfonate ((PFOS) or Perfluorooctane Acid (PFOA), widely used to impregnate breathable functional clothing. Prof. Wittkowski used these examples to explain the need and the challenges of proactive consumer protection, a concept which is followed strongly by the European Community.

One of the purposes of the Organizers of this conference series is the idea of “Thinking out of the Box”, an inbuilt series of talks with related topics out of different scientific fields not directly connected to analytical challenges. This interesting part of the conference showed up with talks from Nobel Laureate Prof. Dr. Robert Huber, giving a summary about his work about the “Beauty and  Fitness for Purpose of Proteins” as well as Prof. Dr. Christoph Kutter, Director of Fraunhofer Research Institution for Microsystems and Solid State Technologies, talking about “Sensors for the Internet of Things”. As a high light for the Chromatography Community the talk of Prof. Dr. Alexander Makarov gave a short cut about the development of the Orbitrap Mass Spectrometer Technology.

During the conference, Restek could present its innovations in  Separation Science. To honor the scientific work of those researchers who showed up with Posters, Restek opened the first poster session with a Champagne Reception to welcome the interested audience.

Restek Corp. especially wants to thank the group of Prof. Markus Fischer, the organizing young scientists are dedicated to make things possible, and the secretary of the IAEAC.

 

Champagne Reception

Preparing the Champagne Reception at the first Poster Session

Poster Session

Interesting Discussions during the Champagne Reception and Poster Session

Hernanisms

Last Sunday, I was sitting in the garden, recapping the experiences of attending the scientific conferences I had the pleasure to visit during this year. One of the honoring sessions has left a quite inspiring impact on my mind. In the beginning of the year, I had the opportunity to listen to one of the most experienced and successful scientists in our Chromatography Community, Dr. Hernan Cortes, who as one of the first researchers was recognizing the power of hyphenated techniques in Chromatography (he even experienced the LC/GC coupling as a helpful technique in solving practical challenges – Sorry, Dr. Cortes that I couldn’t believe that the evening before). He was honored with the  LCGC Europe’s Lifetime Achievement Award during HTC in Ghent/Belgium in January.

Hernan Cortes  always has an open heart and an open mind for young Scientists. Therefore his laudation talk had a clear focus on how to help Young Scientists to develop their skills, as they are the future not only for our field of Science. As my daughter is studying Biology at the University of Cologne in the moment, I was electrified by one of his slides, showing some bullet points he called his “Hernanisms”, which he claimed not be developed only by himself, but were taken over from him as some of the keys of his success. So I asked him to provide this slide to me, because I liked to share these ideas with my young daughter.

Re-reading these “Hernanisms”, I thought it would be valuable to share them with you as well, hoping that you may transfer these ideas to your young colleagues as well.

I have copied these  Bullet Points below (my explaining comments in Grey, for I cannot recap his complete talk).

  • Never stop learning and teaching.
  • Normal is a setting on a washing machine
  • No set roadmap.
    • There is no “normal” path
  • Intelligence, hard work, luck.
    • –Two out of three don’t make it.
  • Continuous improvement.
  • Karma, help others and be true to yourself.
  • Happiness percentage (in everything you are doing, in the end happiness should win over frustration)
  • Time management.
    • The only commodity you can never get back (….is wasted time ).

Restek is very aligned with these Short Cuts for our company also is dedicated to the idea of supporting young scientists, as our RASP (Restek Academic Support Program) may show.*

Thank you, Dr. Cortes for your inspiring work and thoughts.

HJM

*Please ask for our RASP program

Flyer RASP_GNOT2495-UNV_RASP_001

Quadrupole Scan Speed and the 8270 Instrument Checkout Mix – Part II – The DFTPP Evaluation

Yesterday’s blog gave an overview of how choosing the wrong scan speed could be detrimental to the tailing factor evaluation. Before someone asks, I thought we’d spend today looking at the impact of scan speed on the decafluorotriphenylphosphine (DFTPP) tune evaluation.

I know I’ve covered this before, but here are the tuning criteria from EPA 8270 D (rev 5) intended for scanning quadrupole instruments:

11.3.1.1 – In the absence of specific recommendations on how to acquire the mass spectrum of DFTPP from the instrument manufacturer, the following approach should be used: Three scans (the peak apex scan and the scans immediately preceding and following the apex) are acquired and averaged. Background subtraction is required, and must be accomplished using a single scan acquired within 20 scans of the elution of DFTPP. The background subtraction should be designed only to eliminate column bleed or instrument background ions. Do not subtract part of the DFTPP peak or any other discrete peak that does not coelute with DFTPP.

HT Figure 1

EPA Method 8270 “Table 3 – DFTPP Key Ions and Ion Abundance Criteria”

To summarize, the tune evaluation consists of a background subtracted average of the 3 apex scans evaluated against the criteria in Table 3. Using the same instrument settings used in the tailing factor evaluation blog, let’s evaluate DFTPP at 3.1 and 5.9 Hz.

First, there is the qualitative chromatographic evaluation.  Figure 1 is an extracted ion chromatogram showing the six major ion fragments of DFTPP, collected at 3.1 Hz. Notice that the green trace of m/z = 442 is out of sync with the other mass peaks. This is also the case, but to a lesser degree, in Figure 2, the extracted ion chromatogram of DFTPP collected at 5.9 Hz. This offset of the high mass peak is due to spectral tilting, which is one of the issues experienced when scanning too slow.

Slide12

Figure 1 – Extracted major DFTPP ion chromatogram collected at 3.1 Hz

Slide19

Figure 2 – Extracted major DFTPP ion chromatogram collected at 3.1 Hz

Spectral tilt occurs when the amount of an analyte entering the detector increases during an individual scan (mass/second). Temperature programing and constant carrier gas flow combined with high efficiency GC columns yields chromatographic peaks with very steep slopes. The 5975C used here scans from high mass to low mass because it takes less energy to reset from low to high. Using DFTPP as an example, it would stand to reason that if you slowly scan from m/z = 500 to m/z 35 while the amount of DFTPP entering the detector is increasing, the resulting value for m/z = 442 would be biased low, and the m/z = 51 value would be biased high. The opposite would be true for the tailing side of the peak, where the amount of DFTPP entering the detector is dropping over time. 3.1 Hz is slow enough to see these effects when the three scans specified by EPA 8270 (the peak apex scan and the scans immediately preceding and following the apex) are inspected individually (Figure 3, Figure 4 & Figure 5)

Slide13

Figure 3 – 3.1 Hz DFTPP scan 1463 (pre-apex)

Slide14

Figure 4 – 3.1 Hz DFTPP scan 1464 (apex)

Slide15

Figure 5 – 3.1 Hz DFTPP scan 1465 (post-apex)

Consider that the mass spectrum for each “scan” is just a bar graph showing the response for each ion. In the absence of spectral tilt, there would be no change in the slope of a line drawn from the top of the bar for m/z = 51 to the top of the bar for m/z 442, as shown in Figure 6, when you compared each scan for a given compound.

Figure 6 -

Figure 6 – 3.1 Hz DFTPP scan 1463 (pre-apex) with trace for visualizing low to high slope shifts

To perform this tilt evaluation for the 3.1 Hz scan rate, we created an X,Y scatter plot, with the responses for m/z = 51 and m/z = 442 entered for each of the three scans used for the DFTPP evaluation (Figure 7). The slopes are very different.

Figure 7 -

Figure 7 – Spectral Tilt evaluation (m/z = 51 and m/z = 442) at 3.1 Hz

Nearly doubling the scan rate to 5.9 Hz does not eliminate the spectral tilt, but it does minimize the effects. Looking back at Figure 2, it is clear the 3 scans used for the DFTPP evaluation are all clustered near the actual peak apex, when the 2 adjacent scans from the 3.1 Hz data were a ways down the front and back of the DFTPP peak (Figure 1). This clustering near the apex is reflected in the ion ratios of the 3 individual scans used for the tune evaluation (Figure 8, Figure 9, and Figure 10)

Figure 8 -

Figure 8 – 5.9 Hz DFTPP scan 2638 (pre-apex)

Figure 9 -

Figure 9 – 5.9 Hz DFTPP scan 2639 (apex)

Figure 10 -

Figure 10 – 5.9 Hz DFTPP scan 2640 (post-apex)

Evaluating the spectral tilt using the same X,Y scatter plot setup used for the 3.1 Hz evaluation shows less variability for the for m/z = 51 and m/z = 442 ions acquired at 5.9 Hz (Figure 11) and a much smaller range of slopes.

Figure 11 -

Figure 11 – Spectral Tilt evaluation (m/z = 51 and m/z = 442) at 5.9 Hz

A box plot of responses for the averaged scans for the 3.1 Hz (Figure 3, Figure 4 and Figure 5) and 5.9 Hz (Figure 8, Figure 9 and Figure 10) evaluations highlights the reduced response variability for virtually all the ions of significance (Figure 12). This is because the faster scan rate allows repeated sampling in a zone where the concentration of analyte in the detector is fairly stable (the pseudo-plateau at the peak apex).

Figure 12 -

Figure 12 – Ion Responses for the 3 DFTPP scans averaged at 3.1 and 5.9 Hz

Over the course of two days, I evaluated 40 DFTPP tune verification runs – 20 at 3.1 Hz, and 20 at 5.9 Hz. One of the 20 tune evaluations collected at 3.1 Hz failed because the ratio of m/z = 68 to m/z = 69 exceeded 2.0. All 20 DFTPP runs collected at 5.9 Hz met all tune criteria. The 68:69 ion ratio was interesting because even though the variance for the 5.9 Hz data was larger than that of the 3.1 Hz data, the median was approximately 0.5 for the 5.9 Hz data though greater than 1.5 for the 3.1 Hz (Figure 13).

Figure 13 -

Figure 13 – Boxplot and Individual Value plot of the m/z = 68 to m/z = 69 ratio for 3.1 and 5.9 Hz DFTPP evaluations (n=20)

Overall, average tune performance between the slow and fast acquisition populations was similar (Figure 14 and Figure 15), even though there were zero failures in the 5.9 Hz data set. I suspect the lower scan to scan variability of the faster scan rate highlighted in Figure 12 makes the occurrence of an evaluation failing outlier less likely.

Figure 14 -

Figure 14 – Boxplot of 3.1 and 5.9 Hz DFTPP Tune Evaluation Ion Ratios (n=20)

Figure 15 -

Figure 15 – Variances for the 3.1 and 5.9 Hz DFTPP Tune Evaluation Ion Ratios (n=20)

Quadrupole Scan Speed and the 8270 Instrument Checkout Mix

In my last blog (here), I promised an update on the impact of the detector scan speed on the tailing factor. I had speculated that a pentachlorophenol tailing factor value of 0.94 was more likely < 1.0 because of the scan rate, rather than column overload.

The examples I put forward here were collected on a system with a different configuration (Table 1), but the basic principles hold true. While most sources will tell you that you need 6 to 7 scans to accurately define a peak, this is only sufficient for reproducible peak areas. 6 to 7 scans are not necessarily sufficient for evaluating peak symmetry.

Instrument Configuration

Table 1 – Agilent 7890A – 5975C GC-MS configuration (Fast and Slow scanning)

Figure 1 and Figure 2 are examples of pentachlorophenol peaks acquired at 3.1 Hz (Slow on Table 1) that show asymmetric peak apex assignment and a resulting bias in the calculated tailing factors. Figure 3 is an overlay of 9 injections that highlights the variability in peak apex assignment when a slow acquisition rate is used.

PCP slow 1

Figure 1 – 5.55 ng Pentachlorophenol (acquired at 3.1 Hz) Tailing Factor Evaluation showing peak apex assignment biased to the rear of the peak, yielding a tailing factor result biased low.

PCP slow 2

Figure 2 – 5.55 ng Pentachlorophenol (acquired at 3.1 Hz) Tailing Factor Evaluation showing peak apex assignment biased to the front of the peak yielding a tailing factor result biased high.

PCP slow overlay

Figure 3 – Overlay of 9 Pentachlorophenol chromatograms acquired at 3.1 Hz showing high variability in peak apex assignment.

We can nearly double the acquisition rate by halving the number of samples that are averaged for each data point. Figure 4 is an example pentachlorophenol peak acquired at 5.9 Hz (Fast on Table 1) that shows little bias in the peak apex assignment. Figure 5 is an overlay of 8 injections that highlights the low variability in peak apex assignment when a fast acquisition rate is used.

PCP fast

Figure 4 – 5.55 ng Pentachlorophenol (acquired at 5.9 Hz) Tailing Factor Evaluation showing minimal room for peak apex assignment bias.

PCP fast overlay

Figure 5 – Overlay of 8 Pentachlorophenol chromatograms acquired at 5.9 Hz showing little variability in peak apex assignment.

Figure 6 and Figure 7 are graphical representations of the tailing factors calculated from the overlays in Figure 3 and Figure 5.

PCP BENZ slow TF graph

Figure 6 – Set of 9 Pentachlorophenol (Figure 3 overlay) and Benzidine tailing factors acquired at 3.1 Hz.

PCP Benz Fast TF graph

Figure 7 – Set of 8 Pentachlorophenol (Figure 5 overlay) and Benzidine tailing factors acquired at 5.9 Hz.

EPA 8270D has initial tailing factor criteria of ≤ 2.0 for both Pentachlorophenol (PCP) and Benzidine (BENZ). As we saw in Figures 1 and 2, using a slow scan speed of 3.1 Hz yielded a range of 0.94 to 1.69 for PCP on a new column, inlet liner and inlet seal combination. It is easy to envision a situation where analyzing a few extraction batches may leave the instrument unsuitable for further sample analysis, even after inlet maintenance. Figure 8 and Figure 9 are statistical demonstrations of reduced variability in the tailing factor calculation for both Pentachlorophenol and Benzidine when the faster acquisition speed is used.

Box Plot Slow Fast

Figure 8 – Boxplot (with labeled medians) of Pentachlorophenol and Benzidine tailing factors under slow and fast acquisition speeds.

ANOVA slow fast

Figure 9 – ANOVA showing reduced variances for both compounds under fast acquisition speed