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SPME Arrow Size = Sensitivity

By now you should know what a SPME Arrow is and that you should be using a SPME Arrow instead of a traditional SPME fiber, because the Arrow is more durable. The same physical dimensions, which make an Arrow more mechanically robust, also afford an increase in sensitivity. Last time, you saw that the phase volumes could increase by approximately 4 to 6x greater on an Arrow. This time, we see how that correlates to an increase in response.

Colton and I took a 100 µm PDMS traditional SPME fiber and a 100 µm PDMS SPME Arrow. We sampled headspace (HS) volatile organic compounds (VOCs) which had been spiked in drinking water at 2.5 ppb, as per method ISO 17943. Everything was equivalent (i.e., equilibration times, extraction times, desorption temperatures, etc…), except for the fibers.

Here is what the respective c-grams look like:

2 minute HS extraction of ISO 17943 VOCs on 100 µm PDMS SPME Arrow and 100 µm PDMS traditional SPME fiber

 

The following are three take-away messages I would like you to have:

  1. Obviously, the SPME Arrow’s response is higher than a traditional fiber, which should be of no surprise. It all makes sense that if you increase your phase volume, you increase your volume of target analyte collected, and thereby increase your analytical response.
  2. It is hard to accurately quantify this by the chromatogram overlay; however, you will have to trust me that the SPME Arrow demonstrated a ~4x increase in response over traditional SPME fibers for most compounds. Low and behold it is right on target with the phase area/volume increase we discussed last time.
  3. I say “on average” in the previous point, because it is important to note that the increase in response for very volatile compounds like vinyl chloride is ~10x on the SPME Arrow vs the traditional SPME fiber. Whereas, a semi-volatile compound like naphthalene only sees a 2x increase in sensitivity on the SPME Arrow compared to the traditional SPME fiber. This has more to do with partial pressures (i.e., Henry’s law constants) of these compounds and therefore their availability in the headspace being the rate-limiting step. If you have not already done so, take a look back at the chromatogram and you will see what I am talking about.

So now you have the option of a more durable and more sensitive SPME product. What more could you want!? Well, stay tuned for next time when I talk about how you can trade in your Arrow’s increase in sensitivity for speed…

Upcoming Restek Cannabis Testing Seminar

10/9 UPDATE- This seminar has been cancelled. Stay tuned for future dates. 

New to chromatography? Looking to brush up on your current analytical skills? Interested in what different types of cannabis testing currently exist?

The analytical market for cannabis is a rapidly growing, burgeoning field. As more and more states legalize cannabis, an increased number of analytical labs and scope of services will be necessary.

If you are involved with a lab that analyzes cannabis by chromatography, then this seminar is tailor-made for you.

Restek will be holding this one-day seminar on Friday, October 13 in Manchester, NH. The registration fee is only $50 for this event.

Chromatographic Foundations for Cannabis Testing Seminar

Join Restek for this three-hour overview of the fundamentals of chromatography for cannabis-testing laboratories. This event will be valuable to any scientist or technician working directly with the chromatographic instruments used for the wide variety of testing performed on cannabis samples. A brief historical example will serve to highlight the foundations common to all chromatographic techniques. Next, this seminar will explore the major differences between the principal types of chromatography — gas chromatography and liquid chromatography — using examples from the cannabis industry to illustrate the advantages of each type. Finally, a survey of sample introduction and detection techniques, including headspace sampling and mass spectrometry, will introduce the audience to the variety of options available to supplement the separations performed by the chromatograph.

Schedule
8:30–9:00 a.m. — Registration & breakfast
9:00 a.m.–12:00 noon — Chromatographic Foundations for Cannabis Testing 
12:00–1:00 p.m. — Lunch

Cost
$50 (includes breakfast, lunch, and course materials)

Outline
— Universal chromatographic foundations
— GC-specific foundations
— LC-specific foundations
— Special topics: Sample introduction & detection
— Questions and answers

Complete details, along with lodging information and registration instructions, can be found here.

We look forward to seeing you!

Cassini-Huygens: 20-Year Mission Accomplished

Copyright: ESA/NASA/JPL/University of Arizona

Cassini’s Probe Huygens Decent to the Surface of Saturn’s Moon Titan. Courtesy: ESA / NASA / JPL / University of Arizona (5)

On October 15th, 1997, the $3 billion spacecraft went on a seven year, two-billion-mile journey to study the planet Saturn along with its moons and rings. After arriving at the Saturnian system, Cassini deployed the 700-pound Huygens probe to its largest moon, Titan. At 100 miles above the surface the aerosol collector pyrolizer (ACP) and gas chromatograph – mass spectrometer (GC-MS) identified the components in the atmosphere (1). Initial observations after touchdown revealed methane rain drenching the hills and staining the ground with streaks left from higher molecular weight hydrocarbons. The probe was only expected to survive for several minutes, however, the battery had enough power to operate on the surface for nearly 70 minutes. While several different manufacturers were evaluated for capillary columns, two of Restek’s MXT columns were chosen; MXT-1 & MXT-1701 in the 0.18mm internal diameter format (2,3).  The most abundant organic compounds on the surface of the planet were evaporating gases such as; ethane, acetylene, cyanogen and carbon dioxide (4). Most interesting was the discovery that the probe’s landing had allowed for an increase of methane indicating the ground contains high amounts of the liquid. Further investigation by the Cassini craft orbiting Titan discovered an ocean of liquid methane 200 miles under the crust. Today Cassini ends its 13-year orbit around Saturn and NASA has steered the craft into the atmosphere. The gravitational forces of the planet accelerated the spacecraft to 75,000 miles per hour, effectively vaporizing it. We are proud to have been able to contribute our columns to such a monumental mission.

Further Reading: Sitting Down with a Chromatography Icon: Dr. Robert Sternberg

http://www.restek.com/Technical-Resources/Technical-Library/Editorial/general_gen_0030

 

  1. DiGregorio, B. E. GC-MS Analysis on Titan Mission. May 2005. Spectroscopy. http://www.spectroscopyonline.com/gc-ms-analysis-titan-mission?id=&sk=&date=&pageID=3

2. Navale, V., Harpold, D. and Vertes, A. Development and Characterization of Gas Chromatographic Columns for the Analysis of Prebiologic Molecules in Titan’s Atmosphere. Anal. Chem. 1998, 70, 689-697. http://pubs.acs.org/doi/abs/10.1021/ac9708598

3. Szopa C., Sternberg R., Rodier C., Coscia D., and Raulin F. Development and Analytical Aspects of Gas Chromatography for Space Exploration. February 2001. LCGC Europe. http://alfresco.ubm-us.net/alfresco_images/pharma/2014/08/22/5eca4f66-0659-4f22-afdb-5a63cb4a2095/article-7464.pdf

4. Niemann, H.B., Atreya, S.K., Bauer S.J., Biemann K., et. al. The Gas Chromatograph Mass Spectrometer for the Huygens Probe. Space Science Reviews. 2002. 104: 553-591. https://link.springer.com/article/10.1023/A:1023680305259

5. Image provided Courtesy of ESA / NASA / JPL / University of Arizona http://www.esa.int/spaceinimages/Images/2017/09/Descent_to_Titan

 

Which LC column should I use for my method?

We understand that with so many products on the market, choosing a column to get started can be difficult. Of course, column selection depends on what type of method you are following and what kind of LC system you have in your lab. Let’s begin by looking at our choices for the more traditional, fully porous particle LC columns.

 

 

If your method is a USP or other compendial method, the preferred column would be one of our Roc HPLC columns. The ROC phase selections we offer encompass most of what you would need for these methods. The Roc columns are very appropriate for any highly regulated work environment where a rugged and long-lasting column is desired and lot to lot reproducibility is critical. Most regulations also allow for slight modifications in column dimensions, as well as other parameters for example, as indicated here in this document from the FDA: https://www.fda.gov/downloads/ScienceResearch/FieldScience/LaboratoryManual/UCM173085.pdf

 

 

If your method is not a USP method, but is very well established for a 3 or 5 µm particle column, simple in design, and requires very high reproducibility, the Roc HPLC columns are still preferred. If you find that the phase or dimensions you need are not offered as a Roc column, the next place to look would be within our Ultra columns. Ultra columns are also available in preparative sizes if you are doing purification work with large sample sizes.

 

 

 

 

 

If you have a challenging method that is written for UHPLC (<2.0 µm fully porous particles) or one that you would like to eventually convert to a UHPLC method, our new Force LC columns are the ideal product. We are excited to offer these columns made with the highest quality silica in a 1.8 µm particle size, as well as 3 and 5 µm for easy method transfer between HPLC and UHPLC systems. Force columns are available in C18, Biphenyl, and Fluorophenyl phases.

 

 

 

 

 

If your method requires an SPP (superficially porous particle) column or you are looking for superior separation and a fast analysis, but you do not have a UHPLC system, the Raptor columns are ideal. Raptor 2.7 µm columns are preferred for large analyte panels such as pesticides, steroids, and drugs, and are typically used with LC-MS/MS.

Raptor 5.0 µm columns won’t be able to separate quite as many analytes, but separation is still better than you would get with a fully porous 5.0 µm column. Often when using LC-MS/MS, less chromatographic separation is acceptable because the MS can identify closely eluting and even coeluting compounds, unless they are isobaric, so a 5.0 µm column is often still useful. Although it depends on the exact dimensions, generally speaking, the 2.7 µm columns are intended for use with LC systems that have a 600 bar (8700 psi) pressure limit, whereas the 5.0 µm columns are intended for those with a 400 bar (5800 psi) limit. Please see the blog post “Should I use a 2.7 or 5 µm Raptor column?” for more discussion of this.

If you need help selecting a guard cartridge and/or holder for any of these, please see the earlier blog post “Which guard cartridges and holders go with which LC analytical columns?”.

To find a listing of all LC columns we offer, you might also find our LC Columns Physical Characteristics Chart useful.

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

 

Should I use LC or GC for my analysis?

Although most analysts already know which approach they need to use for analysis, this is occasionally a topic of discussion.  While this blog post is not meant to give an absolute answer for each specific application, I hope to provide some tips to steer you in the general direction towards a solution, if this is your dilemma. I will focus here on characteristics of the analyte compound, which should be the primary concern. Other factors, such as cost and detection methods will not be discussed in this blog post.

 

To use GC for analysis:

First of all, the analyte must be volatile.  This is because it will need to exist in a vapor state in order to partition between the carrier gas stream (mobile phase) and the stationary phase inside the column.  While it depends somewhat on the choice in column, generally the compound should have a boiling below about 400°-500°C (at atmospheric pressure of 760 mm Hg).  For this reason, most GC analytes are smaller compounds with a molecular weight of less than 1000.  An example of a compound that works well for GC analysis is naphthalene, which has a boiling point of 217.9°C.  Although we have many examples of analyses that include naphthalene, here is a chromatogram that represents one of the most common applications (EPA method 8270):

http://www.restek.com/images/cgram/gc_ev1418.pdf

In order for partition to occur in the vapor state, the molecule must also remain intact. Ideally, it should not decompose upon heating.  In other words, it must be thermally stable (not thermally labile).  For example, riboflavin decomposes between 278-282°C and is generally not analyzed using GC.  Often in cases like this, GC analysis can be done if the compounds are derivatized.

 

Molecules that can be analyzed by GC or LC:

There are some compounds that could be analyzed equally well by GC or LC.  Bisphenol A is a good example of this.  Here is a link to example chromatograms for both LC and GC analyses of this:

http://www.restek.com/chromatogram/search?s=bisphenol%20A

Some other good examples include compounds like nitrobenzene:

http://www.restek.com/chromatogram/search?s=nitrobenzene

Sometimes analytes that need to be derivatized for GC analysis do not need derivatization for LC.  Chlorophenoxy acid herbicides are a good example of this. Here is an example of GC analysis for these herbicides (derivatized to methyl ester form):

http://www.restek.com/images/cgram/gc_ev00091.pdf

And an example of LC analysis (underivatized):

http://www.restek.com/images/cgram/lc_ev0355.pdf

Although there are many examples of compounds that can be done either way, LC is considered more universal and generally does not require derivatization as often.

 

To use LC for analysis:

Analytes for LC need to be soluble in a suitable mobile phase, but they do not need to be volatile at all. As a result, compounds range from small to very large.  As is the case for GC, most applications for LC are for organic molecules. Although it may be possible under certain circumstances for some inorganic compounds to be analyzed by LC, this would be beyond the scope of what Restek products can accomplish and will not be discussed here.

LC analysis is usually more difficult for the smallest of molecules, particularly if they coincide with the solvents in the mobile phase, for example methanol, acetonitrile or water.  Also, compounds that exist as gases at room temperature cannot be analyzed by LC, or at least it would not be practical.

In LC, to allow partitioning between the liquid mobile phase and the stationary phase inside the column, a compound must be reasonably soluble in the mobile phase and it must have some affinity toward the stationary phase. The phrase “like dissolves like” is very applicable when considering solubility. A compound’s chemical interaction toward a stationary phase could occur in several different ways.  If interested in reading more on this topic, I suggest reading USLC Column Selection and Mobile Phase Adjustment Guide. As discussed in the guide, four of the primary mechanisms are dispersion, polarizability, hydrogen-bonding and cation exchange.

As in GC, an analyte for LC also must remain intact and not decompose. Fortunately, thermal stability is not a concern for LC, since analysis can usually be performed at or near room temperature.  I mentioned earlier that riboflavin does decompose upon heating.  We find that riboflavin is analyzed fairly easily by LC, though, as shown in the following chromatogram:

http://www.restek.com/images/cgram/lc_ff0558.pdf

While decomposition is not common with LC analyses, ionization in aqueous solution occurs quite often. Consequently, such analytes are affected dramatically by the pH of the mobile phase.  To control these affects, most analysts will use buffer in the mobile phase. If interested in reading more on this topic, please refer to the following:

When should you use a buffer for HPLC, how does it work and which one to use?

New Advice on an Old Topic: Buffers in Reversed-Phase HPLC

 

Resources available:

A good tool to use that is at your disposal is Restek Searchable Chromatogram Library.  To look for example analyses for compound(s) of interest, simply type their name or CAS number in the search box. You may see examples for the analysis done by LC or GC, or perhaps both.  If the compounds of interest tend to decompose, become reactive or are difficult to detect, you may see examples of the analysis done by derivatization.

If interested in reading more on this topic, here are some articles that may be helpful:

http://bitesizebio.com/29109/run-fly-comparison-hplc-gc/

http://www.news-medical.net/life-sciences/Liquid-Chromatography-versus-Gas-Chromatography.aspx

http://lab-training.com/2014/04/02/what-are-the-differences-between-gc-and-hplc/

 

I hope you find this helpful. Thank you for reading.

Don’t forget about your lab’s moisture traps/filters, especially when it’s summer

One of the common issues which arise in laboratories during the summer is moisture in the gas lines. Although I never quite understood how this would happen when using gas cylinders, it does, especially when a manifold system is used, or if the length of tubing from the gas source to the instrument is longer than several feet/meters.  Common observations when this happens are:

  1. Unstable instrument detectors (especially TCDs, HIDs, ECDs and mass specs).
  2. Less than ideal chromatography (shifting retention times, unusual peak shapes, etc.).
  3. Unusual activity issues in the injection port and/or ghost peaks.

 

So what is an inexpensive way to minimize these issues, especially in the summer when many locations experience hot and humid conditions? Install an indicating moisture trap/filter on all gas lines.

 

So how would you know which trap filter would be best for you laboratory? Ask yourself the following questions:

Q1. What is the gas?

Q2. Is the gas line tubing copper or stainless steel?

Q3. What is the maximum gas flow rate the trap/filter may be subjected to?

Q4. What is the maximum gas pressure the trap/filter may be subjected to?

 

Here are my suggestions to aid in trap/filter selection based upon the questions listed above:

S1. Generally speaking, the traps/filters we sell are for use with laboratory grade (high purity) inert gases.  If you plan to use a Restek trap/filter with a corrosive, flammable or reactive gas, email us first at support@restek.com

S2. Most use traps/filters containing brass end-fittings with copper tubing and stainless steel end-fittings with stainless steel tubing.  For additional information on the topic, I suggest you review I need a fitting, but which one?

S3. Published flow rates can vary among manufacturers, so make sure you are aware of exactly what a published value represents.  For example, is the published flow rate the maximum flow rate the trap/filter can handle, or is it the maximum flow rate in which the trap/filter can effectively clean (scrub) the gas?  In the context of this post, what is the maximum gas flow rate through the trap/filter that can effectively remove much of the moisture?

S4. For obvious safety reasons, make sure the published maximum trap/filter pressure is not exceeded.

 

Several of the popular indicating moisture filters are shown below.

 

Indicating Moisture Trap

 

 

Restek Super Clean Ultra-High Capacity Moisture Filter

 

 

Don’t forget the baseplate.

Of course we also sell a wide variety of other filters including non-indicating moisture filters, and others for the removal of oxygen and/or hydrocarbons.   To view all of your options, click on the link below.

http://www.restek.com/Supplies-Accessories/GC-Accessories/Gas-Purification

 

Or for removal of a specific contaminant(s), the links below may be quicker to navigate:

Moisture Removal

Oxygen Removal

Hydrocarbon Removal

Multiple Gas Removal

 

You may also find these links (below) informative. If you have any questions, you may email me directly or technical service at support@restek.com .

 

SDS’s for 22010, 22011, 22014 and 22015 (Indicating Oxygen & Moisture traps/filters)

Several things you may not know about our Super-Clean® Gas Filters

Contents inside your baseplate trap

Indicating Oxygen and Moisture Traps-“Hey, it looks like my trap arrived partially spent”

Did I just break my hydrocarbon trap (22012 and/or 22013)?

SDS (MSDS) for VICI® Mat/Sen® Gas-Specific Purifier Modules

Is the column shown on the COA for a chemical reference standard the best column for these compounds?

 

In some cases the answer is “yes”. In most cases the answer is “no”.

 

So why would we choose a column that is not the optimal for a specific standard?

It’s because of the large number of reference standards we sell. Each mix needs to have its own validated method. If we were to test each standard with their own column and/or method, production would slow to a crawl.

 

Stated another way by a product development specialist:

In order to provide certified reference materials, each product manufactured at Restek needs to be analyzed using a validated method.  It is impractical to have an optimized, validated method for each manufactured standard and be able to deliver certified reference materials to our customers that meets their needs in terms of cost and delivery time.

 

Because some customers are not aware of this, most chromatograms contain following statement (printed directly below the chromatogram):

This chromatogram represents a general set of testing conditions chosen for product acceptance. For optimal results in your lab, conditions should be adjusted for your specific instrument, method, and application.

Also keep in mind that we may sell a standard which was designed for a GC method, but actually test it using a HPLC column, and vice versa. An example of this would be Restek #31607 which was designed for EPA Method 8095 (Explosives by Gas Chromatography) but was tested at Restek using HPLC.

 

So how would you know which would be the best column choice? I suggest first looking through our application chromatograms using the online search feature http://www.restek.com/chromatogram/search/

The way I use it is the following: I type in a compound name, followed by a comma, then a space, then another compound name…  For example, if I type methane, ethane, propane, butane the following link shows the results I receive: http://www.restek.com/chromatogram/search?s=type:GC::methane::ethane::propane::butane

 

You can also try ProEZGC http://www.restek.com/proezgc   Please note that this is only for GC columns, and is limited to capillary liquid phase partition (Rtx-1, Rtx-5, Stabilwax, etc…) and not capillary solid phase adsorption (PLOT) or packed or micropacked.

 

In summary, you should not assume that the column and/or instrument method parameters on the chemical reference standard COA are for the optimal analysis of that reference standard. Instead, one of our other references, such as the chromatogram search tool and/or ProEZGC are better choices for choosing the correct column and method parameters.

 

Below are additional links which you may find useful, especially the chromatograms. Thank you for reading.

Industry Pages – Environmental Solutions

Industry Pages – Foods, Flavors & Fragrances Solutions

Industry Pages – Clinical, Forensic & Toxicology Solutions

Industry Pages – Food Safety

Industry Pages – Pharmaceutical Solutions

Industry Pages – Medical Marijuana

Industry Pages – Petrochemical & Chemical Chromatography

 

Cross reference lists for GC columns and other useful links

With everyone in such a hurry these days, when you cannot find what you need in a few minutes, how often do you give up and move onto your next project?   I’m the same way, so when I find a way to group commonly referenced information into one convenient area, I usually do it.  Below is one of these areas if you are trying to find the correct Restek column from a description listed in a method or cross reference another manufacturer’s GC column to a Restek column.  I hope it saves you time like it does for me.

 

GC Column Cross-Reference: Columns by Phase

USP Phase and Support Cross Reference Chart

GC capillary columns for the European Pharmacopoeia methods

1s 5s Waxes Column Cross-Reference Tool

ASTM Petrochemical Method Chromatography Product Guide

Organic Volatile Impurities: Retention Time Index

Can’t find the GC column you are looking for? Try using our Online Tools

Column Selection Poster

Structures for Capillary Column Phases

Upcoming revisions to three wastewater methods reviewed at NEMC 2017

Monday morning, representatives from the EPA’s wastewater office reviewed the significant changes for the three major GC wastewater methods: 608.3, 624.1, and 625.1.

The overarching goals were to modernize the methods and bring the language and QC requirements in line with the methods from the drinking water and hazardous waste offices. The methods have been modified to include capillary columns, and the QC requirements have been updated to reflect this. Maximum calibration RSDs have been dropped to 20% from 35%, and they want to see full list QC spikes.

The most disappointing change is the requirement that the CCV be prepared from a 2nd source. The 500 and 8000 series methods have recently and explicitly moved away from this practice because a 2nd source doesn’t so much verify ICAL stability (precision) as it does accuracy. I’m including the relevant sections for 8270D Rev 5 below:

9.3.2 There must be an initial calibration of the GC/MS system as described in Sec. 11.3. In addition, the initial calibration curve should be verified immediately after performing the standard analyses using a second source standard (prepared using standards different from the calibration standards). The suggested acceptance limits for this initial calibration verification analysis are 70-130%. Alternative acceptance limits may be appropriate based on the desired project-specific DQOs. Quantitative sample analyses should not proceed for those analytes that fail the second source standard initial calibration verification. However, analyses may continue for those analytes that fail the criteria with an understanding these results could be used for screening purpose and would be considered estimated values.

11.4.3 The initial calibration (Sec. 11.3) for each compound of interest should be verified once every twelve hours prior to sample analysis, using the introduction technique and conditions used for samples. This is accomplished by analyzing a calibration standard (containing all the compounds for quantitation) at a concentration either near the midpoint concentration for the calibrating range of the GC/MS or near the action level for the project. The results must be compared against the most recent initial calibration curve and should meet the verification acceptance criteria provided in Secs. 11.4.5 through 11.4.7.

You’ll notice that the ICV acceptance criteria when using a 2nd source is 70-130%, while the daily (or 12 hour) CCV acceptance criteria is ± 20%. This is because the 2nd source brings additional uncertainty.

I’m making the revised methods available for anyone to review.

Method 608.3 – Organochlorine Pesticides and PCBs by GC/HSD

Method 624.1 – Purgeables by GC/MS

Method 625.1 – Base/Neutrals and Acids by GC/MS

A little extra information for installing/conditioning a new packed/micropacked column

Even though we include instruction sheets with most of our packed/micropacked columns which contain enough information for most customers, some customers (many who are new to using packed columns) contact us routinely asking for some additional assistance. As a result, I decided to add some additional information to our current instruction sheets hoping it helps those customers who may benefit from the extra guidance.  I hope you find them useful.

If you have any additional questions or concerns, email me directly or support@restek.com

 

For 1/8″ OD and 3/16″ OD packed   2017 Packed Instructions 2.0mm 3.2mm

For 0.95mm OD and 1/16″ OD micropacked  2017 Micropacked 0.75mm 1mm

 

You may also find these links useful.

Micropacked Columns: 0.53mm ID Instruction Sheet (PDF)

Molecular Sieve 5A & 13X packed columns – Installation / Conditioning / Helpful Hints

ShinCarbon columns – Installation / Conditioning / Helpful Hints

How do I condition a new packed or micropacked column?

 

Maximum temperatures of packed columns – Porous Polymers

Maximum temperatures of packed columns – Liquid Phases

Maximum temperatures of packed columns – Hydrocarbon Analysis

Maximum temperatures of packed columns – Molecular Sieves

Maximum temperatures of packed columns – Sulfur Gases

 

How to Condition a New Capillary GC Column

PLOT Columns Instruction Sheet (PDF)