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October 2001
Vol. 10, No. 10,
pp 23–24, 26.
Instruments & Applications
The origin of specification

opening artWhen buying any new piece of equipment, dare to compare.

Your laboratory needs a new gas chromatograph. You contact the various vendors and request literature on their latest equipment. Each sends you a beautiful, 16-page, full-color glossy brochure and perhaps a black-and-white specification sheet for their instrument. Your first instinct is to study the brochure and file the data sheet for reference. I propose that this may not be the best approach. Let’s pull those instrument specifications out of the drawer and examine how they can play an important role in helping you choose equipment.

Why Look at Specs?
The simple answer is that it makes sense as an educated consumer of laboratory equipment. Specifications essentially are the résumé of an instrument. Just as a résumé guides you in an interview with a job candidate, the specification sheet is a starting point from which to begin your evaluation of an instrument. A thorough study of either document will provide you with both answers and questions.

For example, even though you have some idea of how the equipment performs, based on your research and what the salesperson has told you, the data sheet can confirm your conclusions. Can the unit achieve the detection limits or other performance criteria required for your analysis? Seeing the data in print provides some assurance. However, note the fine print at the bottom of the page, which says, “specifications are subject to change without notice.”

You can likewise determine which features are standard and which are optional. Because today’s equipment is complex, it is very easy for salespeople to imply, and customers to infer, that a function is standard when it actually requires an upgrade from the basic unit. Does a packed column injector include an adapter for wide-bore columns? Can the column oven heat or cool that fast without an add-on accessory?

Reviewing specification sheets helps in formulating questions about vendors’ equipment. How will the unit meet your needs and how does it compare with other systems on the market? In addition, a close examination will uncover which features are important enough to be emphasized by the manufacturer and perhaps lead you to reexamine your requirements.

What’s in a Spec?
As an intelligent purchaser, you can learn various facts from reading instrument specifications. Some are obvious; others are subtle. The former are meant to provide information, and the latter are meant to influence your purchasing decision. These two objectives affect how specifications are created, written, and presented.

First, instrument specification sheets provide basic facts such as physical dimensions, electrical requirements, performance, and configuration data. Manufacturers sometimes seem to provide more details than you really need. However, their objective is to answer questions that would otherwise have to be addressed by a salesperson.

Second, data sheets are marketing tools. They highlight features that a vendor believes are special to the product or that provide an advantage over competitors’ products. (Have you noticed how well a salesperson knows the competitors’ “inadequate” specs?) Sometimes it is easier to find these points in a specification sheet than in a brochure. These usually are located in an introductory paragraph, as the initial items in a section, or under an all-encompassing “other” category.

If these features are significant or unique, they can become weapons in the “spec wars”— the use of specifications to eliminate or lock out the competition from the selection process. You may say that this would be a natural consequence of comparing specifications. Obviously, each instrument has certain areas in which it outshines the competition (e.g., injector temperature ranges), and these should be considered in any evaluation. Unfortunately, vendors take this approach to an extreme by introducing attributes that provide an obscure performance advantage (e.g., how many GC methods actually require seven oven program rates?) or an almost hypothetical advantage (e.g., how important is storing your gas chromatograph at –40 ºC?). An interesting exercise is to try and find those parameters targeted toward a deficit of the competition. Examples that I have seen in GC include the number of pneumatic control zones and the mass range for mass spectrometers.

Ultimately, of course, end users must sort through everything and determine which specifications are relevant to their requirements.

Having encouraged you to closely examine specifications, I must now warn you that it may not be an easy task. Comparing specifications from one vendor with another can be difficult for a variety of reasons.

Inconsistent nomenclature describes similar, if not identical, instrument features. The terms digital flow control, electronic flow control, and programmable pneumatic control all appear in discussions on GC carrier gas flow control. Manufacturers are obviously trying to differentiate their offerings, but it places a burden on the user to determine the exact function being specified. Other discrepancies occur because of a lack of precision in language. Linearity, linear dynamic range, and dynamic range are used as the technical names for the same detector property.

Different test conditions often are used by manufacturers to create specifications. This is most evident in the sensitivity numbers quoted for detectors. For example, the detection limit for a flame ionization detector can be calculated by using propane, butane, or diphenyl (1,1 biphenyl), depending on the vendor. Even more important, the chromatographic parameters that produced the number—capillary or packed column, flow rate, and injection size—are rarely listed. Are conditions important? Remember, each vendor is trying to present an instrument in the best possible light and will naturally optimize the conditions for peak performance. This applies to any calculated value such as oven cool-down time or autosampler reproducibility.

Different units are used to express the same specifications. This occurs often in minimum detection limits for detectors (g vs g/s for electron capture detectors).

Thus, specification writing can be an inexact science. Although organizations such as the American Society for Testing and Materials (ASTM) and the International Union of Pure and Applied Chemistry (IUPAC) have published guidelines for terminology, testing, and results reporting, they are not always followed.

Obtaining Instrument Specs
  • Contact your local salesperson for information.
  • Call the toll-free number and specifically request the specification sheets. If you simply ask for literature about an instrument, you will probably only receive a brochure.
  • Check the manufacturers’ Web sites. Most vendors now post specifications for download from their Web sites. A few still do not, either because they worry that this informs competitors about their products or out of fear that customers use the data primarily to exclude vendors from consideration.
Evaluation Process
After you have assembled all the literature you need (see sidebar,“Obtaining Instrument Specs”) and carefully reviewed it, create a side-by-side comparison table. In the first column, list all the features important to you. (You may want to include a miscellaneous section for those lockout items unique to a vendor.) Across the top of the remaining columns put the names of the instruments you are considering. (See Table 1 for an abbreviated example of a GC comparison table.)

Now complete the chart by putting the data you have extracted from each model’s specification sheet under the model’s name. Analyze the results as you would for an experiment. Draw conclusions, look for outliers, and see where you need more data. It would be extraordinary to complete this table on the first attempt. Now is the time to interview the vendors’ representatives for clarification and fill in the gaps in your chart.

Obtain details on what a specification represents. Is the “height” the height of the GC or the GC plus autosampler shown in the photo?

Confirm that the specifications are current for the unit you would be purchasing, especially those critical to your analyses. Determine exactly what you’re buying and what it can do.

Request backup data on how values were determined. Many vendors publish technical notes on the evaluation of their equipment that contain the experimental conditions used to produce specifications. Ask if they can supply installation guidelines. These documents provide performance criteria for their service personnel during instrument setup. Comparing these numbers to specification data will provide a reality check on how applicable the specifications are for everyday operation.

Ask for an explanation on those highlighted special features to determine if they are important to you. What is “automatic column characterization”, and is it a necessity in your next gas chromatograph?

Don’t hesitate to question salespeople about whether they offer a feature found on a competitor’s data sheet. The information may be available but not listed, or it may be described differently in their literature. If it is not offered, the salesperson will welcome the opportunity to discuss your requirement for this option. At the end of this process, you will have greatly increased your knowledge and understanding of the different instruments and you will have established a good baseline for comparing one unit with another.

The Final Word
Does this mean you can now choose all your instruments solely from their specification sheets? Of course not. But a specification sheet is an effective tool for evaluating laboratory equipment and a good first step to knowing what you are purchasing.

Acting as a résumé for an instrument, specification sheets help confirm that the equipment described can carry out the basic functions required for your analysis. As marketing literature, they can point out what special attributes make the unit different from, and perhaps better than, that of the competition. A specification analysis, as part of a total instrument appraisal weighing factors such as reliability, support, and user interface, can ensure that you have made an educated buying decision.

Further Reading
Leslie Ettre’s article, “Nomenclature for Chromatography (IUPAC Recommendations 1993),” Pure Appl. Chem. 1993, 65, 819–872 is available on the Web at www.merck.de/english/services/chromatographie/iupac/chrnom.htm.

ASTM has published several standards for GC-related equipment, copies of which may be purchased at the group’s Web site, www.astm.org. These standards include

  • E355-96 Standard Practice for Gas Chromatography Terms and Relationships
  • E516-95a Standard Practice for Testing Thermal Conductivity Detectors Used in Gas Chromatography
  • E594-96 Standard Practice for Testing Flame Ionization Detectors Used in Gas or Supercritical Fluid Chromatography
  • E697-96 Standard Practice for Use of Electron-Capture Detectors in Gas Chromatography

Keith S. Kleman is a marketing consultant and freelance writer living in Holmes, NY. Send your comments or questions regarding this article to tcaw@acs.org or the Editorial Office 1155 16th St N.W., Washington, DC 20036.

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